英文原文：
Rehabilitation of rectangular simply supported RC beams with shear deficiencies using CFRP composites
Ahmed Khalifa a,＊, Antonio Nanni b
a Department of Structural Engineering，University of Alexandria，Alexandria 21544，Egypt
b Department of Civil Engineering，University of Missouri at Rolla，Rolla，MO 65409，USA脑神经衰弱
Received 28 April 1999；received in revid form 30 October 2001；accepted 10 January 2002江西高考成绩
Abstract
The prent study examines the shear performance and modes of failure of rectangular simply supported reinforced concrete(RC) beams designed with shear deficiencies。The members were strengthened with externally bonded carbon fiber reinforced polymer （CFRP）sheets and evaluated in the laboratory. The experimental program consisted of twelve full—scale RC beams tested to fail in shear. The variables investigated within this program included steel stirrups, and the shear span-to—eff
ective depth ratio, as well as amount and distribution of CFRP。The experimental results indicated that the contribution of externally bonded CFRP to the shear capacity was significant. The shear capacity was also shown to be dependent upon the variables investigated。Test results were ud to validate a shear design approach, which showed conrvative and acceptable predictions.错误!2002 Elvier Science Ltd. All rights rerved.
Keywords: Rehabilitation; Shear；Carbon fiber reinforced polymer
1. Introduction
Fiber reinforced polymer (FRP) composite systems, compod of fibers embedded in a polymeric matrix, can be ud for shear strengthening of reinforced con—crete （RC) members ［1–7]. Many existing RC beams are deficient and in need of strengthening。The shear failure of an RC beam is clearly different from its flexural failure。In shear, the beam fails suddenly without sufficient warning and diagonal shear cracks are consid—erably wider than the flexural cracks ［8］。
The objectives of this program were to：
1. Investigate performance and mode of failure of simply supported rectangular RC beams with shear deficien—cies after strengthening with externally bonded CFRP sheets。
2。Address the factors that influence shear capacity of strengthened beams such as：steel stirrups，shear span—to—effective depth ratio (a/d ratio）, and amount and distribution of CFRP。
3. Increa the experimental databa on shear strength-ening with externally bonded FRP reinforcement。
4. Validate the design approach previously propod by the authors [9］。
For the objectives，12 full-scale，RC beams designed to fail in shear were strengthened with different CFRP schemes. The members were tested as simple beams using a four-point loading configuration with two different a/d ratios.
2. Experimental program
2。1. Test specimens and materials
Twelve full-scale beam specimens with a total span of 3050 mm。and a rectangular cross-ction of 150—mm—wide and 305—mm-deep were tested. The specimens were grouped into two main ries designated SW and SO depending on the prence of steel stirrups in the shear span of interest.
Series SW consisted of four specimens。The details and dimensions of the specimens designated ries SW are illustrated in Fig. 1a。In this ries, four 32-mm steel bars were ud as longitudinal reinforcement with two at top and two at bottom face of the cross—ction to induce a shear failure. The specimens were reinforced with 10-mm steel stirrups throughout their entire span。The stirrups spacing in the shear span of interest，right half，was lected to allow failure in that span。
科技的名言名句Series SO consisted of eight beam specimens, which had the same cross-ction dimension and longitudinal steel reinforcement as for ries SW. No stirrups were provided in the test half span as illustrated.
Each main ries （i.e. ries SW and SO) was subdivided into two subgroups according to shear span-to—effective depth ratio。This was lected to be a/d = 3 and 4, resulting in the following four subgroups: SW3;SW4; SO3; and SO4.
The mechanical properties of the materials ud for manufacturing the test specimens are listed in Table 1.Fabrication of the specimens including surface preparation and CFRP installation is described elwhere [10］。
2。2. Strengthening schemes
One specimen from each ries (SW3-1，SW4-1, SO3-1 and SO4-1）was left
without strengthening as a control specimen，whereas eight beam specimens were strengthened with externally bonded CFRP sheets following three different schemes as illustrated in Fig. 2。
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In ries SW3，specimen SW3—2 was strengthened with two CFRP plies having perpendicular fiber directions (90°/0°）. The first ply was attached in the form of continuous U-wrap with the fiber direction oriented perpendicular to the longitudinal axis of the specimen （90°)。The cond ply was bonded on the two sides of the specimen with the fiber direction parallel to the beam axis（0°）.This ply ［i.e. 0°ply］was lected to investigate the impact of additional horizontal restraint on shear strength。
In ries SW4, specimen SW4-2 was strengthened with two CFRP plies having perpendicular fiber direction (90°/0°）as for specimen SW3—2.
Four beam specimens were strengthened in ries SO3。Specimen SO3—2 was strengthened with one-ply CFRP strips in the form of U—wrap with 90°-fiber orientation。The strip width was 50 mm with center—to—center spacing of 125 mm. Specimen SO3-3 was strengthened in a manner similar to that of specimen SO3—2, but with strip width equal to 75 mm。Specimen SO3-4 was strengthene
d with one-ply continuous U-wrap （90°）。Specimen SO3-5 was strengthened with two CFRP plies (90°/0°）similar to specimens SW3—2 and SW4—2.
