Causation of Near-Surface ASR at Concrete Slabs-on-Grade

Alkali-silica reaction (ASR) in concrete structures is a well-known damage mechanism. The mechanism of near-surface ASR (NSASR) differs from traditional ASR in that the reaction occurs relatively quickly, occurs only near the top surface of a concrete slab, extrudes gel onto the surface of the slab, and does not result in widespread cracking of the slab. Evaluations of residential and commercial structures in Central Texas have determined the presence of this mechanism. Through recent case studies, this paper will discuss factors that cause and/or exacerbate NSASR and the patterns of NSASR distress. These case studies involve slab-on-grade foundation systems with many similar conditions, but with some important variation in the factors which contribute to NSASR. This paper will also discuss in situ testing and material testing performed for the case studies and how the results of these tests correlate with some common theories related to the causation of NSASR in slab-on-grade foundation systems. Finally, the paper will discuss potential future laboratory testing to study the NSASR mechanism and the effects of specific causation factors.