What is the Problem?
The foundation of a residential structure is affected by many forces acting on it from different directions. Engineering calculations show that concrete can be in compression or tension simultaneously. To avoid the failure of the concrete foundation, steel is added to the foundation to increase the carrying capacity of the concrete, principally in tension. The American Concrete Institute (ACI) in Section 318 of their code specifies that the rebar in a slab-on-grade foundation must be placed at the midpoint of the depth of the foundation. For example, in a four-inch foundation slab, the rebar mat would normally be placed at a two-inch depth. Also, the ACI Concrete Code calls for the steel reinforcement in the concrete beams at all exterior and interior beams to have sufficient cover to avoid exposing the steel to the effects of moisture penetrating corrosion. Once the steel is exposed to the effects of chemical-laden moisture, corrosion starts taking effect.
What is the Solution?
Many contractors, because of the lack of a product(s) that places the steel at the correct depth in a uniform and efficient manner, do not install the steel reinforcement correctly. Many times, concrete laborers use stone (in different sizes) or pieces of broken bricks to support the steel mat (or cage in foundation beams) at the perpendicular intersection or rebar piece. Since the pieces of stone or bricks are not uniform in size, the placement of the rebar ends up uneven. Another problem is that while the foundation is being prepared the workers step on the installed steel to go across the foundation and this sometimes leads to the steel rebar falling off the support (stones or bricks). During the placement of the concrete, the rebar might end up not at the mid-depth level as prescribed in specifications. There are concrete foundation supports out in the market that are not strong enough to withstand the weight of the concrete laborers and the weight of the rebar supports. Some of the supports perform well under ideal circumstances. But these supports, either metal or plastic, normally have a single installation purpose.
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1. Uniform Building Code. 1997 The International Conference of Building Officials. Whitetier, California
2. The Southern Building Code Conference International (SBCCI). Birmingham, Alabama Chapter 23, Section 2307.1
3. CABO. 1995 Joint collaboration of BOCA, ICBO, and SBCCI. One and Two Family Dwelling Code., Chapter 4
4. ACI. American Concrete Institute. Concrete Design Code 1995 edition, Section 318