Near-surface mounted (NSM) FRP rod strengthening method

Một phần của tài liệu Shear strengthening of reinforced-concrete beams using advanced composite materials (Trang 57 - 60)

CHAPTER 2 BACKGROUND AND LITERATURE REVIEW

2.4 Near-surface mounted (NSM) FRP rod strengthening method

Very limited experimental and analytical studies are conducted to date on the use of NSM FRP rods for shear strengthening of RC beams. A review of experimental studies on shear strengthening of RC beam with NSM FRP rod is reported as follows:

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De Lorenzis and Nanni (2001)

De Lorenzis and Nanni (2001) studied the performance of RC beams strengthened in shear with NSM CFRP rods. A total of 8 T-beams were tested. The cross-sectional dimensions of the beams were 152 mm × 355 mm and the span was 3000 mm. The parameters studied were: (i) spacing of rods, (ii) inclination of rods, (iii) presence of an anchorage in the flange, and (iv) presence of internal steel stirrups. The specimens were tested under four-point loads with a shear length ratio a/d=3. With the exception of the specimen with transverse steel, which failed in flexure, all the specimens strengthened in shear with FRP NSM rods failed in shear. Failure was due either to bonding failure of one or more NSM rods, or to splitting of the concrete cover of the longitudinal reinforcement. The results showed that the gains in capacity due to external strengthening can be as high as 106%. The shear capacity of the strengthened beams can be increased by decreasing the spacing of the NSM rods, by anchoring the rods into the flange, or by changing the inclination of the rods from vertical to 45 degrees. This work shows that the use of NSM FRP rods is an effective technique to enhance the shear capacity of RC beams.

De Lorenzis and Nanni (2002)

De Lorenzis and Nanni (2002) investigated the bond between NSM FRP rods and concrete.

The test variables were the bonded length, diameter of the FRP rod, type of FRP material, surface finish of the rod and size of the groove. Twenty two specimens were tested to investigate the effect of the mentioned factors on the bond behavior.

Barros and Dias (2005)

Barros and Dias (2005) presented results of tests carried out on twelve RC T-beams. An experimental program composed of RC T-beams, with a certain percentage of steel stirrups, was carried out to evaluate the effectiveness of NSM with CFRP laminates as a shear strengthening technique. The effect of the percentage and the inclination of CFRP are discussed. Besides, the main results and failure modes are presented and analyzed. According

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to the results of their study, the configuration with laminates at 60° was the most efficient among other shear strengthening arrangements.

De Lorenzis and Teng (2007)

De Lorenzis and Teng (2007) provided a critical review of existing research in NSM strengthening subject, identifying gaps of knowledge, and outlines directions for further research. De Lorenzis and Teng (2006) focused on research work on the structural aspects of NSM strengthening of concrete structures.

Rizzo and De Lorenzis (2009)

Rizzo and De Lorenzis (2009) conducted a research program on shear strengthening of RC beams with NSM reinforcement. A total of nine, 2.0 m-long RC beams with a rectangular 200 mm×210 mm cross-section were tested to analyze the influence on the structural behaviour and failure mode of selected test parameters, i.e. type of NSM reinforcement (round bars and strips), spacing and inclination of the NSM reinforcement, and mechanical properties of the groove-filling epoxy. One beam strengthened in shear with externally bonded FRP laminates was also tested for comparison purposes. All beams had internal steel flexural and shear reinforcement, designed to ensure that unstrengthened and shear- strengthened beams would all fail in shear. The steel tension and compression reinforcement consisted respectively of four and two steel deformed bars with 22 mm nominal diameter.

The steel shear reinforcement consisted of closed double-legged stirrups. One half of each beam starting from mid-span was taken as the “test side”, while the other half was designed as the “strong side”. Only the test side was strengthened in shear with FRP systems and appropriately instrumented with strain gages to monitor the strain distribution in the internal steel stirrups and in the shear strengthening system, as detailed later. CFRP ribbed round bars in epoxy-filled grooves were used as NSM shear reinforcement. The test variables included bar spacing and inclination angle, and anchorage of the bars in the flange. The NSM reinforcement produced a shear strength increase which was as high as 106% in the absence of steel stirrups, and still significant in presence of a limited amount of internal shear reinforcement. Overall, the increase in shear capacity was about 16% for the beam

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strengthened with externally bonded U-wrapped laminate, and ranged between 22% and 44%

for the beams strengthened with NSM reinforcement.

Dias and Barros (2008)

Dias and Barros (2008) conducted an experimental test series on T cross-section RC beams, with different percentages of steel stirrups. The influence of both the percentage and the CFRP strip inclination was discussed. The experimental program consisted of 12 RC T beams. The beams were reinforced to guarantee shear failure mode for all the tested specimens. Beams were tested under three point load configuration to limit shear failure in only one of the beam shear spans. The experimental program consisted of one beam without any transverse steel reinforcement; one beam with transverse steel reinforcement of φ6 spaced at 300 mm; one beam with transverse steel reinforcement of φ6 spaced at 130 mm;

and nine FRP strengthened beams with transverse steel reinforcement of φ6 spaced at 300 mm. The FRP strengthened beams included different CFRP percentages of CFRP laminate on the tested shear span. For RC beams with each of the three percentages of CFRP laminates there were three inclinations for the laminates equal to 90°, 60°, and 45°.

Một phần của tài liệu Shear strengthening of reinforced-concrete beams using advanced composite materials (Trang 57 - 60)

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