Mitigating alkali silica reaction in recycled concrete
Concrete made with recycled concrete aggregate (RCA) that had shown alkali silica reaction (ASR) distress was evaluated. It was found that ASR mitigation was needed to prevent continued expansion. Several mitigation methods were evaluated and compared to conventional concrete using the testing procedures of ASTM C 1260/1567 and ASTM C 1293. Pore solution analysis and Thermal Gravimetric Analysis (TGA) were also conducted. It was found that fly ash, silica fume and ground granulated blast furnace slag (GGBFS) were effective in mitigating ASR in RCA concrete. Compared to conventional concrete, slightly higher dosages were typically required. The ASTM C 1260/1567 test correlated well with ASTM C 1293 with few exceptions. Pore solution and TGA revealed that calcium hydroxide and alkalis are reduced by fly ash, silica fume and GGBFS substitution. Calcium depletion during the pozzolanic reaction is a sufficient condition for ASR arrest, but the alkali reducing effect appears to be more pivotal than calcium depletion. Lithium, as well as the other mitigation strategies, required higher dosages with RCA concrete than conventional concrete. The pore solution lithium to alkali ratio was found to be lower and at one year, reaching equilibrium at approximately 50 to 60 percent of the original dose. Modifying the soak solution of ASTM C 1260 resulted in higher levels of lithium in the test samples and made the test less conservative. Topical application of lithium nitrate solution showed reduced surface expansion of pavement blocks however there was no effect on inside expansion. Lithium was found to penetrate about 25mm.
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