Critical Threshold for Concrete Moisture Content
by Paul Potts
This article is about prescriptive specifications and focuses on 100-mm (4-in.) interior concrete slab on grade for schools, offices, hospitals, libraries, and other commercial buildings that will receive moisture-sensitive or bonded floorcoverings.
A basic specification stating the requirements for strength and slump is the simplest way to specify concrete for interior slab on grade—for example, 24 MPa (3500 psi) compressive strength concrete with a 100-mm (4-in.) slump. This simple performance specification serves well for many applications. The strength satisfies the structural engineer’s requirements and the slump reduces the cost of placement. It is called a performance specification because it states how the concrete is to perform, but says nothing about the mix design, proportions, or ingredients.
Choosing to write a performance specification is a policy decision. It leaves the actual mix proportions up to batch plant operators, who will select a formula from their library of previously proven mixes. By writing a performance specification, the architect-engineer or specifier gives up any refinements to the design, but shifts liability for performance of the product away from themselves to the contractor and batch plant operator.
When the required performance of concrete becomes more complex than the simple requirements used in the above example, a design professional may prefer to write a prescriptive specification with all the details of how they would like the concrete designed, proportioned, and perform in actual applications. A prescriptive specification may include:
- water-cement (w/c) ratio (i.e. the weight of mixing water divided by weight of cementitious materials);
- minimum cement content;
- maximum allowable cement substitute;
- maximum size and minimum quantity of coarse aggregate;
- air content;
- fiber; and
- any allowable admixtures.
Some batch plant operators and concrete contractors are opposed to the designer writing a complete prescriptive specification, because it limits their options and overlooks their experience in proportioning concrete. This author would recommend the designer write a complete prescriptive specification and collaborate with the concrete company during post-bid interviews to discuss any suggestions. I suggest doing this at postbid before contracts are signed.
In addition to the basic properties of workability, strength, and economy, the desired characteristics of hardened interior slab-on-grade for commercial buildings are:
- flatness (i.e. minimal curling);
- minimal long term shrink cracking; and
- moisture content that will not unduly delay the start of moisture-sensitive floor coverings.1
A low w/c ratio mix design and proper quantity, sizing, and grading of coarse aggregates and proper curing methods are the basics to achieving these results.2
A mix designed with a low w/c ratio (at or below 0.45 w/c) results in a flatter concrete surface with less curling and long-term shrinkage cracking; it reduces by months the time required to get the relative humidity (RH) of the slab to a point where it is safe to apply moisture-sensitive floor coverings.3
The most direct way to reduce moisture content in concrete is specifying a low w/c ratio mix design. (Concrete with low water-cement ratios will require a mid-range water reducer to improve workability). The compressive strength of concrete is directly related to the w/c ratio—as the strength goes down water-cement ratio goes up putting more water in the concrete.4
Many structural engineers are comfortable with 1588 kg. (3500 lb.) slab-on-grade concrete for commercial buildings. True, a 1588 kg. (3500 lb.) compressive strength concrete is adequate for applied loads in commercial buildings (and is very workable), but it will have a w/c ratio of 0.60, which has too much surplus water. The extra water increases the shrinkage potential of concrete making it prone to cracking and curling; and the added moisture takes extra months to evaporate to a level acceptable to apply moisture-sensitive floorcovering adhesives.
Critical water threshold for hydration
In May 2010, the National Precast Concrete Association (NPCA) published an article entitled, “Water-to-Cement Ratio and Aggregate Moisture Corrections” in its Precast Magazine.5 “[T]o achieves complete hydration, all the pores within the system must be filled with water throughout the hydration reaction,” wrote Adam D. Neuwald. “If a w/c ratio of 0.36 were used, the pores would not remain full during the entire reaction; thus, to achieve 100 percent hydration, a minimum w/c ratio of 0.42 is required.”
The moisture content of freshly mixed concrete must be at a critical threshold for the optimal performance—neither too little water nor too much. There must be enough water to completely hydrate the cement and keep up with water lost to evaporation in the hours before curing is started, but not so much as to leave a large surplus behind that will evaporate, causing the slab to contract exacerbating shrink cracking and curling.
The concrete mixture require a minimum 0.36 w/c ratio to completely hydrate the cement, but extra water above 0.36 is needed to improve workability and make up for the water lost to bleed water and evaporation. Experts differ on the specific critical threshold, but most conclude it is around 0.42. w/c ratio. The optimal maximum water-cement ratio for floors on grade in commercial buildings is 0.45 w/c ratio. This mix design results in stiffer, harder-to-place concrete that will need a medium range water reducer to aid with placement.6
Extra moisture in concrete over and above what is consumed by hydration with cement is locked in the gel and capillary pore structure and takes months to evaporate to a level acceptable to apply moisture-sensitive floorcoverings. Surplus water is necessary to an extent to improve workability, but when added in excessive amounts, simply to ease placement, is a detriment to the desirable qualities of concrete.7
Negative consequences of surplus mix water include:
- its eventual evaporation causes the slab to contract excessively, worsening long-term shrink cracking;
- loss of surplus water at joints and edges of the slab, and poor aggregate selection, worsens curling; and
- surplus moisture locked in the pores of the concrete evaporates only slowly over time, maintaining an elevated core RH that delays the installation of moisture-sensitive floor coverings.
