15th  International  Brick  and  Block    Masonry  Conference  

 Florianópolis  –  Brazil    –    2012  

 

IMPROVING THE THERMAL RESISTANCE OF LOAD-BEARING PERFORATED FIRED-CLAY BRICKWORK

Zoubeir Lafhaj1; Christophe Chapiseau2; Fayçal El Fgaier1,2; Ibrahim Lemniei2

1 Ecole Centrale de Lille, Laboratoire de Mécanique de Lille (CNRS UMR 8107), Villeneuve d’Ascq, 59651 Cedex, France 2 Briqueteries du Nord, 9ème Rue, Port Fluvial, Lille, 59003 Cedex, France

ABSTRACT

The primary objective of this study was to improve the thermal resistance of perforated brickwork. The bricks, measuring 220 x 220 x 65 mm3 and used in load-bearing masonry, are produced by Briqueteries du Nord (BdN) located in northern France.

The study focus was twofold: first, modifying the perforation configuration of the brick to reduce thermal bridges, and second, inserting insulating material into the perforations. Two types of insulating materials were analyzed in this regard: perlite and cork. Three different binders were also studied for their performance in holding these materials in the perforations: grey cement 52.5, Baticem mortar 12.5 and lime.

A detailed experimental study was conducted in relation to this article with a view to identifying the insulator/binder combination with optimal thermal properties. Baticem mortar exhibited the highest thermal performance of the binders studied. Finally, a numerical simulation was carried out to study the effect of insulator quantity and thermal conductivity as well as perforation size on the thermal resistance of bricks.

Keywords: Perforated brick, thermal resistance, perlite, binder

1. INTRODUCTION

Reducing energy consumption in the building sector is a key policy priority for the industrialized nations. As an illustration, the building sector in France consumes more than 42% of final energy and generates nearly one-quarter of that country’s greenhouse gas emissions [ADEME, French Environment and Energy Management Agency (2010)]. Creating energy savings in this sector constitutes a major economic and ecological challenge. Performance requirements to be reached over the coming years appear ambitious and are expressed in France through adoption of the new RT 2012 thermal regulation, which promotes the widespread construction of low-energy buildings (“BBC”) while reducing the average primary energy consumption in new buildings by two-thirds. Parallel to the evolution of thermal regulation, held on the “Environment Round Table”, which began in October 2007 and defined the key points of government policy on ecological and sustainable development issues for the coming five years. Thus begins a stage looking at the technical, legal and administrative aspects, which will serve to assess how best to implement all the measures decided upon.

The new thermal regulation is more “performance-oriented” than previous regulations in this area and is summarized in three different parameters: Bbiomax, or bioclimatic need, which represents the minimum energy efficiency of construction. Measured as a score, this indicator rates the quality of a building’s design and insulation separately from the heating system and with a focus on bioclimatic design (solar and light sources, inertia). It replaces the Ubat coefficient of RT 2005, which took into account only a building’s insulation performance; Cepmax, a coefficient of maximum primary energy demand, with a maximum value of 50 kWh/m² per year on average for the five uses of heating, hot water, lighting, cooling and auxiliaries (fans, pumps); and Tic, a reference indoor temperature to gauge summer comfort, which must not be exceeded during five consecutive hot days. These requirements impose safeguards and reference values for systems and products including reference values for heat loss (U values) through walls, which must remain below an average value of 0.36 W/(m2.K). Against this backdrop, the building envelope remains an effective means and delivers significant return on investment to generate energy savings in buildings, as demonstrated by Oral et al. (2004) in an envelope approach simultaneously combining thermal, acoustic and esthetic performance with energy savings and occupant health. This approach cites the various parameters associated with the external environment, the building design and the comfort conditions having impact on envelope design. Multiple research studies have outlined various options for improving the thermal performance of building materials with the objective of increasing the thermal resistance of walls. Incorporating an insulating material into hollow brick perforations is one such solution for improving the thermal performance of these building materials, as was demonstrated in a study conducted by Zukowski & Haese (2010) on the experimental and numerical characterization of a vertically perforated brick measuring 248 x 300 x 249 mm3. The brick perforations were filled with perlite in order to increase thermal resistance, and the total heat loss value U for this type of wall was less than 0.29 W/m2.K. Next, the use of additives in the raw material admixture provides another promising approach for improving the thermal performance of masonry bricks. Topçu & Isikdag (2007) studied the effects of perlite-clay proportions on the physical, mechanical and thermal properties of bricks produced from these admixtures. They demonstrated that thermal conductivity decreased from 0.4 W/m.K in standard clay bricks to 0.185 W/m.K in bricks containing 30% perlite. Similarly, Veiseh & Yousefi (2003) studied the addition of polystyrene (PS), as a pore-forming material, to clay to produce light bricks offering optimal thermal performance taking into account the mechanical strength of these products. Numerous additives have been tested in building materials. Additives are to be selected as part of an approach supporting sustainable development and waste reclamation. Such is the case in a study conducted by Sutcu & Akkurt (2009) on incorporating residue from paper recycling into brick production in order to reuse this waste as a durable material, in which the thermal conductivity of the samples decreased by nearly 50% with waste content of 30%. Demir (2008) has also explored the potential usability of multiple types of organic waste (sawdust, tobacco and grass residue) in fired-clay bricks. With its high cellulose fiber content, this waste material increased the porosity of clay bodies fired at 900°C and subsequently also increased their thermal resistance. Another type of waste was reused by Chiang et al. (2009) in masonry bricks: rice straw, an agricultural residue high (more than 90%) in silica and offering good insulating properties that was mixed with sludge from drinking water treatment to produce building materials with various porosity values depending on the firing temperature.

