Low Concrete Cube Results on a First-Floor Slab
On a first-floor slab pour of 20 m³, the site uses M20 concrete batched on site. The cement bags have been stored for six weeks in an open-sided shed; some bags are lumpy. The mixer operator adds about 15 litres of extra water per cubic metre because the concrete 'fights' in the pump. Curing begins the next day and stops after three days because the labour is shifted. The slab spans 3.5 m between beams. Twenty-eight-day cube results are 14.2, 15.0 and 16.1 N/mm². No trial mix was tested for the actual materials. The contractor argues that the average is above 15 N/mm², so the slab is 'close enough' and proposes to start floor finishes immediately while the issue is reviewed. The examiner asks the trainee to assess the results and recommend action.
1. Evaluate the contractor's argument. Is the concrete acceptable for M20?
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The contractor's argument is not acceptable. M20 is a characteristic strength: the design assumes that 28-day cube results will mostly exceed 20 N/mm², with only a small permitted proportion below. Results of 14.2, 15.0 and 16.1 N/mm² are 20–29% below 20 N/mm², and the spread is significant. An average of 15.1 N/mm² is not the acceptance test; averaging three low results does not make them adequate. Finishes should not start because they would conceal a slab whose load capacity is in doubt. The correct action is to treat the slab as non-conforming, obtain a structural assessment, and decide on testing and remediation before any floor finish.
2. Identify the likely causes and explain how each reduced strength.
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Extra water is the strongest cause: 15 L/m³ raises the water/cement ratio, increases capillary voids, and reduces compressive strength; it also increases shrinkage and bleeding. Lumpy cement indicates damp storage and partial hydration, so the effective cement content is lower than the batch record shows. Curing for only three days stops hydration early, especially near the surface, so the cubes and the slab do not develop full strength. Site batching without a verified mix design adds variation. The combination explains both the low average and the wide spread.
3. What should be done before floor finishes? Include testing and possible outcomes.
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Stop the finishes and issue a non-conformance. Confirm cube identity and testing records. Use rebound hammer and ultrasonic pulse velocity to map weak zones, then calibrate with cores from representative locations. Send results to the structural engineer with the actual slab span, loads and reinforcement details. Outcomes: if the in-situ strength is adequate for the actual loads, the engineer may accept with a reduced factor or a condition survey; if not, options include increasing the section, adding reinforcement, or demolishing and recasting. Record the decision and do not allow a thick screed to be used as a cosmetic fix.