Paraffin impregnated lightweight aggregates
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.
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