Views:464 Author:MARIAN PRUTEANU* and MARICICA VASILACHE Publish Time: 2017-11-16 Origin:Site
In the current context, where the climate changes effects are developing continously, with a permanent intensification, the energy efficiency of buildings became a starting point in current building design. The energy efficiency of a building is directly influenced by the thermal performances of the envelope. Therefore, designing of enveloped elements whose global thermal resistance exceed the required minimum values, is a mandatory measure. Increasing thermal performances of enclosure mansonry walls; it can be made by thermal insulating or by using masonry blocks with low thermal conductivity. In this category of building materials it can be found also autoclaved aerated concrete blocks.
The paper presents some experimental determination of thermal conductivity for AAC blocks manufactured in our country. The measurements ware made in the Laboratory of Building Physics within Faculty of Civil Engineering and Building Services from Iasi. Furthermore, the equivalent thermal conductivity of an AAC masonry was determined by using an FEM software and mathematical calculus.
Key words: energy performance; AAC masonry block; thermal conductivity, climate chamber.
In cold season, the indoor hygrothermal comfort conditions are accomplished with high fossil fuels consumption and with a negative environmental impact, through GHG emissions.
Decreasing the residential energy consumption and rasing energy efficient buildings are strategic objectives of the European politics. The EU legislation presents a set of pecularities regarding this class of buildings, according to the specific climate conditions and to the building type, establishing an annual heating energy consuption between zero and 50... 75 kW./m2/year (C107 - 1997... 2011).
Designing this type of buildings assumes using of building materials with low thermal conductivity (lower than 0.1 W/m.K) or very thick thermal insulations.
Using external walls with distribute thermal resistance can be an alternative for increasing the envelope level of thermal protection and decreasing the thickness of required thermal insulation.
2. AAC Blocks and Walls
The autoclaved aerated concrete (AAC) masonry is used for external walls due to the material advantages, referred to its low density, to the voluminous character of the blocks, thereby higher building speed, and to its favourable water vapour permeability.
Thermally, load-bearing or non load-bearing AAC walls are defined by distributed thermal insulation. The types of materials used and theirs characteristics are presented in Table 1.
Used for load-bearing or non load-bearing walls, these blocks are viable and durable alternatives.
3. Samples Analysis and Numerical Simulations for a Masonry Panel
With the specific intention of place them in thermal insulation category (referred to their density below 500kg/m3 and thermal conductivity below 0.1 W/m.K0 new AAC blocks was manufactured, which are lighter than the ones presented in Table 1 and are designed to be bulk-production manufactured. For evaluating the thermal properties of the blocks and walls a set of experiemental measure ments war taken regarding:
a) dry density of the block and walls;
b) thermal conductivity for AAC blocks and equivalent thermal conductivity of the AAC masonry;
c) the behaviour on mass transfer.
The research involved the used of a Twin Climate Chamber manufactured by Feutron Klimasimulation GmbH, Germaniy, comm. - no.9004 2861 and a heat flux-meter for the determination of the AAC blocks thermal conductivity. Furthermore, the numerical simulation used for the determination of equivalent thermal conductivity of the AAC masonry was taken in the ANSYS Workbench 12.0 software.
3.1. The AAC Blocks Thermal Conductivity Determination
The Twin Climate Chamber (Fig.1) create two different environments (warm and cold), defined by relative humidity (RH) and the temperature. In the warm chamber the RH and the temperature varies between 10% ... 95% respectively 5%... 100 centigrade and in the cold chamber the RH and the temperature varies between 15%...95%, respectively, 45%... 100 centigrade.
The measurements methor uses SR EN ISO 8990:2002 and SR EN 1946-3:2004.
For measuring the heat flow intensity and the surface temperature was used a TRSYS01 Huksefluex heatflux meter and a Testo 606 electric humidity meter. The AAC blocks, with de dimension 600x150x250 mm, category I, GBN 25 (SR EN 771-4/2004; SR EN 771-4/2004/A1-2005) were placed in the space between the two climate chambers using a guard ring. The position of the heat flux plates and of the thermocouples is shown in Fig.2.
In order to drying them, the AAC blocks were placed in the climate chamber at a temperature of 80 centigrade and relative humidity of 10% for 72 h. The resulting RH of the blocks was 5.5% as determined using the lectrical humidity meter Testo 606.
After that, the dried blocks were placed in the space between the chambers and the heat flux sensors and the thermocoulples war installed. As is known, the thermal conductivity of the blocks can be determined, knowing the intensity of the heat flux crossing the specimen, the surface temperatures, as well as the thickness of the specimen.
Test duration was of 20h for the following parameters:
a) warm chamber air temperature was 40 centigrade and relative humidity 10%;
b) cool chamber air temperature was 20 centigrade and relative humidity 10%; and of 20h for following parameters:
c) warm chamber air temperature was 40 centigrade and relative humidity 60%;
d) cold chamber air temperature was 30 centigrade and 60 centigrade relative humidity.
Heat flow direction was perpendicular to the surface of the blocks from the hot face to the cold one.
3.2. The Determination of Equivalent Thermal Conductivity of the AAC Masonry
Numerical simulation was performed using the program ANSYS Workbench 12.0. Characteristics of the AAC blocks are presented above.
To build masonry panels it was used a mortar for joints type M5 having thermal conductivity of 0.87 W/m.K. There was two variants studied, refering to the thickness of the horizontal and vertical joints, respectively, 3 and 5 mm. The configuration of the AAC masonry panel is shown in Fig. 7.
It can be noticed a negative effect of the mortar joint on the thermal conductivity of the masonry panel when its thickness and its length increase. Therefore, the conductivity of the panel is increased from 0.127 to 0.138 W/m.K and from 0.152 to 0.163 W/m.K, with the increase of the mortar joint thickness.
4. The Determination of the AAC Masonry Wall Thermal Inertia
The thermal inertia coefficient of a building element is determined by the following relation:
The obtained valued classify the AAC masonry walls in buildings element with average thermal massivity, as D is ranged between 4 and 7.
The experiemntal measurements conducted of AAC blocks and masonry walls have highlighted a number of issues on the thermal performance of the material. Dry density of the blocks (390 kg/m3) and its thermal conductivity are characteristics superior than the one of the common AAC blocks and recommend the tested blocks for exterior walls with good hygrothermal behavior. Dry density of the masonry wall determined is 440 kg/m3 for mortar joints of 3 mm thickness and 488 kg/m3 for mortar joints of 5 mm thickness, with 34.8%... 27.75 lower than the values of common AAC blocks presented in Table 1 (C 107/0 - 2002).
Thermal conductivity of the AAC masonry walls is direct influenced by the mortar joints thickness, the volume occupied by the mortar and the thermal conductivity of the mortar. The obtained values by numerical simulation indicate that the tested AAC masonry walls have an increase thermal resistance, thereby the thickness of additional thermal insulations, necessary to obtain the minimum global resistance, is lower than in common external walls.