[Univ of Cambridge] [Dept of Engineering]

Saturated permeability of concrete as influenced by cracking and self-sealing

by N. Hearn

The dependence of water permeability of saturated concrete on its solid constituents and the curing regime has been widely discussed in the literature. This paper addresses two other factors which affect permeability: interaction of the permeant with clinker residue and cement hydrate in the concrete, and cracking of the cement matrix due to shrinkage and load-induced stresses. The interaction of water with the cement matrix results primarily in the dissolution of hydration products, particularly calcium hydroxide, and subsequent redeposition and crystallization downstream. This redeposition and crystallization, in turn, causes blockage of flow passages, which is observed as self- sealing. The self-sealing effect is especially pronounced after microcracking of the concrete structure, which opens up flow passages and at the same time exposes a large surface area of previously unexposed hydrates. Shrinkage cracks have a considerably greater effect on permeable porosity, than do load-induced cracks. This effect is shown by a significant increase in permeability after drying, often by two orders of magnitude, while load-induced cracks (even at 80 per cent of the failure stress) have little effect on permeability after the load is removed.

Testing was performed with permeameters, designed and build for this study, capable of measuring permeabilities of the order of K = 1015 m/s, with automated data acquisition, minimal sample preparation and rapid specimen change.

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