**Jacek Tomków \* , Dariusz Fydrych and Grzegorz Rogalski**

Faculty of Mechanical Engineering, Gda ´nsk University of Technology, G. Narutowicza Street 11/12, 80-233 Gda ´nsk, Poland<EMAIL_ADDRESS>(D.F<EMAIL_ADDRESS>(G.R.)

**\*** Correspondence<EMAIL_ADDRESS>Tel.: +48-58-347-1863

Received: 19 September 2019; Accepted: 14 October 2019; Published: 16 October 2019

**Abstract:** This paper presents examinations of the role of the bead sequence in underwater welding. Two specimens of wet welded layers made by covered electrodes with the use of normalized S355G10+N steel were welded by a reasonable bead sequence. For each specimen, metallographic macro- and micro-scopic tests were done. Then, Vickers HV10 hardness measurements were conducted for each pad weld in the welded layer. The results show that welding in the water environment carries many problems in the stability of the welding arc, which influences the properties of the welds. The effects of refining and tempering the structure in heat-affected zones of earlier laid beads was observed, which provides a reduction of hardness. The possibility of applying two techniques while welding the layer by the wet method is described. It is stated that a reasonable bead sequence can decrease the hardness in heat-affected zones up to 40 HV10. Tempering by heat from next beads can also change the microstructure in this area by tempering martensite and can decrease susceptibility to cold cracking.

**Keywords:** underwater welding; covered electrodes; wet welding; cold cracking; pad welding

### **1. Introduction**

The most common method of underwater welding is wet welding. The welder and the welding area are in direct contact with the surrounding environment. The process is often carried out by flux-cored arc welding (FCAW) [1–4]. The most common is welding by covered electrodes, which is cheaper and easier [5,6].

The water environment can generate significant problems during welding. For welders, the biggest problems are instability of the welding arc and limited visibility [7,8], which can result in poor quality of welded joints. From the metallurgical point of view, the high cooling rate, the high hydrogen content in deposited metal, and residual stresses have the greatest influence on the quality of underwater welded joints.

The high cooling rate leads to the formation of hardened structures in the heat-affected zone (HAZ) [9,10]. This is one of the reasons for residual stresses in welded joints after the welding process is completed. The next problem in underwater conditions is high diffusible hydrogen content in deposited metal, which is much higher than welding in air [11]. Three factors are responsible for high susceptibility of underwater welded joints to cold cracking. This type of cracking is located in the HAZ and in welds of joints made in water along the fusion line in the overheated area of the HAZ [6,12]. Sometimes, they can also be found in the welds [13].

Experiments in the field of welding processes in the water environment are developing annually, because underwater processes are used more in engineering [14].

Attempts to reduce the imperfections in welded joints made in the water are made in different ways. Much attention is devoted to improving the stability of the welding arc in underwater conditions and assessing the metal transfer in the welding arc, using the following methods: visual sensing [15], in situ imaging [16], mechanical constraints [17], and ultrasonic waves [3,8]. The other research trend is developing filler materials for welding: modifying the flux coating and wires of coated electrodes [18] and applying a waterproof coating [19]. Several articles present the results of technological methods to improve the weldability of steel: diffusible hydrogen content in deposited metal [11,20], the use of austenitic deposits [9,21], preheating [22], and multilayer welding, including the temper bead welding technique [6,12,13,23]. Controlling the distribution of heat input by controlling the sequence and positioning of the beads is considered to be particularly promising, because it leads to a reduction in the grain size and provides decreased hardness and increased impact strength [24–26]. Different variants of bead sequences are used to weld hard-to-weld metals such as cast iron, nuclear steel, aluminum, quenched and tempered steel, and thermomechanically-treated steel [27–35].

For marine and offshore constructions, high-strength, low-alloy (HSLA) steels are widely used [36]. These constructions may undergo failure during exploitation in marine conditions [37–40]. They often have to be repaired in underwater conditions due to the high cost of transport to air conditions. However, the water environment makes repair welding more difficult, therefore, there is still a search for ways to improve the quality of underwater welds.

The aim of the research was to check the influence of the role of bead sequence on the structure and properties of layers using the wet welding method for specimens welded with covered electrodes.

#### **2. Materials and Methods**

### *2.1. Materials Used*

For tests, normalized S355G10 + N steel plates with dimensions of 16 × 100 × 100 mm were chosen as the base metal (BM). This steel is often used as a material for offshore structures that may require repairs in the water environment. The carbon equivalent (CeIIW) is 0.385 according to the International Institute of Welding (IIW). As a filler material, ISO 2560-A: E 38 0 R 11 [41] rutile electrodes with a diameter of 4.0 mm were used. They produce good plasticity of welds, which may be helpful to avoid cold cracking. The chemical compositions of the materials used are listed in Table 1 and the mechanical properties in Table 2.

### *2.2. Welding Process*

For welding, the manual metal arc (MMA) wet welding process was chosen. The schema of this process is presented in Figure 1.

