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高性能预应力混凝土桥的预应力损失比较

Comparison of Prestress Losses for a Prestress Concrete

Bridge Made with High-Performance Concrete

P .J.Barr,A.M.ASCE 1;B.M.Kukay 2;and M.W.Halling,A.M.ASCE 3

Abstract:Five prestressed concrete girders made with high-performance concrete were instrumented using vibrating-wire strain gages.Their behavior was monitored for three years from the time of casting.The measured change in concrete strain at the centroid of the prestressing strands was used to evaluate changes in prestress.The total measured prestress loss was as large as 28%of the total jacking stress.Due to the higher stresses,this loss is larger than would be expected for a girder made with conventional-strength concrete.The observed values of prestress losses were compared with values calculated using the recommended AASHTO LRFD and NCHRP 18-07procedures.The AASHTO LRFD method overpredicted the average prestress losses for the highly stressed Span 2girders by 20%while the NCHRP method underpredicted the average losses by 16%.The NCHRP method was found to be more inclusive and adaptable to regional construction.The calculated NCHRP Span 2losses were found to be within 10%of the average measured losses when the elastic shortening losses were calculated based on measured data and differential shrinkage was calculated based on continuous beams.DOI:10.1061/͑ASCE ͒1084-0702͑2008͒13:5͑468͒

CE Database subject headings:Prestressed concrete;High-strength concrete;Bridge design;Shrinkage;Creep;Bridges,concrete .

Introduction

High-performance concrete ͑HPC ͒is a class of concrete that pro-vides enhanced performance properties such as increased strength or improved durability for a given application.Additives such as microsilica,fly ash,chemical admixtures,or other materials,in-cluded individually or in various combinations,are substituted for portions of materials in conventional concrete to obtain these spe-cial characteristics.The high strength would be beneficial in pre-cast,prestressed girders to obtain ͑1͒the use of fewer girders per span;͑2͒longer spans;or ͑3͒girders with reduced height where grade clearance is a problem.Other structural members would be reduced,resulting in less weight.The use of HPC in the bridge deck may also enhance the corrosion resistance properties of the concrete to the intrusion of corrosive chemicals.

Despite the apparent benefits of using high-performance con-crete,there are reasons why it has not yet been widely used in prestressed members.Current methods for calculating prestress losses,such as those advocated by the American Association of State Highway and Transportation Officials ͑AASHTO 2004͒and the Precast Prestressed Concrete Institute ͑PCI 1975͒,were based on the observed behavior of conventional concrete,with strengths usually below 41.4MPa ͑6,000psi ͒.In particular,several equa-tions for calculating prestress losses are empirically related to

concrete strength.Because incorrect calculations of prestress losses can lead to unacceptable service performance,these losses need to be accurately predicted.

The appropriateness of applying current design methods to high-performance concrete needs to be evaluated.For example,elastic shortening and creep are two major components of the total prestress loss.Because a HPC girder will almost certainly be more highly stressed than one made with conventional concrete,the magnitude of those loss components will probably increase.It is unknown whether this difference in prestress loss is adequately taken into account by present methods of analysis.In response to these concerns,several research projects have been performed in order to quantify the response of prestress concrete girder bridges fabricated with high-performance concrete.Ahlborn et al.͑1998͒compared the AASHTO LRFD design provisions with the mea-sured response of two long-span,high-strength composite pre-stressed bridge girders.They found that the design specifications overestimated the high-strength concrete modulus of elasticity re-sulting in underpredicted elastic shortening losses and overpre-dicted the creep and shrinkage losses.Roller et al.͑1995͒performed an experimental investigation with four high-strength concrete bridge girders.Two of the girders were used to evaluate the early-age flexural properties and the remaining two were used to determine the long-term behavior.The researchers concluded that prestress concrete girders made with high-strength concrete can be expected to adequately perform if designed according to the AASHTO standard specifications .Kowalsky et al.͑2001͒in-strumented four prestress high-performance concrete bridge gird-ers in North Carolina.These researchers found that the elastic shortening and creep losses were major contributors to the total losses with shrinkage losses much less so.The larger than ex-pected elastic shortening and creep losses were attributed to a lower modulus of elasticity than predicted.The total prestress losses ranged from 12.9to 19.1%of the initial jacking stress.Other HPC bridge research can be found in Shams and Kahn ͑2000͒,Lopez et al.͑2003͒,and Waldron ͑2004͒.

The Washington State Department of Transportation

1Associate Professor,Dept.of Civil and Environmental Engineering,Utah State Univ.,4110Old Main Hill,Logan,UT 84332-4110.2

Ph.D.Candidate,Dept.of Civil and Environmental Engineering,Utah State Univ.,4110Old Main Hill,Logan,UT 84332-4110.3

Associate Professor,Dept.of Civil and Environmental Engineering,Utah State Univ.,4110Old Main Hill,Logan,UT 84332-4110.

Note.Discussion open until February 1,2009.Separate discussions must be submitted for individual papers.The manuscript for this paper was submitted for review and possible publication on August 14,2006;approved on November 16,2007.This paper is part of the Journal of Bridge Engineering ,V ol.13,No.5,September 1,2008.©ASCE,ISSN 1084-0702/2008/
5-468–475/$25.00.

Note.Discussion open until February 1,2009.Separate discussions must be submitted for individual papers.The manuscript for this paper was submitted for review and possible publication on August 14,2006;approved on November 16,2007.This paper is part of the Journal of Bridge Engineering ,V ol.13,No.5,September 1,2008.©ASCE,ISSN 1084-0702/2008/5-468–475/$25.00.

468/JOURNAL OF BRIDGE ENGINEERING ©ASCE /SEPTEMBER/OCTOBER 2008

J. Bridge Eng., 2008, 13(5): 468-475

D o w n l o a d e d f r o m a s c e l i b r a r y .o r g b y H e b e i U n i v e r s i t y o f

E n g i n e e r i n g o n 03/12/16. C o p y r i g h t A S C E .

F o r p e r s o n a l u s e o n l y ; a l l r i g h t s r e s e r v e d .

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