Smaller Reinforcing Is Better in Thin Slabs

Question:
Can I use any size reinforcing steel in my countertops?

Answer:
Generally speaking, no. The size of reinforcing steel in a concrete countertop is an important consideration because countertops made with rebar too large in scale for the slab thickness are susceptible to telegraph cracking.

When concrete shrinks due to drying, high tensile stresses develop around proportionally oversized reinforcing. These stresses cause cracks that run along the length of the steel. Smaller-diameter reinforcing has less of an effect on the concrete, so the same amount of concrete shrinkage develops much lower tensile forces, thus dramatically reducing the likelihood of cracks caused by the steel. Because all concrete shrinks to some degree, and shrinkage occurs over a long period of time, the occurrence of telegraph cracking may not appear until long after a countertop is installed.

In addition, oversized reinforcing occupies so much space inside a thin countertop slab that very little cover is left between the steel and the slab surface. Generally, reinforcing is fabricated in a grid arrangement, with strands running along the length of the slab and overlapped strands running across the width of the slab. When stacked, larger rebar can take up half of the total slab thickness, while smaller rebar occupies much less space.

In addition to causing large stress concentrations in the concrete (and therefore increasing the likelihood of telegraph cracking), using oversized reinforcing actually decreases the load capacity of the concrete. In the drawings, the same concrete is reinforced with equal amounts of steel reinforcing (based on cross-sectional area). The top drawing shows one piece of 3/8-inch-diameter (#3) reinforcing steel while the bottom drawing shows four pieces of 3/16-inch structural reinforcing wire. The cross-sectional areas are the same, so the tensile capacity of the steel is the same. But because the four pieces of wire can be located lower in the slab, the load capacity of the wire-reinforced concrete is now 13% (lower reinforcing layer) to 78% (upper reinforcing layer) greater than the slab with the single #3 rebar, even though the same amount of steel is used in the concrete. In actuality, the structural wire has a higher strength than the rebar, so the difference in capacity is even greater.

Rebar that is too big also places a significant amount of the steel closer to the visible surface of the countertop instead of down near the bottom of the slab. For example, a grid made from 3/8-inch-diameter rebar held only 1/4 inch away from the bottom (the bare minimum cover for such a size) places the top of the rebar at the midpoint of the slab, leaving only 1/2 inch of concrete cover between the steel and the visible surface. A grid made from 3/16-inch-diameter wire held 1/4 inch away from the bottom of the slab leaves 7/8 inch of concrete between the surface and the steel, a 75% increase in cover. More cover means less likelihood of telegraph cracking.