Thermal Conductivity, Concrete

Residential Construction. Owing to rising energy costs, the cost and low thermal conductivity are of prime importance in wall and ceiling insulation of residential buildings. The combination of insulation efficiency, desirable stmctural properties, ease of appHcation, abiHty to reduce air infiltration, and moisture resistance has led to use of extmded polymeric foam in residential constmction as sheathing, as perimeter and floor insulation under concrete, and as a combined plaster base and insulation for walls. 

Another aspect of epoxy resin mortar floorings which needs careful attention is that their coefficients of thermal expansion are approximately three times that of concrete. This, coupled with the relative low thermal conductivity of epoxy mortar, can cause stresses to be induced at the resin mortar/concrete interface under conditions of thermal shock (e.g. thermal cleaning), resulting in break-up of the flooring due to initial failure in the concrete. Two approaches have been tried to overcome this problem ... 

A fire occurs in a 3 m cubical compartment made of 2 cm thick. Assume steady state heat loss through the concrete whose thermal conductivity is 0.2 W/m2 K. By experiments, it is found that the mass loss rate, m, of the fuel depends on the gas temperature rise, AT, of the compartment upper smoke layer ... 

An investigation was carried out on concrete containing up to 30% of PE foam waste in order to optimise the thermal conductivity of the concrete. An amorphous aggregate of ash-slag waste was utilised to decrease the thermal conductivity of the concrete. 5 refs. 

Vermiculites are formed by the decomposition of mica. They contain layers of water and magnesium ions in place the potassium ions. When heated to 800°C-1100°C, vermiculite expands because of the conversion of the water to gas. The expanded vermiculite has a low thermal conductivity and density, and is used as a thermal and sound barrier as well as an aggregate in lightweight concrete. It is also used as a moisture-retaining soil conditioner in planting. 

In sulphur concretes, the mechanism of deterioration caused by frost action has been attributed to entirely different causes to those above. The material has low permeability to moisture and as water is not used in mixing, it was not considered that water played a major role in deterioration. Sulphur has a very high coefficient of thermal expansion (a - 55 x 10 6/°C) and low thermal conductivity (0.27 W/m K). Hence the poor durability performance in cyclical freezing and thawing has been attributed to the development of high stresses due to thermal gradients (5,... 

The inherent flammability and low melting point of sulfur impose some limitations of SC use. Flammability can be controlled to some extent by the use of additives, and it is fortunate that the DCPD types of additives used to improve the durability of SC also impart a degree of fire resistance. Sulfur concretes are in any case considerably less of a fire hazard than wood. Because of the low thermal conductivity, heat penetration is slow, and SC can survive short exposures to fire without serious damage. Sulfur concretes do not support combustion, and flame spread is essentially zero. 

The inherent properties of the fire-resistant mastic may be exploited for many other applications. On the basis of low thermal conductivity, the mastic forms a suitable coating on metal sheet buildings which prevents condensation, reduces noise coefficient, and retards radiated heat. Another property which is sometimes important is that the mastic does not support mildew. For this and other reasons, the mastic has been applied on the interior of concrete storage granaries to prevent the grain from mildewing in the area adjacent to the concrete walls. 

A concrete slab 15 cm thick has a thermal conductivity of 0.87 W/m - °C and has one face insulated and the other face exposed to an environment. The slab is initially uniform in temperature at 300°C, and the environment temperature is suddenly lowered to 90°C. The heat-transfer coefficient is proportional to the fourth root of the temperature difference between the surface and environment and has a value of 11 W/m2 °C at time zero. The environment temperature increases linearly with time and has a value of 200°C after 20 min. Using the numerical method, obtain the temperature distribution in the slab after 5, 10, 15, and 20 min. 

Values of the thermal conductivity, k, have been determined in the present work with a thermal conductivity probe (24). It has long been known that sulfur has a low thermal conductivity although the values are even lower in such materials as PVC and expanded polystyrene. Sulfur-bonded composites made with inexpensive fillers such as soil and sand have thermal conductivities which are below those of typical portland cement concrete but with values higher than those of sulfur itself. The values for the composites are, however, still low as may be seen by comparison with the values for conductors such as steel and copper (Table II). 

Consider a 20-cm-ihick large concrete plane wall k 0.77 V/in °C) subjected to convection on both sides with r, = 27"C and A, = 5 W/m °C on the inside, and = 8°C and A2 = 12 W/m °C on the outside. Assuming constant thermal conductivity with no heat generation and negligible radiation, [a) express the differential equations and the boundary conditions for steady one-dimensional heal conduction through the wall, (A) obtain a relation for the variation of temperature in the wall by solving the differential equation, and (c) evaluate the temperatures at the inner and outer surfaces of the wall. 

