Interpreting a Sieve Analysis for Concrete Sand Acceptance
A site receives 40 m³ of fine aggregate for a C-25 structural concrete pour scheduled the next morning. The storekeeper accepts it because the delivery challan states "FM 2.6, within specification". A quick sieve analysis on a 500 g sample gives cumulative percent passing: 4.75 mm = 98%, 2.36 mm = 92%, 1.18 mm = 78%, 600 µm = 55%, 300 µm = 22%, 150 µm = 6%. The concrete mix was designed with a sand of fineness modulus 2.6 and the water demand was fixed on that basis. The site has no other sand and the pour cannot be postponed. The technician must decide whether to use the sand as delivered, adjust the mix, or reject.
1. Calculate the fineness modulus from the given data and compare it with the challan claim. State the practical consequence for the mix.
Read the worked answer
Fineness modulus = sum of cumulative percentages retained on the standard sieves (4.75, 2.36, 1.18, 600 µm, 300 µm, 150 µm) divided by 100.
Cumulative percent retained:
- 4.75 mm: 100 − 98 = 2
- 2.36 mm: 100 − 92 = 8
- 1.18 mm: 100 − 78 = 22
- 600 µm: 100 − 55 = 45
- 300 µm: 100 − 22 = 78
- 150 µm: 100 − 6 = 94
Sum = 2 + 8 + 22 + 45 + 78 + 94 = 249. FM = 249/100 = 2.49.
The challan claim of 2.6 is close but the measured sand is finer (lower FM). A finer sand has more surface area, so it demands more water for the same workability. If the mix water is kept at the design value, the slump will fall and the concrete may be placed stiff and under-compacted. If water is added to restore slump, the water-cement ratio rises and strength drops. The technician must not simply accept the challan figure.
2. Recommend a course of action for the night before the pour, giving at least two options and the reasoning behind the chosen one.
Read the worked answer
The measured FM of 2.49 versus the design 2.6 is a small but real shift. Options:
- Use as delivered with a small water/binder adjustment. Increase water slightly to restore workability, then compensate by adding cement to hold the water-cement ratio constant. This preserves strength but raises cost and needs a quick trial mix.
- Reject and source alternative sand. Safest technically, but the pour cannot be postponed and no other sand is on site - so this is impractical unless a nearby source can deliver overnight.
- Use as delivered without adjustment. Cheapest but risks low slump and poor compaction, or an uncontrolled water addition that lowers strength.
Recommended: run a trial mix with the delivered sand, adjust water to the target slump, and add cement to keep w/c constant; verify with a slump test and cube samples. This is a judgement call: the deviation is modest (0.11 FM), so rejection is disproportionate, but blind acceptance ignores the water-demand shift. The technician should also record the deviation and inform the engineer.