**Figure 3.** a Typical damage of shear walls at the "soft story" during The Kozani 1995 Earthquake in Greece. b. Typical damage of "short" columns during The Kozani 1995 Earthquake in Greece.

## **2. Main applications for dealing with earthquake structural damage utilizing fiber reinforcing plastics**

In section 1.2 a brief description of earthquake structural damage that usually develops in slabs, beams, columns or shear walls of multi-story R/C buildings was presented. In this section, the use of fiber reinforcing plastics (FRP) will be discussed in a way dealing with the correspond‐ ing damage. These FRP materials behave in tension almost elastically till their ultimate state that for the material itself is the breaking of the fibers in tension; they do not develop any forces in shear or compression. The value for the modulus of elasticity is approximately 240Gpa for carbon fibers, 200Gpa for steel fibers and 80Gpa for glass fibers ([8], [17]).

**Figure 4.** Flexural and shear FRP reinforcement for a T-beam.

**Figure 5.** Flexural and shear FRP reinforcement for a column.

The ultimate axial strain values given by the manufacturers reach values in the range of 2%. Consequently, sheets made by these materials, despite their relatively small thickness which is usually below 0.2mm for one layer, can develop substantial tensile forces in the direction of their fibers (see figures 4 and 5). This property accompanied with their low weight and the very easy external application to structural elements, by attaching them on the external surfaces by proper organic or inorganic matrices, results in their being used as effective lon‐ gitudinal or transverse reinforcement for such structural elements that are in need of strengthening (see figures 4, 5 and [12], [15], [16]). However, the following limitations exist for this type of application. The first limitation springs from the fact that the ultimate axial strain value of the order of 2% for the material of the fiber cannot be reached for all the fi‐ bers together in a sheet due to the actual conditions of the attachment that results in nonuniform distribution of the strain field ([13], [18], [19]). Thus, for design purposes, the adopted limit axial strain value is usually below ½ of the ultimate axial strain value for the fiber material. The second limitation results from the way the tensile forces which develop on these FRP sheets can be transferred. When the transfer of these forces relies solely on the interface between the FRP sheet and the external surface of the reinforced concrete structur‐ al elements, the delamination (debonding) mode of failure of these sheets occurs, due to the relatively low value of either the ultimate bond stress at this interface or the relatively low value of the tensile strength of the underlying concrete volume. This mode of failure is quite common and it occurs in many applications well before the corresponding FRP sheets devel‐ op tensile axial strains in the neighborhood of values mentioned before as design limit axial strains (approximately of the order of 1%). Consequently, there is need of alternative ways in order to transfer these tensile forces apart from the simple attachment, in order to en‐ hance the exploitation of the FRP material potential. This will be presented and discussed in the following paragraphs ([11], [13], [20], [21], [22], [23]).

## **2.1. Upgrading the flexural capacity of slabs**

**Figure 4.** Flexural and shear FRP reinforcement for a T-beam.

68 Fiber Reinforced Polymers - The Technology Applied for Concrete Repair

**Figure 5.** Flexural and shear FRP reinforcement for a column.

The ultimate axial strain values given by the manufacturers reach values in the range of 2%. Consequently, sheets made by these materials, despite their relatively small thickness which is usually below 0.2mm for one layer, can develop substantial tensile forces in the direction of their fibers (see figures 4 and 5). This property accompanied with their low weight and the very easy external application to structural elements, by attaching them on the external surfaces by proper organic or inorganic matrices, results in their being used as effective lon‐ gitudinal or transverse reinforcement for such structural elements that are in need of strengthening (see figures 4, 5 and [12], [15], [16]). However, the following limitations exist for this type of application. The first limitation springs from the fact that the ultimate axial strain value of the order of 2% for the material of the fiber cannot be reached for all the fi‐ bers together in a sheet due to the actual conditions of the attachment that results in nonThe main cause of damage in this case is the fact that the flexural capacity cannot meet the demand. Fiber reinforced plastics either in the form of sheets or laminates are externally at‐ tached to the slab either at the top or at the bottom surface (positive or negative bending moment demand). Such a scheme was utilized in the upgrading of the seismic capacity of the slabs of a 4-story R/C building built in 1933 and subjected to the Kozani, 1995 earthquake in Greece ([2], [24]). For this strengthening scheme it was planned to make use of a certain type of Carbon Fiber Reinforcing Plastic laminates (CFRP) with a cross-section 50mm x 1.2mm. These laminates could be applied in-situ either on the upper or lower surface of the concrete slabs with the use of a special epoxy paste. An extensive experimental parametric study was performed on this type of attachment by testing a series of specimens prior to ap‐ plying the best attachment detail to prototype slab specimens. The tests were performed at the laboratory of Strength of Materials of Aristotle University and utilized twin prismatic concrete specimens with dimensions 100mm x 100mm x 150mm each. These twin concrete prisms were joined with these CFRP laminates; they were attached on the two opposite sides of each twin concrete prism. These specimens were then subjected to such a loading as would force the laminates to be detached from the concrete surface. This is shown in figures 6a to 6d. A variety of attachments between the CFRP laminates and the twin concrete prisms were tried. The simplest form of attachment was the one employing only epoxy paste be‐ tween the CFRP laminates and the concrete surface (figures 6a and 6b). Next, a variety of bolting arrangements were utilized with or without the epoxy paste. Figures 6c and 6d de‐ pict such an attachment of the laminates whereby the epoxy paste is combined with one bolt on each side of the twin concrete prism. The bond strength in the first case was found equal to 2.70Mpa whereas in the second case equal to 4.40Mpa, which represents an increase of 63%. The maximum bond strength that was achieved throughout these tests reached the val‐ ue of 7.20Mpa, which represents an increase of 167% from the simple attachment of the lam‐ inates only with epoxy paste. These findings were utilized with slab specimens that were taken from the actual mezzanine slab of the prototype structure; they were cut from parts of the slab where an opening would be formed for a new staircase.

**Figure 6.** Bond tests of the used CFRP laminates

**Figure 7.** Loading arrangement of the slab specimens in flexure.

The Use of Fiber Reinforced Plastic for The Repair and Strengthening of Existing Reinforced Concrete Structural Elements Damaged by Earthquakes http://dx.doi.org/10.5772/51326 71

taken from the actual mezzanine slab of the prototype structure; they were cut from parts of

the slab where an opening would be formed for a new staircase.

70 Fiber Reinforced Polymers - The Technology Applied for Concrete Repair

**Figure 6.** Bond tests of the used CFRP laminates

**Figure 7.** Loading arrangement of the slab specimens in flexure.

**Figure 8.** Flexural behavior of Ermionio slab specimen retrofitted with two CFRP laminates either bonded only with epoxy paste or with epoxy paste and bolts.

**Figure 9.** Flexural failure of the Ermionio slab specimen retrofitted with two CFRP laminates bonded only with epoxy paste. The CFRP laminates were debonded from the concrete slab in this case.

Figure 7 depicts the loading arrangement that was used to subject these slab specimens to four-point flexure. Initially, this was done for a specimen without any retrofitting that reached a maximum bending moment value equal to 8.84KNm. Then two laminate strips, each having a cross section of 50mm x 1.2mm, were attached on the bottom surface of this slab specimen, which was reloaded to flexure as before. Figure 8 depicts the obtained flexu‐ ral behavior of this retrofitted slab specimen. At this stage, the CFRP laminates were attach‐ ed to the specimen only with the use of epoxy paste. This time the specimen reached a maximum bending moment value equal to 20.89KNm, which represents an increase equal to 136% when compared to the maximum bending moment value of the un-retrofitted slab. This retrofitted slab failed in the form of the debonding of the CFRP laminates (figure 9). The debonding of the two CFRP laminates was followed by a sharp decrease in the flexural bearing capacity together with the formation of a plastic hinge at mid-span in the form of excessive plastic deformations of the old steel reinforcing bars at the bottom of the slab as well as crushing of the concrete at the top of the slab. Next, two new CFRP laminates were reattached to the same slab specimen; this time epoxy paste was used again together with bolts penetrating through the slab and securing the attachment of these CFRP laminates. The flexural bearing capacity of the slab this time reached the same maximum bending mo‐ ment value as the one observed when only epoxy paste was used for the attachment of the laminates; they exhibited satisfactory performance without any signs of failure either in the form of debonding or fracture. This is shown by the bending moment versus mid-span de‐ flection curve plotted for this test in figure 8. Despite the satisfactory performance of the CFRP laminates, no increase in the flexural bending capacity could be achieved due to the slab damage from its previous loading history; however; the performance of the anchors for the attachment of the CFRP laminates was satisfactory and the flexural behavior of the slab exhibited a much larger range of deformability (figure 8).

#### **2.2. Upgrading the flexural and shear capacity of beams**

The upgrading of the shear / flexural capacity will be presented in this section together with the corresponding modes of failure of reinforced concrete (R/C) beam specimens including repair schemes with Fibre Reinforcing Plastics (FRP) ([13], [25], [26], [27], [14], [28], [29]). The specimens were constructed and tested at the laboratory of Strength of Materials and Struc‐ tures of Aristotle University. The applied load and the deflections of the specimen at three points were monitored throughout the experiments. Moreover, strain gauges were applied at selective locations of the FRP strips employed in the repaired specimens in order to moni‐ tor their state of stress during the loading sequence. In what follows, the most important ex‐ perimental results are presented together with a discussion of the observed performance in order to highlight the role of the FRP strips in the observed behavior. It must be stressed that in all the studied beam specimens closed loop FRP hoops could be applied; this cannot be done in T-beam sections [19].

#### *2.2.1. The initial reinforced concrete beam specimen and its observed behavior.*

**The virgin beam specimen:** The virgin specimen BEAM-1 is a typical R/C beam, shown in figure 10, of rectangular cross-section of 200 mm x 500 mm and length l = 3700mm, having longitudinal reinforcement 3Φ20 at the top and 3Φ20 at the bottom side with closed stirrups as transverse reinforcement Φ8/250 ([14]). The selected longitudinal and trans‐ verse reinforcement together with the loading arrangement, shown in figure 11, will lead the behavior of this virgin specimen to be dominated by the shear rather than the flexu‐ ral mode of failure.

136% when compared to the maximum bending moment value of the un-retrofitted slab. This retrofitted slab failed in the form of the debonding of the CFRP laminates (figure 9). The debonding of the two CFRP laminates was followed by a sharp decrease in the flexural bearing capacity together with the formation of a plastic hinge at mid-span in the form of excessive plastic deformations of the old steel reinforcing bars at the bottom of the slab as well as crushing of the concrete at the top of the slab. Next, two new CFRP laminates were reattached to the same slab specimen; this time epoxy paste was used again together with bolts penetrating through the slab and securing the attachment of these CFRP laminates. The flexural bearing capacity of the slab this time reached the same maximum bending mo‐ ment value as the one observed when only epoxy paste was used for the attachment of the laminates; they exhibited satisfactory performance without any signs of failure either in the form of debonding or fracture. This is shown by the bending moment versus mid-span de‐ flection curve plotted for this test in figure 8. Despite the satisfactory performance of the CFRP laminates, no increase in the flexural bending capacity could be achieved due to the slab damage from its previous loading history; however; the performance of the anchors for the attachment of the CFRP laminates was satisfactory and the flexural behavior of the slab

The upgrading of the shear / flexural capacity will be presented in this section together with the corresponding modes of failure of reinforced concrete (R/C) beam specimens including repair schemes with Fibre Reinforcing Plastics (FRP) ([13], [25], [26], [27], [14], [28], [29]). The specimens were constructed and tested at the laboratory of Strength of Materials and Struc‐ tures of Aristotle University. The applied load and the deflections of the specimen at three points were monitored throughout the experiments. Moreover, strain gauges were applied at selective locations of the FRP strips employed in the repaired specimens in order to moni‐ tor their state of stress during the loading sequence. In what follows, the most important ex‐ perimental results are presented together with a discussion of the observed performance in order to highlight the role of the FRP strips in the observed behavior. It must be stressed that in all the studied beam specimens closed loop FRP hoops could be applied; this cannot

**The virgin beam specimen:** The virgin specimen BEAM-1 is a typical R/C beam, shown in figure 10, of rectangular cross-section of 200 mm x 500 mm and length l = 3700mm, having longitudinal reinforcement 3Φ20 at the top and 3Φ20 at the bottom side with closed stirrups as transverse reinforcement Φ8/250 ([14]). The selected longitudinal and trans‐ verse reinforcement together with the loading arrangement, shown in figure 11, will lead

exhibited a much larger range of deformability (figure 8).

72 Fiber Reinforced Polymers - The Technology Applied for Concrete Repair

**2.2. Upgrading the flexural and shear capacity of beams**

*2.2.1. The initial reinforced concrete beam specimen and its observed behavior.*

be done in T-beam sections [19].

**Figure 10.** Dimensions (in mm) and structural details of specimen *Beam-1*. Longitudinal reinforcement: 3Φ20 at the top and 3Φ20 at the bottom. Transverse reinforcement: Φ8/250.

**The behavior of BEAM-1:** The specimen was simply supported at two point supports and was subjected to loads at two points symmetrically spaced at a distance 1150mm from each support; the mid-part had a length of 700mm. This loading was gradually increased, being monitored all the time. Moreover, throughout the loading sequence till the failure of the specimen, the cracking pattern was monitored together with the deflections of the beam at three points (mid-span and under each point load). As can be seen in figure 11, the shear cracking pattern is predominant at the two sides of the beam, whereas the middle part, as expected, is dominated by the flexural behavior. Finally, the dominant failure mode was from shear, as was planned during the design of this specimen. Figure 12a depicts the ob‐ served shear cracking patterns. In order to facilitate the repair of this specimen the loading was stopped after a decrease in the bearing capacity of the specimen was observed. In this way, the permanent deformations of this specimen at this stage were rather limited and the shear mode of failure was not allowed to fully develop.

