Impact of Admixture on Heat Gain through Mixed Concrete Roof into a Building

Dr. B. M. Suman, Principal Technical officer, CSIR- Central Building Research Institute, Roorkee

Experimental investigation has been carried out to study thermal performance of various admixtures and their mixed concretes with different ratio and density. There are nine admixtures mixed with concrete for studying their thermal performance. Some of them are Fly ash, Perlite Powder, EPS beads etc. The study of sixteen mixed concretes were carried out and compared with thermal performance of cement concrete. Heat gain were computed for 11.5 cm thick roofing sheets made up of all the mix concretes and cement concrete. According to this study thermal performance of EPS concrete with 30% EPS beads found to be better than all other concretes. The AAC performance was next to EPS concrete followed by the performance of perlite mix concrete with 50% expanded perlite. The other good performer is the vermiculate concrete reducing 41.5% heat gain when compared to cement concrete. It is also found from the study that the mixed concrete with lower overall thermal transmittance allows minimum heat flow across it.

Introduction

Number of thermal insulation materials with fine particles is available to mix with concrete to improve thermal behaviour of mixed concrete. The admixtures are mixed in different ratio with concrete to enhance their thermal characteristics for energy efficiency points of view. Heat gain principle through building fabrics depends upon temperature difference between outside and inside of a building. High temperature difference allows higher heat flow into the building. Heat flow always takes place from high temperature to low temperature. In this process temperature difference is not the only criteria of heat flow but solar radiation, absorptivity and emissivity, external heat transfer coefficient are also influencing the heat ingress into the building. By combining these parameters and outdoor air temperature a new parameter known as sol air temperature which is responsible for heat flow into the building. The conduction heat transfer through structure is of great importance in civil engineering problems. Such problems include, energy efficient building design, thermal load of structures due to diurnal variations of temperature, planning and design of building for thermal comfort, design of radiation shield and other exposed structures for solar thermal loading etc. The knowledge of thermal conductivity and other thermal transport properties of construction material involved in the process of heat transfer are essential in predicting the temperature profile and heat flow through the material. Most of the commercial and residential buildings are now air condition building. Therefore consumption of power and electricity is increasing exponentially. One of the ways of achieving energy conservation in building construction is by introduction of thermally insulated admixture with concrete for building application. Number of concrete manufactures supply such mixes concrete but no one provide relevant data on thermal behaviour of their products. Therefore an experimental investigation was undertaken to understand the influence of various admixtures on density and thermal performance of mix concretes. Number of studies [1, 2] has been made to know the effect of ceiling insulation on the electricity consumption and creating conducive indoor thermal environment. Some of the studies carried out [3, 4] in South Africa revealed that insulated houses are not only warmer in winter but also cooler in summer months. Density of building material plays a key role in determining thermal performance of building section because a masonry building material with low density has decreased thermal conductivity. The derived thermal properties like overall thermal transmittance and subsequently heat gain through the material depend upon its thermal conductivity. Heat gain [5] through roofing sheet will be lower, if thermal conductivity of the material used in roofing sheet is also lower. The mix concrete [6] of low thermal conductivity is useful for building insulation. Bouguerra [7] et. al reported that thermal conductivity of light weight concrete changes considerably with its porosity and density. Roof is the main contributor of heat gain into the building. Sustainable building design [8-10] with mixed cement concrete prepared by mixing admixture with thermal insulation property can be developed. Foam concrete, Vermiculite concrete, Perlite concrete and EPS concrete are some example of such materials. Sol air temperature (Tsol) is the main factor, responsible to heat flow from outside to inside of a building which includes the effect of outdoor air temperature and solar radiation of the place, surface properties like reflectivity, emissivity, and absorptivity of building section. In the present study, sol air temperature will remain same for all cases of study for determining heat gain into the building.

NBM&CW October 2015

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