Concrete Slabs on Grade
The problems associated with concrete slabs are many. Shrinkage, curling, porosity, flatness and dustiness are a few of the problems we face in our efforts to install an acceptable floor.
The concrete industry tries very hard to overcome these problems with their formulations. It is not just cement, water, and aggregates that are put in the cake mix. All concrete mixes have one or more additives called admixtures blended into the mass. For example, there are accelerators, retarders, water reducers, air entraining chemicals (for freezing conditions), and latex modifiers. Most retarders, used in warm weather, slow down the initial set of the concrete to let hydration continue. Most retarder admixtures are also water-reducers, which lowers porosity. Accelerators are used in cold weather situations because concrete hardens slowly at temperatures below approximately 50˚ F.
Approximately one-half of the total amount of shrinkage that will ever occur in concrete happens in the first 60 days after casting, as shown in curves in most concrete text books. The remaining half of shrinkage happens over approximately 20 years. For practical purposes, shrinkage is complete within the first three years after casting. So if a floor is placed within the first several months and shrinkage cracks have developed before the floor is placed, there is a real possibility that significant widening of the cracks will occur after the floor is in place.1
Shrinkage is a basically a result of a high water cement ratio. A high water cement ratio also results in lower strength concrete. High water content can also cause “bleeding” on the surface, as the aggregates and cement particles will start to settle, causing a weak surface. Most concretes have w/c ratios of 0.4 to .07. That is 4.5 to 8 gallons of water per 94-pound bag of cement.2 The simple equation for a w/c ratio is to divide the lbs. of cement into the pounds of water being used. We in flooring would like a ratio of 0.5 to .06 for best density and permeability.
Types of Portland Cement
Since most of us have always considered cement as just cement, here is an overview of cements available. • Type I normal cement — General purpose, widely used in slabs on grade. • Type II moderate sulphate resistant — Used in coastal or marine areas. • Type III high early strength cement — Generally develops more heat in hydration and more shrinkage. • Type V high sulphate resistant — Used when exposed to high-sulphate conditions such as areas where soils or ground water have a high sulphate content.
The use and installation of vapor retarders continues to be problematical. To use or not to use? To place sand on top or place concrete direct to retarder on soil? I have seen failures where lateral moisture migration into the sand layer over the retarder resulted in a constant wet environment that had no place to go but up and affected the flooring installation.
The popular polyethylene film (10 mil) is low in cost and easily installed but it is almost impossible to seal the film at overlaps. Plus, it is easily punctured, some times deliberately.
There are many misconceptions about alkali, about what it is, how it “moves”, where it comes from, how to measure it, and how to handle it. First, you should understand how important alkalinity is. In our own instance, if the pH of our blood were lowered one unit, we would die.3 There are many other examples of the importance of alkali from farming to manufacturing processes. Our concern is in floor covering installation, since the result of high pH can be damaging to our work. Concrete is normally high in pH (12+). This is a necessity to protect the reinforcing materials. As long as these salts remain inert in solid form, we have no difficulties. One misconception is that the removal of surface salts, which are high in alkalinity, and deposited there due to bleed water and concrete troweling, is best removed by hydrochloric (muriatic) or acetic acid. But this can result in leaving water-soluble calcium salts. The best way to remove salts from the surface is to clean with water. Salts within the slab can be carried when moisture is present. Another point to remember is that salt deposits are deposited primarily from the slab itself and not from the soil.4 A pH reading of up to 9 indicates a suitable slab surface for your installation. A quick reading can be obtained with pH paper/distilled water or chipping the slab slightly and putting a few drops of phenolthalein down. If it turns red and the pH paper turns purple this indicates that both moisture and alkali are present.
Carbonation is a chemical reaction in which carbon dioxide in the air or water reacts within the concrete to form carbonates. Carbonation increases drying shrinkage and lowers the alkalinity of the concrete. If the concrete does not have steel re-enforcing it is of little concern to the flooring contractor. If the concrete does contain steel and carbonation (lowering alkalinity) reaches the steel, corrosion of the slab begins. Excessive carbonation is an indication of poor quality concrete, poor ingredients, inadequate curing, or poor surface densification. Sprayed phenolphthalein is used for discovery.
Footnotes 1 Ken Bondy, Consulting Structural Engineer 2 Concrete Craftsman Series CCS-, ACI 3 J. E. Nelson, Chemtrix Inc. 4 AWI Research Department