Ordering Concrete for a Lift of Columns

Twelve columns, two loads, and a joint nobody drew

The lift is twelve columns, 450 square, standing 3.6 m from the top of the kicker to the underside of the beam soffit. That is 0.729 m³ apiece and a shade under nine cubic metres for the whole floor — less concrete than a modest driveway, and roughly a truck and a half. It is also, load for load, the order on the frame most likely to cost somebody a fortnight. A driveway that comes up short gets the screed rail pulled in and finished a little narrower. A column that comes up short has a horizontal plane at 3.1 m that nobody detailed, nobody roughened, nobody removed laitance from, and nobody on site is authorised to accept.

The two ways of getting it wrong are not symmetrical, which is the thing worth internalising before anyone rings the plant. Over-order and you have paid for concrete that now has to go somewhere legal — a cost, an argument with the supplier's terms, and a washout problem, but all of it settled the same week. Under-order and you have created a structural non-conformance in a compression member at the point where its reinforcement is most congested, and it is settled by an engineer's assessment, a possible cut-back to sound concrete, and a paper trail that follows the building. Money on one side, the frame on the other.

So the order has four things to get right, and only the first is arithmetic. What volume is actually going into the forms. How much of a delivered cubic metre is really there once the plant's yield is accounted for. How fast the concrete is allowed to arrive without bursting a form that fills in four minutes. And how long the gap between two trucks can be before the first layer stops taking a poker. The rest of this is those four questions in the order you have to answer them.

The clock started at the plant, not at your gate

ASTM C94/C94M puts a limit on how long a load may take: discharge is to be completed within one and a half hours, or before the drum has turned 300 revolutions, whichever comes first, counted from the moment the mixing water met the cement and aggregates. Those limits can be waived by the purchaser where the concrete is still of a slump and workability that lets it be placed without adding water — a waiver, note, not a default, and one that belongs in writing before the pour rather than in a conversation at the gate.

Almost none of that window belongs to you. Batching, loading, the drive, the queue at the gate, the wait while the skip goes back up on the crane, the pump finishing somebody else's job on the far side of the site — all of it is spent time. A second truck standing at the kerb for forty minutes because the first is discharging slowly has burned nearly half its window before the chute has swung out, and it is the load whose last cubic metre goes into the heads of the last four columns.

Inside a single column the same clock runs faster and matters more. Concrete goes into a column in layers the poker can reach through — ACI 309R, Guide for Consolidation of Concrete, describes the vibrator penetrating into the layer beneath so the two are consolidated as one body rather than stacked. The moment the underlying layer has stiffened past the point where the poker will re-fluidise it, you have a cold joint, whether you accepted one or not. It does not announce itself; it appears when the form comes off, as a line across the section at exactly the height the last barrow ran out.

Heat shortens everything. ACI 305.1, Specification for Hot Weather Concreting, is the document that governs how a hot delivery is managed, and a retarding admixture to ASTM C494/C494M — Type B, or Type D where water reduction is wanted with it — is the usual purchased answer. Both are order-stage decisions: the retarder has to be batched in at the plant, and the concrete temperature you are prepared to accept is a specification line, not something to negotiate with a driver holding a thermometer.

What an unplanned joint costs is worth knowing in advance, because it changes how much standby time looks worth paying for. ACI 301, Specifications for Structural Concrete, requires construction joints to be located where the contract documents show them or where the engineer approves them, which is another way of saying an accidental one is a non-conformance from the moment it forms. The assessment that follows may accept it, may require the column to be cut back to sound concrete and re-poured against a prepared surface, and will in any case involve the engineer who detailed the lap splices that cross the joint. Set that against the cost of holding a truck for half an hour and the arithmetic stops being close.

Writing the order so the plant can price what you meant

A concrete order is not a strength and a volume, and treating it as one is how a column lift ends up with the slab mix. ASTM C94/C94M sets the ordering options out formally — the purchaser specifies by prescription, by performance, or by a defined split of the two — and that choice determines who owns the problem when the cylinders come back low. In the United Kingdom the equivalent framework is BS 8500-1, which defines designated, designed, prescribed and standardised prescribed concretes and lists what the purchaser has to state for each. Both agree on the underlying point: the plant supplies what the order describes, and any property you did not name is one you did not buy.

