TEST ON THE SEISMIC PERFORMANCE OF FRAME JOINTS WITH PRE-CAST RECYCLED CONCRETE BEAMS AND COLUMNS

Transcription

1 TEST ON THE SEISMIC PERFORMANCE OF FRAME JOINTS WITH PRE-CAST RECYCLED CONCRETE BEAMS AND COLUMNS Jianzhuang XIAO (1), M.M TAWANA (1), and Pujin WANG (1) Department of Building Engineering, Tongji University, Shanghai , China Abstract This paper mainly studies the behavior of the side joint in a frame structure made of pre-cast recycled concrete. There are three different types of steel reinforcement connections, which are by welding, by beam connector and by column connector. The pre-cast frame structure is exposed to loading and an observation on the behavior of the concrete frame side joint is made, this includes the joint cracking, further development of the crack, yielding, and the eventual failure of the joint. An analysis was carried out on these three different reinforcement connection forms. Further more an analysis on the failure mode, load-displacement, ductility, strength, and stiffness of the pre-cast frame structure was carried out. The research findings show that pre-cast frame structure s bearing capacity is similar to that of normal concrete. The ductility, energy consumption and other properties satisfy the design requirements of concrete structures. Keywords: Recycled concrete, precast, reinforced concrete frame, seismic behaviour 1. INTRODUCTION How to deal with massive building waste has become a problem to every government in a society with accelerating development. Recycled aggregate concrete provides a proper solution. While the preparation methods of the recycled concrete are different from those of the conventional concrete and preparation conditions influence the properties of the recycled aggregate concrete greatly, it is advisable to produce it in precast factories in large-scale[1]. Prefabricated components conforms to the future industrialization. There are great prospects in its development. Assembly structures will be greatly utilize prefabricated recycled concrete. Therefore this research is based on recycled aggregate concrete and prefabricated structures, probing behaviour of prefabricated frame joints made of recycled concrete and analyzing it s feasibility. 2. TEST OVERVIEW 2.1 Specimen design 773

2 Specimens used were of three kinds of connections. The beam and column cross-sectional areas are 400mm 200mm and 300mm 250mm respectively. Beams and columns are precast with 100% recycled aggregate concrete of strength C30. Joint sections are later made with 100% recycled aggregate concrete of strength C40. A set (3 concrete test cubes) of concrete are reserved for each specimen with scale of mm3. Specimens and test cubes are cured with the same conditions. The concrete cubes strength was tested a day before the test of specimen was carried out. The results conforms with the research of recycled aggregate concrete by Li Jiabin [2]. Table 1: Mechanical properties of recycled concrete of specimen (C30) No. of Specimen Days (d) Cube Strength f cu, k (N/mm 2 ) Compressive Axial Compressive Strength f = 0.76 f (N/mm 2 ) ck, cu, k Elastic Modulus E c (N/mm 2 ) PJ PJ PJ Table 2: Mechanical properties of joint (C40) No. of Specimen Days (d) Cube Strength f cu, k (N/mm 2 ) Compressive Axial Compressive Strength f = 0.76 f (N/mm 2 ) ck, cu, k Elastic Modulus E c (N/mm 2 ) PJ PJ PJ Table 3: Cross-section and reinforcement of specimen No. of specimen PJ-1 PJ-2 PJ-3 Crosssection dimension (mm 2 ) Beam Longitudinal Reinforcement 5Φ 25 Stirrup Φ 8@ 50 Crosssection dimension (mm 2 ) Column Longitudinal reinforcement 4Φ 25 Stirrup Φ 8@ 50 Joi nt Stir rup Nu ll 774

3 Fig. 1~Fig. 4 and Table 3 shows the reinforcements of the specimen and connections 柱子 梁 2 25 Fig. 1: Reinforcement of beam-column Fig. 2: Reinforcement of PJ-1(Welding) 20 ) ) Fig. 3: Reinforcement of PJ-2(beam connection) Fig. 4: Reinforcement of PJ-3(column connection) 2.2 Loading system Test experiments were performed in a seismic laboratory of the civil engineering department of Tongji University. The laboratory has large-scale reacted truss and portal frames. The bottom of the column is fixed on static pedestal. The top of the column is hinged to the large-scale reacted truss. Load is imposed by a 10t and 5t hydraulic jacks. A steel plate is placed on the top of the column which is exposed to the load point to prevent partial 775

