Ordering concrete for a slab: specifying strength, exposure and slump

The concrete order is the last point at which a builder controls what goes into a slab. Once the truck has batched and is on its way, the mix is a fact, and the job is to accept it or refuse it. Yet many small builders still order concrete the way they always have, by asking for the usual mix, and then sign the docket without reading it. The result is a slab that may be perfectly adequate, or may be carrying a mix that was never designed for the job, and nobody can say which until the concrete has set.

This article sets out what a concrete order for a residential slab has to carry: the strength, the exposure, the workability for the placing method, the admixtures, the aggregate, and the volume and timing. It also explains what to check on the delivery docket before the first load is accepted. It does not cover curing or finishing, which are separate decisions with their own risks.

The Cement Concrete and Aggregates Australia website is a useful reference for the terms suppliers use, and it helps a builder read a docket without needing a laboratory. The point is not to become a concrete technologist. It is to make sure the order says what the job needs, and that the docket proves it was delivered.

What the order has to carry

A complete order states the strength grade for the element being poured, the exposure conditions the concrete will face, the slump or workability that suits the placing method, the maximum aggregate size, and any admixtures the design calls for. It also states the volume, the delivery window and the placing method, because the supplier needs those to batch the mix and to plan the trucks. An order that says only the quantity and the date leaves the supplier to choose everything else, and the supplier’s standard mix may not be the one the drawings require.

Write the order from the drawings and the specification, not from memory. The engineer’s details and the project specification are the source of the strength and exposure requirements. If either is missing or unclear for a particular element, stop and ask before the order is placed. A missing specification is far cheaper to resolve on paper than to resolve with a slab that has already been poured.

Strength: match the drawings, not the habit

Strength is specified for each element, and different parts of the same house may carry different grades. Footings, a ground-supported slab, a suspended slab and a retaining wall are not necessarily the same mix, and an order that uses one grade for all of them is a shortcut that may not be safe for every element. If the drawings show different requirements, the order must show each one separately, and the docket must match each order to its element.

A higher grade than specified is sometimes offered as a safe choice. It is not automatically a safer one. A stronger mix changes the cost, the workability, the heat generated as it sets and the way the concrete shrinks as it dries. Any change from the specified grade has to be agreed with the engineer or the designer in writing before the order is placed, and the agreement should go on the job file alongside the docket.

The strength grade is also the main thing a certifier or an inspector will ask about on a docket. A docket that shows a grade different from the one on the drawings invites questions that take time to answer, even if the concrete turns out to be fine. Get the grade right on the order and check it on the way in.

Where a lower-carbon mix is being considered, the low-carbon concrete options in Australia article sets out the trade-offs. Any such mix still has to meet the specified grade and exposure, and the substitution still needs the written agreement described above.

Exposure: where the concrete will live

Exposure describes the conditions the concrete will face for the life of the building: contact with the ground, wetting and drying, salt in the air near the coast, and any chemical or aggressive environment on the site. A slab that will sit in wet ground, or a footing close to the sea, needs a mix that resists those conditions over decades, and that is a property of the mix, not of the finish or the paint on top of it.

The exposure requirement is normally set by the designer or the engineer for the site, and the order should refer to it directly. Do not assume that a mix suitable for a dry inland block is suitable for a coastal lot a few streets from the beach. The conditions around the site, including neighbouring structures and any known contamination, should be considered when the exposure is set. If the site conditions are unclear, ask the designer before the order goes in.

A common mistake is to correct a poor exposure choice by thickening the slab or adding more reinforcement, without changing the mix. That does not solve the problem. The mix defines how the concrete resists the environment, and the cover defines how far the steel is from that environment. Both have to be right, and neither can make up for the other.

Slump: workability for the placing method

Slump is a measure of how stiff or fluid the concrete is when it is delivered. A higher slump is easier to pump, spread and compact, but it usually means more water in the mix, and water is what reduces the strength and durability of the finished concrete. A lower slump is harder to place but gives a stronger, more durable result when it is compacted properly. The right workability is the one that suits the placing method, not the one that is easiest for the crew on the day.

Agree the workability with the supplier before the order, and state the placing method on the order. A pump needs a mix that moves through the hose without segregating or blocking. A chute or a barrow needs a mix that flows without drying out in transit. A mix designed for one method can be a poor match for another, and the supplier can only adjust it properly if the method is known in advance.

Safe Work Australia’s guidance on concrete pumping is a reminder that pumping is a high-risk activity in its own right, with set-up, hose handling and pressure all needing control. The pump operator and the supplier should know the placing method and the mix before the truck arrives, because a mix that is wrong for the pump can cause a blockage that is dangerous to clear.