**Figure 11.** Flexural and shear cracking patterns for specimen *Beam-1*. Prevailing final mode of failure the shear.

**Evaluation of the observed behavior for specimen BEAM-1:** Figure 12b depicts the ob‐ served behavior in terms of shear force (Q). In the same figure the horizontal lines of differ‐ ent colours indicate the shear capacity of this particular specimen, as was predicted following the procedure suggested by various researchers ([30], [31], [32]) as well as by ei‐ ther the Greek Code [9] (without safety factor) or by the Euro-code [7] (with safety factor). Figure 12c depicts the observed behavior in terms of bending moment (M). Again, the hori‐ zontal lines of different colours in this figure indicate the flexural capacity of this specimen as predicted by well established formulae as well as by the corresponding code procedures (without safety factor). Bearing in mind that the mode of failure is dominated by shear and not flexure the comparison between the shear and flexural limit states predicted in this way confirms the observed behavior.

**Figure 12.** a Cracking patterns of Beam-1. b Evaluation of Shear Behavior (BEAM-1). c Evaluation of Flexural Behavior (BEAM-1).

#### *2.2.2. Repaired beam specimen BEAM 1R-1 and observed behavior.*

**Description of Specimen BEAM 1R-1:** After the end of the loading sequence of specimen BEAM-1, this specimen was repaired against shear failure in the way shown in figure 13. Closed loop CFRP (type C1 – 23) hoops of two layers (of nominal thickness 0.133mm per layer), having width bslice = 75mm, were attached externally on the specimen spaced at distan‐ ces of s=275mm. This was very easy to apply to this specimen and the only necessary treat‐ ment was to round up the corners of the specimen. However, in order to apply exactly such a repair scheme in prototype conditions one is faced with the considerable difficulty in drilling the appropriate holes at the top R/C slab that is usually monolithically connected to the beam at its top part. Moreover, the attachment of the CFRP layers in closed loop hoops will cause considerable difficulty. Alternatively, the CFRP shear reinforcement may not be of the closed loop hoop type but of the open loop U-shaped strips with or without anchors. The shear and flexural cracking that developed in specimen Beam-1 from the previous loading sequence was left without any repair (by epoxy resins or other means). An external layer of resin was used to paint the areas of the cracking in order to be able to monitor the activation of these cracks during the new loading sequence [14]. The mechanical properties of the employed CFRP layers (type C1 – 23), as they are given by the manufacturer, are : Ultimate stress fu = 3800 Mpa, Young's Modulus Ef = 230 Gpa, Limit strain εu = 1.8 %, thickness tf = 0.133 mm.

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ther the Greek Code [9] (without safety factor) or by the Euro-code [7] (with safety factor). Figure 12c depicts the observed behavior in terms of bending moment (M). Again, the hori‐ zontal lines of different colours in this figure indicate the flexural capacity of this specimen as predicted by well established formulae as well as by the corresponding code procedures (without safety factor). Bearing in mind that the mode of failure is dominated by shear and not flexure the comparison between the shear and flexural limit states predicted in this way

**Figure 12.** a Cracking patterns of Beam-1. b Evaluation of Shear Behavior (BEAM-1). c Evaluation of Flexural Behavior

**Description of Specimen BEAM 1R-1:** After the end of the loading sequence of specimen BEAM-1, this specimen was repaired against shear failure in the way shown in figure 13. Closed loop CFRP (type C1 – 23) hoops of two layers (of nominal thickness 0.133mm per layer), having width bslice = 75mm, were attached externally on the specimen spaced at distan‐ ces of s=275mm. This was very easy to apply to this specimen and the only necessary treat‐ ment was to round up the corners of the specimen. However, in order to apply exactly such a repair scheme in prototype conditions one is faced with the considerable difficulty in drilling the appropriate holes at the top R/C slab that is usually monolithically connected to the beam at its top part. Moreover, the attachment of the CFRP layers in closed loop hoops will cause considerable difficulty. Alternatively, the CFRP shear reinforcement may not be of the closed loop hoop type but of the open loop U-shaped strips with or without anchors. The shear and flexural cracking that developed in specimen Beam-1 from the previous loading sequence was left without any repair (by epoxy resins or other means). An external layer of resin was used to paint the areas of the cracking in order to be able to monitor the activation of these cracks during the new loading sequence [14]. The mechanical properties of the employed CFRP layers (type C1 – 23), as they are given by the manufacturer, are : Ultimate stress fu = 3800 Mpa,

Young's Modulus Ef = 230 Gpa, Limit strain εu = 1.8 %, thickness tf = 0.133 mm.

*2.2.2. Repaired beam specimen BEAM 1R-1 and observed behavior.*

confirms the observed behavior.

74 Fiber Reinforced Polymers - The Technology Applied for Concrete Repair

(BEAM-1).

**Figure 13.** Repaired specimen BEAM 1R-1 The newly activated cracks are shown with bold lines. Dominant mode of failure the one at the compression zone of the mid-part of the specimen governed by flexure.

**Observed Behavior of Specimen BEAM 1R-1:** The main shear crack at the right side gave signs of becoming active for vertical load of 177KN (Q=88.5KN) whereas the same occurred for the left side main shear crack for vertical load of 216KN (Q=108KN). This meant that the corresponding CFRP shear type hoops started to develop stresses resisting part of the shear. Crushing was observed at the compressive upper side of the mid-part of this specimen un‐ dergoing flexure, when the vertical load reached 373KN. This was accompanied by the acti‐ vation of the flexural cracks starting from the bottom side of the beam. These vertical flexural cracks developed even further for a maximum load of 414KN extending all the way from the bottom side to the crushed upper side, with the specimen reaching its flexural ca‐ pacity. The shear mode of failure of the previous specimen was contained without further development by the applied CFRP hoops. The final condition of this specimen is depicted in figure 13. The observed behavior in terms of shear force (Q) and bending moment (M) is de‐ picted in figures 14a and 14b. The maximum observed shear force was Q = 207 ΚN, and the maximum observed bending moment was M= 238 ΚNm.

**Figure 14.** Observed shear behavior of BEAM 1R-1. 14b. Observed flexure of BEAM 1R-1.

In these figures 14a and 14b, the observed behavior before the repair of this specimen is also plotted for comparison with maximum observed shear force Q = 183.5 ΚN, and maximum observed bending moment M= 211 ΚNm. From this comparison it can be concluded that the applied CFRP hoops resisted a shear force of 23.5KN without signs of any distress. The suc‐ cessful increase (13%) in the shear bearing capacity of this specimen by this applied CFRP repair scheme, which leads to the exhausting of the specimen's flexural capacity and to reaching the desired flexural mode of failure, demonstrates the potential of such repair schemes for prototype conditions. However, there are construction difficulties in prototype conditions which were already pointed out before. Moreover, as was remarked before, the shear capacity of this scheme was not reached as the specimen survived the applied loading sequence in terms of shear. A subsequent reloading of specimen BEAM 1R - 1, with addi‐ tional repair elements, namely specimen BEAM 2R -1, is presented in the following. The ob‐ jective is to define by measurements the shear bearing capacity of the applied CFRP-hoops in this repair scheme.

**Figure 15.** The variation of the CFRP-Hoop strain with the measured shear force for BEAM 1R-1.

**Evaluation of the contribution of the CFRP for specimen BEAM 1R-1:** During the loading sequence of Beam 1R-1, described above, the strains that developed at the mid-height of the third CFRP-hoop from the right end of the specimen (see figure 13) were monitored throughout the test by a strain gauge attached in this location.  Unfortunately, beyond this load, the strain gauge became inactive due to a crack going through it. The variation of the axial (vertical) strain of CFRP-hoop with the applied shear force is depicted in figure 15. As can be seen in this figure, during the loading of specimen Beam 1R-1 the variation of the measured CFRP-hoop strain with the ap‐ plied shear force is almost linear with different slope in two distinct ranges; the first range for shear force values up to Q=150KN and the second range for shear force values larger than Q=150KN. By extrapolating the second linear strain trend up to the maximum meas‐ ured shear force value of 207KN a maximum strain value equal to 1.43 10-3 is found for the CFRP-hoop, which is also depicted in figure 15 (see also figure 14a). By employing the rela‐ tionship: *Vf* <sup>=</sup> *<sup>A</sup>hoop*(*<sup>h</sup>* /*s*)*Ef <sup>ε</sup><sup>f</sup> eff*

where *A hoop*=3*9.9mm2* the cross-section of one CFRP hoop, *h=500mm* the height of the beam, *s=275mm* the spacing of the hoops, *E <sup>f</sup>* =*230Gpa* the Young modulus of the applied CFRP, *ε <sup>f</sup> eff* =*1.43 10 -3* the effective CFRP strain as was mentioned before, the value of the shear resisted by the applied CFRP-hoops can be found equal to *V <sup>f</sup> = 23.86KN.*

When this value, which was found from direct strain measurements of the CFRP-hoop, is compared with the value of *V fa* = 23.5KN, found by subtracting the shear resisted by the virgin specimen BEAM -1 from the shear resisted by the repaired specimen BEAM 1R-1 with the CFRPhoops, a very good agreement is observed, which further validates the shear resisted by the applied CFRP-hoops. Using the software developed by Triantafillou [33] a prediction for the maximum shear force equal to 44KN is found to be resisted by the CFRP-hoop arrangement applied in BEAM 1R-1 (figure 13). The measured value of approximately 24KN and the ob‐ served shear performance of this specimen are in agreement with this predicted upper limit (44KN) for the CFRP-hoop shear resistance of the employed repaired scheme.

## **2.3. Repaired specimen Beam 1R-2**

In these figures 14a and 14b, the observed behavior before the repair of this specimen is also plotted for comparison with maximum observed shear force Q = 183.5 ΚN, and maximum observed bending moment M= 211 ΚNm. From this comparison it can be concluded that the applied CFRP hoops resisted a shear force of 23.5KN without signs of any distress. The suc‐ cessful increase (13%) in the shear bearing capacity of this specimen by this applied CFRP repair scheme, which leads to the exhausting of the specimen's flexural capacity and to reaching the desired flexural mode of failure, demonstrates the potential of such repair schemes for prototype conditions. However, there are construction difficulties in prototype conditions which were already pointed out before. Moreover, as was remarked before, the shear capacity of this scheme was not reached as the specimen survived the applied loading sequence in terms of shear. A subsequent reloading of specimen BEAM 1R - 1, with addi‐ tional repair elements, namely specimen BEAM 2R -1, is presented in the following. The ob‐ jective is to define by measurements the shear bearing capacity of the applied CFRP-hoops

76 Fiber Reinforced Polymers - The Technology Applied for Concrete Repair

**Figure 15.** The variation of the CFRP-Hoop strain with the measured shear force for BEAM 1R-1.

**Evaluation of the contribution of the CFRP for specimen BEAM 1R-1:** During the loading sequence of Beam 1R-1, described above, the strains that developed at the mid-height of the third CFRP-hoop from the right end of the specimen (see figure 13) were monitored throughout the test by a strain gauge attached in this location.  Unfortunately, beyond this load, the strain gauge became inactive due to a crack going through it. The variation of the axial (vertical) strain of CFRP-hoop with the applied shear force is depicted in figure 15. As can be seen in this figure, during the loading of specimen Beam 1R-1 the variation of the measured CFRP-hoop strain with the ap‐ plied shear force is almost linear with different slope in two distinct ranges; the first range for shear force values up to Q=150KN and the second range for shear force values larger than Q=150KN. By extrapolating the second linear strain trend up to the maximum meas‐ ured shear force value of 207KN a maximum strain value equal to 1.43 10-3 is found for the CFRP-hoop, which is also depicted in figure 15 (see also figure 14a). By employing the rela‐

in this repair scheme.

tionship: *Vf* <sup>=</sup> *<sup>A</sup>hoop*(*<sup>h</sup>* /*s*)*Ef <sup>ε</sup><sup>f</sup>*

*eff*

As described in section 2.2.2. before, a middle part compressive zone crushing was observed as a limit state for specimen BEAM 1R-1. After unloading this specimen the crushed con‐ crete was removed and the whole area cleaned till the compressive longitudinal reinforce‐ ment became visible. The reshaping of the orthogonal form of this part of the beam was achieved with a paste from a mixture of cement and epoxy resin, which becomes hard in one day attaining a compressive strength of 30 MPa. The thickness of this layer of paste was 20mm above the compressive longitudinal reinforcement and the corners were given a smooth curvature. This upper zone was next reinforced with layers of CFRP (C 1-23), as shown in figure 16, in order to increase its compression capability through confinement. In addition, three layers of GFRP (G-60 AR) were attached at the bottom side of the specimen to enhance through tension its flexural capacity beyond the one provided by the existing longitudinal reinforcement (figure 16).

**Figure 16.** Specimen BEAM 1R-2 with CFRP layers attached in a way as to contain the compressive zone of the middle part as well as additional GFRP layer at the bottom side of the specimen extending the whole span.

