CONCRETE BEHAVIOUR AND CRACK CONTROL OF PRESTRESSED CONCRETE BEAMS WITH FIBRES

Transcription

1 BEFIB2012 Fibre reinforced concrete Joaquim Barros et al. (Eds) UM, Guimarães, 2012 CONCRETE BEHAVIOUR AND CRACK CONTROL OF PRESTRESSED CONCRETE BEAMS WITH FIBRES Alena Kohoutková *, Iva Broukalová * * Czech Technical University in Prague, Department of Concrete and Masonry Structures Thákurova 7, Prague, Czech Republic akohout@fsv.cvut.cz, web page: Keywords: SFRC; prestressed; beam. Summary: The paper presents SFRC (steel fibre reinforced concrete) prestressed girder. Two pieces of the girder shall serve as a precast foot-bridge. Basic information about the design and full-scale tests is given. Two types of tests were performed load-test where deflections of the girder were followed and test of anchors for the railing. The girder was analysed in finite element numerical simulation. Brief description and results of FE simulation are presented. 1 INTRODUCTION Significance of fibres in FRC (fibre reinforced concrete) is not only in improvement of the performance of FRC in comparison to the plain concrete but also in application in members with rebar reinforcement reinforced concrete structures and prestressed structures. FRC provides higher ductility, profitable layout of cracks, eventually higher tensile strength; some types of fibres may prevent excessive cracking due to shrinkage at early stages of concrete hardening. Utilisation of fibre reinforced concrete is efficient particularly in precast members. Production control in precast plants provides better quality of concrete mixture and guaranteed material properties of SFRC. In present bad economic situation the producers market strategy aims to higher efficiency of the production and advantageous offers for customers. The decrease of the price of product can lead to increasing sales and thus earnings for the producer. Nevertheless the utilisation of fibre reinforced concrete in precast members and in structures in general is not very often in the Czech Republic although it may bring benefits regarding savings of workability, decrease of rebar steel consumption and favourable behaviour of the structural element. Presented precast girder for the foot-bridge is one of the rare exceptions of application of SFRC in structural use. 2 FOOT-BRIDGE PRESTRESSED SFRC GIRDER In the production program of SMP precast plant from the Vinci Group are already two products from FRC. The manufacturability of FRC precast members has been verified; the SMP precast plant has gained knowledge and skills in production of precast structural elements and proved competitiveness of FRC products. This experience is a basis of enterprise to manufacture a larger FRC element a prestressed foot bridge without conventional rebar reinforcement. The foot-bridge has a solid cross-section with dimensions ca 400 mm x 1000 mm. The decision not to design thin-walled element was given by the demand of producer that the foot bridge shall embody only pre-stressed reinforcement without any other rebar reinforcement. Minimizing of conventional bounded rebar reinforcement diminishes workability and time consumption in the manufacturing of the element and thus the manufacturing becomes cheaper.

2 2.1 Lay-out of foot-bridge The foot-bridge is designed from two parallel girders with a gap between them. The two girders are connected at end supports by monolithic cross-beams. Length of the girders is 14 meters; depth is designed as 1/35 of the length, i.e. 0.4 m. The mid gap is 100 mm wide and it is covered by a composite grid. The foot bridge is drained in the mid gap. The inner sides of the girders are furnished with drip edges so that the water would not flow down along the sides of the girders. Asphalt based waterproofing of the bridge is assumed. The bridge railing is anchored in outer sides of the girders. Wave shaped anchors were tested in pull-out test. The manipulation lifting anchors are steel bar loops. The steel bar loops are anchored in SFRC and via safety-pin in a form of mild rebar reinforcement. The rebars stick out from the face end of the girders and they will provide lapping of girders in end cross-beams. The girders with cross-section in a shape of irregular hexagon (see fig. 2, 3, 4) are made from SFRC with compressive strength that corresponds to concrete class C 55/67 XF4, XD3, XC4 with 40 kg of steel fibres per cubic meter. The steel fibres with diameter 0.8 mm are 60 long; their tensile strength is 1200 MPa. Proportions of the SFRC mixture are in the table 1. Table 1 SFRC mix composition Component Weight per m 3 Cement CEM I kg/m 3 Fine coarse kg/m 3 Aggregate kg/m 3 Aggregate kg/m 3 Water 167 kg/m 3 Steel fibres 40 kg/m 3 Grounded lime Microsilica Plasticiser The girder is pre-tensioned; prestressing is provided with ten strands Ø15.7 mm with strength 1570/1770 MPa. Full prestressing is applied at midspan of the beam; at the ends the tendons are partially separated. Figure 1 Longitudinal elevation of the half of the girder 2

3 Figure 2 Lay out of the foot-bridge cross-section Concreting was performed in the precast plant SMP CZ, a.s. in the Czech Republic. The concreting procedure was recommended in the direction from end of girders to the centre. The SFRC flew in below the strands and the whole section is totally homogeneous. The concreting was accompanied by compression tests on cubes 150/150/150 mm and four-point bending test on prisms 150/150/700 mm. 3

4 Figure 3 End section of the girder Figure 4 End section of the girder with ten prestressing strands and mild rebar reinforcement for anchoring of the precast girder to in-situ cross-beams 4

