Experimental investigation of lightweight composite deck slabs

Transcription

1 Experimental investigation of lightweight composite deck slabs T. Luu, E. Bortolotti ArcelorMittal Liege R&D, Bd de Colonster B57, 4000 Liege, Belgium B. Parmentier Belgian Building Research Institute, Avenue P. Holoffe 21, 1342 Limelette, Belgium X. Kestemont Argex, Kruibeeksesteenweg 227, 2070 Zwijndrecht (Burcht), Belgium M. Briot ArcelorMittal Construction Belgium, Parc Industriel des Hauts-Sarts, 4040 Herstal, Belgium J.-C. Grass ArcelorMittal Construction France, BP25, Strasbourg-port du Rhin, France Abstract Composite floor systems using light gauge steel sheeting are gaining reputation worldwide. These composite slabs contribute to faster, lighter and economical construction of buildings. With lightweight aggregate concrete, the composite deck slabs offer henceforth new assets such as higher unpropped span during construction, supplementary reduction of self weight of the floor (up to 40% compared to the normal concrete), lasting heat insulation, etc. However, the longitudinal shear behaviour of composite deck slab must be studied by experimental methods to justify the combination of the steel deck and lightweight concrete and to evaluate the m-k values as defined by EN This paper presents a complete experimental investigation conducted on the lightweight composite deck slabs associating steel deck and pumpable lightweight concrete. Keywords: Steel deck, composite deck slab; lightweight concrete 1. Introduction The composite deck slab, combining steel deck and concrete poured in-situ, is already highly recommended by contractors and project managers for the construction or renovation of multi-storey car parks and tower buildings. The steel decks remain permanently in place as an integral part of the floor system. They act as the formwork while concreting, and in the composite slab they act as the tension reinforcement. This type of flooring results in faster construction, lighter floor, and offers high level of performances. In addition, they also have other advantages such as easy handling, a good ceiling surface and convenient ducting for

2 routing utility services. Finally, the thin sheeting is extremely light and hence can be transported conveniently, handled and placed easily by the contractor. However, the longitudinal shear behaviour of composite deck slab must be studied by experimental methods to justify the combination of the steel deck and concrete and to evaluate the m-k values as defined by EN [1]. An experimental study has been carried out on the lightweight composite floor Cofraplus 77 LS associating Arval steel deck and Argex lightweight concrete based on expanded clay aggregate [2]. These concretes are pumpable, representing a revolution in the world of lightweight concrete. Difficult-to-access construction sites and the construction of skyscrapers are therefore much easier today. The weight reduction is about 40% compared to the standard concrete and for similar structural performances. The impact on the structures and foundations are therefore significant. Two lightweight concrete classes (LC16/18 D1.6 and LC35/38 D 1.8) were used to investigate the influence of concrete strength on longitudinal shear capacity. More than 30 composite slab specimens were cast in fully supported conditions. These specimens were then tested according to EN [1] at the Belgian Building Research Institute. This paper is an experimental evaluation of m-k values for the lightweight composite floor. Using the values of m and k as determined by experimental tests, ultimate load carrying of the lightweight composite slab can be calculated. The lightweight composite deck slabs represent a promising combination which opens up new fields of applications from reducing dead weight of high rise buildings to projects with a tough foundation base, or in case of renovation. 2. Experimental investigation on lightweight composite slabs Two extreme cases were tested for shorter shear span loading and longer shear span loading. Simply supported spans of 2.5 m and 4.5 m were tested to failure (including cyclic loading). For each shear span loading and for each slab depth, a set of three specimens was used. The first one was tested to failure under static monotonic loading. The other two specimens were tested for cyclic loading for 5000 cycles, followed by a static test. The details of the steel deck are given in table 1. Fig. 3 shows the cross sectional view of the steel deck. Thickness (mm) Yield strength (MPa) Table 1. Properties of the steel deck. Weight (kg/m²) Height h t (mm) Width B (mm) Steel area A p (mm²/m) Figure 1. Cross section of the embossed sheet.

