BREATHABLE CONCRETE FOR LOW ENERGY BUILDINGS Jimmin Wong, Fredrik P. Glasser and Mohammed S. Imbabi School of Engineering & Physical Sciences, University of Aberdeen King’s College, Aberdeen AB22 8YH, United Kingdom ABSTRACT The present paper reports the early outcomes of research to develop a new type of air permeable, breathable concrete. Pre-cast concrete blocks can be made possessing the necessary permeability and strength for handling and construction of energy efficient breathing buildings. The authors demonstrate how the tortuous permeation channels in this new type of material can be designed and optimised to provide adequate structural strength, act as a ventilation source, and significantly reduce thermal conductivity. Pre-cast breathable concrete blocks thus provide an elegant and sustainable building material. INTRODUCTION Conventional wisdom suggests that, in order to reduce energy usage for heating a building must be air tight, and more insulation must be used. This has led to inadequately ventilated buildings and high levels of volatile organic compounds and other pollutants indoors, thus contributing to sick building syndrome. Dynamic insulation, which has been investigated by the authors elsewhere [1, 2], provides an alternative strategy for reducing energy usage by reducing U-Value as a function of air flow rate. A breathing building envelop incorporating dynamic, as opposed to conventional, insulation layers acts as a heat exchanger, ventilation source and air filter. As outdoor air is drawn into the building, either actively or passively, it is pre-heated (or cooled) and scrubbed of airborne pollutants. Dynamic insulation thus allows the ventilation rate to be increased without cost penalty, improves indoor air quality and cleans up the external environment. Breathable concrete for monolithic breathing wall construction, possessing the required combination of strength and permeability, is currently being developed at Aberdeen University. Using graded aggregate and ordinary Portland cement, a target compressive strength of 10 MPa for concrete elements having 0.6 m2/Pa.hr permeability has been achieved. The thermal performance of this newly developed material is clearly important. The present paper reports our theoretical predictions of the static and dynamic thermal performance of breathable concrete, and compares these against conventional concrete. Concrete is a multi-phase material which consists of mortar, aggregate (coarse and fine) and entrapped air. The thermal conductivity of concrete, as shown in various studies, is influenced by the following factors [3 - 4]: a) Thermal conductivity of the aggregate: aggregate with low thermal conductivity produces low thermal conductivity concrete and vice versa. b) Density of the concrete: the density of the concrete is strongly related to its porosity, since air has a very low thermal conductivity. c) Moisture content: since water has a higher thermal conductivity than air, if the voids are full of water the concrete will have higher thermal conductivity than in a dry state. In our study it is assumed that both the conventional and breathable concrete are dry. For a 0.2 m thick conventional concrete block of density 2200kg/m3 the U-value is 8W/m2 K [5]. PHYSICAL PROPERTIES OF BREATHING CONCRETE Breathing concrete is borrows from a well-known product, no-fines concrete [6]. The main difference between them is that in the breathable product, the interconnectivity of voids to form permeation channels within the structure needs to be precisely controlled and optimised by manipulating the volume and rheology of cement paste and size distribution of aggregate. In order to allow air flow through the breathing structure to play a significant role in heat transfer, the internal structure of breathing concrete must be highly porous; since the strength of concrete is strongly influenced by its porosity, breathing concrete has lower compressive strength than conventional concrete. However through careful manipulation of mix design parameters, the strength of this new building material is sufficient for handling on site and use as building block in low rise or residential building. The properties of breathing concrete developed by Aberdeen University are summarized in Table 1. Table 1 Properties of breathing concrete for