Use of paraffin impregnated lightweight aggregates to
improve thermal properties of concrete panels
Concrete structures can be subjected to high temperatures
occasionally. Depending on their purpose, some structures may be required to
prevent heat from escaping to the surrounding areas and some to prevent heat
from entering the interior. There are several factors affecting thermal properties
of concrete; for example, water to cement ratio, moisture content, aggregate
type, or porosity. One of the most effective concepts to improve thermal
properties is to incorporate large amount of voids (or pores) into concrete.
High porosity can be achieved by either mix porous lightweight aggregates into
concrete mixture or use high porosity cement paste (aerated concrete). The
concept of using high porosity to slow down heat transmission comes from the
fact that heat travels in the air at a slower rate than in solid materials.
However, there is also an alternate method to improve
thermal insulation of concrete by increasing its thermal storage. Instead of
slowing down the rate of heat transfer, the heat is allowed to travel into
concrete. Rather than letting the heat flow through quickly, the heat is
captured or stored inside the concrete. This can be achieved through the use of
phase change material.
The method of using phase change materials (PCMs) to improve
thermal storage properties was first introduced sometime around the end of
World War II. The PCMs are the materials that are capable of changing its phase
from solid to liquid (or liquid back to solid) at a certain level of
temperature. During the phase changing process, a certain amount of energy
(heat) is taken or released into the surrounding environment. In the solid to
liquid phase changing stage (melting process), the temperature of PCMs will
increase slowly with the rising ambient temperature. When the temperature
reaches the PCMs’ melting point, the PCMs will absorb large amounts of heat at
near constant temperature. During this period, the heat will be absorbed
without a significant rise in temperature until all PCM is transformed to the
liquid phase. On the other hand, when the ambient temperature decreases, the
solidification begins, the PCM will release its stored latent heat. Several
researches show that the PCMs’ ability to absorb the heat during the phase
change process is beneficial in term of storing the heat inside the structures
and slowing down the rate of heat transmission (shifting the temperature peak
period).
In general, there are three groups of PCMs: organic,
inorganic and eutectic. The organic group is often referred to materials like
paraffin and some fatty acids. Organic materials generally exhibit the
following properties: congruent melting (melting and freezing repeatedly
without phase segregation and consequent degradation of their latent heat of
fusion), and self nucleation (they crystallize with little or no supercooling
and usually non-corrosiveness). Inorganic materials are further classified as
salt hydrate and metallics. These PCMs do not supercool appreciably and their
heats of fusion do not degrade with cycling, but they do exhibit high volume
change. Eutectic group refers to a compound between at least two or more
components, each of which melts and freezes congruently forming a mixture of
the component crystals during crystallization. Eutectic materials always melt
and freeze without segregation since they freeze to an intimate mixture of
crystals, leaving little opportunity for the components to separate. On melting
both components liquefy simultaneously, again with separation unlikely.
For construction materials such as concrete, the organic
group is the most widely accepted because of its ability to maintain its
properties after being subjected to numbers of temperature cycles (without
segregation or efficiency degradation). The organic group can be divided into
paraffin and non-paraffin groups. The paraffin group seems to have more
advantages than the non-paraffin group due to its cost effectiveness, wide
melting point range, high inertness, high stability (less volume change) and
durability.
There are a few methods to incorporate PCM into concrete.
The first method is to directly mix it with concrete mixture as one of the
constituent materials. However, this method seems to have a problem related to
the leakage of PCM on the surface after being subjected to numbers of high
temperature cycles. Another technique is the encapsulating technique. It was
designed primarily to solve the leakage problem. Using this technique, the
paraffin is encapsulated inside small sphere capsules and then mixed with
concrete. This technique is in fact quite effective in terms of protecting the
leakage. However, there are some drawbacks in term of cost effectiveness and
the manufacturing process.
The impregnation technique which uses heat and pressure at
different levels and combinations are proposed instead of using the
encapsulation technique to insert PCM into porous lightweight aggregates. The
process is expected to be more cost effective and simpler to manufacture than
the encapsulating technique. The aggregates with the highest impregnation
levels are then used in the concrete mixture by partially replacing parts of
normal aggregates from 25 to 100 % by volume. The concrete samples are tested for
heat insulation and sound properties (transmission loss) to investigate the
effect of PCM on concrete.
Civil Scholar is a Blog by Aravind Samala on Construction, Environment, Technology, Trends and Innovation.