How to Calculate Moisture Correction in a Concrete Batching Plant

One of the most common complaints at a concrete plant is: “We dosed exactly the right water, but the concrete came out wet.” The recipe is checked and the water quantity is correct. Still the slump is off and strength drops. The cause is almost always in the same place: the moisture in the aggregate was not taken into account.

The recipe aggregate is dry — the aggregate in the bin is not

Concrete recipes are written in the laboratory on a dry-weight basis. On site, aggregate sits in open stockpiles, gets rained on and draws moisture from below. Moisture in 0-5 sand is often between 3 and 6 %; in gravel it is lower, but not zero.

Two separate errors hide between those two sentences:

1. Too little aggregate. If you weigh 620 kg of wet sand instead of 620 kg of dry sand, part of that weight is water; the real amount of aggregate is below target.
2. Too much water. The water carried in by the aggregate is added on top of the recipe water. The water/cement ratio rises and strength falls.

Both errors work in the same direction: the concrete becomes both weaker and wetter.

The calculation

The correction has two steps. First the wet weight to be weighed is found:

Wet aggregate = Dry aggregate × (1 + moisture % / 100)

Then the water brought in by the aggregate is deducted from the recipe water:

Water from aggregate = Wet aggregate − Dry aggregate
Water to add         = Recipe water − Σ(water from aggregate)

Example: C20/25, 1 m³

Material	Dry (kg)	Moisture	To weigh (kg)	Water brought (kg)
Sand 0-5	620	4 %	644.8	24.8
Sand 0-3	340	4 %	353.6	13.6
Gravel I (5-12)	480	0.5 %	482.4	2.4
Gravel II (12-22)	420	0.5 %	422.1	2.1
Total	1860	—	1902.9	42.9

If the recipe says 175 kg of water, the water to dose on the scale is:

175 − 42.9 = 132.1 kg

Those 42.9 kilos are water that never appears on the delivery note but decides the strength of the concrete. Without the correction the effective water/cement ratio rises from 0.583 to 0.726 — the concrete you poured as C20/25 looks right on paper but does not meet its class.

How is moisture measured?

Manual measurement. A sample is weighed, dried in an oven or on a stove, and weighed again. The difference is the moisture. It is done once a day at the start of the shift and repeated after rain. Simple and cheap, but it does not catch changes during the day.

Moisture probe. A sensor mounted at the bin outlet or above the belt reads moisture continuously. It needs investment; at plants that produce a lot and have fluctuating moisture it pays for itself quickly.

Whichever you use, what matters is not measuring — it is getting the measured value into the weighing targets of every batch. You cannot expect the operator to multiply four fractions by hand for every batch and then deduct from the water scale, let alone with the same care for a whole shift.

What should the software do?

In a properly designed plant automation, moisture is entered once per order (or read from the probe) and the rest is automatic:

• Each fraction’s weighing target is raised according to the moisture
• The total water from the aggregate is calculated and deducted from the water scale target
• The effective water/cement ratio is shown live on screen
• Both the dry and the weighed values are written to the production record and can be audited later

Without these four, moisture correction is not really “being done” — it is just being guessed in good faith.

Summary

• The recipe is dry weight, the silo is wet — they are not the same number
• Wet aggregate up, water down; the effective water/cement ratio stays constant
• At 4 % moisture, the correction in a typical 1 m³ batch is about 43 kg of water
• The calculation can be done by hand but cannot be repeated for every batch; it is the software’s job

If you are looking for plant automation that does moisture correction in every batch by itself: DKT CPlants — concrete batching plant software. For asphalt: DKT APlants.