Hot-Weather Raft Pour with Delayed Transit Mixers
You are the site engineer for an 18 m by 12 m by 0.45 m raft foundation for a two-storey vocational institute in Pune. The design is M25 with 40 mm cover; the total concrete is about 97 cubic metres. The batching plant is 25 km away. On the pour day the ambient temperature is 38 °C, humidity is low, and the first two transit mixers arrive 95 minutes after batching with a slump of 70 mm against the specified 150 ± 25 mm at discharge. The contractor wants to add 20 litres of water per 6 cubic metre drum and continue. Only one vibrator is working; a second is expected in four hours. The raft has a 1.5 m wide construction joint planned at mid-length but no waterstop. The client’s inspector is absent.
1. Decide whether to accept, reject, or conditionally accept the concrete. Explain using slump, time, temperature, and strength risk.
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The concrete should not be accepted as delivered. A slump of 70 mm is well below the specified lower limit of 125 mm, and the mix is already 95 minutes old at 38 °C. Adding 20 litres per 6 cubic metres raises the water content by about 3.3 litres per cubic metre, which increases the water-cement ratio and reduces strength and durability. Retempering by adding water at site is not a substitute for a properly designed admixture dose.
A conditional acceptance would be defensible only if the plant confirms the mix design, a compatible plasticiser is added under a controlled dose, the slump is remeasured, and cubes are taken from the revised mix. Even then, the 95-minute age and hot weather make the risk high for a raft with 40 mm cover.
My decision is to reject the two loads, record the time, temperature and slump, and require the plant to adjust the mix with a retarder or plasticiser and improve delivery. The client inspector should be informed immediately, even if absent.
2. Plan the pour and cold-joint strategy with one vibrator, a delayed second vibrator, and no waterstop at the planned joint.
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With one vibrator, the safe output is limited. I would not start a 97 cubic metre pour without a reliable second vibrator and a standby. The pour should be sequenced in strips, working away from the planned joint so that the joint remains a deliberate stop, not an accidental cold joint.
Because there is no waterstop, the planned joint must not be left as a open cold joint. Before continuing, the joint should be formed with a keyway or roughened surface, cleaned of laitance, and provided with an approved waterstop or hydrophilic strip if the design requires watertightness. If the design does not require a waterstop, the joint still needs a clean, roughened, saturated surface and a bonding grout or approved bonding agent.
Practical plan: reduce the pour rate, keep one vibrator on the active face, arrange a standby vibrator from another site, monitor concrete age and temperature, and stop at the planned joint if the second vibrator does not arrive. Do not create an unplanned cold joint in the middle of the raft.
3. What records and communications would you complete before the pour restarts?
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Records: delivery dockets with batching time, arrival time, slump, temperature and quantity; rejection notes for the two loads; updated mix design and admixture dose; vibrator availability; joint preparation photographs; and cube sampling plan.
Communications: a written site instruction to the contractor to stop the pour, a message to the client engineer explaining the risk, and a revised pour sequence with a restart time. The contractor should confirm in writing that the joint preparation and waterstop decision are accepted.
The key judgement is that programme pressure does not justify placing low-slump, old concrete in a raft. A short delay is cheaper than a weak, porous raft. The restart should be authorised only when the mix, vibration and joint details are under control.