Cement and Fly-Ash Reconciliation for an M25 Raft
At a BHEL site, the store reconciliation for an M25 raft is due. The month's accepted concrete volume is 480 m³. The approved mix design uses 390 kg/m³ cement, 70 kg/m³ fly ash, and 1.2% admixture by mass of cementitious material. The contract allows 2.0% cement wastage and 1.5% fly ash wastage. Stock records: cement opening 1,500 bags of 50 kg, receipts 3,400 bags, closing 860 bags; fly ash opening 8.0 t, receipts 40.0 t, closing 12.0 t; admixture opening 2 drums, receipts 14 drums, closing 3 drums, each drum 205 L. The store keeper says the excess is due to rich mix at the batching plant. The cube results show mean strength 31.5 MPa against the M25 requirement. The client is withholding payment for the excess cement.
1. Calculate the theoretical and allowable consumption of cement, fly ash and admixture, and compare with actual issues. Present the variances.
Read the worked answer
Cement: theoretical = 480 × 390 = 187,200 kg = 3,744 bags. Allow 2%: 3,744 × 1.02 = 3,818.9 bags. Actual = 1,500 + 3,400 - 860 = 4,040 bags. Excess over allowable = 221.1 bags (11.06 t); excess over theoretical = 296 bags (14.8 t). Fly ash: theoretical = 480 × 70 = 33.6 t. Allow 1.5%: 34.10 t. Actual = 8 + 40 - 12 = 36.0 t. Excess over allowable = 1.90 t. Admixture: cementitious = 390 + 70 = 460 kg/m³. At 1.2%, dosage = 5.52 kg/m³, about 5.52 L/m³ if density is 1.0. Theoretical = 480 × 5.52 = 2,649.6 L = 12.93 drums. Actual = 2 + 14 - 3 = 13 drums = 2,665 L. Variance is only +0.6%, so admixture is consistent with the mix design.
2. Is the rich-mix explanation consistent with the cube results and the admixture variance? What other causes should be investigated, and what should be done about the client's withholding?
Read the worked answer
A uniformly richer mix would raise cement, fly ash and admixture together. Cement is 7.9% over theoretical and fly ash 7.1%, but admixture is only 0.6% over theoretical. That pattern does not fit a uniformly richer mix. The mean cube strength of 31.5 MPa is 26% above the M25 characteristic value, which suggests over-cementing or a lower water/cement ratio, but it does not by itself explain the missing bags. Investigate batch records against weighbridge data, calibration of cement and fly-ash hoppers, bags used for other works, spillage and moisture, returns not credited, unrecorded issues and physical stock verification. The client should not withhold automatically; a joint reconciliation and physical stock check should come first. If unaccounted issue is proved, recover the cost; if it is a record error, correct the ledger.
3. Calculate the cementitious content per m³ and comment on strength and durability. Compute the excess cement cost at ₹380 per 50-kg bag.
Read the worked answer
Cementitious content = 390 + 70 = 460 kg/m³, well above the 320 kg/m³ minimum for M25 in moderate exposure. Strength and durability are therefore not at risk from low binder; the issue is cost and the higher heat of hydration and shrinkage of a rich mix. Excess cement over allowable = 221.1 bags. At ₹380/bag, excess cost = 221.1 × 380 ≈ ₹84,000. That is large enough to justify an audit. A rich mix may explain part of the strength but not the admixture mismatch, so treat this as a cost-control and records problem rather than a structural safety problem.