different degree of filling. Degree of filling Strength (MPa) Permeability (m2/Pa.hr) Concrete porosity 0.5 10.8 0.60 0.32 0.6 18.2 0.32 0.28 0.7 25.0 0.18 0.22 (water / cement ratio = 0.25) The degree of filling is defined as the ratio between volumes of cement paste to the volume of pores constituted by the packing of (uncemented) aggregate particles in a confined mould volume, assuming that the pores are interconnected. The static thermal conductivity of breathing concrete is determined by the type of aggregate, and cement being used. In order to compare the thermal performances, three different types of aggregates are being analysed for their thermal performances in breathing concrete. The thermal properties of the main constituents [7] for breathing concrete are listed in table 2. Table 2 Thermal conductivity of main constituents in breathing concrete. Type of Constituents Thermal conductivity (W/m.K) Cement paste 0.7 Air 0.025 Quartz 5.18 Granite 2.91 Limestone 2.56 STATIC HEAT TRANSFER OF BREATHABLE CONCRETE Thermal conductivity of breathing concrete, unlike other single phase materials, consists of four components: cement paste, with entrapped air or water in pores, aggregate and air in open voids the channels. Breathing concrete is highly voided and the effect of micro pores in cement mortar on thermal conductivity becomes negligible. Since breathing concrete is a heterogeneous material, its thermal conductivity will lie between that of its components depending upon the volume fraction of each component and its distribution [8]. The simplest model of thermal conductivity for a mixture of n components arranged in parallel to each other is obtained as λn = φ1λ1 + φ 2 λ2 + ⋅ ⋅ ⋅ ⋅ ⋅ ⋅ φ n λ n (1) λ is thermal conductivity (W/mK) of the material, φ is volume fraction of material and the sum of φ is unity. In this arrangement the overall heat conduction will be predominated by the thermal conductivity of the better conductor. In the case where the components are in series to each other, the thermal conductivity becomes λn = λ1λ2 ⋅ ⋅ ⋅ ⋅ ⋅ ⋅λn φ1λ2 λ3 ⋅ ⋅ ⋅ λn + φ 2 λ1λ3 ⋅ ⋅ ⋅ λn + φ n λ1λ2 ⋅ ⋅ ⋅ λn −1 (2) In this arrangement, the thermal conductivity of the material will be dominated by the poorest conductor. Assuming that the cement paste is coated evenly on the aggregate throughout the structure, the solid matrix and adjacent void are distributed equally; the heat transfer is in the opposite direction of air flow which shows in Figure 1. Figure 1 2D-Microscopic view of breathing concrete In one dimensional heat conduction, the heat conduction path will depend on the phases encountered. If we analyse a finite section of breathing concrete which involve only two aggregate bind together by cement paste as in figure 2, the heat flow that passes through the air without any contact to the solid phase represents heat flow through pores. Since the air thermal conductivity is low, majority of heat flow will be going through the solid phases which consist of cement paste and aggregate. At the bridging between two aggregates, the aggregate is being pulled apart due to surface tension of cement paste; part of the heat flow through solid phase will be diverted through cement paste bridging. Figure 1 can be simplified into a equivalent heat flow through various resistances as figure 2. Figure 2 Heat flow on a finite section of breathing concrete and its equivalent heat resistance. Since the thermal conductivity of air is much lower than in the solid phases, the parallel heat path through air can be ignored. Assuming that breathing concrete is in dry state, heat transfer is mainly through conduction and no significant heat convection and radiation is taking place [9]. Combining expressions (1) and (2), the expression for thermal conductivity for breathing concrete is given by (3), which is similar to the heat transfer model proposed by Bonacina for lightweight concrete. λcomposite = [φ a λ a + φ m λ m ] + (φ a + φ m ) λ m λv φ v λ m + φ m λv (3) Subscripts a, v and m represent aggregate, void and cement paste respectively in (5). The composite U-values for breathing concrete of 0.2 meter thickness composed from different types of aggregate and having different degrees of cement paste filling are given in Table 3. Table 3 U-value (W/m2K) of breathing concrete produce from different