Low Cube Results on a Hot-Weather RCC Slab
An RCC roof slab for a 12 m × 9 m hall is being cast with a nominal M20 mix (1:1.5:3) in mid-May, when the ambient temperature is 41 °C with a dry wind. Concrete arrives at the hopper at 32 °C. The slump is 25 mm, the mason calls it unworkable, and about 20 litres of water per cubic metre is added at the hopper; the slump then reads 70 mm. Cement bags have stood for five weeks on a damp floor under a tarpaulin and several are lumpy. The slab is cured by ponding for three days, after which the water is drained because the next trade needs the area. The 28-day cube results are 14.2, 15.0 and 16.1 N/mm². The structural consultant says the cubes were probably bad and wants the slab accepted as it is.
1. Explain why adding 20 litres of water per cubic metre at the hopper is a serious error. Quantify the effect on strength and durability using the water–cement ratio.
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For a nominal M20 mix the cement content is roughly 320–340 kg/m³, and the design water–cement ratio is about 0.50, i.e. about 160–170 litres per m³. Adding 20 litres raises this to about 0.56–0.57.
Strength falls as the water–cement ratio rises - Abram's law. At this level, an increase of about 0.06 typically costs 15–20% of the 28-day strength, roughly 27 MPa down to about 22–23 MPa.
Durability suffers too: the extra water that is not used in hydration leaves capillary voids, so permeability, drying shrinkage and chloride ingress all increase, and the cover protects the steel less well. There is a second, worse problem - water poured in at the hopper is never uniformly mixed, so some concrete is far wetter than the tested average. If the concrete has begun to stiffen, adding water is retempering, which breaks down the gel structure.
The important judgement: this error is real but it cannot by itself explain 15 MPa. Something else is also wrong.
2. List the factors in this scenario that could have contributed to the low cube strength, and rank the two most probable.
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- Cement stored damp for five weeks with lumpy bags - partially hydrated cement loses strength, and lumps mean unreliable batching.
- Curing cut to three days - M20 needs at least 7 days; three days in 41 °C means hydration stops early and strength loss can be 20% or more.
- Water added at the hopper - as above, 15–20% loss.
- Hot weather and dry wind - rapid evaporation, higher concrete temperature, plastic shrinkage and less water available for hydration.
- 25 mm slump before the water addition - poor compaction and honeycombing are likely, reducing cube and in-situ strength.
- Volume batching, with damp sand bulking not accounted for - cement content per m³ may be lower than assumed.
Ranking. The two most probable are the damp-stored cement and the three-day curing. Their combined effect can halve strength and explain a drop to 15 MPa. The water addition explains only a part of it. This ranking matters because the remedy targets both, and because blaming only the water lets the storage and curing failures continue on the next floor.
3. Recommend a course of action to determine whether the slab is structurally acceptable, naming the tests and the acceptance logic.
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Cube failure is a legitimate signal and cannot be dismissed by opinion. Proceed in stages:
- Non-destructive tests - rebound hammer (IS 13311 Part 2) and ultrasonic pulse velocity (Part 1) - to map uniformity and locate weak zones. These are indicative only and cannot alone justify acceptance.
- Core testing (IS 516 / IS 456). Take at least three 50–75 mm cores from critical, randomly chosen locations, avoiding reinforcement, and test in compression. Acceptance logic: the average core strength should not be less than 0.85 fck, and no single core less than 0.75 fck. For M20 that means an average of at least 17 MPa and no core below 15 MPa.
- If cores fail, require a load test on a representative panel as per IS 456, with acceptance based on deflection and recovery, or require strengthening/redesign.
- Check the remaining cement for lumps and test it, and verify the batching records.
Record all results. If the slab is accepted, apply a protective coating and note reduced durability in the file; do not accept it merely because the consultant doubts the cubes.