Thermal conductivity is the ability of the material to absorb heat. It can also be defined as the ratio between the heat flux and the temperature gradient. Unit of thermal conductivity is joules per second per square meter.
Conductivity of concrete depends on its composition. In case of saturated concrete, conductivity ranges from 1.4 to 3.6 joule/meter square per second. Generally, conductivity is not affected by the concrete density, but as the conductivity of air is very low therefore, conductivity of lightweight concrete varies with the variation in concrete density.
Conductivity also depends on the natural composition of aggregates like mineral properties of aggregates. Orientation of crystals also affects the conductivity. Conductivity increases with the increase in crystal properties of aggregates.
There is another important aspect that plays an important role is the degree of saturation of concrete. This is so because of the high conductivity of water than air. If the moisture content of lightweight concrete increases 10% then as a result conductivity increases about one-half.
In concrete, cement has the highest thermal conductivity. Conductivity of water is lower than half of the conductivity of cement paste. So, if water content will be lower in the fresh mix than there will be higher conductivity of hardened concrete.
Effect of temperature on thermal conductivity is very little at the room temperature. But, as the temperature rises from normal room temperature, the conductivity of phenomenon becomes more complex. Almost up to the 60 degree centigrade rise in conductivity is very slow with the loss of water. But, as the temperature reaches 120C conductivity decreases very sharply.