For the tests, 2 specimens were made in the water (0.15 m depth) with 5 beads in each. The time between welding each bead was 120 s. Specimens were welded by different techniques. In sample S1, the beads were laid one after the other from bead 1 to bead 5. In sample S2, the beads were not laid in order. These 2 techniques were chosen because they are representative of welding techniques. In S1, a bead was tempered only by the bead welded later. In S2, beads 1 and 2 were tempered by 2 beads. The use of other techniques, such as 14253, would obtain only one bead (bead 2), which

will be tempered by 2 others. That may not have allowed preparation of the planned experiments. The schemas of welding techniques for S1 and S2 specimens are presented in Figure 2. The welding parameters were chosen according to preliminary tests. They provided stability of the welding arc in the experimental conditions and allowed heat input values higher than 0.90 kJ/mm. From the point of view of weldability in underwater conditions, these values may reduce the susceptibility to cold cracking. It has been assumed that the heat input of the tempering bead should be higher than that of the tempered bead. The parameters for both specimens are shown in Table 3. In the case of manual underwater wet welding, the ability to control the value of heat input is very limited. Hence, differences between heat input values of subsequent beads are not significant (0.24 and 0.39 kJ/mm for specimens *Materials*  S1 and S2, respectively) from the point of view of metallurgical transformations. **2019**, *12*, x FOR PEER REVIEW 3 of 10

**Figure 1.** Wet welding by covered electrodes schema. 1: water vapor, 2: gas bubbles, 3: flux coating, 4: wire, 5: melting flux coating, 6: liquid metal, 7: electric arc, 8: base materials, 9: slag, 10: gas from melting flux coating. **Figure 1.** Wet welding by covered electrodes schema. 1: water vapor, 2: gas bubbles, 3: flux coating, 4: wire, 5: melting flux coating, 6: liquid metal, 7: electric arc, 8: base materials, 9: slag, 10: gas from melting flux coating.

underwater wet welding, the ability to control the value of heat input is very limited. Hence, differences between heat input values of subsequent beads are not significant (0.24 and 0.39 kJ/mm for specimens S1 and S2, respectively) from the point of view of metallurgical transformations.

**Table 3.** Welding parameters for specimens S1 and S2.

**Table 3.** Welding parameters for specimens S1 and S2. **Bead No.

*Materials* **2019**, *12*, x FOR PEER REVIEW 4 of 10

for specimens S1 and S2, respectively) from the point of view of metallurgical transformations.

**Figure 1.** Wet welding by covered electrodes schema. 1: water vapor, 2: gas bubbles, 3: flux coating, 4: wire, 5: melting flux coating, 6: liquid metal, 7: electric arc, 8: base materials, 9: slag, 10: gas from

For the tests, 2 specimens were made in the water (0.15 m depth) with 5 beads in each. The time between welding each bead was 120 s. Specimens were welded by different techniques. In sample S1, the beads were laid one after the other from bead 1 to bead 5. In sample S2, the beads were not laid in order. These 2 techniques were chosen because they are representative of welding techniques. In S1, a bead was tempered only by the bead welded later. In S2, beads 1 and 2 were tempered by 2 beads. The use of other techniques, such as 14253, would obtain only one bead (bead 2), which will be tempered by 2 others. That may not have allowed preparation of the planned experiments. The schemas of welding techniques for S1 and S2 specimens are presented in Figure 2. The welding parameters were chosen according to preliminary tests. They provided stability of the welding arc in the experimental conditions and allowed heat input values higher than 0.90 kJ/mm. From the point of view of weldability in underwater conditions, these values may reduce the susceptibility to cold cracking. It has been assumed that the heat input of the tempering bead should be higher than that of the tempered bead. The parameters for both specimens are shown in Table 3. In the case of manual underwater wet welding, the ability to control the value of heat input is very limited. Hence,

**Figure 2.** Scheme of welding process in (**a**) specimen S1 and (**b**) specimen S2. 4 216 30.0 5.41 1.20 5 216 29.0 4.58 1.39 3 220 28.8 5.88 1.05 4 216 30.0 5.41 1.20 5 216 29.0 4.58 1.39

#### **Figure 2.** Scheme of welding process in (**a**) specimen S1 and (**b**) specimen S2. *2.3. Examination Procedure 2.3. Examination Procedure*

