Mechanisms affecting the development of alkali-silica reaction in hardened concretes exposed to saline environments

Abstract

In circumstances where concretes containing UK aggregates with reactive siliceous components are exposed to high concentrations of NaCl there is conflicting evidence as to whether salt ingress enhances susceptibility to alkali-silica reaction (ASR). There is also uncertainty over the mechanisms involved. The research undertaken was intended to elucidate these issues. Expansion tests with concrete prisms and cores immersed in 2 and 7 M NaCl solutions at 20 and 38°C showed that significant ASR expansion and cracking could be induced in specimens containing the more highly reactive UK aggregates at alkali levels well below 3 kg/m3 Na2Oeq, which is the present recommended UK limit for minimizing the risk of ASR. Even concretes containing aggregates of lower reactivity (chert-bearing gravels, etc.) were found to exhibit significant ASR expansion when exposed to NaCl solution in specimens of 3–4 kg/m3 Na2Oeq. The role of NaCl ingress in exacerbating the development of ASR in concretes of varied composition was studied by thermoanalytical techniques and petrography combined with electron probe microanalysis. The mechanisms were found to depend on several features of the environment (NaCl concentration, temperature, etc.) and the composition of the concrete (type of reactive aggregate, alkali content, cement mineralogy, etc.). A simplified reaction scheme was proposed to account for the results obtained.

Ingress of NaCl in concrete with alkali reactive aggregate: effect on silicon solubility

“…According to Bérubé et al [3], leaching of potassium during exposure of concrete to NaCl solutions results in a reduction in pH of the concrete pore solution, SiO 2 solubility and consequently less ASR damage. Nevertheless, as described previously [4][5][6][7][8][9][10][11], NaCl ingress in hardened concrete can promote damage due to ASR significantly. Moreover, NaCl increases capillary porosity which accelerates transport [13,14] and also affects the phase composition of the hydration products [8,9,15] as well as, in particular, the composition of the pore solution [15].…”

“…Investigations of the concrete revealed the presence of alkali silica gel in the cracked concrete indicating the occurrence of a damaging alkali-silica reaction [2]. The enhancement of ASR damage in concrete structures and pavement exposed to deicing salt has also been observed in many other countries and has been the subject of numerous laboratory and field investigations [3][4][5][6][7][8][9][10][11].…”

“…In investigations with concrete structures and laboratory concretes, Sibbick and Page [8,9] assessed the effect of NaCl exposure on ASR in concrete with regard to the alkali content of the concrete and the C 3 A content of the cement. The authors proposed a reaction scheme in which, firstly, the ingress of NaCl increases the NaOH concentration of the pore solution owing to the reaction of chloride with the C 3 A hydration products and portlandite.…”

“…The SiO 2 network disintegrates forming a loose gel structure [5,12]. According to the reaction scheme of Sibbick and Page [8,9] a fluid alkali-rich silica gel is formed which takes up calcium and water and is able to develop an expansion pressure.…”

“…However, the effect of NaCl ingress on the composition of the concrete pore solution and the dissolution rate of SiO 2 was not considered in the laboratory and field investigations mentioned above [3,4,8,9]. According to Bérubé et al [3], leaching of potassium during exposure of concrete to NaCl solutions results in a reduction in pH of the concrete pore solution, SiO 2 solubility and consequently less ASR damage.…”