How to Order Concrete: m³ Calculation, Pump Selection and Pour Day

Ordering concrete looks like a job closed with a single phone call; yet most of the problems on pour day stem from incomplete information given at the time of ordering. A correct order means settling five things in advance: quantity, class, consistency, access and timing.

1. Quantity: m³ calculation and allowance

Concrete is sold by volume. The calculation is simple — the volume of the element to be poured — but two items are constantly forgotten on site: formwork allowance and ground losses. Formwork bulging, an uneven base and the junctions between elements require slightly more concrete than the theoretical volume.

• Slab: area × thickness. Thickness is verified by measuring on site.
• Shear walls and columns: section area × height × count.
• Foundation: blinding and raft are calculated separately.

Too little concrete means a cold joint in the middle of the pour; too much is wasted. Checking the quantity together in advance is the cheapest step — you can get approximate figures from our concrete calculator.

2. Class and exposure: check the design

The concrete class (C25/30, C30/37 and so on) is set in the structural design; it is not something to decide over the phone. The second heading, as important as the class, is the exposure class: the building’s proximity to the sea, the chemistry of the groundwater and freeze–thaw conditions directly affect the mix.

In buildings near the coast, chloride attack is the main cause of reinforcement corrosion. On such jobs a low water/cement ratio and adequate cover are as decisive as the strength class.

3. Consistency (slump): pump or chute?

The slump value describes the flowability of the concrete. High slump places easily but must not be achieved by arbitrarily adding water — adding water permanently lowers strength. The right consistency is set at the plant through admixtures and mix design.

• If the pour is by pump, the consistency is planned accordingly.
• Densely reinforced elements need a more flowable mix.
• On sloping surfaces, overly flowable concrete runs; the consistency is reduced.

Do not add water on site. If there is a consistency problem, the solution comes from the plant.

4. Access: the route the truck will take

This is the heading that wastes the most time because it is not discussed when ordering:

• The width, gradient and turning radius of the road determine whether the mixer truck can get in.
• The area where the pump is set up must be level and firm; space is left for the outriggers.
• Overhead power lines restrict the reach of a boom pump.
• A mobile pump is preferred on a narrow plot, a boom pump over long distances.

On sites with uncertain access, a survey before the pour is cheaper than waiting on pour day.

5. Pour day: sequence and timing

Concrete races against time from the moment it leaves the plant. In hot weather that time is shorter. The pour plan must answer three questions: how many trucks, at what interval, in what order?

1. Formwork, reinforcement and services are checked before pour day.
2. Vibrators and spare equipment are kept ready on site.
3. If samples are to be taken (for quality records), this is planned in advance.
4. Curing materials must be on site before the pour ends.

After the pour, curing is essential for the concrete to reach its design strength; we cover this in detail in our concrete curing article.

Frequently asked questions