We often get questions from concrete contractors that relate to common and uncommon concerns. Here we discuss three such questions.
Clear cover for reinforcing bars used in slabs-on-ground
Plans call for a 5-inch-thick slab-on-ground with #5 reinforcing bars spaced at 12 inches on center each way with the surface of the top bar located 2 inches below the slab surface to help control curling as recommended in Section 14.9 of ACI 360R-10 ("Guide for Design of Slabs on Ground"). The building inspector states that reinforcing bars must have 3 inches of clear cover from the base as stated in Section 7.7.1 of ACI 318-08 ("Building Code Requirements for Structural Concrete") for concrete cast against and permanently exposed to earth. Earth isn't defined in ACI 318-08 so the inspector says he considers the granular base the slab rests on to be earth. As seen in Figure 1, there's a problem in meeting the inspector's requirement.
FIGURE 1. For a design slab thickness of 5 inches, requiring 3-inch cover to the bottom bar and 2-inch cover for the top bar leaves no room for the mat of #5 bars each way. Even if bars are placed at the tolerance extremes, the slab must be at least 5 1/2 inches thick.
With allowable tolerances, the 3 inches of clear cover can be reduced to 25/8 inches, and the location of the reinforcing steel can be reduced to 15/8 inches from the surface per Sections 2.2.2 and 2.2.1, respectively, of ACI 117-10 ("Specifications for Tolerances for Concrete Construction and Materials and Commentary"). Thickness of the two layers of #5 bars is fixed at 1¼ inches. So even if the bars are placed precisely to take maximum advantage of the tolerances, the slab must still be exactly 51/2 inches thick to meet the designer's intent for the top bars to be located 2 inches below the slab surface. There is no plus tolerance on slab thickness (ACI 117-10) so the contractor could plan on placing a 51/2-inch-thick slab.
In summary, meeting the inspector's expectation based on the engineer's design requires an extra half inch of concrete thickness if the steel is placed in an exact location (no tolerance) and the thickness is at least 51/2 inches. A change order would be required to reimburse the contractor for extra concrete costs, but the impossible task of placing the reinforcing steel and concrete slab with no tolerance isn't likely to happen. What now?
The first obvious solution to the problem is that ACI 318-08 does not apply to slabs-on-ground. Section 1.1.7 states this explicitly:
This Code does not govern design and construction of slabs-on-ground, unless the slab transmits vertical loads or lateral forces from other portions of the structure to the soil.
But what if the slab does transmit vertical loads or lateral forces from other portions of the structure? There is still a way to deal with this problem, as described in the Commentary for Section 7.7 of ACI 318-10:
Alternative methods of protecting the reinforcement from weather may be provided if they are equivalent to the additional concrete cover required by the Code. When approved by the building official under the provisions of 1.4, reinforcement with alternative protection from the weather may have concrete cover not less than the cover required for reinforcement not exposed to weather.
Because placing a vapor retarder immediately beneath the slab-on-ground can be considered as alternative protection, when approved by the building official, that solution is probably the best because it also restores the tolerances by allowing a depth of cover of 3/4 inches on the bottom of the ground slab. A change order would still be needed to cover the additional cost of the vapor retarder.