**Table 3.** Welding parameters for specimens S1 and S2. **Bead No. I (A) U (V) Vsp (mm/s) Ql (kJ/mm)**  Specimen S1 1 220 21.8 5.95 1.03 2 216 29.5 6.17 1.03 3 224 26.8 5.88 1.02 After welding, specimens were cut to cross-sections, and macroscopic and microscopic metallographic examinations were conducted in accordance with the EN ISO 17637:2011 [42] standard, with the use of 4% Nital. Finally, Vickers HV10 hardness measurements were taken in accordance with EN ISO 9015:2011 [43], by using a Sinowon V-10 stand (Sinowon, Dongguan, China), with a measurement error of ± 3 HV10. The investigated S355G10 + N steel is classified as material group 2.1 in accordance with EN ISO 15614-1:2017 [44]. The maximum hardness values of HAZ recommended by this standard cannot exceed 380 HV10. After welding, specimens were cut to cross-sections, and macroscopic and microscopic metallographic examinations were conducted in accordance with the EN ISO 17637:2011 [42] standard, with the use of 4% Nital. Finally, Vickers HV10 hardness measurements were taken in accordance with EN ISO 9015:2011 [43], by using a Sinowon V-10 stand (Sinowon, Dongguan, China), with a measurement error of ± 3 HV10. The investigated S355G10 + N steel is classified as material group 2.1 in accordance with EN ISO 15614-1:2017 [44]. The maximum hardness values of HAZ *2.3. Examination Procedure*  After welding, specimens were cut to cross-sections, and macroscopic and microscopic metallographic examinations were conducted in accordance with the EN ISO 17637:2011 [42] standard, with the use of 4% Nital. Finally, Vickers HV10 hardness measurements were taken in accordance with EN ISO 9015:2011 [43], by using a Sinowon V-10 stand (Sinowon, Dongguan, China), with a measurement error of ± 3 HV10. The investigated S355G10 + N steel is classified as material

Macroscopic tests were prepared for cross-sections of both specimens. Microscopic tests were prepared for the base material in welds and in specific areas of the samples. The first areas were located in the HAZ in the axis of each weld. The second areas were located where the HAZ from the previous weld was overlaid by the HAZ from welds deposited later. Schematic views of the areas of microscopic testing are presented in Figure 3. HV10 hardness measurements were prepared for cross-sections of both specimens in the BM, each weld, and the HAZ of each weld in their axis according to the schemas presented in Figure 4. recommended by this standard cannot exceed 380 HV10. Macroscopic tests were prepared for cross-sections of both specimens. Microscopic tests were prepared for the base material in welds and in specific areas of the samples. The first areas were located in the HAZ in the axis of each weld. The second areas were located where the HAZ from the previous weld was overlaid by the HAZ from welds deposited later. Schematic views of the areas of microscopic testing are presented in Figure 3. HV10 hardness measurements were prepared for crosssections of both specimens in the BM, each weld, and the HAZ of each weld in their axis according to the schemas presented in Figure 4. group 2.1 in accordance with EN ISO 15614-1:2017 [44]. The maximum hardness values of HAZ recommended by this standard cannot exceed 380 HV10. Macroscopic tests were prepared for cross-sections of both specimens. Microscopic tests were prepared for the base material in welds and in specific areas of the samples. The first areas were located in the HAZ in the axis of each weld. The second areas were located where the HAZ from the previous weld was overlaid by the HAZ from welds deposited later. Schematic views of the areas of microscopic testing are presented in Figure 3. HV10 hardness measurements were prepared for crosssections of both specimens in the BM, each weld, and the HAZ of each weld in their axis according

**Figure 3.** Schematic view of areas of microscopic testing: (**a**) specimen S1, (**b**) specimen S2. **Figure 3.** Schematic view of areas of microscopic testing: (**a**) specimen S1, (**b**) specimen S2. **Figure 3.** Schematic view of areas of microscopic testing: (**a**) specimen S1, (**b**) specimen S2.

**3. Results and Discussion Figure 4.** Hardness distribution points: (**a**) specimen S1, (**b**) specimen S2. **Figure 4.** Hardness distribution points: (**a**) specimen S1, (**b**) specimen S2.

#### *3.1. Macroscopic Testing*  **3. Results and Discussion 3. Results and Discussion**

#### Macroscopic testing showed that both specimens were welded according to the assumed *3.1. Macroscopic Testing 3.1. Macroscopic Testing*

research plan (Figure 2). No imperfections were found in cross-sections of specimens S1 and S2. The characteristic cross-sectional sizes of all beads had similar dimensions, resulting from small differences in the values of heat input (Table 3). A factor that complicates predicting the effects of multi-pass welding is dilution, which affects, among other aspects, hardenability. Dilution values were calculated with the methodology presented in the literature [45,46]. The cross-sectional area of the fusion zone for each pass was used, i.e., D = Abase/A, where A is the total molten area and Abase is Macroscopic testing showed that both specimens were welded according to the assumed research plan (Figure 2). No imperfections were found in cross-sections of specimens S1 and S2. The characteristic cross-sectional sizes of all beads had similar dimensions, resulting from small differences in the values of heat input (Table 3). A factor that complicates predicting the effects of multi-pass welding is dilution, which affects, among other aspects, hardenability. Dilution values were calculated with the methodology presented in the literature [45,46]. The cross-sectional area of Macroscopic testing showed that both specimens were welded according to the assumed research plan (Figure 2). No imperfections were found in cross-sections of specimens S1 and S2. The characteristic cross-sectional sizes of all beads had similar dimensions, resulting from small differences in the values of heat input (Table 3). A factor that complicates predicting the effects of multi-pass welding is dilution, which affects, among other aspects, hardenability. Dilution values were calculated with the methodology presented in the literature [45,46]. The cross-sectional area of the fusion zone for each

