Principal Investigator Franz-Josef Ulm
The question of premature concrete deterioration by the Alkali-Silica Reaction (ASR) has long haunted researchers, engineers, and infrastructure administration ever since Stanton in 1940 reported the "Expansion of concrete through reaction between cement and aggregates." The structural ASR-deterioration can be attributed, on the micro-structural level, to the formation of a hydrophilic gel from reactive silica in the aggregates, alkalis from the cement clinker, and water in the concrete pore solution. In the presence of water the reaction products swell, creating an increasing internal pressure in localized regions of the cementitious matrix, that induce deformation, initiating micro- to macro cracking, excessive expansion, misalignment of the structure, etc.
The critical nature of the alkali-silica reaction (ASR) on premature concrete deterioration requires the quantitative assessment, in time and space, of the chemomechanical impact of ASR expansion on the dimensional stability of concrete structures. In particular, the coupled problem of heat diffusion and ASR kinetics can be critical, as the ASR is a thermoactivated chemical reaction. The quantitative analysis of these coupled effects on both material and structural level is the main objective of this paper. Starting from the governing micromechanisms of ASR expansion, a chemoelastic model is developed that accounts for ASR kinetics and the swelling pressure exerted by the ASR reaction products on the skeleton. This chemoelastic model is a first-order engineering approach to capture timescale and magnitude of ASR expansion. It is shown that the realistic prediction of ASR structural effects requires the consideration of two timescales: (a) A latency time associated with the dissolution of reactive silica; and (2) a characteristic time associated with the ASR product formation. In addition, a dimensional analysis of the governing equations reveals that the ASR deterioration of ''massive'' concrete structures is driven by the simultaneous activation of heat diffusion and reaction kinetics within a surface layer defined by a characteristic ASR heat diffusion length. In turn, in ''slender'' structures, it is the simultaneous activation of moisture diffusion and ASR kinetics that drives the surface layer delamination. This is illustrated through finite-element case studies of ASR effects in structures of different dimensions: a concrete gravity dam and a bridge box girder.