American Society of Concrete Contractors Publishes 46th Position Statement: w/cm Ratio for a Trowel Finish

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The American Society of Concrete Contractors (ASCC), St. Louis, MO, has published its forty-sixth 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 (NRMCA) 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.

The ASCC is a non-profit organization dedicated to enhancing the capabilities of those who build with concrete, and to providing them a unified voice in the construction industry. Members include concrete contracting firms, manufacturers, suppliers, and others interested in the concrete industry, such as architects, specifiers, and engineers. There are approximately 710 member companies in the United States and 12 foreign countries.  For more information visit the website at www.ascconline.org or call (866) 788-2722.

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