Solid cures for concrete floor problems

Follow these steps to understand common concrete floor problems and ensure lasting slab construction and repair.

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.

The area around joints is also vulnerable to damage. A concrete floor can deflect at the joints if there is a void between the slab and subgrade. This problem typically begins when material-handling equipment carrying a heavy load approaches a slab joint. When the slab has diminished bearing capacity, the load deflects the slab downward into the void, sometimes only a fraction of an inch, which exposes the edge of the adjacent joint shoulder to the full impact of the load. Even with its strength and durability, concrete is no match for this type of repetitive tensile stress, and spalling begins. If the problem isn’t remedied, the condition will worsen, become more dangerous for vehicular traffic and potentially increase the cost of repairs.

Ensuring successful repairs

Even if the deterioration or floor damage has become visible, instituting a concrete repair program immediately can help reduce the effects. The first step in a successful repair is determining the root cause of the problem based on its visible effects and the extent of the damage. This is critical because the damage from the underlying root cause is often more extensive than what is visible.

A condition survey is a good method for determining the cause, evaluating the damage and selecting a repair strategy. The survey should provide as much information as possible about the nature, extent and location of the problem areas. During the survey, damaged areas are sounded, photographed and mapped to scale. Selecting a qualified, experienced team to perform the condition survey and subsequent repairs is extremely important. Use a team consisting of facility manager, plant engineer and concrete-repair professional to determining the most appropriate repair strategy.

When curling is evident, it might be necessary to stabilize the slab. Slab stabilization or undersealing requires a detailed understanding of the geotechnical issues. Test borings can gather information about the subsoil and its capacity. If the soil has adequate bearing capacity, slabjacking can return the concrete to its original surface elevation. It involves pumping a stiff grout under pressure through holes in the slab drilled at grid intervals. The grout fills the void and pressure forces the slab upward. Another process — cavity-filling — injects grout below the slab under low pressure to fill the cavity completely and provide intimate contact between the subgrade and slab. The purpose of this type of undersealing is to stabilize the slab only it won’t reposition the pavement to its original elevation.

Next, address the condition of the joints and adjacent concrete. Spalled joints can be repaired to provide many years of trouble-free service. As discussed above, the first step in repairing a joint is to stabilize the slab. The second step is to drill a series of holes along both sides of the joint through the full depth of the slab and into the void. Then, a flowable, cementitious grout is then pressure-pumped through the holes to fill the void. Caution must be exercised during this process. Too much pressure and overfilling causes the slab to upheave.

Next, the joint edges must be reconstructed to produce a smooth riding surface. Surface preparation is the most critical factor here. The basic requirement for concrete reconstruction is the removal of unsound concrete. For spalled joints, this means parallel saw cuts on each side of the affected area. Concrete between the saw cuts is then removed mechanically with care to avoid additional damage to the surrounding areas. The process then repeats until it exposes “sound” substrate. The criteria for soundness is the inability to dislodge intact coarse aggregate in the concrete matrix, the concrete begins to fracture and the surrounding pores in the matrix are clean and free of dust. After a thorough cleaning, a concrete-repair material is installed to reconstruct the joint edges. Install the repair material flush with the saw cuts where it meets the existing surface. This technique provides a smooth transition when wheel loads cross that point.

Maintain the existing joint cavity and extend it into the repaired area by saw-cutting a new joint into the replacement material the day after installation. Keep the saw cut as narrow as possible to minimize the exposure of the restored edges to the hard-wheel impact from material-handling equipment. Because high or low spots could provide a point of impact that might lead to further spalling, this technique provides a smooth transition for loads crossing the joint.

Final steps to success

After making necessary repairs, establish a long-term maintenance program. The slab surface treatment products available can provide protection against abrasion, impact, water absorption and even some chemicals. Evaluating the loads and traffic volume on the slab will help determine the best product for your facility’s floor. The new joint edges need long-term support to protect them against impact, chipping and spalling. Install an appropriate joint filler into the saw-cut joint slot to assist in providing this protection.

Another key component in a long-term maintenance plan is consulting with an experienced, reputable concrete-repair professional. Those with the right experience can develop a plan that will add years of life to your existing floor slab and help you achieve the greatest return on investment. If it is cost prohibitive to repair the slab, the best solution is to replace it. A good repair professional can help you make the best decision for your situation.

Tom Kline is the engineering services division manager in Structural Preservation Systems Inc.’s Houston office. Contact him at (281) 478-5300.