Using recycled concrete aggregate (RCA) as a replacement for natural aggregate in new concrete is a promising way to increase the overall sustainability of new concrete. This has been hindered, however, by a general perception that RCA is a sub-standard material because of the lack of technical guidance, specifically related to long-term durability, on incorporating RCA into new concrete. The goal of this research was to determine whether current testing methods (namely, ASTM C1260) for assessing natural aggregate susceptibility to alkali-silica reactivity could be used to assess the potential reactivity of concrete incorporating RCA. Seven different RCA sources were investigated. It was determined that ASTM C1260 was effective in detecting reactivity, but expansion varied based on RCA processing. Depending on the aggregate type and the extent of processing, up to a 100 % increase in expansion was observed. Replicate testing was performed at four university laboratories to evaluate the repeatability and consistency of results. The authors recommend modifications to the mixing and aggregate preparation procedures when testing the reactivity of RCA using ASTM C1260.

This paper presents the results of tests relating the expansion of concrete specimens with alkali-silica reaction (ASR) to applied constant stresses, both tensile and compressive, and reinforcement ratio. The results were compared with those found by other researchers and to two proposed mathematical models. Additionally, the effects of rate of expansion, specimen size and shape, and orientation of restraint were investigated. It was concluded that size of specimen and orientation of restraint were important factors, and that the relationship between restraint and expansion could be modelled mathematically but the amount of data required restricted the model's practical use.

Type of Report and Period Covered Sponsoring Agency Code Supplementary Notes AbstractThe primary goal of this research project was to investigate the long-term durability of concrete incorporating recycled concrete aggregate (RCA) through accelerated laboratory testing. Overall it was found that modifications to standard aggregate testing and characterization standards were necessary for testing RCA. This included modifications to standard tests including ASTM C 128, C 305 and C 1260. It was found that the potential for alkalisilica reactivity did exist for new concrete containing RCA. The characteristics of the RCA also had a profound effect on ASR related expansion. RCA with a higher content of reactive coarse or fine aggregate (compared to paste fraction) exhibited greater reaction and would therefore require higher levels of mitigation. Based on testing in this research project precision and bias statements in ASTM C 1260 (for virgin aggregate) do not apply to RCA. Recommendations for future work are also included in this report.17.

It is well established that tension stiffening, the contribution of the concrete in tension to the stiffness of a reinforced concrete member, is a significant parameter in deflection calculations and this is recognised by design codes such as BS 8110 and Eurocode 2. These codes make provision for tension stiffening effects and also distinguish between short- and long-term behaviour. What is not known, however, is how rapidly tension stiffening decays with time or how significant this rate of decay might be for deflection calculations. This paper reviews the current approaches to tension stiffening in BS 8110 and Eurocode 2 and then summarises a major experimental investigation which was undertaken to explore time dependent aspects. Revisions to BS 8110 and Eurocode 2 are proposed as a result of this work. Further refinements for modelling tension stiffening effects in reinforced concrete members are also described.

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