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3204.006
Valore, Rudolph C., Jr.
Thermophysical Properties of Masonry and its Constituents
Book
The International Masonry Institute
1988
I. 158; II. 103
Y
I. THERMAL CONDUCTIVITY OF MASONRY MATERIALS Over 1000 European and North American thermal conductivity test values of masonry materials reported in the literature were analyzed. Basic nondesign conductivity-density relationships were obtained by linear regression for separate groups of dry materials with similar mineralogical or morphological characteristics. Materials were bricks, ordinary and lightweight concretes, rocks (as aggregates), granular fills, and cement pastes and mortars. Conductivity “moisture factors” are given for each group, usually amounting to 4 to 6 percent increases in conductivity per one percent of moisture, by weight. Equilibrium moisture contents for groups of materials in “protected” and “unprotected” exposures for groups of materials in “protected” and “unprotected” exposures at mean relative humidities of 60 and 80 percent, respectively, combined with moisture factors, provide multiplying factors for converting basic nondesign conductivities for dry materials to “practical” values for materials in service.Models are presented which provide equations for calculating conductivity of composite materials. The Cubic Model yields values in good agreement with test data for sanded lightweight aggregate concretes and various other composites. Conductivity-density relationships were usually well-defined but ordinary “normal weight” concretes provided no useful correlation. For these materials calculations based on conductivities and volume fractions of components will yield more accurate values.
It is shown that present standards are in need of revision. During that process consideration must be given to heterogeneity in masonry materials as it relates to required sizes and numbers of test specimens in providing conductivity values that are representative of materials in a structure.
II. THERMAL TRANSMITTANCE OF MASONRY WALLS
Thermal conductivity values from Part I were used in series-parallel (“isothermal planes”) calculations of thermal transmittance or U-values of 193 concrete block and 20 brick test walls. Net densities ranged from 37 to 142 pcf (600 to 2270 kg/m3). Solid, hollow (with cores empty and filled with granular or premolded insulation), and multicore masonry units were included. Calculation consisted in dividing a wall into layers perpendicular to heat flow, calculating the thermal resistance of each layer (consisting of any number of parallel segments of different materials including air-spaces and mortar joints) and summing these values to obtain the total resistance of a wall. Agreement of calculated and test U-values was good: the mean deviation of calculated from test values was 6.0 percent. U-values can be reduced to suitable levels by reducing densities and conductivities of materials and thermally bridged fractional wall areas, and by increasing thicknesses of walls and insulation.