Common mistakes in concrete floor slab construction can be avoided with proper base preparation, mix design, placement, finishing, and curing. If these steps are performed correctly, the owner can expect an attractive, durable product.
Standard concrete floor slab thickness in residential construction is 4 inches. Five to six inches is recommended if the concrete will receive occasional heavy loads, such as motor homes or garbage trucks.
To prepare the base, cut the ground level to the proper depth to allow for the slab thickness. Remove all organic material and large hard objects such as stones and tree roots to a depth of at least 4 inches. If building up the grade is necessary, use gravel or sandy soil, and compact the final base with a vibratory plate or similar device. The edge forms may be any straight material that can be secured into position. Consider plastic or metal forms if consistently straight lumber is not available. Set a string line using grade stakes or batter boards to establish a square, level reference before positioning the formwork.
As for the concrete mix, it must meet compressive strength requirements (usually 3000 pounds per square inch) without measures that cause excessive shrinkage. Because water increases shrinkage and cracking, a plasticizer is preferable to achieve desired slump. Also, consider including fibers to control plastic shrinkage cracking. Higher strength and entrained air may be required for exterior slabs exposed to freezing weather or deicing chemicals. When in doubt ask the concrete supplier for a recommended mix.
Always avoid adding water at the jobsite in excess of 1 to 2 gallons per cubic yard. If additional slump is truly necessary, ask the mixer truck driver how much water can be added without taking the concrete out of specification.
Distribute the concrete around the slab area as close to its final position as possible, and then rake it into position. Consolidate low-slump mixes with a handheld vibrator or use a vibratory screed. Finish with the minimum force and strokes of the float necessary to achieve a smooth surface.
Create control joints no farther apart than 24 to 30 times the slab thickness and at no time greater than 15 feet along both the width and length of the slab by pressing a 1-inch deep grooving tool into the surface. Joint spacing greater than 15 feet requires the use of load transfer devices such as dowels or dowel plates. For slabs that require long joint spacing or no joints, steel reinforcement is recommended. It will increase the potential for random cracking, but will hold cracks tightly to ensure good structural performance.
Proper curing conditions are critical, and the curing method must be put in place as soon as the finished surface can resist damage. The concrete must not be allowed to freeze or dry out. Place a curing compound over the surface, or provide appropriate moist curing. If there is a risk of freezing, cover the slab with an insulator, such as insulating blankets or a 4-inch-thick layer of straw that is weighted down to prevent it from blowing away. Leave the insulator in place until the concrete achieves a strength of at least 500 psi. This usually occurs within a few days.
- Pieter VanderWerf is president of Building Works Inc. (www.buildingworks.com), a consulting firm that helps companies introduce new construction products. Terry Collins is concrete construction engineer with the Portland Cement Association (www.cement.org), which promotes the use of concrete and other cement-based products.