the molten area of the BM [45]. The calculations showed that dilution in S2 is lower than in S1, which

the fusion zone for each pass was used, i.e., D = Abase/A, where A is the total molten area and Abase is the molten area of the BM [45]. The calculations showed that dilution in S2 is lower than in S1, which

pass was used, i.e., D = Abase/A, where A is the total molten area and Abase is the molten area of the BM [45]. The calculations showed that dilution in S2 is lower than in S1, which may result in the structures and hardness for both specimens [47]. The results of macroscopic testing are presented in Figure 5. The dilution in S1 and S2 is presented in Table 4. *Materials* **2019**, *12*, x FOR PEER REVIEW 5 of 10 may result in the structures and hardness for both specimen

**Figure 5.** Results of macroscopic testing: (**a**) specimen S1, (**b**) specimen S2. **Figure 5.** Results of macroscopic testing: (**a**) specimen S1, (**b**) specimen S2.

**Table 4.** The dilution in the S1 and S2. **Table 4.** The dilution in the S1 and S2.

#### *3.2. Microscopic Testing 3.2. Microscopic Testing*

The results of microscopic testing are presented in Figure 5. Microscopic testing of base material showed that the investigated S355G10 + N steel consisted of fine-grained pearlite and fine-grained ferrite with layers (Figure 6a). The beads consisted of bright fine-grained ferrite arranged in columns, from which grew acicular ferrite at the boundaries of dendrites. Inside them were fine ferrite grains (Figure 6b). These structures are typical for joints made in underwater welding of HSLA steel. In the HAZ of each weld in S1 (regions A, C, E, and G), brittle structures such as bainitic and martensitic were found (Figure 6c). The structures in the HAZ in S2 were the same in areas A, E, and I. In the HAZ of welds 1 and 2 (areas C and G), the structure indicated the presence of refined and tempered low-carbon martensite mixed with normalization structures with fine ferrite and pearlite (Figure 6d), which is a result of the heat influence from welds laid later (Figure 2b). These types of structures have lower susceptibility to cold cracking. The areas where the HAZ of one bead overlapping the HAZ of the bead laid earlier (areas B, D, F, and H) in both specimens are characterized by normalization structures with fine ferrite and pearlite (Figure 6e). While formation and decomposition of austenite occurred twice in regions B, D, F, and H, it happened once in areas A, C, E, and G, followed by inprocess tempering with the heat of the next bead. Cracks were found in each HAZ of S1 (Figure 6c), which resulted in brittle structures in this area. In S2, cracks were found only in the HAZ of welds 3, 4, and 5 (Figure 6f) and in area H (Figure 6e). This cracking could have occurred before weld number 5 was laid, which tempered the HAZ of weld 2. Tempering from welds laid later could not repair the microcracks that occurred during the welding of the previous bead and could even propagate these cracks, which is similar to the effect of the temper bead welding technique [12,13]. The location and the length of the cracks in both specimens are summarized in Table 5. The longest cracks were observed in HAZ of welds prepared with the lowest values of heat input. The results of microscopic testing are presented in Figure 5. Microscopic testing of base material showed that the investigated S355G10 + N steel consisted of fine-grained pearlite and fine-grained ferrite with layers (Figure 6a). The beads consisted of bright fine-grained ferrite arranged in columns, from which grew acicular ferrite at the boundaries of dendrites. Inside them were fine ferrite grains (Figure 6b). These structures are typical for joints made in underwater welding of HSLA steel. In the HAZ of each weld in S1 (regions A, C, E, and G), brittle structures such as bainitic and martensitic were found (Figure 6c). The structures in the HAZ in S2 were the same in areas A, E, and I. In the HAZ of welds 1 and 2 (areas C and G), the structure indicated the presence of refined and tempered low-carbon martensite mixed with normalization structures with fine ferrite and pearlite (Figure 6d), which is a result of the heat influence from welds laid later (Figure 2b). These types of structures have lower susceptibility to cold cracking. The areas where the HAZ of one bead overlapping the HAZ of the bead laid earlier (areas B, D, F, and H) in both specimens are characterized by normalization structures with fine ferrite and pearlite (Figure 6e). While formation and decomposition of austenite occurred twice in regions B, D, F, and H, it happened once in areas A, C, E, and G, followed by in-process tempering with the heat of the next bead. Cracks were found in each HAZ of S1 (Figure 6c), which resulted in brittle structures in this area. In S2, cracks were found only in the HAZ of welds 3, 4, and 5 (Figure 6f) and in area H (Figure 6e). This cracking could have occurred before weld number 5 was laid, which tempered the HAZ of weld 2. Tempering from welds laid later could not repair the microcracks that occurred during the welding of the previous bead and could even propagate these cracks, which is similar to the effect of the temper bead welding technique [12,13]. The location and the length of the cracks in both specimens are summarized in Table 5. The longest cracks were observed in HAZ of welds prepared with the lowest values of heat input.

