When a concrete subfloor cracks even a little bit, as is the case here, it can compromise the appearance of the resilient floor it supports. It is therefore essential that the concrete is mixed properly and allowed adequate time to cure before the flooring is installed.

If there is one solid fact about concrete it is this: it is prone to cracking. Cracking is a natural, expected and accepted behavior of this venerable building material. It is also a condition that can wreak havoc on the installation of resilient flooring. But while cracking issues may be inevitable it is often unclear who bears the responsibility: The designer? The company that poured the concrete? The general contractor? The flooring installer? Depending upon the conditions it could be any one or all of them.

All concrete is prone to cracking and curling and there is a wide range of reasons why. It could be the way the concrete is mixed or cured, or perhaps it is the structural integrity or volume changes of the slab. Whatever the specifics, it is an issue people in the flooring business come across all the time. Let's look at each of these items in detail.

Concrete mix design: The lower the compressive strength of the concrete, the more prone the slab is to cracking. Still, the tendency of the industry is to lower the compressive strength of the concrete. Instead of specifying a 4,000 psi or greater compressive strength concrete, the decision is made to do with value-engineering slabs rated at 3,000 psi or less. ASTM F-710 requires a water/cement ratio of 0.40 - 0.45 for concrete slabs to receive resilient floor covering. The flooring industry, meanwhile, will accept a slab water/cement ratio to 0.50. A 0.50 water/cement will yield a compressive strength of about 4,200 psi. The national average is 0.58 and much higher in some areas of the country, about 3,300 psi. A gallon of water, about 3 percent, added to a cubic yard of concrete will increase the cracking potential by about 10 percent.

Curing of the concrete slab: Keeping the moisture content of the slab uniform from top to bottom is key, and it can be accomplished with proper curing and acclimation. Otherwise shrinkage of slabs that dry too fast on the surface can lead to curling, much like a dried sponge. Moisture in a slab will migrate to the cool, wet side (bottom) of the slab. If the surface dries and shrinks, and the bottom remains unchanged, the slab can curl and then crack.

Structural integrity: The structural conditions of both the support settlement and flexural stresses can contribute to cracking. As a building is loaded, there is both settling and flexural movement.

Volume changes: Expansion and contraction due to thermal changes and/or drying shrinkage also contribute to cracking. These types of cracks can be controlled by reinforcement steel and proper control joint spacing (saw cuts). Remember: All concrete is susceptible to movement from temperature and humidity change coefficients on the job site. For this reason acclimation of a job-site concrete slab is imperative to the success of any floor installed on the site.

From the time the slab is constructed, about 80 percent of cracks will typically appear within the first six months. The other 20 percent will crop up in year two and even as long as three years.

So what can a flooring contractor do to prevent cracks from creating problems after the fact? Is there a way to prevent the subfloor from undermining the resilient floor installed on it? Unfortunately, this is an area where the flooring industry is remiss, but there are several things that can be done to help prevent this, beginning with proper moisture testing. Enlist the services of an independent testing entity that can deliver reliable test results to ASTM standards. The competence of the people doing the test is crucial because moisture testing performed by uneducated parties can produce errors as high as 80 percent-and these errors can be either false-positives or false-negatives.

Also, stay within the moisture standards set forth by the flooring manufacturer. While they may seem conservative, they are based on many tests and they take into consideration some of the variables found at installation sites

Of course it is a good idea to be represented at pre-construction meetings and ensure that the people responsible for the floor covering are heard from (and understood). There may be some who will try and dismiss you as a prima donna, but they are not the ones who have to handle the callback if the job goes bad. Insist on proper protocol and let the other trades know your requirements.

Be aware that the floor you install is also likely to change the moisture condition of the slab below. Once a concrete slab is covered by an adhered flooring material, the moisture content throughout the slab will shift. Moisture at the bottom of the slab is going to equalize throughout the thickness of the slab. When a slab goes into equilibrium there is going to be surface expansion. While it is a minor amount of movement, the cracks filled with a cementitious patching material coupled with the closure of the crack will push the patching compound up creating a "vein-like" ridge (see photo).

Cracks that are more than .035" (a loose fitting credit card) will lose their aggregate interlock and start to create a ledge effect. Temperature and humidity changes during and after the installation will also contribute to the movement of the concrete at the crack. Make a determination if the crack is active (still moving) or dormant.

Repairing Cracks in Concrete

If the concrete cracks are active there is no fixing them. If you try they will either reoccur in the same place or alongside the area you repaired-whichever is weakest. There are several methods of repairing dormant cracks in concrete. They are as follows:
  • Epoxy Injection - a concrete bonding system that can work on cracks as small as .002"
  • Routing and Sealing - a non structural repair to concrete cracks
  • Stitching and Doweling - the use of metal staples designed for concrete cracks. Holes are drilled and the metal staples are epoxy set into the concrete, just under the surface of the concrete.
  • Gravity Filling - a gravity filling with a low viscosity urethane, epoxy or a high-molecular weight methacrylate (HMWA)
  • Grouting - Polyurethane used to stop water leaks, mainly used on walls
  • Dry Packing - hand placement of low water content mortar
  • Overlay and Surface Treatments - removal of concrete surface by chipping or bushing on either side of the crack and replacing with a high compressive patching compound.
  • Autogenous Healing - lets the concrete heal its self through carbonation. For cracks .010" or less

    Of these the most popular methods involve the epoxy injection, routing and sealing or stitching and doweling.

    So to answer the first question, regarding who's responsible for cracking. We all are. The design community has a huge communication gap between Division 3, which specifies the concrete and Division 9 which specifies the floor finish. Too often you have people specifying a design even if they have no idea what, if anything will be applied to the surface of the concrete. Time is money on a construction site and everyone wants to speed things along. The bottom line is that you can only accelerate the construction process so far before the quality of the finished product is compromised. And no floor installer who takes pride in his work will allow that.

    SIDEBAR: Whose job is it anyway?

    So who is responsible when a resilient floor is compromised by the cracked concrete in the subfloor? To avoid fingerpointing after the fact, keep in mind the specific obligations assigned to each party involved in a construction project:

    General contractor
    Ensures that the concrete is properly cured and that subcontractors are delivering a concrete slab to the designer's specifications. Makes certain that others involved in the project understand and follow the specifications set forth by the designers. Coordinates job schedules to ensure that specifications are not being neglected in favor of shortcuts.

    Concrete contractor
    Responsible for the delivery of quality concrete. The material must be poured and finished without compromising the quality of the slab by over-watering the mix.

    Flooring contractor
    Makes certain the subfloor is ready for the flooring installation and follows the specifications set down by the manufacturer of the flooring material. Never allows himself to be coerced into breaking those specifications due to pressure from the general contractor and job schedules.