Concrete cube compressive strength testing

What is concrete cube testing for?

Cube testing is the standard check that the concrete supplied to your project reaches the strength it was specified to reach. Cubes are made from a sample of the fresh concrete, cured under controlled conditions and crushed at set ages, and the strength is compared with the requirement in the specification. It is a quality control and conformity test on the concrete as delivered, which is why the specimens are protected from site conditions rather than left to experience them.

Which standards apply to concrete cube testing?

The compression test is usually specified to BS EN 12390-3, with the specimens made and cured to BS EN 12390-2 and the moulds and specimen tolerances covered by BS EN 12390-1. Conformity of the concrete itself is normally judged against BS EN 206 and the UK complementary standard BS 8500. Highways and infrastructure contracts sometimes call up a different clause, and we follow whatever your specification names.

Why are cubes tested at 28 days?

28 days is the reference age at which concrete strength classes are defined, so it is the age conformity is normally judged on. Concrete gains strength quickly at first and then much more slowly, and by 28 days most mixes have reached a strength that is stable enough to design against. Earlier ages are still tested, commonly at 7 days, as a check that the mix is developing as expected rather than as the acceptance figure.

What happens if a cube is tested early or late?

The actual age at test is recorded and stated on the report, and the result is read against that age rather than the one that was asked for. Concrete keeps gaining strength, so a cube crushed late reads high and one crushed early reads low, which matters when the figure is being used for acceptance. The standard allows only a small tolerance around each nominal age, so a test date that has to move should be agreed rather than allowed to drift.

What do you check on a cube before it goes in the machine?

Each cube is identified, weighed and measured, and its dimensions, squareness and the flatness of the loaded faces are checked against the tolerances in BS EN 12390-1. The condition of the specimen is noted, including honeycombing, chipped arrises or signs that it has dried out. A cube outside the dimensional tolerances can still be tested, but the standard sets out how that is handled and the deviation is stated on the report.

How quickly is the load applied when a cube is crushed?

The load is applied at a constant controlled rate, set by BS EN 12390-3 at 0.6 ± 0.2 MPa per second, and increased without shock until the cube fails. The rate matters, because loading too quickly makes the strength read high and loading too slowly makes it read low. The maximum load is recorded and the compressive strength is reported to the nearest 0.1 MPa, alongside the age at test and the failure pattern.

What does an unsatisfactory cube failure look like?

A satisfactory cube failure is roughly symmetrical, with all four moulded faces cracking about equally and little damage to the faces that were against the platens. An unsatisfactory failure is one-sided, or leaves a face largely intact, or shows a corner breaking away, and it usually points to how the specimen was made, positioned or handled rather than to the concrete. We record the failure type with the result so it can be taken into account.

Do you test cylinders as well as cubes?

Yes, where the specification calls for cylinders rather than cubes. UK practice is built around the 100 mm and 150 mm cube, but concrete strength classes in BS EN 206 are written as a pair, so a C25/30 means 25 MPa on a cylinder and 30 MPa on a cube. The two are not interchangeable and there is no single conversion factor, so test the specimen shape your specification is written against rather than converting afterwards.

How do you know the compression machine is reading correctly?

Compression machines used for cube testing are verified against a traceable reference at set intervals, and the performance requirements for the machines themselves are given in BS EN 12390-4. Verification covers the accuracy of the load reading across the working range rather than at a single point, and the platens are checked for flatness and alignment, because a worn or badly aligned platen skews the result. Calibration records are available if your client or auditor asks for them.

How soon after the test age will we have the result?

Cubes are tested on their due date and the result normally follows shortly afterwards, once the figures have been checked and the report issued. Nothing can bring a 28-day result forward, so the reporting date is set by the test age rather than by the laboratory. Where a result is needed urgently, for striking formwork or opening a slab to loading, say so when the cubes are booked in and we will confirm what is achievable.

Does your accredited schedule include cube compressive strength testing?

Hixtra holds UKAS accreditation to ISO/IEC 17025 for the tests listed in its schedule of accreditation, number 8996. Accreditation is specific to the listed methods and scope, so it applies to particular methods rather than to every service. Reports state the method used and the accreditation status of each result, which matters if the cubes are being submitted as evidence of conformity. Ask us to confirm the position for your testing before you book.

What do you need from us to quote for cube testing?

Send the site postcode, the number of cubes, the testing ages required and the specification or testing schedule if there is one. Tell us whether you want us to sample and make the cubes on site or whether you will deliver specimens to us, and roughly how often through the programme. Cost is driven by the number of specimens and visits, travel, the testing ages and any curing, collection or out-of-hours working involved.