*3.2. Hardness Measurements*

*Materials* **2019**, *12*, x FOR PEER REVIEW 6 of 10

**Figure 6**. Results of microscopic testing: (**a**) base material, (**b**) weld, (**c**) heat-affected zone (HAZ) in S1, (**d**) HAZ of weld number 1 (area C) in S2, (**e**) overlapping of two HAZs (area H) in S2, (**f**) HAZ of weld 3 (area A) in S2. Etch: 4% Nital. **Figure 6.** Results of microscopic testing: (**a**) base material, (**b**) weld, (**c**) heat-affected zone (HAZ) in S1, (**d**) HAZ of weld number 1 (area C) in S2, (**e**) overlapping of two HAZs (area H) in S2, (**f**) HAZ of weld 3 (area A) in S2. Etch: 4% Nital.

**Table 5.** The length of cracks in specimens S1 and S2. **Table 5.** The length of cracks in specimens S1 and S2.

The investigated S355G10 + N steel is classified as material group 2.1 by EN ISO 9015-1:2011. In accordance with the EN-ISO 15614:1 standard, the maximum hardness of the HAZ cannot exceed 380

### *3.3. Hardness Measurements*

The investigated S355G10 + N steel is classified as material group 2.1 by EN ISO 9015-1:2011. In accordance with the EN-ISO 15614:1 standard, the maximum hardness of the HAZ cannot exceed 380 HV10 for material group 2.1. The hardness in welds for both specimens was lower than that in the HAZ, which is typical for welding HSLA steel in the water environment [5]. Measurements in the HAZ showed significant differences in hardness distribution in the two tested specimens. The HV10 values in the HAZ were higher in S1 than in S2, which is connected with the different dilution values in both specimens. It is stated in the literature [45], that the higher dilution leads, for example, to more martensite in the welded joint, which is a brittle structure. In S1, most of the measured values did not fulfill the criterion of EN ISO 15614-1 standard of 380 HV10. The welding technique used in S2 allows reduced hardness in the HAZ, especially for welds that have been tempered by heat from welds laid later. The reduction of hardness by welding with the technique used in S2 can reduce the probability of cold cracks occurring in the HAZ. The differences of HAZ hardness are associated with the transformation cycle and tempering effect, which is characteristic for multi-pass welds [48]. *Materials* **2019**, *12*, x FOR PEER REVIEW 7 of 10 HV10 for material group 2.1. The hardness in welds for both specimens was lower than that in the HAZ, which is typical for welding HSLA steel in the water environment [5]. Measurements in the HAZ showed significant differences in hardness distribution in the two tested specimens. The HV10 values in the HAZ were higher in S1 than in S2, which is connected with the different dilution values in both specimens. It is stated in the literature [45], that the higher dilution leads, for example, to more martensite in the welded joint, which is a brittle structure. In S1, most of the measured values did not fulfill the criterion of EN ISO 15614-1 standard of 380 HV10. The welding technique used in S2 allows reduced hardness in the HAZ, especially for welds that have been tempered by heat from welds laid later. The reduction of hardness by welding with the technique used in S2 can reduce the probability of cold cracks occurring in the HAZ. The differences of HAZ hardness are associated with the transformation cycle and tempering effect, which is characteristic for multi-pass welds [48].

The hardness distribution in each point is presented in Figure 7. The results shown in this figure are arranged in order of bead appearance (from left to right) in specimens, e.g., "second weld" means this is the second weld from the left. The average results of hardness measurements in each area are presented in Table 6. The hardness distribution in each point is presented in Figure 7. The results shown in this figure are arranged in order of bead appearance (from left to right) in specimens, e.g., "second weld" means this is the second weld from the left. The average results of hardness measurements in each area are presented in Table 6.

**Figure 7.** Hardness measurement results for specimens S1 and S2. **Figure 7.** Hardness measurement results for specimens S1 and S2.

**Table 6.** Average HV10 hardness measurements results. **Table 6.** Average HV10 hardness measurements results.

#### S2 173 384 249 340 236 257 256 379 259 385 282 176 **4. Conclusions**

found.

**4. Conclusions**  The results of the prepared experiments show that the technique used has a significant role in The results of the prepared experiments show that the technique used has a significant role in the process of repairs in underwater conditions. The technique used for preparation of specimen S1 can cause a need for further repairs, because it allows cold cracks to form.

the process of repairs in underwater conditions. The technique used for preparation of specimen S1 can cause a need for further repairs, because it allows cold cracks to form. The prepared experiments allow us to draw the following conclusions:

sequence of the beads during the process.

The prepared experiments allow us to draw the following conclusions: 1. Water as a welding environment causes cold cracks to occur in the HAZ of wet welded HSLA steel. The susceptibility of steel to cold cracking can be reduced by changing the 1. Water as a welding environment causes cold cracks to occur in the HAZ of wet welded HSLA steel. The susceptibility of steel to cold cracking can be reduced by changing the sequence of thebeads during the process.