## *2.3.1. Observed Behavior of Specimen BEAM 1R-2*

This specimen was reloaded in a way similar to the one employed for the preceding speci‐ mens BEAM -1 and BEAM 1R-1. Figures 17a and 17b depict for all three specimens the shear and flexural behavior, respectively. In figure 17b the predictions of the yield moment for specimen BEAM-1 and the ultimate moment of specimen BEAM 2R-1 are also indicated. Due to an increase in the flexural capacity for specimen BEAM 2R-1 when compared to the pre‐ ceding specimen BEAM 1R-1, resulting from the described repair scheme for specimen BEAM 2R-1, the shear failure mode prevailed this time. This mode of failure was accompanied by the rupture of the CFRP hoops as well as by crushing of the concrete at the compressive shear transfer zone neighboring the point of loading (figure 18). As expected, this specimen (BEAM 1R-2) exhibited an increase in its flexural capacity when this is compared with the flexural limit state attained during the preceding test of specimen (BEAM 1R-1). This fact demon‐ strates that the repair scheme described before, aimed to increase the flexural capacity of specimen BEAM 1R-2 proved to be successful. The observed maximum shear force during this test was Qαν = 215.8ΚN whereas the maximum bending moment was Mαν = 248.2ΚNm. Comparing these values with the ones observed during testing the original virgin BEAM-1 it can be concluded that the applied repair scheme resulted in an increase of the order of 18% for the shear capacity and more than 18% for the flexural capacity; the exact increase in the flexural capacity could not be measured as the shear failure for BEAM 1R-2 prohibited this specimen from reaching its flexural limit state. The predicted flexural capacity of BEAM 2R-1 is listed in Table 1.

**Table 1.** Predictions of flexural capacity for BEAM 2R-1.

**Figure 17.** Shear behavior of all specimens. b. Flexural behavior of all specimens.

It must be pointed out here that the way the enhancement of the compressive zone capacity was achieved through the compressive zone confinement, attained from the CFRP layers, cannot be applied so easily in prototype conditions due to the presence of an R/C slab at this part of a prototype beam. Table 2 lists the total vertical load values which were measured during important developments in the observed behavior of specimen BEAM 1R-2. More‐ over, figure 19 depicts the observed mode of failure in a detailed way.

**Table 2.** Important developments in the behavior of specimen BEAM 1R-2 during loading.

*2.3.1. Observed Behavior of Specimen BEAM 1R-2*

78 Fiber Reinforced Polymers - The Technology Applied for Concrete Repair

is listed in Table 1.

**Table 1.** Predictions of flexural capacity for BEAM 2R-1.

**Figure 17.** Shear behavior of all specimens. b. Flexural behavior of all specimens.

This specimen was reloaded in a way similar to the one employed for the preceding speci‐ mens BEAM -1 and BEAM 1R-1. Figures 17a and 17b depict for all three specimens the shear and flexural behavior, respectively. In figure 17b the predictions of the yield moment for specimen BEAM-1 and the ultimate moment of specimen BEAM 2R-1 are also indicated. Due to an increase in the flexural capacity for specimen BEAM 2R-1 when compared to the pre‐ ceding specimen BEAM 1R-1, resulting from the described repair scheme for specimen BEAM 2R-1, the shear failure mode prevailed this time. This mode of failure was accompanied by the rupture of the CFRP hoops as well as by crushing of the concrete at the compressive shear transfer zone neighboring the point of loading (figure 18). As expected, this specimen (BEAM 1R-2) exhibited an increase in its flexural capacity when this is compared with the flexural limit state attained during the preceding test of specimen (BEAM 1R-1). This fact demon‐ strates that the repair scheme described before, aimed to increase the flexural capacity of specimen BEAM 1R-2 proved to be successful. The observed maximum shear force during this test was Qαν = 215.8ΚN whereas the maximum bending moment was Mαν = 248.2ΚNm. Comparing these values with the ones observed during testing the original virgin BEAM-1 it can be concluded that the applied repair scheme resulted in an increase of the order of 18% for the shear capacity and more than 18% for the flexural capacity; the exact increase in the flexural capacity could not be measured as the shear failure for BEAM 1R-2 prohibited this specimen from reaching its flexural limit state. The predicted flexural capacity of BEAM 2R-1

**METHODOLOGY MU [KN m] Mode of failure**

Ultimate stress strain method **301.26** Failure from compression of the top zone and of tension of the

FRPFLEX **319.02** Failure of compression zone

It must be pointed out here that the way the enhancement of the compressive zone capacity was achieved through the compressive zone confinement, attained from the CFRP layers,

GFRP

**Figure 18.** Shear failure accompanied by the rupture of the CFRP stirrups as well as of crushing of the concrete com‐ pressive zone at the shear transfer area neighboring the point of loading.

**Figure 19.** Mechanism of shear failure during the limit state of specimen BEAM 1R-2.

#### *2.3.2. Evaluation of the contribution of the CFRP-hoops for BEAM 1R-2*

As was done for specimen BEAM 1R-1, during the loading sequence of Beam 2R-1 the axial (vertical) strains that developed at the mid-height of the third CFRP hoop from the right end of the specimen (see figure 13) were again monitored employing a strain gauge attached in this location. This strain gauge remained operational throughout the whole load sequence measuring a maximum axial strain value ε<sup>f</sup> eff = 0.67 % when the total vertical load reached its maximum value of 435.6KN (Q=215.8KN). By subtracting the shear resisted by the virgin specimen BEAM -1 (Q=183.5KN) from the shear resisted by the repaired specimen BEAM 2R-1 (Q=215.8KN), a value of Vfa = 32.3KN is found as being resisted by the used repair scheme. As mentioned before, using the software developed by Triantafillou [33] the maxi‐ mum shear force of 44KN is found to be resisted by the CFRP hoop arrangement applied in BEAM 2R-1. Moreover, employing the maximum strain measurement and the relationship *V <sup>f</sup> = A hoop (h/s) E <sup>f</sup> ε <sup>f</sup> eff* the shear resisted by the applied CFRP-hoops can be found equal to *V <sup>f</sup> =* 111.8*KN.* This discrepancy dictates further investigation on the shear resisting mechanisms that developed during the limit state of specimen BEAM 2R-1 (see figure 19).

The maximum axial strain measurement equal to 0.67 % was recorded at this particular CFRP-hoop location during maximum applied load just before the rupture of this CFRPhoop and the final development of the shear failure for specimen BEAM 2R-1. The variation of the applied shear force with the measured CFRP hoop strain is depicted in figure 20; it is almost linear with different slope in two distinct ranges; the first for a shear force value from zero to Q=205KN and the second for a shear force value larger than Q=205KN up to Qαν = 215.8ΚN, when the rupture of the CFRP occurred.

The Use of Fiber Reinforced Plastic for The Repair and Strengthening of Existing Reinforced Concrete Structural Elements Damaged by Earthquakes http://dx.doi.org/10.5772/51326 81

**Figure 20.** The variation of the CFRP-hoop strain with the measured shear force for BEAM 2R-1.

**Figure 21.** Numerical simulation of the dowel action.

#### *2.3.3. Shear capacity of specimen BEAM 2R-1 based on the observed failure mechanism*

Based on the observed shear failure mechanism, which is depicted in figure 19, the shear ca‐ pacity of BEAM 2R-1 will be estimated as will be described below. It is assumed that the shear is resisted in this case by the following parts.

One closed steel stirrup.

The compressive zone for shear transfer next to the point of load application at the top of the main shear crack.

## Two CFRP-hoops.

**Figure 19.** Mechanism of shear failure during the limit state of specimen BEAM 1R-2.

80 Fiber Reinforced Polymers - The Technology Applied for Concrete Repair

*2.3.2. Evaluation of the contribution of the CFRP-hoops for BEAM 1R-2*

measuring a maximum axial strain value ε<sup>f</sup>

215.8ΚN, when the rupture of the CFRP occurred.

*<sup>f</sup> = A hoop (h/s) E <sup>f</sup> ε <sup>f</sup>*

As was done for specimen BEAM 1R-1, during the loading sequence of Beam 2R-1 the axial (vertical) strains that developed at the mid-height of the third CFRP hoop from the right end of the specimen (see figure 13) were again monitored employing a strain gauge attached in this location. This strain gauge remained operational throughout the whole load sequence

its maximum value of 435.6KN (Q=215.8KN). By subtracting the shear resisted by the virgin specimen BEAM -1 (Q=183.5KN) from the shear resisted by the repaired specimen BEAM 2R-1 (Q=215.8KN), a value of Vfa = 32.3KN is found as being resisted by the used repair scheme. As mentioned before, using the software developed by Triantafillou [33] the maxi‐ mum shear force of 44KN is found to be resisted by the CFRP hoop arrangement applied in BEAM 2R-1. Moreover, employing the maximum strain measurement and the relationship *V*

111.8*KN.* This discrepancy dictates further investigation on the shear resisting mechanisms

The maximum axial strain measurement equal to 0.67 % was recorded at this particular CFRP-hoop location during maximum applied load just before the rupture of this CFRPhoop and the final development of the shear failure for specimen BEAM 2R-1. The variation of the applied shear force with the measured CFRP hoop strain is depicted in figure 20; it is almost linear with different slope in two distinct ranges; the first for a shear force value from zero to Q=205KN and the second for a shear force value larger than Q=205KN up to Qαν =

that developed during the limit state of specimen BEAM 2R-1 (see figure 19).

*eff* the shear resisted by the applied CFRP-hoops can be found equal to *V <sup>f</sup> =*

eff = 0.67 % when the total vertical load reached

The contribution of the bottom side longitudinal reinforcement (dowel action).

The contribution of the compressive zone is found from the relationship: Vc = 0.3 (fc)2/3 (2)1/2 x b. Where fc the concrete compressive strength, x the height of the compressive zone (as estimated from the flexural behavior), b the width of the beam. This leads to a shear force equal to Vc = 46.42 kN at the initiation of concrete compressive zone crushing.

The contribution of the assumed two CFRP-hoops is equal to Vf = 18234 ε<sup>f</sup> eff [ΚN]. Accord‐ ing to the strain measurements this contribution at the initiation of crushing of the compres‐ sive zone is equal to Vf = 73.54ΚN.

The contribution of the dowel action of the longitudinal reinforcement will be based on the geometry of this particular failure mechanism which was measured in detail at the laborato‐ ry. For this purpose, a simple numerical simulation of this dowel action was formed, which is depicted in figure 21. The longitudinal reinforcement is supported by a steel stirrup (left support) and by the CFRP-hoop (multiple mid-supports). All the supports are simulated by springs with the corresponding elastic / post-elastic and cross-sectional properties the actual steel stirrup and CFRP-hoop possess.

Due to the volume of concrete the rotations at the two ends of this dowel action numerical model were restrained; moreover, the employed simulation provided the capability for the longitudinal reinforcement to develop plastic hinges with parameters based on the crosssectional area and yield stress of the employed longitudinal reinforcement, which were measured at the laboratory.

The limit state contribution of the dowel action of the longitudinal reinforcement, as result‐ ed from this numerical simulation, was found equal to Vd = 37.67 KN. The contribution of the steel stirrup ( Vw ) is based on the assumption that its strain is the same as the measured strain of the neighboring CFRP hoop. The various contributions based on all the above as‐ sumptions are listed in Table 3 together with the corresponding total measured value. As can be seen a very good comparison is reached between the total estimated shear resistance and the corresponding measured value. It must be pointed out that the estimated value was based on the exact observation of the failure mechanism as well as on the measurements of the CFRP hoop strains. In the same table a predicted shear resistance value is also listed based on the prediction of the maximum shear resisted by the CFRP-hoops by the software developed by Triantafillou [33] (maximum predicted Vf value equal to 44KN is found).

**Table 3.** Shear resistance contributions of the various parts of specimen BEAM 2R-1 at the initiation of the compressive zone crushing.

The variation of the estimated shear contributions of the CFRP-hoops (Vf), the steel stirrup (Vw), the dowel action of the longitudinal reinforcement (Vd) as well as the sum of all these (Vf+Vw+Vd) are plotted in figure 22 together with the measured total shear force (VR) for specimen BEAM 2R-1. The common basis for the plotted displacements between the estimat‐ ed contributions and the measured values is found from the correspondence of the load and strain measurements during testing. In addition, in the same figure, the variation of the contribution of the compressive concrete zone is plotted by subtracting from the total meas‐ ured shear force (VR) the contributions of the steel stirrups and the CFRP hoops (Vc=VR-Vw-Vf). After the formation of the shear cracks this contribution includes the dowel action of the longitudinal reinforcement. This results in a maximum value Vc = 84.14 ΚN, which is in agreement with predictions based on formulas suggested by various researchers. More‐ over, it can be seen that the summation of the various contributions is in good agreement with the total measured shear force if the contribution of the compressive zone is added, after the formation of the shear cracks and up to the initiation of the crushing of the compres‐ sive zone concrete.

**Figure 22.** Shear resistance contributions, extracted from the mechanism of shear failure for specimen BEAM 2R-1.

#### **2.4. Concluding observations**

The contribution of the assumed two CFRP-hoops is equal to Vf = 18234 ε<sup>f</sup>

= 73.54ΚN.

82 Fiber Reinforced Polymers - The Technology Applied for Concrete Repair

developed by Triantafillou [33] (maximum predicted Vf

2 CFRP Hoops **Vf** [ΚN]

sive zone is equal to Vf

steel stirrup and CFRP-hoop possess.

measured at the laboratory.

