Low Cube Results and Site Water Addition on an Office Slab
On a five-storey office project in Ulaanbaatar, C25/30 ready-mixed concrete is supplied to columns and slabs. At the 12th placement, 28-day cube results for the slab are 22.5, 24.0 and 25.5 MPa against the specified characteristic strength of 30 MPa. Delivery tickets show slump 180 mm at the plant; the site slump test was 220 mm. The pump operator added about 30 litres of water per truck because the mix 'looked stiff'. Cubes were stored in an unheated site hut and tested by an uncertified local lab. Curing consisted of one day under plastic, followed by no wet curing in windy, dry weather. The contractor wants to accept the slab because 'only three cubes failed' and asks for payment. The engineer must decide on investigation, acceptance and payment.
1. Was the site water addition acceptable? Explain its effect on the concrete and on the test results.
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No. Adding 30 litres per truck on site is not permitted unless the mix design and the engineer approve it. It raises the water/cement ratio, lowers strength and durability, increases bleeding and shrinkage, and can cause the very slump loss the operator was trying to fix. The correct response is to reject a mix that is outside the agreed slump and retemper only with an approved admixture under the plant's instructions. The difference between 180 mm at the plant and 220 mm at site also points to a supply or testing problem, not a reason to add water.
2. What investigation is needed before deciding whether the slab complies?
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Investigation: review batch records, cement and admixture certificates, aggregate moisture, batching accuracy, delivery times, site water additions, sampling and cube storage. Check the laboratory's accreditation and the cube curing regime; cubes stored in an unheated hut and tested by an uncertified lab are not reliable evidence. Then test the hardened slab by cores from representative locations, with cover, rebound or ultrasonic tests as supporting information. The engineer should use the core results and structural analysis to decide whether the slab meets the design intent. A root-cause report should separate batching, transport, site addition and curing failures.
3. How should the engineer handle acceptance and payment if the cores are also low?
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Disposition and payment: hold payment for the affected slab and do not accept it merely because only three cubes failed. If cores and analysis show adequate strength and durability, the engineer may accept with a documented deviation and any required durability measures. If not, require repair, strengthening or demolition and recasting at the contractor's cost. Payment should follow the accepted disposition. A corrective action plan is required before the next placement.