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 (floor flatness or bumpiness) values of 50 to 60 and FL (floor levelness) values from 35 to 45 or more for placements of 20,000 square feet and larger. What separates an excellent contractor from the competition is attention to detail and consistency, especially when finishing near edges, blockouts, drains, cleanouts, and other penetrations. Except for the best finishers, this detail work is fast becoming a lost art.
Training and education
CONCRETE CONSTRUCTION should be a must read for all key concrete construction personnel. Every flatwork contractor should become familiar with ACI 302.1R “Guide for Concrete Floor and Slab Construction.” Many of the people who wrote the guide are currently contractors or have been in the past. Your key personnel also should become ACI Certified Concrete Flatwork Finisher/Technicians and also obtain the Commercial/ Industrial Certification.
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 experienced finishers. Finally, contractors should attend practical seminars that cover basic concepts of slab design so they can protect themselves from problems that could occur due to improper design but for which they will receive the blame. Although the ACI publication ACI 360R “Design of Slabs on Ground” sounds like it is for designers only, it's a practical publication that received much input from contractors, and it can help you protect yourself.
After concrete is placed and initially screeded for superflat floors, workers use long aluminum straightedges to restrike the concrete. This process is repeated after each pass with a bullfloat.
Credit: Robert M. Simonelli
Preparation, equipment, and tools
For slabs on ground, FL numbers are primarily controlled by how well the forms are set and by the initial strike-off of the concrete. FF numbers are primarily determined by finishing operations. Make sure there is backup for all key equipment and materials. This includes a backup concrete plant on important placements, extra form-work 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- to 12-foot check rods or channel floats should be used for bull floating, rather than traditional 3- 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. Bump cutters ranging from 10- to 14-foot lengths (modified highway straightedges) are not used as much as they should be because many contractors are achieving adequate FF numbers by only using riders and pans. However, the proper use of bump cutters will produce better floors and higher FF numbers.
Workers are using bump cutters following a pass with a ride-on trowel. Notice the worker restraightening the edges as well.
Credit: Robert M. Simonelli
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 adequate—if defects are not easily noticeable, they will be missed. Pipe penetrations, cleanouts, drains, and the like must be checked to assure they are plumb, level, and at the correct elevation. This will have a direct reflection on 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 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 the slab surface can become contaminated with windblown dirt or debris.
Tips for finishing
With modern finishing equipment, timing of the middle and later steps is not quite as critical as it was in the past when only walk-behind trowels were used. However, there is still a concern in starting power floating with pans too early. Pans spread out the load of a riding trowel more than float shoes, so many finishers start floating earlier with pans. But starting too early can increase the chance for surface delaminations, map cracking, and excessive crazing. Although most flatwork contractors start with riders and pans, some start power floating with float shoes to “read” the floor, then switch to pans, avoiding any problems with hot spots that can be created by starting with pans.
The arrival of nonoverlapping trowel ride-on finishing machines made it possible to use pan floats. Using pans make it possible to produce flatter floors more routinely.
Power floating should not start until bleed water has evaporated or has been removed by a hose. The traditional rule of thumb about power floating is still a good one: Begin when a footprint leaves a 1/8- to ¼-inch impression. Early finishing with machines produces “windrows” or slings mortar.
Generally floating should start near edges, walls, and openings because these areas will set faster, especially when exposed to sunlight. The first power float pass should be made by the most experienced finishers to determine areas that are setting too slow or too fast. Extra care must be taken with the boundaries of very slow setting areas because they can transition within 1 to 2 feet and are likely to develop delamination, as well as problems with aesthetics and tolerances. On large placements, notify the concrete supplier of wide variations in set time as soon as possible so the cause can be determined and corrected. Significantly different set times from concrete load to load (and sometimes within the same load when mixing is inadequate) are one of the main causes of floor surface problems.
For higher F-numbers, use bump cutters before and after float passes to cut off ridges and high spots and to fill in valleys and low areas. Some of the bump cutting should be done at a 45-degree angle to the ridges and troughs to more effectively eliminate them, especially if walk-behinds are being used instead of riders. The first two power float passes and all trowel passes should be perpendicular to the previous passes, and a definite, planned pattern should be used.