How to Compute the Percentage of Steel in Reinforced Concrete

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Concrete is very resistant to compression, but when it comes to tension, it needs the some help. Tension is the opposite of compression, so that would include any forces that tend to pull the concrete apart. For that reason, construction workers reinforce concrete with steel rebar, which is short for reinforcing bar. As the concrete dries, it grips the rebar tightly, and together the rebar and concrete provide the structural resistance to both compression and tension needed for construction. That is particularly important when concrete is spanning large distances.

Calculate the cross-sectional area of one unit of steel. For rebar, one stick of rebar would be a unit, and you can find that with the formula A = 0.012n², where A is the area and n is the nominal size of the rebar. For example, the cross-sectional area of No. 6 rebar is A = 0.012*6² = 0.43 square inches.

For welded wire mesh, one unit would be one strand of the mesh's wire, and the area is A = w/100, where w is the mesh's "W" number. For example, the cross-sectional area of 6x6 W1.4/1.4 wire mesh is A = 1.4/100 = 0.014 square inches.

Find the number of units of steel in a 1-foot cross section of concrete. To illustrate, if you were to cut through a slab with a single layer of rebar spaced 4 inches on center, you would cut through three sticks for every foot of the cut. That's summed up in the following formula: U = 12L/s, where U is the number of units, L is the number of layers, and s is the spacing. For example, if you have two layers of rebar 4 inches on center, U = 12*2/4 = 6.

For one layer of 6x6 W1.4/1.4 wire mesh, U = 12*1/6 = 2, which means there are two strands of wire in a 1-foot cross section.

Calculate the percentage of reinforcement with the formula P = 8.33UA/t, where P is the percentage and t is the thickness. For example, the percentage of reinforcement of a 30-inch slab with two layers of No. 6 rebar spaced 4 inches on center is P = 8.33_6_0.43/30 = 0.72 percent.

For one layer of 6x6 W1.4/1.4 wire mesh in a 4-inch slab, P = 8.33_2_0.014/4 = 0.058 percent, which is only enough for crack containment. To truly reinforce concrete, the percentage has to be a minimum of 0.5 percent.

About the Author

Mike Gamble started writing professionally in 2011 for Demand Media Studios. Having worked as a line mechanic, landscaper, custodian, carpenter, web developer and disk jockey, he hopes to bring fresh insight into the topics he writes about from a variety of experiences.

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