Non-Conforming Aggregate Moisture and Batch Adjustment
A concrete plant is producing a C30/37 mix. The approved mix design assumes a sand moisture content of 5.0%. On a particular morning the plant operator notes that the sand stockpile has been exposed to rain overnight and the moisture probe reads 8.5%. The operator continues batching to the original mix design without adjusting the batch weights, reasoning that 'the mix design is approved, so we follow it.' The first three truckloads are delivered to a site and placed. The site engineer later reviews the batch records and notices the discrepancy. The concrete appears workable and no slump problems were reported.
1. Explain the consequence of batching without adjusting for the higher sand moisture.
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The approved mix design is based on saturated-surface-dry (or oven-dry) aggregate weights. When sand moisture rises from 5.0% to 8.5%, the sand carries an extra 3.5% of its mass as free water into the mix.
Consequences:
- The effective water/cement ratio increases. The extra free water is not accounted for, so the actual w/c ratio is higher than designed. This reduces strength and durability.
- The batch weights are wrong. If the operator follows the design weights literally, the amount of sand solids is effectively reduced (because part of the weighed sand is water), and the total water is higher.
- The concrete may still look workable. A higher w/c ratio often increases slump, so the concrete can appear fine on site while being weaker than specified - which is exactly why the batch records matter.
The operator's reasoning is flawed: following the design weights without moisture correction does not follow the mix design; it departs from it.
2. What should the engineer do about the three delivered loads, and what control should be put in place?
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Immediate action on the three loads:
- The loads were batched with an unaccounted water addition, so their actual w/c ratio is uncertain. The engineer should treat them as potentially non-conforming.
- Assess the risk: request the batch records, calculate the likely excess water, and consider strength verification (e.g. cubes or cores) for the affected elements. If the excess water is significant, the concrete may not meet the specified strength.
- Decide whether the affected elements need investigation or acceptance based on the strength evidence, not on the fact that the concrete looked workable.
Control measures:
- Moisture correction must be applied at batching. The plant must measure aggregate moisture (probe and/or oven-dry checks) and adjust batch water and aggregate weights accordingly.
- Calibrate and verify moisture probes, especially after rain.
- Record moisture readings on the batch records so the correction is traceable.
- Cover or drain stockpiles to reduce moisture variation.
Key point: the mix design is only valid when the moisture correction is applied; the control failure is at the plant, and the consequence is a potential strength shortfall on site.