Moisture-sensitive adhesive failures
In previous years, solvent-based adhesives were impervious to moisture, but these products are now banned by the Clean Air Act because they are volatile organic compounds (VOCs) that contribute to ground level ozone or smog. Solvent-based adhesives have been replaced by water-based products that degrade in the presence of moisture. Thus, the problems we are faced with today to get the concrete dry enough to apply the floor covering.
Moisture-sensitive adhesives fail when surplus moisture trapped in the pores of the concrete is released over time and rises to the adhesive bond layer degrading the adhesive. To protect themselves, adhesive manufacturers now require concrete be dried to a low central core RH before the adhesive is applied or the warranty is voided. If a properly placed vapor barrier is not in place, groundwater from under the slab brought to the surface by capillary action will keep the slab moist forever.8
Curing concrete and the drying period
Concrete does not harden by drying, but rather the cement in concrete needs moisture to hydrate and become hard. As there is limited surplus water to lose to evaporation in dryer mixtures, with low w/c ratios (i.e. 0.45 w/c ratio and below) curing must begin as soon as the concrete is hard enough to resist roughing the surface.9
Curing keeps critical water from evaporating before the hydration of the cement is complete. However, after seven, the curing products must be removed so evaporation of surplus water can begin. Extended curing times (beyond 10 days) unnecessarily delays the start of the drying of concrete.10
The best curing product for slabs that will receive moisture-sensitive adhesives is polyethylene film that can be removed after 10 days without residual effect. Water curing and burlap water curing adds new moisture into the slab and should be avoided when possible. Additionally, membrane-forming curing compounds are not recommended for slabs receiving moisture-sensitive floorcoverings, because it is difficult to predict when they will dissipate (short of sandblasting them off).11
Long-term shrinkage cracking
Above a critical threshold, water is the enemy of good concrete proportioning. For this article’s purposes, that threshold is a w/c ratio of 0.45. – water above this ratio incrementally leads to more long-term shrinkage cracking and curling, and delays floor covering installation.
Long-term (as opposed to plastic) shrinkage cracking occurs when the loss of excess water causes the slab to shrink, but it is prevented from contracting uniformly by the friction with the subgrade or mechanical penetrations. By way of illustration, if a 30 x 30-m (100 x 100-ft), 100-mm (4-in.) thick concrete slab was poured on a perfectly flat slippery surface where the concrete could shrink uniformly, no shrink cracking would occur.12
Long-term shrinkage cracks will telegraph through bonded floor coverings such as thin set cementitious or epoxy terrazzo. Concrete with 0.45 w/c ratio produces strong 2268 kg (5000 lb.) compressive strength concrete with minimal long term shrink cracking. Mixtures with higher water-cement ratios above 0.50 have much more water and thus greater shrinkage potential. Lab work done by the Massachusetts Institute of Technology (MIT) shows for each one percent decrease of mixing water, concrete shrinkage decreases by two percent.13
Minimizing cement paste to reduce total water
Coarse aggregates for concrete are irregular in shape, causing gaps between the individual stones of the same size even when compacted tightly together. In concrete terminology, these gaps are called void spaces that must be filled with cement paste to make high quality concrete. Reducing the void space reduces the cement and water (cement paste) requirement—a basic goal of designing concrete for floors as explained in this article.14
The void space can be reduced when larger maximum size aggregates are used and two or more aggregate sizes are combined. Larger allowable maximum size graded aggregates ranging between 9.5 and 38 mm (3/8 and 1 ½ in.) have less void space than smaller maximum size aggregate ranging between 9.5 and 19 mm (3/8 and 3/4 in.), for example. The larger the maximum size stone allowed in a graded mixture, the less paste required.15 But, the size of the largest aggregate must be consistent with physical restraints of formwork and other building elements.
Size and quantity of aggregates and paste content
Aggregates, the least expensive material in concrete (other than water), make up 60 to 75 percent of the total volume of wet concrete—they contribute significantly to the economy of concrete as a building material. The volume relationship between aggregates and cement paste is such that increasing the quantity of aggregate decreases the quantity of cement paste needed to make a cubic meter (cubic yard) of concrete.