Further research has been conducted in our laboratory, where work has been ongoing for years on the reclamation of treated waste in masonry bricks. For example, one study was carried out on the reuse of polluted river sediments, which were treated and stabilized using the Novosol® process, as demonstrated by Lafhaj et al. (2008), and then incorporated into an admixture at a 15% substitution rate to partially take the place of quartz sand. The resulting bricks were subjected to a series of qualification tests (mechanical strength, freeze/thaw, water absorption, sanitary quality). The study results, presented in the work of Samara et al. (2009), demonstrated that the bricks containing treated sediments met criteria and environmental requirements pursuant to existing standards for masonry bricks. Building materials are consequently a high priority insofar as they have to meet the requirements of the new regulatory texts. Our laboratory has been working for several years in this regard with Briqueteries du Nord, located in northern France, on developing and improving the thermal performance of a perforated brickwork used in load-bearing masonry and measuring 220 x 220 x 65 mm3 (see Figure 1). This type of brick occupies an important place on the brickwork market and also presents 70% of the manufacturer production (BdN). The study focus of this work is twofold: first, the perforation configuration of the brick, and second, the impact on thermal resistance of the insulator/binder admixture inserted into the perforations. Three different binders (grey cement 52.5, Baticem mortar 12.5 and lime) were tested in relation to each insulator in terms of holding the latter in the perforations. After a technical and economic study, two types of insulators were analyzed in this regard: perlite and cork.

Figure 1: Brickwork for load-bearing masonry

(220 x 220 x 65 mm3)

2. MATERIALS AND METHODS 2.1. Characterization of Insulating Materials Figure 2 illustrates the microstructure of perlite grains and cork grains viewed through a scanning electron microscope (SEM). Perlite is made up mainly of silica (SiO2). It offers high (greater than 90%) porosity, which gives it insulating properties. It also has average thermal conductivity of 0.051 W/m.K. Cork is a cellular material made up of tiny pentagonal or hexagonal closed cells. These cells immobilize air to create an insulating material with average thermal conductivity of 0.036 W/m.K.

(a)

(b)