5 3 TESTING 3.1 Full-scale test The girder was subjected to the load test according to Czech standards. The deflection and cracking were followed. The tested 14 meters long beam was tested as a simply supported beam with span 13.6 m. The load was applied by four panels with dimensions 3x1.2x0.22 m and load ca 20 kn. The panels were placed on two beddings each at distance 1.4 m from the centre of the girder. The panels load was applied in four steps after placing of each additional panel a deflection was measured in three transversal sections at both supports and at midspan. At each section two measuring points were located on the sides of girder; that means 6 measuring point altogether. 2.8 m 13.6 m Figure 5 Set-up of the load test of the girder (cross = measurement point on both sides of the girder) Loading of two panels stands for straining approximately equal to normal operating loads for common pedestrian bridges. Loading with third and fourth panels verified behaviour of overloaded structure. For this loading (due to four panels, i.e. ca 80 kn at midspan) the stresses at the bottom edge of the beam get close to tensile therefore cracking or other defects were monitored. Naked eye observing before, during and after test did not discover any cracking, damage and other abnormalities in the girder behaviour. Measurements were compared to deflections calculated assuming elastic theory and deflections calculated in numerical simulation (fig. 7). Table 2 Comparison of deflections measured in the load-test, calculated in hand and calculated in numerical simulation Loading Deflection Deflection calculated Deflection measured determined in assuming simple in the load-test [mm] numerical simulation elastic theory [mm] [mm] 1 panel [20 kn] panels [40 kn] panels [60 kn] panels [80 kn]

6 10:00:00 11:00:00 12:00:00 13:00:00 14:00:00 15:00:00 16:00:00 Defletion [mm] BEFIB2012: Alena Kohoutková and Iva Broukalová. Figure 6 Setting of the last panel in the load test of the girder Time Deflection at midspan - right side Deflection at midspan - left side Deflection - elastic theory Deflection - numerical simulation Figure 7 Comparison of theoretical assumptions and real deflections measured in the load test 6

7 3.2 Testing of anchors for railing Wave shaped anchors for railing posts were tested in pull-out test. Two tests of railing anchors were performed. In the first pull-out test the anchor failed due to rupture of the steel bar in the body of the anchor. In the second pull-out test the steel bar ruptured at the connection of the bar with the screw chasing nut (fig. 7). The failures occurred at loads 7.5 times higher than guaranteed loadbearing capacity of the anchors (i.e. 90 kn and 92 kn respectively). The failure mode was different than failure mode of reference concretes without fibres. In concretes without fibres the crack occurs at the tip of anchor and the anchor is usually pulled-out of the concrete body. The confinement of fibres prevents cracking in concrete and changes the failure mode from pull-out to rupture of steel anchor. Figure 8 Testing of the railing anchors. Figure 9 Railing anchor after the test picture of the ruptured steel rebar 7

8 4 ANALYSIS OF THE GIRDER The girder of the footbridge was designed by methods of linear elasticity. Also non-linear calculations were performed by finite element method. Non-linear analysis was performed in ATENA program developed especially for analyses of concrete structures. The load-test was simulated in nonlinear analysis. A quarter of the girder was modelled in the program. Supporting was chosen so that the model would describe real supports and conditions of symmetry of the girder and loading. The FE mesh is created by the mesh generator from regular 3D bricks; the dimension of the FE brick element was 0.1 m. The pre-stressing cables were modelled as discrete reinforcement. Concrete with dispersed reinforcement was input as a homogeneous material with properties corresponding to real behaviour of steel fibre reinforced concrete (SFRC). Material parameters of SFRC were determined based on conventional tests of cubes (compressive strength) and prisms (flexural test) by means of inverse analysis. ATENA program offers several implemented material models. For modelling of behaviour of the girder implemented material model 3D NLC2 (NonLinear Cementitious 2) was used. Particular values of material parameters are listed in the table 3. Table 3 Material parameters for non-linear simulation in ATENA program Elastic modulus 38 GPa Tensile strength 4,2 MPa Compressive strength 68 MPa Fracture energy 106 N/m Softening compression Reduction factor of compressive strength in cracked concrete 0,8 The numerical model had the same set-up as the real load-test. The loading was applied in small increments until the value of load equal to the real load-test was reached. Numerical simulation of the load test proved results of the real loading. Minimal number of cracks was calculated in the simulation. The cracks had thickness up to 0.05mm these hair cracks are almost invisible. Figure 10 Stresses determined in numerical simulation 8

9 5 CONCLUSIONS The results of the pilot plant proved potential of steel-fibre-reinforced-concrete for use in prestressed structures without additional rebar reinforcement. The SFRC in the foot-bridge was homogeneous and the girder had sufficient resistance. Non-linear analysis shows possibilities of numerical simulation in the designing of fibre reinforced structures. ACKNOWLEDGEMENT The contribution was elaborated with support of project FR-TI2/496 and SGS11/106/OHK1/2T/11 using the findings from GA CR 103/09/1788. REFERENCES [1] PONTEX: Report of the Load-test [2] Batal I., Brejcha V., Fidranský P., Němec P.: Drátkobetonový nosník pro lávky pro pěší výroba a zkoušky, Proceedings Betonářské dny [3] Červenka, V., Jendele, L., Červenka, J.: ATENA Program Documentation Červenka Consulting, 2005 [4] Kohoutkova A., Broukalova I.: Utilisation of Fibreconcrete in Structural Elements for Increasing of Durability, Proceedings Life Cycle Assessment, Optimisation, Behaviour and Properties of Concrete Structures, Brno 2008 [5] Kohoutkova A., Broukalova I.: Simulation as a Tool for Decision - Making Process for Applications of FRC in Precast Elements, Proceedings of The 3rd Central European Congress on Concrete Engineering, Innovative Materials and Technologies for Concrete Structures, Visegrad 2007 [6] Veselý V., Kohoutková A., Vodička J., Smiřický S., Krátký J.: Branded Fibreconcrete from Development to Practical Usage, Proceedings 6th International Conference Fibre Concrete Technology, Design, Application, Prague