Rice Lake CB-2 Concrete Batch Controller Version 2.0 - Manual (Page 81)

Installation/Operation Manual - Appendix 77
10.12 Moisture Specifications
10.12.1 Cement/Water/Aggregate Mix Specifications
When defining a mix, you specify the amount of cement, water, and aggregates per yard of delivered product.
The specified amount of water is the amount needed to react with the cement (free water).
Aggregates may (and probably will) contain moisture (the moisture content of the cement and admixtures is
ignored). They can contain both absorbed and free water. Absorbed water is NOT available to react with the
cement – free water is.
Each material has an associated “Absorption Factor”. This is the maximum moisture level at which there is no
free water. If a material is below its absorption factor, some of the added water will be absorbed by the material
and will not be available to react with the cement. If it is above its absorption factor, some of the water in the
aggregate will be available.
The CB2 will automatically adjust added water based on the current moisture levels in the aggregates, either
adding or subtracting as needed.
10.12.2 Oven Dry vs. Saturated Surface Dry Mix Designs
When specifying a mix design, the amount (by weight) of an aggregate per yard of delivered product is specified.
For a particular batch, the amount by weight is dependent on how much moisture is present, This may change
from day to day, but we don’t want to have to continuously change our mix designs to account for these
differences.
To solve this problem, we specify mix designs in terms of material with a specified moisture content and then
adjust that weight based on the current moisture content when producing product.
There are two recognized standard moisture levels – Oven Dry (OD) and Saturated Surface Dry (SSD). Oven
Dry contains no moisture at all. Saturated Surface Dry Contains moisture at the material’s “Absorption Level”.
Mix designs may specify the amount of aggregate per yard in either Oven Dry or Saturated Surface Dry weights.
(Right now, the only way to use Oven Dry weights is to specify Minnesota DOT as the regulatory agency.)
10.12.3 Example of Weight Adjustments Based on a Saturated Surface Dry Mix Design
This example shows how the CB2 adjusts the designed aggregate and water amounts based on actual moisture
content for a mix design using SSD aggregate weights.
Suppose for simplicity’s sake we have a design that calls for 1000 pounds of gravel, 500 pounds of cement, and
400 pounds of water per yard of concrete. Suppose also that the gravel has an absorption factor of 6% and a
current moisture content of 4%.
As the design is based on SSD weights, we could use the specified weights if the current moisture level was 6% -
the same as the absorption factor - but it is not – we are 2% short. So, how much gravel would we need to add at
4% moisture content to get the same amount of oven dry gravel as 1000 pounds of SSD gravel?
1000 pounds of SSD gravel contains 943 pounds of OD gravel (1000 / (100 x (1 + .06)). 943 pounds of OD
gravel at 4% moisture is 981 pounds. (943 x 1.04). So, to get the same amount of gravel, we need to reduce the
1000 pounds per yard to 981 pounds per yard.
OK, but we are not finished. When we add water to the mix, the aggregate will absorb enough of it to bring its
moisture content up to the SSD level. This water is not available to chemically react with the cement, but,
because the mix design is SSD based, the amount of water specified to be added is what is needed for this
reaction. We need to adjust the amount of water to be added to account for this.
Actually, this is pretty simple. We know that at SSD, the aggregate is 1000 pounds per yard and that we are
adding aggregate at 981 pounds per yard, so how much extra water do we need to bring it up to its 1000 pounds
per yard? 19 pounds per yard (1000 – 981).
So, for this simple case we wind up with an actual batch calling for 981 pounds per yard of gravel, 500 of
cement, and 419 of water.