ASTM C138

The test method for the density, yield and air content of fresh concrete, measured by filling a container of known volume and weighing it. Yield is the point of it on site: the volume a batch actually produces, against the volume it was ordered as.

A batch that yields short is the usual explanation for a pour that ran out before the slab did. It is a measurement procedure, not an acceptance criterion.

What the result has to be is set by the project specification or the design code, not here. A yield result is only as good as the consolidation: an under-rodded sample traps air, reads light, and reports a yield the batch did not have.

The same weighing gives an air content, worked out from the difference between the density measured and the density the mix should have with no air in it. That is the gravimetric method, and it is one of three — the pressure method and the volumetric method measure the same property by other means, and they do not all suit the same concrete.

Relative yield is the figure that settles an argument: the volume the batch actually produced against the volume it was ordered as, where 1.00 means the load delivered what the ticket said. Below 1.00 the pour runs short before the slab is full, and the shortfall is found at the far end rather than at the truck.

Cement content per unit volume falls out of the same measurement, because the batch weights are known and the volume has just been measured. A mix that yields high has spread its cement over more concrete than the design intended.

It depends entirely on a container of known volume, and that volume is a calibration rather than a nameplate. A measure that has been dropped, dented or simply not re-calibrated reports a yield error that belongs to the equipment, not the concrete.

It measures the concrete as delivered. Water added on site after the test invalidates it, which is why the test is taken at the point of discharge and the ticket records what happened afterwards.

Obtained from ASTM International. What ASTM publishes

This site holds no edition list for ASTM C138, so a citation here names ASTM C138 rather than a year. The publisher above has the current designation.