A computational fluid dynamics (CFD) model is developed to study thermal performance of hollow autoclaved aerated concrete (AAC) blocks in wall constructions of buildings under hot summer conditions. The goal is to determine size and distribution of cavities (within building blocks) that reduce heat flow through the walls and thereby lead to energy savings in air conditioning. The model couples conjugate, laminar natural convective flow of a viscous fluid (air) in the cavities with long-wave radiation between the cavity sides. Realistic boundary conditions were employed at the outdoor and indoor surfaces of the block. A state-of-the-art building energy simulation programme was used to determine the outdoor thermal environment that included solar radiation, equivalent temperature of the surroundings, and convective heat transfer coefficient. The CFD problem is put into dimensionless formulation and solved numerically by means of the control-volume approach. The study yielded comprehensive, detailed quantitative estimates of temperature, stream function and heat flux throughout the AAC block domain. The results show a complex dependence of heat flux through the blocks on cavity and block sizes. In general, introducing large cavities in AAC blocks, being a construction material of low thermal conductivity, leads to greater heat transfer than the corresponding solid blocks. Several small cavities in a block may lead to small reductions in heat flux, but the best configuration found is a large cavity with a fine divider mesh in which case heat flux reductions of 50% are achievable.
Graphene is a Nano-material whose tensile strength is about two-hundred times higher as compared to steel. The effects of graphene-concrete composite on the properties of concrete have been investigated in this paper. A mix with a water-to-cement ratio of 0.4 was designed. The graphene powder was introduced as 0.5 percent, one percent, one-and-a-half percent and two percent (designated as GC2). The control mix was prepared without mixing the graphene powder. The same size and type of aggregates were used for all these concrete mixes. For each mix, resistance to acid (H2SO4) and sulphate (Na2SO4) attacks, compressive strength, water absorption, compressive strength development over time, compressive stress-strain relationship, split tensile strength, and modulus of rupture were determined. It was found that the addition of graphene increased the acid and sulphate resistance, and absorption. The acid resistance of concrete improved for all the employed mixes. The highest improvement was achieved for the GC2 mix. On the other hand, an overall decrease in water absorption of the mixes was noticed. Moreover, the addition of graphene increased the bulk and apparent density of the mix and decreased the volume of permeable voids.
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