Concrete contractors know that a lower water-cement ratio is a key design attribute for higher-strength concrete. But there can be applications for which design professionals should avoid specifying a mixed design with too low of a w/cm. One application that requires a good pre-pour discussion about the correct w/cm ratio is interior troweled concrete floors.  

To increase abrasion resistance to floors that will be exposed to forklift traffic, design professionals may suggest a low w/cm. If the w/cm is too low, the concrete contractor can have difficulty in providing a quality surface finish.

As with many important industry discussions, The American Society of Concrete Contractors (ASCC), St. Louis, MO, publishes a position statement that provides the concrete contractors’ point of view on actual topics that come up in the field. Concrete contractors are urged to use these documents for reference in their preplacement meetings with design professionals, ready-mixed concrete producers, and testing labs.

The topic of what w/cm ratios should be used on concrete floors is discussed in ASCC’s Position Statement 46, “Water-Cementitious Material Ratio for Concrete to Receive a Trowel Finish.” ASCC members have reported that many project specifications require a maximum w/cm for interior concrete, often as low as 0.40. This range of w/cm ratio is undesirable for the surface finish.

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 the concrete member. This figure varies from N/A to 0.55, 0.50, 0.45, and 0.40.

Position Statement 46 urges design professionals to refer to ACI 302.2 R-06, “Guide for Concrete Slabs that Receive Moisture-Sensitive Flooring Materials” for better guidance on the proper w/cm. This document 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” also 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.

Position Statement 46 includes additional references from ACI, as well as the National Ready Mixed Concrete Association and ASTM International.

Position Statement 46 provides a great resource.  ASCC recommends 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.

You can review Position Statement 46 at ASCC Position-Statements.