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Figure 2 - Aggregate moisture content plays a part in concrete workability. If aggregates are too dry, they will absorb (steal) water from the mix. If aggregates are too wet, the excess moisture must be subtracted from the intended mix water quantity. (Photo courtesy of PCA)

The absorption and surface moisture of aggregates are simple yet critically important aspects of producing concrete that consistently achieves the specified or targeted strength. The fundamental relationship between water-to-cement ratio and strength starts with correcting for the moisture contribution or absorption by the aggregates.

Aggregates are somewhat like small, hard sponges. Mix design calculations are conducted assuming aggregates are in a saturated surface dry (SSD) condition, meaning their absorption is satisfied and no water is taken from or added to the mix. If their absorption is not satisfied, these "sponges" steal water from the designated quantity of mix water, reducing the slump of the concrete. If the "sponges" are excessively wet (beyond that amount to satisfy the absorption), the extra water must be subtracted from the quantity of batched mix water. Otherwise, the target w/c ratio is exceeded and strengths will decrease (Figure 2).

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