The Environmental Degradation of Cement-Based Materials Due to Alkali-Silica Reaction: Case Studies

Abstract

The environmental degradation of infrastructures is widely widespread. Every material or combination of it is subjected to a specific type of deterioration, which can occur in combination with other damaging effects. Structures are subjected to physical, chemical, mechanical and biological attacks. The monitoring, the diagnostic and the restoration are expensive processes that often can be mitigated by a correct material planning and design. Concrete structures are affected by freeze / thaw attack, acid and water leaching and sulphate deterioration. Every mechanism takes place at different rates depending upon the concentration of the aggressive pollutants and the environmental parameters, such as the humidity and the temperature. Alkali-silica reaction is known to happen between the alkali content of the cementitious binders and the amorphous quartz-rich stone aggregates. It usually takes a relatively long time to occur. In this work, cableway piles exposed to adverse atmospheric condition on a mountain as well as a support wall of a freeway 80 meters high pile were investigated with respect to the microstructure, the residual expansion in a NaOH-rich solution and the damage. Expansive gel was found around some stone aggregates. This caused the formation of wide cracks. These latter created a net-like structure on the concrete surface and spalling of the concrete cover was observed. Immersion tests in an alkali-rich solution also exhibited a potential residual expansion of the concrete, which was in turn related to the microstructure and the mineralogy of the cementitious material. The humidity, the aggregates and the reactive alkalis interact on a longterm basis over 50 years and largely control the extension of the degradation. Even with widely opened cracks, the AAR reaction potential may not be still completely stopped. Repair mortar applied on concrete may act as a barrier against the reaction, but it is not be able to completely mitigate the AAR effect.