Steel Stirrup **Vw** [ΚN]

compressive zone crushing.

ing to the strain measurements this contribution at the initiation of crushing of the compres‐

The contribution of the dowel action of the longitudinal reinforcement will be based on the geometry of this particular failure mechanism which was measured in detail at the laborato‐ ry. For this purpose, a simple numerical simulation of this dowel action was formed, which is depicted in figure 21. The longitudinal reinforcement is supported by a steel stirrup (left support) and by the CFRP-hoop (multiple mid-supports). All the supports are simulated by springs with the corresponding elastic / post-elastic and cross-sectional properties the actual

Due to the volume of concrete the rotations at the two ends of this dowel action numerical model were restrained; moreover, the employed simulation provided the capability for the longitudinal reinforcement to develop plastic hinges with parameters based on the crosssectional area and yield stress of the employed longitudinal reinforcement, which were

The limit state contribution of the dowel action of the longitudinal reinforcement, as result‐ ed from this numerical simulation, was found equal to Vd = 37.67 KN. The contribution of the steel stirrup ( Vw ) is based on the assumption that its strain is the same as the measured strain of the neighboring CFRP hoop. The various contributions based on all the above as‐ sumptions are listed in Table 3 together with the corresponding total measured value. As can be seen a very good comparison is reached between the total estimated shear resistance and the corresponding measured value. It must be pointed out that the estimated value was based on the exact observation of the failure mechanism as well as on the measurements of the CFRP hoop strains. In the same table a predicted shear resistance value is also listed based on the prediction of the maximum shear resisted by the CFRP-hoops by the software

> Dowel action of longitudinal reinforcement (3Φ20) **Vd** [ΚN]

Measured total shear resistance **210.9KN** Predicted total shear **227.5KN** [ref. 33]

**Table 3.** Shear resistance contributions of the various parts of specimen BEAM 2R-1 at the initiation of the

**53.94 73.54 37.67 46.42 211.57**

The variation of the estimated shear contributions of the CFRP-hoops (Vf), the steel stirrup (Vw), the dowel action of the longitudinal reinforcement (Vd) as well as the sum of all these (Vf+Vw+Vd) are plotted in figure 22 together with the measured total shear force (VR) for specimen BEAM 2R-1. The common basis for the plotted displacements between the estimat‐ ed contributions and the measured values is found from the correspondence of the load and strain measurements during testing. In addition, in the same figure, the variation of the

eff [ΚN]. Accord‐

value equal to 44KN is found).

Total estimated shear resistance **VR** [ΚN]

Compressive Zone of Concrete **Vc** [ΚN]

**a1.** The technique of repairing reinforced concrete beam elements by attaching CFRP layers has been examined in the laboratory. It was found that using CFRP-hoops, spaced at the areas of the beam that resist the shear forces, was a quite successful repair technique in up‐ grading the shear capacity of beam specimens by inhibiting the shear mode of failure thus favouring the development of the flexural mode of failure.

**b1.** The repairing of the examined specimens by attaching GFRP layers to complement the tensile longitudinal reinforcement together with CFRP confinement of the compressive zone at these parts of the beam that undergo flexure was again a quite successful technique in up‐ grading the flexural capacity of the beam specimens. However, this must be properly con‐ trolled in order to prohibit the development of the undesired shear mode of failure.

**c1.** In order to be able to successfully control the shear and flexural capacities and modes of failure, after the application of CFRP repair schemes, reliable tools are necessary that suc‐ cessfully predict the bearing capacities of the repaired structural elements. It was demon‐ strated by this work that whereas this is possible for the flexural capacity it is not so straightforward for the shear capacity unless the basis of the shear capacity estimate is founded on a realistic failure mechanism that takes into account the realistic contributions of all the parts that are mobilized through this failure mechanism.

**d1.** It must be pointed out that the examined repair schemes for upgrading the shear resist‐ ance through CFRP stirrups will not be so easily applied in prototype conditions where the presence of a reinforced concrete slab, cast monolithically with the upper part of the beam, must be confronted.

**Figure 23.** R/C cross-sections with one side longer than the other (h/b>1.5) and CFRP partial confinement.

**Figure 24.** Bridge Pier compression failure mode.

## **3. Partial Confinement**

strated by this work that whereas this is possible for the flexural capacity it is not so straightforward for the shear capacity unless the basis of the shear capacity estimate is founded on a realistic failure mechanism that takes into account the realistic contributions of

**d1.** It must be pointed out that the examined repair schemes for upgrading the shear resist‐ ance through CFRP stirrups will not be so easily applied in prototype conditions where the presence of a reinforced concrete slab, cast monolithically with the upper part of the beam,

**Figure 23.** R/C cross-sections with one side longer than the other (h/b>1.5) and CFRP partial confinement.

all the parts that are mobilized through this failure mechanism.

84 Fiber Reinforced Polymers - The Technology Applied for Concrete Repair

must be confronted.

**Figure 24.** Bridge Pier compression failure mode.

## **3.1. The use of confinement for upgrading the flexural and shear capacity of columns.**

A very common application of externally applied fiber reinforced plastics (FRP) is confining parts of structural members. This can be easily envisaged for columns with their cross-sec‐ tion approximating a square shape [34]. This is done by wrapping around the structural ele‐ ment one or more that one layers of FRP sheets together with the proper resine in-order to bond them on the surface of the structural member as well as to bond the FRP sheets be‐ tween themselves. The main points of attention here are the following [35]:

Preparation of the surface of the existing structural member in order to prohibit premature delamination (debonding) of the attached FRP sheets.

Localized rupture of the FRP jacket due to abrupt change of curvature at the corners, to buckling of reinforcing bars, or to excessive dilation of concrete.

Proper application of the resin in order to achieve sufficient bond between the FRP sheets and the surface of the existing structural member.

Proper application of the resin and enough overlapping length of the wrapped layers of the FRP sheet around the existing member in order to prohibit any unwrapping mode of failure.

The proper wrapping of the FRP multilayer sheet around the existing structural member (column or beam) introduces a passive confinement in the same way as steel stirrups do on such concrete members. Whereas the wrapping of FRP sheets around columns or beams of rectangular shape with width / height ratio no greater than 2 can be easily materialized this is not so easily applicable in the following cases:

In columns with rectangular cross-sections having a width / height ratio greater than 2. It will be discussed later on in this section how this difficulty was confronted in the laboratory.

In T- beams whereby the presence of the slab prohibits the wrapping of the FRP sheets around the cross-section.

**Table 4.** Characterization of the effectiveness of the partial confinement.

The beneficial effect of such a confinement on the compressive strength of axially loaded structural members is well known and will not be repeated here. Moreover, the function of these externally applied FRP sheets as transverse reinforcement for both columns and beams is also evident in the same way closed steel stirrups function in reinforced concrete sections. Acting as external transverse reinforcement FRP sheets fully wrapped around beams or col‐ umns can be utilized to upgrade their shear capacity (figure 5). This has been already dis‐ cussed in the beginning of section 2. Again, for T-beams the presence of the slab prohibits the formation of closed loop transverse reinforcement by the FRP sheets. If the FRP sheets are applied as transverse reinforcement in an open loop formation they are susceptible to a premature delamination type of failure (see figure 4). This will be further discussed in sec‐ tion 6 together with the way this difficulty was confronted in the laboratory.

The upgrading of reinforced concrete (R/C) cross-sections, with one side rather longer than the other (h/b > 1.5), by partial application of CFRP (Carbon Fiber Reinforcing Plastic) con‐ finement is investigated here (figure 23). This partial application of CFRP confinement is aimed at the retrofitting of bridge-pier type R/C cross-sections in order to prohibit, up to a point, the development of premature compressive failure at the base of the pier due to com‐ bined compression and flexure from seismic loads (see figure 24 and [36], [37]). The per‐ formance of such structural elements was studied extensively in the past ([38], [39]). This type of partial confinement may also be applied to upgrading vertical structural members with non-accessible sides. Design guidelines for rectangular FRP jackets applied on rectan‐ gular columns have been proposed with the limitation that the cross-sections have aspect ra‐ tio h/b < 1.5 [37]. For higher aspect ratios it is recommended to design a circular or oval jacket. However, it is expected that for rectangular cross sections with aspect ratios larger than 1.5 the radius of a circular or rectangular jacket will be too large and will result in inef‐ fectual confinement and will prove costly and impractical. For this reason it is desirable to investigate alternative schemes for increasing the confinement of rectangular cross-sections with relatively large aspect ratio without resorting to complete circular or oval jackets. Such a scheme is studied here using CFRP layers that do not extend all around the cross-section (figure 23 "*partial confinement*").

**Figure 25.** Initial cross-section with h/b ratio = 2.5. 25b. Test specimens without and with partial CFRP confinement.

To compensate for the fact that the CFRP layers do not enclose the cross-section entirely, an‐ chorage of these layers must be provided, as shown schematically in figure 23. To this end, a laboratory investigation was carried out to study the effectiveness of such partial confine‐ ment together with alternative anchorage schemes. As will be explained, this effectiveness was tested by subjecting the specimens only to compressive loads. Despite this limitation, as will be demonstrated from the results of this investigation, the most significant aspects of the critical factors for this "*partial confinement*" scheme were brought to light.

## *3.1.1. The formation of test specimens*

The beneficial effect of such a confinement on the compressive strength of axially loaded structural members is well known and will not be repeated here. Moreover, the function of these externally applied FRP sheets as transverse reinforcement for both columns and beams is also evident in the same way closed steel stirrups function in reinforced concrete sections. Acting as external transverse reinforcement FRP sheets fully wrapped around beams or col‐ umns can be utilized to upgrade their shear capacity (figure 5). This has been already dis‐ cussed in the beginning of section 2. Again, for T-beams the presence of the slab prohibits the formation of closed loop transverse reinforcement by the FRP sheets. If the FRP sheets are applied as transverse reinforcement in an open loop formation they are susceptible to a premature delamination type of failure (see figure 4). This will be further discussed in sec‐

The upgrading of reinforced concrete (R/C) cross-sections, with one side rather longer than the other (h/b > 1.5), by partial application of CFRP (Carbon Fiber Reinforcing Plastic) con‐ finement is investigated here (figure 23). This partial application of CFRP confinement is aimed at the retrofitting of bridge-pier type R/C cross-sections in order to prohibit, up to a point, the development of premature compressive failure at the base of the pier due to com‐ bined compression and flexure from seismic loads (see figure 24 and [36], [37]). The per‐ formance of such structural elements was studied extensively in the past ([38], [39]). This type of partial confinement may also be applied to upgrading vertical structural members with non-accessible sides. Design guidelines for rectangular FRP jackets applied on rectan‐ gular columns have been proposed with the limitation that the cross-sections have aspect ra‐ tio h/b < 1.5 [37]. For higher aspect ratios it is recommended to design a circular or oval jacket. However, it is expected that for rectangular cross sections with aspect ratios larger than 1.5 the radius of a circular or rectangular jacket will be too large and will result in inef‐ fectual confinement and will prove costly and impractical. For this reason it is desirable to investigate alternative schemes for increasing the confinement of rectangular cross-sections with relatively large aspect ratio without resorting to complete circular or oval jackets. Such a scheme is studied here using CFRP layers that do not extend all around the cross-section

**Figure 25.** Initial cross-section with h/b ratio = 2.5. 25b. Test specimens without and with partial CFRP confinement.

tion 6 together with the way this difficulty was confronted in the laboratory.

86 Fiber Reinforced Polymers - The Technology Applied for Concrete Repair

(figure 23 "*partial confinement*").

The initial cross-section, which formed the basis of the tested specimens, is depicted in fig‐ ure 25a with an aspect ratio h/b equal to 2.5. This is a rectangular cross section of a bridge pier model structure, which was tested both at the laboratory (figure 25a) and at the Volvi-Greece European Test Site in the framework of the European project Euro-Risk [36]. This cross section was intentionally designed to develop flexural mode of failure at the base of the pier; moreover, it was desirable to find ways to retrofit such specimens by prohibiting premature compression failure at the base by means of partial CFRP confinement. The effec‐ tiveness of the partial CFRP confinement is studied by subjecting the tested specimens only to pure compressive loads. This type of stress field is expected to develop at the base of such vertical members under combined vertical loads and seismic actions, where undesired com‐ pression failure may develop. In order to limit the maximum level of compressive loads re‐ quired to bring to failure such a cross-section with the loading arrangements available, the tested specimens had a cross section (figure 25b) of 200mm by 300mm instead of 200mm by 500mm of the initial cross-section ( figure 25a and figure 25b) for the bridge pier specimens tested both at the laboratory and at the test site under combined compression and flexure [36]. Moreover, in order for the tested specimens to form compression failure at the same part of the cross-section where such failure would develop at the base of the initial bridge pier model, one part of the tested cross-section was left identical to the initial cross-section (the one that is marked in figure 25b as weak) whereas the remaining part was strengthened both with longitudinal and, in particular, with transverse reinforcement (the one marked in figure 25b as strong).

In this way, with the compression capacity of the weak part being smaller than that of the strong part, the compression failure was expected to develop at the weak part. This proved to be correct during the experiments, as will be shown in the following sections. The CFRP partial confinement was applied at the weak part, as is shown at the right hand side of fig‐ ure 25b. Then, by studying the resulting bearing capacity and mode of failure under com‐ pression of the tested specimens (with or without partial confinement) the effectiveness of such a repair scheme could be demonstrated and classified as listed in table 4.

## *3.1.2. Construction of test specimens*

Ten identical specimens were constructed and eight of them were used in the current experi‐ mental sequence (see table 5). All specimens were reinforced in the same way and were cast at the same time with the same mixture aiming for similar plain concrete strength values. The reinforcing details are depicted in figure 26. As can be seen, the total height of the speci‐ mens was 1600mm. They had a mid-part height of 580mm that was left to develop the com‐ pression failure (figure 26, part denoted as CROSS SECTION A). The cross section of this mid-part was the one shown in figure 25b, including the two distinct parts (the weak and the strong). The two edges of the specimens, with a height of 510mm each, were confined during the experiment with strong steel brackets covering these parts from all sides thus prohibiting any compressive failure developing at those two edges (see figure 27).

