Finding the ultimate strength of concrete is an important investigation measure used across the built environment to ensure our buildings and other constructions remain structurally sound. When it comes to remedial work, for example, the results from a concrete sample strength test may prove crucial for project compliance as well as the green lighting process for any future works.
With this in mind, ensuring the highest level of accuracy of these tests should be of paramount importance to developers. However, work at SOCOTEC has shown that some of the preferred testing specifications for strength testing of concrete in the UK may not be the most effective or reliable. Of course, best practice in this area is codified by the British Standards Institution. In the recently revised BS EN 12504(1) – the current British Standard responsible for testing concrete in structures – there are a number of techniques suggested for the capping of a specimen.
Differing methods, differing results
To prepare a cored concrete sample for compressive strength testing, the cylindrical sample’s end must be made to be flat, uniform and perpendicular to the applied load. This process, completed prior to the test, is known commonly as ‘capping’. One common method of capping, which is referenced in the current British Standard, and appears in many specifications for high-profile civil engineering projects, is core end grinding. This is the process in which a machine is used to grind away the irregularities of the sample ends so that both are flat to the specified tolerances and ready for testing. Alternatively, capping using sulfur-based mortar caps is another common technique. At SOCOTEC, the preferred method has always been to cap using high alumina cement (HAC) – one of the methods described in the superseded British Standard (BS 1881 Part 120(2)).
In 2011, while working on a large, high-profile London railway construction project, we were surprised to see that the testing specification mentioned Core End Grinding as the preferred technique for capping samples. Our experts at SOCOTEC, then ESG, felt strongly that this method would leave aggregate high spots on the samples. If this were true, then the validity of the given ‘ultimate strength’ would be in question.
To test our hypothesis, it was agreed with one of the main contractors that we would test different methods of capping on a few initial trial mixes for the sprayed concrete. We took a number of cores from each test panel and divided them into sets, preparing them with three different methods, Grinding, Capping with HAC, and Sulfur capping. Following the tests, the results showed that HAC capping produced the ultimate strength of the concrete where Grinding and Sulphur capping actually created influences that reduced the strength values.
At this moment we feel the cause of this variation is due to the aggregate and matrix being of a different strength quality therefore eroding the matrix to a higher degree allowing the aggregate to protrude by an extremely minor amount but in most cases enough to influence the failing load. Also to a lesser degree the peripheral aggregate pieces are held in place by the HAC cap.
With these results being known, we can confidently say that if core end grinding is chosen as the end preparation method for testing concrete, the results could – in theory – show weaker strength assessment or even non-compliance. With the HAC method, this may not be the case.
Of course, this has major implications for the construction and remediation sector, especially when structural assessments and investigations are undertaken. For example, when it comes to the potential restoration of a historical building, testing using core end grinding could result in the needless reassessment of the structure. By simply ensuring that the true ultimate strength of a concrete specimen is found during the investigation process, developers can avoid unnecessary costs or long delays to projects.
Despite our results, we are still finding that core end grinding and sulfur capping are widely used methods for many testing authorities in the construction and infrastructure development sectors. This undoubtedly becomes a major concern in cases where two samplers are undertaking assessments on the same structure but using different end preparation techniques.
In my opinion, there needs to be a standardised approach to preparing concrete samples to prevent confusion, inaccuracy and unnecessary costs and works, and this approach must be HAC. BSEN 12504 should be revisited to create a more accurate and consistent overall approach.
If you have any questions about our research, or would like to find out more about SOCOTEC’s concrete testing services, visit https://www.socotec.co.uk/our-services/infrastructure-energy/concrete-testing/.
1. BRITISH STANDARDS INSTITUTION, BS EN 12504. Testing concrete in structures. Cored specimens. Part 1 – Taking, examining and testing in compression. Part 2 – Non-destructive testing. Determination of rebound number. Part 3 – Determination of pull-out force. Part 4 – Determination of ultrasonic pulse velocity. BSI, London.
2. BRITISH STANDARDS INSTITUTION, BS 1881. Testing concrete. Part 120 – Method for determination of the compressive strength of concrete cores. BSI, London, 1983, withdrawn.
*This article was first published in Concrete magazine.