Figure 2: Microscopic observation: (a) Perlite (b) Cork 2.2. Characterization of Binders Three different binders were selected for this study. The first is the Portland Cement CEM I 52.5R, which is more than 90% clinker. This clinker is an admixture made up 75% of limestone and 25% of silica. In general, clinker consists more specifically of alite Ca3Si (50 – 65%), belite Ca2Si (15 – 20%) and aluminate Ca3Al (5 – 15%). The second binder is the Baticem Mortar 12.5, which is made up of Portland cement, gypsum (CaSO4), limestone filler and additives serving mainly as water-repellent air carriers. These additives are particularly interesting in that their water-repellent properties help to considerably improve the behavior of admixtures exposed to moisture and humidity. The aerial Lime is the last binder used in this work. It is a white slaked lime CL90 (CL: calcium lime – 90% limestone CaCO3). In this study, the mechanical properties of these binders are not addressed, since the binders are inserted into the brick perforations and consequently not subjected to stress. 2.3. Methodology The samples were prepared from various insulator/binder admixtures. They were then cast in parallelepipedal molds measuring 40 x 40 x 10 mm3 as illustrated in Figure 3. A constant volume of insulator (perlite/cork) was used for the various samples corresponding to the volume of the mold used (16 000 mm3). However, the binder volume was variable, with the maximum volume corresponding to total filling of the free space in the mold. Following preparation of the various samples, the mold was placed between two plates, one hot and one cold, and connected to two flowmeters and thermocouples on all sides. It was presumed that the heat flux through the sample was unidirectional. The equivalent thermal conductivity of the admixture was determined using the fluxmetric method in accordance with the NF EN 12664 (2001) standard. It represents the average of a series of five tests conducted on samples with the same insulator/binder proportions. Figure 4 illustrates the experiment configuration for one sample.

Figure 3: Preparing insulator/binder

admixtures

Figure 4: Configuration for determining

the thermal conductivity of the admixture 3. RESULTS AND ANALYSIS 3.1. Results for Various Admixtures: Insulator/Binder Proportions 3.1.1. Perlite/Binder Admixtures Figure 5 illustrates the evolution of thermal conductivity in the admixtures based on the perlite weight ratio for the various binders.

0

0,05

0,1

0,15

0,2

0,25

0,3

0,35

0,4

0,45

0 10 20 30 40 50 60 70 80 90 100

CementLimeMortar

Ther

mal

cond

uctiv

ity (W

/m.K

)

Perlite weight (%) Figure 5: Thermal conductivity of admixture according

to perlite weight ratio with different binders

All three resulting experimental curves exhibit the same profile. At a low perlite weight ratio, between 0 and 10%, the thermal conductivity decreases dramatically. At 14 to 70%, a slight decrease is observed among all binders. Above 70%, thermal performance values tended to move closer together as they converged toward the thermal conductivity of perlite. Baticem mortar exhibited the highest thermal performance of the binders studied. The perlite/cement and perlite/lime admixtures exhibited nearly the same values, differing only slightly. 3.1.2. Cork/Binder Admixtures Following the same procedure as with the perlite admixtures, Figure 6 shows the results obtained for the various cork/cement, cork/lime and cork/mortar samples.

The thermal conductivity of the admixtures decreased as the quality of cork increased. The lime and the Baticem mortar demonstrated similar results, although the mortar performed better in terms of improved thermal resistance.

0

0,05

0,1

0,15

0,2

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0,35

0,4

0,45

0 10 20 30 40 50 60 70 80 90 100

CementLimeMortar

Ther

mal

con

duct

ivity

(W/m

.K)

Cork weight (%) Figure 6: Thermal conductivity of admixture according

to cork weight ratio with different binders 3.2. Numerical Model After studying the thermal performance of the various admixtures, the thermal behavior of the brick was modeled in two dimensions with the objective of arriving at a perforation configuration offering maximum thermal resistance while taking into account the insertion of an insulator/binder admixture into the perforations. 3.2.1. Assumptions and constraints The modeling and study were conducted in two dimensions, with steady flow, on a quarter brick (symmetric conditions). The thermal flux was considered unidirectional running perpendicular to the wall surface as depicted in Figure 7. Modeling was carried out using the Abaqus application and finite-element analysis. A quadrilateral grid was used in this regard since it best represented the brick geometry.

parfaites entre la terre cuite et l’isolant (aucune résistance thermique à l’interface). Ce modèle est soumis à un gradient de température de 10°C entre ses faces,flux thermique en régime permanent est calculé en chaque point du maillage. Le schéma alvéolaire étant en deux dimensions, la modélisation et l’étude se font également en deux dimensions. Le flux thermique dans la brique est donc considéré comme invariant par translation verticale. De plus, les modèles alvéolaires étudiés possèdent deux axes de symétries comme sur l’exemple suivant (brique 22x22cm classique sans isolant) :

Figure 1 : Schéma alvéolaire en deux dimensions d’une brique 22x22cm Pour des raisons de symétries, la modélisation du schéma alvéolaire se fera donc sur un quart de brique :

Figure 2 : Modélisation d’un quart de la brique précédente avec Abaqus avec les conditions aux limites en température.

parfaites entre la terre cuite et l’isolant (aucune résistance thermique à l’interface). Ce modèle est soumis à un gradient de température de 10°C entre ses faces, puis le flux thermique en régime permanent est calculé en chaque point du maillage.

