A new approach for the mix design of (patch) repair mortars

Abstract

The failure of most repairs in concrete structures is mainly manifested in the form of cracking and/or debonding of the repair layer. These two repair mechanisms could lead to the continuous corrosion of reinforcing steel in concrete. Cracking and debonding have been attributed to the high differential shrinkage that exists between the concrete substrate and repair layer as well as the lack of adherence, which commonly results from poor workmanship. It is crucial that restrained shrinkage mechanisms of patch repairs be understood so that the abovementioned effects can be prevented. The performance of patch repairs can be improved through the optimization of their mix design. Thus, it is common to use admixtures, non-cementitious compounds and polymers. However, promising technologies such as expansive admixtures that counteract shrinkage have found little application to date.Polymers do not chemically react with the cementitious matrix. They, however, have been reported to modify the microstructure in cementitious materials, consequently, resulting in improved properties with respect to mechanical, chemical and durability aspects. They increase chemical interactions with mineral phases, which improves the bond between the mortar and the substrate. Besides this, expansive cements are types of cement whose volume increases, unlike plain Portland cement where the volume typically decreases with time. This property relates to the use of expansive admixtures like quicklime, which exhibits an enormous volume increase upon hydration. This property leads to the reduction of the shrinkage, which is increased by the combination of the quicklime to a shrinkage-reducing admixtures (SRA).Using polymer technology and expansive admixtures, it is possible to develop a new approach for the mix design of repair mortars that combines the positive effects of polymer latex (increasing of bond strength) and expansive admixture (reducing the shrinkage) properties to reduce cracking and debonding failures in patch repairs.