**Figure 26.** Reinforcement details common to all specimens.

**Table 5.** Test specimens with their corresponding concrete strength.

The partial CFRP confinement was attached only on the three sides of these specimens cov‐ ering the weak part and leaving the fourth side (of the strong part) free without any CFRP layers (figure 25b). There were 8 virgin specimens, namely 1, 1a, 3, 3a, 4, 4a, 5, and 5a (Table 5). These specimens were tested in their virgin state in which some of them were without partial CFRP confinement whereas the rest had the partial CFRP confinement applied to them from the beginning. The second column of table 5 indicates the partial confinement condition of the virgin specimens. The testing sequence of these virgin specimens is signi‐ fied as Test 1. Most of these specimens were repaired after they had reached their limit state during their previous test. In all the repaired specimens the CFRP partial confinement was applied. Throughout these series of experiments the nominal thickness of the employed CFRP layers was 0.176mm with a given Young's Modulus E=350GPa. The measured maxi‐ mum axial CFRP strain was approximately 1%.

The reinforcing details are depicted in figure 26. As can be seen, the total height of the speci‐ mens was 1600mm. They had a mid-part height of 580mm that was left to develop the com‐ pression failure (figure 26, part denoted as CROSS SECTION A). The cross section of this mid-part was the one shown in figure 25b, including the two distinct parts (the weak and the strong). The two edges of the specimens, with a height of 510mm each, were confined during the experiment with strong steel brackets covering these parts from all sides thus

prohibiting any compressive failure developing at those two edges (see figure 27).

**Figure 26.** Reinforcement details common to all specimens.

88 Fiber Reinforced Polymers - The Technology Applied for Concrete Repair

Specimen 5 Test 1 5+(2) CFRP layers

**Table 5.** Test specimens with their corresponding concrete strength.

Specimen 5a Test 1 No

CFRP

Specimen 1 Test 1 No Test 2 3+(2) CFRP 28.0 Specimen 1a Test 1 3 CFRP layers Test 2 3 GFRP 25.8 Specimen 3 Test 1 No Test 2 3+(2) CFRP 27.6 Specimen 3a Test 1 5 CFRP layers Test 2 5 CFRP 27.6 Specimen 4 Test 1 5 CFRP layers 27.7 Specimen 4a Test 1 5 CFRP layers 27.7

Confinement **Repaired Specimens Plain Concrete Strength (Mpa)**

27.6

25.8

Test 2, 5+2 CFRP Test 3, 5+2 CFRP Test 4 7 CFRP

Test 2, 7 CFRP Test 3 7+4 CFRP

**Virgin Specimens**

**Figure 27.** Specimen with confining steel brackets at the edges and partial CFRP confinement at the mid-part

This sequential test number of the repaired specimens is signified as Test 2 (for the 1st re‐ pair), Test 3 (for the 2nd repair) etc. (see column 3 of Table 5). In the same column the num‐ ber of CFRP layers used in the partial confinement for the repaired specimens is also indicated. The total number of specimens, virgin and repaired, was seventeen. In Table 5 the unconfined concrete compressive stress is also listed, found from cylinders with diameter 150mm and 300mm height; these cylinders were obtained during the casting of each virgin specimen. The anchorage of the CFRP layers was applied along the two long sides, which were common to both the weak and the strong parts. The main load that was applied was axial compression, although in limited specimens the axial compression was combined with bending, which is not reported here. From the observed behavior, the effectiveness of the applied partial confinement could be deduced. As shown in table 4, this judgment was based on the level of the bearing capacity combined with the type of compression failure that was formed (at the weak or strong part). Moreover, the observed behavior of the vari‐ ous parts of the test specimens, such as the CFRP layers and their anchorage, helped to iden‐ tify the factors that bear an adverse or beneficial influence on these aspects of the behavior.

#### *3.1.3. Critical parameters and their variation:*

The following parameters were critical (see results contained in Table 6).

#### *1 st . The type of anchorage of the CFRP layers, 2 nd . The number of the CFRP layers.*

The second parameter becomes critical only if the first parameter performs satisfactorily. A large number of specimens ( 1, 1a, 3, 3a, 4 and 4a) exhibited failure of the anchorage of the partial confinement thus limiting the effectiveness of the partial confinement to low levels. Consequently, the increase in the compressive bearing capacity in this case was relatively modest. The compressive load was monitored by continuous sampling throughout the test. The "*Average Stress at failure*" listed in table 6 is found by dividing the compressive load by the gross cross-sectional area of each specimen. Listed in table 6 are certain particulars of the anchorage of the CFRP layers. Anchors-1 signifies bolts of 7mm diameter and 60mm length that do not penetrate the cross-section in its whole width, whereas Anchors-2 signifies bolts of 10mm diameter that penetrate the whole section (220mm length).For specimens 5 and 5a, due to the improvement of the anchoring details of the partial confinement, the increase in the compressive bearing capacity of these specimens was 50% higher than the observed bearing capacity of similar specimens with no partial CFRP confinement. When the maxi‐ mum compressive bearing capacity achieved with the partial confinement is compared with the corresponding plain concrete strength value, a 230% increase can be observed. More‐ over, the compressive failure in this case involved both the weak and the strong part thus improving substantially the effectiveness of the partial confinement and allowing the em‐ ployment of a large number of CFRP layers.

The Use of Fiber Reinforced Plastic for The Repair and Strengthening of Existing Reinforced Concrete Structural Elements Damaged by Earthquakes http://dx.doi.org/10.5772/51326 91

This sequential test number of the repaired specimens is signified as Test 2 (for the 1st re‐ pair), Test 3 (for the 2nd repair) etc. (see column 3 of Table 5). In the same column the num‐ ber of CFRP layers used in the partial confinement for the repaired specimens is also indicated. The total number of specimens, virgin and repaired, was seventeen. In Table 5 the unconfined concrete compressive stress is also listed, found from cylinders with diameter 150mm and 300mm height; these cylinders were obtained during the casting of each virgin specimen. The anchorage of the CFRP layers was applied along the two long sides, which were common to both the weak and the strong parts. The main load that was applied was axial compression, although in limited specimens the axial compression was combined with bending, which is not reported here. From the observed behavior, the effectiveness of the applied partial confinement could be deduced. As shown in table 4, this judgment was based on the level of the bearing capacity combined with the type of compression failure that was formed (at the weak or strong part). Moreover, the observed behavior of the vari‐ ous parts of the test specimens, such as the CFRP layers and their anchorage, helped to iden‐ tify the factors that bear an adverse or beneficial influence on these aspects of the behavior.

*3.1.3. Critical parameters and their variation:*

90 Fiber Reinforced Polymers - The Technology Applied for Concrete Repair

ployment of a large number of CFRP layers.

The following parameters were critical (see results contained in Table 6).

*1 st . The type of anchorage of the CFRP layers, 2 nd . The number of the CFRP layers.*

The second parameter becomes critical only if the first parameter performs satisfactorily. A large number of specimens ( 1, 1a, 3, 3a, 4 and 4a) exhibited failure of the anchorage of the partial confinement thus limiting the effectiveness of the partial confinement to low levels. Consequently, the increase in the compressive bearing capacity in this case was relatively modest. The compressive load was monitored by continuous sampling throughout the test. The "*Average Stress at failure*" listed in table 6 is found by dividing the compressive load by the gross cross-sectional area of each specimen. Listed in table 6 are certain particulars of the anchorage of the CFRP layers. Anchors-1 signifies bolts of 7mm diameter and 60mm length that do not penetrate the cross-section in its whole width, whereas Anchors-2 signifies bolts of 10mm diameter that penetrate the whole section (220mm length).For specimens 5 and 5a, due to the improvement of the anchoring details of the partial confinement, the increase in the compressive bearing capacity of these specimens was 50% higher than the observed bearing capacity of similar specimens with no partial CFRP confinement. When the maxi‐ mum compressive bearing capacity achieved with the partial confinement is compared with the corresponding plain concrete strength value, a 230% increase can be observed. More‐ over, the compressive failure in this case involved both the weak and the strong part thus improving substantially the effectiveness of the partial confinement and allowing the em‐

**Table 6.** *Summary of test results together with the basic specimen characteristics*.*\* As % of the corresponding plain concrete strength.*

*3.1.4. Instrumentation to obtain the average stress-strain behavior*

Apart from monitoring the compressive load, the deformations of the mid-part were also continuously recorded throughout each experiment with displacement measurements taken at each side of the cross-section. Eight displacement sensors (two at each side) were em‐ ployed to record the deformations of the mid-part. Although the deformation of this midpart was far from uniform, as can be seen from the obtained displacement measurements of the weak and strong parts (figure 29), the average axial displacement, which was found by averaging the measured displacement values at all four sides of each specimen, is mostly used here as an indication of the deformability of each specimen. By dividing this average axial displacement by the height of the mid-part an average axial strain could also be ob‐ tained in this way. The following discussion of the observed behavior of each specimen is based on diagrams of average axial stress versus average axial strain found from the previ‐ ously described averaging process. More detailed study on the obtained non-uniform de‐ formability for each specimen will be carried out at a future stage. An additional measurement that was obtained during the experimental sequence was the axial strain that developed at the CFRP layers of the partial confinement of the mid-part. These CFRP strain measurements are an additional indication of the effectiveness of the partial confinement.

**Figure 28.** a Partial confinement of low effectiveness. b. Partial confinement of low effectiveness.

**Figure 29.** Specimen 4, Test 1 (5 layers CFRP) Comparison with Specimen 1 Test 1 (no confinement).

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**Figure 30.** a Virgin Specimen without confinement. b. Failure of anchor bolts. 30c Low effectiveness of partial confine‐ ment. c. Tensile failure of CFRP Layers. Considerable effectiveness of partial confinement.

## **3.2. Discussion of the results**

at each side of the cross-section. Eight displacement sensors (two at each side) were em‐ ployed to record the deformations of the mid-part. Although the deformation of this midpart was far from uniform, as can be seen from the obtained displacement measurements of the weak and strong parts (figure 29), the average axial displacement, which was found by averaging the measured displacement values at all four sides of each specimen, is mostly used here as an indication of the deformability of each specimen. By dividing this average axial displacement by the height of the mid-part an average axial strain could also be ob‐ tained in this way. The following discussion of the observed behavior of each specimen is based on diagrams of average axial stress versus average axial strain found from the previ‐ ously described averaging process. More detailed study on the obtained non-uniform de‐ formability for each specimen will be carried out at a future stage. An additional measurement that was obtained during the experimental sequence was the axial strain that developed at the CFRP layers of the partial confinement of the mid-part. These CFRP strain measurements are an additional indication of the effectiveness of the partial confinement.

92 Fiber Reinforced Polymers - The Technology Applied for Concrete Repair

**Figure 28.** a Partial confinement of low effectiveness. b. Partial confinement of low effectiveness.

**Figure 29.** Specimen 4, Test 1 (5 layers CFRP) Comparison with Specimen 1 Test 1 (no confinement).

#### *3.2.1. Partial confinement of low effectiveness*

In figures 28a and 28b the obtained behavior of specimen1 (Test 2) with 3 CFRP layers and specimen 3a (Test 1) with 5 CFRP layers is compared with specimen 1 test 1 (no partial con‐ finement). Specimen 1 test 2 was formed from specimen 1 test 1 by repairing the failed speci‐ men 1 test 1 with special (low shrinkage) concrete as well as with 3 layers of CFRP forming the partial confinement. This repaired specimen failed in compression with almost the same capacity as the previously-tested virgin test with no partial confinement, but with larger de‐ formability. The effectiveness of the partial confinement is low and it is due to the failure of the anchor bolts of the applied confinement. In figure 28b the observed behavior of speci‐ men 3a Test 1 is also included. This is a virgin specimen that had a 5-CFRP layer partial con‐ finement. Despite the increase in the CFRP layers, the observed effectiveness of the partial confinement is low as was for specimen 1 test 2, again because of the failure of the anchor bolts. In figure 29 the observed behavior of specimen 4 test 1 is depicted. This was a virgin specimen with 5 CFRP layers partial confinement. In this case, a certain alteration was ap‐ plied in the anchor bolts, by increasing their length. However, this was not sufficient to im‐ prove accordingly the effectiveness of the partial confinement, which was again linked to the failure of the anchor bolts. The failed virgin specimen without the partial confinement is shown in figure 30a whereas figure 30b depicts the failure of the anchor bolts for the "low effectiveness" partial confinement.

## *3.2.2. Partial confinement of considerable effectiveness*

In figures 31a and 31b the obtained behavior of specimen 5 (Test 3) with 5 (+2) CFRP layers is compared to specimen 1 test 1 (no partial confinement). Specimen 5 test 3 was formed by repairing a previously failed virgin specimen with special (low shrinkage) concrete as well as with 5 layers of CFRP forming the partial confinement. An additional two (+2) CFRP lay‐ ers were applied at the part of the section where the anchor bolts were placed. This repaired specimen failed in compression with a modest increase (31%) in its capacity when compared with the capacity of the virgin unconfined specimen. The effectiveness of the partial confine‐ ment in this case was classified as considerable. This was due to an alteration in the anchor‐ ing of the partial confinement which proved to be relatively successful. The limit state for this specimen commenced with the tensile failure of the CFRP layers at the central zone and was accompanied, as expected, by a consequent compressive failure of the neighboring weak part of the section. This is depicted in figure 30c where the anchor bolts, which were left intact, are also shown. In figure 31b, the comparison is extended to include specimen 4 (Test 1), in which the partial confinement exhibited low effectiveness due to the failure of the anchor bolts.