4 destruction (Fig 5). The design of this test is the same with the test carried out by Zhu Xiaohui[3], this enabled us to compare cast-in-situ concrete and precast concrete. GANTRY JACK DISPLACEMENT GANTRY JACK DISPLACEMENT REACTED TRUSS DISPLACEMENT Fig. 5: Test device 2.3 Loading mode The axial compressive ratio nk in this test is 0.30, designed axial compressive ratio is fck, 23.4 n= nk = 0.30 = 0.42 (1) fc 16.7 This test is a quasi-static test, in the displacement control loading phase a repeated load is imposed on the specimen. Low repeated loading is adopted, data is recorded and observation for cracks is done. In all tests carried out, an axial load N is first imposed on the top of the column to conform with the designed axial compressive ratio. The load N remains unchanged during the whole test experiment. The beam section is imposed a repeated load (Fig 6). 776

5 LOAD ON BEAM LOAD ON COLUMN Δ Δ Δ Δ 0 Δ Δ Δ DISPLACEMENT OF BEAM REPEATED TIME Fig. 6: Loading mode REPEATED TIME Load-displacement loading mode is used in this test. After the axial load is imposed, firstly the load on the beam is imposed until a first crack appears at the end of the beam, then unload; continued load is imposed with reasonable steps. In this test the first crack appeared when the load on the beam was 20kN, and the following loads were added at intervals 40kN, 60kN, 70kN, 80kN, 90kN This was continued until the upper reinforced yielded with the displacement Δ, at this moment load addition control began using the displacement loading mode, which were 2Δ, 3Δ, 4Δ, each loading interval was repeated twice so as to analyze the ductility and energy consumption. When the capacity of the beam falls to below 85% of its maximum capacity, the test experiment is discontinued. 2.4 Crack formation and development Under fixed column pressure and recycled beam load, the specimen is beam hinged failure mechanism. The beam forms a plastic hinge first, which satisfies weak beam strong column failure mode. The dimensions of the specimen are the same, except the connection forms are different, but the cracks of the three specimen are nearly the same. There are still some differences among them, and these are as follows:- (1) Since the reinforcement and beam size are the same, the crack load of the specimen are nearly the same. But where the crack appears differently because of the effect of the difference created by concrete casting difference. The crack of PJ-1 and PJ-3 are nearer to the joint, which is 3~5cm; while PJ-2 occurs 15cm from the joint. That may be due to the connections on the beam is weaker and slides between bolt and reinforcements. (2) Cracks on the joint are 45 o clockwise to the axis of the beam, and less damaged in the joint zone than on the beam. That s because the load is in one-direction(downward), energy consumption of the joint can not be fully achieved. (3) Different connections have different failures. Proper reinforcement working well with concrete in PJ-1, leads to cracks appearing at the outer side of the joint (Fig 7a). While for PJ-2, weakness of the reinforcement at connection on the beam is serious, so the yielding platform is short and the reinforcement is pulled off, characteristic of brittle fracture. Therefore it s not recommended in practical engineering projects (Fig 7b). Failure of PJ-3 is 777

6 the same with PJ-1 except the cracks at the bottom of the column are along the reinforcement (Fig 7h). (4) In ductility, PJ-1 has the most beam displacement in failure, which is 187mm. Its concrete on the bottom section of the beam was crushed and fell off from the beam, and on the outer side, concrete cracks are at angle of 45 o along the axis. This proves connections of PJ-1 and PJ-3 have better strength. The displacement of PJ-2 is 51mm, with failure occurring at connections, and its ductility and energy consumption is the worst in all the three specimens. PJ-3 has a displacement of 144mm. Its column damage is worse than the other two specimens, but the column can still bear much load even after the beam fails. And it s recommended for use in practical engineering projects. (a)pj-1 crack on the outer side of joint (b)pj-1 failure mode (c)pj-2 failure mode (d) PJ-2 pulled off reinforcement at connection (e) PJ-3 failure on the side of joint (f)pj-3 failure at back of joint 778

