Site Mix Adjustment Calculator
Enter the lab mix and moisture content to get real batch weights.
Inputs
Result
Formula
| Wet sand = dry sand × (1 + moisture) | same for gravel |
| Water added = lab water − dry sand×m − dry gravel×m | keeps w/b constant |
| Batch weight = per m³ × (cement per batch ÷ cement per m³) | batch sized to whole bags |
C30 lab mix: cement 340, sand 700, gravel 1200, water 180 kg/m³; sand 3% and gravel 1% moisture. Wet sand 721, wet gravel 1212 kg, aggregate brings 33 kg of water, so add only 147 kg. For a two-bag (100 kg) batch the factor is 0.294: sand 212 kg, gravel 356 kg, water 43 kg.
FAQ
Why not just follow the lab mix and add water as needed?
Water/binder ratio governs strength. Adding water without deducting what the aggregate already carries raises w/b by a real margin - roughly 20 kg extra water per cubic metre costs 3-5 MPa - and promotes bleeding and cracking. Always solve for water first.
How often should moisture be measured, and what about rain?
Once per shift normally, every two hours after or during rain, sampling surface and core separately. The top 10 cm of an open sand pile can run 3-5 points wetter than the core; sampling only the surface over-deducts water and the mix comes out dry.
The calculated water comes out negative - what is wrong?
Three usual causes: the lab water figure assumed a superplasticiser that is not being added on site; the moisture reading is too high (often free water on the sample after rain); or a decimal slip in the aggregate weights. Check moisture first, then the admixture dose.
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Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.