**Figure 31.** Partial confinement of considerable effectiveness. 31b. Partial confinement of considerable effectiveness.

#### *3.2.3. Very effective partial confinement*

In figure 32a, the behavior of specimens 5 (Test 4) and 5a (Test 2) is compared with the be‐ havior of specimens 1 test 1 (with no partial confinement) and 5 (Test 3) discussed before. Specimens 5 (Test 4) and 5a (Test 2) were formed by repairing previously failed specimens with partial confinement of 7 layers of CFRP. Moreover, all the anchoring of their partial confinement was made with bolts going through the whole width of the strong part of the repaired section (see table 6 and figure 25). As can be seen in figure 32a, a substantial in‐ crease (40%), in the bearing capacity as well as in the deformability, resulted from the descri‐ bed partial confinement for these two specimens. Their behavior was in this way better than the behavior of specimen 4 (Test 1) which was classified before as one of considerable effec‐ tiveness of the partial confinement. Based in this increased capacity and deformability of specimens 5 (Test 4) and 5a (Test 2) they were classified as specimens with very effective partial confinement.

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**Figure 32.** Very effective partial confinement. 32b. Very effective partial confinement.

*3.2.2. Partial confinement of considerable effectiveness*

94 Fiber Reinforced Polymers - The Technology Applied for Concrete Repair

the anchor bolts.

*3.2.3. Very effective partial confinement*

partial confinement.

In figures 31a and 31b the obtained behavior of specimen 5 (Test 3) with 5 (+2) CFRP layers is compared to specimen 1 test 1 (no partial confinement). Specimen 5 test 3 was formed by repairing a previously failed virgin specimen with special (low shrinkage) concrete as well as with 5 layers of CFRP forming the partial confinement. An additional two (+2) CFRP lay‐ ers were applied at the part of the section where the anchor bolts were placed. This repaired specimen failed in compression with a modest increase (31%) in its capacity when compared with the capacity of the virgin unconfined specimen. The effectiveness of the partial confine‐ ment in this case was classified as considerable. This was due to an alteration in the anchor‐ ing of the partial confinement which proved to be relatively successful. The limit state for this specimen commenced with the tensile failure of the CFRP layers at the central zone and was accompanied, as expected, by a consequent compressive failure of the neighboring weak part of the section. This is depicted in figure 30c where the anchor bolts, which were left intact, are also shown. In figure 31b, the comparison is extended to include specimen 4 (Test 1), in which the partial confinement exhibited low effectiveness due to the failure of

**Figure 31.** Partial confinement of considerable effectiveness. 31b. Partial confinement of considerable effectiveness.

In figure 32a, the behavior of specimens 5 (Test 4) and 5a (Test 2) is compared with the be‐ havior of specimens 1 test 1 (with no partial confinement) and 5 (Test 3) discussed before. Specimens 5 (Test 4) and 5a (Test 2) were formed by repairing previously failed specimens with partial confinement of 7 layers of CFRP. Moreover, all the anchoring of their partial confinement was made with bolts going through the whole width of the strong part of the repaired section (see table 6 and figure 25). As can be seen in figure 32a, a substantial in‐ crease (40%), in the bearing capacity as well as in the deformability, resulted from the descri‐ bed partial confinement for these two specimens. Their behavior was in this way better than the behavior of specimen 4 (Test 1) which was classified before as one of considerable effec‐ tiveness of the partial confinement. Based in this increased capacity and deformability of specimens 5 (Test 4) and 5a (Test 2) they were classified as specimens with very effective

**Figure 33.** Strengthening stirrups - strong part. 33b. Failure of the CFRP layers of the mid-part.

The limit state of specimen 5 (Test 4) was accompanied with the failure of the stirrups of the strong part whereas the limit state of specimen 5a (Test 2) was accompanied with the tensile failure of the CFRP layers at the central zone. In figure 32b the observed behavior of speci‐ men 5a (Test 3) is compared with that of specimens 5 (Test 4) and 5a (Test 2) previously dis‐ cussed. Specimen 5a Test 3 had a partial confinement formed by 7 layers of CFRP. Moreover, the area sustaining the anchor bolts was strengthened by additional four (+4) lay‐ ers of CFRP. Apart from the anchor bolts going through the whole width of the strong part of its section, the stirrups of this strong part were also strengthened (figure 33a). This time the increase in the bearing capacity was 46%; the increase in the deformability was also quite substantial. Thus, the behavior of this specimen is also one with very effective partial con‐ finement. The observed limit state was accompanied with the compressive failure of the CFRP layers at the mid-part near the steel brackets (figure 33b).

## **3.3. Concluding remarks**

**a2.** The undesired compression failure expected to develop in the base of vertical members with reinforced concrete cross sections having h/b ratio larger than 1.5 under combined vertical loads and seismic actions is studied through specially formed specimens subjected to uni‐ form compression. The retrofitting of such specimens with partial CFRP confinement is aimed at prohibiting, up to a point, such compression failure. This type of partial confinement may also be applied to upgrading vertical structural members with non-accessible sides.

**b2.**. From the results of the experimental investigation with identical specimens, with or without this type of partial CFRP confinement, the successful application of such partial confinement was demonstrated. An increase of almost 50% was observed in the compres‐ sion bearing capacity of some of the tested specimens. Moreover, the deformability of these specimens was substantially increased, demonstrating the effectiveness of this type of parti‐ al confinement.

**c2.** It was found from the experimental sequence that critical factors for this increase were the type of anchorage of the CFRP partial confinement and the number of CFRP layers. Success‐ ful anchoring of the CFRP layers allowed this partial confinement to become effective and to permit the use of a larger number of CFRP layers. In the present study alternative anchoring schemes were tried with limitations imposed by the geometry of the model cross-section. Similar limitations imposed by the geometry and the reinforcement of the cross-section will also dictate the design of such an anchoring scheme for a prototype cross-section. Further investigation on the performance of such prototype anchoring arrangements may be necessary.

## **4. Upgrading the flexural capacity of (R/C) vertical members**

## **4.1. Flexural upgrading**

The upgrading of the flexural capacity of reinforced concrete (R/C) vertical members with externally applied CFRP layers as means of tensile reinforcement is investigated here (see figure 34 and [4], [6], [11], [16], [22], [34], [40], [41])

The CFRP layers are applied at the two opposite sides of R/C specimens which were con‐ structed for this purpose (figures 35a, 35b). They were designed with such longitudinal and transverse reinforcements shown in figure 35b that the flexural limit state would prevail. The specimens were tested to develop the ultimate flexural behavior with the corresponding damaged region concentrated near their joint with the foundation. These specimens had the foundation block fixed at the strong reaction frame of the laboratory of Strength of Materials and Structures of Aristotle University and were subjected to simultaneous constant vertical load as well as horizontal cyclic imposed displacements approximating thus the seismic ac‐ tions. This loading sequence was applied prior to the application of any CFRP layers and was repeated again after these CFRP layers were attached to the damaged specimens. The observed upgrading of the flexural capacity in terms of ultimate overturning moment is pre‐ sented and discussed.

**3.3. Concluding remarks**

96 Fiber Reinforced Polymers - The Technology Applied for Concrete Repair

al confinement.

**4.1. Flexural upgrading**

sented and discussed.

figure 34 and [4], [6], [11], [16], [22], [34], [40], [41])

**a2.** The undesired compression failure expected to develop in the base of vertical members with reinforced concrete cross sections having h/b ratio larger than 1.5 under combined vertical loads and seismic actions is studied through specially formed specimens subjected to uni‐ form compression. The retrofitting of such specimens with partial CFRP confinement is aimed at prohibiting, up to a point, such compression failure. This type of partial confinement may

**b2.**. From the results of the experimental investigation with identical specimens, with or without this type of partial CFRP confinement, the successful application of such partial confinement was demonstrated. An increase of almost 50% was observed in the compres‐ sion bearing capacity of some of the tested specimens. Moreover, the deformability of these specimens was substantially increased, demonstrating the effectiveness of this type of parti‐

**c2.** It was found from the experimental sequence that critical factors for this increase were the type of anchorage of the CFRP partial confinement and the number of CFRP layers. Success‐ ful anchoring of the CFRP layers allowed this partial confinement to become effective and to permit the use of a larger number of CFRP layers. In the present study alternative anchoring schemes were tried with limitations imposed by the geometry of the model cross-section. Similar limitations imposed by the geometry and the reinforcement of the cross-section will also dictate the design of such an anchoring scheme for a prototype cross-section. Further investigation on the performance of such prototype anchoring arrangements may be necessary.

The upgrading of the flexural capacity of reinforced concrete (R/C) vertical members with externally applied CFRP layers as means of tensile reinforcement is investigated here (see

The CFRP layers are applied at the two opposite sides of R/C specimens which were con‐ structed for this purpose (figures 35a, 35b). They were designed with such longitudinal and transverse reinforcements shown in figure 35b that the flexural limit state would prevail. The specimens were tested to develop the ultimate flexural behavior with the corresponding damaged region concentrated near their joint with the foundation. These specimens had the foundation block fixed at the strong reaction frame of the laboratory of Strength of Materials and Structures of Aristotle University and were subjected to simultaneous constant vertical load as well as horizontal cyclic imposed displacements approximating thus the seismic ac‐ tions. This loading sequence was applied prior to the application of any CFRP layers and was repeated again after these CFRP layers were attached to the damaged specimens. The observed upgrading of the flexural capacity in terms of ultimate overturning moment is pre‐

**4. Upgrading the flexural capacity of (R/C) vertical members**

also be applied to upgrading vertical structural members with non-accessible sides.

#### *4.1.1. The virgin beam specimen*

This specimen is shown in figure 35a with a rectangular cross-section of 200 mm x 500 mm and height h = 1815mm (from its top to the upper surface of its foundation). 8Φ6 was the longitudinal reinforcement that extended with no splices and was anchored to the founda‐ tion block and Φ6/100 closed stirrups the transverse reinforcement. The detailing of the cross-section is shown in figure 35b. The selected longitudinal and transverse reinforcement together with the loading arrangement, will cause the behavior of this virgin specimen to be dominated by the flexural rather than the shear mode of failure. More details are given in references [40] and [41].

**Figure 35.** a Pier A. b. Cross-section and reinforcing details of Pier A.

#### *4.1.2. The Loading sequence*

The foundation of the specimen was anchored at the strong floor of a reaction frame. The specimen was then subjected to a simultaneous constant vertical load and a cyclic horizontal displacement with increasing amplitude in time, utilizing servo-electronically-controlled dy‐ namic actuators. The frequency of this cyclic displacement was in some tests 0.1Hz whereas in other selected tests it became 1.0Hz. The vertical load was kept constant at 95KN. The horizontal imposed cyclic displacement was applied in 13 groups of continuously increasing amplitude. Each group included 3 full cycles of constant amplitude. This time history of the imposed displacement is depicted in figure 36a. The amplitude in this picture is non-dimen‐ sional and is given as a percentage of the final maximum amplitude of this cyclic loading. In the experimental sequence the maximum displacement horizontal amplitude was initially relatively small (2mm); as testing progressed, in subsequent cyclic loading sequences it reached values of 20mm to 25mm. The imposed displacements were measured at the loca‐ tion of the horizontal actuator which was placed at a height of 1400mm from the top of the foundation (figure 36b). Figures 36b and 36c depict this experimental set-up. As can be seen the specimen was placed in the strong reaction frame, having its foundation block, with di‐ mensions in plan 1000mm by 1000mm and 300mm height, fully anchored to the strong floor. The horizontal and vertical actuators, as part of this strong reaction frame, applied the load‐ ing sequence described above. Instrumentation was provided in order to measure the varia‐ tion of the applied horizontal and vertical loads as well as the most important aspects of the displacement field that resulted from the application of these loads to the specimen. The horizontal displacements of the specimens at the top were monitored as well as the displace‐ ments of the specimen at the region near the foundation block in order to identify the flexu‐ ral, and shear deformations as well as the plastic hinge behavior at this part of the specimen. In figure 36b the two vertical sides of the specimen where the CFRP layers were attached are indicated together with the region where the anchoring arrangements were placed as part of the current investigation. Throughout these series of experiments the nominal thickness of the employed CFRP layers was 0.176mm with a given Young's Modulus E=350GPa. The measured maximum axial CFRP strain was approximately 1%. These CFRP layers were placed at these locations

**Figure 36.** Imposed horizontal displacement. 36b. Experimental set-up. 36c Pier A placed at the strong reaction frame.