3 Two pumpable lightweight concretes LC16/18-D1.6 and LC35/38-D1.8 used for concreting were studied to observe the influence of concrete strength on shear capacity. The lightweight concrete characteristics are given in table 2. Table 2. Characteristics of lightweight concretes (according EN206-1). Concrete class f lck (cylinder) (MPa) f lck (cubes) (MPa) Density D (kg/m³) LC16/18-D LC35/38-D The dimensions of composite slab specimens are given in table 3. These specimens were cast with the profiled sheet as the base. The casting was carried out in fully supported conditions. Table 3. Geometry of the composite slab specimens Steel deck Cofraplus 77 LS Lightweight concrete class LC16/18-D1.6 LC35/38-D1.8 Slab depth h t (mm) Slab width B (mm) Span L (mm) Total length L tot (mm) Shear length L s (mm) The tests on composite deck slab were carried out according to Annex B.2 of Eurocode 4 at Belgian Building Research Institute [3]. Detailed description of the experimental set-up is presented on [4]. 3. Experimental results Typical load and end slip vs. midspan deflection curves are shown in the figure 2 and figure 3 for monotonic and cyclic test, respectively. Figure 2. Load or end slip vs. midspan deflection for static monotonic test. Figure 3. Load or end slip vs. midspan deflection for cyclic test.

4 Influence of concrete strength on shear capacity The test results obtained with two lightweight concrete classes LC16/18-D1.6 and LC35/38- D1.8 are reported on figure 4. The test results are similar for composite slabs cast with these two lightweight concrete classes. The influence of concrete strength on shear capacity of composite slab is negligible Vt (kn) Evaluation of m-k values Span L (mm) LC16/18-D1.6 LC35/38-D1.8 Figure 4. Influence of concrete strength on shear capacity. The main objective of the present testing program was to determine the m-k values which define the shear bond of the lightweight composite deck. The recommended design equation for shear bond of composite deck slabs is given in EN : map V A t p V = Bd + k t p or = m + k (1) BLs Bd p BLs which is in form of an equation for a straight line 0,35 0,30 y = mx + k. V / (B.d ) [N/mm²] 0,25 0,20 0,15 0,10 0,05 y = 183,9x - 0,0944 0,00 0,0000 0,0005 0,0010 0,0015 0,0020 0,0025 Ap / (B.Ls) [/] Figure 5. Characteristic curve (m-k curve). The m-k values obtained are m = and k = N/mm²

5 4. Example of application Project of multi-storey buildings : - Composite deck : Cofraplus 77 LS sheeting thickness 0.75 mm, - Concrete : Pumpable lightweight concrete LC16/18-D1.6, - Number of spans : double spans, - Live load : 2.5 kn/m² (for office areas), - Props during construction : without props. The admissible span of lightweight composite deck slab Cofraplus 77 LS is 4.15m. Figure 6. Lightweight composite slab Cofraplus 77 LS. q h t L L Figure 7. Double spans of Cofraplus 77 LS. The comparison of dead weight between concrete slab and lightweight concrete slab is given in table 4. The latter has a weight saving of 47% which has a very significant impact on the structures and foundations. Table 4. Self weight of lightweight composite slab compared to concrete slab. Slab depth Cofraplus 77 LS Concrete slab Weight savings 14 cm 1.78 kn/m² 3.36 kn/m² 47 % The m-k values of the profiled steel deck when used with normal concrete are m = and k = N/mm²

6 5. Conclusion The lightweight composite deck slab Cofraplus 77 LS associating steel deck and pumpable lightweight concrete has been experimentally investigated. This new product has been included in the Arval sales program. Thanks to the use of pumpable lightweight concrete, the dead load of the slab may be reduced by 40% compared to the normal concrete for similar structural performances, implying weight and cost savings for structure and foundation. It also leads in increasing the unpropped span of the composite deck during the construction stage. These composite deck slabs with pumpable lightweight concrete find their advantages for all types of buildings, especially in multi-storey buildings and renovation. 6. References [1] EN (2005), Design of composite steel and concrete structures - Part 1-1: General rules and rules for buildings. [2] Lightweight concrete Argex : [3] Parmentier B. (2007), Détermination de la résistance au cisaillement longitudinal par essays de flexion 4 points sur planchers mixtes acier-béton léger - Cofraplus 77 L, Technical report, Belgium. [4] Luu T. et al. (2008), Composite deck slabs with lightweight aggregate concrete, 5 th European conference on steel and composite structures, pp , Graz, Austria.