Containment of internal expansion caused by alkali-aggregate reaction with ultra-high performance fiber reinforced concrete

In severe climatic condition, concrete structures can undergo various damages including alkali-aggregate reaction (AAR) which generally induces internal swelling and cracks in the reactive aggregates and surrounding cement paste. This study proposes an innovative repair technique which consists in casting a thin UHPC layer directly on the affected concrete substrate. For such repair, once UHPC is poured in the formwork, self-induced tensile stresses developed in UHPC by its restrained shrinkage on the concrete substrate. Later, the UHPC repair sustains further tensile stresses resulting from additional AAR internal expansion. The main objective of this research project was to evaluate the UHPC behavior under restrained shrinkage conditions and to assess the available strain margin to contain the AAR expansion in different UHPC repair configurations. A special device adapted to ring specimens was developed to first simulate the restrained shrinkage and second generate the AAR expansion. Experimental results showed that a repair in high performance concrete (HPC) cracks under restrained shrinkage and thus cannot confine internal expansion. For the UHPC repairs, no cracks developed under restrained shrinkage and very limited internal tensile stress was generated in the repair. The expansion applied thereafter on the UHPC repairs showed that the UHPC tensile strain available to confine AAR expansion without loss of watertighness is inversely proportional to the fiber content in the UHPC, i.e. 0.28%, 0.2% and 0.17% for fiber contents of 2%, 3% and 4% in volume respectively. Addition of 1.2% of rebar in the UHPC repair extends by 30% the tensile strain available to confine the AAR, while presence of cold joints in this reinforced repair reduced this performance.