## **4.2. Observed Behavior - Results**

#### *4.2.1. The behavior of Pier A*

*4.1.2. The Loading sequence*

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placed at these locations

The foundation of the specimen was anchored at the strong floor of a reaction frame. The specimen was then subjected to a simultaneous constant vertical load and a cyclic horizontal displacement with increasing amplitude in time, utilizing servo-electronically-controlled dy‐ namic actuators. The frequency of this cyclic displacement was in some tests 0.1Hz whereas in other selected tests it became 1.0Hz. The vertical load was kept constant at 95KN. The horizontal imposed cyclic displacement was applied in 13 groups of continuously increasing amplitude. Each group included 3 full cycles of constant amplitude. This time history of the imposed displacement is depicted in figure 36a. The amplitude in this picture is non-dimen‐ sional and is given as a percentage of the final maximum amplitude of this cyclic loading. In the experimental sequence the maximum displacement horizontal amplitude was initially relatively small (2mm); as testing progressed, in subsequent cyclic loading sequences it reached values of 20mm to 25mm. The imposed displacements were measured at the loca‐ tion of the horizontal actuator which was placed at a height of 1400mm from the top of the foundation (figure 36b). Figures 36b and 36c depict this experimental set-up. As can be seen the specimen was placed in the strong reaction frame, having its foundation block, with di‐ mensions in plan 1000mm by 1000mm and 300mm height, fully anchored to the strong floor. The horizontal and vertical actuators, as part of this strong reaction frame, applied the load‐ ing sequence described above. Instrumentation was provided in order to measure the varia‐ tion of the applied horizontal and vertical loads as well as the most important aspects of the displacement field that resulted from the application of these loads to the specimen. The horizontal displacements of the specimens at the top were monitored as well as the displace‐ ments of the specimen at the region near the foundation block in order to identify the flexu‐ ral, and shear deformations as well as the plastic hinge behavior at this part of the specimen. In figure 36b the two vertical sides of the specimen where the CFRP layers were attached are indicated together with the region where the anchoring arrangements were placed as part of the current investigation. Throughout these series of experiments the nominal thickness of the employed CFRP layers was 0.176mm with a given Young's Modulus E=350GPa. The measured maximum axial CFRP strain was approximately 1%. These CFRP layers were

**Figure 36.** Imposed horizontal displacement. 36b. Experimental set-up. 36c Pier A placed at the strong reaction frame.

The yield stress of the longitudinal reinforcements was equal to 344.8MPa whereas the con‐ crete strength was equal to 21.2MPa. Based on these mechanical characteristics and the cross-section reinforcing detailing, predictions of the limit-state flexural behavior for this virgin specimen (without CFRP) were obtained in terms of the M-N interaction. The limitstate flexural behavior of a specimen with 2 layers of CFRP attached to both vertical sides of a repaired specimen is also obtained, based on the assumption that the cross-sections under flexure remain plane even with the attachment of the CFRP layers. The development of flexural cracks took place as shown in figure 37. At this horizontal cross-section the meas‐ ured flexural behavior of Pier A is depicted in figure 38, which presents the variation of the resulting bending moment at this cross-section of the specimen against the rotation of the same cross section,. The rotation was obtained from measurements made by displacement transducers that were placed at the two sides of the pier monitoring the relative vertical dis‐ placement between the upper part of the specimen and its foundation block. Additional in‐ strumentation was provided to monitor the rocking or the sliding of the foundation block itself, which proved to be non-significant. The observed maximum bending moment value compares quite well with the corresponding predictions of the limit-state flexure for this cross-section. The red line in these plots represents the corresponding envelope curve.

**Figure 37.** Observed flexural cracks for Pier A.

**Figure 38.** The measured flexural response of Pier A in terms of bending moment - plastic rotation.

#### *4.2.2. Repaired specimen Rep-1 Pier A and observed behavior*

The damaged Pier A was repaired in a way depicted by figures 39a,b. The resulting speci‐ men is designated as Rep-1 Pier A. Two layers of CFRP were attached at each vertical side of the pier's cross-section. This attachment extended to a height of 1100mm from the founda‐ tion surface as well as at the top surface of the foundation block depicted in figure 39b. Moreover, in order to increase the bond strength between the CFRP layers and the top sur‐ face of the foundation block two double T steel sections were placed at these locations. A force normal to the bond surface at this location was applied by bolting these double T steel sections to the foundation block utilizing pre-stressing rods. Due to space limitations no fur‐ ther details are given here.

**Figure 39.** Specimen Rep-1 Pier A. Specimen 39b Rep-1 Pier A, attachment of CFRP layers. 39c Debonding of the CFRP layers from the right side of the repaired specimen Rep-1 Pier A.

## *4.2.3. Observed Behavior of Specimen Rep-1 Pier A*

**Figure 38.** The measured flexural response of Pier A in terms of bending moment - plastic rotation.

The damaged Pier A was repaired in a way depicted by figures 39a,b. The resulting speci‐ men is designated as Rep-1 Pier A. Two layers of CFRP were attached at each vertical side of the pier's cross-section. This attachment extended to a height of 1100mm from the founda‐ tion surface as well as at the top surface of the foundation block depicted in figure 39b. Moreover, in order to increase the bond strength between the CFRP layers and the top sur‐ face of the foundation block two double T steel sections were placed at these locations. A force normal to the bond surface at this location was applied by bolting these double T steel sections to the foundation block utilizing pre-stressing rods. Due to space limitations no fur‐

**Figure 39.** Specimen Rep-1 Pier A. Specimen 39b Rep-1 Pier A, attachment of CFRP layers. 39c Debonding of the CFRP

*4.2.2. Repaired specimen Rep-1 Pier A and observed behavior*

100 Fiber Reinforced Polymers - The Technology Applied for Concrete Repair

layers from the right side of the repaired specimen Rep-1 Pier A.

ther details are given here.

The loading sequence, described before, was applied to this repaired specimen. During in‐ creasing of the imposed horizontal displacement the debonding of the CFRP layer at the right side of the pier occurred as depicted in figure 39c. The obtained flexural behavior is depicted in figure 40 in terms of applied horizontal load against the imposed horizontal dis‐ placement at the top of the pier. If this behavior is compared with the corresponding behav‐ ior of the virgin specimen Pier A, a modest increase in its bearing capacity of approximately 20% can be observed as a result of the applied CFRP repair scheme, despite the debonding of the CFRP layers. The red line in this figure represents the corresponding envelope curve.

**Figure 40.** The measured flexural response of Rep-1 Pier A in terms of Horizontal load - Horizontal Displacement.

#### *4.2.4. Repaired specimen Rep-2 Pier A and observed behavior*

The damaged Rep-1 Pier A was repaired again in a way which is shown in figures 41a,b. The CFRP layers that were separated from the concrete at the right side of Rep-1 Pier A were reattached, both with the appropriate resin and additional 2mm diameter small bolts of 35mm length; these bolts were spaced at regular 50mm intervals along all the height of the CFRP layer at both sides. Moreover, the edge of the double T steel section neighboring the location of the CFRP layer at the pier-foundation joint was machined to form a curvature so that it provided a relatively uniform contact between the steel section and CFRP layers at this location. The locations of the bolts were first marked and drilled and then the bolts were applied including the corresponding plastic anchoring inserts. The specimen repaired in this way is designated as Rep-2 Pier A.

**Figure 41.** a The repair of specimen Rep-1 Pier A. b. The repair of specimen Rep-1 Pier A. c. Fracture of CFRP for speci‐ men Rep-2 Pier A.

#### *4.2.5. Observed Behavior of Specimen Rep-2 Pier A*

The loading sequence, described in section 2, was also applied for this repaired specimen. During the increasing of the imposed horizontal displacement the fracture of the CFRP lay‐ ers at the right side of the pier occurred as depicted in figure 41c. This fracture occurred at the location where the pier joins the foundation block. Figure 42a presents the variation of the measured horizontal load versus the horizontal displacement, whereas the resulting bending moment against the plastic hinge rotation is plotted in figure 42b. The plastic hinge which was formed during testing specimen Rep-1 was not repaired in any other way, but with the means described before. The fracture of the CFRP layer depicted in figure 41c took place at the same cross section. As was mentioned before the plastic rotation was measured by displacement transducers that were placed at the two sides of the pier. Because of the dis‐ location of one of these transducers the rotation angle after that was not monitored. This is noted at the bottom left hand side of figure 42b. Despite this lack of rotation measurements after the dislocation, the bending moment capacity remains approximately at the same lev‐ els, as deduced from the variation of the horizontal load (figures 42a and 43a). The effective‐ ness of the applied repair schemes for specimens Rep-1 and Rep-2 can be seen in figures 43a and 43b in terms of envelope curves. In figure 43a the comparison is made in terms of hori‐ zontal load – horizontal displacement whereas in figure 43b this is done in terms of bending moment-rotation. As can be seen, the effectiveness of the CFRP layers is inhibited by the de‐ bonding of the CFRP layers; thus although a considerable increase is achieved in the bearing capacity, this is not sustained in terms of displacement because of the premature debonding of the CFRP. The improvement of the bonding with the use of the employed bolting scheme for Rep-2 resulted in an increase of the bearing capacity both in terms of load and displace‐ ment and resulted in the fracture of the CFRP.

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**Figure 42.** The measured flexural response of Rep-2 Pier A in terms of horizontal load - horizontal Displacement. 42b. The measured flexural response of Rep-2 Pier A in terms of bending moment - plastic rotation.

**Figure 41.** a The repair of specimen Rep-1 Pier A. b. The repair of specimen Rep-1 Pier A. c. Fracture of CFRP for speci‐

The loading sequence, described in section 2, was also applied for this repaired specimen. During the increasing of the imposed horizontal displacement the fracture of the CFRP lay‐ ers at the right side of the pier occurred as depicted in figure 41c. This fracture occurred at the location where the pier joins the foundation block. Figure 42a presents the variation of the measured horizontal load versus the horizontal displacement, whereas the resulting bending moment against the plastic hinge rotation is plotted in figure 42b. The plastic hinge which was formed during testing specimen Rep-1 was not repaired in any other way, but with the means described before. The fracture of the CFRP layer depicted in figure 41c took place at the same cross section. As was mentioned before the plastic rotation was measured by displacement transducers that were placed at the two sides of the pier. Because of the dis‐ location of one of these transducers the rotation angle after that was not monitored. This is noted at the bottom left hand side of figure 42b. Despite this lack of rotation measurements after the dislocation, the bending moment capacity remains approximately at the same lev‐ els, as deduced from the variation of the horizontal load (figures 42a and 43a). The effective‐ ness of the applied repair schemes for specimens Rep-1 and Rep-2 can be seen in figures 43a and 43b in terms of envelope curves. In figure 43a the comparison is made in terms of hori‐ zontal load – horizontal displacement whereas in figure 43b this is done in terms of bending moment-rotation. As can be seen, the effectiveness of the CFRP layers is inhibited by the de‐ bonding of the CFRP layers; thus although a considerable increase is achieved in the bearing capacity, this is not sustained in terms of displacement because of the premature debonding of the CFRP. The improvement of the bonding with the use of the employed bolting scheme for Rep-2 resulted in an increase of the bearing capacity both in terms of load and displace‐

men Rep-2 Pier A.

*4.2.5. Observed Behavior of Specimen Rep-2 Pier A*

102 Fiber Reinforced Polymers - The Technology Applied for Concrete Repair

ment and resulted in the fracture of the CFRP.

**Figure 43.** a Comparison of flexural response of repaired Rep-2 Pier A with repaired specimen Rep-1 Pier A and virgin specimen Pier A in terms of Horizontal load – Horizontal displacements. b. Comparison of flexural response of repaired Rep-2 Pier A with virgin specimen Pier A in terms of bending moment - plastic rotation.

From figure 43b the effectiveness of the repair scheme utilized in specimen Rep-2 Pier A can be deduced compared with the virgin specimen Pier A in terms of bending moment - plastic hinge rotation. The achieved increase in bending moment is almost 50% and the flexural be‐ havior in terms of rotation is satisfactory up to the fracture of the CFRP layer at the joint between the pier and the foundation. Comparing the maximum value of the bending mo‐ ment (83KNm) that was resisted by specimen Rep-2 Pier A with the one predicted (130KNm), as an ultimate bending moment that the cross-section reinforced by the applied CFRP layers can ideally resist, it can be concluded that the fracture of the CFRP did not al‐ low this ideally maximum bending moment value to be reached. One reason for this is the way the anchoring of the CFRP layers at the foundation block is achieved with the utiliza‐ tion of the double T steel sections described before. Despite the machining of the edge of these double T steel sections the development of high stress levels concentrated at the CFRP layers at these locations is the cause of the observed CFRP fracture. Moreover, such an an‐ choring scheme is quite impractical for prototype conditions. For these reasons a different anchoring scheme was investigated in order to address both these shortcomings that were found to be critical in the repair schemes investigated so far. This is presented next in a sum‐ mary form.

#### *4.2.6. Repaired specimen Rep-3 Pier A and observed behavior*

Figure 44a depicts the outline of this anchoring scheme and figure 44b depicts the applica‐ tion of this anchoring scheme for repair specimen Rep-3 Pier A, whereby the CFRP layers are folded around a cylinder that is anchored to the foundation [40]. Moreover, in order to avoid the separation (detachment) of the CFRP layers from the sides of the specimen, as ob‐ served in specimen Rep-1, the bolting scheme that was used in specimen Rep-3 was further improved by using bolts and washers of larger size (due to space limitations no details are given here). In addition, to avoid the development of shear failure at the bottom part of the pier, three horizontal CFRP closed hoops were attached in this region of the specimen (see figures 44c, 44d and [40], [41], [42]).

**Figure 44.** a New anchoring device. b. New anchoring device. c. Specimen Rep-3 Pier A. Figure d. Specimen Rep-3 Pier A. Figure e. Partial fracture of CFRP layers at the cylindrical part of anchoring device

The Use of Fiber Reinforced Plastic for The Repair and Strengthening of Existing Reinforced Concrete Structural Elements Damaged by Earthquakes http://dx.doi.org/10.5772/51326 105

**Figure 45.** Comparison of flexural response of repaired specimens Rep-3, Rep-2, Rep-1 and virgin specimen. 45b. Comparison of flexural response of repaired Rep-3, Rep-2 and virgin specimens.

