Q: We recently heard about "controlled-permeability formwork," which drains water away from the surface of freshly placed concrete, improving surface appearance and reducing the lateral pressure of the concrete against the form. Isn't there a danger that too much water will escape and not enough will be left for proper hydration of the cement?
A: Most concrete mixes have excess water, over and above that needed for cement hydration, simply to make it easier to handle and place the mix. The evidence we have found suggests that only this water, or part of it, is withdrawn through controlled-permeability formwork or form liners. The liners drain water by gravity; there is no vacuum effect. One of the strongest demonstrations that sufficient curing moisture remains is the well-documented improvement in durability of the concrete surface. In the article "Introducing Controlled-Permeability Formwork" (Concrete Construction, February 1991, pp. 198-202) author Tom Harrison indicates a reduction of about 20% in the water-cement ratio at the concrete surface as one reason for the improved durability.
One manufacturer of a controlled-permeability membrane points out that the membrane itself holds about a half liter of water per square meter, and this water remains available for curing as long as the forms stay in place. After the forms are removed, the denser concrete surface tends to retain moisture more than do conventionally formed concrete surfaces. However, specified or recommended curing practices should be followed after form removal, particularly in hot, dry climates.
European tests of several different concrete mixes, some containing fly ash and some with ground slag, showed reductions in carbonation, surface absorption and chloride diffusion, and significant improvements in surface strength and frost resistance when the controlled-permeability liner was used. For both wet-cured and uncured concretes, surface strengths were as much as five times those of conventionally formed concrete, suggesting an excellent level of cement hydration. Other improvements can be attributed to reduced porosity at the concrete surface. Thus, it appears that the amount of water drained away by a controlled-permeability liner does not have an adverse effect on the concrete.
In answer to the question discussing the effect of controlled-permeability formwork on curing, you say: "Most concrete mixes have excess water over and above that needed for cement hydration, simply to make it easier to handle and place the mix. The evidence we have found suggests that only this water, or part of it, is withdrawn through controlled-permeability formwork or form liners. The liners drain water by gravity; there is no vacuum effect." Actually, the only water removed by absorptive forms (or vacuum processing) is the water that could (and would) bleed if the setting of the concrete was delayed until all the cement and aggregate had settled as much as it could and the bleedwater was on top. This effectively reduces the water-cement ratio, and hence increases concrete strength.
- Bryant Mather, Department of the Army Waterways Experiment Station Corps of Engineers Vicksburg, Miss.