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Description: This doctoral thesis documents development of an electrical conductivity test method for determining permeability of concrete. The thesis comprehensively reviews concrete porosity and transport properties, including permeability to water, chloride diffusion, electrical conductivity and resistivity, wicking action, and sorption. Effects of water:cement ratio and use of supplementary cementitious materials are considered as well. This technical document will be of interest to advanced concrete technologists interested in concrete durability issues. This report is also found on DVD021.
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. This thesis was developed as part of the educational requirements under a fellowship grant from PCA.
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