Concrete slabs on grade can be found in nearly every single industrial, commercial, and residential building. Whether they exist below a layer of flooring material or are exposed, slabs on grade pro-vide foundation for all building foundations.

The Portland Cement Association says that concrete slabs on grade can be as simple as a residential driveway placed and finished by hand or as complex as a super-flat industrial floor installed with laser-guided screeds and power trowels.

Regardless of the intended use, the engineering principles remain the same. Essentially, quality materials combined with good design and expert workmanship yield the best concrete slab. The following information and references from the PCA serve as a guide for all three of these essentials.

Placing a Bonded Floor Overlay: If a floor needs to carry greater loads than it was designed for, it can often be upgraded with a bonded overlay. A well-bonded overlay can give the floor the added thickness it needs to support the additional weight. However, placing a bonded overlay presents many difficult challenges. Here are a few tips from PCA’s publication, “Resurfacing Concrete Floors” that should help ensure success:

Keep the water content of the overlay as low as possible to minimize shrinkage and curling. The concrete should have a water-cement ratio of 0.45 or less and a minimum cement content of 600 lb/cubic yards (360 kg/cubic meters). The maximum aggregate size should be no more than one-third the thickness of the overlay.

Saw control joints to the full depth of the overlay directly over the underlying floor joints. An overlay joint and an un­derlying joint may begin and end at the same place, but they often are not aligned perfectly along the entire length of the joint. Sawing the overlay joint to its full depth reduces the chances of reflective cracking in the overlay in areas where the joints are not perfectly aligned.

Proper curing is even more important in bonded resurfacing than in ordinary concrete work because of the potential for rapid, early drying of the thin con­crete overlay due to its high surface-to-volume ratio. Use a fog spray immediately after finishing, if necessary, to protect against rapid drying, and cover with wet burlap, plastic sheets, or water proof paper as soon as they can be placed without marring the surface.

Design for moisture sensitive floor coverings?

The choice of slab design for moisture sensitive floor coverings should include the following considerations:

• Concrete water-cement ratio;

• Sub-base moisture conditions;

• Mineral and chemical admixtures;

• Concrete curing; and

• Concrete drying environment.  

For slab construction of this type, the water cement ratio should be maintained at 0.4. The sub-base moisture conditions should be considered to determine if a vapor retarder will be required or if granular cushion will be sufficient to resist the en­trance of moisture into the slab. A water reducer may be considered for use with a low water-cement ratio concrete to pro­mote workability and to aid in ease of consolidation.

Curing practice may be altered to ac­commodate an early dried condition (three day moist cure). Proper ventilation and low relative

humidity environment are recommended for the drying conditions. Moisture related problems are unique with every slab that is placed. First you will need to consider the ground water and drainage conditions for each site. This information will determine if a va­por retarder will be required. In most cases, if a vapor retarder is not required, a 28-day air drying of the slab should prove to be adequate as preparation for placement of the floor covering.

Should a retarder be required, things get a little more complicated. There are a number of different ways to do this and each has its strengths and weaknesses. A vapor retarder placed below a blotter layer (a layer of sand or granular material used to allow moisture to evacuate the slab from both faces) minimizes curling, yet may act as a moisture reservoir to promote higher vapor pressures.

A vapor retarder in direct contact with the bottom of the slab does not pro­vide this reservoir but forces the conve­nience water from the initial placement to evacuate through the top of the slab only. This may substantially change the water cement ratio in the upper surface of the slab. This in turn may make for a weaker finished surface for the floor and will increase the shrinkage rates at the upper surface of the slab promoting curling.

Some designers have adopted the practice of using the vapor retarder at the bottom contact surface of the slab, a low water cement ratio with water reducers to control the workability of the concrete mixture, and a mat of steel in the upper half of the slab to restrain shrinkage and with that to control curling.

Durability is the ability of concrete to resist weathering action, chemical attack, and abrasion while maintaining its desired engineering properties. Dif­ferent concretes require different de­grees of durability depending on the exposure environment and the proper­ties desired. Concrete ingredients, their proportioning, interactions between them, placing and curing practices, and the service environment determine the ultimate durability and life of the concrete.

Exposure Conditions and Deteriora­tion Mechanisms:

Durability of concrete can be ad­dressed by two approaches. The first is called the “prescriptive” approach, where designers specify materials, pro­portions, and construction methods based on fundamental principles and practices that exhibit satisfactory performance.

The second is called the “perfor­mance” approach, where designers iden­tify functional requirements such as strength, durability, and volume changes, and rely on concrete producers and con­tractors to develop concrete mixtures to meet those requirements. Performance specifications define performance for a given exposure and life expectancy, and include tests, which are tied to the field performance of concrete.

Refer to NRMCA’s Performance-Based Specifications for Concrete for details. Very often a specification will contain prescriptive as well as perfor­mance elements. FSM