Evaluation of Concrete Grinding Residue (CGR) Slurry Application on Vegetation and Soil Responses Along Nebraska State Hwy 31. Final Report.

摘要

Diamond grinding is a concrete pavement restoration technique that corrects irregularities such as faulting and roughness on old concrete pavements and extends the life of pavement. Cooling water used during the diamond grinding of concrete pavement highways generates slurry consisting of water, concrete and aggregate residue (CGR). Recently, disposal of CGR in Nebraska changed from unregulated roadside discharge to a National Pollutant Discharge Elimination System (NPDES) permit (NDEQ, 2010). The permit is designed to control pollutant levels being land applied as a result of the spreading of CGR slurry. According to NPDES permit, the CGR primary pollutant is its alkalinity and the amount of CGR that can be roadside applied is restricted to 5 dry tons/acre or the agronomic liming rate whichever is lower. The Nebraska Department of Roads (NDOR) is concerned that existing agronomic rate calculation methods were developed to minimize the active ingredient (lime) application, as such, there is also need to evaluate maximum discharge rate of CGR for cost efficiency. Therefore, this research was established to evaluate rates that will maximize the CGR discharge rate without adverse effects on roadside vegetation and soil. We conducted a two-year study to evaluate the effect of CGR application on soil chemical properties, existing vegetation, and rainfall runoff. The study was conducted on roadsides of NE State HWY 31 at mile post 36 (MM36) in 2013 and mile post 34 (MM34) in 2014 on loam and silt loam soils, respectively. CGR slurry rate was 0, 10, 20, 30, and 40 dry tons per acre, with 40 dry tons/acre considered to be the maximum CGR rate that would be applied to roadside foreslope. Vegetation, soil, and runoff were evaluated before CGR application and one month and one year after CGR application at both mile posts. The CGR effective calcium carbonate equivalent (ECCE) ranged from 13 to 28%. The results showed that application of CGR slurry at 5 dry tons/acre, as limited by the NPDES permit1, on medium to fine textured roadside soil does not have adverse effects on existing vegetation, soil chemical properties, and water quality. Moreover, the results indicated that a uniform application (i.e. uniform spread) of CGR of up to 40 dry tons/acre on loam and silt loam soils did not negatively affect existing vegetation, soil chemical properties, and runoff volume and chemistry. The highest CGR application increased soil calcium, sodium and pH in short term (one month) but did not persist after one year of CGR application. While CGR discharge of up to 40 dry tons/acre can safely be applied in a uniform layer one time to roadsides with medium textured soils, there needs to be caution applying these high rates to coarser soils (sandy soil). Such soils may respond differently due their lower ability to retain cations or buffer pH changes. In addition, this study evaluated the CGR application after one time application based on the premise that grinding extends the life of pavement and with high likelihood that regrinding will not occur again on the same highway segment. As such, the findings of our study on soil, vegetation, and runoff water quality, should not be extended for multiple and frequent application of CGR application at the same location. This study showed that it is plausible to apply CGR slurry at rates up to 40 dry tons/acre on medium to fine textured soil without negative effects and provides evidence that rate higher than the current regulated limit of 5 dry tons/acre may be applied on roadside with similar soil characteristics as this study. However, application rates must also consider the ECCE and moisture of the CGR to adjust rate even in medium to fine textured soils. We recommend NDOR develops a quick field method to estimate the ECCE during the grinding process so that application rates can be adjusted appropriately.