In ries SO4, two beam specimens were strengthened。Specimen SO4—2 was strengthened with one—ply CFRP strips in the form of U—wrap similar to specimen SO3-2. Specimen SO4—3 was strengthened with one—ply continuous U-wrap （90°) similar to SO3—4。
2。3. Test t-up and instrumentation
All specimens were tested as simple span beams subjected to a four-point load as illustrated in Fig。3。A universal testing machine with 1800 KN capacity was ud in order to apply a concentrated load on a steel distribution beam ud to generate the two concentrated loads。The load was applied progressively in cycles, usually one cycle before cracking followed by three cycles with the last one up to ultimate. The applied load vs。deflection curves shown in this paper are the envelopes of the load cycles.梦见生了个女儿是什么意思
Four linear variable differential transformers （LVDTs) were ud for each test to monitor vertical displacements at various locations as shown in Fig. 3. Two LVDTs were located at mid—span on each side of the specimen. The other two were located at the specimen supports to record support s
ettlement。
For each specimen of ries SW, six strain gauges were attached to three stirrups to monitor the stirrup strain during loading as illustrated in Fig。1a. Three strain gauges were attached directly to the FRP sheet on the sides of each strengthened beam to monitor strain variation in the FRP。The strain gauges were oriented in the vertical direction and located at the ction mid-height with distances of 175, 300 and 425 mm，respectively, from the support for ries SW3 and SO3. For beam specimens of ries SW4 and SO4，the strain gauges were located at distance of 375，500 and 625 mm, respectively，from the support.
3。Results and discussion
In the following discussion, reference is always made to weak shear span or span of interest.
3。1. Series SW3
Shear cracks in the control specimen SW3-1 were obrved clo to the middle of the shear span when the load reached approximately 90 kN。As the load incread, additional shear cracks formed throughout，widening and propagating up to final failure at a load of 253 kN
In specimen SW3—2 strengthened with CFRP (90°/0°）, no cracks were visible on the sides or bottom of the test specimen due to the FRP wrapping. However，a longitudinal splitting crack initiated on the top surface of the beam at a high load of approximately 320 kN.
The crack initiated at the location of applied load and extended towards the support. The specimen failed by concrete splitting at total load of 354 kN. This was an increa of 40% in ultimate capacity compared to the control specimen SW3-1. The splitting failure was due to the relatively high longitudinal compressive stress developed at top of the specimen，which created a transver tension, led to the splitting failure. In addition, the relatively large amount of longitudinal steel reinforcement combined with over—strengthening for shear by CFRP wrap probably caud this mode of failure. The load vs。mid—span deflection curves for specimens SW3—1 and SW3-2 are illustrated ，to show the additional capacity gained by CFRP.
The maximum CFRP vertical strain measured at failure in specimen SW3—2 was approximately 0.0023 mm/mm, which corresponded to 14％of the reported CFRP ultimate strain。This value is not an absolute becau it greatly depends on the
整合营销策划>我要制作相册location of the strain gauges with respect to a crack。However，the recorded strain indicates that if the splitting did not occur, the shear capacity could have reached higher load。
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Comparison between measured local stirrup strains in specimens SW3—1 and SW3-2 are shown in Fig。6. The stirrups 1, 2 and 3 were located at distance of 175, 300 and 425 mm from the support, respectively. The results showed that the stirrups 2 and 3 did not yield at ultimate for both specimens. The strains (and the forces) in the stirrups of specimen SW3—2 were，in general，smaller than tho of specimen SW3-1 at the same level of loading due to the effect of CFRP。
3。2。Series SW4
In specimen SW4-1，the first diagonal crack was formed in the member at a total applied load of 75 kN. As the load incread, additional shear cracks appeared throughout the shear span。Failure of the beam occurred when the total applied load reached 200 kN. This was a decrea of 20% in shear capacity compared to the specimen SW3-1
In specimen SW4-2，the failure was controlled by concrete splitting similar to test specimen SW3—2. The total applied load at ultimate was 361 kN with an 80％increa in shear capacity compared to the control specimen SW4-1. In addition，the measured strains in the stirrups for specimen SW4-2 were less than tho of specimen SW4-1. The applied load vs. mid-span deflection curves for beams SW4-1 and SW4—2 are illustrated . It may be noted that specimen SW4—2 resulted in greater deflection when compared to specimen SW4-1.
When comparing the test results of ries SW3 specimens to that of ries SW4, the ultimate failure load of specimen SW3-2 and SW4—2 was almost the same。However, the enhanced capacity of specimen SW3-2 （a/d=3) due to the addition of the CFRP reinforcement was 101 kN, while specimen SW4-2 （a/d=4）was 161 kN. This indicates that the contribution of external CFRP reinforcement may be influenced by the ayd ratio and appears to decrea with a decreasing a/d ratio。Further, for both strengthened specimens (SW3-2 and SW4—2），CFRP sheets did not fracture or debond from the concrete surface at ultimate and this indicates that CFRP could provide additional strength if the beams did not failed by splitting。
3.3. Series SO3
Fig。8 illustrates the failure modes for ries SO3 specimens。That details the applied load vs。mid—span deflection for the specimens.
