Review of Studies concerning Effects of Unbound Crushed Concrete Bases on PCC Pavement Drainage

Abstract

Recycled concrete aggregate (RCA) products are sometimes used as replacements for virgin aggregate products in concrete pavement structures. Recent concerns have centered on the deposit of RCA-associated fines and precipitate suspected of reducing the drainage capacity of RCA base layers and associated drainage systems. Environmental concerns have focused on the relatively high pH of the effluent produced by untreated RCA base layers. Several studies have examined these concerns and others; the results of some of these studies have not been published or publicized. The most relevant of these studies are summarized herein. These research efforts demonstrate that calcium-based compounds are present in most recycled concrete aggregates in quantities sufficient to be leached and precipitated in the presence of carbon dioxide. Precipitate potential appears to be related to the amount of freshly exposed cement paste surface. Thus, selective grading or blending with natural aggregates can reduce, but not eliminate, precipitate problems. It was also noted that insoluble, noncarbonate residue makes up a major portion of the materials found in and around pavement drainage systems. Washing the RCA products before using them in foundation layers appears to reduce the potential for accumulation of dust and other fines in the drainage system, but probably has little effect on precipitate potential. Field studies have shown that precipitate and insoluble materials can significantly reduce the permittivity of typical drainage fabrics but that attention to drainage design details can minimize the effects of these materials on pavement drainage.