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(3)
1, breakage of severalprestressing tendons occurred, as described in Section3.3. The second beam (Beam 2) was then fabricatedusing a lower target pretension value for the tendons.The nominal test beam design was obtained accordingto AASHTO Type 2 beam speci®cations [25]. The stan-dard cross-section of an AASHTO Type 2 beam is shownin Fig. 1. AASHTO standard allowable stresses areTable 1Manufacturer-supplied properties of Leadline CFRP cableCharacteristic Leadline GA-D10Matrix material EpoxyCarbon ®ber volumefraction65%Cable diameter 10 mm 0.394 inNominal cross-section area 71.8 mm30.1113 in2Intact cross-section area(measured)69.5 mm20.1075 in2Ultimate tensile load 186 kN 41 000 lbUltimate tensile stress 2600 MPa 377 ksiLongitudinal thermalexpansion0.68 ´ 10ÿ6/°C 0.38 ´ 10ÿ6/°FLongitudinal Young'smodulus147 GPa 21.4 MsiExtension at break 1.60%Matrix material 1.6Table 2Manufacturer-supplied properties of C-Bar GFRP rebarCharacteristic C-Bar #12 grade B type 1Fiber type E-GlassMatrix material Recycled PET ®ber matrix urethane modi®ed vinylester shellFiber volume fraction 70%Bar diameter 12 mm 0.47 in.Nominal cross-sectional area 113 mm20.175 in2Design ult. tensile strength 713 MPa 103 ksiModulus of elasticity 42 GPa 6.1 MsiTable 3Concrete unit mix designMix ingredient Beam 1 Beam 2Lone star green castle type III (kg) 386 363Micro silica solids (kg) 38.6 21.8Fine agg. (Martin Marietta, Columbus) (kg) 565 583Coarse agg. #8 Ls. (Martin Marietta, Columbus) (kg) 676 699Super plasticizer sikament 300 (kg) 7.8 6.9Water (kg) 127 134 shown in Table 4 in terms of the fully cured compressivestrength of the concrete, fc, and the compressive strengthat the time of load transfer, f 0ci. While the characteristictensile strength values presented in Table 4 were devel-oped for conventional strength concrete f 0c 641 MPa,they were used here with higher-strength concrete toobtain guiding values for characteristic tensile strengths.The AASHTO Type 2 beam section is typically used inconjunction with an integral concrete deck that contrib-utes to the structural performance; however, the limitedscope of this program precluded the use of a represen-tative bridge deck. The beam design was therefore basedsolely on the cross-section shown in Fig. 1.A beam analysis was applied in a parametric study toarrive at candidate designs for which the ultimate beamstrength was limited by bending strength, and bendingstrength was limited by the tendon strength.
The ®nalnominal design was 12.19 m long with an allowable live load of 133 kN based on a two-point loading spaced at2.50 m. (The test con®guration featured a 11.79 m freespan between end supports, as shown in Fig. 2.) Thedesign featured eight prestressing tendons with a cent-roid 102 mm above the bottom of the beam (see Fig. 1),giving an eccentricity of 300 mm below the neutral axis.The test beam was reinforced with vertically alignedshear stirrups. C-Bar brand glass ®ber reinforced poly-mer (GFRP) composite stirrups were used to achieve asteel-free design for Beam 1. The same stirrup shape andspacing were used for both beams. The shear stirrupdesign was constrained by the shape limitations ofC-Bar mentioned earlier. Composite C-shaped bars wereused in a double-C con®guration, as shown in Fig. 1.The two vertical tendons of the double-C shape resistopening forces across shear cracks while the ends keepthe stirrups anchored in the concrete. Shear stirrupswere spaced conservatively so that the test beams wouldfail in bending, not in shear. Fig. 3 shows the variablespacing used for the rebar stirrups in the test beam.Beam 2 featured steel rebar stirrups except for the high-shear zone on one half of the beam where C-Bar wasused (see Fig. 3).Cracking-load and ultimate-strength analyses wereperformed to predict the characteristic failure momentsfor the cross-section designs of the two beams. For bothtypes of analysis, a pure bending response was modeled.Material strains were set proportional to the curvatureof the neutral axis and the vertical distance from theneutral axis. For the initial cracking analysis, linearelastic properties were used for the concrete with anassumed modulus of 27 600 MPa, and a concrete mod-ulus of rupture of 0:62f 0cp (MPa) per Table 4. For theultimate load (cracked-beam) calculation, non-linearcompressive stress/strain behavior of the concrete wasmodeled. A characteristic stress±strain curve was ob-tained for high-strength concrete with f 0c 67:6MPa[26]. The stress values from this curve were scaled up toachieve the as-measured values of f 0c . Axial stresses inthe cross-section were numerically integrated to com-pute bending moment and axial load as a function ofcurvature, and an iterative scheme was used to deter-mine the proper neutral axis location corresponding tozero axial load. Concrete creep and shrinkage losses were assumed to produce a 13% loss of the initial tendonprestress. The elastic response to pretension loadtransfer was included in the analyses. The moment±curvature relations thus constructed were used in nu-merical beam analyses to predict ultimate loads andde¯ections for four-point bend tests.