The declining pH of waters exposed to pervious concrete

Jonathan Nicolas Thomle

The focus of this study is on the pH changes over time in various stormwaters in contact with pervious concrete that has aged under varying ambient air restrictions. Elevated pH levels may be a concern if exfiltrated directly to sensitive waters. This study was conducted to aid designers by determining the rate of pH decline under various conditions. For this study, laboratory prepared pervious concrete specimens exposed to three different levels of ambient air restriction, were periodically tested for pH using four different testing methods; infiltrating either deionized water or tap water through the specimens, or soaking the specimens in either deionized water or tap water. Obvious trends in pH decline were observed. Greater exposure to ambient air significantly increased the rate of pH decline. The tap water tests represented more typical stormwaters and had much lower pH readings than the deionized water tests. The pH of samples representative of typical, in-place conditions, declined sufficiently in well under a year for most typical field conditions. In addition, this study also includes the declining pH of water in contact with pervious concrete exposed to carbonate laden waters. The increased rate of carbonation is desired in many primary and secondary life applications of concrete in order to make the life cycle of concrete structures more carbon neutral. Most carbonation rate studies focus on concrete exposed to ambient air or air enhanced with increased levels of carbon dioxide under various conditions. A possible alternative mechanism for accelerated carbon sequestration in concrete was investigated in this research based on the pH change of waters in contact with pervious concrete. The results indicate that the pH of water in contact with pervious concrete exposed to carbonate species laden waters declined more quickly than when exposed to ambient air, which may indicate an increased rate of carbonation. It is theorized that the proposed alternative mechanism reduces the limiting rate effect of carbon dioxide dissolution in water and diffusion into the micro pores of the cement paste.

The declining pH of waters exposed to pervious concrete

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