Tips for placing concrete slabs on grade
Grade Preparation
Grade preparation involves setting the grade elevation and creating a level and well compacted surface. Compaction can be obtained using a compactor, sprinkler, or both. During summer months it is beneficial to spray down the grade with water just before placing the concrete on the grade. This is beneficial for the curing process, and helps prevent shrinkage cracks, especially in dry, hot, windy conditions.
Placing concrete
Placing the concrete is typically done right from the truck. As soon as possible, screed off concrete with a screed board (a 2" x4" is commonly used), and immediately bull float. It is critical to bull float the slab as soon as possible after screeding, delays in bull floating may cause surface problems later. The slab is usually ready to be edged after the concrete has been bull floated. If this is an exterior slab, a broom finish is recommended. A broom finish is obtained by pulling a broom across the surface of the concrete in one uniform direction. The broom finish is applied after all bull floating, grooving, edging and mag work has been done. If you are unsure as to when to broom your concrete, try a small sample piece next to the edge that you can easily fix with a hand mag if needed. Smooth troweled (interior) finishes require more time and labor, experienced help is recommended when attempting to finish floors.
Curing Concrete
After concrete has been placed, and finished, the process of curing the concrete begins. Curing concrete is basically controlling conditions that are in direct contact with the concrete to help it obtain maximum durability. One of the most common curing practices is to apply a curing agent to the exposed surfaces of the concrete. Curing agents are easily applied by either sprayer or roller, and are readily available from your local LC Redi-Mix location. Curing compounds allow the concrete to utilize its own moisture to obtain maximum durability. For the best results, most curing compounds should be applied after finishing operations have been complete, and normal traffic will not damage the surface of the concrete. Another method of curing concrete is to cover the concrete with a sheet of plastic, this will basically do the same thing as the curing compound by keeping the concrete in a moist condition. If plastic is used to cure the concrete, clear plastic is recommended over black plastic. It is recommended that all exterior concrete be sealed every year to help fight against deterioration from salt and chemicals. Concrete sealers are also available at your local concrete supplier.
Controlling Cracks
Concrete cracks are usually controlled by one of two different methods. The first is cutting the concrete in locations where it is likely to crack with a specially designed saw. Concrete saws can be rented from most rental stores. The other method is called 'tooling' or 'grooving' joints in the concrete. This practice is done while the concrete is in the plastic state. If concrete is not tooled, it is strongly recommended that it be cut no more than 24 hours after it has been placed to prevent uncontrolled shrinkage cracks. The recommended distance between joints is usually two to three times the thickness of the slab in feet. If a 4" thick slab is being placed, joints should be no more than eight feet to 12 feet apart.
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