2. The technique used for the preparation of specimen S2 allowed the structure in the HAZ of some welds to be changed. In this area, the structure indicated the presence of refined and tempered low-carbon martensite mixed with normalization structures with fine ferrite and

**Author Contributions:** Conceptualization, J.T. and G.R.; Methodology, J.T., D.F. and G.R.; Formal Analysis, J.T. and D.F.; Investigation, J.T. and G.R.; Writing—Original Draft Preparation, J.T., D.F. and G.R.; Writing—Review and Editing, J.T. and D.F.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.


*4.4.3 Magnesium*

Magnesium is the second most abundant intracellular cation and is involved in about 300 biochemical reactions related to anabolic and catabolic actions in the body, such as glycolysis and protein and lipid metabolism [55–56]. The Women's Health Initiative Observational Study (*n* = 3713 postmenopausal women, aged 50–79 years) reported that intake of dietary magnesium was independently and inversely associated with plasma concentrations of hs-CRP, IL-6, and sVCAM-1 in postmenopausal women after an adjustment for multiple variables including dietary fiber, fruit, vegetables, folate, and saturated and trans-fatty intake [57]. Another study done by Guerrero-Romero and Rodriguez- Moran found that low serum Mg levels were independently related to elevated CRP concentration, in nondiabetic, nonhypertensive obese subjects (*n* = 371) [15, 58].


**2. Improvement of mechanical properties and durability**

Improvement of mechanical properties and durability of cementitious materials is mostly obtained by their nanostructure modification, that is, the incorporation of nanomaterials into cement matrix. Nanoparticles possess a high chemical reactivity due to a high surface area and can promote the growth of cement hydration products. Nanomaterials employed in cementitious composites, up to date, are nano-silica, nano-titania, nano-iron oxide, nano-alumina, nanoclay particles, carbon nanotubes, graphene oxide (GO), and graphene nanoplatelets (GNPs) [7].

Nano-silica (nano-SiO2 ) is proved to enhance the compressive, tensile, and flexural strength of OPC pastes and mortars [8]. The addition of nano-SiO2 effects in denser cement paste microstructure with improved porosity thus leads to a decreased water penetration and sorptivity [8] and therefore to the reduction of calcium leaching [9]. Nano-alumina (nano-Al2 O3 ) matches the performance of nano-silica—it leads to a more compacted microstructure of cementitious composites, decreases their porosity, and enhances the compressive strength [10, 11]. It is worth noting that nano-alumina was proved to improve concrete performance at both elevated and low temperatures [10, 11]. The incorporation of nano-titania (nano-TiO2 ) may lead to the enhancement of compressive and flexural strength as well as to the improvement in the resistance to chloride penetration due to a refined pore structure of the composites [12]. The impact of nano-titania addition on the performance of cementitious composites at an elevated temperature turned out to be comparable to composites incorporating nano-alumina [13].

has been modified several times over the years. Today, it can be defined as *"the application of scientific knowledge to manipulate, control and restructure matter at the atomic and molecular level in the range of 1–100 nm to exploit the size-dependent and structure-dependent properties and phenomena distinct from those at different scales"* [3]. Basically, nanotechnology is based on the statement that we can change any property of any material with reducing at least one dimension

While nanotechnology has attracted attention in many fields of science and technology, including chemistry, electronics, medicine, or biology, its application in civil engineering, up to date, remains limited [1, 5–7]. These days, searching in SCOPUS database for the terms *"nanomaterial"* AND *"civil engineering"* within titles, abstracts and keywords of published papers returns only 18 document results. The RILEM TC 197-NCM Report [5] has highlighted, for the first time in 2004, the potential applications of nanotechnology in construction materials. The e-mail survey carried out among researchers, construction professionals, and large construction companies was the basis of reported information. The report revealed that little awareness of nanotechnology applications in construction is an effect of insufficient information on this subject. Therefore, nanotechnology was perceived as expensive and highly complex, thus discouraging potential customers. However, the results of the survey have shown that nearly 100 research projects carried out by respondents were based on nanotechnology.

Since then, research introducing nanotechnology in civil engineering has followed mainly

This chapter presents a review of the achievements of nanotechnology in Structural Engineering with special emphasis on improved physical parameters of structural materials

Improvement of mechanical properties and durability of cementitious materials is mostly obtained by their nanostructure modification, that is, the incorporation of nanomaterials into cement matrix. Nanoparticles possess a high chemical reactivity due to a high surface area and can promote the growth of cement hydration products. Nanomaterials employed in cementitious composites, up to date, are nano-silica, nano-titania, nano-iron oxide, nano-alumina, nanoclay particles, carbon nanotubes, graphene oxide (GO), and graphene nanoplatelets (GNPs) [7].

and their potential in strengthening repairs and Structural Health Monitoring.

**2. Improvement of mechanical properties and durability**

The potential nanotechnology applications were pointed out as follows:

of this material into the nanoscale [4].

116 New Uses of Micro and Nanomaterials

• understanding nanostructure of materials, • nanostructure modification of materials,

• environment-friendly applications [5].

these abovementioned development paths.

