Development of discontinuous capillary porosity in concrete and its influence on durability

Durability of concrete is largely determined by its resistance to the transport of water and ionic solutions. In the absence of gross cracks, the volume and connectivity of the capillary pore system controls the rate of fluid ingress. In this study, early-age transport properties, pore structure and compressive strength were measured. Saturated water permeability tests were performed to investigate whether the findings of Powers, Copeland and Mann in 1959 regarding capillary discontinuity in cement pastes were valid for modern concrete mixtures. The results indicate that concrete with water to cement ratio less than 0.70 has a discontinuous pore structure, verifying the 1959 published results.; In concrete, an interfacial transition zone forms at the aggregate-cement paste boundary. No evidence of a percolated interfacial transition zone at the macro scale was found for any of the transport properties investigated.; Since saturated water permeability is difficult to measure, an electrical conductivity test method was developed for use on concrete between 1 and 28 days of age. Both the volume and connectivity of the pore structure as well as the conductivity of the pore solution influence bulk conductivity. To separate out the effects of the changes in pore solution composition from the development of the pore structure, the electrical conductivity of the pore solutions was measured independently. The changes in concrete conductivity over time were found to be related to changes in permeability, porosity and indirectly to strength. As expected, a marked decrease in transport properties after the first few days indicates a reduction in volume and connectivity of the capillary pore system. Changes in pore solution conductivity were found to have a minor effect on bulk conductivity. The conductivity method appears promising for the development of efficient test methods to determine minimum required curing for various concretes. High performance concrete develops significant "impermeability" in the first few days, indicating that curing requirements of 7+ days may be overly conservative from a transport perspective.