7 (g)pj-3 under column cracks along the reinforcement (h)pj-3 failure mode of the beam Fig. 7: Failure mode (i)pj-3 failure mode of the joint 3. ANALYSIS OF THE RESULTS 3.1 Load-Displacement curve of beam Conclusions are drawn from the three figures (Fig 8): (1) Before cracks appear at the joint or only when little and narrow cracks appear, which is when the load is less than 60% of the maximum load, the area of the curve is little. And when unloading, the curve becomes a line, which shows the joint is in elastic stage. (2) When controlling loading by displacement, the bearing capacity improves, that s because the reinforcement is strengthened. (3) During displacement controlled loading, the area of the curve becomes bigger with bigger displacement, which means energy consumption of the joint increases. (4) The curve of PJ-2 shows the beam connector reinforcement. The strengthening stage of the reinforcement is short because of stress concentration effect and it fails at sudden without obvious deformation. Load (kn) Load (kn) Displacement (mm) Displacement (mm) (a)pj-1 s P-Δ curve (b) PJ-2 s P-Δ curve 779

8 Load (k N) Displacement (mm) (c) PJ-3 s P-Δ curve Fig. 8: P-Δ curve of the three 3.2 Skeleton curve The skeleton curve is the envelope of the peak point of every step. It comprehensively shows the relationship between load and deformation, and it shows the properties of the specimens at different stages. The skeleton curve of the three specimens is shown in Fig 9: ) N ( k d o a L Welded reinforcement Beam connection Column Connection Normal Concrete Frame Displacement (mm) Fig. 9: Skeleton curve of the three Observation: (1) At the very beginning of loading (before elastic limit point), load-displacement curve 780

9 is a straight line, and the skeleton curve is also a straight line. When the beam cracks, the skeleton curve bends, the increase of the load begins to be less than that of the displacement. And the stiffness of the specimen declines. After the yielding of the reinforcement, an obvious inflection point of the skeleton curve can be seen from the figure. The general trend is less load increasing but more displacement, the stiffness of the beam is decreasing. (2) Skeleton curves of PJ-1 and PJ-3 are almost the same, while the displacement of PJ-2 is much less. (3) Cracking point, yield point, peak load point and displacement limit point can be seen obviously from the skeleton curve, which shows that the specimen goes through elastic, elastic-plastic and limit damage stages. 3.3 Stiffness Degradation Stiffness of the overall conversion is defined as the ratio of beam load (kn) and beam displacement, to study the change of stiffness of the specimen in one-way low cyclic loading. The stiffness at characteristics of the load points are set in table 4, the stiffness degradation is shown in figure 10. As drawn from table 4 and figure 10: (1) Stiffness degradation of the three specimens is obvious and almost on the same curve. But PJ-2 fails earliest because of weaker reinforcement. And the degradation of PJ-1 is faster than that of PJ-3. (2) Stiffness degradation of PJ-1 and PJ-3 are both faster at the beginning of loading, and then the degradation slows down with the increase of displacement of the beam and plastic increase. Table 4: Stiffness at characteristic points No. of Specimen PJ-1 PJ-2 PJ-3 Crack load/crack displacement P cr /Δ cr Yield load/yield displacement P y /Δ y Max load/max displacement P max /Δ max Max load/max displacement

10 Fig. 10: Stiffness Degradation for all specimens 3.4 Energy Consumption Table 5~Table 7 show energy consumption under different load rating. And we can see that: (1) At the beginning of loading, energy consumption is little because the specimen is in elastic state. (2) In the two cycles under the same load level, the latter cycle consumes less than the first one because of damage accumulation. (3) Energy consumption of the beam increase with the increase of displacement. Table 5: PJ-1 Energy Consumption Energy Load State Quantity Recycle Time Consumption (kn mm) P=20kN P=40kN P=60kN P=70kN P=80kN Load Control P=90kN P=100kN P=110kN P=120kN P=125kN Δ=34mm Displacement Control Δ=51mm Δ=68mm Δ=85mm

