Transverse flexural and torsional strength of Prestressed Precast Hollow-Core Slabs

Transcription

1 Tailor Made Concrete Structures Walraven & Stoelhorst (eds) 2008 Taylor & Francis Group, London, ISBN Transverse flexural and torsional strength of Prestressed Precast Hollow-Core Slabs A. Pisanty Faculty of Civil and Environmental Engineering, Technion, Haifa, Israel ABSTRACT: The transverse flexural and pure torsional strength of Prestressed Precast Hollow-Core Slabs (PPHCS) were investigated experimentally. Two series of tests were conducted and their results reported, relating to: a. the lateral (perpendicular to span) flexural strength, and b. the pure torsional strength. The development of a machine for the testing of slab cuts under pure torsion is presented and explained in detail. Evidence of existing reliable tensile/shear strength was produced. A size effect was detected indicating a higher strength with the reduction of the slab thickness, under both bending and torsion. 1 INTRODUCTION Information about the transverse flexural and torsional resistance of prestressed, precast hollow-core slabs (PPHCS) is essential for design and the expected performance of these slabs, designed as one way spanning elements, however, employed in slabs and horizontal decks under circumstances of uneven loads and load distributions, therefore transverse flexure and torsion are inevitable. The geometry of PPHCS, in section, as a result of the industrialized manufacturing system, imposes difficulties on the investigation of particular, narrow defined, zones of their sections, therefore most experimental investigations were conducted observing their global behavior as one way single spanning element, in service or ultimate states. This was done by all past code drafts or chapters in codes and to a large extent in the fib bulletin 6 by Van Acker et al (2000). To the most, transversal behavior of a one way spanning series of PPHCS was regarded as a chain of non reinforced segments, interconnected by hinges with a vague knowledge of their transversal strength or torsional resistance. A recently published experimental and analytical investigation by Broo et al (2007), amongst the very few to employ powerful FE analysis programs, might allow a better insight into the stress state of these complex topology sections, however the presently published evidence still is with regards to the global element behavior. Experimental investigations aimed at a better understanding of the micro behavior of PPHCS were conducted by Pisanty ( ). The web shear strength was investigated by Pisanty (1992). The tensile strength of the web was reported also by Pisanty & Regan (1991). Lateral bending stiffness was assessed experimentally by Pisanty (1997). Last a testing device was developed and tests were conducted in order to obtain an insight of the torsional resistance Pisanty (1998). In spite of the lack of reinforcement (other than prestressing) a level of resistance to both lateral bending and torsion was exhibited. 2 EXTRUDED PPHCS The manufacturing method of extruded PPHCS differs from other production systems, where the concrete quality over the thickness of the slabs is fairly uniform. The process of extrusion results in reduced concrete quality from the bottom layer (where the prestressing reinforcement is embedded) towards the top layer. This fact requires a particular consideration as to the location of the tensile zone in the investigated model and the imposed stress state when dealing with extruded PPHCS. Proof was provided by Pisanty (1991) that the tensile strength of the bottom layer is higher compared to that of the top layer, testing for flexural bending al cuts from extruded PPHCS in the direction of the span. 3 TRANSVERSE FLEXURAL STRENGTH Transverse flexural strength tests on extruded PPHCS were carried out following the classical Rilem flexural bending test of two point loaded segments at 1/3 of the span (Fig. 1). For this purpose from 1200 mm wide slabs segments were cut having a length equal to their thickness (whereby prestressing was neutralized): 8 specimens 527

2 Figure 1. Standard Rilem flexural bending test. Figure mm thick slab specimen after rupture. Table 1. Tests results for transverse flexural bending tensile stress at rupture (MPa). 300 mm 250 mm 200 mm Spec. no. Figure 2. Figure mm thick slab specimen on test set. 300 mm thick slab specimen on test set. from 300 mm slabs, 4 specimens from 250 mm slabs and 4 specimens from 200 mm slabs. Characteristic concrete strength for all test specimens was 60 MPa measured in 100 mm cubes For the testing half of the specimens were positioned so that the bottom layer (originally containing the prestressing strands) was at the tensile face (marked in table 1). The second half of the specimens were positioned so that the original top layer would be at the tensile face (marked in table 1). Illustration of both positions may be seen at Figs. 2 (250 mm) and 3 (300 mm). In Fig. 4 a 300 mm specimen after rupture is given. Tests were controlled using a load cell having an accuracy of 10 N, and the average loading rate was 50 N/sec (duration was 4 5 minutes). The tests results are presented in table 1. Two tendencies are clearly demonstrated: a. tensile strength grows with diminishing overall thickness. b. there is a very substantional difference between the tensile strength of the top and bottom layer of the slabs (as previously mentioned the manufacturing process is such that the layer containing the prestresing strand 19/1/B /2/B /3/B /4/B /5/B /6/B /7/B /8/B /9/B /10/B /11/B /12/B /1/B /2/B /3/B /4/B 4.39 Average is the bottom layer). The 250 mm thick slabs results do not exactly fall in line, however it is believed that an increased number of test specimens of both 200 and 250 mm thickness should improve the average and demonstrate well a systematic development of the trends cited here. In terms of characteristic tensile strength f ctm the results indicate 75% to 100% for the bottom layer and 60% to 80% for the bottom layer, depending on the slabs thickness. 4 TESTING MACHINE FOR TESTING TORSION STRENGTH A simple machine for testing torsional strength was developed. It allows testing of 1200 mm long specimens (cuts from 1.2 m wide slabs) of a square section up to 300 mm thick. The main parts are three concentric frames (Fig. 5) installed at 440 mm distances that 528

