ACI Materials Journal November/December 2015
Newswise — Cement Concrete is a composite materials comprising cement, aggregates (coarse and fine) and water along with admixtures (chemical and/or powder). As per Portland Cement Association (PCA) concrete is a mixture of paste and aggregate. The paste is a combination of cement and water that binds the coarse and fine aggregate particles to form a solid mass called as concrete. Concrete has wide ranging applications such as building construction, transportation infrastructure and power plants and water resources projects. With growing concerns of project cost and time overruns along with sustainability and environment pollution, has led to research and development of high performance concrete (HPC). As per P. Kumar Mehta (2006), high performance of concrete can be attributed to high constructability, high resource productivity, high strength, cost effectiveness and eco-friendly properties. Self-compacting concrete (SCC) is one of the popular type of high performance concretes. Speedy construction, better mechanical and durability properties make SCC a classic example of high performance concrete.
One of the important factors in ensuring a maintenance free concrete infrastructure is the service life performance of concrete especially the durability performance. Amongst the various durability factors, the environmental parameters (thermal as well as moisture) play a crucial role in deciding the service life performance of cement concrete. The resistance offered by concrete to environmental factors depends on its thermal properties and porosity. The various thermal properties that play an important role are coefficient of thermal expansion (CTE), specific heat of concrete and thermal conductivity (k) of concrete. The paper presents the results of the experimental investigation undertaken to understand the effect of powder additions (fly ash) and its varying dosage, as well as the type of fine aggregate, natural sand (NS) as well as crushed sand (CS), on the thermal conductivity values of SCC. The aggregates (coarse as well as fine) used in the concrete mix were of granite origin. The normal range of thermal conductivity values for concrete mix prepared with granite aggregates is 2.6 to 2.7.
Depending on the application of the concrete specimen a particular k value is desired. Concrete specimen subjected to high ambient temperature conditions requires high k values for quick dissipation of heat to prevent build-up of temperature stresses. Concrete specimens used for insulation applications require low k values. Most of the studies undertaken by various research groups measure the k value of concrete for elevated temperatures. The objective of these studies was the assessment of fire resistance properties of concrete. The study undertaken in the paper measures the k values of SCC mixes at ambient service temperature of 86oF to 176oF (30oC to 80oC) using steady state method. This study is significant in particular for concrete applications in road infrastructure where the above mentioned temperature ranges are prevalent. The k value of concrete in road application influences the thermal stress development and resistance of concrete specimen to the stress.
From the study undertaken it was observed that the k values of SCC were influenced by the density of the concrete mix, type of fine aggregate added in the concrete mix and the varying dosage of the fly ash. Microstructure studies (using intermediate scanning electron microscope, SEM) were undertaken to study the relative positioning of various concrete ingredients with respect to each other. The microstructure of SCC mixes revealed two distinct phases viz. aggregate phase and matrix phase. Both these phases influence the density and mechanical performance of the concrete specimen. The composition and layout of these two phases differed in the SCC mix with NS as well as CS. From the studies undertaken following points were inferred;
i. Denser the concrete, higher the thermal conductivity of concrete.ii. The k values of SCC mix with CS were higher (6.25% more at a temperature range of 86oF to 104oF and 2.38% more at 158oF to 176oF) than the k values of SCC mix with NS.iii. Fly ash addition lowered the density of the matrix phase in SCC mix. This resulted in the drop of k values of SCC mixes with NS as well as CS, when compared to the respective control mixes with NS and CS. However, SCC mixes with CS and fly ash had relatively higher k values as compared to NS mixes.
The research can be found in a paper titled “Thermal Conductivity Studies for Self-Consolidating Concrete with Sand and Fly Ash Variation”, published by ACI Materials Journal.
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