The line a column changes is the aggregate. ACI 318 limits the nominal maximum aggregate size to no more than three quarters of the clear spacing between reinforcing bars, one fifth of the narrowest dimension between form faces, and one third the depth of a slab. In a 450 column the form-face limit is no constraint at all — a fifth of 450 is 90 mm — so the one that governs is the clear bar spacing, and it governs hardest at the lap splice, where the bars projecting from the lift below run alongside this lift's cage for another forty diameters or so and the clear spacing is at its worst on the whole element. Aggregate that cannot pass arches across the bars and leaves a void underneath, invisible until stripping. Dropping from 20 mm to 14 or 10 mm is a different order line, a different price and often a different day's notice — and where the cage is truly congested, a self-compacting mix is the better purchase than a smaller stone, because it also removes the need to get a poker into a space that has no room for one.

Splitting it into loads, and where to keep the flexibility

Twelve columns at 0.729 m³ is 8.75 m³, and with five per cent it is 9.19. Against a six cubic metre truck that is not two even loads, and the way you divide it is a real decision rather than an administrative one. A mixer is loaded to its rated mixing capacity, which is a good deal less than the drum's gross volume, so the plant's whole-load figure is the ceiling and part loads are what you pay a premium for.

The rule that saves jobs is to put the certainty at the front and the flexibility at the back. Send a full load first, get it into the ground, and use what you can see — how much went into the first eight columns, how much came back in the skip, whether the heads are taking more than you allowed — to fix the second load before it is batched. Ring the plant while the second truck is still a phone call away from mixing. Ordering two equal loads of 4.6 looks tidy on paper and throws away the only correction opportunity in the whole sequence.

Part-load charges, minimum load charges and standing time are commercial terms that vary by supplier and by region, and this site does not publish rates for them because there is no honest single figure to publish. Ask for them by name when the order is placed, and ask one more question at the same time: whether the plant will hold a part load in reserve for a stated window. On a lift where the alternative is a cold joint in a compression member, standing time bought in advance is cheap insurance, and it is far easier to arrange the day before than from a site phone at half past ten.

The tail end deserves its own line of the plan, because it is where discipline usually fails. The last 0.3 m³ that tops the final column heads is exactly the quantity somebody will offer to make up out of bagged mix from the container. A bagged mix is a different strength, a different aggregate, an unknown water content and no delivery ticket, and putting it into the top of a structural column — the part of the element that carries the beam reaction directly — is the single worst place on the frame to improvise. If the load is going to be short, the honest options are another part load, or a planned joint at a level the engineer has approved.

1. Compute the volume per column type and add the types together, rather than averaging sizes.
2. Add the head over-fill, the skip residue, the test specimens and — if pumping — the priming material, and let that set the waste percentage instead of a habit.
3. Divide by the truck's rated capacity to find the whole-load count, and let the remainder be the last load, not a share of both.
4. Book the interval between loads from the discharge rate you can actually sustain, allowing for the crane cycle or the pump's other commitments.
5. Confirm the retarder, the aggregate size and the consistence at the same time as the volume, since all three change what the plant has to batch.
6. Agree standby or a held part load before the day, and get the supplier's part-load terms in the same conversation.

Standing the loads down in order

By the morning of the pour the ordering decisions are all spent and what is left is holding the sequence. The two things that will actually go wrong are a truck arriving before the gang is ready for it and a truck arriving after the previous layer has stiffened, and both are managed from the same place: one person with the delivery times, the discharge rate and the plant's number, who is not also on the vibrator.

The work around the pour is regulated as well — OSHA 29 CFR 1926 Subpart Q, Concrete and Masonry Construction, in the United States, and the equivalent national regulations elsewhere. That is not an ordering decision, but it is why the sequence below has somebody watching the forms rather than only the clock.

1. Check the form clamps and ties against the pressure the pour rate produces, before the first load arrives rather than while it waits.
2. Read the batch time on the ticket, not the arrival time, and write the discharge deadline on it in front of the driver.
3. Take the acceptance samples from the load as required by the specification, and cast the specimens before the load is committed to the forms.
4. Place in layers the poker will reach through, and vibrate into the layer below rather than alongside it.
5. Watch the level in the last four columns against what remains in the drum, and ring the plant while an adjustment is still possible.
6. Wash out into the contained basin, and dose the liquid before it is left standing overnight.