Specimen preparation

Mortars with a low water/cementitious materials (w/cm) ratio and high cement content were prepared to simulate high-performance concrete. The mixture design is shown in Table 1. A melamine-based high-range-water reducer (HRWR) (4 % solid by mass of cement) was used to obtain a usable flow. The sand was prepared by crushing and grading coarse aggregate as described in ASTM Test Method for Potential Alkali Reactivity of Aggregates (Mortar-Bar Method) (C1260) (non-reactive) or by combining four grades of fused silica sand (reactive) to obtain the specified grading. Consequently, both sets of aggregates had the same size distribution. The only difference between the two sets of samples prepared was in the reactivity: the fused silica aggregates were very reactive (greater than 0.1 % expansion at 14 days (ASTM C1260)), and the other aggregates were a non-reactive limestone (0.01 % expansion at 14 days (ASTM C1260)). The fused silica sand was provided in four gradings, namely 4-10, 10-20, 20-50 and 50-100 (sieve size as defined in ASTM E11). The silica fume was obtained in a slurry and its characteristics are described in Ferraris et al. 1998. An inert silicon carbide powder was also used as a cement replacement. This powder was UNASIL 1200¹, silicon carbide used for polishing. This has been demonstrated to be an inert material in the hydration process.²

Cylindrical test specimens, 38 mm in diameter and 279 mm long (1.5 in x 11 in) were cast using an ASTM Type I portland cement with a high alkali content (approx. 1.2 % Na₂O equivalent by mass). As the high alkali content of the cement was expected to result in a high alkali concentration and pH in the pore solution, this cement was also expected to induce ASR with reactive siliceous aggregates. The high temperature used, 50 ± 3ºC, should also have promoted the alkali-silica reaction. After casting, the specimens were cured for 24 h in 100 % RH at 20 ± 5ºC, and then at 50 ± 3ºC for another 24 h. The stress test was initiated immediately after this second 24-hours curing period, which also served to slowly equilibrate specimen temperature, avoiding thermal shock in going from room temperature to the 50 ºC bath.

¹Brand names and names of manufacturers are identified in this report to adequately describe the experimental procedure. Such an identification does not imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the material identified is necessarily the best available for the purpose.