Q: I've heard guys on jobsites talking about yield and they don't mean “get out of the way.” Is this something I should care about?
A: Yield measures volume (in cubic feet) of concrete. A better number to know is relative yield. It's important because you expect a load of concrete to fill a certain volume in your forms or slabs. If the ready-mix producer says he sent 8 cubic yards (216 cubic feet) and you receive 210 cubic feet, you can use relative yield to find out why.
Calculate yield by determining the density (or unit weight) of the mix, then check the batch ticket to determine the total weight (or mass) of all the materials that went into the batch. Calculate the yield and air content of the mix by using ASTM C 138, “Standard Test Method for Density (Unit Weight), Yield, and Air Content (Gravimetric) of Concrete.”
Unit weight is determined using a 0.25-cubic-foot air meter bucket—good for coarse aggregate up to 1 inch. For larger aggregate, use a 0.5-cubic-foot container. Weigh the empty container, fill it with concrete removing any entrapped air, and determine the weight of the concrete as the difference. That weight divided by the volume is the density in pounds per cubic foot.
Yield is determined as batch weight divided by density. This is converted to relative yield, which is the total volume of the concrete received divided by the volume the producer intended to provide. If relative yield is 1.00, then it's exactly the same. But this is expressed as cubic feet per cubic yard. There are 27 cubic feet in 1 cubic yard, so if relative yield is 27.00, you got the volume the producer said he shipped.
If relative yield is more than 27.00, you've received more, which usually means the air content is higher than specified. A 1% change in air content changes the relative yield by 0.27 cubic feet/cubic yard. If relative yield is lower, the air content might be low.
Say the batch weights show that a cubic yard of concrete weighs 3944 pounds. The unit weight test indicates 0.25 cubic feet of concrete weighs 36.2 pounds. Divided by 0.25, the result is a cubic foot weight of 144.6 pounds, so relative yield is 3944/144.6=27.28 cubic feet/cubic yard.
Many concrete experts say that the single best onsite check is unit weight and yield. Learn how to use both to get the concrete you expect.