Question: We're thinking of developing a 6-inch-thick precast concrete residential wall system made with lightweight aggregates. For good insulating properties we want a low density and high R-value, but we also need to reach an adequate compressive strength. How are concrete density, R-values, and strength interrelated?

Answer: The R-values in the table above, based on data from the 1993 ASHRAE Handbook (American Society of Heating, Refrigeration, and Air-Conditioning Engineers), are for 6-inch-thick concrete walls of differing densities.ACI 213R-87, "Guide for Lightweight Structural Aggregate Concrete," divides lightweight concretes into three categories. Structural concrete has a density ranging from about 90 to 120 pounds per cubic foot and compressive strength from 2500 to more than 6000 psi. Moderate-strength concrete has a density between 50 and 90 pcf and a compressive strength in the 1000- to 2500-psi range. Low-density concrete commonly used for insulation purposes has a density ranging from 20 to 50 pcf and a strength of 100 to 1000 psi.You might also want to consider using preformed foam. Replacing 20% to 50% of the concrete volume with preformed foam produces concrete with densities in the 75- to 120-pcf range that can reportedly be used in structural precast operations (See "Expanding Possibilities with Preformed Foam," The Concrete Producer, August 1998, pp. 565-567).

R-Values for Concrete

Density R-Value
Normal weight--150 pcf 1.22
Structural lightweight--110 pcf 1.85
Lightweight aggregate--80 pcf 2.47
Lightweight aggregate--60 pcf 3.46
Lightweight insulating--30 pcf 6.30
Lightweight insulating--20 pcf 8.35