3 Figure 5. Conceptual design of the testing machine. Figure 6. Sections A-A (left) and B-B (right). serve as vertical supports. Two end frames are fixed by welding on a horizontal frame section A-A (Fig. 6) and the central frame is supported vertically on a segment of a circular arch, however free to rotate section B-B (Fig. 6). A specimen is inserted and fixed in position at the two end frames that serve as supports for torsion. On the central frame the specimen is supported vertically as well.at an eccentricity of 750 mm a vertical load is applied. The spans of 440 mm are extremely small, so that pure bending and corresponding shear are negligible. The actual stress state is one of almost pure torsion. The load cell accuracy is 10 N. To allow for carefully monitored loading, load rate may be kept to a low 50 N/second. It should be clear that since the tested specimens are short the effect of prestressing is lost, therefore a non reinforced specimen is actually tested. An actual view of the machine may be seen in Figs.7&8. 5 TESTING UNDER PURE TORSION 18 specimens were cut from 1200 mm wide slabs: 10 specimens from 300 mm, 5 from 250 mm and 3 from 200 mm thick slabs. Specimen s length was equal to the slab thickness. Tests were conducted with the aid of the machine described previously, monitored by a Figure 7. Overview of the testing machine (top). load cell having an accuracy of 10 N, at an average rate of 50 N/second. In the case of testing for torsion consideration about top or bottom layer is irrelevant. Fig. 9 and Fig. 10 show fractured specimens of 200 mm and 300 mm thick slabs respectively. An inclined crack is clearly visible in both cases though the complexity of the sections does not allow a simple link between inclination angle and any of the section properties. The tests results are given in Table 2. In view of the complexity of the ruptured inclined section, which is difficult to define, a simple model was adapted, given 529

4 Figure 8. Overview of the testing machine (side). Table 2. Fracture stress results (MPa) tested in pure torsion. Slabs Thickness Test Sample 300 mm 250 mm 200 mm 19/ / / / / / / / / / / / / / / / / / Average Standard deviation Figure mm thick specimen. Figure 11. Simplified model for shear stress distribution. Figure mm thick specimen. in Fig. 11. Obviously the model does not represent the real rupture mode, however it does provide a simple assessment tool. A uniform stress distribution is assumed over both layers, in opposite directions. The shear stresses so derived are given in Table 2. Had a parabolic distribution been assumed for the shear stresses a maximum shear stress 50% higher would be obtained. It was decided not to search for an optimum or better suited stress distribution considering the fact that any distribution along a section as given in Fig. 11 is not representing the true rupture section, so that Table 2 provides values that can be useful for design and also provide an insight for a size effect. Stresses are in the range of 40% to 75% of the average concrete tensile strength f ctm. Again it is clear that torsion resistance grows with reducing thickness of the slabs. Again it is seen that 250 mm slabs exhibit (probably due to geometry of the section reasons) some small weakness. The standard deviation for the shear stress results due to torsion is of the order of magnitude of that for concrete tensile strength, however, the standard deviation for 250 mm slabs is considerably higher. 6 CONCLUSIONS Experimental analysis of the transverse flexural bending and of pure torsion of Prestressed Precast Hollow 530

5 Core Slabs was carried out and the results presented here. In flexural bending a reliable average rupture strength of 75% to 100% of f ctm for the bottom layer and 60% to 80% of f ctm for the top layer were demonstrated (300 mm reducing to 200 mm thick slabs). It is possible that improved manufacturing methods resulting in better uniformity may reduce these differences. Torsion rigidity was proved to exist also. Tests results show maximum shear stresses of 40% to 75% of f ctm (again growing with reduced thickness) in a very simplified model. The testing machine developed for the torsion tests proved to be efficient, for testing full scale specimens, reliable and simple. Both tests series provide useful tools for design and research. ACKNOWLEDGEMENT The technical and financial sponsoring of Spancrete Israel and the invaluable assistance of Eng. Yoram Gotlieb M.Sc. P.E. are gratefully acknowledged. REFERENCES Broo, H., Lundgren, K. & Engstrom, B Shear and Torsion in Prestressed Hollow Core-Units: Finite Element Analyses of Full-ScaleTests. Structural Concrete, fib 8(2): Pisanty, A.& Regan, P.E Direct Assessment of the Tensile Strength of the Web in Prestressed, Precast, Hollow- Core Slabs. Materials & Structures, RILEM, 24(144): Pisanty, A The Shear Strength of Hollow Core Slabs. Materials & Structures, RILEM, 25(148): Pisanty, A Assessment of the Lateral Bending Stiffness of Prestressed, Precast, Hollow-Core Wide Slabs. (Res ) National Building Research Station, Final Report, Nov. 1997, (in Hebrew). Pisanty, A Development of Testing Machine for Investigation of thetorsional Resistance of Prestressed, Precast, Hollow-Core slabs. Report to Spancrete Israel, (In Hebrew). Van Acker, A. et al Special design considerations for precast prestressed hollow core floors. Fib bulletin 6. International Federation for Structural Concrete, Lausanne. 531

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