Also, aggregates in concrete must float in a matrix of paste and be completely coated with cement paste. Larger maximum-size coarse aggregates have less surface area than an assortment of smaller coarse aggregates; the larger aggregate requires less cement paste to coat all the surfaces, reducing the paste and thus the total water requirement of the mixture.16
Therefore, greater quantities of graded coarse aggregates and larger allowable maximum size coarse aggregates reduce the paste requirement for a concrete mixture, reducing the shrinkage potential.17 For these reasons, careful prescription of the maximum size and grading of coarse aggregates has a significant impact on the total water content in concrete.
There are more benefits of greater quantities of course aggregate. Aggregates have a lower shrinkage potential than cement paste; as the quantity of coarse aggregate is increased in the mixture, the shrinkage potential of the slab is reduced. Additionally, large coarse aggregate exerts an internal drag on the shrinkage movement of concrete, counteracting the shrinkage potential.18
Where are contractions joints necessary?
Hand-tooled or saw-cut contraction joints in concrete floors are for purely aesthetic purposes. They guide concrete to shrink crack in uniform patterns more pleasing to the eye, instead of unpredictable random cracks. From another perspective, contraction joints are the source of most curling in concrete slabs. The question then becomes, do you always need contraction jointing? The answer is no.
Concrete is perfectly flat when random cracking is allowed instead of tooling contraction joints. For this reason, it may be beneficial to omit contraction jointing altogether under carpeting, linoleum, and floating wood gymnasium floors. Concrete allowed to randomly crack must be made from a low-shrinkage mix design and incorporate welded wire reinforcement to keep the joints tightly together. While synthetic fiber mesh is not a substitute for welded wire reinforcement in this application, steel fibers in the right proportions will work.
A few extra tips
Concrete for anchored wood gymnasium floors should not exceed 1814 kg (4000 lb.) as power-actuated anchors will not penetrate higher-strength concrete. (The anchor will just knock a spall out of the surface of the concrete.)
Inspectors and construction administrators should be aware of the difficulty in removing rinse-water from the mixer drum that has been left over from a previous truckload (if it is ignored when refilling the truck, it becomes unaccounted for mix water). The inspector should review this issue with the batch plant operator to come to an understanding of how leftover water from the last load will be accounted for in the fresh load.
There is a critical threshold for water in concrete—there must be enough water to hydrate the cement, but not so much as to worsen shrinkage cracking and curling, or exacerbate delays in the application of moisture-sensitive floorcoverings. A certain amount of excess water is needed to aid in placement and completely hydrate the cement allowing for water lost to evaporating before curing is started. But, placement cannot become the driving force in determining the amount excess water, if moisture sensitive adhesives are the concern. Workers must not be allowed to hinder the overall schedule by adding water at the site. The designer must anticipate this problem and add a minimum requirement for midrange water reducer to the specification.
The surest way to stay safely within these parameters is to specify a low water-cement ratio for the concrete and supplement the benefits of a low w/c ratio with careful selection of size and quantities of a graded aggregate. However, one must be mindful a low w/c ratio mix design requires a timely application of curing materials.
1 For more, see page 24 of the American Concrete Institute’s (ACI’s) Guide for Concrete Slabs that Receive Moisture-sensitive Flooring Materials, reported by ACI Committee 302.
2 This comes from the Portland Cement Association’s (PCA’s) Volume Changes of Concrete (1967), specifically the Robert F. Ytterberg article, “Control of Shrinkage and Curling in Slabs on Grade.”
3 See page 26 of the resource cited in note 1.
4 This comes from page 77 in the PCA Design and Control of Concrete Mixtures.
5 Visit precast.org/2010/05/water-to-cement-ratio-and-aggregate-moisture-corrections.
6 Optimal maximum w/c ratio for commercial buildings is 0.45. For more, see Ken Hover’s Summer 2002 article in Concrete News. Tech Stuff: Curing and Hydration
7 See Researchgate Publication 229748, Chapter 6.11, “Cements as Porous Materials,” by Jeffrey Thomas and Jeffrey Chen (Northwestern University’s Departments of Materials Science and Engineering, and Civil Engineering).
8 For more about capillary action in concrete, see National Ready Mixed Concrete Association (NRMCA) CIP-28, Concrete Slab Moisture. Visit www.nrmca.org/aboutconcrete/cips/28p.pdf.
9 See page 4 in the PCA Design and Control of Concrete Mixtures.
10 See note 3.
11 Ibid, page 22.
12 PCA Design and Control of Concrete Mixtures Caper 13, “Volume Changes of Concrete,” on page 151.
13 For more, visit www.engr.psu.edu/ce/courses/ce584/concrete/library/cracking/dryshrinkage/dryingshrinkage.
14 See PCA Design and Control of Concrete Mixtures, page 33.
15 Ibid, pages 31 and 32.
16 Ibid, page 35.
17 See PCA’s Concrete Floors on Ground, page 22.
18 Ibid; the size and amount of coarse aggregate can be found in Chapter 4 (page 21).