【文章內(nèi)容簡(jiǎn)介】 l of a laboratory building where the average temperature is expected to remain relatively constant with time.Experimental results and discussionUpon loading, the immediate midspan deflections of the beams relative to the measured selfweight deflections were and mm ( and in.) for the control and CFRPstrengthened beams, respectively. Numerous flexural cracks were observed extending from the tension face of the control beam, predominantly over the region of constant bending moment between the point loads. No flexural cracks were observed in Beam 2 at first loading. Such cracking would be hard to see unless significantly wide, due to the existence of the FRP strips. This is because the tensile strain in the FRP will be constant in the constantmoment section. If there were a few wide flexural cracks, the strain, and thus the stress, in the FRP would dramatically vary between the FRP still bonded to the concrete and the FRP stretching across such cracks. Equilibrium of the FRP strip has to be maintained。 therefore, rapid and large variations in force in the strip are not possible, as there is no mechanism to acmodate such stress variation. Hence, the concrete in the constantmoment section has to crack with a large number of very thin cracks so that the strain in the FRP remains constant. Several months after loading, a thin flexural crack could be seen extending from the tension face of Beam 2 close to the beam midspan. The CFRP strips significantly increased the cracking moment capacity of Beam 2. Although not tested, the additional FRP reinforcement was also expected to increase the ultimate moment capacity of the beam.The longterm midspan deflections (total deflection minus the initial deflection) of both beams are shown in Fig. 2, plotted versus time after loading up to 2470 days of loading. The relative slip movements versus time at each end of one of the CFRP strips bonded to Beam 2 are shown in Fig. 3. Several observations can be made:1. The longterm deflection of Beam 2 is significantly less than that of Beam 1. Longterm deflection due to concrete creep is a function of the immediate deflection, which depends on crosssectional stiffness and cracking status. Beam 1 cracked much sooner than Beam 2.2. The longterm deflection of Beam 2 constitutes a larger proportion of its immediate deflection pared with Beam 1. Plevris and Triantafillou2 observed a similar response in beams externally reinforced with FRP strips pared to their control specimen (no FRP).3. The midspan deflection for Beam 2 does not appear to be “catching” that of Beam 1. That is, at first appearance, there is no indication that the CFRP reinforcement is unloading, thereby being ineffective against the sustained load.4. The rate of increase of deflection changes with time. This is particularly evident when pared to the smooth curves of predicted longterm deflections discussed and presented in the following sections (Fig. 2, 4, and 5). Similar trends occur for both beams. The periods of reduced rate of increase of deflection coincide with the summer months and those of increased creep coincide with the winter months. In the calculation of creep coefficient using CEBFIP,10 the creep coefficient reduces as the relative humidity increases, implying a reduced rate of creep during periods of higher relative humidity. The relative humidity was not recordedduring the current tests. However, relative humidity was recorded during creep tests by Hall and Ghali,6 conducted in the same laboratory as these tests. Their results showed relative humidity varying between 5 and 50%, with a summertime average of approximately 35% and a wintertime average of approximately 10%. The observed changes in creep rate are thus believed to be seasonal and depend on changes in relative humidity.Fig. 2—Experimentally measured longterm midspan deflection versus time after loading (recorded up to 2470 days) and as predicted for both beams using CEBFIP10 and ACI11 models.Fig. 3—Relative slip versus time at each end of one CFRP strip (Beam 2) as measured and as predicted using FE model.5. Some relative slip occurred between the concrete and CFRP strip at the strip ends soon after loading, as shown in Fig. 3. Since then, the movement at one end of the strip has essentially stabilized and only a relatively small gradual movement has occurred at the other end. The significant scatter in the slip readings, particularly late in the data record, is thought to result from temperature variations in the strain gauges between dates of reading (the beams are beside an airconditioning outlet) and repeated reconnection/disconnection of the measurement instrumentation for these gauges. The maximum