These fine cracks are very small but show signs of moisture migrating up through them.


It’s time to answer some new letters from readers. This month, we will talk about various issues related to concrete and concrete slabs as they pertain to the installation of resilient flooring.


Q: I am confused by the terminology for the types of concrete joints and what they do and whether they can be covered or not?

A: I find a lot of confusion as it relates to concrete joints. There are basically four types of concrete joints: Isolation, Expansion, Contraction or Control (saw cut), and Construction. All joints are subject to movement until the slab is dry and stabilized. Even then, the isolation, expansion and construction joints are prone to movement and should be honored. Control joints will stabilize and can be gone over, but an installation begun too early will result in joint show-through.  

To avoid confusion for nomenclature for joints I have gone to a different terminology: Dormant or active. One way to determine if a joint is active is to spread plaster-of-Paris across the joint and allow it to dry. Return a week later to see if there is a fine crack along the joint. If there is a crack, it is active. If no crack appears, it is dormant.


Q: How long will new concrete tend to crack?

A: To quote a friend: “All concrete cracks and that’s a fact.” About 80% of concrete cracking occurs during the first year and the final 20% occurs two to three years after. If there is still cracking after three years, it is generally a sign that something structural is causing it.    


Q: What factors determine concrete movement?

A: There are two major factors that cause concrete to move: temperature and moisture (humidity). Concrete will move twice as much due to changes in temperature, compared to moisture. For example, if an exterior concrete slab receives rain, the moisture on the slab’s surface will cause the slab to curl down. Once the concrete slab dries, the slab will relax and flatten out.


Q: What is the best way to prepare a concrete surface that has an adhesive on it?

A: This is a tough question, as the response will vary depending on the adhesive on the concrete and the adhesive to be used. Some adhesives require complete removal while others can be applied over a thin adhesive residue. The compatibility of a new adhesive with an old adhesive is always a concern when residue of the old adhesive is remaining.

When the control joints are too far apart the concrete will make its own joints. These will generally be intersecting cracks.

Q: In several of your articles you have referred to concrete slab temperature rather than ambient temperature. Manufacturers do not make any references to slab temperatures; why do you feel it is so important?

A: All too often, I am on an installation site where the heat is supposed to be on and the slab is supposed to be stabilized. I find that the ambient temperature in these situations is about where it is supposed to be, yet the slab is about 10 degrees F or more cooler. This tells me the slab is not acclimated. Starting an installation over a cold, un-acclimated slab is asking for problems with crack and joint show-through as well as difficulty working with materials and adhesives. The slab temperature does not lie about acclimation.


Q: How does moisture move in concrete?

A: Water as a liquid moves to the cold wet side of the slab. In order for moisture to move out of the slab it must convert to a vapor. Once converted, the moisture vapor will seek the warmer, dryer side of the slab. For example, if the slab is warmer than the ambient temperature the moisture, due to the loss or lack of heat, the moisture in the air (humidity) will migrate into the slab.


Q: With curing and sealing compounds used on concrete being very difficult to remove, can an installation be completed over them?

A: Flooring manufacturers and ASTM F-710 “Specifying Concrete Slabs to Receive Resilient Floor Coverings” specify not to use any type of curing, sealing or parting compound on the surface.

Flooring manufacturers do not and cannot keep up with the curing, parting and sealing compounds that are used on the surface of the concrete. These products impact how an adhesive bonds to the concrete and, in some cases, are incompatible with adhesives.

Curing compounds are put on the slab to hold the moisture in the slab and are designed to degradate over a period of time. Unfortunately, some are put on very heavily and some lightly, which means some will release the moisture quickly and some slowly.  

Sealers are designed to be used as a walking surface to control dusting, not serve as a substrate for flooring materials. Parting compounds also known as “bond breakers” are designed for tilt-up construction, so the walls can be poured on the slab and then tilted up into place. If any of these compounds are used they must be removed prior to the installation of flooring materials. Failure to do so can result in an adhesive failure.


Q: Can you explain the American Concrete Institute’s (ACI) Flatness and Levelness issues as compared to the traditional 1/4” in 10’ requirements?

A: The American Concrete Institute (ACI) has established floor flatness (FF) and a floor levelness (FL) method of measuring how flat and level a concrete slab. The FF and FL factors are designed to replace the 1/4” in 10’ we are most accustomed to and are a logarithmic scale. These numbers are calculated so that the higher the number, the flatter and level the concrete floor. The ACI current specification for a slab on grade is FF 35 / FL 25.

To help one understand this, an FF 20 is twice as flat as an FF10 and only half as flat as an FF40. Unfortunately, the average slab in North America is FF 18. There is no correlation between the floor flatness factors and the straightedge tests in 6’ or10’.  Remember the concrete industry has super flat slabs at FF 300, so it can be done. The bad thing about a concrete slab is the floor flatness tests are normally done about two weeks after the concrete hardens, and as the concrete dries the floor flatness factors change.


I hope I have answered some of your questions about concrete. If you have any other questions regarding concrete or any other resilient flooring installation, drop me a line.