Accelerated Laboratory Testing for Alkali-Silica Reaction Using ASTM 1293 and Comparison with ASTM 1260

A great deal of work on alkali-silica reactivity (ASR) has been reported since1940. ASR is a reaction in which certain aggregates react with the highly alkaline pore solution of concrete. As the name of the reaction implies, the reactive aggregate contains silica. However, not all siliceous aggregates are reactive.In general, the aggregates that cause harmful reactions in concrete are those containing amorphous silica (glasses and opal), unstable crystalline polymorphs of silica (cristobalite and tridymite), poorly crystalline forms of silica (andesite and rhyolite), and microcrystalline quartz-bearing rocks (quartzite and greywacke). The highly alkaline pore fluids of concrete are able to depolymerize the reactive silica present in the aggregates, forming products of different compositions in the concrete pores. In the presence of moisture, the reaction products (gel products) change in volume and may expand to such a degree that the concrete tensile strength is reached and the material cracks. The cracks allow water to enter the material, thus affecting its integrity, and several processes of deterioration may take place.Alkali-silica reactivity of aggregates in California may create problems if no preventative measures are used. Some tests used to screen aggregates for potential alkali-silica reactivity are ASTM C 1260 and ASTM C 1293. Both tests are accelerated test methods, meaning that the tests put the materials in conditions that increase the rate of reaction compared to the rate at which it would occur in the field. In some cases, accelerated tests may cause reactions that would not occur in the field.ASTM C 1260 is aggressive because of the high temperature and the high concentration of hydroxide used in the test. A criticism of ASTM C 1260 is that it may classify good aggregates as reactive. Results are obtained from ASTM C 1260 in 14 days.A more realistic, yet still accelerated test is ASTM C 1293. The lower concentration of hydroxide and the lower temperature at which the test is run are not as aggressive as the ASTM C 1260 conditions. ASTM C 1293 is also considered to be more representative of actual field conditions since the test is performed on concrete specimens, unlike ASTM C 1260, which is performed on mortar specimens. However, the major drawback of this test is that it requires a year to complete.ASTM C 1293 requires high-alkali content cement. This cement was difficult to find in the Western region of the United States, and had to be shipped from Pennsylvania. In addition to problems caused by the lack of a readily available local source for this cement, the cost of shipping such a material from the east coast imposes a substantial cost burden. Because of these two factors, a replicate set of the experiment was preformed following ASTM C 1293, but with the substitution of low-alkali cement for high-alkali cement. Sodium hydroxide was added to increase the cement alkali content to 1.25 percent, which is exactly the same amount that is required for the unmodified version of ASTM C 1293. The mixing procedures were performed according to ASTM C 192.The Partnered Pavement Research Center has previously reported results for several California cements and aggregates using ASTM C 1260.