gfrc panels

Oliveira et al.

Architectural panels: experience in Brazil

39 GFRC ARCHITECTURAL PANELS: EXPERIENCE IN BRAZIL L.A. OLIVEIRA and C.T.A. OLIVEIRA Department of Civil Construction Engineering, Engineering, Escola Politécnica, Universidade de São Paulo, Brazil

F. PISCIOTTA and A.S. YAMAGUTI PAVI do Brasil

SUMMARY: This paper concerns the application of GFRC architectural façade panels on a major project – a 20-storey residential building in São Paulo, Brazil. The specific adaptations made in the design of components, connections and joints to suit Brazilian construction trade practices and local conditions are outlined. Data on fire resistance, thermal transmittance and sound insulation of GFRC component are shown. Requirements for global performance analysis of GFRC facade systems and the development of technical Brazilian standards are discussed. Keywords: GFRC, sandwich panels, performance analysis, sound insulation, thermal transmittance, fire resistance, Brazil, façade panels.

INTRODUCTION

Since 2001, when glassfibre reinforced concrete (GFRC) facade commercial technology was introduced in Brazil, its employment for multi-storey buildings has been increasing, mainly in São Paulo city. It is well known that standards and codes of practice are fundamental to established credibility and provide assurance for specifiers, customers and contractors, besides improving competitiveness of GFRC with other technologies and materials. In this regard, the ABNT1 subcommittee CE 18:316.01 has been working on GFRC standards since July 2001. The draft standards cover procedures proc edures for f or basic productio production n control control tests, including including methods for acceptance of raw materials, calibrating equipment and determining the phys physica icall prop propert erties ies of the the matr matrix ix and and comp compos osit ite. e. With Within in a few month monthss a set of seven seven draft draft standards will be submitted to public consultation, and it is expected that these standards will be issued by the end of 2003. The subcommittee is aware of establishing standards related to practices for design, installation and performance analysis of GFRC components mainly related to durability in tropical climates2 . The next step will focus on codes of practice for design and procedures for accelerated and natural ageing tests. As reported by Vieira (2001), despite the difficulties of entering the new Brazilian market, there has been a good level of cooperation with local technicians and clients.

1

ABNT (Tech (Technica nicall Standard Standardss Brazilian Brazilian Association) is the main national standards as sociation dev oted to the writing and publication of standards and specifications for materials and products. 2 Tropical climate is typical over the major part of Brazilian territory, with with average temperatures arou nd 30°C and average relative humidity ranges from 70 to 80%. . Edited by J.N. Clarke and R. Ferry. GRC 2003: Proceedings of 12th Congr es ess s of th e GRCA, October October 2003, Bar celona, Spain The Concrete Society, Century House, Telford Avenue, Crowthorne, RG45 6YS, UK, on behalf of the GRCA. Ref: GRC21, ISBN 1 904482 04 X.

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During the past two years about 1000 bath units and 24,000 m 2 of GFRC architectural panels have been installed in residential and commercial buildings in São Paulo (Pavi do Brasil, 2003). This demonstrates that there is an increasing demand for GFRC products in the Brazilian construction market. But the technology transfer was not enough for the growth of the GFRC industry in Brazil. This approach must consider the cultural differences, local climatic conditions, local trade practices and construction traditions. Therefore the aims of the present paper are to outline the design process and analyse some performance data of the GFRC facade panel system on a multi-storey residential building under construction in São Paulo city at this time.

