8 EXTRA LIGHT GRC SANDWICH ELEMENTS FOR ROOFING IN INDUSTRIAL BUILDINGS

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1 8 EXTRA LIGHT GRC SANDWICH ELEMENTS FOR ROOFING IN INDUSTRIAL BUILDINGS MARICA DELLA BELLA and DIEGO CIAN Precompressi Centro Nord S.p.A., Italy SUMMARY: Secondary roofing elements, complementary to the main precast prestressed concrete roof elements with aerial geometry, are normally made of concrete or, better, of metal sandwich panel due to the required lightness. The application of GRC sandwich panels with internal lightening made of polyurethane, rock wool or glass wool for the manufacture of roof elements is presented. The use of GRC allows the elements to be light in weight and also fireresistant. Furthermore, this paper outlines the use of such elements together with main precast concrete roof elements, which provide the required uniform rheological behaviour and uniform aesthetic surface appearance. Having defined the mix design and manufacturing process for the industrial production of GRC elements, able to withstand roof loads, criteria for the design have been assessed through prototype tests. The first applications of very light elements with span up to 6.0 m and weight of 0.4 kn/m 2 are presented. Installation details of GRC elements for curved roofing and lateral closures are also reported. Keywords: Geometry, GRC, impact tests, industrial buildings, lightweight elements, precast, roofs, sandwich elements, structural, testing, waterproofing. INTRODUCTION Until years ago roofing elements in industrial buildings were precast double-tee units or sloped roof beams with ribbed slabs without any special aesthetic requirement, as shown in Figures 1 and 2. After the precast elements have been erected, the roof extrados is normally covered with an insulating layer and then protected with a waterproofing sheet or fibrecement units. Now the market more and more requires better architectural design and aesthetic features, including the structure and finish of industrial buildings. This market demand, together with the necessity to minimise in-situ finishing operations after precast structure erection, induced precasters to design and manufacture new roof elements of high architectural quality. Such elements present special aerial geometry and are delivered with perfectly finished and painted surfaces, including insulating and waterproofing layer. GRC 2003: Proceedings of 12th Congress of the GRCA, October 2003, Barcelona, Spain. Edited by J.N. Clarke and R. Ferry. The Concrete Society, Century House, Telford Avenue, Crowthorne, RG45 6YS, UK, on behalf of the GRCA. Ref: GRC21, ISBN X. GRC 2003 Paper 8: page 1

2 Figure 1: Double-tee unit. Figure 2: Precast roofing of industrial building. Typical sections are shown in Figures 3 and 4, with spans up to 30 m; these special roof elements are usually supported on longitudinal beams and positioned parallel at distances between 2 and 6 m apart, according to the span. Figure 3: Aliant. Figure 4: Europlan. GRC 2003 Paper 8: page 2

3 Between the main roof elements, secondary roof elements or special transparent window elements are normally used. Secondary elements are usually thin precast concrete sections, due to the necessity of minimising weight, or steel sandwich panels; the latter are very light but with unsatisfactory aesthetic and material characteristics, rather different from those of the main concrete elements. The use of GRC is a very valuable market alternative to the secondary elements used up to now, due to its particular characteristics: lightness, strength, waterproofing, durability and aesthetic features. STRUCTURAL APPLICATION OF GRC GRC has been largely used in the construction market since the 1970s, but its main application has been limited to architectural façades sometimes with large dimensions and special shapes. Such elements are often designed for heavy service loads due to wind pressure, which may be very strong, up to 3.0 kn/m 2 in high-rise buildings (see examples in Figures 5 and 6), similar to the floor service load of a residential building. Figure 5: GRC roof of high-rise building. Figure 6: GRC cladding elements of high-rise tower. GRC 2003 Paper 8: page 3

4 The design stress in such elements requires that the material strength and durability should be typical of a structural material, as may be employed also for precast floors. For this purpose, Precompressi Centro Nord promoted experimental research and tests to determine the mechanical properties of GRC, such as tension and compression resistance, Young s modulus, creep and shrinkage and stress strain behaviour, at early stages and after ageing. Through the European CRAFT Research STRUCTUA-GRC with other Italian and Portuguese precast companies and with the support of the Universities of Lisbon and Barcelona, a direct investigation of a large number of GRC compositions has been made with the purpose of improving the mechanical and durability characteristics of structural GRC. The optimisation of mix design has been achieved and the premix technique has been improved as an industrial manufacturing process. Figure 7. Special apparatus for testing GRC sample. Figure 8. Stress strain diagram for LOP and MOR determination of GRC samples. GRC 2003 Paper 8: page 4

