{"id":15705,"date":"2016-03-04T14:52:49","date_gmt":"2016-03-04T19:52:49","guid":{"rendered":"http:\/\/www.constructionspecifier.com\/?p=15705"},"modified":"2016-12-15T11:27:31","modified_gmt":"2016-12-15T16:27:31","slug":"dry-concrete-for-moisture-sensitive-floorcoverings","status":"publish","type":"post","link":"https:\/\/www.constructionspecifier.com\/dry-concrete-for-moisture-sensitive-floorcoverings\/","title":{"rendered":"Dry concrete for moisture-sensitive floorcoverings"},"content":{"rendered":"
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Photo courtesy Great Lakes Floor Covering<\/span><\/figcaption><\/figure>\n

by Paul Potts
\n<\/strong>There are two major sources of moisture in fresh concrete\u2014excess mixing water left over after hydration of cement, along with natural groundwater beneath the concrete that moves to the surface by capillary action. This article describes how to control the sources of moisture in concrete slabs, minimize drying wait time to meet floorcovering manufacturer\u2019s requirements, and determine when the slab meets those requirements.<\/p>\n

Concrete is a combination of cement, water, and aggregate; the specific proportions are called the mix design. When water and cement are combined, a chemical reaction called hydration occurs and then continues as long as there are particles of cement and new water to feed the process. Curing concrete is the procedure for keeping water in the concrete to prolong hydration until the optimal qualities of concrete have been realized. The longer the curing process continues, the better the concrete.<\/p>\n

Excess water (i.e.<\/em> what is needed for hydration) is figured into the mix as an aid in placement. If this excess water is not added, the concrete will be too stiff to place economically. Once hydration has progressed far enough to produce the desirable qualities of hardened concrete, the excess water must be allowed to evaporate for the concrete to become dry enough for moisture-sensitive floorcoverings. This is called the drying stage. Together, curing and drying equal the interminable \u2018wait time.\u2019<\/p>\n

The excess water gathers in the pores of the concrete, first as water when the concrete is wet, and then as water vapor that produces a level of relative humidity (RH) in the concrete.\u00a0<\/sup>(For more, visit www.59.167.233.142\/publications\/pdf\/Moisture.pdf<\/a>). The RH of concrete is a good barometer of when the concrete is safe for moisture-sensitive floorcoverings, such as vinyl tile, resilient sheet flooring, linoleum, vinyl-backed carpet, and cork and wood flooring.<\/p>\n

During the drying stage, excess moisture evaporates at the surface of the concrete, while moisture deeper in the slab moves up near the surface, where it too gets whisked away by an exchange of relative humidity. The drying stage is a straightforward process\u2014assuming there is no groundwater replenishing the moisture in the slab, nothing is atop the slab preventing evaporation, nothing is rewetting the concrete from the surface, and the ambient air humidity is less than the humidity in the concrete. As long as adverse factors are absent, the wait time for the concrete to dry is directly related to the water-cement (w\/c) ratio and the RH around the concrete.<\/p>\n

Water-cement ratio and total water
\n<\/strong>The w\/c ratio and the total water content have the most direct effect on the amount of excess water that must evaporate before moisture-sensitive floorcoverings can be applied. The w\/c ratio is the weight of water divided by the weight of cement in a unit of concrete. Wait time can be reduced significantly by specifying a low w\/c ratio and using less cement. Low water-cement ratios and less cement decreases the amount of mix water and, thus, reduces drying time. Excess water can also be reduced by using the largest recommended graded coarse aggregates.<\/p>\n

A water-cement ratio of 0.45 is optimal for slabs on grade. It produces high-quality concrete with shorter drying times. A higher ratio of 0.50 increases long-term shrink cracking and can take a month longer to dry to the floorcovering manufacturer\u2019s requirements.<\/p>\n

Concrete with a w\/c ratio of 0.45 will be quite stiff coming off the truck and the contractor may want to add a plasticizer for convenience. However, under no circumstances should extra water be added onsite.<\/p>\n

Six sacks of concrete (40 kg [94 lbs] per sack) for commercial building slabs on grade is unnecessary if 5.5 sacks will suffice. More cement at a given ratio requires more water, adding to the drying wait time.<\/p>\n

