ASTM C138: Density (Unit Weight), Yield, and Air Content (Gravimetric) of Concrete

What the ASTM C138 test measures

ASTM C138 is the density test for fresh concrete, often called the unit weight test. A technician fills a container of precisely known volume with concrete, weighs it, and from that one careful measurement gets three answers: the density of the concrete in pounds per cubic foot, the yield. That is whether the producer's cubic yard is actually a cubic yard. A calculated air content as a cross-check on the pressure or volumetric meter.

It is the quiet accountant of the fresh-concrete tests. Slump gets the attention and air gets the arguments, but unit weight is the test that tells you whether you received the volume you paid for and whether the mix in the truck matches the mix on the approved design, ingredient for ingredient.

Below, we walk through what one weighing yields, how the yield arithmetic works on a real ticket, and where this quiet test saves real money.

Why density and yield matter

Density first. Every concrete mix has an expected fresh density, typically around 140 to 150 pounds per cubic foot for normal-weight structural concrete. A load reading meaningfully light usually means extra air or extra water; a load reading heavy can mean low air or a batching change. Either way, density drift is a mix drift, caught while the load is still on the truck.

Yield is the money question. Concrete is sold by volume but batched by weight. Dividing the total batched weight on the ticket by the measured density gives the volume actually delivered. If a 10 yard ticket calculates to 9.6 yards delivered, you are paying for concrete that never arrived, and on a big placement that gap compounds fast. Yield testing is how the shortage is found, documented, and corrected while the pour is still young.

Structural designs that depend on weight, lightweight decks, heavyweight shielding, mass balance on marine work, make density itself an acceptance property, and this is the test that verifies it.

Density also quietly audits the mix itself. Cement, water, rock, sand, and air each have known weights; if what is in the drum matches the approved design, the density lands where the design says it should. When it does not, something in the recipe moved, and the number says so before the cylinders ever get the chance to, 28 days too late.

How the ASTM C138 test works

From a composite sample per ASTM C172, the technician fills a calibrated measure, a rigid bowl whose exact volume is established in the laboratory, in three layers, rodding each 25 times and tapping the sides to close the voids, matching the consolidation to the mix's slump. The surface is struck off dead level with a flat plate, the outside is wiped clean, and the full measure is weighed on a calibrated scale.

Weight of concrete divided by volume of the measure is the density. From there the arithmetic fans out: density times 27 gives weight per cubic yard. Total batch weight from the ticket divided by density gives yield; and comparing measured density to the theoretical air-free density of the mix gives gravimetric air content. Ten minutes at the tailgate, three specification answers.

Calibration is the quiet backbone: the measure's volume is verified in the laboratory by water filling at known temperature, the scales carry current calibration. Both facts appear in the project file, because a density is only as good as the volume it was divided by.

What the numbers mean

Read against the approved mix design, the density confirms the load is the concrete the engineer accepted. Read against the batch ticket, the yield confirms the volume. Read against the theoretical density, the gravimetric air flags a disagreement with the meter reading before anyone stakes a rejection on a single instrument.

Trends carry the most value. A morning of loads all running a pound light is a plant conversation; one odd load is a retest. Because our technicians run unit weight alongside slump, air, and temperature on the same sample, the crosschecks are automatic and disagreements surface in minutes.

Who needs ASTM C138 testing

Contractors placing large volumes need yield verified early in the pour. That is because a shortage discovered in the quantities reconciliation is unrecoverable, while one discovered on truck five gets fixed at the plant by truck eight. Owners paying by the yard need the delivered-volume record. Engineers on lightweight and heavyweight designs need density verified as a structural property, and producers benefit too: a documented, correct yield defends them against shortage claims as often as it catches one.

Common questions about ASTM C138

How we help with ASTM C138 testing

Unit weight rides in our standard fresh-property rotation with calibrated measures and scales, and yield calculations are run against the actual batch tickets on the spot rather than back at the office. On volume-critical work, paving, mats, structural lightweight decks, we build a yield-check schedule into the coverage plan so the accounting happens during the pour, when it can still change the outcome.

A worked example

Say the batch ticket shows 39,150 pounds of total material batched for a nominal 10 cubic yard load, and the measured fresh density is 146.2 pounds per cubic foot. Delivered volume is 39,150 divided by 146.2, about 267.8 cubic feet, which is 9.92 cubic yards. That is a healthy yield, within a percent of nominal. Had the density read 150.9 with the same ticket, delivered volume calculates to 9.61 yards, a 4 percent shortage worth a same-day conversation with the plant. The arithmetic is simple; the value is having the calibrated measurement and the ticket in the same technician's hands at the same tailgate.