SOLUTION Hot- and cold-water pipes run parallel to each other in a thick concrete layer. The rate of heat transfer between the pipes is to be determined. Assumptions 1 Steady operating conditions exist. 2 Heat transfer is two-dimensional (no change in the axial direction). 3 Thermal conductivity of the concrete is constant. 

Hot- and cold-water pipes 8 m long run parallel to each otlier in a thick concrete layer. The diameters of both pipes are 5 cm, and the distance between the centerlines of the pipes is 40 cm. The surface temperatures of the hot and cold pipes are 60 C and I5°C, respectively, Taking the thermal conductivity of the concrete to be k — 0.75 W/m °C, determine the rate of he,at transfer between the pipes. Amwer 3C6 V/... 

The very numerous and widely utilized group of insulating materials comprises natural and artificially prepared highly porous subtances. These substances can be employed as unformed granular insulations, as refractory concretes or as shaped Ware. For the highest temperatures and special purposes, use can be made of hollow spheres of fused AI2O3 which have a thermal conductivity about 3 times lower than that of dense AI2O3. 

Keywords wood-chip concrete, recycling of used timber, special cement paste, injection method of cement paste, wood-chip packing ratio, strength, specific gravity, thermal conductivity... 

Fig. 8 Packing ratio of wood-chip vs. thermal conductivity of wood-chip concrete...

The thermal conductivity of wood-chip concrete was measured with the guarded hot plate method at JTCCM. The relationship between the packing ratio of wood-chip and the thermal conductivity is shown in Fig. 8. After the figure, it is known that the thermal conductivity decreases linearly when the packing ratio is increased, and the moisture content has a great influence on thermal conductivity of wood-chip concrete. 

The thermal conductivity of wood-chip concrete is 0.2 0.36 W/mK in the air dried state. 

The thermal conductivity k is a transport property whose value for a variety of gases, liquids, and solids is tabulated in Sec. 2. Section 2 also provides methods Tor predicting and correlating vapor and liquid thermal conductivities. The thermi conductivity is a function of temperature, but the use of constant or averaged values is frequently sufficient. Room temperature values for air, water, concrete, and copper are 0.026, 0.61, 1.4, and 400 W/(m K). Methods for estimating contact resistances and the thermal conductivities of composites and insulation are summarized by Gebhart, Heat Conduction and Mass Diffiision, McGraw-Hill, 1993, p. 399. 

Sulfur. To assess whether sulfur can be used as a partial oxidant for propane, exploratory experiments have been made at Worcester Polytechnic Institute in which propane and propane-helium mixtures were saturated partially with sulfur at atmospheric pressure and then passed over commercially available chromia-alumina catalysts. No methyl-acetylene was detected by thermal conductivity gas chromatography using a 20-foot squalane column, but significant amounts of methyl and ethyl mercaptans were found. Figure 7 illustrates the nature of the gaseous products obtained. Continued experimentation will establish the system more concretely. No coking data or results from sustained operation are available as yet. The results shown in Figure 7, while preliminary, show that the reaction... 

Full depth sand-asphalt-sulfur pavement structures should reduce the depth of frost penetration into the subgrade in low temperature regions and thus reduce frost damage to pavements. The coefficient of thermal conductivity of these mixes is approximately one third the value for asphalt concrete. 

Stage 2 The temperatures gradually descend from their peak values because the thermal conductivity and first-term water cooling make the release of heat within the concrete after the temperatures reach their peak values. 

Other properties. The coefficient of thermal expansion of concrete is about 10 °C . That of ferritic steels is not very different (about 1.2 X 10 as in usual carbon-steel reinforcement) that of austenitic steels is higher (about 1.8 X 10 ) austenitic-ferritic steels are in an intermediate position. The higher thermal expansion of austenitic and duplex stainless steels is not believed to cause any problems in concrete, and no cases of damage due to differential expansion have been reported [6]. Furthermore, the thermal conductivity of austenitic stainless steel is much lower than that of carbon steel and thus the increase in temperature throughout the steel is delayed. 

Expanded polystyrene foam (EPS) has different applications, because of its physical form (beads) and properties (higher permeability to water and less effective adhesion to facing materials than polyurethane). The expansion gases, pentane and steam, escape fairly rapidly from the foam, so the thermal conductivity of the foam filled with air is about twice that of the best polyurethane foam—a 50 mm thick slab of foam has a U-value of 0.5-0.6 Wm K . EPS mouldings can be used as shutters (formwork) for pouring concrete in a composite wall. The two EPS layers are connected at intervals to fix the thickness of the concrete. Extruded polystyrene foam (XPS) is used in plank form for insulation imder the concrete floor of houses, and in roofing panels. 

---