5 GEOMETRY OF ROOF ELEMENTS Secondary roof elements may be flat or curved, according to the type of main element to which are coupled, with a thickness of 50 to 100 mm according to the span and design load, and with a weight around 0.50 kn/m 2. As shown in Figure 9 and 10, such roof elements, made of GRC, look like a sandwich with two GRC layers. Rock wool or glass wool lightening is placed in between, and fibres are oriented perpendicular to the surface, in order to connect the inner and outer skins to each other. Such a composite section is fundamental for the overall structural robustness of the element and its shear resistance. Longitudinal ribs are designed to be incorporated internally within the depth of the element and in some cases also raised externally to increase the flexural strength for longer spans, while transverse ribs are usually used internal only, to increase the transverse distribution capacity. This sectional design consistently increases the overall stiffness without a significant increase in weight. The lateral edge of the roof elements is designed to form a longitudinal joint and to avoid any rainwater infiltration from the roof. Figure 9: Section of GRC curved roof elements. Figure 10: View of roof element. GRC 2003 Paper 8: page 5

6 The manufacturing technique uses only premix GRC through the following procedure: First, a 10 mm GRC layer is cast and vibrated on the mould, reinforced with glassfibre grid, mesh mm. Then, glasswool, wetted with acrylic resin, is placed on the intrados for better adherence and connection of the two GRC layers. Finally, after positioning additional stainless or galvanised reinforcing steel corresponding to the longitudinal ribs, casting of the upper layers and raised ribs is completed. After wet curing for one day under polyethylene sheet the roof elements are demoulded and put in storage. Steel reinforcement is designed in accordance with ULS verification, without taking into account the glass fibre s contribution, as required by the Italian Building Code. Figure 11: Assembly of secondary roof elements. LOADING TESTS Prototypes have been manufactured and tested under equivalent service load (about 2.0 kn/m 2 ), and then loaded to failure, as shown in Figures 12 to 16, demonstrating the high structural performance of the elements. During tests at the SLS waterproofing of the elements has been verified by means of four cylinders, sealed and filled with water, and by checking the water level for two months. The water absorption was negligible, confirming the impermeability of the upper GRC surface, of a suitable mix design with acrylic resin, without any additional waterproof sheet. The production of such secondary roof elements is completed by tailor-made head-closure elements as shown in Figures 17 and 18. GRC 2003 Paper 8: page 6

7 Figure 12: Test at SLS with water cylinders of first GRC prototype. Figure 13: Test at SLS of second prototype. Figure 14: Load-deflection diagram. Figure 15: Test at ULS. Figure 16: Water cylinders for waterproofing test. GRC 2003 Paper 8: page 7

8 Such elements, manufactured by the same procedure as the secondary roof elements, have been tested according to the specific impact test to ensure safety in case of a person falling on the roof. Figure 17: Closure GRC element from inside. Figure 18: Assembly of closure between two main roof elements. GRC 2003 Paper 8: page 8

9 Figure 19: Impact test on GRC roof element. CONCLUSIONS Having established the mix design and manufacturing process for structural GRC, the way is open to the production of GRC precast elements of large dimensions subject to significant loads, not only for architectural façades and special shapes, but also for other secondary structural elements in precast buildings. Lightness, architectural value and surface finish, combined with structural performance, are important plus-factors for the market. Extensive and accurate testing of GRC material characteristics and on prototypes, together with optimisation of the premix technique and manufacturing process, may guarantee a qualified industrial production of such GRC structural elements. The first applications are now underway in Italy, just after starting industrial factory production. These innovative GRC structural elements are covered by patents and the authors are available for any further clarification or details. grc@gruppocentronord.it ACKNOWLEDGEMENT The authors thank Ing. F. Finzi for his great scientific contribution and technological guidance in GRC material development and industrial application. GRC 2003 Paper 8: page 9