Pretensioned CFRP tendons have been usedin draped con®gurations, although in one project, con-ventional small-radius steel rollers were found to causesplitting of the tendons [21]. The steel rollers were re-placed with large-radius polymer guide channels as a ®x.The issue of safety in the face of grip failures has beenraised as new gripping technology is being applied in the®eld [9].2.5. Transfer lengthCareful tests of transfer length have been performedfor selected FRP prestressing products [7,13]. No un-usual behavior was discovered compared to steel ten-dons, though the characteristic transfer lengths di€erfrom steel. Because there is variability in FRP tendoncomposition and surface deformations, the transferlength must be determined for the speci®c product underconsideration.2.6. Thermal expansion mismatchThe coecient of thermal expansion (CTE) values forconcrete and steel are similar, approximately 10 ´ 10ÿ6/°Cfor concrete and 11 ´ 10ÿ6/°C for steel. For an epoxy-matrix CFRP tendon, typical longitudinal and transverseCTE values are in the neighborhood 70 ´ 10ÿ6/°C and30 ´ 10ÿ6/°C, respectively. Temperature will a€ect bend-ing stress levels, and concerns about concrete crackingdue to transverse expansion of the reinforcement havebeen raised.
A concern over beam end splitting was ad-dressed in one ®eld application by the addition of helicalFRP reinforcements in the end regions [21]. The e€ects ofthermal expansion mismatch have been studied bothanalytically [22] and experimentally [22,23] for the case ofunprestressed FRP rebar.2.7. Ductility concernsFRP tendons lack the ductility under extreme loadingexhibited by steel. Thus, a CFRP-prestressed beam maysimultaneously provide greater ultimate load capacityand lower energy absorption than a similar steel-pre-stressed design. Tendons have been distributed over alarge range of eccentricity values to achieve progressivefailure [14]. Recent work has advanced the ability topredict the de¯ections of CFRP-prestressed beamsduring progressive tension cracking, including loadcycling [24].3. Prestressed bridge beam test programTwo 12.19 m long AASHTO Type 2 beams weredesigned, fabricated, and tested to destruction in four-point bending. The two beams shared the same geo-metric con®guration for prestressing tendons and shearreinforcement, but featured di€erent concrete formula-tions and di€erent tendon pretension levels. Beam 1su€ered from a design calculation error and failure ofsome prestressing tendons, but was salvaged as a usefultest specimen. Fiberglass rebar was used for shearreinforcement in Beam 1, and was also used in a shear-critical region of Beam 2, the remainder of whichfeatured steel shear reinforcement. This section docu-ments the material properties, beam design, beamfabrication, and beam testing. A discussion of the testresults is included
.3.1. MaterialsThe composite prestressing material was Leadlinecable, manufactured by the Mitsubishi Kasei of Japan. Leadline cable features unidirectional carbon ®bers inan epoxy matrix. Surface deformations are milled intothe surface in a helical pattern. Manufacturer-suppliedproperties and characteristics of Leadline are shownin Table 1. The epoxy matrix has a glass transitiontemperature, Tg, of 120°C (248°F).Marshall Industries of Lima, Ohio, manufactured thecomposite rebar shear stirrups used in the program. Thecomposite rebar, named C-Barä, is fabricated using acontinuous hybrid pultrusion/compression moldingprocess. C-Bar consists of an inner core of unidirectionalE-Glass ®bers embedded in a PET matrix, and an outerlayer composed of sheet molding compound withchopped ®ber mats embedded in urethane-modi®edvinyl ester.
Uniform deformations on the surface inhibitlongitudinal movement of the bar. According to themanufacturer, tests have shown that C-Bar resists de-gradation when exposed to deicing salts, seawater, andwastewater. Table 2 provides the mechanical propertiesof C-Bar, obtained from product literature. C-Bar canbe fabricated with curves having a 51 mm inside radius,where all curves must be in the same direction (noreversed curves or S-shapes). #4 bar steel (414 MPayield strength) shear stirrups were also used in the pro-gram, as described in Section 3.3.Two di€erent high-strength concrete formulationswere used to fabricate the two beams. Twenty-eight daycompressive strength cylinder tests showed a compres-sive strength of 86.3 MPa for the ®rst beam and71.1 MPa for the second beam. Mix designs, formulatedby Anderson Concrete of Columbus, Ohio, are providedin Table 3.3.2. Design and ultimate strength analysisThis section documents the procedures used to obtainthe prestressing and shear reinforcement speci®cationsfor the nominal test beam design, and to predict theultimate strength capacity of the as-built beams. Twobeams were built during the program. Using the nomi-nal material properties for Leadline cable and assuming69.0 MPa compressive strength for the concrete, theprestressing for ®rst beam (Beam 1) was designed.During fabrication of Beam 复合材料高强度混凝土的试验研究桥梁英文文献和中文翻译(2):http://www.751com.cn/fanyi/lunwen_33192.html