aggregate Degree of filling Us(Quartz) Us(Granite) Us(Limestone) 0.5 7.0 4.7 4.3 0.6 8.3 5.5 5.0 0.7 10.3 6.5 5.9 DYNAMIC HEAT TRANSFER OF BREATHABLE CONCRETE Breathing concrete developed by Aberdeen University was tested for air permeation rate by putting the cylindrical sample into an air loop which is shown schematically in figure 3. In this apparatus, air is supplied to the test section using an electric axial fan operating in a closed loop. The pressure drops between inlet and outlet of test samples are measured using a differential pressure gauge. The flow rate was measured at the outlet of the flow magnifier in order accurately to test the sensitivity of permeability when pressure was varied. The permeability κ (m2/Pa.hr) of breathing concrete of different degrees of filling (DF) versus pressure drop is given by figure 4. Figure 3 Schematic sketch of permeability test rig As can be seen from figure 4, as the pressure drop increases, the permeability decreases slightly. This is because shear resistance of concrete particles against fluid flow increases as flow velocity increases. 0.7 Pressure Drop VS Perm eability for breathing concrete Permeability, κ (m^2/Pa.hr) 0.6 0.5 DF-0.5 DF-0.6 0.4 DF-0.7 0.3 0.2 0.1 0 0.00 10.00 20.00 Pressure drop (Pa) Figure 4 Experimental pressure drop versus permeability for breathing concrete It has been shown by Taylor et. al. [1] that the dynamic U-value for breathing envelope is given by Ud = va ρ a ca exp(v a ρ a c a R) − 1 (4) va is the air flow velocity (m/s) entering breathing concrete, ρ a is density of air, ca is specific heat of air and R is the resistivity of breathing wall. Derivation from Darcy equation showed that velocity entering breathing concrete at certain pressure drop is given by (5); where κ , is the permeability obtained from experimental data, ∆p is the tested pressure drop and L is the thickness of sample. v= κ∆p (5) 3600 L Combining (4) and (5), the dynamic U-value of breathing concrete is given by (6). Ud = κ ⋅ ∆p ⋅ ρ a c a κ 3600 ⋅ L[exp( ∆pρ a c a ) − 1] λ (6) The dynamic U-value of breathing concrete at different degrees of cement paste filling and produced from different aggregate type is shown in figure 5. The bold line is a reference of static U-value for conventional concrete. The differential pressure across the breathing concrete was increased slowly from 5 Pa to 20 Pa. As the differential pressure increased, the air flow velocity also increased and this effectively lowered the U-value of the breathing envelope. For breathing concrete having lower permeability, i.e. higher degree of pore filling, the material will have a higher U-value for two reasons: (a) lower air flow for the same pressure drop has impeded the heat recovery process in the breathing envelope; (b) breathing concrete with low permeability has a higher specific mass hence is a better conductor. The Effect of Flow Velocity to Dynamic U-value 10 DF-0.5 (quartz) 9 DF-0.5 (granite) Conventional concrete U-value (W/m^2 K) 8 DF-0.5 (limestone) 7 DF-0.6 (quartz) 6 DF-0.6 (granite) 5 DF-0.6(limestone) 4 DF-0.7 (quartz) 3 DF-0.7 (granite) 2 DF-07(limestone) 1 0 0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 Flow velocity(m /s) Figure 5 The effect of flow reat on dynamic U-value of of different breathable concretes. CONCLUSIONS From the above discussion and derivation, the following conclusions can be drawn: a) Breathing concretes that have higher permeability are better insulators in the dynamic condition. b) Breathing concretes with 0.5 degree filling are better dynamic insulators than the other two candidates; this implies less cement is needed per unit volume, and production costs are lower. c) The type of aggregates being used plays a secondary role in controlling the dynamic Uvalue of breathing concrete. REFERENCES [1] Taylor BJ, Cawthorne DA, Imbabi MS. Analytical Investigation of the Steady State Behaviour of Dynamic and Diffusive Building Envelopes. Building and Environment 1996;31(6):519-525. [2] Taylor B, Imbabi M. The application of dynamic insulation in buildings. Renewable Energy 1998 0;15(1-4):377-382. [3] Uysal H, Demirboga R, Sahin R, Gul R. The effects of different cement dosages, slumps, and pumice aggregate ratios on the thermal conductivity and density of concrete. 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