Some causes of unsatisfactory concrete test results

The two most common kinds of failure are:
• failure to get the required strength, the concrete being otherwise apparently good;
• structural failures, such as honeycombing, sandy patches, and cracking.
Failure to get the right strength in cubes taken from a concrete pour can sometimes have a very simple cause. Among such causes are the following:
• the cube was not compacted properly;
• it was left out all night in hard frost or dried out in hot sun;
• there was a mix-up of cubes and a 7-day old cube was tested on the assumption it was 28 days old;
• the cube was taken from the wrong mix.

Such simple errors are not unusual and must be guarded against because they cause much perplexity and waste of time trying to discover the cause of a bad test result. The concrete must be fully compacted in the mould, which is kept under damp sacking until the next day when the mould can be removed and the cube marked for identity. It is then best stored in water at ‘room temperature’ for curing until sent to the test laboratory. If poor cube test results appear on consecutive batches, an error in the cement content of batches may be suspected, or else the quality of the cement itself.
Honeycombing is most usually caused by inadequate vibration or rodding of the concrete adjacent to the face of formwork. Sometimes too harsh a mix is used so there are insufficient fines to fill the trapped interstices between coarse aggregate and formwork, or the larger stones cause local arching. Sand runs – patches of sandy concrete on a wall surface which can be scraped away with a knife – can be due to over-vibration near a leaking joint in the formwork which allows cement and water to pass out of the mix. One simple, and not infrequent, cause of poor concrete is use of the wrong mix due to a ‘failure of communication’ with the batching plant operator or ready-mix supplier.
An experienced concreting foreman should be able to detect a ‘wrong mix’ the moment it is discharged.