While ACI 318 requires a maximum w/cm for durability, it has a companion specified compressive strength, f’c, for each level of w/cm. This is a recognition that w/c cannot be reliably measured and verified for conformance to the requirement, while compressive strength can (with test cylinders). ACI 318 provides commentary related to the requirement that concrete mixtures must be proportioned to comply with the maximum w/cm and the other requirements based upon the anticipated exposure classification of the structural concrete, as follows:
Maximum water-cementitious material ratios (w/cm) of 0.40 to 0.50 that may be required for concretes exposed to freezing and thawing, sulfate soils or waters, or for corrosion protection of reinforcement will typically be equivalent to requiring f’c of 5000 to 4000 psi, respectively. Generally, the required average compressive strengths, f’cr, will be 500 to 700 psi higher than the specified compressive strength, f’c.
The idea here is that specifying a compressive strength that will achieve the desired durability will automatically ensure that the maximum w/cm is not exceeded. ACI 318 goes on to caution the designer not to specify a w/cm and a compressive strength that are inconsistent, say w/cm of 0.45 and f’c of 3000 psi. Going back to Table 1, if you need a 0.45 w/cm mix for durability, you should specify concrete with a compressive strength closer to 5500 psi. Since the usual focus during inspection is on strength, a 3000 psi concrete mixture might result in concrete with a w/cm that is higher than desired and therefore a lower durability concrete.
Care is taken to proportion concrete mixes that achieve desired performance characteristics of compressive strength and resistance to damage from freezing and thawing, sulfate exposure, and corrosion.
These performance characteristics are vulnerable when adding water above the design limitations. The project’s specified design strengths should closely relate to proven concrete performance at the maximum permitted w/cm.
Bill Meek is technical services manager for Cemex’s Midwest Cement Division. Jim Turici is director of technical services, Cemex’s U.S. Cement Division.This article originally appeared in Concrete Construction, TCP’s sister publication.
Higher Water = Lower Strength
Let’s consider a ready-mix truck containing 10 cubic yards of air-entrained concrete mixture (containing 564 pounds of portland cement per cy) that has been proportioned at the maximum design w/cm of 0.45 (dashed arrows on Fig. 1). Upon arrival to the jobsite and prior to placement, 33 gallons of water are added to the 10 cubic yard load, resulting in a w/cm of approximately 0.50 (solid arrows on Fig. 1). The resulting 28-day compressive strength of the mixture prior to the water addition was 4250 psi and the resulting compressive strength realized was 3750 psi. This equates to a drop of 500 psi or a strength loss of nearly 12 percent.