A pour returns one low cube. Is the concrete rejected, investigated or accepted? This worked example shows how the arithmetic and the procedure actually run.
Cube results arrive as a list of numbers and are usually filed without being read. They are the main evidence that the structure has the strength the design assumed, and they follow a procedure: sampling at the right frequency, curing under controlled conditions, testing at defined ages, and acceptance based on averages and minimum values rather than on any single cube. The example below shows how a low result is handled without panic and without a quiet decision to ignore it.
The situation
An illustrative villa project pours a suspended first floor slab of 60 cubic metres in a single morning. The specified characteristic strength is 30 megapascals at 28 days. The specification requires one sample set per 30 cubic metres of each mix placed in a day, with each set giving cubes for testing at 7 days and at 28 days. The owner receives the laboratory report three weeks after the pour and finds one result that looks low.
Assumptions (illustrative)
- Pour volume: 60 cubic metres of a single specified mix.
- Specified characteristic strength: 30 megapascals at 28 days.
- Sampling rate: one sample set per 30 cubic metres, giving 2 sets for this pour.
- Each set: 2 cubes tested at 7 days and 2 cubes tested at 28 days.
- Illustrative 28 day results, set A: 36.0 and 35.2 megapascals.
- Illustrative 28 day results, set B: 27.4 and 33.8 megapascals.
Step 1: Work out how many samples the pour should have produced
60 cubic metres divided by the sampling rate of one set per 30 cubic metres gives 2 sample sets. With 4 cubes per set, 2 for 7 days and 2 for 28 days, the pour should have generated 8 cubes in total, of which 4 are tested at 28 days.
Before reading any strength figure, check the count. A pour that produced fewer samples than the specification requires has an evidence gap, and no amount of good results from the samples that do exist closes it.
Step 2: Calculate the result for each sample set
A sample result is normally the average of the cubes in that set. Set A averages 36.0 plus 35.2, divided by 2, which is 35.6 megapascals. Set B averages 27.4 plus 33.8, divided by 2, which is 30.6 megapascals.
Set B contains the worrying cube at 27.4, but as a sample it averages 30.6, which is above the specified characteristic strength. The individual low cube has not automatically failed anything yet.
Step 3: Check the spread inside the low set
The two cubes in set B differ by 33.8 minus 27.4, which is 6.4 megapascals, or 6.4 divided by 30.6, about 21 per cent of the set average. Cubes from one sample should behave almost identically, and a spread of this size points at the cubes rather than at the concrete.
A wide spread within a single sample usually indicates poor sampling, bad compaction of the cube, damage, or curing that was not controlled. It is a signal to examine the laboratory records before blaming the concrete.
Step 4: Apply the acceptance criteria as a pair of tests
Typical acceptance works on two conditions applied together: an average condition, where the mean of a defined group of consecutive results must exceed the characteristic strength by a margin, and an individual condition, where no single result may fall more than a stated amount below it. Here the mean of the two sets is 35.6 plus 30.6 divided by 2, which is 33.1 megapascals, comfortably above 30. The lower sample result, 30.6, is also above the characteristic value.
The pour passes on the sample results even though one cube was low, because acceptance is based on sample results and group averages. The exact margins are set by the specification and the governing standard for your project, so read them rather than assuming.
Step 5: Investigate the low cube rather than filing it
Pull the records for that sample: the time it was taken, the truck it came from, the slump and temperature recorded at discharge, how the cubes were compacted, when they were demoulded, and the temperature of the curing tank. The 7 day result for the same set is a useful cross check, since a normal 7 day result followed by a weak 28 day result points firmly at handling after casting.
Most single low cubes are explained by sampling or curing, not by weak concrete. Documenting the explanation is what makes the acceptance defensible if anyone questions the structure years later.
Step 6: Decide what would have happened if the set had failed
If set B had averaged, for example, 24.0 megapascals, the pour would fail the individual sample condition. The response sequence is then defined: review the records, test additional cubes if any remain, carry out non destructive testing such as rebound hammer readings for comparison, then core the element and test the cores, and finally have the structural engineer assess the actual capacity against the real loads.
A failed cube does not mean demolition. It means a structured investigation ending in an engineering decision to accept, strengthen or remove, and that decision belongs to the designer, not to the contractor or the laboratory.
What changes the answer
- Sampling frequency: a specification requiring a set per smaller volume would have produced more evidence and a clearer picture of this pour.
- Curing of the cubes: tank temperature outside the controlled range can move a 28 day result by a wide margin in either direction.
- Testing age: a result at a later age can be permitted for some mixes, which changes what a 28 day figure means.
- Mix consistency: results from several pours of the same mix tell you more than any single pour, because they show whether the plant is in control.
- Element sensitivity: a low result in a lightly loaded slab and the same result in a heavily loaded column are not the same problem.
What the numbers tell you
- Acceptance is based on sample results and group criteria, not on the lowest individual cube.
- Count the samples before reading the strengths, because missing samples are the real failure.
- A wide spread inside one sample points at the cubes and their handling, not the truck.
- Low results have a defined investigation route ending with the structural engineer, not with an argument.
- The cube report is a structural record and belongs in the permanent handover file with the pour it refers to.
Limits of this example
Every figure above is an illustrative assumption used to show the method, not a quote, valuation or market rate. Costs, rates, fees and regulations change and must be confirmed with the relevant authority, lender or supplier before you commit.
Domira builds and supervises projects across Saudi Arabia, and this is the level of detail its site teams work at. Send the drawings or the site, and Domira will tell you what the specification has to say before anyone prices it.
Frequently Asked Questions
What happens if a concrete cube test fails?
The specification sets a sequence: review sampling and curing records, test any remaining cubes, carry out comparative non destructive testing, then take cores from the element and test them. The structural engineer assesses the real capacity against the design loads and decides whether to accept, strengthen or remove the work. A single low cube rarely leads to demolition but always requires a documented decision.
How many concrete cubes should be taken per pour?
The sampling rate comes from the project specification and the governing standard, usually expressed per volume of each mix placed per day with a minimum per pour. Confirm the figure in your specification before the structure starts, because sampling cannot be recovered after the event. Missing samples mean the strength of that concrete has no direct evidence at all.
Why do 7 day concrete results matter if acceptance is at 28 days?
Seven day results give early warning. They indicate whether the mix is developing strength as expected, which lets you stop pouring the same mix if something is wrong rather than discovering it three weeks and several floors later. They also help diagnose a later low result, since a normal early figure followed by a poor 28 day figure usually points at cube handling.
Who should take and cure the concrete cubes on site?
Sampling should be done by trained personnel following the standard method, and the cubes should be cured in controlled conditions, not left in the sun beside the pour. Using an accredited laboratory that attends site, takes the samples and controls curing removes the obvious conflict of interest and produces results that stand up if they are ever challenged.
Have a Property Question?
Domira can help you navigate property, financing and investment decisions in Saudi Arabia. Speak with our team for an initial consultation.
