【正文】 beams strengthened with carbon fiber sheets, plates, and fabric, an cpoxy resin with an ultimate strain of % was used (Epoxy B). The mcchanical properties of the adhesives provided by their manufactures are shown in Table 3.StrengtheningThe 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 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 (Hsystcm), two beams were fabricated and tested for each configuration to verify the results. Table 4 summarizes the test beams.InstrumentationThe 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 beamThe control beam had a yield load of kN ( kips) and an ultimate load of kN ( kips). The beam failed by the yielding of steel, followed by pression 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). Beam Group ABeam Group A contains ihc heains strengthened at the bottom face only. Figure 6 to 11 show the test results for these beams. The results of Beams H501 and H751 were very close to those of H502 and H752, respectively, and hcc, the discussions concerning these beams arc focused on the last two to avoid repetition. The ductility of each beam is observed by calculating the ductility index as the ratio between the deflection of the beam at failure and its deflection at yield.Figure 6(a) shows the loadvcrsusmidspan deflection diagram for Beam Cl, in which the carbon fiber sheet was used for strengthening. The beam yielded at a load of kN ( kips) and failed at a load of kN ( kips) due to rupture of the carbon fiber sheet. From this figure, it should be noted that, although ductile behavior is experienced, only a 4% increase in the yield load pared with that of the control beam was achieved. A ductility index of was experienced. Figure 6(b) shows the load versus carbon fiber strain at midspan.Figure 7(a) shows the loaddeflection response for Beam C2. This beam was strengthened using the pultrudcd carbon fiber plate. The beam showed no yielding plateau ( ductility index) and had a sudden failure at kN ( kips) due to sheartension failure at the end of the plate. Although an increase in load of 61% was obtained, the failure was brittle. Figure 7(b) shows the load versus carbon fiber strain at midspan. The maximum recorded strain of carbon fiber plate at failure was %, which indicates that 24% of the capacity of the plate was utilized.The loaddeflection response of Beam C3 is shown in Fig. 8(a). Beam C3 was strengthened by two layers of the carbon fiber fabric. The beam yielded at a load of kN ( kips) and failed by fabric debonding at a load of kN ( kips) before showing any significant yielding plateau similar to that of the control beam. A ductility index of was achieved. From Fig. 8(b), it should be noted that the maximum recorded carbon fiber strain at failure was %, which indicates that approximately 48% of the fabric capacity was utilized.Figure 9(a) shows the loaddeflection response of Beam H502. This beam was strengthened with developed I mm thick hybrid fabric. A yield load of kN ( kips) was experienced (a 19% increase in yield load over that of the control beam). It should be noted from Fig. 9(b) that the fabric had a strain of % when the beam yielded. The beam experienced a ductility index of and failed by total rupture of the fabric at an ultimate load of I kN ( kips). Figure 9(c) shows the beam at failure.Figure 10(a) shows the loaddeflection response for Beam H752. The beam was strengthened with mmthick developed hybrid fabric. The beam yielded at a load of kN ( kips) and exhibited a ductility index of before total failure occurred from the debonding of the fabric at an ultimate load of kN ( kips). It is noticed that, although final failure was from the debonding of the fabric, it happened after achieving a reasonable ductility. Figure 10(b) shows that the fabric had a strain of % when the beam yielded. Figure 10(c) shows the beam at failure.Figure 11 and Table 5 pare the results from Beam Group A. The following arc observed: 1. Beams C1 and 11502 exhibited relatively good ductile behaviors. Beam 11502, however, showed a higher yield load than Beam Cl. This is because the developed hybrid fabric was designed so that it has a higher initial stiffness than the carbon fiber sheet。 hence, it contributed to strengthening more effectively than the carbon fiber sheet before the steel yielded。2. Although the carbon fiber fabric had an ultimate load several times greater than the yieldequivalent load of the mmthick hybrid fabric, Beam H752 showed a similar behavior to Beam C3. up to its yield. Beam H752, however, exhibited a reasonable yielding plateau, and Beam C3 did not。 to current carbon fiber strengthening materials, the de