A strength dispute arrives with evidence. A volume dispute arrives with an empty form and two people who are each certain. The crew says the last truck was light. The plant says the batch weights are on the ticket and the ticket balances. Both can be telling the truth, because the volume that was produced at the plant and the volume that disappeared into the work are two different quantities, measured in two different places, and only one of them is measured at all on most jobs.
Yield is the quantity that connects them. It is worth understanding before the pour that runs short, not during it.
What yield actually is
Yield is the volume of fresh concrete produced from a known quantity of materials. That is the definition in the scope of ASTM C138/C138M, which “covers determination of the density of freshly mixed concrete” and “gives formulas for calculating the unit weight, yield or relative yield, cement content, and air content of the concrete.” In Canada the equivalent gravimetric density-and-yield procedure sits in the CSA A23.2 series alongside the air, slump and sampling methods your spec already names.
The important consequence of that definition is a matter of units. Ready mixed concrete is batched by mass and sold by volume. Nothing at the plant measures cubic metres directly. The batch is weighed, and the volume it produced is calculated: total batch mass divided by the density of the fresh concrete. NRMCA’s CIP 8 on yield discrepancies sets out the arithmetic in exactly that form, with a worked example — a batch totalling 14,640 kg at a measured density of 2,430 kg/m³ yields 6.0 m³.
Read that equation backwards and the point lands. If nobody measured density, nobody measured yield. The number of cubic metres on the ticket is a calculation using an assumed density, and the only way to confirm it is to put fresh concrete in a measure and weigh it.
Where the volume actually goes
The test exists because the two common explanations for a short pour have nothing in common.
A genuine yield shortfall happens at the plant, and it is a batching or air problem. Material masses differ from target within permitted tolerances, air content lands at the low end of its range, and the batch produces slightly less volume than the design assumed. CIP 8 is direct about who fixes that one: real yield discrepancies, measured by density, are corrected by the producer.
An apparent shortfall happens on site, and it is a geometry problem. CIP 8 lists the causes and quantifies the worst of them: a 3 mm difference in a 100 mm slab is a three percent shortage — one cubic yard on a 32 cubic yard order. Form deflection under concrete pressure, settlement into irregular subgrade or granular fill, over-excavation, spreading forms, spillage, waste, and the small quantities diverted daily into mud sills and incidental footings all add to the same column. ASTM C94 is quoted in that document for the conclusion, and the wording matters when a credit is being argued: apparent shortfalls from waste, spillage, over-excavation, spreading forms, air loss or settlement are “none of which are the responsibility of the producer.”
One more line in CIP 8 deserves to be in every estimator’s head: hardened concrete in place may be about two percent less than its fresh volume. Measuring the finished slab and working backwards will always understate what was placed.
Why the contract makes this worse
The commercial structure of most concrete work guarantees that someone absorbs the difference, because the two sides of the transaction are measured on different bases.
Look at how a Canadian public owner pays. Manitoba Infrastructure’s cast-in-place concrete specification states that “The supplying and placing of cast-in-place concrete will be measured on a volume basis,” and then defines that volume: “The volume of cast-in-place concrete to be paid for will be the total number of cubic metres computed from the neat lines on the Drawings.” The owner pays theoretical geometry. The supplier invoices discharged volume. Everything between the two — deflection, over-excavation, subgrade settlement, waste — sits in the contractor’s margin whether or not anyone named it in the bid.
That same specification shows what does and does not get tested. It requires that each batch be accompanied by “a time slip issued at the batching plant, bearing the time of batching,” requires the supplier to keep “a record of the actual proportions used for each concrete pour,” and lists density among the properties trial batch data must report — density, air content, temperature and slump. It does not require a field yield calculation on any load. The data that would settle a volume dispute is available and almost never collected.
How the density test is run
The procedure is short, and the discipline is in the sampling rather than the weighing.
CIP 8 follows ASTM C94 in calling for samples from three different truck mixers, with the results averaged — a single load tells you about a single load, not about the mix. The measure is filled in three layers, each layer rodded 25 times, with the sides tapped by mallet to close the voids left by the rod. The surface is struck off with a flat plate, the outside of the measure is cleaned, and the filled measure is weighed. Density is the mass of concrete divided by the known volume of the measure. The scale has to be accurate and level, which on a jobsite is a real constraint rather than a formality.
