STUDY ON SEISMIC BEHAVIOR OF RECYCLED CONCRETE FRAME JOINTS UNDER LOW CYCLIC LOAD

Transcription

1 STUDY ON SEISMIC BEHAVIOR OF RECYCLED CONCRETE FRAME JOINTS UNDER LOW CYCLIC LOAD Guoliang BAI (1), Liu Chao (1) (2), Shengwei JIA (1), and Zonggang QUAN (3) (1) School of Civil Engineering, Xi an Univ. of Arch. & Tech., Xi an, China 7155; (2) School of Science, Xi an Univ. of Arch. & Tech., Xi an, China 7155; (3) Xi'an Research and Design Institute of wall & Roof Materials, Xi an, China 7161 Abstract: To understand mechanical characteristics and seismic behaviors of recycled concrete frame joints, the low cyclic loading tests were carried out on three frame interior joints made of recycled aggregate concrete with recycled coarse aggregate replacement rate %, 5% and 1% respectively. The mechanical characteristics, crack development, failure mode, hysteretic curve loops and ductility of joints with different recycled coarse aggregate replacement rates and same axial compression ratio are comparatively analyzed in this paper. The result indicates that failure process of recycled concrete frame joints is similar to that of ordinary concrete frame joints, and the ductility and energy dissipation ability of the former is slightly variant to those of the latter. The frame joints made of recycled aggregate concrete with suitable replacement rate can be applied in building structure, but the frame joints construction must be strengthened. Keywords: recycled concrete; frame joint; failure mode; ductility 1. INTRODUCTION Recycled concrete is cleaned waste concrete, replaced by a certain percentage which is a part or all of recycled concrete aggregate with post-classification [1]. The present research on recycled concrete frame joints such as: Literature [2] and Literature [3] do some research on joints seismic performance under low cyclic load. Since the little research about recycled concrete frame, this paper does a set of recycled concrete frame joint seismic performance under low reversed cyclic load test. Through research the results of the recycled concrete frame joint with different replacement, the ductility and energy dissipation of the recycled concrete frame joint are studied. 638

2 2. TEST OVERVIEW 2.1 Specimen design The main consideration of the experiment is the influence of the change on the rate of recycled concrete aggregate. Recycled coarse aggregate replacement rates were %, 5%, 1% in the frame joints. To study shear capacity of the joint core area, each specimen is designed with the principle of strong specimens, weak joints. It means that the shear capacity of beam and capacity of column are greater than shear capacity of the joint core area, so that shear failure of joints should be occurred. The section size and reinforcement of specimens are the same, as shown in Figure 1. The performance indexes of the coarse aggregates used in this experiment are shown in Table 1. Artificial mix proportion of concrete strength grade is based on the C3 intensity ratio, with the average measured intensity ratio shown in Table 2. Reinforcement is HRB335 grade and the measured intensity meets the requirement. Table 1: Coarse aggregate performance Moisture content Water absorption Crushed index Specimen (%) (%) (%) Natural aggregate Recycled aggregate Fig. 1: Specimen size and reinforcement Table 2: Concrete Mixture Specimen Quality Mixture f Cement: Water: Sand: Natural aggregate: Recycled aggregate cu (MPa) f c (MPa) J- 475:163:54:118: J-5 475:184:54:59: J-1 475:23:54:: Test loading and test content Seismic performance of recycled concrete frame joints is used quasi-static load test method, testing in the Structure and Seismic Laboratory of Xi'an University of Architecture and Technology. The joints are loaded on the column-side, to consider that the P-Δ effect could not be ignored under axial pressure. The axial pressure is loaded by hydraulic jack and counter-force beam, and the standard axial compression ratio is.15. Horizontal load is loaded by electro-hydraulic servo actuator. The experimental data are collected by 1-channel 7V8 data acquisition instrument, and the entire test process is controlled by 639

3 microcomputer and M281 servo controller, as shown as in Figure 2. The loading system is controlled by load - displacement hybrid loading system [4]. The main measuring points: (1) the horizontal load and the corresponding displacement of specimen column-side in every class of cyclic loading; (2) the shear deformation of the core area of the joints; (3) the rotating angle of beam- column plastic hinge area; (4) the relative rotating angle θ of beam-column; (5) concrete strain of joints-core area; (6) longitudinal reinforcement strain and stirrups strain of beam and column, set as seen in Figure 3; (7) load-displacement hysteretic curve; (8) concrete cracking, crack development and distribution of the joints; (9) the local deformation and failure; (1) shear capacity of joints. Fig. 2: Loading device Fig. 3: Sensor location 3. TEST RESULTS AND ANALYSIS 3.1 Test phenomenon Observed by experiment, the failure phenomenon and failure process of 5% and 1% replacement rate of the recycled concrete joints are similar with the natural concrete joints to have first crack, full-crack, ultimate-capacity and failure of the four stages: (1) in the beginning of loading, the structure is flexible working conditions. With the load increasing, the first bending cracks appeared in the core area which closes to the beam. Then the crack appeared which came toward to the core area of joint. As the load added to 7~8 kn, the first diagonal crack appeared in the core area of joint, which called the cracking load; (2) with the cyclic load increasing, so many diagonal cracks have appeared in the core area of the joint which is divided into a number of diamond pieces. Then those diagonal cracks shape a pair of main diagonal crack, this moment structure is in the elasto-plastic; (3) as the cyclic load increasing, the width of the main diagonal cracks are getting widening, meanwhile deformation of specimen increases faster than the increase of the load. The structure is in the plastic stage, and horizontal load P and the shear capacity are maximum. (4) after the maximum horizontal load, the load reduces with the deformation increasing and large areas of concrete has spalled in the core area. When the horizontal load reduces down to the 85% of maximum, the joint fails. Each stage for characteristic pattern of the three specimens is shown in Table 3. 64

