Conclusions

New equipment for monitoring water movement in cement-based materials with sub-millimeter resolution has been demonstrated. The initial experiments presented in this paper suggest that:

• 1) because of the rapid rearrangement of water due to capillary forces, small specimens of fresh cement paste dry out in a relatively uniform fashion as opposed to exhibiting the sharp drying front commonly observed in porous materials,
• 2) the w/c=0.45 cement pastes cured for several days or exposed to a milder drying environment do exhibit a somewhat sharp drying front (perhaps due to the density gradient established by the initial settling of the paste), and
• 3) in layered cement paste composites exposed to drying, water preferentially moves from the less dense to the more dense paste; this mechanism is also observed in sealed specimens as empty internal porosity (self-desiccation) is created due to chemical shrinkage.

The implications of these observations for the field curing of concrete have been discussed. At early ages, the movement of water from less dense paste regions to regions of higher density could produce or intensify local flaws. The ITZ regions in a concrete are one naturally occuring area of higher w/c ratio that could contain large empty pores due to self-desiccation and water rearrangement. Proper consideration of pore structure and capillary forces can lead to the optimization of the curing of high-performance concrete. Controlled permeability formwork and saturated lightweight aggregates are two practical examples of the application of these principles.

The results also have implications for the application of repair materials to existing concrete. If the concrete to be repaired is relatively dry and contains a fine pore structure, it may "draw" out the needed curing water from the repair material, leading to insufficient strength development and cohesive failure of the latter. Conversely, a repair material with a fine pore structure may be able to draw needed curing water from the saturated original material to offset any water lost to the external environment.