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Compressive Strength Test - Unsatisfactory Failure

posted Apr 3, 2022, 7:52 PM by jeffery jim

1. Compressive strength test is a test which is conducted on concrete specimen in order to check their compressive strength and its progression through time; usually 7th day and the 28th day after concrete were batched. This time vs. strength representation ensure that concrete achieved targeted characteristic strength and to tackle any possible underlying issues on the concrete for mitigation and remedial works. It means that particular batch of concrete maybe suitable as the specimen represent the concrete population is acceptable and will perform as desired or intended.

2. Nevertheless, most time, civil and structural engineers have no qualm about this. It is assumed that the laboratory have followed all procedures and the test is representative of the actual concrete casted at site. This is not true.
3. In most cases, the third party or internal laboratory does not know what happened as they are just operators and general workers. What ticked me the most when it comes to attention to details is their sketchy illustration on the failure mode. If you are used to this test, sketches we got from them are "worms" in a diagram. It is not a proper representation and serves no purpose.

4. When concrete cube specimen are crushed, they need to achieve the satisfactory explosion which renders the common outcome under compression. In certain cases, there is/are hairline crack(s). This mean, there is/are issue(s) with the whole procedure. Although BS EN 12390 and/or BS 1881 do not sanction this kind of occurrence as a failure, there is a need to conduct investigation. It may cause some reduction of actual compressive strength.

5. There are a few precursors to this issue. This issue is rarely discussed and hardly answered in forums across the internet. Hence, I am going to share with you the issues which triggered such failure especially with the formation of tension crack line(s).

6. Among the issues that may triggered or contribute independently or a combination of issues are as follows;

A. The wrong mix of concrete constitute involving coarse aggregate size envelope. It can be the source of the coarse aggregate with geological inheritance which performs badly under uniaxial load. It could also be the shape of the aggregate which is too flaky or too elongated. This usually happen in academia and the introduction of new components or substrates into the mix.

B. The straightness or squareness of the specimen mould is no longer suitable and lapse the allowed deviation of 1% to 5%. Hence, the load applied is not spread or propagated uniformly. This issue usually occurs for extracted cores in cylinder shape and commonly ignored for square mould.

C. The piston of the machine is no longer distributing the load appropriately and have issues with its hydraulic properties or pump and arbitrary movement of the liquid. This triggered lopsided pressure across the surface.

D. The force control pacer could be faulty. In this case it can be analog or digital control. This loading of pressure in a rapid manner and trigger undesired outcome. For JKR project it should be between 0.4 to 0.6 while BS allows up to 1.0.

7. With these being said, yearly calibration is required and trust me, calibration sometimes did not tackle this problem. Therefore, it is only the outcome from crushing test will give you a clear tale-tell signs about these precursors.