United States Department of Transportation
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Bond between smooth prestressing wires and concrete : finite element model and transfer length analysis for pretensioned concrete crossties.
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2014-04-03
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NTL Classification:AGR-INFRASTRUCTURE-INFRASTRUCTURE;AGR-INFRASTRUCTURE-Railroads;NTL-RAIL TRANSPORTATION-RAIL TRANSPORTATION;
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Abstract:Pretensioned concrete ties are increasingly employed in railroad high speed
and heavy haul applications. The bond between prestressing wires or strands and
concrete plays an important role in determining the transfer length of pretensioned
concrete members, but little research was done to characterize the transfer length in
terms of steel reinforcement and concrete factors for railroad concrete ties. Federal
Railroad Administration is sponsoring a comprehensive test program at Kansas State
University (KSU) aimed at quantitatively correlating prestressing steel and concrete
variables with the transfer length of pretensioned concrete crossties, and Volpe Center
has been applying the data obtained in the KSU test program to develop bond models
that can be used in transfer length prediction and failure analysis of concrete ties.
This paper describes finite element (FE) model development related to the
smooth prestressing wire whose dominant bonding mechanisms with concrete are
chemical adhesion and friction. The commercial FE software Abaqus is employed,
and the steel-concrete interface is discretized with cohesive elements. A user bond
model is developed within the elastoplastic framework and implemented for
axisymmetric and 3D cohesive elements. The bond model defines constitutive
relations in terms of normal and shear stresses vs. interfacial dilation and slips. The
bond behavior is initially linear elastic, followed by adhesion and friction that are
governed by a yield function and a plastic flow rule specific for the smooth wireconcrete
interface. The main bond material parameters are normal and shear elastic
stiffness, initial adhesive strength, plastic slip at which adhesion first breaks
completely, and coefficient of friction. Except for the coefficient of friction, which is
determined with reference to the open literature, the bond parameters are calibrated
from untensioned pullout tests and pretensioned prism tests conducted at KSU. The
calibrated bond parameters exhibit a dependence on the nominal compressive strength
of concrete at the time of pretension release. Because considerable concrete creeping
has been observed in the periods between pretension release and concrete strain
measurement in the test program, an additional concrete material parameter, basic
creep compliance, can be calculated and applied to adjust the concrete surface strain
data. The user bond model is then validated with transfer length data measured on
actual concrete crossties made with a smooth prestressing wire in a tie manufacturing
plant.
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