Oven-Dry vs. Saturated-Surface-Dry Basis in a Moisture Dispute
A ready-mix producer delivers coarse aggregate to a paving job. The supplier's shipping ticket states 0.8% absorption and 1.4% total moisture, and the batch plant used those values to adjust water. The contractor's technician takes a sample, splits it, and obtains: oven-dry mass 4,860 g; saturated-surface-dry (SSD) mass 4,975 g; as-received mass 4,940 g. The technician reports the sample as 'within the supplier's 1.4% moisture' and approves the load. The plant records show the mix was batched with 178 kg/m3 of added water against a design of 165 kg/m3, and the first truck showed slump 150 mm versus a 90 mm target. The contractor wants to reject the load on the grounds that the aggregate was over-wet.
1. Compute total moisture and absorption on a dry basis from the technician's data, and state whether the technician's approval was defensible.
Read the worked answer
Absorption = (SSD − oven-dry)/oven-dry × 100 = (4,975 − 4,860)/4,860 × 100 = 2.37%.
Total moisture (dry basis) = (as-received − oven-dry)/oven-dry × 100 = (4,940 − 4,860)/4,860 × 100 = 1.65%.
Free moisture = total moisture − absorption = 1.65 − 2.37 = −0.72%, i.e. the aggregate is still below SSD and would absorb water from the mix, not donate it.
The technician compared 1.65% against the supplier's 1.4% and called it 'within tolerance,' but he compared a dry-basis moisture to a ticket value that is meaningful only alongside its own absorption figure. The supplier's 0.8% absorption is inconsistent with the measured 2.37% - a discrepancy large enough to invalidate the batching adjustment. The approval was not defensible: the correct conclusion is that the absorption input to the batch computer was wrong by roughly 1.6 percentage points, which changes the effective water content materially.
2. Reconcile the moisture result with the observed high slump and the 13 kg/m3 water overrun. What is the most probable explanation?
Read the worked answer
The measured aggregate is below SSD, so it should have reduced slump, not raised it. The high slump and water overrun therefore cannot be attributed to free water on this aggregate. The plausible causes are: (a) the batch plant used the ticket's 1.4% moisture as if it were free water, crediting water that was not there, and simultaneously under-credited absorption - a double error that misallocates water; (b) the sample is not representative (the split may have been taken from a drier crust while the core of the stockpile was wetter); or (c) water addition or aggregate moisture in the silo bins differs from the delivered stockpile.
The defensible engineering position is that the aggregate data contradict the slump explanation, so the load should not be rejected on 'over-wet aggregate' grounds without resampling. The correct action is to resample per the stockpile sampling plan, re-determine moisture and absorption, and audit the batch computer's moisture/absorption entries before dispositioning the truck.