Before placing and finishing a slab, some critical work must be done to help ensure that you have a successful placement. It has been my experience that the finisher foreman tends to be left out of the process and typically is the one responsible for the overall finish of the floor. They are usually placing slabs daily and it's hard for them to leave a placement for a meeting. In these cases, the finisher foreman usually has to rely on a project manager or concrete superintendent to ensure that these critical elements are handled. However, it is best for them to be included. A properly conducted job and preslab meeting is very important. Placement rates (yards per hour), jobsite access, mix design, slump and slump adjustment, drain elevation, and slopes are a few of the items that should be discussed. All onsite personnel should be present and prepared to give their input. The American Society of Concrete Contractors (ASCC) has a very detailed pres-lab meeting agenda available as a reference document.
Slab Mix Design
When the overall aesthetics of a concrete floor are as critical as its flatness and levelness, care should be taken in the development of the mix design, as it plays a critical part in the construction of a concrete floor. There are many factors to consider: coarse aggregate, fine aggregate, natural sand versus manufactured sand, cement content, and cementitious material, to name a few. Examples of cementitious materials include fly ash and slag.
A workable slump must be specified and allowing onsite adjustments is crucial to achieving a flat and level floor. Consistency is the key; slumps that range more than 1 inch can cause a 45-minute to 1½-hour set differential. Floors that have areas of differential set created by varying slumps create problems for finishers. To achieve a high-quality floor, initial set should proceed in the direction that it was placed. Finishers struggle with this on a daily basis, an issue that tests their abilities as concrete finishers. It is very difficult to go back across already finished concrete without damaging the surface. Using steel backed poly blades can help eliminate this problem.
On every project an independent testing lab should check the concrete mix for air content. Hard-troweled concrete floors should not contain more than 3% air. Excessive amounts can cause blistering or delamination. According to ACI 302.1R-04, “The total air content of normal weight concrete should exceed 3% only if the concrete is subject to freezing and thawing cycles under service conditions, and the concrete floor slab is not to receive a hard trowel finish.”
On about 90% of slab-on-grade placements, contractors use laser-guided screeds and ride-on power trowels. There is very little finishing that is not controlled mechanically any-more. Many younger, good, concrete finishers have spent little time hand screeding concrete floors with straight-edges or using walk-behind power trowels. This is the result of advancements made in the manufacturing of equipment. Manufacturers have turned riding trowel machines into a science. The advancement of these machines has made finishing concrete much easier then years ago.
Ride-on trowels increase the quality by providing much flatter floors than with walk-behind machines. This was achieved with the introduction of the nonoverlapping riding trowel with pans. These machines do about 80% of the critical work on a floor, correcting problem areas and making them flatter. They have improved so much over the years that some finishers hardly even look at the floor while running a machine, a sign of the impact that good equipment has on today's production of concrete floors.
Years ago to achieve high FF numbers, the use of a bump cutter was the most critical step. Today's equipment has all but eliminated its use except on highly specialized floors. Contractors these days can routinely get FF values in the 50s, 60s, and 70s range with just a laser screed and four to six ride-on trowels on a typical floor placement. With this increase in quality also comes a big boost in production because the equipment does most of the work. The same size crew compared to 20 years ago now can produce double, if not triple, the amount of work. But, has the equipment evolution come at a price? If laser screeds and ride-on trowels weren't available, could finishers today still produce a quality floor?
Big Box Finishing
Due to the equipment evolution, timing is not as critical as it once was. Finishers can cover so much square footage in such a short time that it is very easy to catch up if you fall behind on a floor. With today's equipment, it's the transition phases that are now more critical than timing. Going from pans to combination or trowel blades, depending on what you use to finish with, while you are “laying the floor in,” is an example of a transition. Some finishers like to “pull a floor down” very tight with pans, causing slab discoloration. A couple of things happen as a result. It makes a floor flatter—the more passes you run with pans on a floor the flatter it will be, which usually is done with the bigger 10-foot machines that weigh more. It also causes a floor to set faster. Several years ago, finishing floors in the wintertime, floating floors before there were pans on the market, and then final troweling it out by hand could last until 1 or 2 a.m. Now panning a floor as tight as possible puts more heat into the floor to help accelerate the finishing process.
Big pan machines, while pulling a floor down very tight, also roll small particles of sand at the surface. These are not pin holes that the finishers missed while finishing the floor as it sometimes appears. They are areas where fine particles of sand have been moved by the speed and torque of today's ride-on power trowels.
Now comes the transition phase from panning to laying the floor. Many finishers panic and run the first blade machine as fast as it can go with the blades tilted at a high angle. There are a wide variety of finishing techniques at this phase—too fast, too slow, blades tilted too high, blades tilted too low, rotor speed too slow, rotor speed too fast, travel speed too slow, and travel speed too fast. The most common mistake, however, is running too fast. The trick to getting the pin holes filled in by running your troweling machine at a speed that doesn't throw surface material across the floor. To do this your blade pitch must be kept relatively flat with only a slight pitch. The machine pass should be run 90 degrees from the previous pass. Finishers are taught to allow some setting time between passes, but an immediate 90-degree pass in the opposite direction will close most, if not all, of these pin holes. So you should lay it down in one direction and immediately make a pass at 90 degrees. Once that is completed, you can return to normal finishing operations and procedures.
The next transition area of concern happens in the final finishing sequence when finishers use the “burn machine” or final ride-on trowel machine to make the final passes. The biggest problem normally seen here is having only one “lay down machine” or one with dirty blades (from running into too wet concrete) or when the second blade machine throws loose concrete off the blades, causing the burn machine (final ride-on trowel) to run over them on the floor. When this happens, dark spots appear all over the floor, making it look like a spotted Dalmatian—the overall appearance being less than desired.