A concrete slab-on-ground is the heart of an industrial facility. The means are at hand to thwart operational disruption in these buildings. We have it within our power to produce and place concrete in a way that will help solve the two most egregious problems associated with slabs on ground: high shrinkage leading to excessive random cracking and curling and uncontrolled vapor transmission. Such a mix simultaneously serves environmental sustainability standards by reducing cement content while achieving the highest serviceability characteristics and the lowest maintenance expenditures.
The adverse consequences of uncontrolled vapor transmission through concrete slabs-on-ground have risen to a dilemma of epic proportions in the construction industry. This problem— driven by fast-track construction, changes in the behavior of modern floor coverings and adhesives, and the restrictions on the use of volatile organic curing compounds— has added impetus to the discussion of vapor barrier positioning under floors, particularly in the office areas of large warehouses and distribution centers destined to receive adhered finishes such as resilient tile and carpet.
After concrete placement the floor slab begins to dry as batch water not consumed by hydration leaves the concrete. This process, known as outflow, is influenced by external factors such as site drainage, ambient temperature, humidity, wind, and direct sunlight. The upward migration of this moisture from within the slab and other moisture from below the slab-on-ground can lead to soluble alkali salts being placed into solution, which will raise the pH at the surface of the slab above the tolerable limits for today’s adhesives. This phenomenon and the moisture itself can lead to the failure of adhered finished materials including, but not limited to, blisters and loss of adhesive bond.
Vapor Barriers
The position of the vapor-retarding membrane under a slab has long been the subject of research and conjecture. In 1981, a research program conducted by Leo Nicholson (described in a September 1981 article in Concrete Construction), used three test slabs, with and without vapor barriers, formulated of concretes with varying cement contents and water-cement ratios. The findings led Nicholson to proclaim that serious plastic shrinkage cracking in slabs-on-ground could be virtually eliminated by placing the slab on a pervious sand layer. The slabs he placed directly on impervious polyethylene suffered “serious cracking.” Nicholson’s results, however, were drawn from three concrete test mixes laden with excessive amounts of water.
ACI Committee 302’s Guide for Slab-on-Ground Construction has historically agreed with Nicholson’s conclusion and cautioned that a vapor-retarding membrane placed directly beneath the concrete slab exacerbates plastic and drying shrinkage and the related cracking and curling, because the bottom of the slab remains wet while the top continues to dry. The 1989 version of ACI 302 advocated avoiding vapor barriers wherever subsurface ground conditions permitted, and otherwise suggested placement of a pervious fill layer between the bottom of the slab and the vapor barrier, despite the fact that a relatively thin layer of porous fill subbase strewn with load transfer devices is all but unmanageable under the wheels of a laser screed.
Beginning in 1996, this publication drew a distinction between the terms “vapor retarder” and “vapor barrier” (with vapor barriers having a water-vapor transmission rating of 0.00 perms). The 1996 edition featured a cross-sectional diagram showing the concrete slab on a layer of compactible granular fill acting as a blotter atop the vapor-retarding membrane (with a minimum 10-mil thickness). Regarding positioning of the “true” vapor retarder, the current document (2004 version of ACI 302) recommends that each installation be evaluated on a case-by-case basis but does direct that the slab be placed directly on a vapor barrier or retarder when a moisture-sensitive floor covering is to be installed.
Water Content
Traditional notions about concrete for slabs on ground do not distinguish between water-cement ratio and actual water content, especially at the point of placement. And mix designs that specify both water-cement ratio and compressive strength are not uncommon, although these two quality markers are not mutually compatible.
With slab-on-ground concrete, water-cement ratio is ultimately derived, not specified. The water content has historically been governed by the water demand of local aggregates and proportioned to produce a placeable mix with an initial slump of 3 to 4 inches. Producers routinely offer a menu of mixes, prepared in accordance with ACI 211 recommendations, featuring an array of ascending strengths coinciding with descending water-cement ratios, in which the water content remains essentially the same. The w/c fraction can be manipulated simply by juggling the numerator and denominator.
