18 TH INTERNATIONAL CONFERENCE ON COMPOSITE MATERIALS CREEP OF FULLY OR PARTIALLY FRP-CONFINED SQUARE OR CIRCULAR CONCRETE COLUMNS Y. S. Ma, Y. F. Wang*, B. Han, M. H. Liu School of Civil Engineering , Beijing Jiaotong University , Beijing , China *Corresponding author (cyfwang@bjtu.edu.cn) Keywords : Concrete columns; FRP; Square; Strap; Creep; Tests. 1 Introduction modified Model B3 [6, 7] for the creep of concrete Externally wrapping fiber-reinforced polymer (FRP) with fly ash or silica fume and Findley’s power law composites around concrete columns has found [8] for the creep of FRP, considering the state of increasingly wide applications in strengthening and triaxial stresses of concrete core, the effectively retrofitting of existing concrete structures. As confined core area, and the interaction between demonstrated by extensive published studies, the concrete core and FRP. uses of FRP jackets, hoops and spirals for transverse confinement are all particularly suited for gaining 2 Materials and experiments better short-term behaviors of concrete columns 2.1 Experimental program under axial compressive loading, which involve the A total of four square specimens in the size of concrete compressive strength, ultimate compressive strain and ductility capacity [1, 2]. However, few 150×150×400mm, wrapped with two layers of unidirectional AFRP laminates fully, and six circular investigations [3, 4] are related to the creep behavior specimens in the size of 150×450mm, wrapped with of FRP wrapped concrete columns (FWCCs), with the limitations as: the loading process is not long two layers of AFRP straps, were tested in laboratory. The AFRP straps were 100mm in width with the enough for developing creep fully; the concerned confining materials are glass FRP (GFRP) and overlap of 150mm and the spacing of 75mm along the column’s axis. One half of them were used to carbon FRP (CFRP), without aramid FRP (AFRP); perform the creep study, and parallel shrinkage test the confining form involved is only FRP jackets; square or rectangular columns confined with FRP was carried out on the other half specimens. The actual creep strains were calculated by subtracting have not yet been tested for creep; and the influence shrinkage values from total time-dependent strains of concrete composition and strength can’t be analyzed by models. of the corresponding specimens under loads. The specimen dimensions and the arrangements of Actually, the fact that structures have been built does the AFRP confinement of the tested specimens are not mean the process is complete. The long-term behavior is an important issue, which reflects the shown in Fig. 1. Also shown in this figure is the effectiveness of FRP confinement in the horizontal evolvement of materials and structures' lives. and vertical planes, which will be discussed later. Especially for concrete and FRP laminates both having obvious creep behavior, the design and detailed analysis of this hybrid column can be performed in safe and economic ways better as long as a thorough understanding of the long-term deformation property is available. This paper focuses on the experimental work on the creep of square concrete columns wrapped with AFRP laminates fully and circular concrete columns wrapped with AFRP straps. Ten square or circular specimens were tested for 312 or 385 days respectively, with the main variables of the composition and compressive strength of the concrete core. For theoretical study, a creep model for this hybrid column was developed based on the Fig. 1. Dimensions and arrangements of AFRP. Model B3 [5] for the creep of plain concrete or
The test variables in this experiment were the were taken on site to check the quality of the composition and compressive strength of the concrete used. One day after casting, the concrete concrete core. Five concrete batches, labeled A, B, C, columns were demolded and cured in a controlled environment of 20±2 ℃ and relative humidity > 95% D and E, were used. Correspondingly, the creep specimens were named as FWCC-A, FWCC-B, for 20 days. Meanwhile, three cubes were prepared FWCC-C, FWCC-D and FWCC-E. The summary of for each group and cured in the same manner as the the specimen test matrix is given in Table 1. column specimens to measure the concrete compressive strength at 28 days. 2.2 Materials and properties Before wrapping the AFRP laminates, square The cementitious materials of concrete core used in specimens were rounded off to have the corner the test were ordinary Portland cement, fly ash and radius of 15mm with a grinding machine to avoid silica fume. River sand was used as the fine local stress concentration. The AFRP laminates were aggregate and crushed granite stone with a wrapped fully or discretely around the columns in a maximum size of 40mm was used as the coarse wet lay-up process, with the main fibers orientated aggregate. The densities of the fine and coarse in the hoop direction of the columns. Effective joints aggregate were 2.65×10 3 kg/m 3 and 2.74×10 3 kg/m 3 were attained by overlapping the laminates 150mm respectively. The composition and compressive in length. The hybrid columns were then cured for 8 strength of the concrete are shown in Table 2, where days, with a controlled temperature of 20±2 ℃ and a the compressive strength was determined based on relative humidity of 95% above. the average measurement of three identical 100mm For measuring the deformation, each creep specimen concrete cubes from the same batch. was instrumented at its mid-height with an The AFRP laminates were used for transverse embedded vibrating wire strain gauge (DI-25) in the confinement, and epoxy resin was used as adhesive. axial direction. The gauges were then connected to The elastic modulus, ultimate tensile strain and an automated data acquisition system. The same thickness per layer of the AFRP laminate given by gauges were also embedded in the other five manufacturer are 118000MPa, 0.017 and 0.286mm companion specimens that remained unloaded to respectively. correct for non-creep related deformations. At the age of 28 days, the creep specimens were 2.3 Testing procedure and instrumentation subjected to sustained axial stresses corresponding The concrete mix was slowly poured into the steel to the stress-strength ratio of 30% for 312 or 385 moulds by vibration, to prevent segregation and days. Shrinkage was tested parallel to creep. All voids from forming. Specific attention was paid to creep and shrinkage specimens were kept in a the corner zones of square specimens so that good controlled temperature of 20 ℃ throughout the test. compaction was ensured at the corners. Slump tests Table 1. Specimen test matrix Specimen Cross-section Dimension (mm) Confinement type Applied stress (MPa) No. of specimens 150 150 400 FWCC-A Square Fully 9.5 2 150 150 400 FWCC-B Square Fully 9.5 2 150 450 FWCC-C Circular Partially 15.1 2 150 450 FWCC-D Circular Partially 21.0 2 150 450 FWCC-E Circular Partially 21.0 2 Table 2. Composition and properties of concrete Composition (kg/m 3 ) Measured 28-day Concrete cubic compressive Slump (mm) Cement Fly ash Coarse Silica fume Water Sand strength (MPa) aggregate A 47.3 37 229.5 40.5 0 105 765.00 1360.00 B 51.1 37 246.5 43.5 0 105 758.90 1349.10 C 65.9 37 394.7 0 0 150 749.87 1124.80 D 73.0 37 451.6 0 79.69 170 760.44 929.43 E 76.2 37 470.5 0 83.04 155 727.45 964.30
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