Le schéma alvéolaire étant en deux dimensions, la modélisation et l’étude se font également en deux dimensions. Le flux thermique dans la brique est donc considéré

invariant par translation verticale. De plus, les modèles alvéolaires étudiés possèdent deux axes de symétries comme sur l’exemple

(brique 22x22cm classique sans

: Schéma alvéolaire en deux dimensions d’une

ur des raisons de symétries, la modélisation du schéma alvéolaire se fera donc sur un quart

: Modélisation d’un quart de la brique

précédente avec Abaqus avec les conditions aux limites

Des valeurs de flux obtenues simplement retrouver les valeurs de résistance thermique correspondant à la brique entière par les formules suivantes [2]:

! : conductivité thermique (W/mK)" : flux thermique surfacique(W/m!)

: épaisseur du modèle (m)#T : gradient thermique appliqué Puis :

: résistance thermique de la brique (m!K/W)

: épaisseur de la brique (m) Dans cette étude,

= 0.11 m #T = T2-T1 = 10 K

= 0.22 m Maintenant que le modèle est en place, il convient avant d’exploiter tout résultd’estimer la précision de cette simulation. 4/ Précision des résultats Le logiciel Abaqus travaillant avec la méthode des éléments finis, les résultats obtenus ne sont que des approximations dont l’erreur dépend du maillage du modèle. utilisé ici est de type quadrilatère car il correspond bien à la géométrie de la brique.

Figure 3 : Maillage du modèle précédent avec une taille de maille de 1,1 mm.

Rth

ebrique

ebrique

4

Des valeurs de flux obtenues nous pourrons simplement retrouver les valeurs de résistance thermique correspondant à la brique entière par

[2]:

: conductivité thermique (W/mK) : flux thermique surfacique(W/m!) : épaisseur du modèle (m)

thermique appliqué (K)

: résistance thermique de la brique

: épaisseur de la brique (m)

Maintenant que le modèle est en place, il convient avant d’exploiter tout résultat, d’estimer la précision de cette simulation.

4/ Précision des résultats

Le logiciel Abaqus travaillant avec la méthode des éléments finis, les résultats obtenus ne sont que des approximations dont l’erreur dépend du maillage du modèle. Le type de maille utilisé ici est de type quadrilatère car il correspond bien à la géométrie de la brique.

: Maillage du modèle précédent avec une taille

Figure 7: Modeling of a perforated brick

A perforated brick is considered the assembly of two homogeneous materials and isotropes: fired clay and an insulator, with thermal conductivity values of 0.77 W/m.K and 0.04 W/m.K respectively. It is presumed that the insulator is inserted effectively into the perforations such that any contact thermal resistance is negligible. Moreover, the construction industry is highly regulated, and it is essential that the bricks under study comply with the various applicable standards, including NF EN 771.1 (2004) and its French counterpart NF P 12-021-2 (2004). The bricks studied are face bricks designed for

load-bearing walls. The standards dictate that for this type of brick, the air space ratio may not exceed 40% and perforation edges may not come within less than 15 mm of the exposed face. Additional production industrial constraints are prescribed by the manufacturer (BdN). It must be possible to produce the brick using existing equipment, and the material between the perforations must be thick enough to withstand die cutting after extrusion. The type of clay used and the outside brick dimensions must also remain constant. 3.2.2. Selected Model Taking into account these various constraints, a simplified brick model with a rectangular perforation configuration was selected (see Figure 8). It was proposed to study the effect of the quantity of insulator on thermal resistance. The quantity of insulator in this regard is determined based on the number of rows of perforations (n) and their thickness (a). Parameter (n) must be an odd number to maintain symmetric modeling conditions with a maximum value of 9 rows of perforations to maintain a partition thickness of approximately 10 mm and minimize the risk of production issues.

