CROSS Safety Report
Concern over concrete test methods used on site
A reporter is concerned about the test method a contractor uses to justify the strength of concrete placed on site.
Key Learning Outcomes
For contractors and the construction team:
- Quality control and competent supervision on site can help to ensure that the structure is built in accordance with the design
- It is good practice to provide all test records to the design engineer/test specifier for their review. This is particularly relevant if the results do not meet the test requirements.
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A reporter worked as a structural designer on a project where a reinforced concrete basement was being constructed under an existing domestic property. The underpinning was performed in a typical hit and miss construction sequence. In the structural drawings it was specified that the contractor should take cube tests to verify the strength of the concrete which they were mixing themselves on site. The reporter attended site when around 50% of the underpinning was complete, at which point they asked to see the results from the cube tests.
The results showed that the strength was over 25% lower than the required strength. It was clear that the contractor was performing the cube tests but was not looking at the results to confirm that they met the specification. Their remedial solution to justify the strength of the underpinning that was already complete was to perform Schmidt hammer tests. This test uses a hammer that measures the rebound of a spring-loaded mass impacting against the surface of a sample. The reporter is not convinced that this is an appropriate way to justify the strength of concrete and would be interested to know other views on how a safety issue like this should be managed.
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