CASE STUDY

This case study deals with a high-standard 20-storey residential building located on a busy street. According to the original schedule a 6000 m2 facade was to be erected in just four months. The building structure consists of columns, beams and floor slabs of reinforced concrete cast-in-place. GFRC composite GFRC panels are being manufactured by the spray-up process. A high-early-strength cement (class V Brazilian cement) and natural silica sand were used. The slurry has a cement:sand ratio of 1, a water:cement ratio of 0.33, minor amounts of admixtures and curing agent. A content of 3-4% (% by weight of slurry) of 25–40 mm-long chopped AR glass fibres was used. Some properties of hardened composite are shown in Table 1. Table 1: Properties of GFRC at 28 days. Property

Range of results

Specific gravity

1900–2100 kgf/m

Modulus of elasticity

15–20 GPa

Compressive strength

60–80 MPa

Flexural strength

15–25 MPa

Tensile strength

7–10 MPa

Thermal conductivity

0.79 W/m°C

3

Panels Two types of panel were designed. The first is a GFRC single-skin panel 15 mm-deep attached to a metal stud frame or directly attached to the concrete structure by means of flex anchors. The single skin panel acts as a covering columns and beams (see Figure 1). The panel has an exposed aggregate outer surface. The face mix consists of unreinforced cement/sand mortar plus set retarder, oxide pigment and 6 mm maximum size decorative aggregate. The exposed aggregate surface is achieved by removing the surrounding paste by high-pressure washing after demoulding.

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connectors

60mm

single skin panel

reinforced concrete structure Elevation

Figure 1: Sketch of a beam covering panel (single skin panel without stud frame).

The second type is a GRFC double-skin panel with a glass wool insulation layer and total depth of 120 mm; these layers are joined by plastic connectors (see Figure 2). This component is a sandwich panel, which acts like an external partition system rather than a cladding system. The sandwich panels must meet the performance requirements of an external partition such as: structural strength and stability, resistance to impact loads and wind loads, watertightness, hygrothermal and acoustic insulation. The inner surface of the sandwich panel is ready to receive any kind of architectural finishes. The outer surface finish is similar to that described above for the single-skin panel. The sandwich panel has a superficial density of about 70 kg/m2 . At each floor 20 sandwich-panel units are used, made up of 10 different panel types. A 4900 mm wide × 2880 mm high panel is the largest panel used in this building. The following discussion will be focused on the sandwich (double-skin) panel rather than the cladding panel. This is due to the fact that the sandwich panel system design incorporates new concepts to fit the Brazilian construction industry requirements and standards. In fact development of this sandwich panel has resulted in a patented Brazilian product.

face mix

insulation (glass wool)

GFRC skin

plastic conector Cross-section Figure 2: Sketch of sandwich panel.

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The sandwich panels were designed to be ventilated (see Figure 5) in order to help eliminate the stress caused by the thermal differential between the exterior and interior face of the panel in fire conditions. The glass wool insulation layer between the GFRC skins improves the thermal properties as well as the fire resistance. The sandwich panel provides 120 min fire resistance to the stability and integrity criteria according to Brasilian standard (NBR 10636 – ABNT, 1989). It must be stated that the insulation criterion was not satisfied. Regarding the thermal properties, from the data described before, it is possible to assess the theoretical thermal transmittance, which is shown in Table 2. Table 2: Thermal transmittance (estimated by the authors). Component

GFRC sandwich panel

Thickness (a) (e = e1 + e3) GFRC skins (m)

0.02

Thickness (a) (e = e1 + e3) GFRC skins (m)

0.09 2

Thermal resistance, R= (1/he) + (1/hi) + (e/ë1) + (e2/ ë2) (m ºC/W) 2

Thermal transmittance, K = 1/ R (W/m ºC)

2.85 0.35

Notes: Interior surface coefficien t of heat transfer he=20 W/m2 ºC (Frota and Schiffer, 1988); Exterior surface coefficient of heat transfer hi=8 W/m2 ºC (Frota and Schiffer, 1988); ë1 = 0,79 W/mºC (GFRC skin) and ë2 = 0,034 W/mºC (glass wool).

