Concrete cube testing checks whether the concrete in your building is as strong as the engineer designed it to be. During each pour, samples are taken, placed in 150 mm steel moulds, cured in water, and crushed in a laboratory, usually at 7 days and 28 days. The 28-day result is the one that counts against the specified grade, such as C25. The 7-day result is an early warning, typically reaching around 60 to 70 percent of the 28-day strength. Together with a slump test on site, cube tests are the cheapest insurance you can buy on a concrete structure.
Why test concrete at all?
You cannot see strength. Concrete that had too much water added, was badly mixed, or used poor sand looks almost the same as good concrete when it sets. Weak concrete in columns, beams and slabs reduces the building's safety margin, and once the frame is up it is very costly to fix. Testing tells you while there is still time to act. See concrete grades and mix ratios for what the grades mean.
Step 1: The slump test on site
Before taking cubes, check workability with a slump test:
- Fill a metal cone (300 mm tall) with fresh concrete in three layers, rodding each layer 25 times.
- Lift the cone straight up.
- Measure how far the concrete drops (the slump).
The engineer specifies a slump range, often around 50 to 100 mm for slabs and beams. A much higher slump usually means extra water was added, which weakens the concrete. Reject or correct the batch before it goes into the structure.
Step 2: Making the cubes
- Take samples from the middle of the batch, not the first or last bit.
- Fill clean, oiled 150 mm cube moulds in layers, compacting each layer by rod or vibrator.
- Level the top, label each cube with date, location and grade.
- Make at least one set (usually three cubes) for each age to be tested, per pour or per set volume of concrete as the specification requires.
- Leave the cubes covered and undisturbed on site for about 24 hours.
Step 3: Curing the cubes
After 24 hours the cubes are taken out of the moulds, marked, and placed in a water curing tank until testing. Cubes left in the sun on site give false low results, and cubes kept in better conditions than the structure can hide problems. Follow the specification and keep a record.
Step 4: Crushing in the laboratory
At 7 and 28 days the lab crushes each cube in a compression machine and records the load at failure. Strength is load divided by area, given in N/mm² (MPa). In Tanzania, testing is available at materials laboratories such as those run by TANROADS, universities and private labs. Ask for a signed report with cube identification, dates, weights and results.
How to read the results
| Age | What it tells you |
|---|---|
| 7 days | Early check. Around 60 to 70 percent of the target 28-day strength is common for ordinary Portland cement mixes |
| 28 days | The result compared with the specified grade |
| 56 days or more | Sometimes used for blended cements that gain strength slowly |
For a C25 concrete under British notation, the target is a characteristic cube strength of 25 N/mm² at 28 days, and the mix is designed to average well above that. Under European notation (C25/30), the cube figure is 30. Your engineer applies the compliance rules in the specification, which look at both the average of results and individual low values.
Cement type matters. Many cements sold in Tanzania are blended (such as CEM II or pozzolana cements), which can gain strength more slowly early on. A slightly low 7-day result with these is not always a problem. See our guide to building materials.
What if cubes fail?
Do not panic, but do not ignore it either. The usual steps:
- Check the testing: were cubes made, labelled, cured and tested correctly?
- Wait for later results if only the 7-day result was low.
- Test the structure itself: rebound hammer (Schmidt hammer) surveys, ultrasonic tests, or cores drilled from the element and crushed.
- Engineer's assessment: the structural engineer checks whether the actual strength is still adequate for the loads.
- Remedial work: if it is not, options range from strengthening to removing and recasting the element.
The cost of all this is why prevention matters: control water, mix properly, compact well and cure properly.
Common causes of low strength
- Too much water added on site to make concrete easier to place.
- Dirty sand or aggregates with silt or clay.
- Wrong mix proportions, often by guessing volumes with buckets.
- Old or damp cement.
- Poor compaction, leaving honeycombing.
- No curing in hot weather.
Arranging testing on your project
Testing works best when it is agreed before the first pour, not on the day:
- Write it into the contract: who makes the cubes, which lab tests them, how often, and who pays. Commonly the contractor makes and delivers cubes, and the cost of testing sits in the Bill of Quantities.
- Choose an independent lab with calibrated machines and signed reports.
- Keep a cube register: pour date, element (for example "first floor slab, grid B to D"), grade, cube numbers and results.
- Share results quickly with the structural engineer, especially the 7-day results.
For small houses, owners sometimes skip testing to save money. Even there, one set of cubes from each slab and column pour is cheap compared with the cost of the structure. See fundi or registered contractor for how supervision changes on smaller jobs.
Frequently asked questions
How many cubes should be taken?
The specification sets the rate, often one set per pour or per given volume of concrete, with cubes for 7 and 28 days. Larger pours need more samples.
Is a 7-day test enough?
No. It is an early indicator. The 28-day result is what is compared with the specified grade.
Does ready-mix concrete need cube tests?
Yes. Ready-mix suppliers test their own production, but the client or contractor should take independent cubes on site too.
What is a core test?
A cylinder of concrete drilled from the actual structure and crushed. It shows the strength of the concrete in place.
Want concrete you can trust?
Vika Construction tests concrete on every structural pour as part of our quality and safety approach. Contact us to discuss your project.
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