Because that result depends entirely on the sample being representative, the step that ruins yield numbers is the same step that ruins air and strength numbers — a sample taken from the first or last of the discharge, or taken without remixing. Our note on how to sample fresh concrete covers the requirement properly, and a yield test built on a bad sample is worse than no test, because it produces a number people then argue from.
| What you measure | What it tells you | What it does not tell you |
|---|---|---|
| Density of fresh concrete | The volume the batch actually produced | Where placed concrete went |
| Batch mass from the ticket | The input side of the yield calculation | Whether the mix was as designed |
| Air content | Why density moved | Whether the air survived placement |
| Slab or form dimensions as built | Site consumption | Fresh volume, which was about 2% higher |
| Neat-line volume from drawings | What the owner pays for | What the pour consumes |
Why air content and yield move together
Air is the variable that connects the two halves of this problem, and it moves yield in the direction people least expect.
Entrained air occupies volume. A batch at the top of its air range produces more cubic metres from the same mass of materials than the same batch at the bottom of its range. So a load that tests low on air also tests low on yield, and a load that tests high on air produces generous volume — which is not good news. CIP 8 treats excess yield as a warning sign when it comes from extra air, extra water, or extra aggregate, or from forms that were not properly filled. Volume gained that way is volume you did not want.
Air also moves after the truck. CIP 8 lists loss of entrained air and reduced air content in hardened concrete among the causes of apparent shortfall, which means a slab can finish with less volume than the fresh concrete that went into it even when the plant delivered exactly what the design called for. That is why the specified air range is a placement constraint and not just a durability one, and why the air content test and its CSA range belongs on the same ticket conversation as yield.
What to do before the pour, not after
Four decisions made in advance remove most volume disputes.
Measure the forms and order above plan volume. CIP 8 recommends ordering four to ten percent above plan volume to cover waste and contingencies. On elevated work, brace forms to limit deflection, because deflection is volume.
Prepare the subgrade to elevation. Compacted and finished to the correct grade, on purpose. Settlement into soft or uneven fill is the single largest unbudgeted consumer of concrete on flatwork.
Check yield early, not at the end. Run density on the first loads, while there is still time for the answer to change what you order. CIP 8 recommends exactly this, and repeating it if problems appear. A yield test on the last truck proves nothing anyone can act on.
Reevaluate volume near the end of placement and call the plant early. The difference between a scheduled top-up load and a short pour is usually thirty minutes of notice. Supply is a logistics problem until it becomes a cold joint.
The paperwork side is simpler than it looks. Keep the delivery tickets, because the batch masses on them are one half of every yield calculation you might need later — our breakdown of the numbers on a concrete batch ticket sets out which fields matter and why.
Order the volume, and the mix, against the real geometry
Most short pours we are asked about were decided at the order, not at the discharge. Send us your project details — pour date, mix type, PSI, volume — and our team will get back to you within 1 business hour with a quote and delivery slot. If the geometry is unusual, say so when you order: we would rather talk through form deflection and subgrade before the schedule depends on a top-up truck. See our custom mixed concrete in Calgary page for mix supply, or read how our volumetric mixers work if the pour is one where ordering to the cubic metre matters.
FAQ
Does a yield test tell me whether the plant short-delivered? It tells you what volume the batch produced from the masses on the ticket. That is the only direct evidence either side has. If measured density shows the batch produced less than the ticket claims, that is a yield discrepancy the producer corrects. If the batch produced what it should have, the volume went into the work.
How many loads do I have to test? The practice NRMCA follows is sampling three different truck mixers and averaging the results, consistent with ASTM C94. One load is a data point about one load.
Can I prove a shortfall by measuring the finished slab? Not reliably. Hardened concrete in place may be about two percent less than its fresh volume, so slab measurement understates what was placed before you even reach the question of subgrade settlement or form deflection.
How much should I order above the drawing quantity? NRMCA’s guidance is four to ten percent above plan volume for waste and contingencies, with the upper end appropriate where subgrade is irregular or forms are likely to deflect.
Why did a load test high on yield? Usually extra air, extra water, or extra aggregate, or forms that were not completely filled. Excess yield is treated as a warning sign rather than a bonus, because the volume arrived at the cost of something the mix was specified to have.
Where does Canadian practice put this test? The gravimetric density, yield and air content procedure sits in the CSA A23.2 series, alongside the sampling, slump and air methods your specification already references. Clause numbering and any project limits come from the standard and from your own specification, so confirm both before a dispute rather than during one.
Sources
- NRMCA, CIP 8 — Discrepancies in Yield — https://www.nrmca.org/wp-content/uploads/2021/01/08pr.pdf
- ASTM C138/C138M, Standard Test Method for Density (Unit Weight), Yield, and Air Content (Gravimetric) of Concrete — scope and abstract — https://store.astm.org/c0138_c0138m-17a.html
- Manitoba Infrastructure, Specification 1030 — Reinforced Cast-in-Place Concrete — https://www.gov.mb.ca/mti/contracts/pdf/manual/1030.pdf