ABCI (1990) roughly classify as a good insulation component one that has a thermal transmittance ( K =1/ R) below 1 W/m2 ºC, medium insulation K between 1 and 1.8 W/m2 ºC and poor insulation K above 1.8 W/m2 ºC. Therefore, according to this estimated value the ventilated panel designed to this construction is a good insulation component. The sound insulation at low frequencies can be estimated by surface mass (weight per unit area). Figure 3 shows the insulation reduction of only one skin of GFRC that depends on its weight per unit area (2000 kg/m3 × 0.01 m = 20 kg/m2 ).

60

H 0 5 1 50 3 0 0 40 1 ) B d ( 30 n o i t 20 a l u s 10 n i d n u o S

0 1

10

100

1000

Weight of panel (kg/m 2 )

Figure 3: Relationship of sound insulation to surface mass (after Cem-FIL GRC technical data).

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It must be pointed out that the value of 29 dB extracted from Figure 3 refers to only one skin of GFRC. Increasing the thickness to 20 mm will increase the reduction by about 35 dB. Further increase in sound insulation depends on the glass wool core. According to Baring (1998), the Sound Insulation Index requirement to class A and B3 facades must be higher than 45 and 40 dB, respectively. Connections The anchorage of the panels to the concrete frame is made through galvanised connectors. This anchorage was designed to support on one level, at two upper points (bearing connections) and held in with some degree of flexibility at two other lower points (lateral connections). Figure 4 shows this arrangement.

ventilated panel lateral connection

reinforced

Elevation

bearing connection

25mm

concrete structure Elevation Figure 4: Sketch of connections of sandwich panel.

The connections will be inserted into pre-drilled or self-drilled holes in hardened concrete (expansion anchors). These connections were designed to accommodate erection tolerances and product tolerances, so the final clearance is about 25 mm. It has to be pointed out that the earlier projects in Brazil employed European standardised connections that allow tolerances of 15 mm. But for a cast-in-place concrete frame in Brazil, the acceptable building frame alignment tolerance is 25 mm. These tolerances for tall buildings are greater than European tolerances; there was 3

Class A refers to high-standard hotels in a particularly noisy location (close to an airport, for example) and class B to commercial and residential buildings.

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therefore a need to develop a new connection suitable for local building conditions. Joints Joints are required to accommodate changes in wall panel (GFRC panel) or structure dimensions caused by changes in temperature, moisture content or load. These joints were assessed to accommodate local wall movements rather than cumulative movements. In the present case, the joints were designed as shown in Figure 5.

Figure 5: Sketch of treatment of joints.

Those joints are based on the open rainscreen principle. According to PCI (1989) the rainscreen principle means the control of the forces that can move water through small openings, rather than the elimination of the openings themselves. These joints have two lines of defence for waterproofing, one is a rain barrier near the exterior face and the other is the sealants, also near the exterior face. The rain barrier was designed to shed most of the water from the joint, and the wind-barrier or airseal should be the demarcation line between outside and inside air pressure, the sealant being near the interior face. In this case, the sealant is near the exterior and functions as a watertight barrier rather than an airseal. The most common joint materials used in Brazil, particularly in this building, are neutral silicones. The main properties of these sealants are: good UV resistance, do not accept painting, movement capability of ±25% limited to 25 mm, service temperature from –30ºC up to +80ºC.

DISCUSSION

The facade performance cannot be ensured by individual panel properties, despite some properties of sandwich panels meeting the local requirements. That performance is directly concerned with the complete structure, including materials and design of joints, connections, architectural design aspects and local environmental conditions to which