## *4.2.7. Observed Behavior of Specimen Rep-3 Pier A*

choring scheme is quite impractical for prototype conditions. For these reasons a different anchoring scheme was investigated in order to address both these shortcomings that were found to be critical in the repair schemes investigated so far. This is presented next in a sum‐

Figure 44a depicts the outline of this anchoring scheme and figure 44b depicts the applica‐ tion of this anchoring scheme for repair specimen Rep-3 Pier A, whereby the CFRP layers are folded around a cylinder that is anchored to the foundation [40]. Moreover, in order to avoid the separation (detachment) of the CFRP layers from the sides of the specimen, as ob‐ served in specimen Rep-1, the bolting scheme that was used in specimen Rep-3 was further improved by using bolts and washers of larger size (due to space limitations no details are given here). In addition, to avoid the development of shear failure at the bottom part of the pier, three horizontal CFRP closed hoops were attached in this region of the specimen (see

**Figure 44.** a New anchoring device. b. New anchoring device. c. Specimen Rep-3 Pier A. Figure d. Specimen Rep-3 Pier

A. Figure e. Partial fracture of CFRP layers at the cylindrical part of anchoring device

*4.2.6. Repaired specimen Rep-3 Pier A and observed behavior*

104 Fiber Reinforced Polymers - The Technology Applied for Concrete Repair

figures 44c, 44d and [40], [41], [42]).

mary form.

The loading sequence, described before, was also applied for this repaired specimen. During increasing of the imposed horizontal displacement the partial fracture of the CFRP layer at the right side of the pier occurred as depicted in figure 44e. This fracture occurred at the lo‐ cation where the CFRP layers are folded around the cylindrical part of the anchoring device of figures 44a,b. In figures 45a and 45b the measured flexural behavior in terms of horizontal load-horizontal displacement and bending moment-rotation envelope curve diagrams for repaired specimen Rep-1, Rep-2 and Rep-3 is compared with the coresponding flexural be‐ havior of the virgin specimen Pier A. Due to space limitations the measured full cyclic re‐ sponse for specimen Rep-3 is not included in this presentation. The following observations can be made on the basis of these diagrams. a) The horizontal load and maximum bending moment bearing capacity of the latest repair scheme, namely specimen Rep-3, exhibits an in‐ crease of almost 100% when compared with the corresponding bearing capacity of the virgin specimen. b) When this comparison is made between the latest repair scheme Rep-3 and re‐ pair scheme Rep-2 the increase is of the order of 25%. Moreover, the new anchoring scheme does not cause an abrupt fracture of the CFRP at the anchoring device. This is reflected in figures 45a and 45b by the fact that the obtained response of specimen Rep-3 extends to rela‐ tively large displacement and rotation values. c) A reduction in the initial stiffness appears to be present in specimen Rep-3 when its obtained cyclic response in terms of the envelope curve of figures 45a,b is compared to the corresponding curves of specimen Rep-2 and the virgin specimen. This must be attributed to the fact that most parts of specimen Rep-3 have been subjected to three full loading sequences except the new parts of the CFRP. d) Rep-1 corresponds to the least effective repair scheme due to the debonding of the CFRP layers.

#### **4.3. Concluding Observations**

**a3.** The upgrading of the flexural behavior of vertical R/C structural elements was investi‐ gated utilizing the possibility of attaching longitudinal CFRP layers that can be stretched de‐ veloping tensile forces externally at the opposite sides of these elements. Critical aspects for their satisfactory performance are the bonding of the CFRP layers as well as the effective transfer of the tensile forces to the foundation block.

**b3.** The bonding of the CFRP layers at the sides can be improved by bolting schemes that, in this way, increase the cooperation between the CFRP layers and the concrete part.

**c3.** The transfer of the tensile forces that develop at the CFRP layers to the foundation is a difficult technical problem. Two schemes were tried. The 2nd scheme is more practical and was successful in mobilizing sufficiently the available capacity of the applied CFRP layers in such a way that it resulted in approximately 100% increase in the flexural capacity when compared to the initial flexural capacity of the virgin specimen before the application of the CFRP layers.

**d3.** The applicability of such an anchoring system must be validated in a more general way before final practical conclusions can be reached.

**e3.** Such an upgrading of the flexural capacity may lead to the appearance of the shear mode of failure. However, the shear capacity can be increased with relative ease by closed CFRP hoops.

## **5. Relevant code provisions - Emphasis in the application of FRP strips for shear strengthening**

During the last fifteen years numerous researchers ([12], [18], [29], [34]) have proposed dif‐ ferent approaches for predicting the behavior of FRPs when they are used for strengthening structural members. Several design guidelines and code provisions adopted some of the proposed approaches. More specifically, the approach of Triantafillou and Khalifa [33] has been adopted by the American Code ACI 440 [11] whereas the model of Chen and Teng ([18], [25]) is utilized in the Greek Code of Interventions [6]. Eurocode 8 part 3 ([7]) follows the guidelines of FIB [8]. In the following paragraphs the aforementioned three Codes are discussed with particular emphasis being given to the application of FRP strips for shear strengthening. All codes recommend a limit-state tensile force that a cross-section (AFRP) of an FRP sheet can sustain. This limit-state tensile force is obtained through equation 2:

$$\left(V\right)\_{FRP} = \frac{A\_{FRP} \cdot \varepsilon\_{FRP} \cdot E\_{FRP}}{\varphi} \tag{2}$$

Where: VFRP: Limit-state tensile force of the FRP strip, φ: safety factor, AFRP: cross section area of a strip of FRP

εFRP: developed axial strains on the FRP strip, EFRP: modulus of elasticity of the FRP strip

The various codes recommend the application of Equation 2 by defining various values for the safety factor (φ) and the limit-state axial strain value (εFRP). These values vary according to the application depending on whether the predicted FRP contribution is for shear, flexure or confinement. In doing so, the basic factors that are varied among the Codes are the safety factor and the allowable developed strain on FRP. Table 7 presents the maximum allowable εFRP values and the recommended safety factor (φ) values according to ACI 440, Eurocode part 3, and the Greek Code for structural interventions, when anchoring is used in the trans‐ fer of FRP strip forces.

veloping tensile forces externally at the opposite sides of these elements. Critical aspects for their satisfactory performance are the bonding of the CFRP layers as well as the effective

**b3.** The bonding of the CFRP layers at the sides can be improved by bolting schemes that, in

**c3.** The transfer of the tensile forces that develop at the CFRP layers to the foundation is a difficult technical problem. Two schemes were tried. The 2nd scheme is more practical and was successful in mobilizing sufficiently the available capacity of the applied CFRP layers in such a way that it resulted in approximately 100% increase in the flexural capacity when compared to the initial flexural capacity of the virgin specimen before the application of the

**d3.** The applicability of such an anchoring system must be validated in a more general way

**e3.** Such an upgrading of the flexural capacity may lead to the appearance of the shear mode of failure. However, the shear capacity can be increased with relative ease by closed CFRP hoops.

**5. Relevant code provisions - Emphasis in the application of FRP strips**

During the last fifteen years numerous researchers ([12], [18], [29], [34]) have proposed dif‐ ferent approaches for predicting the behavior of FRPs when they are used for strengthening structural members. Several design guidelines and code provisions adopted some of the proposed approaches. More specifically, the approach of Triantafillou and Khalifa [33] has been adopted by the American Code ACI 440 [11] whereas the model of Chen and Teng ([18], [25]) is utilized in the Greek Code of Interventions [6]. Eurocode 8 part 3 ([7]) follows the guidelines of FIB [8]. In the following paragraphs the aforementioned three Codes are discussed with particular emphasis being given to the application of FRP strips for shear strengthening. All codes recommend a limit-state tensile force that a cross-section (AFRP) of an FRP sheet can sustain. This limit-state tensile force is obtained through equation 2:

*VFRP* <sup>=</sup> *AFRP* <sup>⋅</sup> *<sup>ε</sup>FRP* <sup>⋅</sup> *EFRP*

Where: VFRP: Limit-state tensile force of the FRP strip, φ: safety factor, AFRP: cross section area

The various codes recommend the application of Equation 2 by defining various values for the safety factor (φ) and the limit-state axial strain value (εFRP). These values vary according to the application depending on whether the predicted FRP contribution is for shear, flexure

εFRP: developed axial strains on the FRP strip, EFRP: modulus of elasticity of the FRP strip

*<sup>φ</sup>* (2)

this way, increase the cooperation between the CFRP layers and the concrete part.

transfer of the tensile forces to the foundation block.

106 Fiber Reinforced Polymers - The Technology Applied for Concrete Repair

before final practical conclusions can be reached.

**for shear strengthening**

of a strip of FRP

CFRP layers.

**Table 7.** Maximum allowable developed strains εFRP and proposed safety factors φ when anchoring is used.Where: ε\* nom: max strain from manufacturer

The allowable limit-state axial strains (εFRP) value defined by the various codes also depend upon the way the FRP sheets are attached upon the structural elements in need of strength‐ ening. According to all code provisions, there are two basic ways of attaching FRP sheets on structural elements. First, they can be attached using an anchoring device or they can be fully wrapped around the structural element. Secondly, they can simply be attached on the surface of the structural element through an organic or inorganic matrix in an open loop Ushaped without any anchoring. When the simple attachment is used without anchoring the debonding mode of failure prevails, whereas when FRP sheets are combined with an an‐ choring device or when they are fully wrapped around a column or a beam, fracture of FRP's occurs. The debonding mode of failure poses a limitation to the FRP axial strains and stresses. This limitation is reflected in all three Codes through a factor (usually designated as kv), that takes values less than 1. This reduction factor depends on the recommended value of the attachment FRP length (effective attachment length). Table 8 lists the basic formulae included in the various codes for calculating this effective attachment length. The most im‐ portant factor in calculating the effective attachment length is the determination of bond strength, which is not easily obtained since it depends on the actual concrete tensile strength and the state of the surface where the FRP is attached; both these parameters can easily vary even for the same structural element.

When the attachment of FRP sheets is combined with an anchoring device a more reliable transfer of forces can be achieved thus resulting in an equally reliable design of the relevant strengthening scheme. Moreover, since the prevailing mode of failure is the fracture of the FRP's, when anchoring of the FRP strips is used, the exploitation of the material of the FRP sheets is enhanced by reaching higher values of axial strains than for the cases governed by the debonding mode of failure. Under all circumstances the maximum allowable strains should not exceed the value presented in table 7. All codes demand the use of a safe anchor‐ ing device that would allow the fracture of FRP sheets without proposing any calculations for the safe design of these anchoring devices. In any case, equation (3) describes this condi‐ tion for the bearing capacity of a permissible anchoring device

**Table 8.** Calculation of effective length. Where VFRP: total force received by FRP, Vanchoring device: total strength of the anchoring device.

$$\mathcal{V}\_{FRP} \leq \mathcal{V}\_{anchoring\text{ }g\text{ }device\text{ }}\tag{3}$$

## **6. Special study for anchoring FRP strips**

In this section results from a recent research effort conducted at the Laboratory of Strength of Materials and Structures of Aristotle University will be briefly presented and discussed ([13], [42]). This research aimed to investigate the effectiveness of a specific FRP strip an‐ choring device by utilising a number of small concrete prismatic specimens, that can house such an FRP strip with sufficient width and length. In all, twelve (12) specimens were inves‐ tigated. For six specimens no surface preparation of the concrete specimen was applied when attaching the FRP layers, whereas six other specimens had their surface treated ac‐ cording to construction guidelines. CFRP layers were attached to all specimens ([28], [43]). The use of the anchoring device was utilised on three (3) specimens with treatment prepara‐ tion and on three specimens without such treatment. All the concrete prisms were fabricated using the same concrete mix and the same internal reinforcement, which was used to pro‐ hibit any accidental failure. The measured cylinder strength of the concrete was equal to 22 MPa. The properties of the used CFRP are listed in Table 9, as given by the manufactures.

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**Table 9.** Properties of the used FRP sheets.

for the safe design of these anchoring devices. In any case, equation (3) describes this condi‐

**(***ntf Ef* **)0.58**

*tj* **2** *f ctm*

*<sup>L</sup> <sup>e</sup>* **<sup>=</sup>** *Ef* **<sup>⋅</sup>** *tf* **4⋅ τmax**

**Table 8.** Calculation of effective length. Where VFRP: total force received by FRP, Vanchoring device: total strength of the

In this section results from a recent research effort conducted at the Laboratory of Strength of Materials and Structures of Aristotle University will be briefly presented and discussed ([13], [42]). This research aimed to investigate the effectiveness of a specific FRP strip an‐ choring device by utilising a number of small concrete prismatic specimens, that can house such an FRP strip with sufficient width and length. In all, twelve (12) specimens were inves‐ tigated. For six specimens no surface preparation of the concrete specimen was applied when attaching the FRP layers, whereas six other specimens had their surface treated ac‐ cording to construction guidelines. CFRP layers were attached to all specimens ([28], [43]). The use of the anchoring device was utilised on three (3) specimens with treatment prepara‐ tion and on three specimens without such treatment. All the concrete prisms were fabricated using the same concrete mix and the same internal reinforcement, which was used to pro‐ hibit any accidental failure. The measured cylinder strength of the concrete was equal to 22 MPa. The properties of the used CFRP are listed in Table 9, as given by the manufactures.

where n: number of FRP layers tf: thickness of FRP Ef: Modulus of elasticity

where Ef: Modulus of elasticity tf: thickness of FRP τmax: bond strength

where

: Modulus of elasticity

: thickness of FRP fctm:tensile concrete strength

Ej

*VFRP* ≤*Vanchoring device* (3)

tj

**Code Effective Length (Le)**

tion for the bearing capacity of a permissible anchoring device

108 Fiber Reinforced Polymers - The Technology Applied for Concrete Repair