In floor finishing, how do we separate the men from the boys and the women from the girls? With the finishing equipment available today, almost anyone can get a decent finish and adequate surface profile tolerances in the interior, wide-open portions of a slab placement. With a properly adjusted laser screed, riding trowels, and pans, many flatwork contractors commonly achieve FF values of 50 to 60 and FL values of 35 to 45 or more for placements of 20,000 square feet and larger. What separates the really good contractor from the competition is attention to detail and consistency, especially with the finishing near edges, block-outs, drains, clean-outs, and other penetrations. Except for the best finishers, this detail work is fast becoming a lost art.
TRAINING & EDUCATION
CONCRETE CONSTRUCTION magazine and similar publications should be “must reading” for all key concrete construction personnel. Every flat-work contractor should become familiar with the American Concrete Institute (ACI) publication ACI 302.1R “Guide for Concrete Floor and Slab Construction.” Many of the people who wrote that guide are currently contractors or have been in the past. Your key personnel should also become ACI Certified Flatwork Finishers. Furthermore, you should consider holding an in-house seminar and training program using the best finishers and outside consultants to pass their expertise on to less qualified finishers. Finally, contractors should attend practical seminars that cover basic concepts of slab design so that they can protect themselves from problems that could occur due to improper design but for which they will receive the blame. Although the new ACI publication ACI 360R “Design of Slabs on Ground” sounds like it is for designers only, it is a practical publication that received much input from contractors, and it can help you to protect yourself.
The importance of a proper preslab construction meeting cannot be overemphasized. Although ACI 302.1R and several published articles have addressed the key issues regarding this meeting, we continue to see that a good one is the exception rather than the rule. The planned placement sizes should be conservative because the placement size can determine the success or failure of that placement.
CONCRETE MIX & TESTING
Contractors should become more knowledgeable about mix designs, especially in terms of how to optimize aggregate combined grading and cementitious materials. ACI 302.1R has a good section on materials and mix analysis. Another source of good, practical information is the article “For the Ideal Slab-on-Ground Mixture” (Harrison, P. J., Concrete International, V.26, No.3, March 2004, pp. 49–55).
Concrete slump and set time must be as consistent as possible. Controlled, onsite additions of water must be allowed, but the procedures should be settled and agreed to at the preslab meeting, if not before. From both a design and construction standpoint, consistently low or reasonably high slumps are preferable to widely varying slumps that are still within spec. Significantly varying slumps can have an adverse effect on F-numbers, durability, and aesthetics. For example, a one-inch difference in slump can change the set time by one hour or more.
To minimize the chances for surface delaminations in troweled slabs with normal weight concrete, the mix should have no air entraining admixture. Even so, because it can be inadvertently added or other mix components can incorporate air into the mix, air content should be checked on the first truck and as often as feasible thereafter (at least every sixth truck). If the air is over 3% and the concrete is already in-place, each finishing step should be delayed as long as possible and the surface should be kept as open as possible, while being careful to use an evaporation retardant or other means to prevent excessive surface drying. These issues are of even more concern when a dry-shake hardener is to be applied to the surface.
Conventional wisdom for decades has held that troweled slabs with air-entrained lightweight concrete are immune from surface de-laminations. However, over the last few years many lightweight slabs have had significant de-laminations. This seems to have occurred primarily for two reasons. Air contents generally have increased in order to ensure the concrete will meet the specified maximum unit weight. Also, the higher the design air content, the harder it is to control. Thus, some of the air contents on problem slabs have been up to 8% or more. The use of riders and pans on suspended slabs probably has played a part when they were used too early in the finishing process (see discussion later on starting power floating too early) and the surface was troweled too tightly at the end.
PREPARATION, EQUIPMENT, AND TOOLS
For slabs on ground, FL (floor levelness) numbers are primarily controlled by how well the forms are set and by the initial strike-off of the concrete. FF (floor flatness or bumpiness) numbers are primarily set by the following finishing operations. Make sure there is backup for all key equipment and materials. This includes a backup concrete plant on important placements, extra formwork and dowels in case an emergency bulkhead is required, and an alternate means of strike-off if the laser screed or truss screed breaks down. For tighter tolerances 8-foot to 12-foot check rods or channel floats should be used for bull floating, rather than traditional 3-foot or 4-foot bull floats.
For larger areas, at least two 10-foot riders are preferable to smaller equipment because of their production and contribution to a flatter surface. Make sure bull floats, bump cutters, pans, float shoes, and blades are true—not warped—and in good condition. Ten-foot to 14-foot bump cutters (modified highway straight-edges) are not used as much as they should be because many contractors are achieving adequate FF numbers by only using riders and pans; but the proper use of bump cutters will produce better floors and even higher FF numbers.
Equipment should be serviced immediately before the placement in order to minimize the risk of slab surface contamination with oil, grease, or hydraulic fluid. Ensure that lighting will be at least adequate—if defects are not easily noticeable, some of them will be missed. Pipe penetrations, clean-outs, drains, and the like must be checked to assure they are plumb, level, and at the correct elevation. This will have a direct reflection of the overall appearance and quality of your floor. All tape should be removed from penetrations during finishing, and the floor surface finished level with the penetration. Ensure that the proper elevation controls have been set to provide the required slopes to the drains. Take whatever precautions are necessary if there is a chance that the slab surface can become contaminated with windblown dirt or debris from adjacent work.