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ASTM C138 · gravimetric yield check
Cement-bag mix yield check
Express a mix design in the unit a site can count: bags per batch, the volume a batch makes, and the concrete each bag of cement is responsible for.
Verified engine
What the engine returns
The bag count is what to hand the mixer operator. The batch yield is the volume that batch should fill, and the yield per bag is the figure to check against reality on the next pour. If the concrete consistently falls short of the batch yield, the cement content being achieved is higher than the design — the mix is richer and more expensive than intended, and that usually means aggregate is being under-measured.
Inputs the result is built from
Cement mass in the batch
Standard bag mass
Design cement content
How the result is derived
MethodBatch-yield check against the mix design’s cement content
StandardASTM C138/C138M — Standard Test Method for Density (Unit Weight), Yield, and Air Content (Gravimetric) of Concrete
GuardA design cement content outside the range normal structural concrete occupies is refused rather than computed. Values below it are usually lean-mix or blinding figures used by mistake; values above it are usually a units error, and either would produce a bag count that looks entirely plausible.
How the yield per bag moves with the design cement content
What yield per bag tells a site team
A mix design specifies cement content: kilograms of cement per cubic metre of finished concrete. It is the right unit for a specification and the wrong one for a mixer, where the material arrives in bags of a standard mass and is counted rather than weighed.
Dividing the batch cement mass by the design cement content gives the volume that batch should produce. Dividing it by the bag mass gives the number of bags in it. Dividing one by the other gives yield per bag: the concrete a single bag of cement is responsible for.
That last figure is the useful one, because it is stable. It does not change with batch size, it can be checked against what actually came out of the mixer, and a site team can hold it in their head. If a batch of six bags does not fill what six bags should fill, the batching has drifted — usually aggregate, usually moisture.
This is a yield CHECK, not a mix design. It takes a cement content that a design already established and expresses it in field units. It cannot tell you whether that cement content was right.
A bag count also assumes the bags are still cement. Cement is hygroscopic: stored off a pallet, unwrapped, or through a humid season, it pulls moisture from the air and air-sets into lumps of partially hydrated clinker. A bag with hard lumps that do not crumble in the hand has already spent part of its binding capacity, and no count of such bags reproduces the designed content.
Batch-yield check against the mix design’s cement content
When this calculation is used
- Converting a designed cement content into a bag count for a site batching sheet.
- Checking a delivered or site-mixed batch against the volume it should have produced.
- Diagnosing a short pour: too few cubic metres from the right number of bags.
- Costing a pour in the unit cement is actually bought in.
Worked example
A site batch is made up with a whole number of standard bags, against a mix design written to an ordinary structural cement content.
The bag count is what to hand the mixer operator. The batch yield is the volume that batch should fill, and the yield per bag is the figure to check against reality on the next pour. If the concrete consistently falls short of the batch yield, the cement content being achieved is higher than the design — the mix is richer and more expensive than intended, and that usually means aggregate is being under-measured.
The figures are produced by the certified engine when the calculator loads, from this pack’s own declared example. Nothing on this page is a stored answer.
What each input represents
Cement mass in the batch
The total cement going into one batch, however it is measured. Where supplementary cementitious materials are used, this is the cement alone unless the design’s content was written on a total-cementitious basis — the two bases are not interchangeable and mixing them shifts the yield without any sign that it has.
Standard bag mass
The mass of one bag as supplied. It is regional: the same nominal bag is a different mass in different markets, and a bag count carried between them is a quiet error.
Design cement content
Kilograms of cement per cubic metre of finished concrete, from the mix design or the durability class being worked to. This is the figure the check is made against — it is an input to the calculation, never an output of it.
Assumptions and limits
- The design cement content is correct and applicable. This page checks consistency with it; it has no opinion about whether the content satisfies a durability class.
- Bags are full and the standard mass is the actual mass. Part-bags and settled or partly hydrated stock are the most common reason a real yield disagrees with this one.
- Yield here is derived from cement content, not measured. ASTM C138 measures yield gravimetrically from the fresh density of the concrete itself; that measurement and this calculation should agree, and where they do not, the measurement wins.
- No allowance is made for material left in the mixer, in the barrow or on the ground. On small batches that loss is a larger fraction than most people expect.
What the guards protect against
- A design cement content outside the range normal structural concrete occupies is refused rather than computed. Values below it are usually lean-mix or blinding figures used by mistake; values above it are usually a units error, and either would produce a bag count that looks entirely plausible.
- The guard is on the design content because it is the only input that carries an assumption. Batch cement mass and bag mass are facts about what is on the ground, and constraining them would refuse legitimate small batches and unusual bag sizes.
- Nothing here refuses a fractional bag count. Batches genuinely are made up in part-bags, and a calculator that pretended otherwise would be rounding a real quantity for tidiness.
Provenance
ASTM C138/C138M — Standard Test Method for Density (Unit Weight), Yield, and Air Content (Gravimetric) of Concrete
Batch-yield check against the mix design’s cement content
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