Research Article | Open Access
Water Permeability of Pervious Concrete Is Dependent on the Applied Pressure and Testing Methods
Falling head method (FHM) and constant head method (CHM) are, respectively, used to test
Research Article | Open Access
Water Permeability of Pervious Concrete Is Dependent on the Applied Pressure and Testing Methods
Falling head method (FHM) and constant head method (CHM) are, respectively, used to test the water permeability of permeable concrete, using different water heads on the testing samples. The results indicate the apparent permeability of pervious concrete decreasing with the applied water head. The results also demonstrate the permeability measured from the FHM is lower than that from the CHM. The fundamental difference between the CHM and FHM is examined from the theory of fluid flowing through porous media. The testing results suggest that the water permeability of permeable concrete should be reported with the applied pressure and the associated testing method.
Pervious concrete is made by eliminating most or all fine aggregates from the concrete mix. Its internal interconnected void space allows storm water to percolate and thus to reduce the amount of run-off. The permeability or the saturated hydraulic conductivity of the pervious concrete signifies its capacity to drain the ponding water from the concrete surface. It quantifies the resistance of the medium to flow and depends only on the characteristics of the porous medium. It is not measured directly but is calculated by measuring its internal states like flux and pressure.
There are several established methods to measure the saturated hydraulic conductivity of porous materials on the basis of Darcy’s law. During the measurement, a pervious concrete sample is subjected to a water pressure that is lower sufficiently to support a laminar flow [1–5]. The associated flowing rate is measured to estimate the permeability. The applied pressure during the measurement can be maintained to be constant, referred to as CHM, or be allowed decaying, called FHM. The CHM measures the permeability of pervious concrete by applying a constant water head on the surface of the sample and by weighting the water volume flowing through the sample at a designed time interval [7, 8]. The FHM allows the water head above the sample surface dropping from a starting level to a designed level and then records the time interval during this dropping process [9, 10]. As the permeability is not measured directly but is calculated by inverting the applied pressure and its associated water flux, the permeability measured from different scales and distinctive experimental setups is likely to render inconsistent values. It thus merits examining whether the pervious concrete’s permeabilities measured from the CHM and the FHM agreed with each other and whether the permeability varies with the applied pressure.
This paper experimentally measures the permeability of pervious concrete under the CHM and the FHM, respectively. Two Portland concrete mixtures are prepared and cored for the water permeability test. The permeability is measured under different water heads to see if the permeability is a function of the applied pressure. The permeability measured from the two methods is compared and it is found that the permeability measured from the FHM tends to be higher than that of the CHM. The fundamental theoretical difference between the two methods is reassessed.
2.1. Sample Preparations
Two concrete mixes consisted of Portland cement, water, and coarse aggregates. Ordinary Type I Portland cement is selected for the experiments. The aggregates used are crushed limestone with a single gradation from 0.5 cm to 1.0 cm. It has an absorptivity of 0.55%, an observed density of 2725.7 kg/m3, and a bulk density of 1348.3 kg/m3. By weight, a cement/aggregate ratio of 1 : 4 is used. One mix has a water-to-cement ratio 0.23 and the other 0.25. Due to its low water content, 2% of water-reducing agent with respect to the cement’s weight is added to aid the workability of the mixture.
Selected mixtures are mixed by the use of a 0.05 m3 rotating-drum mixer. The aggregate is firstly cast in the mixer, and then the estimated absorbed water is spilled on the aggregate for 1 min mixture to ensure that the aggregate surface is wet. The cement is afterwards cast to the mixer for another 1 min mixture with the wet aggregate. Finally, the amount of the water needed for the desired water-to-cement ratio is added to the mixer for another 2 min mixture.
The concrete mixtures are cast in slender wood moulds with a dimension of 150 × 150 × 860 mm3. All mixtures are placed by lightly rodding 10 times in the three layers to ensure a uniform compaction in each lift. Mixtures in the mould are leveled by spatulas without any vibration compaction. They are moved to an air-conditioned, foggy room with 20°C and with more than 95% relative humidity for 3-day curing. After that, they are demolded and the pervious concrete blocks are restored in the air-conditioned room for another 25-day curing. Each cured block is drilled for six cores with a diameter of Φ 100 × 100 mm (Figure 1).
The drilled cores are vacuum-washed to eliminate the drilling-left debris and then are submitted to the basic property’s te