Development of structure-property relationships for hydrated cement paste, mortar and concrete

A mechanistic approach was adopted to develop models for the mechanical properties of hydrated cement paste, mortar and concrete. These models reflect the fundamental structure of concrete, and represent a departure from the predominantly empirical models available for cement-based materials. The intrinsic elastic modulus and fracture toughness of hydrated cement paste were determined based on the interatomic interactions between calcium silicate hydrate (C-S-H) particles, which are the primary binder among the hydration products of cement. The elastic modulus model of hydrated cement paste was developed by introducing the effect of elliptical capillary pores into the corresponding intrinsic model. The fracture toughness model of hydrated cement paste was also developed by introducing the energy dissipation associated with the C-S-H/C-S-H debonding and the phononic frictional pullout of calcium hydroxide (CH) crystals, with the latter phenomenon found to be the major contributor. The strength of hydrated cement paste was determined using the elastic fracture mechanics principles. Experimental results and available empirical models were used to validate the models. Mechanical models for cement mortar were developed by introducing the effects of fine aggregate on hydrated cement paste. The elastic modulus model of cement mortar accounts for the positive effects of the high elastic modulus fine aggregates, and the negative effects of the interfacial transition zone. The fracture toughness model of cement mortar was developed by introducing the energy dissipation associated with the pull-out of fine aggregates into the fracture toughness model of hydrated cement paste. The strength model of cement mortar was developed using the elastic modulus and fracture toughness models based on fracture mechanics concepts, with due consideration given to the effects of fine aggregates in restrained shrinkage microcracking of cement mortar. Experimental results and empirical models were used to verify the mechanistic models of cement mortar. The mechanistic models of concrete were developed based on the corresponding models of cement mortar by introducing various effects of coarse aggregates. The fracture toughness model of concrete accounted for the energy dissipation associated with pullout of coarse aggregates from cement mortar. The approach to development of elastic modulus and strength models was similar to that used for development of the corresponding models of cement mortar. The predictions of the mechanistic models of concrete compared favorably with experimental results. The mechanistic models of concrete were tailored for application to high-volume fly ash concrete, and were used to determine the effects of high-volume replacement of cement with coal fly ash on the mechanical properties of concrete. Some comparisons were also made between the predictions of the mechanistic models and the test data available for high-volume fly ash concrete.