American Society of Concrete Contractors Publishes 46th Position Statement

The American Society of Concrete Contractors published its 46^th Position Statement, “Water-Cementitious Material Ratio for Concrete to Receive a Trowel Finish.” ASCC position statements clarify the concrete contractors’ point of view for architects, engineers, owners and others.

Position statement 46 explains that the ACI 318-19 Building Code requires a maximum water-cementitious material ratio (w/cm) in accordance with the severity of the anticipated exposure of members, and varies from N/A to 0.55, 0.50, 0.45 and 0.40. Many specifications, however, require a maximum w/cm for interior concrete, often as low as 0.40, which is undesirable for the surface finish.

ACI 302.2 R-06, “Guide for Concrete Slabs that Receive Moisture-Sensitive Flooring Materials,” recommends a w/cm of 0.50 as the best compromise between drying rate and finishing performance. ACI 302.1R-15 “Guide to Floor and Slab Construction” states that specifying a maximum w/cm for concrete workability is not useful, and further recommends a w/cm in the range of 0.47 to 0.55 for concrete floors to receive a trowel finish.

The position statement includes additional references from ACI, as well as the National Ready Mixed Concrete Association and ASTM International. It concludes with the recommendation that concrete contractors should encourage specifiers to consider ACI 302.1R-15 and ACI 302.2R-06 for w/cm for concrete to receive a trowel finish.