# Evaluation of the Performance of Previous Concrete Inspired by CO\({}_{2}\)-Curing Technology
## 3 Results and Discussion
### Runoff Characteristics  
Clay containing water with a turbidity of around 800 NTU was passed through the concrete samples for ten repeat cycles. Figure 7 shows the change in this water turbidity at the end of each cycle and the consequent reduction in the percolation rate due to sediment blockage. The clayey water turbidity at the end of the first cycle was reduced from 800 to 645 NTU. However, the reduction in this water turbidity in subsequent cycles was not significant regardless of the different curing conditions used in the preparation of the sample. The percolation rate was found to gradually reduce with increasing test cycles. The reduction in percolation rate at the end of the experiment reached 86% and 91% in the case of non-carbonated and carbonated samples. As illustrated in Figure 7, clay particles may have occupied the dead-end pores of the concrete matrices during passage, which was the reason for the minor reduction in turbidity and the rate of water percolation. Clayey particles are associated with each other by the van der Waals attractions between them, and the increase in their densities severely reduces water percolation in pervious concrete [45]. Clay-induced pore clogging is ten times greater than that caused by sand [46]. The size of the pores in the pervious concrete is in the range of 2-8 mm [2]. Pervious concrete reduces surface runoff and maximum flow by 30% and 70%, but during service, the pores in this concrete gradually become clogged with silt, sand, and organic matter, affecting hydraulic performance [47]. These clogged particles also accumulate fine pores in the matrix, and when dried, they form a hard crust that seals the pores. The level of water percolation through pervious concrete is reduced due to these processes, causing surface overflow and ponding when infiltration rates are less than the intensity of rainfall [48]. Cleaning methods that are effective in removing clogged particles from pervious concrete are vacuum cleaning, air compression, and hydro washing. The accumulated sediments at the shallow depths of the pervious concrete pavement are removed with a vacuum cleaner, followed by air compression to expel the grains from the middle section, and finally, hydro washing to remove the adhering sediment and unclog the interconnected pore channels. This combination has been shown to recover water percolation by up to 90% [45]. Figure 8 illustrates the amount of calcium ions released from pervious concrete samples due to the passage of distilled water and HNO3 solution for ten repeat cycles. This figure also presents the change in their pH at the end of each cycle. The initial pH of the distilled water and HNO3 solution was measured to be around 3.21 and 6.94. This examination was carried out mainly to reveal the leaching behavior of carbonated concrete when exposed to acidic species in the simulated runoff. Calcium ions were found to be released from the porous concrete matrices regardless of the curing conditions and characteristics of the percolating water. However, this removal of calcium ions from concrete samples was found to be relatively less when the pervious samples were cured in a CO2-rich environment. The pH results correlate well with the calcium removal information. The  
Figure 7: Change in turbidity and percolation rate due to the passage of clay containing water. dissolved sources of calcium ions were mainly derived from Ca(OH)2 and CSH. The pH of the HNO3 solution and the distilled water that passed through the non-carbonated samples was found to be 11.44 and 11.63 at the end of the experiment. In the case of carbonated samples, the pH values of these solutions were 9.91 and 10.16, respectively. The pH values were found to gradually increase with cycles. The cement surfaces in the flow channels of the pervious concrete are subjected to coupled calcium leaching and hydraulic abrasion due to infiltrating runoff [49].