Four minutes on the ramp
The drum is turning, the pump is primed, and the gang is standing in whatever weather the morning brought while the driver holds a clipboard out of the cab window. Everything still open about this load has to close in the time it takes to walk to the chute and back. After that the concrete is either in the forms or it is somebody's disposal problem, and the ticket in your hand becomes the only surviving account of what was ever in the drum.
Signing is not a courtesy to the driver. The ticket is the delivery record, the quantity receipt, and — where water goes in at the site — the document that moves responsibility for a change to the mixture from the producer to the purchaser. ASTM C94/C94M, Standard Specification for Ready-Mixed Concrete, builds most of the commercial relationship around that one page: what must be printed on it, what the purchaser may require be printed on it, and which entries the receiver makes and initials. Disputes about a low break, a pour that ran a bay short, or a wall mix that ended up in a slab are all settled months later by reading these same tickets, at which point the boxes left blank are the ones that decide the argument.
Two questions genuinely belong to the gate, in the strict sense that they are answerable there and unanswerable afterwards. The first is thermal: the load has arrived at some temperature, and that temperature governs how much finishing window exists and whether the specification permits placing it at all. The second is volumetric: the ticket claims a number of cubic metres, and the only honest way to test the claim is to weigh a sample of the material and divide. Consistence, air content and strength matter as much or more, but the first two belong to a sampling regime with its own agreed location and the third is settled twenty-eight days later by a testing house.
What is printed, and what you had to ask for
A ready-mixed ticket carries two classes of information, and knowing which class a line belongs to is worth more at the gate than reading any single line. One class is mandatory under ASTM C94 and prints on every ticket without anyone requesting it: the identity of the plant and of the ticket, the truck, the purchaser and the job, the class or designation of concrete, the volume claimed, and the time the batch was made. The other class exists only because the purchase order asked for it — the batch weights of each ingredient, the admixtures and their dosages, the revolution counter reading when water first met cement, the water withheld at the plant against aggregate moisture. That second class is precisely what the yield arithmetic further down this page needs, which is a strong argument for asking at the order stage instead of standing at the ramp discovering it was never requested.
The most useful habit at the window is to read the class designation before anything else on the page. A ticket printed against the wrong mixture is not rare: a plant running six designs into one job across a single morning will eventually send the wall mix to the slab, and the driver has no way to know. It is the only failure available at the gate that costs nothing to catch and a great deal to miss, and catching it means comparing the printed designation against the pour card in your other hand rather than against a memory of what was ordered on Monday.
| Entry | Where it comes from | What it settles |
|---|---|---|
| Plant, ticket serial number, date, truck number | Printed on every ticket | Traceability, when a cylinder breaks low and three trucks are candidates |
| Purchaser, job, and the class or designation of concrete | Printed on every ticket | Whether this drum holds the mixture this element was designed around |
| Volume of concrete claimed | Printed on every ticket | The quantity being invoiced, and the figure the yield check tests |
| Time of batching, when water first met cement and aggregate | Printed on every ticket | Where the discharge limit stands, in minutes rather than impressions |
| Water added at the site, signed or initialled by the receiver | Entered at the gate, by you | Who owns the change to the water-cement ratio of record |
| Batch weights of cement, aggregates, water and admixtures | Supplied when the purchaser requires it | The total batch mass, without which no yield can be computed at all |
| Revolution counter reading at the first addition of water | Supplied when the purchaser requires it | The half of the discharge limit that elapsed time does not describe |
The clock started when the water met the cement
ASTM C94 caps the interval between batching and complete discharge two ways at once, and both ceilings apply: an hour and a half of elapsed time, or three hundred revolutions of the drum, whichever arrives first, counted from the moment mixing water reached the cement and aggregate. Neither clock starts when the truck reaches the site. A load batched at the plant, sent to a job that could not take it, and diverted to yours has spent both allowances on the road, and the revolution counter is the only witness to the second one. A drum turning at agitating speed for forty minutes in traffic accumulates revolutions exactly as it does under the chute.
