Nine-Week-Old Cement and a Slab Pour Tomorrow
A co-educational senior secondary school in Jhalawar is adding a 4.5 m × 3.6 m room with an RCC roof slab 125 mm thick and a 1.2 m wide chajja. In mid-June, 140 bags of OPC 43 grade were delivered and stacked twelve bags high on a rammed-earth floor inside a corrugated-sheet shed; the stack touches the north wall and there is no dunnage under it. Nine weeks of monsoon later, the mason opens bags for the slab pour scheduled for tomorrow morning. Most bags contain soft lumps that crumble between the fingers; about fifteen bags contain hard, stone-like lumps that cannot be crushed by hand. The slab needs roughly 4.2 m³ of M20 concrete in one continuous pour, and the mason says he will sieve the cement and carry on. A fresh consignment of 40 bags of OPC 43 grade arrived last week and is stacked on timber pallets; further supply is five days away. The supervisor has to decide by evening whether the pour proceeds.
1. Decide whether the stored cement may be used in the M20 slab. State the checks you would apply, the pass/fail rule for each, and your final decision with reasons.
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Step 1 – classify the lumps. Rub a sample from each damaged bag between the palm and fingers. Lumps that crush back to powder are harmless (aerated, not hydrated) and may be used in non-structural work. Lumps that resist hand pressure, or ring when struck, are hydrated cement and have lost binding power. Fifteen hard-lump bags are unusable for structural concrete; the soft-lump bags are doubtful but not automatically condemned.
Step 2 – check what is still verifiable. Weigh a few bags (nominal 50 kg, tolerance ±1 kg). Sieve a sample on a 90-micron sieve - residue above about 10% indicates lumping/fineness loss. Read the manufacture date on the bags; storage beyond three months, and far sooner in a damp shed, carries measurable strength loss.
Step 3 – the decisive constraint is time. A soundness (Le Chatelier) and cube test result cannot be produced before tomorrow morning. Designing an RCC slab on unverified cement is not a judgement you can defend later; the cost of a failed slab is many times the cost of cement.
Step 4 – check whether the good cement is actually enough. At roughly 320 kg/m³ for M20, 4.2 m³ needs about 1,344 kg = 27 bags; add 5–8% site wastage and you need 29–30 bags. The 40 fresh pallet-stored bags are sufficient, so the shortage argument collapses.
Decision: reject the shed stock for the RCC slab, cast with the fresh cement, and defer the pour only if batching/plant is not ready.
2. Write the site instruction you would issue covering storage, the rejected bags and prevention of a repeat, and explain the reasoning behind each clause.
Read the worked answer
Instruction (site instruction book, dated and signed by supervisor and contractor's engineer):
- Do not use shed-stock cement in any RCC member - slab, chajja, beam, column - until 7-day and 28-day cube results (IS 516 moulds, three cubes per lot, sampled at 10% of bags) are available and accepted.
- Segregate the fifteen hard-lump bags and mark them 'REJECTED – NOT FOR STRUCTURAL WORK'. Return them to the supplier against the invoice; if return is refused, use the sieved material only in non-structural mortar for masonry or as a filler, never in load-bearing concrete.
- Rebag and store the remaining stock on 150 mm timber dunnage, minimum 600 mm clear of walls, not more than 8–10 bags high, in a stack no wider than two bags so bags are not squeezed; first-in-first-out with a stock register showing date of receipt.
- Draw cement strictly against issue slips and reconcile bag counts with concrete volume poured.
Reasoning: clauses 1 and 2 isolate the unverified risk; clause 3 attacks the cause (ground moisture and wall contact are what hydrated this stack, not age alone); clause 4 makes the next lapse visible in the accounts rather than at the pour. Clauses are written as verifiable actions, not advice, so non-compliance is measurable.