Figure 8: Model studied (dimensions in mm)

Figure 9 illustrates the evolution of thermal resistance in the brick based on the insulator ratio.

Figure 9: Impact of the insulator ratio on the brick’s thermal

resistance for different numbers of rows of perforations   Figure 9 demonstrates that thermal resistance increases with the quantity of insulator for different values of (n). Taking into account the constraints, which limit the insulator ratio to 40%, the optimal model for the configuration is obtained at 9 rows of perforations. The effect of insulator thermal

conductivity on the thermal resistance of the brick was subsequently studied for this model. The results are illustrated in Figure 10. Thermal resistance varies significantly with the thermal conductivity of the insulator. For example, it increased from 0.77 m2.K/W to 0.91 m2.K/W by decreasing conductivity from 0.08 W/m.K to 0.05 W/m.K, corresponding to an 18% improvement. This justifies the interest in experimental study of various insulator/binder admixtures.

0,7

0,8

0,9

1

1,1

1,2

1,3

1,4

1,5

0 0,01 0,02 0,03 0,04 0,05 0,06 0,07 0,08

Ther

mal

resis

tanc

e of

bric

k (m

.K /

W)

Thermal conductivity of insulator (W/m.K)

2

 Figure 10: Evolution of the brick’s thermal resistance based

on the thermal conductivity of the insulator 4. CONCLUSION Based on the experimental study of various admixtures, it was determined that the binder offering the best thermal performance was the Baticem mortar. Binders also played a significant role in the cohesion and strength of the admixture in the perforations during the manual handling of bricks. The thermal conductivity of the insulator/binder admixture had a significant effect on the thermal resistance of the bricks. Decreasing conductivity from 0.08 W/m.K to 0.05 W/m.K caused thermal resistance to increase by 18%. This work will be completed by a subsequent study to investigate the effect of moisture on the thermal properties of different admixtures. REFERENCES

ADEME (2010) : Chiffres Clés du « Bâtiment – Énergie – Environnement », édition 2010.

Chiang, K-Y., Chou, P-H., Hua, C-R., Chien, K-L., Cheeseman, C. Lightweight bricks manufactured from water treatment sludge and rice husks. Journal of Hazardous Materials 171 (2009) 76–82.

Demir, I. Effect of organic residues addition on the technological properties of clay bricks. Waste Management 28 (2008) 622–627.

Lafhaj, Z., Samara, M., Agostini, F., Boucard, L., Skoczylas, F., Depelsenaire, G. Polluted river sediments from the North region of France: treatment with Novosol® process and valorization in clay bricks. Construction and Building Materials 22 (2008) 755–762.

NF EN 771-1 (2004) : Specification for masonry units – Part1 : Clay masonry units.

NF P 12-021-2 (2004) : Specification for masonry units – Part1 : Clay masonry units – National addition to NF EN 771-1.

NF EN 12664 (2001) : Thermal performance of building materials and products — Determination of thermal resistance by means of guarded hot plate and heat flow meter methods — Dry and moist products of medium and low thermal résistance.

Oral, G.K., Yener, A.K., Bayazit, N.T. Building envelope design with the objective to ensure thermal, visual and acoustic comfort conditions. Building and Environment 39 (2004) 281–287.

Samara, M., Lafhaj, Z., Chapiseau, C. Valorization of stabilized river sediments in fired clay bricks: Factory scale experiment. Journal of Hazardous Materials 163 (2009) 701–710.

Sutcu, M., Akkurt, S. The use of recycled paper processing residues in making porous brick with reduced thermal conductivity. Ceramics International 35 (2009) 2625–2631.

Topçu, I.B., Isikdag, B. Manufacture of High heat conductivity résistant clay bricks containing perlite, Building and Environment 42 (2007) 3540–3546.

Veiseh, S., Yousefi, AA. The Use of Polystyrene in Lightweight Brick Production. Iranian Polymer Journal, 12 (4), 2003, 323-329.

Zukowski, M., Haese, G. Experimental and numerical investigation of a hollow brick filled with perlite insulation. Energy and Buildings 42 (2010) 1402–1408.