20 width (2.6 kips/in.) and an ultimate load of 0.78 kN/mm (4.4 kips/in.) arc observed. 

BEAM TESTS 

Beam details  Thirteen reinforced concrete beams with cross-sectional dimensions of 152 x 254 mm (6x10 in.) and lengths of 2744 mm (108 in.) were cast. The flexure reinforcement of the beams consisted of two No. 5(16 mm) tension bars near the bottom, and two No. 3 (9.5 mm) compression    bars near the top. To avoid shear failure, the beams were over- reinforced for shear with No. 3 (9.5 mm) closed stirrups spaced at 102 mm (4.0 in.). Five beams were formed with rounded corners of 25 mm (1 in.) radius to facilitate the installation of the strengthening material on their sides and bottom faces without stress concentrations. Figure 4 shows the beam dimensions, reinforcement details, support locations, and location of loading points. The  steel used was Grade 60 with a yield strength of 415 MPa (60.000 psi), while the concrete compressive strength at the time of testing the beams was 55.2 MPa (8000 psi).  Strengthening materials  The developed hybrid fabric was used lo strengthen eight beams. Two different thicknesses of fabric were used. The first (H-systcm, t = 1.0 mm) had a thickness of 1.0 mm (0.04 in.), and the second (H-systcm, / = 1.5 mm) had a thickncss of 1.5 mm (0.06 in.). Four other beams were strengthened with three currently available carbon fiber strengthening materials: 

1) a uniaxial carbon fiber sheet with an ultimate load of 0.34 kN/mm (1.95 kips/in.);  

2) two layers of a uniaxial carbon fiber fabric with an ultimate load of 1.31 kN/mm (7.5 kips/in.) for the two layers combined: and

 3) a pultrudcd carbon fiber plate with an ultimate load of 2.8 kN/mm (16 kips/in.). The tested load-strain diagrams for all these materials are shown in Fig. 5. Table 2 shows the properties of the strengthening materials, including the developed fabric. 

Adhesives  

For the hybrid fabric, an cpoxy resin (Epoxy A) was used to impregnate the fibers and as an adhesive between the fabric and the concrete surfacc. This epoxy had an ultimate strain of 4.4% to ensure that it would not fail before the failure of the fibers. For the beams strengthened with carbon fiber sheets, plates, and fabric, an cpoxy resin with an ultimate strain of 2.0% was used (Epoxy B). The mcchanical properties of the adhesives provided by their manufactures are shown in Table 3.  Strengthening  The beam bottom faces and sides were sandblasted to roughen the surface. The beams were then cleaned with acetone to remove din. Two strengthening configurations were used: 1)    strengthening material on the bottom face of the beam only (Beam Group A); and 2) strengthening material on the bottom face and extended up 152 mm (6 in.) on both sides to cover approximately all the flcxural tension portions of the beam (Beam Group B). The strengthening was installed for 2.24 m (88 in.), centered along the length of the beam. The cpoxy was allowed to cure for at least 2 weeks before the beams were tested. For the beams strengthened with the developed hybrid fabric (H-system), two beams were fabricated and tested for each configuration to verify the results. Table 4 summarizes the test beams.  Instrumentation  The FRP strain at midspan was measured by three strain gages loeated at the bottom face of the beam. The steel tensile strain was measured by monitoring the strain on the side surfacc of ihc beam at reinforcing bar level using a DEMEC    (detachable mechanical gage) with gage points for Beam Group A, while strain gages were used for Beam Group B. The midspan deflection was measured using a string potentiometer. The beams were loaded using a hydraulic actuator. The load was measured by means of a load cell. All the sensors were connected lo a data acquisition system to scan and record the readings. 

TEST RESULTS AND DISCUSSION

Control beam  The control beam had a yield load of 82.3 kN (18.5 kips) and an ultimate load of 95.7 kN (21.5 kips). The beam failed by the yielding of steel, followed by compression failure of concrete at the midspan. Test results for the control beam arc shown in the figures of the test results of the strengthened beams (Fig. 6 through 15).