• functional films and coatings, • smart structures and devices,

However, the most studied nanomaterials to be used in cementitious composites are carbonbased nanomaterials. Until 1985, only two allotropic forms of solid carbon had been known, these had been diamond and graphite, which both feature covalently bonded networks [14]. In 1985, the new era of carbon nanomaterials had begun, when fullerenes—molecules composed of 60 carbon atoms, C60—had been discovered [14, 15]. It was less than 6 years later when it turned out that carbon atoms can also form cylindrical tubes. In 1991, Iijima [16] observed first the multi-walled carbon nanotubes (MWCNTs) and then in 1993, Iijima and Ichihashi [17] reported single-walled nanotubes (SWCNTs).

 The diameters are in the range of 1.4–100 and 0.4–3 nm for MWCNTs and SWCNTs, respectively. Young's modulus for SWCNTs and MWCNTs is equal to ca. 1 and 0.21 TPa, respectively, while the tensile strength for both types of CNTs reaches 500 and 10–63 GPa [15].

Manufacturing of cementitious composites incorporating carbon-based nanomaterials is an extremely challenging task due to the crucial problem of obtaining a homogeneous dispersion of a nanomaterial within cement matrix. Carbon-based nanomaterials are prone to form aggregates and bundling as an effect of both their high hydrophobicity and strong van der Waals forces [18–20]. Nonuniformly dispersed nanoparticles strongly influence the workability and microstructure of cement composites and hinder the ongoing hydration; thus, it is of significant importance to adopt an appropriate treatment to obtain a sufficient consistency and dispersion of nanomaterial within cement matrix.

Several different attempts to obtain a homogeneous dispersion of CNTs in cement mix were reported, including carboxylation of CNTs [21], that is, special treatment to attach carboxylic acid to their surface or functionalization of CNTs with COOH groups [22, 23]. Nevertheless, the main approach employed to fabricate cement-CNT composites is, clearly, stirring and ultrasonication of aqueous dispersion of CNTs with various types of surfactants, such as polycarboxylic acid-based superplasticizers [23–25], anionic sodium dodecyl sulfate [20, 26], sodium dodecyl benzene sulfonate [27], nonionic polyoxyethylene(23) laurylether [20], Gum Arabic [22, 28], polyacrylic acid polymer [22], and cetyltrimethylammonium bromide [27], to name a few, or solvents, for instance, acetone [29]. It is worth noting that the studies on CNTs dispersion [20, 30] have shown that the most beneficial dispersion is the one with a CNT-tosurfactant ratio of 1:1–1:5.

nucleation effect [46–48]. As a consequence of this 3D network, the microstructure of cement composites is visibly densified with a higher crystal growth and less prominent microcracks. Furthermore, also brittle crystals of ettringite are hardly observed [49]. The addition of GO remarkably refines the porosity, reducing the critical pore size and the volume of macropores [48, 50]. For the reason of reduced porosity, the incorporation of even small amount of GO into cementitious composites leads to a decreased sorptivity [50, 51]. The decrease up to 8 and 44% has been reported for initial and secondary sorptivity, respectively [50]. Therefore, cement-GO composites feature with a tremendously reduced ingress of chlorides. Even the marginal addition of graphene oxide of 0.01 wt% may effect in significant decrease of chloride penetration depth from 26 to 5 mm [51]. Interestingly, the addition of GO and its acceleration effect on cement hydration lead to a higher drying shrinkage at early stages of hydration. Nevertheless, since drying shrinkage depends on the tension of capillary pores, which are highly reduced in composites reinforced with graphene oxide, drying shrinkage after 28 days

Nanomaterials in Structural Engineering http://dx.doi.org/10.5772/intechopen.79995 119

Some attempts [19, 52] of introducing graphene nanoplatelets (GNPs) to improve the barrier properties and enhance the durability of cementitious composites have been reported. In this respect, this low-cost graphene derivative matches the performance of graphene oxide in concrete. The addition of 1.5 wt% of GNPs contributes to pore refinement, reducing the critical pore diameter and the average void size, thereby decreasing the water permeability, chloride diffusion, and chloride migration by 80, 80 and 40%, respectively [52]. It is worth noting that according to various authors, the addition of GNPs does not improve [52] or may even, to

Electrical properties of carbon-based materials in structural engineering are drawing attention of scientists for many years, giving hope for smart materials and self-monitoring structures. One of the first attempts of using carbon-based materials in concrete was made almost three decades ago when cut carbon fibers were mixed with concrete for traffic monitoring and weighting in motion [54]. The results were promising; however, this solution had never been

The development of science and technologies during recent years has brought new nanomaterials as graphene or carbon nanotubes with even more interesting properties, also electrical. Former experiences in structural engineering materials but also in other areas of science as medicine or aviation encouraged scientists to return to the concept of self-monitoring materials for smart structures. Clearly, carbon nanotubes are the most studied carbon nanomaterial

Typically, various types of sensors are used to evaluate structural health, including optical fibers, strain gauges, and piezoresistive sensors. However, these sensors possess some serious limitations and disadvantages, such as high cost, poor durability, low sensitivity, and insufficient compatibility with concrete and expensive peripheral equipment [6, 55]. The

is then reduced [47].

some extent, deteriorate [53] the strength of concrete.