The limit is waivable, and the wording of the waiver matters. C94 allows it to be set aside where the concrete is still of such workability that it can be placed without adding water — which is a statement about the material in front of you, not about how far behind schedule the pour is. The engineered version of the same relief is a hydration-stabilising admixture dosed at the plant for a known haul, and that is a decision taken before the truck loads rather than argued at the ramp. Other regimes frame the same problem differently: EN 206, Concrete — Specification, performance, production and conformity, leans on the producer's declared consistence retention rather than a universal number, and AS 1379, Specification and Supply of Concrete, states its own. Whichever governs the job, a load that has been waiting is not automatically a bad load; it is a load whose consistence has to be established rather than assumed.
Whose heat is it
The temperature reading is the easiest measurement on site to take badly. ASTM C1064/C1064M, Standard Test Method for Temperature of Freshly Mixed Hydraulic-Cement Concrete, asks for the sensor to sit with at least 75 mm (3 in) of concrete around it in every direction, to be left in place for a minimum of two minutes or until the reading stops moving, and for the whole measurement to be finished within five minutes of taking the sample. A probe waved through the falling stream at the chute reads the air, the steel and the concrete in unknown proportions, and it reads them low on a cold morning and high on a hot one — always in the direction that makes the load look acceptable.
What the number is compared against comes from the specification, not from the weather. ACI 305.1, Specification for Hot Weather Concreting, carries a maximum as-delivered concrete temperature, widely written into projects at 35 °C (95 °F) and tightened below that where hauls are long or sections are thin. ACI 306.1, Specification for Cold Weather Concreting, works the opposite end with minimum as-placed temperatures banded by the section's least dimension, running from roughly 13 °C (55 °F) for thin members down to about 5 °C (40 °F) for mass pours, on the reasoning that a thin section sheds its own heat of hydration long before a raft does. Read the project specification for the governing figure and read the ACI documents for why the figure sits where it does, because the second is what lets you argue for a variation on a particular day.
Once the reading is in, the interesting question is not what the temperature is but which ingredient owns it. The heat balance in ACI 305R, Guide to Hot Weather Concreting, resolves that cleanly. Aggregate is around three quarters of the batch mass, so a stockpile baking in the sun dominates the answer and a stockpile shaded or sprinkled the night before is the cheapest temperature control that exists. Cement leaves the silo hot and holds that heat for days, but it is a fifth of the mass at roughly a fifth of water's specific heat, so even a very hot silo shifts the result by a degree or two. Water carries more than four times the specific heat of the solids per kilogram and is only about seven percent of the mass, which is why chilling the batch water helps and rarely helps enough. Ice is the lever that actually moves things, because melting a kilogram of it absorbs about eighty times the heat that warming a kilogram of water by one degree does.
None of that is a site action. You cannot cool the load in the drum, and nothing in the arithmetic changes what is standing in front of you — it either meets the specified ceiling or it does not. What the arithmetic changes is the next eight trucks. A balance run at the gate, using the batch weights off the ticket and a genuine stockpile temperature rather than a guessed one, is a plant instruction: shade the coarse aggregate, sprinkle it, charge part of the batch water as ice, or start the pour at four in the morning. Run it as a diagnosis and hand the result to the plant while the second load is still being batched.
The thermometer says what this load is; it says nothing about which ingredient owns the number or what the plant would have to change to hit the ceiling on the rest of the day's trucks. The balance describes the mixer rather than the chute, so the gap between its answer and your gate reading is the haul, the drum friction and the sun on the barrel — worth knowing separately on a long delivery.
Cementitious content of one batch, as batched.
Temperature of the cement as it leaves the silo.
Coarse and fine aggregate together, dry, for the same batch.
Temperature of the stockpile at the depth the loader is digging.
Surface water carried on the stone, as a percentage of its dry mass.
Water added at the plant, excluding whatever the aggregate brought with it.
Temperature of the liquid water as batched.
How much of the batch water is charged as flaked or crushed ice instead of liquid.
The as-delivered temperature the specification allows or requires.
Predicted concrete temperature
81.5 °F
A prediction of the mix temperature leaving the plant. Haul time, drum friction and sun on the barrel all add to it before discharge, and none of them is in this balance.
- Same batch with no ice substituted
- 81.47 °F
- Difference from the target
- -4.53 Δ°F
- Ice charged
- 0 lb
- Liquid water charged
- 330 lb
- Free moisture riding on the aggregate
- 122.4 lb
- Ice that would reach the target on its own
- 0 lb
- Water temperature that would reach the target
- 88.67 °F
They open the calculator with your figures already in it
Concrete Mix Temperature and Ice Substitution Calculator: 81.47 °F — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
What this calculation does not cover
- Predicts the temperature at the mixer, not at the point of placement.
