Effectiveness of Alkyl Alkoxy Silane Treatment in Mitigating Alkali-Silica Reaction

The possibility of using an alkyl alkoxy silane (AAS) (iso-butyltriethoxysilane) in mitigating alkali-silica reaction (ASR) has been investigated. Cement mortars were prepared with reactive aggregates of basaltic and siliceous origin. AAS was applied to the surface of test specimens. The accelerated mortar bar method (ASTM C1260) was employed for the detection of ASR-related expansions. In addition to the length change, total water absorption (ASTM C642) of mortars and penetration depth of AAS were also measured. Three different curing procedures (short, medium, and long prewaiting periods) were applied to AAS-treated mortar bars before the ASTM C1260 tests. To understand the possible mechanism of prewaiting periods on water repellency of AAS-treated mortars, Fourier-transform infrared spectmscopy (FT-IR) analyses were also performed. Test results indicated that the success of AAS treatment mainly depends on a prewaiting period. AAS treatment is more effective if prewaiting periods are lengthened. In case of shorter precuring periods, mortar contains water, and in the presence of water, silane is very hydrolyzable. Once the silane hydrolyzes, it quickly bonds to substrate and this prevents its penetration to the deeper regions of mortar. In addition to the penetration problem, in the presence of water, the layer of silane molecules on the surface of mortars could not form strong Si-O and Si-O-Si bonds, and unbonded silane molecules detached or washed out. To effectively mitigate with ASR-related expansions, a minimum of 28 days air curing is mandatory. Longer precuring periods will permit the formation of strong oxygen bridging bonds between the silane molecules and mortar substrate, which provide a surface with stable hydrophobic character. In conclusion, it is shown that AAS treatment can be more effective when applied to older concrete structures.