When it comes to choosing the “right tools for the job,” the prudent contractor does his homework before getting in over his head. Today, choosing tools incorrectly, or not including the needed tools in the bid for a project, can financially sink a contractor when repairs are needed because of poor slab placement. In some cases, multimillion dollar lawsuits may result. These scenarios do happen and are avoidable when a job is carefully evaluated and the right tools are used.
When following the normal sequence of constructing the slab, striking off, and then finishing, a concrete contractor encounters many types of tools and equipment needed to make the job successful.
Constructing the slab
Subgrade installation is a very important step because it supports the slab on grade. Many tools are available to owners, developers, designers, and contractors for properly constructing the building pad. These tools include a laser and laser-guided skidsteers, rod and level style receivers, and the 3-D scanning method currently in development. Protecting the finished building pad from exposure to heavy vehicle traffic and severe weather, especially rain and freezing temperatures, is paramount. The construction of large slabs that require extremely flat and level high-tolerance FF/FL or superflat F-min concrete surfaces must also have a closely graded building pad tolerance. If the subgrade is not uniformly flat and level, then the possibility of random problems with slab thickness (specifically thinness) occurs and the random statistical testing method is unreliable.
When constructing random-traffic floors, many contractors are familiar with big-box construction, as well as the well-known tools for placing and finishing this type of floor. These tools include a laser-guided screed for placement followed by finishing the slab with large ride-on power trowels with float pans, which produce extremely high FF/FL results. However, FF/FL and F-min floors have no correlation and a good FF/FL result can land a contractor in unwanted scenarios when the floor does not perform to the owner's expectations for material handling efficiency.
So what exactly is a defined-traffic floor? Well for starters, it's a floor that allows a particular vehicle—in this case, a high-speed forklift, turret truck, or high-bay crane—to operate at its maximum-designed speed while operating effortlessly with minimal preventive maintenance for the life of the structure. A properly designed floor and minimal preventive maintenance can keep material handling equipment (MHE) running for decades, while operating efficiently and safely without unscheduled down-time. However, an inferior floor can cause problems immediately as operations begin, escalating thereafter, and diminishing the life of MHE and the floor to a few years or less.
When it comes to defined-traffic floors there are two different ways to place and finish the slab. The procedures needed to achieve satisfactory results depend on the specified F-min number, usually determined by rack height. Typically a superflat floor that requires F-min numbers between 65 to 100 or higher would only be poured in narrow strips between 11 and 20 feet. Only as the F-min number decreases, can the strip pour width increase.
For large floors with F-min numbers less than 65, usually in the 50 to 60 range, experienced contractors have been known to place and finish between 20,000 and 50,000 square feet at a time, with minimal corrective grinding in the wheel paths of the defined-traffic lift trucks. Placements of this size usually require control joints every 10 to 15 feet in each direction depending on slab design. These are made by early-entry sawcutting with a walk-behind saw. This compensates for the amount of shrinkage over the large area and prompts cracks to occur directly into the sawcut (or control joints).
When large slabs are placed, plastic shrinkage cracking can occur before the slab will allow foot traffic. It's no surprise that while placing and finishing operations continue on the pour-out side of a slab, the pour-start side is already shrinking and cracking; hence the need for control joints. Defined-traffic slabs are installed in a different manner for a specific reason. First the strips are narrow, so tools, such as laser-guided or truss vibrating screeds, have less area to cover, which results in greater control of surface tolerances.
Second, the small areas are heavily reinforced with continuous rebar in the longitudinal direction to prevent inevitable shrinkage cracks from opening. Since there are no construction or control joints in the transverse direction of the narrow width of slab or perpendicular to the vehicles path of travel, curling cannot create speed bumps in the aisle at the joints the vehicles would otherwise encounter. The problem with a construction or control joint in the aisle is that every part (both sides of each new independent slab) along each cut will encounter the curling phenomena due to differential curing.
Nonetheless, there are “tools” on the market, such as round, square, and flat plate dowels, that can assist in eliminating some of this unwanted curling. The slab is, however, designed to expand and contract in the lateral direction, which happens under the center of the racking where it doesn't affect the forklifts. Dowels are another important tool that when strategically incorporated into the design can provide a load transfer mechanism between each narrow strip allowing for expansion and contraction while eliminating vertical slab movement. Without these devices, aggregate interlock is not enough to stop movement over time with the massive weight placed on the slab or the constant pounding force of the MHE.