Alkali-aggregate reaction under the influence of deicing salts in the Hokuriku district, Japan

Concrete cores taken from highway bridges and culverts undergoing alkali-silica reaction (ASR) were investigated petrographically by means of core scanning, point counting, polarizing microscopy, scanning electron microscopy (SEM), X-ray diffraction analysis (XRD), electron-probe microanalysis with energy-dispersive spectrometry, in conjunction with wet chemical analyses and expansion tests. Field damage was roughly proportional to the content of andesite in the gravel aggregates due to the presence of highly reactive cristobalite and tridymite. Electron-probe microanalyzer analysis of unhydrated cement phases in the concrete revealed that the cement used had contained at least 0.5% to 1.0% alkali (Na2Oeq) and that both the aggregates and the deicing salts had supplied part of the water-soluble alkali to concrete toward the threshold of producing ASR (Na2Oeq 3.0 kg/m3). An accelerated concrete core expansion test (1 M NaOH, 80 °C) of the damaged structures mostly gave core expansions of >0.10% at 21 days (or >0.05% at 14 days), nearly comparable to those of a slow expansion test with saturated NaCl solution (50 °C, 91 days) which produced Cl-containing ASR gel.