Concrete arrives on site as a wet mix. Its strength develops over the following weeks, out of sight, inside the floor or structure it was poured into. Concrete cube testing is the method the construction industry uses to measure that strength directly, so that an engineer can confirm the concrete delivered matches the concrete that was designed.
This article explains what a cube test is, how the cubes are made, cured and crushed, which British Standards govern the process, and how to read the results. It is written for site managers, clients and anyone who has been handed a cube test certificate and wants to know what the numbers mean.
In brief
- A cube test measures the compressive strength of concrete by crushing a standard sized cube of it in a laboratory press.
- Cubes are made from concrete sampled on site, cured in water at a controlled temperature and tested at 7 and 28 days.
- The 28 day result is compared with the characteristic strength of the specified concrete class, for example C32/40.
- The process is governed by BS EN 12390 and BS 8500 in the UK.
What a concrete cube test measures
Concrete is very strong in compression, and compressive strength is the property that most design calculations rely on. A cube test measures it by applying a steadily increasing load to a cube of hardened concrete until the cube fails. The load at failure, divided by the area of the cube face, gives the compressive strength in newtons per square millimetre (N/mm², also written as MPa).
The result matters because concrete strength cannot be assessed by looking at it. Two loads from the same plant can differ if the water content, the cement content or the aggregate grading has drifted. The cube is a witness sample: it was made from the same concrete, on the same day, and cured under known conditions, so it tells you what that concrete is capable of.
How concrete cubes are made
Cube making is deceptively simple, and most poor results trace back to poor sampling or curing rather than to poor concrete. The method is set out in BS EN 12390-2 and follows these steps.
Sampling
A sample is taken from the delivery in accordance with BS EN 12350-1, drawn from the middle of the discharge rather than the first or last concrete out of the truck. The sample is remixed before use so it represents the load.
Filling the moulds
The concrete is placed into rigid steel moulds, normally 150 mm cubes in the UK, although 100 mm cubes are permitted for mixes with smaller aggregate. Each mould is filled in layers and compacted with a tamping bar or vibrating table so no air voids remain.
Finishing and marking
The top surface is struck off level and each cube is marked with a unique reference, the date and the pour location. The marking is what links a laboratory result back to a specific area of floor months later.
Initial curing
The filled moulds are kept undisturbed, covered and protected from extremes of temperature for the first 24 hours, then demoulded.
Water curing
The demoulded cubes are placed in a curing tank at 20 degrees Celsius, plus or minus two degrees, until the day of test. Water curing ensures the cube develops its strength under ideal conditions, giving a consistent benchmark from one project to the next.
At least three cubes are normally cast from each sample so that there is one for the 7 day test, one for the 28 day test and a spare. Where the specification asks for 14 or 56 day results, additional cubes are cast at the same time.
How many cubes are needed
There is no single answer, because the sampling rate is set by the specification for the project rather than by a fixed rule. BS 8500 and BS EN 206 describe conformity testing in terms of samples per volume of concrete, and engineers commonly specify one sample for every fixed quantity of concrete placed, or one per pour, whichever gives more. A large warehouse floor poured over several days will therefore generate a set of cubes for each day, while a small structural pour may need only one sample.
What matters more than the number is that each sample is genuinely representative and that its cubes can be traced back to the correct pour.
The 7 day and 28 day tests
Concrete does not reach its full strength when it sets. It continues to gain strength as the cement hydrates, quickly at first and then more slowly. Testing at fixed ages allows the rate of strength gain to be checked against what is expected for the mix.
| Test age | Typical strength reached | Purpose |
|---|---|---|
| 7 days | Around 60 to 70 per cent of the 28 day strength | Early warning. A low 7 day result flags a problem while the pour is still fresh in everyone’s mind. |
| 28 days | The specified characteristic strength | The reference result used to confirm compliance with the design. |
| 14 or 56 days (optional) | Intermediate or long term | Used where an engineer wants to track the curve in more detail or where slower gaining cements are used. |
On test day the cube is removed from the tank, wiped, measured and placed in a compression testing machine with the load applied to the sides that were cast against the mould, not the trowelled top. The load increases at a controlled rate, in accordance with BS EN 12390-3, until the cube fails. The machine records the maximum load, and the laboratory calculates the strength.
How to read a cube test result
A test certificate will show the cube reference, the casting date, the test date and age, the density, the failure load and the compressive strength. The figure to look at is the 28 day strength, compared against the characteristic strength of the concrete class that was specified.
Concrete classes are written with two numbers, for example C32/40 or C40/50. The second number is the characteristic cube strength in N/mm²; the first is the equivalent cylinder strength used in European design codes. A C32/40 mix is therefore expected to return cube results at or above 40 N/mm² at 28 days. Conformity rules in BS EN 206 allow for the natural variation between cubes, so an engineer will look at the pattern of results across a pour rather than at any single cube in isolation.
A result a little below the target is not automatically a failure of the floor. Concrete in a well cured floor often outperforms the cube, and cubes that were left drying on site or knocked during transport will understate the true strength. This is why careful making and curing, and the spare cube, are so important.
What happens if a cube fails
A low result prompts an investigation rather than an immediate verdict. The spare cube is tested, the supplier’s batch records are checked for the load in question and the affected area of the pour is identified from the cube marking. If doubt remains, the engineer may call for cores to be cut from the floor itself and tested, which gives a direct measurement of the in place concrete.
In most cases the investigation confirms that the floor is adequate. Where it does not, the options range from restricting loading on the affected area to localised replacement. The value of cube testing is that these conversations happen at 7 or 28 days, before racking, plant and finishes have been installed on top of the floor. Our article on fixing cracks in concrete floors covers what to do when a defect appears later in a floor’s life.
Why cube testing matters for concrete floors
A commercial or industrial floor is designed to carry a specific loading pattern: racking legs, forklift wheels, mezzanine columns or HGV axles. The design assumes a particular concrete strength, and everything from the reinforcement layout to the laser screed pour thickness is calculated on that basis. Cube testing closes the loop by confirming the assumption held.
It is also a matter of record. Cube certificates form part of the handover documentation for the building and are the first thing an engineer or insurer will ask for if a question about the floor arises years later. Together with a sound curing regime, they are the evidence that the floor was built as designed.
If you need cube testing carried out on a project, whether or not Technic is placing the concrete, our concrete cube testing service page explains how independent testing to British Standards is arranged and reported.
Frequently asked questions
Q.What size is a concrete test cube?
In the UK the standard cube is 150 mm on each side. A 100 mm cube may be used where the maximum aggregate size is small enough, and the strength result is adjusted accordingly.
Q.Why are cubes cured in water?
Water curing at a controlled temperature gives every cube the same ideal conditions, so results are comparable between projects and laboratories. It also mirrors the curing that a well protected floor receives on site.
Q.What is the difference between a cube test and a cylinder test?
Both measure compressive strength. The UK has traditionally used cubes, while much of Europe and North America uses cylinders. The two give different numbers for the same concrete, which is why concrete classes quote both, for example C32/40.
Q.Can I test the concrete in the floor rather than a cube?
Yes. Cores can be cut from the hardened floor and crushed, and rebound hammer surveys give a non destructive indication. Cores are usually reserved for investigating a low cube result because they leave a hole that has to be made good.