5.4 Yield, Relative Yield, and Batch Comparison
Key Takeaways
- Yield converts measured density and total batch mass into the actual volume of concrete produced.
- Relative yield compares actual yield with the design or specified yield, often one cubic yard or one cubic meter basis.
- Yield errors commonly come from unit conversion mistakes, wrong batch mass, uncorrected moisture assumptions, or a bad density.
- C138 yield is a batching and production control clue, not a substitute for following project specifications.
Yield Links Density To Batch Volume
Density tells how much a known volume of fresh concrete weighs. Yield uses that density to estimate how much concrete volume the whole batch produced. This matters because concrete is ordered, batched, delivered, and paid for by volume, while the plant often controls materials by mass.
The basic yield idea is actual volume = total batch mass / measured density. If the total batch mass is in pounds and density is in pounds per cubic foot, the first answer is cubic feet. To express the result in cubic yards, divide cubic feet by 27. In metric work, compatible kilograms and kilograms per cubic meter produce cubic meters.
The total batch mass must represent the batch being evaluated. That includes the materials that make up the concrete, on the correct mass basis. Moisture corrections, added water, returned concrete, or partial loads can make a ticket value differ from the actual material mass being analyzed. On the exam, use the values provided and keep the units visible.
Yield and relative yield summary:
| Term | Meaning | Calculation concept |
|---|---|---|
| Density | Net mass per unit volume of fresh concrete | C138 measured value |
| Actual yield | Actual volume produced by the batch | Total batch mass divided by density |
| Design yield | Intended volume for the batch | Usually from mixture proportions |
| Relative yield | Actual yield divided by design yield | Shows overyield or underyield |
| Yield per batch | Batch-specific result | Must match batch mass and density units |
Relative yield is a ratio. A relative yield of 1.00 means the batch produced the intended volume. A value greater than 1.00 suggests overyield. A value less than 1.00 suggests underyield. The interpretation depends on project tolerances, mixture design, and batching records, but the math is a comparison of actual to intended volume.
Overyield does not automatically mean the concrete is better. It may indicate extra water, excess air, lower cement content per cubic yard, batching error, or density lower than expected. Underyield may indicate too little air, missing water adjustment, more dense materials, or an error in batch weights. C138 provides a field signal that the producer and inspector can compare with records.
A common written-exam trap is to multiply total batch mass by density. That makes the unit problem worse because pounds times pounds per cubic foot is not a volume. Another trap is forgetting to divide by 27 when converting cubic feet to cubic yards. A third trap is using design density instead of measured density when the question asks for actual yield.
Yield calculations inherit all density problems. If the measure was not full, if the concrete was underconsolidated, or if exterior paste was included in the mass, the yield result is unreliable. The calculation may be mathematically correct from bad input, but the field result is still not defensible.
Exam workflow for yield problems:
- Identify total batch mass and its unit.
- Identify measured density and its unit.
- Divide mass by density to get volume.
- Convert volume only if the requested unit requires it.
- Compare actual yield with design yield for relative yield.
- Ask whether a density or unit error could explain a strange answer.
C138 yield is useful because it connects the small test measure to the whole truck or batch. Treat it as a controlled calculation tied to a controlled physical test, not as a standalone arithmetic exercise.
A batch has total mass in pounds and density in pounds per cubic foot. What unit results when total mass is divided by density?
What does relative yield compare?
Which mistake most directly causes an incorrect yield calculation?