**3. Self-monitoring materials**

implemented in large scale.

for self-monitoring applications in concrete.

CNTs can enhance both the compressive and flexural strength of cementitious composites up to 50 [22] and 87% [24], respectively. The addition of CNTs also improves both the fracture energy and flexural toughness [31]. Young's modulus of cement mortars containing 0.1 wt% of CNTs can be even 100% higher compared to reference samples [24]. According to SEM images, the interaction between cement hydration products and CNTs is observed [32]. CNTs increase the crack bridging capacity of cementitious composites, acting as networks between the crack and the pores [23, 31, 33]. Moreover, nanoindentation investigation indicates that CNTs contribute to a higher growth of strong C–S–H phase [30]. CNTs act as the nanofiller of voids and thus reduce the total pore volume of cement paste [21, 23, 30, 32]. Interestingly, the addition of CNTs decreases the drying shrinkage of composites. Indeed, the authors [34] attributed this behavior to the reduction of micropores. It is worth noting that the influence of CNTs on the microstructure, porosity, and thereby mechanical properties of cementitious composites is highly dependent on the quality of their dispersion within cement matrix as well as on the type of surfactant to be used. Several studies show that the addition of CNTs may also deteriorate the properties of cementitious nanocomposites [28, 29, 33].

Over the past decades, graphene—another carbon allotrope, which is a single, planar, twodimensional carbon layer [35]—has attracted considerable attention in science and technology, while its extraordinary properties have been extensively studied by various research groups. Especially, due to its outstanding mechanical [36] and electrical properties [37], graphene has emerged as the most promising nanomaterial for smart structures. Graphene is known to exhibit the intrinsic tensile strength of 130 GPa with a corresponding strain of 0.25, while its Young's modulus may be estimated at 1 TPa [36].

Nevertheless, studies on graphene-cement composites remain, up to date, limited due to the abovementioned perplexing problem of obtaining a uniform dispersion of a nanomaterial within cement matrix. For this reason, over the past years, special attention was paid to one of graphene derivatives, that is, graphene oxide (GO). Graphene oxide is highly dispersible in water [38] and therefore, as was assumed, also in cement mix. However, several studies [39–41] show that calcium ions present in cement paste negatively affect graphene oxide dispersion due to the chemical cross-linking phenomena. To circumvent this problem, different approaches have been persuaded, including the sonication of graphene oxide with polycarboxylate superplasticizer [42, 43] or silica fume [39, 40], which provide surface modification of nanomaterial and thereby separate graphene oxide nanoplatelets from calcium ions.

However, various cementitious composites incorporating graphene oxide, with or without surface modification, have emerged as materials with improved microstructure, mechanical properties, and durability. With the dosage of 0.03–0.05 wt% of GO, the increase up to 47, 61, and 79% has been reported for compressive [44], flexural, and tensile-splitting strength [45], respectively. The strengthening mechanism of GO in cement matrix is attributed to the chemical reaction between -COOH groups attached on the GO flakes and calcium ions from calcium hydroxide present in cement; thus, a 3D network structure is formed. Moreover, graphene oxide promotes and accelerates the growth of cement hydration products due to the nucleation effect [46–48]. As a consequence of this 3D network, the microstructure of cement composites is visibly densified with a higher crystal growth and less prominent microcracks. Furthermore, also brittle crystals of ettringite are hardly observed [49]. The addition of GO remarkably refines the porosity, reducing the critical pore size and the volume of macropores [48, 50]. For the reason of reduced porosity, the incorporation of even small amount of GO into cementitious composites leads to a decreased sorptivity [50, 51]. The decrease up to 8 and 44% has been reported for initial and secondary sorptivity, respectively [50]. Therefore, cement-GO composites feature with a tremendously reduced ingress of chlorides. Even the marginal addition of graphene oxide of 0.01 wt% may effect in significant decrease of chloride penetration depth from 26 to 5 mm [51]. Interestingly, the addition of GO and its acceleration effect on cement hydration lead to a higher drying shrinkage at early stages of hydration. Nevertheless, since drying shrinkage depends on the tension of capillary pores, which are highly reduced in composites reinforced with graphene oxide, drying shrinkage after 28 days is then reduced [47].

Some attempts [19, 52] of introducing graphene nanoplatelets (GNPs) to improve the barrier properties and enhance the durability of cementitious composites have been reported. In this respect, this low-cost graphene derivative matches the performance of graphene oxide in concrete. The addition of 1.5 wt% of GNPs contributes to pore refinement, reducing the critical pore diameter and the average void size, thereby decreasing the water permeability, chloride diffusion, and chloride migration by 80, 80 and 40%, respectively [52]. It is worth noting that according to various authors, the addition of GNPs does not improve [52] or may even, to some extent, deteriorate [53] the strength of concrete.