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the components are subjected. Further in-site analysis and laboratory tests will be necessary to provide accurate results on this matter. The stability of the facade depends on panel integrity as well as on connection performance and durability, which is directly assessed by the connection’s corrosion resistance. Another important aspect that must be considered is the maintenance of the connections, which aims to prevent corrosion problems especially under Brazilian climatic conditions. Despite the good thermal and acoustic properties of the panel presented, these results are not sufficient to ensure good performance of the facade. Therefore, other factors, such as joints, architectural design and the local climatic conditions must be considered. Since São Paulo city has high temperature gradients, the thermal inertia of the panel must be analysed, especially in buildings without air conditioning. The facade fire resistance could not be analysed only by the panel integrity, stability and insulation. Other factors must be taken into account: passive connection protection and joint treatment that prevents passage of flame or hot gases for a given fire protection rating. It must be noted that Brazilian requirements are to be applied to the whole construction (facade system), that is, all these factors have to fit the Brazilian legislation (Decreto Estadual S.P. 46076/2001 – IT 08/01) 4 . The watertightness of the facade must be ensured by the joint performance that mostly depends on the elastomeric joint sealants and on the joint geometry configuration. The sealant used in this construction was silicone, and it should be noted that most silicones available in the Brazilian market have a service life about 20 years, according to the manufacturers. The physical and mechanical characteristics of ageing GFRC are not yet wellknown, so there is a need for comprehensive scientific and technological researches to ensure the long-term properties of GFRC components, considering local construction trade practices, climatic conditions and quality of materials.

CONCLUSION

This case study represents the major GFRC panel project under construction in Brazil at the present time. The modifications made in the design process have been essential for making the GRC technology consistently more competitive in cost and in technical performance the cladding system currently used in high-standard residential buildings in Brazil: architectural precast concrete panels. It must be stressed that the successful use of GFRC architectural components in the near future in Brazil depends on the consolidation of national standards and codes of practice. This is very important in providing performance levels to be met by GFRC components and the quality control level GFRC manufacturer should attain. As important as the standards and codes of practice is the necessity for the GFRC component design process to be understood as an integrated process. According to the integrated process approach it is desirable that GFRC panel design starts simultaneously with the architectural project to provide a cost-effective alternative for the facade

4

According to Brazilian criteria, the fire resistance of the facade system of a multi-storey residential building, 30 to 80 m high, should be 120 min.

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system, speed construction, increase productivity, avoid additional work at the jobsite and ensure the long-term properties of the facade system. In doing so, a designer can take advantages of GFRC material properties in order to enhance some of the environmental building performance requirements such as: possibility of reducing heating, cooling or ventilation energy consumption and costs of operation through the use of a suitable performance building envelope, obtain an design process that focuses on future demountability and adaptability to changing needs in the future. All these factors are fundamental in establishing confidence on the part of architects in specifying GFRC in their projects and in attracting economic interest on the part of owners and contractors in adopting GRFC components in their buildings. There is still a lot of work to be done.

REFERENCES

ABNT (1989) Divisórias sem Função Estrutural – Determinação da resistência ao fogo, NBR 10636. Rio de Janeiro, 1989. ABCI, Manual técnico de alvenaria. Projeto/PW. São Paulo, Editores Associados, 1990. BARING J.G. O desempenho acústico das vedações verticais em edifícios. In: Seminário de vedações verticais, 1998. Anais. São Paulo, EPUSP, 1998, pp.113–125. Cem-Fil International Ltd. GRC Technical Data – Vetrotex Cem-Fil. CORPO DE BOMBEIROS, Decreto Estadual S.P.46076/2001 – Instrução Técnica nº 08/01. Segurança estrutural nas edificações – resistência ao fogo dos elementos de construção. São Paulo, 2001. Available at www.polmil.sp.gov.br/ccb. FROTA A.B. and SCHIFFER S.T.R. Manual de conforto térmico. São Paulo, Editora Nobel, 1988. VIEIRA A. Starting production of GFRC structural elements in Brasil. In: International Glassfibre Reinforced Concrete Congress, 12th Proceedings. GRCA International, Dublin, 2001, pp.53–57. PAVI DO BRASIL. Painéis arquitetônicos de GFRC. Catálogo técnico. São Paulo, 2003. PRECAST CONCRETE INSTITUTE (PCI), Architectural precast concrete. 2nd ed., Chicago, 1989.

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