Published: Jan 1990
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A lightweight structural concrete was developed for use in exterior walls of low-rise residential and commercial buildings. The lightweight concrete has a unit weight of 800 kg/m3 (50 pcf), a compressive strength of 13.8 MPa (2000 psi), and a thermal conductivity of 0.23 W/m · K (1.6 Btu · in./h · ft2 · °F). Lightweight concretes have not been previously developed with this combination of low density and moderate strength. The most commonly used concrete, normal weight concrete, has a unit weight of approximately 2320 kg/m3 (145 pcf), a compressive strength in the range of 17 to 41 MPa (2500 to 6000 psi), and a thermal conductivity of 1.7 to 2.3 W/m · K (12 to 16 Btu · in./h · ft2 · °F).
The portland cement concrete developed for this project can be used to combine structural, thermal insulation, and heat storage capacity functions of exterior walls in one element. For many climates this concrete can be used without additional insulation as a complete wall system in lowrise buildings.
Heat transfer characteristics of two 200-mm (8-in.)-thick, full-size wall assemblies were evaluated using a calibrated hot box (ASTM C 976). One test specimen, designated Wall L, was a 200mm (8-in.)-thick wall constructed entirely of the newly developed lightweight structural concrete. The second specimen, designated Wall S, was the same as the first except for a 150-mm (6-in.)-high normal weight concrete strip running horizontally across the wall at midheight. The horizontal strip simulates a floor slab extending through an exterior wall.
Overall thermal resistances of Walls L and S, respectively, are 0.92 and 0.83 m2 · K/W (5.2 and 4.7 h · ft2 · °F/Btu) at 24°C (75°F). Thermal resistance of Wall S is 11% less than that for Wall L.
Tests under dynamic temperature conditions provide a measure of thermal response for selected temperature ranges. Dynamic response includes heat storage capacity as well as heat transmission characteristics of the wall assembly. Results from a 24-h period, sol-air temperature cycle showed that heat storage capacity of the low density concrete delayed heat flow through the test specimen. Average thermal lag for the 200-mm (8-in.)-thick lightweight concrete wall was 6 h.
Thermal and physical properties of the lightweight concrete were also measured on small-scale specimens. Concrete thermal conductivity, thermal diffusivity, specific heat, compressive strength, flexural strength, splitting tensile strength, shear strength, modulus of elasticity, drying shrinkage, and freeze/thaw resistance were determined.
Laboratory test results provide information on the thermal and physical performance of the new lightweight concrete.
calibrated hot box, energy, heat transmission, lightweight concrete, structural concrete, thermal conductivity, thermal mass, thermal resistance
Van Geem, MG
Senior Research Engineer, Fire/Thermal Technology Section, Construction Technology Laboratories, Inc., Skokie, IL