Design of Concrete Bridge Beams Prestressed with CFRP Systems (2019) / Chapter Skim
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From page 6... ...
6 2.1 Current Applications of CFRP in Prestressing A survey of state departments of transportation (DOTs) and the District of Columbia was conducted to gather information on the state of practice of CFRP prestressing.
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Literature Review and Current Design Practices 7 six precast concrete girders. CFRP cables were used in four girders and CFRP bars were used in pretensioning the other two girders (Rizkalla and Tadros, 1994)
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8 Design of Concrete Bridge Beams Prestressed with CFRP Systems unbonded beams does not accurately reflect the governing mechanics of these elements. Most of the models reported in the literature for estimating the force in the unbonded tendons at the ultimate load were derived for prestressing steel with the assumption that the yield strength will not be reached and the elastic modulus of the tendon can be used to estimate the increase in tendon stress.
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Literature Review and Current Design Practices 9 ultimate to that when the reinforcement yields. Since CFRP reinforcements do not yield, ductility cannot be defined in this manner.
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10 Design of Concrete Bridge Beams Prestressed with CFRP Systems The results showed that stress relaxation losses were linearly related to the logarithm of the time; the best fit line was obtained by regression analysis and expressed by the following formula: Relaxation % 0.056 0.396 log 24 (Eq.
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Literature Review and Current Design Practices 11 where db is the diameter (in.) , fr is the rupture strength (psi)
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12 Design of Concrete Bridge Beams Prestressed with CFRP Systems 2.4 Experimental Investigations Reported in Literature 2.4.1 Flexural Behavior of CFRP Prestressed Beams Due to the relatively low modulus of elasticity of CFRP compared to that of steel, larger curvatures and deflections are induced under the same level of flexural load for beams prestressed with CFRP tendons than those prestressed with steel tendons (ACI 440.4R-04, 2011)
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Literature Review and Current Design Practices 13 can provide the same level of deformability as those with steel strands. However, the beams prestressed with bonded CFRP tendons do not redistribute the moment as there is no reserved capacity once the cable ruptures at the plastic hinge above the center support.
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Authors # o f P re st re ss ed C F R P te st s Beam Details Prestressing Details Failure Mode Section Shape Beam Span Section Depth Jacking Stress Tendon Profile Bond Type R ec ta ng ul ar I' sh ap ed T ee o r do ub le T ee B ox ≤ 15 ft .
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Authors # of v al id te st Beam Details Prestressing Details Failure Mode Section Shape Beam Span Section Depth Jacking Stress Tendon Profile Bond Type R ec ta ng ul ar I' sh ap ed T ee o r do ub le T ee B ox ≤ 15 ft 15 30 ft > 3 0 ft h ≤ 10 " 10 " -2 0" > 2 0" ≤ 0.
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16 Design of Concrete Bridge Beams Prestressed with CFRP Systems The effect of freeze–thaw cycles on prestressing CFRP tendons was studied solely and/or in combination with other environmental effects by various researchers (Mashima and Iwamoto, 1993; Tannous, 1997; Micelli and Nanni, 2004; Mertol et al., 2006)
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Literature Review and Current Design Practices 17 to 100% of the cracking load. The beams survived 2 million cycles with little effect on beam stiffness.
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18 Design of Concrete Bridge Beams Prestressed with CFRP Systems Stress relaxation of the prestressing CFRP tendons was found to be linearly related to the logarithm of the time and to have a direct relationship to the level of initial prestressing. Therefore, several analytical models were developed by using the curve fitting methods (presented in Section 2.3.3)
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Literature Review and Current Design Practices 19 accordance with CAN/CSA S806-12: Design and Construction of Building Components with FibreReinforced Polymers (Canadian Standards Association, 2017) specifications for all test beams.
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20 Design of Concrete Bridge Beams Prestressed with CFRP Systems prestressed beams. For the same level of initial prestressing force, CFRP prestressed beams have similar deflection to those of prestressed steel beams (Braimah et al., 2006; Zou, 2003b, Zou, 2003c)
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Literature Review and Current Design Practices 21 some design guidelines (see Table 2.6)
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22 Design of Concrete Bridge Beams Prestressed with CFRP Systems 2.5.3 Prestress Relaxation Losses For evaluating the relaxation loss (RL)
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Literature Review and Current Design Practices 23 prestressing CFRP tendons.
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24 Design of Concrete Bridge Beams Prestressed with CFRP Systems concrete (i.e., strain compatibility is satisfied)
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Literature Review and Current Design Practices 25 For a rectangular section: (Eq.
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26 Design of Concrete Bridge Beams Prestressed with CFRP Systems Prestressed Beams with Unbonded Prestressed CFRP Tendons The design strength of post-tensioned beams with unbonded tendons is determined by estimating the stress in the tendon at failure, fp, which is calculated as the summation of the effective prestress, fpe, and the additional stress due to loading, Dfp (fp = fpe + Dfp)
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Literature Review and Current Design Practices 27 2.6.1 Database of Experimental Investigations A database of experiments on CFRP prestressed beams was compiled (Table 2.3)
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28 Design of Concrete Bridge Beams Prestressed with CFRP Systems Inc rea se in the Be am Si ze Figure 2.3. Experimental moment capacities versus predicted values by analytical procedure for bonded CFRP prestressed beams.
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Literature Review and Current Design Practices 29 The relatively low COV values (0.13 to 0.15) indicate that the current model formulations for flexure can reasonably predict the capacity.
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