4 (a) J- (b) J-5 (c) J-1 Table 3: Each stage characteristic value Fig. 4: Failure pattern Crack Full-crack Ultimate-capacity Failure P(kN) P(kN) (mm) P(kN) (mm) P(kN) (mm) J J J The Load - Displacement hysteretic curve loops and skeleton curve The load-displacement hysterics curve loops of each specimen are shown in Figure 5. The characteristics of hysterics curve loops: at the beginning of loading, hysterics curve loops process along the line cycle. As unloading, residual deformation is almost existent and the structure is flexible working conditions. When the structure has yielded, hysteric curve loops have shaped minor inverted S-type which is not obvious. As the load-system has been controlled by displacement, the deformation of specimen has increased faster than the increasing of loading. At this time unloading-residual deformation has turned up and the hysteric curve loops have shaped inverted S-type which is obvious. This is typical shear failure of the joint core area. While beam longitudinal reinforcement of the joint core area core occur bond-slip, the stiffness of joint occurs degradation. As the horizontal load added to the ultimate load, the residual deformation of joints have increased fleetly and the speed of stiffness degradation increases. The hysteric curve loops have shaped inverted S-type which is very obvious. The three specimens skeleton contrastive curve is shown in Figure 5. Morphologically, the skeleton contrastive curves of three frames joints are similar the replacement rate 5%, 1% of the frame joints are similar to the joints of ordinary concrete. The inflection point and yield step of these joints are obvious, especially the replacement rate 1% of the frame joint. The shear capacity of recycled concrete joints is less than the shear capacity of natural concrete joint. 641

5 P(kN) 15 1 P(kN) P(kN) (a) J- (b) J P(KN) (c) J % 5% 1% (d) Skeleton Curve Fig. 5: Hysteretic curves and skeleton curve 3.3 Ductility and energy dissipation Joint ductility is a very important measurement index which reflects non-elastic deformation capacity of the specimen. The ductility factor is shown in Table 4, and the calculation formula of displacement ductility factor has used the following [5] : u µ = Here, ultimate-load. y u mean that the horizontal displacement of the column top as the loading is at mean that the horizontal displacement of the column top as the loading is y at yield-load. The yield-load is the one of the inflection point of the P- skeleton curve, and yield-displacement is the displacement which is relative to the inflection point. Table 4: The displacement ductility factor of joints Specimen y (mm) u (mm) J J J As can be seen from Table 4, the displacement ductility factors of replacement rate 5%, 1% of recycled concrete frame joints are similar to the ordinary concrete frame joint. The structure is in the seismic energy field when earthquake occur. As the seismic energy is input into the structure, there is a series process of energy absorption and dissipation of structure. When the structure gets into the plastic state, the dissipation capacity is important µ 642

6 for the seismic capacity. The energy absorption of the specimen is reflected by the acreage which is shaped with the hysteretic curves; the energy dissipation of the specimen is reflected by the acreage which is shaped with the unloading hysteretic curves. The energy value Q is calculated by the area of every hysteretic loop, and the larger energy value Q means the well energy dissipation. The energy Q-displacement of joints contrastive curve is shown as Figure 6. As shown as Figure 6, replacement rate 5% and 1% of the recycled concrete frame joints has the same energy dissipation capacity. So the recycled concrete frame joints using in the structure is applicable. Q (kn mm) % 5% 1% 4. CONCLUSIONS Fig. 6: Energy dissipation Seismic behavior of the recycled aggregate replacement rate 5% and 1% recycled concrete joints and natural concrete joint is studied by test experiments under low cyclic loading. The following conclusions are drawn: (1) The test phenomena of the recycled concrete joints was observed under cyclic loading, there are four stages crack, full-crack, ultimate-capacity, failure. (2) As shown as the P- curve, the hysteric curve loops have shaped inverted S-type which is very obvious and it is typical shear failure of joint core area. The skeleton contrastive curves of recycled concrete frame joints and natural concrete frame joint are similar. (3) By the calculation of hysteric loop area, the energy dissipation of recycled concrete frame joints meets the requirements, so the recycled concrete frame joints use in structures is applicable. (4) The joint failure pattern is the shear failure of core area, and the energy dissipation must meet the higher requirements. To avoid the shear failure of core area to occur, this paper advises that reinforcement of joint core area should be strengthened to improve the shear capacity. ACKNOWLEDGEMENTS This work was supported by the National Technology Support Foundation of China, (No. 28BAK48B5) and The Key Project of Natural Science Basic Research Plan in Shanxi Province (29JZ12). The National Key Laboratory of Structural Engineering at Xi an Univ. of Arch. & Tech. (Xi an, P.R. China) is thanked for supporting Technology and Equipment. REFERENCES [1] Jinfeng Shi, Lili Wang, Hao Wang etc. Study on waste coarse aggregate concrete made with recycled concrete, Fly Ash Utilization, Vol 35, No.2 pp ,

7 [2] Corinaldesi V. Moriconi G. Behavior of beam-column jionts made of sustainable concrete under cyclic loading, Journal of Materials in Civil Engineering, Vol 39, No.4 pp , 26. [3] Jianzhuang Xiao, Xiaohui Zhu. Study on Seismic Behavior of Recycled Concrete Frame Joints, Journal of TongJi University(Natural Science), Vol 33, No.4 pp , 25. [4] JGJ11 96, Specification for seismic test method. Ministry of conctruction, P. R. China,1996. [5] Jiuru Tang. Seismic design of reinforced concrete frmne joints. Nanjing: Southeast Uiversitv Press,