Reinforced concrete beam bending reinforcement experimental study
(lanzhou jiaotong university, in gansu, civil engineering college lanzhou 730070)
Abstract: carbon fiber because of its light weight, high strength, corrosion resistance, anti-fatigue performance and so on is superior performance, so CFRP reinforcement in concrete has extensive applications, in recent years in academic and engineering caused wide interest. Based on carbon fiber composite reinforcing steel reinforced concrete beam of experimental data, make a series of analysis, the research of the experimental beams preloading characteristics of load and the influence of the rigidity.
Keywords: reinforced concrete beam; CFRP; Reinforcement; Preloading; Load characteristics; stiffness
1.. Experiment scheme
The experiment were 18 root beam, according to the longitudinal reinforcement rate into A, B two groups. The size of the beam for 150 mm wide, high 250 mm long, 2500, sectional shape, size and reinforcement is shown in figure 1 below. This experiment is the design of concrete strength for C25, concrete compressive strength is 23 MPa, longitudinal reinforcement yield strength for 360 MPa, elastic modulus for 200 GPa, CFRP tensile strength for 3550 MPa, elastic modulus for 235 GPa,, the single CFRP thickness of 0.111 mm.
The experiment test content includes: displacement (cross, the distribution of two support beam position), carbon fiber strain, beam longitudinal reinforcement strain, the strain of concrete and crack development situation.
The experiment process: the simply supported; The length of 2000 mm, pure bending length for 500 mm; CFRP paste length of 1800 mm, width is 120 mm, no additional anchor measures. Loading method: the hydraulic jack reverse two symmetric loading.
2. The experimental results
2.1 the failure process
16 the beams appeared two kinds of failure mode: (1) CFRP plate tensile; (2) the middle caused cracks CFRP plate detachment.
2.2 CFRP dissection of the failure process
From the experiment phenomenon see (figure 3), longitudinal reinforcement yield increased with load after, distribution of the Lord near loading beam bending shear fractures (figure 4 a) width increase quickly. When the Lord curved shear fractures to a certain width (usually in a 1.0 mm or so), the interface cracks on both sides formed the free surface concrete, concrete crack between both sides lost interaction, the Lord curved shear fractures to end a range of concrete was cracking (figure 4 b, the crack formation called time inclined cracks). When loading to certain degree. Time with the main diagonal crack bending shear fractures fellowship form a triangle prism form concrete block. When the time of the diagonal crack width to a certain extent, fiber cloth from the cracks in the concrete cover and at the end of the interface start detachment (figure 4 c). With the load increase, in the period of pure bending concrete cover yield more along the longitudinal reinforcement of approximation of horizontal crack, concrete cover has been stripped of the tendency; Cut section bending in concrete cover depth within the scope of The Times diagonal crack also significantly increased, local also appeared along the longitudinal reinforcement of horizontal crack. FRP sheet material finally detachment, form the triangle prism form of block is still attached to the concrete in fiber sheet material (figure 4 d).
To sum up. CFRP plate can peel divided into three phases: the initial stage, stable stripping stage stripped completely stripped and stage. (1) the initial stripping stage: the Lord bending shear fractures to set the width of the-the degree, in the aft form a TiaoCi inclined cracks, CFRP plate start stripping; (2) the stable stripping stage: CFRP plate after start stripping, with the load increasing CFRP plate to the plate to peel range at the end of the passage direction; (3) complete detachment stage: when stripping goes on to range from sheet material ends about 200 mm, in not load cases, when the fiber cloth in a few minutes of time continuous detachment. When eventually stripped. Because of elastic fiber cloth can release in a moment, in the period of pure bending and loading the concrete cover happened near different degree of fall off, tensile steel exposed.
3. The result analysis
3.1 load-deflection relationship
A1, A2, B1 and B2, four groups of load-deflection curves are respectively as shown in figure 5 to figure 8 shows. The figure of the gods. and
Common reinforced concrete beam similar, CFRP reinforcement of cutting-Hollywood derived.a deflection curve is divided into three stages, namely elastic stage, with crack stage and yield stage. Each phase of the corresponding characteristics respectively load, load cracking yield load and limit load.
And common reinforced concrete beam different is, CFRP reinforced beams will destroy dissection occurred, test records the CFRP plate stripping the initial when load (). This charge is cutting through testing the phenomenon of observation certain. In CFRP plate before departure general will appear the following three features: (1) longitudinal reinforcement happen yield; (2) the load bending cut near crack width increase quickly; (3) when loading to a certain load, CFRP plate will send out a clear and crisp sound. But now can't judge in which specific local dissection occurred, reload 2-3 level will in time at the end of the inclined cracks appear around throughout the paste of the width of the dissection, at this point in the region by hand or other good thing striking the CFRP plate will find empty drum sound, results will CFRP plate appear the full width along the cross section of stripping before load level as stripping load ().
3.2 CFRP plate strain curve
From 9 to 12 figure can be seen, the CFRP plate showed good linear elastic properties, and this with the material itself is the performance of the match. In the longitudinal reinforcement yield before, with the specimens under preloading reinforcement, CFRP plate load-the slope of the same basic strain curve. This shows that the test of the epoxy resin with almost the same proportion and the dosage, strengthening quality is reliable, the test scheme and test right loading. From the strain curve to see, in the same level of the load, in good condition of reinforcement specimens (in addition to the individual specimen outside) the fiber strain are to be higher than under preloading the reinforcement of specimens.
4. Experiment results
4.1 preloading feature the effect of load
From the figure 13 and figure 14 can see, preloading size on the influence of the load characteristics are not obvious. In addition, the test data from table 1 statistical analysis found that: the group with intact for comparison specimens beams, each group in certain specimens under preloading the reinforcement of specimens its yield loads and loads of good beam yield ratio of 0.98 for the mean respectively A1, A2 group 0.98, 1.02 group B1, B2 group 1.06, the variation coefficient were 0.02, 0.03, 0.04 and 0.05; Each group must be shown in this paper.the top specimens of the reinforcement of ultimate load and intact beam limit load of the mean value ratio respectively 1.01 group A1, A2 group 1.01, 1.11 group B1, B2 group 1.13, the variation coefficient were 0.01, 0.04, 0.01 and 0.02.
So from the above analysis, we can find that, in this experiment, the specimens strengthening preloading the reinforcement of the size of the load characteristics after the little influence.
4.2 the influence of the rigidity of preloading
From the graph 5 to figure 8 and table 1 can see, preloading size of group A and B group of reinforced beam tangent stiffness influence each are not identical: group B of the tangent stiffness of reinforced beams line by preloading significantly greater than the effect of group A specimen, figure 15 to figure 16 in two groups of tangent stiffness specimens by preloading size comparison of influence more directly reflected the results.
To sum up, preloading size specimens of strengthening characteristics after the load effect is small, but for stiffness for both in cracks in yield with stage or stage, must be under preloading the reinforcement of the beam tangent stiffness than in good condition the stiffness of reinforced beam.
. Design specification for concrete structures [S]. Beijing: China architecture &building press, 2002.
. ZhaoTong, XieMin, wear self-improvement. Of carbon fibre reinforced concrete beam subjected to bending capacity of experimental study [J]. Architectural structure. 2001, 30  : 11 to 15.
. DengZongCai. CFRP reinforced concrete beams increase bending mechanical properties [J]. China journal of highway and transport, 2001, 9 (2) :
. CECS 146-2003, carbon fiber strengthened concrete technology procedures [S].