A simplified design approach to prevent shrinkage cracking in patch repairs

This paper outlines two procedures for determining the interfacial shrinkage stresses in a repair patch. The first is an analytical approach based on the analogy of a bimetallic strip undergoing contraction (shrinkage). The second is a semi-empirical procedure based on strain monitoring of in situ repairs to in-service bridges. The procedures determine conversion factors to relate the specified properties of the repair materials to their in situ properties in a field repair patch. For example, the shrinkage of a repair patch is influenced by the volume–surface effect, site temperature and relative humidity which are not considered in repair material specification. Creep is initiated in situ by differential shrinkage stresses in the repair material and is determined by adopting an effective elastic modulus approach. Both procedures require the basic material properties (elastic modulus, shrinkage, creep) and geometrical details (width, depth) of the repair patch. The analytical approach incorporates the repair material creep coefficient to predict the interfacial tensile stresses. Alternatively, it uses a less rigorous, elastic approach that omits creep. The creep approach provides higher accuracy whereas the elastic approach overestimates stresses since relaxation by creep is neglected. The elastic approach is recommended for design due to its simplicity and the in-built factor of safety provided by the overestimation of tensile stress. The semi-empirical approach uses an expression derived from long-term field data to determine the strain (and consequently stresses) at the interface of the repair patch and the substrate concrete. The procedures predict the maximum interfacial tensile stress during the service life of a repair patch. They can be used to design crack-free repair patches and optimise repair material selection through a better understanding of the interaction between the repair patch and substrate concrete.