From the Technical Expert: Concrete Properties and Slab Preparation
The water-cement ratio determines the quality of the concrete, i.e. the amount of water in the mix compared to the amount of Portland Cement. The lower the percentage of water used, the higher the quality of the concrete, i.e. the compressive strength.
Most slabs are finished with a steel trowel (smooth) finish to accommodate vinyl carpet, and wood. Ceramic tile and dimensional stone require a broom finish. Portland Cement: a cement may be composed as usually 60 percent lime, 25 percent silica, 5 percent alum with minor amounts of iron oxides, gypsum, etc.
How do you get "satisfactory" concrete? First the ratio of constituents should be determined and consistent. The water/cement ratio should be controlled as this is an important factor. Of course more water is needed then is necessary for proper hydration and placement. This extra water needed for workability is called "free" water, even though you pay for it.
I have been asked a few times what slump is. Slump is a measuring factor to determine if the mixture contains the right amount or does it have excess water. This is measured by using a cone that is 12 inches tall, and has a 4-inch hole on the top and an 8-inch hole opening on the bottom. The cone, on the jobsite, is filled with fresh concrete, set down and then lifted. The difference between the 12-inch cone height and the resultant settling measurement is and indication of how much the concrete "slumped."
Generally the slump for a floor slab should be no more than 5 inches. Also generally, the lower the slump figure obtained it will result in stronger (PSI) concrete.
Hydration: The next step and extremely important to achieve satisfactory concrete is hydration, again, another question I often receive. Hydration is the chemical process resulting from the presence of water, within the mix that develops strong and satisfactory floor slabs. Hydration continues as long as water is present inside the slab, even thorough you may think the slab is dry by visual means or by touching the top. The hydration reaction may sometimes, in some construction modes, go on for 20 years.
For superior concrete, minus excessive drying cracking, hydration must be allowed to continue for a definite period of time. To do this, a concrete placement company must prevent rapid water loss. How is this accomplished? Most efforts involve concrete curing compounds of which there are many. Contrary to its name, concrete curing compounds do not cure the concrete. They are designed to slow down the water loss so that the concrete mass through continued hydration will cure itself. Other methods are the use of plastic sheeting, water ponding, fogging, wet burlap and in the past, (I don't believe this is done anymore) straw.
The ideal situation for hydration is to prevent water loss for 28 days, at which time the concrete has obtained approximately 80 percent of its strength. Concrete is alkaline by nature and remains alkaline. When in the curing process the surface may have a 12.5 alkaline reading. However, after completion the surface of the slab returns to neutral.
Sometimes when there is excess water and a weak friable surface is left, there may be dry alkaline salts remaining on the surface. A good practice is to use water to properly prepare the surface (i.e. remove alkaline salts).
Strength: For commercial floor slabs the American Concrete Institute (ACI) in ACI 302 1R-96 Guide for Concrete Floor Slabs state that the slab should have a compressive strength of 3500 PSI (pounds per square inch).
Prepping: Prepping concrete slabs for vinyl is extensively covered in ASTM-F710. Standard Practice For Preparing Concrete Floors To Receive Resilient Flooring.
Basically all contaminants, such as oil, drywall compounds, wax, paint, etc. must be removed. Do not use solvents as they may still exist in the concrete and attack your installation adhesive and/or your vinyl in the form of bubbles. The use of a grinder may be necessary.
Expansion joints filled with elastomeric caulk are constructed to control slab movement. Vinyl installed over these joints, even if covered with cementitious underlayment, may buckle when the slab moves. Use suitable expansion joint covers that are available.
All irregularities in the concrete slab must be corrected to eliminate telegraphing. This may be done, according to the severity, by using Portland cement patches or levelers or by grinding.
Ceramic Tile and Dimensional Stone
These products require a more severe degree of correct flatness and also require a broom finish to the slabs surface to allow the bonding mortar to make a physical "grab."
It is important not to install over concrete curing compound, as like, above it limits physical "grab" of the bonding mortar.
A quick check of porosity is to drop a few beads of water on the concrete slab. If the beads do not readily dissipate into the concrete, some type of surface barrier is present. This may require bead blasting.
For natural stone, the Marble Institute recommends a crack isolation membrane over all concrete slabs.
For thin-set ceramic installation, the concrete sub floor variation should not exceed a plane of 1/4-inch in 10 feet. Undulations should not exceed 1/16 per foot within a 10 foot span.
Expansion joints cannot be covered. The expansion joint must be carried up through the ceramic tile or stone joints which are then filled with flexible caulk, as per TCA detail EJ171-03.
Concrete on-grade slab preparation is especially critical. Of extreme importance is moisture vapor control. Wood is hygroscopic; that is, it absorbs and releases moisture constantly.
Most wood producers require a moisture meter reading of 4.5 or below to indicate a safe level of moisture. Others require the moisture dome test with a maximum of 3 lbs. of water after 24 hours within a thousand square feet.
There is an on going effort within the industry to develop moisture barriers. At this point in time, some wood producers recommend installing sheet vinyl with a felt back prior to installing the wood flooring. Others recommend a 2-part epoxy coat, while others have tried latex based coatings. There is now an effort to introduce urethane as a barrier.
Water is wood's worst enemy. Prepare!