11 Table (continued) Δ=102mm Δ=119mm Δ=136mm Δ=153mm Table 6: PJ-2 Energy Consuming Energy Load State Quantity Recycle Time Consumption (kn mm) P=25kN P=63kN P=73kN Load Control P=83kN P=93kN P=103kN Δ=31mm Δ=35mm Displacement Control Δ=39mm Δ=43mm Δ=47mm Δ=51mm

12 Table 7: PJ-3 Energy Consumption Energy Load State Quantity Recycle Time Consumption (kn mm) P=20kN P=40kN P=60kN P=70kN Load Control P=80kN P=90kN P=100kN P=110kN P=120kN Δ=25mm Δ=42mm Displacement Control Δ=59mm Δ=76mm Δ=93mm Δ=110mm Joint Cracking Capacity The ordinary concrete joint bearing capacity formula raised by Tang Jiuru[4]is: N V = jcr, 0.8ftk, bh j j f bh (2) tk, c c The calculated results and results from the test experiments were compared, see table below: Table 8: Calculated and tests cracking capacity Specimen t Calculated result Test result V, c t c NkN ( ) f, ( MPa ) V, ( kn ) PkN ( ) V, ( kn ) V tk jcr PJ PJ PJ Note: (1) Tensile strength of recycled concrete formula: ftk, = 0.24 fcu, k (2) Relationship between t Vjcr, and load P: Vjcr, = T Vcol We can see from table 8 that it s feasible to use the ordinary concrete formula for recycled jcr jcr jcr, 784

13 concrete, and the surplus factor is more than Joint Derformation In this test the shear stress ( τ )-shear deformation ( γ ) curve of the core joint was ploted. Shear stress is calculated by formula (3), and shear angle is calculated by formula (4). The meaning of the symbols is shown in figure 11. L τ = P L beam column bh (2) d γ = (3) 2 bh There is no stirrup in the joint zone, shear deformation can therefore be considered as plain concrete s shear deformation. As it can be seen from Fig. 13a~Fig 13c, joint damage is not severe in all the three specimens. γ of PJ-1 is the smallest because of welding on the beam, which has good connection between concrete and reinforcement. When the beams fails, the joint is still bearable. That agrees with the assumption of strong joint weak component. h Δ2 d b Δ1 γ T PL C P PL/2 PL/2 N1 Vj N2 P*L1/L2 P*L1/L2 Fig. 11: Calculation of γ Fig. 12: Force of joint core Fig. 13 (a): τ γ curve of PJ-1 joint core Fig. 13 (b): τ γ curve of PJ-2 joint core 785

14 Fig. 13 (c): τ γ curve of PJ-3 joint core 4. CONCLUSIONS The failure mode of recycled aggregate concrete frame under one-direction low cyclic loading is strong column weak beam mode. Recycled aggregate concrete frame has good bearing capacity, deformation capacity and energy consumption capacity. It is able to meet the current code for structural frame design and it s feasible in actual projects. Different connections effect greatly on the bearing capacity of recycled aggregate concrete frames. After a comprehensive consideration of bearing capacity, economy, ease of construction, it is found that reinforcement connection at the column joint (PJ-3) is recommended. ACKNOWLEDGEMENTS This work was granted by the Ministry of Science and Technology (No. 2008BAK48B03) and the Ministry of Education (No. NCET ), and are both highly appreciated. The authors acknowledge other members of the research group who have been engaged in this research project. REFERENCE: [1] Xiao Jianzhuang, Recycled Concrete(2008)[M]. Shanghai: China Building Industry Press, [2] Li Jiabin, Research on Basic Properties of Recycled Concrete(2004.6) [D]. Tongji University, [3] Zhu Xiaohui, Study on Seismic performance of Recycled Concrete Frame Joint(2005.4)[D]. Tongji University, [4]Tang Jiuru, Anti-seismic reinforced concrete of frame joint[m]. Nanjing: Southeast University Press. 786