Prestressing of reinforcing bars in concrete slabs due to concrete expansion induced by Alkali-Silica Reaction

Alkali-silica reactions (ASR) in concrete bridges have been a major concern worldwide for many decades. In Denmark, several bridges are severely damaged due to ASR and over 600 bridges have the potential to develop ASR in the future. The majority of these bridges are slab-bridges. Despite the many cases, experimental research on structural safety and residual load carrying capacity of ASR-damaged bridges is limited. As ASR causes severe cracks in the concrete, which may affect the concrete compressive and tensile strength, concerns have been directed towards the residual shear capacity. Yet, against all expectations, existing experimental studies have shown that the residual shear capacity of ASR deteriorated specimens is not significantly reduced compared to similar undamaged specimens - even in cases with severe ASR-induced crack formation. These results are often explained by a favourable prestressing effect in the reinforcement induced by ASR expansion. In order to develop a general shear model, the ASR-effects need to be taken into account. Unfortunately, the number of studies focusing on the actual quantification of this prestressing effect is limited. Additionally, the studies are mostly based on relatively small laboratory specimens or beams and structural elements with shear reinforcement. This paper provides the first plausible explanation on why these results are unrepresentative for slab elements. Additionally, this paper quantifies the prestressing effect by analysing full-scale slabs, obtained by severe ASR-damaged bridges located in Denmark. The results show that in the early stage, the ASR-induced prestressing effect develops simultaneously with the ASR-induced cracks. Subsequently the cracks develop further, while the prestressing effect reaches a maximum. The results and existing studies are used to discuss an existing assessment method.