Flat floors are increasingly necessary in applications other than distribution centers. Many warehouses have advanced to sophisticated levels, using unmanned forklifts that often must reach products that may be stacked on racking 30 feet tall or even higher. Super-flat and level floors lower the maintenance needed on material handling vehicles and the floors as well. The result is an increase in the demand for flatter floors from warehouses to retail operations.
To ensure efficiency and safety with material handling vehicles, a properly specified concrete floor must include tolerances to control the floor surface bumps and dips. This is referred to as the flatness and levelness or rideability as controlled by F-numbers. This floor profile control needs to be addressed at the outset of the project, and the intended use of the facility must be an important consideration.
The floor flatness and the intended use of the facility will affect the efficiency of material handling operations throughout the life of the facility and can negatively impact personnel safety and vehicle and floor maintenance. When choosing the design criteria for a long-term flat floor slab, the options must be carefully evaluated in order to choose the best system to ensure the initial cost, longevity, and lowest maintenance for the facility.
Industrial slab design options
When designing and producing a flat floor for warehouse, retail, food service, logistics, or office use, the options should be evaluated based on the material handling equipment tolerances, ceiling height, load capacity requirements, soil conditions, desired longevity of floor levelness, and the anticipated maintenance expense. ACI 360R-10 provides information on the design of slabs on ground and a general comparison of types.
Unreinforced Concrete is simple to construct and generally less expensive. The disadvantages are the floors must be closely jointed, there’s more opportunity for slab curling and joint deterioration, and the designer should consider load transfer at the joints. Aggregate interlock is not a viable technique for load transfer when heavy material handling vehicles are used.
Concrete Reinforced for Crack-Width Control limits crack widths. The disadvantages are it may be more expensive than an unreinforced slab, must be closely jointed, there is more opportunity for curl, load transfer is needed, and the reinforcement can actually increase the amount of random cracking.
Continuously Reinforced Concrete using reinforcing bars or welded wire reinforcement can reduce the number of sawcut joints and curling with sufficient reinforcement. There is less change in flatness and leveling with time. The disadvantages are it requires a high amount of reinforcement and typically produces numerous fine cracks throughout the slab.
Shrinkage-Compensating Concrete has construction joints from 40 to 150 feet; sawcut contraction joints are eliminated. There is negligible curl. The disadvantages are it requires reinforcement, and rigorous control of placement sequence and shrinkage allowance. It must be placed in a controlled environment, can be expensive, and requires a contractor experienced with this type of concrete.
Post Tensioned Slabs can have construction joints from 100 to 500 feet. Sawcut contraction joints can be eliminated and there is negligible curl. There is improved long-term flatness. It can decrease slab depth (with increased flexural strength). Additionally, it can be used on poor soils. The disadvantages are it is a more demanding installation, requiring an experienced contractor, as well as extra inspection. The system can be expensive.
Steel Fiber-Reinforced Concrete(SFRC) can increase impact and fatigue loadings and is relatively simple to construct. Increased joint spacing is possible with SFRC slabs. The disadvantages are fibers can be exposed and corrode. Adjustments may need to be made for mixing, placing, and finishing. The system can be expensive.
Synthetic Fiber-reinforced Concrete uses macrosynthetic fibers to provide resistance to impact and fatigue loadings. The fibers are simple to use and noncorrosive and provide significant reduction of plastic shrinkage cracking. Micro synthetic fibers help control plastic shrinkage cracks, but not long term drying shrinkage cracking, and the joint spacing is the same as for unreinforced slabs.
Structural Slabs can carry structural load and can reduce or eliminate sawcut joints. The disadvantages are they may have numerous fine or hairline cracks if the reinforcement stresses are sufficiently low. The system can be expensive.
Ductilcrete Slab Systems is a new design-build system that provides increased load-bearing capacity, 70% fewer joints, reduced fatigue from load, negligible curl, decreased slab thickness, decreased shrinkage, decreased differential shrinkage, and unrestricted placement size (which can have a positive impact on schedules).