【正文】 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 relative slip at the ends of the gauged CFRP strip within 2470 days was in the order of mm ( in.)—an average of approximately 60 microstrain over the length of the strip—which implies an average loss of tensile stress of approximately 9 MPa ( psi) in the strips, and translates to an average loss of tensile force of approximately 1 kN ( kips) per strip. Fifty percent of this movement occurred in the first 3 weeks after loading. This observation agrees with those reported by Choi et al.,confirming that the majority of epoxy creep occurs in a relatively early time period.ANALYTICAL PREDICTION OF DEFLECTIONSAnalytical predictions of the beam deflections were made in an attempt to identify any features of the behavior not obvious from the experimental results alone. First, the simplified procedures of CEBFIP10 and ACI11 were used. These procedures focus on accurate modeling of concrete creep—and thus deflection—without considering the effect of creep of epoxy. A stepbystep intime analysis and an FE model were therefore developed to examine the bined effect of creep of the concrete and of the epoxy on the longterm deflection.Analytical prediction of deflections using CEBFIP and ACIApproaches based on the CEBFIP Model Code 1990 and the ACI Committee 209 remendations11 were used by Hall and Ghali6 with the former being shown to achieve good agreement with experimental results for concrete beams reinforced with steel bars and concrete beams with GFRP bars in place of the steel bars. Both approaches aim at estimating longterm deflections due to the effects of creep and shrinkage in the concrete. The methodologies are described in detail by Hall and Ghali6 and Masia et al. Fig. 4—Deflections predicted by stepbystep in time model versus experimentally measured deflections for Beam 1 (reinforced with steel only), including effect of concrete creep and tension stiffening of concrete (238。 328 psi).Fig. 1—Test specimens, test setup, and strain distribution.The two beams were cast together and stored—fully supported—for 10 months before the CFRP strips and GFRP wraps were applied. One beam (Beam 1) was designated as a control reinforced concrete.INTRODUCTIONIn recent years there has been much research on the use of fiberreinforced polymers (FRPs) to strengthen existing concrete structures. One popular application, used widely in practice, is to bond FRP strips externally to the tension face of reinforced concrete (RC) beams to increase flexural capacity.The FRP strips are typically bonded directly to the prepared concrete surface using an epoxy adhesive. The strips may be anchored mechanically near their ends or supported by additional shear reinforcement, usually in the form of Ushaped FRP sheets. If the beam is subsequently loaded with sustained loads, creep in the epoxy adhesive could take place and would allow the FRP strips to unload, leaving them ineffective against the sustained load. Similarly, if the strips are prestressed as recently remended by some researchers (for example, Ye et ), creep in the epoxy may relieve some of the initial force. Hence, although the FRP strips can still assist in supporting additional live load, the increased sustained loads may exceed the capacity of what could effectively bee the original unstrengthened beam.Research into the