Many architects and engineers are not practiced in specifying concrete by the water-cement ratio formula, so they specify strength and slump alone. Contractors bidding the project should check the cast-in-place concrete specification and the structural drawings for the water-cement ratio. If it is not stated, they should bring it to the attention of the architect-engineer during a pre-bid or pre-award conference.<\/p>\n

\"Low
Low water-cement (w\/c) ratio material is stiff and the contractor may want to add a plasticizer for convenience when offloading it from the truck.<\/span>
Photos courtesy Paul Potts<\/span><\/figcaption><\/figure>\n

Retarders and barriers
\n<\/strong>Moisture from under the slab enters the concrete by capillary action\u2014the ability of a liquid to flow in narrow spaces (pores) without the assistance of, and in opposition to, external forces like gravity. Moisture entering the concrete from under the slab will make it almost impossible to get the concrete dry enough for moisture-sensitive floorcoverings.<\/p>\n

If groundwater is present in the building footprint, the architect-engineer should design subsurface drainage to route the water elsewhere. Regardless of flowing water or a high perched watertable, it is prudent to design for a moisture retarder in all circumstances, or manufacturers may void their warranties in the case of moisture-related problems.<\/p>\n

American Concrete Institute (ACI) 302.1, Guide for Concrete Floor and Slab Construction,<\/em> differentiates between vapor retarders and vapor barriers. Retarders are plastic sheets 254 \u00b5m (10 mil) thick, with a perm rating less than 0.2085 metric perms (0.3 U.S. perms). Vapor barriers are 381-\u00b5m (15-mil) thick plastic sheets with perm ratings of 0.00695 metric perms (0.01 U.S. perms). Almost all sheet vapor protectors used in the past have been vapor retarders by this definition.<\/p>\n

In the past, moisture protection has rarely been carefully selected, placed, or maintained. However, with modern water-based adhesives substituting for solvent-based adhesives, floorcovering failures have become more financially threatening and have brought more attention to the proper installation of a sheet vapor retarder. Thicker sheets mean punctures caused by equipment or workers are less likely. Both careful installation and paying close attention to overlapping seams and holes in the sheets caused by infrastructure penetrations will ensure protection provided by the vapor retarder is more reliable.<\/p>\n

The ACI 302 guide now recommends vapor retarders be placed directly under the concrete, with a cushion layer of sand between it and the aggregate subgrades. This sand cushion prevents punctures in the sheet and minimizes long-term shrink cracking in the concrete by allowing the slab to slip on the smooth surface without being restrained by the irregular aggregate.<\/p>\n

To ensure proper construction of the vapor retarder, the 152-mm (6-in.) overlapped seams must be sealed with tape. This tape should also be used to carefully seal punctures and holes around piping and conduit. Even a small pinhole in the moisture retarder will be a continuous source of moisture in the slab.<\/p>\n

Curing of concrete
\n<\/strong>Evaporation is the process by which water changes from a liquid to a gas or vapor. It is the primary pathway water uses as it moves from the liquid state to atmospheric vapor.<\/p>\n

Making concrete requires the hydration of cement by exposing it to water and heat\u2014the chemical reaction continues as long as there are cement particles in the presence of water. Curing prevents moisture from prematurely evaporating, keeping the chemical reaction going until the best qualities of hardened concrete are achieved.<\/p>\n

After concrete starts to harden, the hydration process begins to slow down. However, if the moisture evaporates too quickly, hydration halts\u2014leaving the concrete under-strengthened. Applied curing measures retain moisture in the concrete and must be kept in place until all the moisture required for developing the optimal characteristics of hardened concrete is used up, but the wait time for applying moisture-sensitive floorcoverings is increased by the same amount of time. The longer the curing measures are kept in place, the better the concrete; however, this also increases the wait time.<\/p>\n

To minimize waiting, curing products and practices must be selected on the basis of their rewetting characteristics and the amount of time they keep the concrete sealed. In the case of slabs for moisture-sensitive floorcoverings, the curing products must dissipate or be mechanically removed to allow evaporation of excess water to begin. Curing products vary as to their rewetting characteristics and how long they must remain in place.<\/p>\n

For this article\u2019s purposes, curing methods can be divided into three classes:<\/p>\n