pH attenuation by soils underlying recycled concrete aggregate road base

Highlights

• High pH leachate from RCA road base can accelerate corrosion of underlying metal

• Soil organic material promotes respiration of CO₂ that attenuates high pH

• Column experiments conducted to investigate pH attenuation in underlying soils

• Results indicate underlying soils attenuate RCA leachate pH to safe levels

• RCA should be placed at least 0.3 m from metal to allow natural attenuation

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.

Introduction

Previous studies have documented accelerated corrosion occurring when galvanized or aluminized metal culverts are placed near crushed limerock (FHWA, 1997; Kuo et al., 2001). This reduces the useful life of the culvert and can result in sudden failure, flooding, roadway damage, traffic delays, safety hazards, and costly emergency repairs (Perrin and Jhaveri, 2004). Leachates emitted from limerock can have a pH greater than 9, a level considered aggressive to aluminized and galvanized metal culverts (Kuenzel et al., 2014); as a consequence, it can initiate corrosion of nearby culverts. In cases where culverts are installed well below the limerock road base, this leachate can be buffered to values below a pH of 9 (and therefore non-aggressive to aluminized and galvanized culverts) by soil acidity and by carbon dioxide (CO₂) emitted from natural soil respiration processes (Setiadi et al., 2006; Akhoondan and Sagüés, 2013). However, this also raises the question of how seepage from RCA and site-specific parameters (e.g., soil depth) may affect nearby soil properties over time. Soils with high (> 9) or low (< 5) pH have been shown to promote corrosion processes in nearby metallic infrastructure (Vu et al., 2012; Tewari and Manning, 2017; Crowder, 2018).

Recycled concrete aggregate (RCA) exhibits many excellent engineering properties and is increasingly being used as a replacement for natural aggregates in road base construction (Chen et al., 2012; 2013; Cardoso et al., 2016; Puthussery et al., 2016). However, RCA leachates are known to exhibit higher pH than those emitted from limerock, which heightens the corrosion concerns described above. Past laboratory studies (Engelsen et al., 2010; Gupta et al., 2018) reported RCA leachate pH values ranging from 10.6 to 12.8 when measured at a liquid-to-solid ratio (L/S) of 10. On the contrary, a few studies over the past decade (Engelsen et al., 2012; 2017; Qin and Yang, 2015; Natarajan et al., 2019) have demonstrated that RCA leachate pH seepage in-situ does decrease over time and that reduction can be linked to factors such as infiltration rate through an overlying layer (e.g., asphalt pavement) and constituents within the underlying soil (e.g., organic matter). Still, the question of whether the elevated pH of RCA leachate can be attenuated under field-like conditions to below aggressive levels to avoid metallic infrastructure corrosion (e.g., culverts) is not well understood and merits an in-depth investigation.

Alkaline RCA leachate can be attenuated to an inert range when it interacts with soil acidity and CO₂ gas. Gupta et al. (2018) performed a laboratory-scale experiment to assess the impact of RCA leachate emanating from subgrade material in Florida roadways and how soil acidity can neutralize its high pH. The results from this study suggested that soil acidity can lower RCA leachate pH but also that this acidity is depleted at high L/S and therefore cannot be relied on for long-term pH control. Laboratory testing and modelling performed by Chen et al. (2020) evaluated how high pH and alkalinity from RCA leachate can be controlled depending on underlying soil properties and found that mineral content and cation exchange capacity (CEC) directly influence neutralization. However, these works did not fully investigate the influence of CO₂ arising from soil respiration and its influence on attenuating high pH RCA leachate. Highly alkaline RCA leachate can be neutralized through a process known as carbonation, where alkali hydroxides (e.g., Ca(OH)₂) are converted to carbonates (e.g., CaCO₃). The rate and extent that carbonation occurs are influenced by external properties (e.g., water infiltration) in addition to the availability of CO₂ (Engelsen et al., 2012; Abbaspour et al., 2016; Abbaspour and Tanyu, 2019).

This study presents the results of a 12-month investigation where different soil conditions were exposed to leachate from an overlying RCA layer at different infiltration rates. The impacts of soil depth and organic matter (OM) content were evaluated to determine the effects that increased CO₂ concentrations had on pH attenuation for highly alkaline leachate from RCA and natural aggregate (crushed limerock). This resulted in the creation and testing of sixteen different column experimental groups for analysis. RCA was used in 12 of these columns, while the remaining four used crushed limerock, a common road construction material in Florida, US. The extent of pH change at different soil depths caused by inherent soil acidity and the influence of CO₂ was evaluated. The ultimate objective of this study is to provide insight as to how close an RCA road base can be placed to underlying metallic infrastructure (e.g., culverts) in soils where depth and OM content vary, which is useful for establishing guidelines for using RCA in roadway construction.