- Aggregate temperature dominates the answer and is the input most often estimated rather than measured.
- Cement and aggregate are treated with one shared specific heat, as ACI's equation does.
- Ice must be fully melted before discharge; beyond roughly three quarters of the batch water that stops being reliable.
Predicted at 27.5 °C (81.5 °F) against a target of 30.0 °C (86.0 °F), this batch has 2.5 degrees Celsius of margin at the mixer, which is 4.5 Fahrenheit degrees. Spend it carefully: haul, drum friction and sun on the barrel all draw on it before the concrete reaches the forms.
The water box, and the initials beside it
There is a box on the ticket for water added at the site, and there is a space beside it for the receiver's initials. The pairing is the whole point. ASTM C94 permits water to be added on delivery once, in a single controlled addition, provided the specified maximum water-cement ratio and the specified consistence are not exceeded, with the drum then turned at least thirty revolutions at mixing speed so the addition is distributed rather than sitting as a slug against the fins. Filling in that box converts an informal act into a record, and initialling it says the site asked for the change and accepted its consequences.
It is worth knowing why loads arrive stiff, because the reason changes the right answer. Plants withhold trim water against the free moisture riding on the aggregate, and on the morning after heavy rain that correction can overshoot in either direction. A high-slag or high-fly-ash mixture behaves differently in the first hour than a plain portland one. A long wait at a gate does what a long wait always does. Against those, adding water to a load already at its specified maximum water-cement ratio is a breach of the mix design whatever the slump cone says, and the honest alternatives are an admixture redose authorised by the supplier or a load that goes back. Where site water comes from a standpipe of unknown provenance or from a plant's recycled water system, ASTM C1602/C1602M, Standard Specification for Mixing Water Used in the Production of Hydraulic Cement Concrete, is the document that decides whether it was water at all.
Weigh a bucket of it and the argument stops
The density test in ASTM C138/C138M, Standard Test Method for Density (Unit Weight), Yield, and Air Content (Gravimetric) of Concrete, is the cheapest instrument at the gate and the only one that produces a number nobody can talk their way around. A calibrated measure, a rod, a mallet, a strike plate and a scale, and a sample taken to ASTM C172/C172M from the middle portion of the discharge rather than from the first or last of it. Size the measure against the nominal maximum aggregate: the small measures most crews carry suit ordinary aggregate sizes, and larger stone needs a larger measure or the result is a measurement of how badly the rod fitted.
Interpreting the answer is where crews reach for the wrong benchmark. Normal-weight concrete generally lands somewhere near 2,240 to 2,400 kg/m³ (140 to 150 pcf), and structural lightweight commonly falls between about 1,440 and 1,840 kg/m³ (90 to 115 pcf), but those bands are wide enough to swallow every problem worth catching. The comparison that finds things is against this plant's own recent results for this same design. On a 2,400 kg/m³ mixture, each percentage point of air is worth roughly 24 kg/m³ (about 1.5 pcf), so a load reading 60 kg/m³ light against last week's figure for the same ticket designation is carrying something like two and a half percent more air than it did — or the aggregate proportions have moved at the plant, which is the same conversation with a different ending.
Most of the error in this test lives in the handling rather than in the arithmetic, and all of it lands in the final figure at full weight, because the calculation is nothing more than a subtraction and a division.
- Sample from the middle of the discharge, compositing across at least two intervals as ASTM C172/C172M requires, and remix the sample in the barrow before filling anything.
- Weigh the measure clean, dry and empty, and use its calibrated volume rather than its nominal one — measures get dropped, and a dented 7 L measure is not a 7 L measure any more.
- Fill in three roughly equal layers, rodding each layer with the standard rod distributed evenly over the surface, and penetrating a short way into the layer beneath on the second and third.
- Tap the side of the measure ten to fifteen times with the mallet after each layer, to close the rod holes and release the air the rodding trapped.
- Strike the top off flat with the plate and wipe the outside of the measure before the second weighing — a smear of mortar on the wall is real mass, and it goes straight into the density.
- Subtract, divide by the measure's volume, and compare the result against the plant's recent figures for this design before comparing it against any published range.
Two masses and one calibrated volume, which is all this test ever produces. The density that comes out is the input every yield dispute on the site will eventually be run through, so it is worth getting off the scale and into a number before the barrow is tipped out.
