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Slab-on-Grade and Concrete Pads
Introduction: Designing a concrete slab-on-grade requires careful attention. Generally, the designer will use a six-inch thick slab for storage loading (150 pounds per square foot live load) and a five-inch thick slab for office loading (80 pounds per square foot live load). A six-inch thick concrete pad can be used for light mechanical and electrical components, however, for heavy chillers, boilers, or transformers, a slab thickness of 12" to 18" is recommended.
Recommendations: For slab on grade and concrete pads, the load is distributed uniformly to the ground by direct contact with the slab. To avoid cracks and settlement of the slab-on-grade, the following should be implemented:
- If the slab-on-grade is located on different depths of compacted fill, the slab should be floated and not tied to perimeter building walls. For a slab-on-grade, which is to be tied to a building, use 2 x 4 keys in the perimeter walls. Also, #4 top bars @ 12" on center, 2'- 6" long should be provided. Top bars must have a minimum cover of 1½ inches of concrete.
- Soil and gravel below the slab-on-grade should be properly tested for 95% compaction. Uneven or improperly compacted fill below the slab-on-grade can cause severe cracks in the slab.
- Welded wire fabric (WWF) should be placed one inch from the top of the slab. During the pouring of the concrete, it is extremely difficult to keep the WWF one-inch from the top of the slab because the WWF is simply rolled over the area and is not supported by chairs and support bars. Generally, with the weight of the concrete, the WWF sits at the bottom of the slab. To avoid this, proper chair heights should be provided or use #3 or #4 bars in lieu of WWF. Using #3 bars will force the contractor to use proper chairs and support bars.
- For concrete pads on grade carrying heavy equipment loads, two layers of top and bottom reinforcement is recommended. Air entrained concrete is also preferred for concrete pads exposed to the weather.
- Control joints should be used with areas not to exceed 400 square feet.
This will help control the shrinkage cracks, which occur as the concrete
slab cures.
For additional information, contact the Consulting Support Service (183A), Kharaiti Abrol @ 202-565-5579.
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Reviewed/Updated: January, 2004