Precast Math: Final concrete density

How much does your concrete weigh? It is a question to which far too many people assume the wrong answer. 150 pounds per cubic foot is the widely accepted density of concrete with reinforcing rebar included. In reality, the final density could be significantly lower. A simple calculation can take the density from a guess to a more precise value of precast concrete applications.

In precast concrete manufacturing facilities, little variance in density for the daily mix design is seen. A quick survey of precast concrete plants in the Eastern U.S. for density of normal concrete mix designs (everyday mix) revealed that the average ranged from 139 pounds per cubic foot to 143 pounds per cubic foot. When asked what density was used for estimating the weight of concrete, it was found 150 pounds per cubic foot was the predominate response.

Analyzing a box structure that measures 36” x 36” x 36” I.D. with six-inch thick walls, top, and bottom will illustrate this example. Calculating the theoretical concrete needed to pour the structure reveals that 63,936 cubic inches or 37 cubic feet of concrete is needed. The density of the concrete used to pour the box was measured at 141 pounds per cubic foot.

The reinforcing steel schedule shows that eighty (80) feet of #3 rebar and sixty-five (65) feet of #4 rebar will be used. Referring to Chart 1 the weight per foot of rebar can be found. The accepted density of rebar is 490 pounds per cubic foot. It is easy to see that reinforcing steel density is well over three times that of normal density concrete.

How does the weight of the reinforcing steel affect the final density? All the reinforcing steel added to the box structure takes the place of concrete. While it does not look like a large enough volume to compensate when seen as a rebar cage, imagine if you melted down all that steel in the form of a cube. Not only do you have a larger volume than you may have expected but you have a volume that weighs more than three times that of concrete to consider.

Knowing the amount linear feet of reinforcing steel used in the product can be used to convert the amount of rebar to a volume.   Out of the total theoretical volume of the structure to be poured in this example of 37 cubic feet, you would need to allocate 0.15 cubic feet to reinforcing rebar. Subtracting 0.15 cubic feet from 37 cubic feet results in 36.85 cubic feet of concrete needed to pour the box structure. Multiply each volume by the appropriate density, combine the weights, and solve for your final density.  As you can see from the example above the final density of concrete does not change significantly from the measured density of concrete. This will vary according to how much reinforcing steel per yard of concrete is used in the product. 200 pounds of reinforcing steel per yard would have resulted in a final density of 146.28 pounds per cubic foot.
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