I am constantly getting questions fromNFTreaders. Some are related to my column and some are general questions about resilient flooring materials, adhesives, installation techniques and concrete issues. I appreciate getting these questions and I try to answer all of them in a timely fashion. Often, I know the answer off the top of my head (“No, the adhesive will not work right in sub-freezing temperatures…Yes, you do need a moisture barrier even for above grade installations.”)
But sometimes, I admit, I’m stumped by your questions so I do a bit of research to find the correct answer. What follows are a few of the more difficult questions I’ve encountered and, more important, the answers.
Question:You have made many references to water to cement ratio in concrete mix designs. Can you further explain this? I still find it difficult to get a grasp on this subject.
Answer:Concrete is always characterized by the mass ratio of water-to-cement (w/c) in the mixture. Unfortunately, the w/c is an abstract number without any particular meaning. Furthermore, this number is inversely related to concrete strength: the lower the w/c ratio the greater the strength, according to Duff Abrams, “Design of Concrete Mixtures,” 1918. People generally understand and can remember direct relationships better than inverse relationships.
In fact w/c has a hidden meaning: it’s directly linked to the spacing between the cement particles in the cement paste. The smaller this spacing, the faster the cement hydrates filling the gaps between the cement particles, the stronger the links created by these hydrates and most importantly, the stronger the concrete. Also, the lower the w/c, the faster concrete dries, as there is less space for water to collect and hide.
ASTM F-710 calls for a water-to-cement ratio of 0.40 – 0.45 w/c ratio. The flooring industry, which is a little more lenient allows up to a 0.50 w/c. A 0.40 – 0.50 water to cement ratio is a compressive strength of 5,300 to 4,200 lbs. per square inch. Surprisingly, the 3,500 psi compressive strength specified by the flooring industry is a 0.58 water-to-cement ratio. And, we wonder why concrete does not dry in a timely fashion.
Question:We have been running into more cases of Ettringite (looks like alkaline salts). Can you explain what causes this phenomenon?
Answer: Ettringite is a mineral, high in calcium sulfoaluminate Ca6Al2(SO4)3(OH)12-26H2O found in Portland cement. When it comes in contact with calcium sulfate screed (gypsum), the Ettringite is formed by sulfate attack between the Portland cement being placed over a gypsum.
The last case of Ettringite I witnessed was a commercial resilient replacement job. The flooring contractor went in and removed the existing flooring and failed to remove the entire gypsum patch. A skim coat of Portland-based patch was used to directly skim coat over the existing gypsum. This sulfate attack takes about 3-6 months to develop. Sulfate is also present in supplementary cementitious materials.
Question:Can you tell me which concrete joints you can install resilient flooring over?
Answer:There are many terms used to describe concrete joints. These include: construction joints, isolation joints, contraction joints, control joints, expansion joints and cold joints. All of these terms are used interchangeably and sometimes incorrectly. With all of the confusion I have adopted the terms for a concrete joint as either being active or dormant. It makes sense that an active joint is going to cause problems for the floor installation. The best way to check is to make a mix of Plaster-of-Paris, spread the mixure across the joint and allow it to dry. Re-check the joint in about a week and look for a fine crack across the hardened mix. If there is a hairline crack in a joint, you will have a problem with the concrete joints showing through the finished floor. The solution is to use a joint cover to allow the joint to move.
Question:Why are flooring manufacturers so set against installing over sealers, curing and parting compounds?
Answer:Let’s look at the intended uses of each of these products:
Sealers are designed for a concrete walking surface and not as an underlayment. Sealers are easily applied and very difficult to remove. Curing compounds are designed to hold the mix water in concrete for hydration purposes.
Curing compounds are supposed to degrade, over a period of time, either through oxidization or ultraviolet degradation, allowing foot traffic to walk the residual off. The problem with curing compounds is they are supposed to be applied in a very thin layer (1–1 ½ mils thick). In many instances the coating is applied too thick and does not degrade in the allotted times and prolongs the start of the concrete drying process, thus delaying the start of any flooring installations.
Concrete slabs with a curing compound need to be mechanically abraded to be sure the residual is gone, especially along walls and where foot traffic is minimal. Even worse is a combination cure and seal product that is difficult to remove. This is normally a solvent-based product and when applied too heavily will retain the residual solvent for a long time. This type of product has been known to attack flooring installation adhesives, even one year after a flooring product was installed over it. The concrete industry is gradually discovering that wet curing works better than a curing compound.
Parting compounds are used primarily in concrete tilt-up construction. These compounds can be oil-, silicone-, wax- or soap-based. Also known as a “bond breaker” they prevent the tilt-up wall, which is poured on the surface of the slab, from sticking to the slab. Bond failures are caused by this residual not being removed from the slab surface.
All of these compounds should not be used or totally removed if one has been used previously. The comment made by some: “it is just one more substrate that is likely to fail” is appropriate.