Water on the truck and on the site

The most common way a good mix goes wrong is water. A driver or a labourer adds water to the drum to make the concrete run more easily, or the crew adds it on the slab to speed up the spread. The concrete looks better for a few minutes, and the finished slab is weaker and more prone to cracking and dusting than the specification intended. The docket may not show the water at all, and the damage does not show until the slab has cured and been tested or, worse, has failed in service.

Refuse water added on site unless it has been recorded and agreed by the person responsible for the design. Make the rule clear to the drivers and the crew before the first truck arrives, and check the slump on arrival if the placing method depends on it. A mix that arrives too stiff to place is a problem to solve by changing the order and the placing plan, not by adding water on the slab.

Reading the delivery docket

The delivery docket is the record of what was placed, and it is only useful if someone reads it. Each truck should be checked against the order before the concrete is accepted. Look at the strength grade, the exposure, the slump, the admixtures and the aggregate size, and confirm that the batch time and the time remaining before discharge are recorded. A docket that does not match the order should be queried before the truck is unloaded, not after the slab is finished.

The checklist below sets out the checks that matter most. It is worth printing and keeping with the docket book, because the moment of acceptance is short and the pressure to pour is high.

Timing the trucks and the time limit

Concrete begins to set from the moment it is batched, and the supplier will state a time limit before it must be discharged. That limit depends on the mix, the weather and the distance from the batching plant, and it is not a suggestion. A truck that waits too long at the site, or a pour that is delayed by a late pump, can leave a mix that is past its limit when the last load arrives. The safe course is to refuse concrete that is past the limit, and to plan the trucks so that the time limit is never tested.

Book the trucks to arrive at a steady rate that the pump and the crew can keep up with. Too many trucks at once leaves concrete waiting in the sun, and too few leaves gaps that can become cold joints where new concrete meets concrete that has already begun to set. Speak to the supplier about the pour rate before the order, and confirm the arrival times the day before.

Quantities, allowances and the margin

Ordering the right volume matters for cost and for the finish of the job. Measure the slab area and its depth, convert the result into a volume, and then add a sensible allowance for spillage, the uneven base and the thickness that is never quite even on a real site. Use your own waste history to set that allowance, rather than a rule of thumb from someone else’s job. Over-ordering wastes money and creates disposal costs, and under-ordering creates a cold joint or a rushed partial pour that is hard to fix.

Check the volume again when the trucks arrive. The docket shows what was delivered, and the total should be compared with the total ordered. A shortfall that is discovered when the last truck is unloaded is a problem that needs a decision on the spot, and the decision is easier when the record is already in front of you.

Worked example: a slab for a granny flat

A builder in the Hunter region is pouring a ground-supported slab for a granny flat in the backyard of an existing house. The engineer has specified the strength grade and the exposure in the drawings, and the placing method is a pump because the access is narrow. The builder orders the concrete by phone, saying the usual mix for a slab. The supplier confirms the order, and the builder does not check it against the drawings.

On the morning of the pour, the first truck arrives with a mix that is stiffer than the pump can take easily. The driver offers to add water to make it run. The builder recognises the problem, refuses the water, and calls the supplier. The supplier confirms that the mix on the truck is a standard mix, not the specified one, and offers a replacement truck with the correct grade and workability. The first load is rejected, the replacement arrives within the time limit, and the pour goes ahead, with the docket for each truck checked against the order.

The lesson is simple. The order must carry the specification, and the docket must be checked against it on arrival. The builder who had ordered by habit would have accepted the first truck, added water, and poured a slab that nobody could later prove was to specification. The builder who wrote the order from the drawings had the record to show exactly what was placed.

Keeping the docket with the job file

Keep every docket, the order sheet and any written agreement for a change of mix with the job file. The docket is the evidence of what was placed, and it is what a certifier, an engineer or an insurer will ask for if the slab is questioned later. Dockets that are thrown in the ute and lost on the way to the office are not evidence of anything.

Where the job needs a sign-off from the engineer or the certifier, check what records they will want before the pour, so that the dockets and the order sheets being kept match what will be asked for. For the CPD side of running a site with this kind of work, the PointCPD guide to how NSW builder CPD works sets out the background to builder CPD in New South Wales.

Ordering it right the first time

A concrete order that states the strength, the exposure, the workability, the admixtures and the timing, and a docket that is checked on arrival, is the cheapest insurance a slab can have. The mix is not the place to save a little time on the phone. Write the order from the drawings, agree any change in writing, refuse water added on site, and keep the record. The few minutes spent on the order and the docket are small compared with the cost of breaking out a slab that was never what the drawings required.

For builders who want to see how the foundations below the slab fit into the same chain of decisions, the guide to strip and pad footings for residential work in NSW is a useful companion. The mix that goes into the footings and the slab has to match the design of both.

Questions builders ask

• Concrete
• Slabs
• Specifications

Published 2 October 2026. General information for builders, not legal, financial or engineering advice for a specific job. Check current requirements with the relevant regulator or standard before relying on them.