The total mass of the measuring container filled with consolidated concrete.
The mass of the empty measuring container.
The calibrated volume of the measuring container.
Unit weight
158.2 pcf
- Net concrete mass
- 37 lb
- Equivalent in pcf
- 158.16 pcf
They open the calculator with your figures already in it
Concrete Unit Weight (Density) Calculator (ASTM C138): 158 pcf — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 158.2 pcf — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- Unit weight is a density measurement, not an air-content test. It cannot separate entrained and entrapped air from the rest of the volume unless you already know the mix's theoretical air-free density, so it does not replace a pressure or volumetric air meter.
- The calculation treats the measure's stated volume as exact. It makes no allowance for a measure that has not been water-calibrated, one that is dented or out of round, or for how the sample was consolidated and struck off — under-rodding leaves voids that read back as low density, and none of that is visible in the arithmetic.
- This is the fresh, plastic density at the point of sampling. It is not the hardened in-service density used for dead-load calculations, which is lower once free water leaves the concrete; for lightweight mixes the equilibrium density is a separate determination.
- One sample, no verdict. The calculator does not compare the result against a design or specified density, apply an acceptance tolerance, or account for where in the load the sample was taken or how long after batching it was drawn.
- The measure volume field is capped at 30 L (about 1 cubic foot). A larger measure — the size used where the nominal maximum aggregate is oversize — cannot be entered; the value is clamped to the cap, which computes the density against a volume you did not enter.
From a density to the cubic metres actually in the drum
The yield check is a single division and it settles a question that otherwise runs on for weeks. Take the total batch mass off the ticket — every ingredient, including any water added at the site and recorded in its box — and divide it by the density just measured. That gives the volume this batch actually made. Divide that in turn by the volume ordered and the result is relative yield, where 1.00 means the drum produced exactly what the ticket claims. This is not an improvised site method: ASTM C94 makes gravimetric determination the basis on which a batch's volume is established, in preference to drum markings, plant design volumes or anyone's recollection of how full it looked.
The direction of the error decides who has a problem. Below 1.00 the load is short, the last bay does not get poured, and the site is buying concrete it never received. Above 1.00 the batch made more volume than it was designed to make, which sounds like a bonus and is not: the same mass of cement is now spread through more cubic metres, so the cementitious content per cubic metre has fallen below what the design leaned on, usually because air content ran above what was assumed. That is a durability and strength finding rather than an invoicing one, and it is the version of the error that never gets noticed because nothing runs out. A band of about two percent either side of 1.00 is commonly applied, but read what the project specification actually names before quoting a tolerance to anybody.
When a load comes up short, the causes are rarely exotic. Air content lower than the design assumed shrinks the volume for the same mass. Aggregate free-moisture assumptions at the plant move the water and the aggregate in opposite directions. Scales drift. And a good part of an apparent shortfall is often concrete that existed but never reached the works: the prime discharged to the skip, the material left standing in a pump line, the washout. Two structural facts help keep the argument honest. The yield check needs the ticket's batch weights, which sit in the requestable class from earlier on this page and must be asked for at the order stage. And a truck's rating is not its drum: ASTM C94 limits a load to 63 percent of the gross drum volume when the drum is mixing and 80 percent when it is only agitating, so nobody should be estimating volumes by looking at the barrel.
Total batch mass over measured density, tested against the volume the ticket claims — that is the entire yield question, and it belongs in the two minutes before the signature rather than in the two months afterwards, when there is nothing left to weigh.
The total mass of all ingredients in the batch.
The batch's measured unit weight, from an ASTM C138 unit weight test.
The volume the mix design was intended to produce.
Relative yield
1.07 (ratio, target 1.0)
The batch produced MORE volume than designed — the mix may be under-yielding on materials relative to the design, or air content is higher than assumed.
- Actual yield
- 1.34 yd³
They open the calculator with your figures already in it
Concrete Relative Yield Calculator (ASTM C138): 1.07 (ratio, target 1.0) — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
What this calculation does not cover
- A relative yield of 0.96 or 1.03 tells you the batch missed its design volume; it does not tell you why. Air content, aggregate moisture, scale calibration and admixture dosage all move yield, and none of them are inputs here — this flags the symptom and leaves the diagnosis to the air meter, the moisture test and the batch records.
