Influence of admixtures on the cracking sensitivity of mortar layers applied to a mineral substrate

Mortar layers used as renderings or adhesives can be exposed to conditions of temperature and humidity that can induce considerable dimensional variations. As these mortar layers are applied to an adhesive substrate, the dimensional variations are restrained and can thus produce significant tensile stress inside the mortar. If such stresses exceed the tensile strength, cracking can occur. This paper proposes a methodology for analysing the sensitivity of mortars to cracking in relation with the characteristics of the substrate. A new test method named the “de-bonded length” test was designed, developed and validated. The effect of both polymeric admixtures (type and quantity) and substrate conditions on the cracking time was analysed. The mechanical characteristics of the mortar were also determined by means of a three-point bending test. Moreover, the cracking time was analysed in regard to flexural strength, flexural modulus and flexural toughness. It follows that the cracking sensitivity depends on the flexural toughness, whatever the quantity and type of polymeric admixtures or the humidity of the substrate at the time of mortar casting.