PH attenuation by soils underlying recycled concrete aggregate road base

Abstract

Recycled concrete aggregate (RCA) is widely used as a replacement for natural aggregates in road base construction worldwide. Still, there are concerns that the high pH of RCA leachate may cause accelerated corrosion of metallic-coated (aluminum or galvanized) steel culverts placed in the underlying soil. Carbonation from CO2 produced in the soil by organic matter (OM) has been considered as a potential mechanism to attenuate RCA leachate pH to below aggressive conditions (pH < 9). Twelve field-scale columns of various lengths and containing compacted RCA underlain by sandy soil were constructed and observed over a 12-month period. Experimental variables included water infiltration rate, soil OM, and soil depth. CO2 concentrations within the soil profile were measured throughout the duration of the experiment, and soil pH was measured at various depths at the end of testing. Only three of the experimental columns exhibited a soil pH greater than 9 (the pH level known to be detrimental to aluminized and galvanized metal) — two of these simulated limited depth (0.3 m) between the RCA base and culvert, and all three contained low OM soils (which is not expected to occur in practice, as soils under typical environmental conditions will typically be much deeper and contain more organic material). The results indicated that even when soil acidity was completely depleted, carbonation could sufficiently buffer soil pH from aggressively basic RCA leachate. Site-specific conditions, like soil OM content and depth to metallic infrastructure, should be considered when utilizing RCA as a base course.