huangyhg

concrete slab - need low moisture for floor finish
i need to specify a structural slab on metal deck. the slab will have an adhered finish that requires a very low moisture content. what can i do to achieve a low moisture content in a timely manner, without waiting months for a curing process? will a vented deck help?
-jcali
you can use vented decks, i believe that they do help speed up moisture release. also, you can usually assume that a 28 day cure will be adequate, but if shorter times are required, moisture tests can be conducted to verify that the amount of moisture release is below that required by your floor covering.
you could spec. low w/c ratio, water reducer, steam cure and then dry it out more with dry heat and the vented deck. just make sure your concrete samples cure the same way. test at 7 days, 14 days and 28 days. don't allow any storage of construction materials on the deck until it meets your moisture content limit.
definitely specify low w-c ratio. use the largest aggregate you can use for the application (this will cut the water demand). use high range water reducing admixture if you need for workability. use vented deck. after initial cure, spread dessicant (dry-sorb or similar) on surface and allow to pick up as much moisture as can be done in your time constraints. clean the floor but don't use water. mop over with highly evaporative solvent if dusty (alcohol will work).
i can't find any manufacturers who make vented deck for a composite structural floor slab (i've looked into nj bouras, vulcraft, and epic). any suggestions? how is wet concrete placed on vented deck without leakage?
-jc
in my experience most floor covering contractors specify a moisture level which simply isn't practical. this has been with ground bearing slabs where there's only one route for the moisture release and when you apply the figures, you're looking at 6 months to achieve the spec. they have then offered a surface treatment which acts as a barrier. this treatment has always come as an extra cost but allows immediate follow on.
i alway feel they are generating extra revenue but without it they won't guarantee the finish.
my experience is similar to yours endeng. in past dealings with flooring people, they have asked for ridiculously low moisture requirements to avoid liability.
-jc
here's a couple of references on the topic:
"use and misuse of moisture-vapor emission tests on on concrete slabs", december 2003 issue of aci concrete international
letters: moisture-vapor emissions testing, july 2004 issue of aci concrete international, pages 13-15.
very interesting and shows how much variation there is between tests and testing kits.
one comment on endeng comment regarding the "upcharge" requested by flooring contractors for application of surface applied vapor barriers. it seems to me to be a "pay me now or pay me later" issue, nothing comes free. if you want to the protection of a surface applied vapor barrier to maintain the flooring guarantee, then specify it up front and then it is included in the orginial bid price.
there have been plenty of law suits over this topic and once you suffer the loss from a floor failure due to water vapor pressure, you'll wish you had spent the dollars, it is cheap insurance!

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