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In our high-tech, fast-paced world, many industrial facilities use highly sophisticated forklifts, automated guided vehicles (AGV) and other advanced material-handling equipment. Accordingly, the condition of the floor slab is a critical component of the facility’s efficient operation. Floor slabs must withstand tough conditions such as impact and abrasion from heavy, moving traffic and static loads, and ideally, the floor must be flat, in proper alignment and in good shape. Unfortunately, because of initial construction techniques, or uses and loads that differ from the original design, slab-on-grade floors can develop problems that often result in costly delays or downtime. However, with a basic understanding of common problems, as well as tips on troubleshooting and repair, a maintenance manager can maximize productivity and reduce downtime.
From the bottom up
A concrete floor should have firm contact with the supporting subgrade so that moving and static loads can be distributed through the slab. This intimate contact can fail, however, when a void forms between the slab and subgrade — exerting stress in the concrete. The void becomes apparent when the floor cracks and settles at a lower level than the adjacent areas or the slab rocks when wheeled loads roll across a joint. Curling at joints, joint failure and heaving are other indications that the desired contact between the slab and subgrade has been compromised.
You can minimize these long-term problems if you follow a few key steps during construction. A thorough examination of the subsoil, including its condition and compaction, is the first step. The subsoil greatly affects the long-term slab settlement. If the slab is constructed over a poorly compacted subbase, cracks, joint failure, heaving and other problems can occur.
Next, it’s important to check for water vapor because moisture migrating through the concrete will cause the floor to “sweat.” To prevent such migration, install a 6-mil polyethylene vapor barrier over the subgrade.
Once water vapor concerns are addressed, the next concern is placement of reinforcement. Pay strict attention to the design drawings, which detail how and where rebar should be installed. If placed too high or too low, the rebar won’t function correctly and will cause the concrete to crack — leading to costly repairs.
Pay attention to the concrete mix composition. The water-to-cement ratio is a critical variable. The proper mixture is important for ensuring the long-term durability of the slab, as well as minimizing the potential for cracks caused by concrete shrinkage.
What you do after pouring the slab is also important. Northern climates present special challenges, especially in the winter. Avoid using unvented propane or kerosene heaters because the fumes they emit cause carbonation that weakens the concrete surface. Additionally, the high concentration of carbon monoxide and carbon dioxide induces a chemical reaction that affects the durability of the concrete and also produces a problem known as dusting.
After the concrete is placed properly, take additional steps to ensure a solid, reliable product. First, it is critical to determine if the slab has cured adequately. Proper curing ensures maximum strength, reduces unplanned material shrinkage cracking and increases long-term durability. Further, the installation of any required floor joints should be completed within the first 24 hours of the concrete placement. Joints should be one-fifth to one-fourth the depth of the slab. This action helps manage cracking by controlling the location of planned concrete material shrinkage cracks. The joint must be deep enough to develop a weakened plane that controls where the concrete is to crack, instead of the slab cracking on its own in an inconvenient location. After a specified curing period — typically seven days — the joints are then sealed with any of various resinous products to protect the embedded reinforcing steel and provide durable joint shoulders.
Playing the hand you’ve been dealt
These steps can help a facility manager who is involved in the initial phase of a new construction project, but many facility managers are responsible for older buildings. These managers have a number of measures that can assist in the evaluation of the facility.
Determine if there has been a change of use in the facility — new uses might produce unanticipated loads on the slab. Obviously, it’s important to ensure that the floor can handle the new loads. Reviewing the initial design documents and performing a preliminary analysis can determine the slab’s load capacity. The owner, contractor, architect and engineer should work together to conduct a thorough evaluation of the existing facility, its intended uses and potential problems.
Common floor-slab problems
A common problem with floor slabs in an industrial environment is curling. This phenomenon occurs when curing conditions are different in the top and bottom portions of a freshly placed concrete slab. The surface dries and cools faster than the moister, warmer slab bottom. This causes the slab to shrink or shorten at the top. Another cause for curling is geotechnical conditions.
When curling occurs, the slab’s corners and edges rise. As the ends curl upward, the load on the subgrade at the center of the slab panel increases and the slab sinks deeper into the subgrade. Concrete can’t withstand the resulting high tensile stresses, so it relieves them by cracking along the panel’s perimeter areas. This phenomenon is frequently observed in cast-in-place concrete floor slabs and can generally be traced back to careless installation or faulty field practices during construction. There are many methods available for reducing or eliminating the chance of curling. However, minimizing the temperature and moisture differentials that are responsible for the curling process are the most important.
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