Investigation of an Old Problem: Optimisation and Cost Effectiveness

Abstract

The Highway Managing Agent’s concerns over widespread cracking reported in many bridges on the network led to special investigations to bridge elements over a motorway in England. The cracking in both pre-cast and in-situ concrete has historically been diagnosed as resulting from alkali-silica reaction (ASR). However, previous data was often inconclusive and sometimes missing. Rather than carry out widespread testing to numerous bridges, the concrete specialist advised combining desk study with visual comparison of conditions across the network, development of a ‘league table’ of conditions, and targeting of site visits and investigations at those structures showing highest levels of reaction and risk to ongoing serviceability.The detailed laboratory investigation concluded that the cracking in the precast concrete elements had been initiated by AAR and that expansive ettringite formation – possibly delayed ettringite formation (DEF) had subsequently occurred in conjunction with AAR contributing to ongoing crack development. AAR was associated with the siliceous component of the limestone coarse aggregate particles and had given rise to abundant gel formation and cracking. Ettringite formation had been exacerbated by unusually high levels of sulphate and moisture ingress. The alkali content of the concrete was low and may have been depleted as a result of long-term moisture ingress. Core expansion testing at 38°C showed there to be a very small potential for further expansion of the concrete. The diagnosis informed the management strategy for numerous nearby bridges showing the same features.