In an attempt to address a longstanding problem with concrete floors that are to receive floor covering, especially resilient or wood coverings, the American Society of Concrete Contractors (ASCC) has released a position statement that begins to address this concern. The position statement, officially entitled Division 3 versus Division 9 Floor Flatness Tolerances, Position Statement #6, addresses the incongruity between the F-number flatness standards typically used by concrete contractors, whose specifications are shown in Division 3 of most jobs and the straightedge standards preferred by flooring contractors that are typically described in specification Division 9.
Some history is in order for those who may not be familiar with the problem. The older straightedge standard says that if a ten foot straightedge is placed anywhere on a floor and one end is pressed down, the other end shall not have more than 1/8 inch or perhaps ¼ inch between the raised end and the surface of the floor, depending on the specified tolerance. This standard is most likely to give the flooring contractor a surface that will make his product present itself as expected. The problem with this standard is that there is no ASTM-approved test method for determining whether or not a contractor has complied with it. The measurements are also taken immediately before the flooring is applied, which may be months after the concrete contractor has finished his work. Concrete contractors hate this standard because of these problems.
Contractors prefer the F-number standard because it is easily measured according to an ASTM test procedure, and it is measured within 72 hours of the placement of the floor slab. The immediate feedback that this test gives allows concrete contractors to make any necessary adjustments in their placing methods before the whole job is completed. The results are clear, quantifiable, and immediate. Concrete contractors love this test. Unfortunately a floor with a wonderful F-number at the time of placement may not be so flat when it comes time to place flooring over it. This is primarily because of curling — the tendency of concrete to dry out more quickly on the surface that on the bottom and thus curl up like the mud left in a puddle after it dries up. Concrete contractors maintain that poor design is the primary cause of curling and is therefore not their fault. They are at least mostly right, but they get blamed for curling anyway.
The ASCC position statement, reflecting the concern of concrete contractors, says that engineers should specify a minimum slab reinforcing of 0.5% of the cross-section area of the slab in each direction to combat likely curling. This is fairly heavy reinforcing equivalent to #5 bars at 12 inches on center both ways in a 5 inch thick slab. Whether or not this recommendation will be accepted by engineers remains to be seen. There is, needless to say, a substantial cost associated with this recommendation. In any case, it is a start in the process of reconciling the concerns of concrete contractors and flooring contractors, and that is a good thing. If you refer back to one of my previous blogs you will see the results of some field tests that are being done to examine this problem and some possible solutions. Although I applaud the efforts of the ASCC, I am hopeful that some more cost-effective solutions may soon present themselves.
David Prizio has over 35 years experience in the fields of general contracting, concrete subcontracting, and civil engineering. He functioned in the capacity of carpenter at Prizio & Prizio while attending college. After graduating from Cal Poly, Pomona in 1975, David Prizio began working as a project manager at the firm. He was responsible for design coordination, cost estimation, construction coordination and administration of commercial general contracting projects. He consulted with clients regarding design, marketing, and financing of building and construction projects.