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TECHNICAL PAPER

macro defect free (MDF) materials, densified systems with small normal strength concrete is higher than lightweight aggregate
particles (DSP), ultra- high-strength concrete (UHSC), and ultra- concrete or lightweight aerated concrete. Thus, it is obvious that
high-performance concrete (UHPC). All these developments, thermal conductivity is related to bulk density for such materials.
realized during the 1980s, can be attributed to chemical and Some of the very early works relied on bulk density and moisture
[4]
mineral admixtures . content of the construction materials for the prediction of their
thermal conductivity through approximate empirical relationship
The current state of the art indicates that compressive strengths in the form tables, chart or equations [31-35] . However, such
as high as 800 MPa are achievable for RPC . Ultra-high-strength density-based estimates often exhibit significant scatter around
[4]
or ultra-high-performance concretes of 200 MPa class, similar the mean value, as the conductivity of solid constituents may
to RPC incorporates fibre reinforcement to enhance ductility vary from one another significantly, even when their specific
and crack-arrestation capability and successfully used in gravities are similar. Thus, estimation of thermal conductivity of
pretensioned structural elements. Other high strength cement- concrete from density is not very reliable. A more fundamental
based composites such as engineered cement composites consideration of heat transfer through porous materials not only
(ECC) have also found industrial usage for both structural and provides better understanding of the phenomena but has also
[3]
non-structural components . The parameter K in Equation 10, resulted in development of more reliable prediction models for
accounts for the relationship between compressive and tensile the thermal conductivity of bricks and cement-based materials
strengths and depends on the testing method, elastic modulus, like concrete.
and surface energy of the pore-free solid, Recent studies on
elastic modulus of solid phases through nano-indentation Heat transfer through concrete and similar porous materials is
techniques and molecular dynamics simulation have provided complex and involves several mechanisms [36-38] . Most important
valuable insights into the elastic modulus of un-hydrated cement of these are: 1) heat conduction in solid materials; 2) heat
clinker (UHC), CH, and C-S-H gel; present as both inner products conduction through pore fluid (air or water); 3) convection heat
(IP) and outer products (OP). Reported elastic modulus values transfer through pore fluid; 4) radiation from solid surfaces
are approximately 137 GPa for cement clinker compounds, of pores, and also 5) evaporation and condensation in the
about 35 GPa for CH, and in the ranges of 26-32 GPa and pores when they are partially saturated with water. Thermal
13-26 GPa for C-S-H IP and OP, respectively [3,28-30] Elastic conductivity is the amount of heat flow under unit temperature
modulus for aggregates range around 100 GPa . The elastic gradient for unit area. The heat flow takes place by some or
[4]
modulus of pore free solid consisting of UHC plays a major role all the above modes of heat transfer. Thus, the measured
in strength. Complete hydration is possible at infinite age and at conductivity is effective or equivalent-thermal conductivity. Effect
w/c greater than 0.36; at lower w/c, space available is insufficient of radiation and convection in pores is negligible in comparison
to accommodate the hydration products as they possess a with other modes of heat transfer at atmospheric pressure and
[37]
higher specific volume than clinker. Thus, at w/c below 0.36, the temperature for pore diameter smaller than 3 mm . Hence, at
capillary porosity is small thereby enhances the strength, further, normal ambient conditions the conduction heat transfer through
the contribution of un-hydrated clinker towards elastic modulus solid skeleton and that through fluid in the pores are the most
also leads to higher strength. A comprehensive modelling of dominant mechanisms influencing the effective conductivity,
both elastic modulus and surface energy of the composite pore particularly when the pores are either completely dry or fully
free solid consisting of UHC, CH, IP, OP, and aggregate is yet to saturated. In pores, partially saturated with water, evaporation
be accomplished and is a current research challenge . and condensation of moisture within the pores also contribute
[3]
[39]
to the effective conductivity significantly . Conduction through
solid skeleton contributes more compared to that through pores
5. THERMAL CONDUCTIVITY OF CONCRETE
as conductivity of air and water is much less. Hence, porosity
AND CEMENT-BASED MATERIALS
plays a major role in thermal conductivity of porous material.
5.1 Equivalent conductivity of porous material General theoretical models for thermal conductivity of porous
and model materials have thus recognized porosity as the major influencing
Thermal conductivity is a critical property for heat transfer factor. From considerations of fundamental mechanisms of
calculations in mass concretes and in building envelopes heat transfer in porous construction material, porosity, thermal
wherein concrete is widely used either as masonry blocks or as conductivity of the solid and nature of the fluid in the pores
structural component. (water or air) are recognized to be the main governing factors,
influencing the effective thermal conductivity. The variation of
Lightweight aggregate concrete is made of porous aggregate the thermal conductivity with porosity under oven dried and fully
and hence, less dense. On the other hand, normal structural saturated conditions for identical material samples as measured
concrete is dense and exhibits lower porosity. Conductivity of at IIT Delhi, are shown in Figures 10 and 11 respectively. As

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