- This is a volume check, not an acceptance test. A batch can land at 1.00 relative yield and still fail its specified strength, slump or air content, and none of those are evaluated here.
- The total batch mass is taken as entered and never verified. It has to be the mass actually batched, with free moisture on the aggregates included — enter the mix design's SSD proportions instead of the batch ticket's real weights and the yield error is invisible to this calculation.
- One unit weight sample, one batch. Yield varies within a load and between loads, and nothing here allows for that variation, for the calibration of the measure the sample was struck off in, or for how well the sample was consolidated.
- The ±2% window used to pass or fail the result is a common working rule, not an acceptance clause — ASTM C138 gives the yield computation, not a tolerance. Whether you were delivered the volume you paid for is settled under the delivery tolerance in the purchase specification, not by this ratio.
What a density reading will not tell you
Density is one scalar, and several different faults move it the same way. A load two percent over on air and a load batched with a lighter aggregate fraction can read identically, and neither of them announces itself. Separating the two needs an air test in its own right — ASTM C231/C231M by the pressure method for normal-weight concrete, or ASTM C173/C173M by the volumetric method where lightweight or highly porous aggregate makes the pressure method unreliable. C138 does return a gravimetric air content, but only once the theoretical air-free density from the mix design is in hand, which is one more line to request rather than one more line to assume.
Nor does it say anything about strength. That belongs to specimens made and cured to ASTM C31/C31M, broken to ASTM C39/C39M, and judged against the acceptance criteria in the project specification and in ACI 318, Building Code Requirements for Structural Concrete. Consistence belongs to ASTM C143/C143M and to the tolerance the specification names around the ordered value. The gate is a filter, not a laboratory. Its job is to stop the loads that are visibly wrong and to leave enough record behind on the rest that a dispute three months later has evidence in it rather than recollections.
Turning a truck around, and what each option costs
Rejection is cheap at the ramp and expensive at every point downstream of it, and the comparison is only honest when both columns are drawn. Sending a load back costs a delivery, a gap in the pour, and possibly a construction joint the drawings never showed. Placing a load that was out of specification costs a core, an assessment, a concession negotiated with the engineer or, at the far end, a demolition — plus a commercial argument whose outcome depends entirely on what somebody wrote on a ticket at seven in the morning.
The worst available choice is the compromise in the middle: taking half a doubtful load, or splitting it across two elements to dilute the problem. That converts one element of known doubt into two of unknown quality and destroys the traceability the ticket had been keeping. Decide the load as a load. The sequence below is the order the checks fall in at the gate, arranged so that the cheapest decision to reverse comes first.
- Read the class designation against the pour card before the driver reverses onto the ramp. The wrong mixture is the one gate failure with no remedy once it is in the forms.
- Read the batch time and the counter reading, and work out what is left of the discharge allowance after the truck is positioned and the line is connected, not before.
- Take the temperature to ASTM C1064/C1064M with the sensor properly buried, and compare it against the ceiling or the floor the specification names for this element.
- Establish the consistence before anyone reaches for a hose, and if water is going in, put the quantity in the ticket's box and initial it at the time rather than at the end of the pour.
- Where the quantity is contested, or the pour looks like running short, weigh a measure of it and run the yield before the truck leaves the site.
- Write the reason for any rejection on the ticket, keep your copy, and telephone the plant while the next load is still being batched, when the information can still change something.
On the clipboard before the first drum turns
What the person signing needs in hand, and the arithmetic that turns a reading at the ramp into a decision.
- The mix designation, copied from the pour card — Wrong class of concrete is the only gate failure with no remedy once it has been placed.
- Batch weights requested on the purchase order — Ask at the order stage; without a total batch mass there is no yield check, only an argument about drum markings.
- A thermometer to ASTM C1064 and a way to bury the sensor — Seventy-five millimetres of concrete around it in every direction, held until the reading stops moving.
- The temperature ceiling and floor named for this element — Cold-weather minimums vary with the section's least dimension, so a raft and a wall do not share one figure.
- A calibrated unit weight measure, rod, mallet and strike plate — The only test at the gate producing a number the plant cannot talk its way around afterwards.
- The plant's recent density results for this same design — A published range catches almost nothing; last week's figure for this mixture catches nearly everything.
Opens the calculators above on one screen with the dimensions from this article already filled in. Quantities only — this site publishes no price list, because local prices vary too much to publish honestly.