The measurements of total load versus center de¯ec-tion are plotted for both test beams in Fig. 7. Bothbeams failed due to tension failure of the CFRP tendonsin the bending zone between the load points. Beam 1 failed at a crack 1.05 m from the mid-length, and Beam2 failed at a crack 0.75 m from the mid-length. Photo-graphs of the failure regions for the two beams areshown in Fig. 8.The ultimate load values are marked with the symbol``´'' in Fig. 7. The measured ultimate live load valueswere 434 and 554 kN for Beams 1 and 2, respectively.These values were greatly in excess of the predictedstrength values of 294 and 418 kN for Beams 1 and 2,respectively. The explanation oered is that the actualstrength of the Leadline cable is signi®cantly greaterthan the manufacturer-supplied value of 2600 MPa. Themeasured ultimate load value for Beam 1 was used withthe ultimate-strength analysis method to calculate theeective strength of the Leadline cable, providing aneective strength value of 3490 MPa. This latter valuewas used to recalculate the ultimate strength value ofBeam 2 to be 569 kN, which is within 3% of the mea-sured strength of Beam 2. It is concluded that the eective ultimate tensile strength of Leadline is ap-proximately 3450 MPa, rather than the quoted value of2600 MPa.The load/de¯ection curves for the two test beamsagree closely up to about 311 kN. The initial (pre-cracking) load/de¯ection slopes were calculated usinglinear regression analyses to be 17.1 and 16.1 kN/mm forBeams 1 and 2, respectively, in the range 22±133 kN.Both beams deviated from a linear response at about178 kN. Audible cracking of the concrete in Beam 2 wasnoted at about of 182 kN. The load at ®rst cracking wasnot noted for Beam 1. Loading of Beam 1 was paused at222 kN for inspection, and loading of Beam 2 waspaused at 185 and 222 kN for inspection.The total tendon preload force was similar for thetwo beams, 712 kN for Beam 1 and 676 kN for Beam 2.The crack-associated loss of bending stiness occurredat about 178 kN for both beams. This is lower thanpredicted crack-load values of 209 kN for Beam 1 and196 kN for Beam 2, computed assuming a 13% total lossof tendon pretension due to creep and shrinkage of theconcrete. Possible explanations for this discrepancyinclude the following:1. the prestress losses were greater than the assumedvalues;
2. the rupture moduli of the concrete formulations wereless than the values predicted by the AASHTO allow-able value of 0:62f 0cp (MPa).An extensive regime of post-cracking strength is evi-dent in Fig. 7. Beam 2 exhibited over 23 cm of centerde¯ection at ultimate load, most of which was achievedin the cracked regime. It is noteworthy that while theCFRP tendon material exhibits a brittle tensile failure,the prestressed test beams that use the CFRP exhibitedlarge-de¯ection, progressive failure that is desirable inconcrete structures.As the ultimate load of Beam 2 was approached, adiagonal crack began to show a noticeable openingdisplacement in the shear region which featured C-BarGFRP shear reinforcement. This crack can be discernedin the Fig. 9. In view of the low elastic modulus of theGFRP (41 400 versus 207 000 MPa for steel), the crack-opening displacement observed during testing wouldlikely have been much less in a steel-reinforced beam.The beam did not fail at this location, however, so thelow stiness of the C-Bar appears not to have aectedthe ultimate strength of the test beam design.4. Summary and conclusionsTwo full-sized AASHTO Type 2 beams were fabri-cated using high-strength concrete and emerging FRPproducts for prestressing and reinforcement. The beamswere tested to ultimate failure in four-point bending.Full documentation of material properties, beam design,and test results is provided. Signi®cant lessons learnedand important observations are summarized in the fol-lowing points:1. The American Concrete Institute committee ACI-440is actively addressing major issues surrounding the in-troduction of FRPs as concrete reinforcement includ-ing prestressing. ACI Subcommittee 440-I on FRPPrestressing is developing a design code for FRP-pre-stressed concrete.2. There are inconsistencies between commercial pro-ducers of CFRP tendons in the way characteristicstrength values are established. Two leading manu-facturers use dierent ratios of allowable stress to ul-timate stress. The ultimate strength of the LeadlineCFRP tendons computed from current test resultsis approximately 3450 MPa, 33% greater than themanufacturer quoted value of 2600 MPa.3. The practice of linking CFRP tendons to steel cablesduring pretensioning (done to complete the length ofprestressing beds and/or to allow the use of standardpretensioning equipment) can induce large twistingdeformation in the CFRP tendon as the steel cableuntwists during tensioning. This may causestrength-reducing stress concentrations where theCFRP tendon exits the grip/anchor.4. Fabrication handling and safety procedures must bescrutinized as CFRP prestressing tendons are adopt-ed. 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(5):http://www.youerw.com/fanyi/lunwen_33192.html