Crack Width Formula for Transversely Post-Tensioned Concrete Deck Slabs in Box Girder Bridges

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Crack Width Formula for Transversely Post-Tensioned Concrete Deck Slabs in Box Girder Bridges Powered By Docstoc
					 ACI STRUCTURAL JOURNAL                                                                                        TECHNICAL PAPER
Title no. 106-S69


Crack Width Formula for Transversely Post-Tensioned
Concrete Deck Slabs in Box Girder Bridges
by Young Cheol Choi and Byung Hwan Oh

Cracking in prestressed concrete members is very critical because
it directly affects not only serviceability, but also the durability of
prestressed concrete structures. However, test data on the cracking
behavior of prestressed concrete structures, especially on the box
girder bridges, are very limited. The purpose of this study is to
investigate cracking behavior and crack width of transversely
prestressed concrete decks in box girder bridges, which have been
frequently adopted in common practice for the design of deck slabs
in box girder bridges. For this purpose, full-scale box girder
segments were fabricated and tested. Major test variables include
magnitude of precompressive stresses and prestressing steel ratios.
The crack widths at the bottom surface of deck slabs were
measured automatically according to the increase of applied loads.
Strains of prestressing strands and nonprestressed reinforcing bars were
also measured during the loading process. Bond characteristics of
post-tensioned prestressing strands were identified from the relation
between prestressing steel strains and nonprestressed ordinary                   Fig. 1—Transverse post-tensioning system of bridge deck in
reinforcing bar strains under the same loading conditions. The                   box girder bridges.
bond effectiveness of prestressing steel was then obtained from
these test data. The effective circumferential perimeter of multiple
strands in a duct that is bonded to concrete is derived and used to
determine the crack width.
   A realistic formula for predicting crack width of prestressed
concrete deck slabs was derived in terms of effective combined
reinforcement ratio and the reinforcing bar stress after decompression.
The proposed equation and test data agrees very well. It is seen,
however, that the existing crack width formulas for general flexural             Fig. 2—Typical longitudinal cracks of top slab in prestressed
beams show large deviations from the present test data of box girders
                                                                                 concrete box girder bridges without transverse prestressing.
and are not directly applicable to the top slabs of box girders due
to different geometries. The proposed equation may be a good base for
the realistic analysis/design of prestressed concrete deck slab structures.      structures under service loads.5-8 It is therefore necessary to
                                                                                 have a realistic prediction equation for crack width under
Keywords: bond; box girder bridges; crack width; deck; post-tensioning; slabs.   applied loading for rational serviceability design of
                                                                                 prestressed concrete box girder bridges.9-11
                      INTRODUCTION                                                  Suri and Dilger5 have shown that, for a crack width equation
   Transverse prestressing of bridge decks was first introduced                  of prestressed members to be rational, the bond characteristics
in box-girder bridges mainly to maximize the length of cantilever                of both prestressed and nonprestressed reinforcements must
overhangs and to reduce the number of webs.1 A post-tensioning                   be considered. In the post-tensioned prestressed members,
system is generally used in the transverse prestressing of                       prestressing steel stresses are transferred from the
bridge decks in box girder bridges (Fig. 1). Recently, it was                    prestressing steel via the surrounding grout to the duct,
reported that many serious longitudinal cracks occurred at                       (containing the prestressing tendon), and then from the duct
the bottom of the top slab of prestressed concrete box girder                    to the structural concrete.11 It is generally difficult to evaluate
bridges that are not prestressed laterally2 (refer to Fig. 2). The               the local bond behavior of prestressing tendon because the
primary reason for the occurrence of longitudinal cracking at the                tendons are partly in contact with the duct and partly with the
bottom surface of the top slab is mainly due to tensile stresses                 surrounding concrete. It is therefore necessary to identify the
arising from loads on the slab. These cracks may affect the                      effective circumference of multistrands in which the strands
serviceability as well as the duprestressed concreterability of                  are bonded to concrete. The purpose of the present study is
prestressed concrete bridges by reducing the stiffness and                       first to investigate the bond behavior of prestressing steel
also by facilitating the corrosion.3-4 Therefore, transverse                     and then to propose a realistic formula for predicting
partial or full prestressing has been a common practice in
the design of concrete deck slabs in box girder bridges.2 The                      ACI Structural Journal, V. 106, No. 6, Novem
				
DOCUMENT INFO
Description: Cracking in prestressed concrete members is very critical because it directly affects not only serviceability, but also the durability of prestressed concrete structures. However, test data on the cracking behavior of prestressed concrete structures, especially on the box girder bridges, are very limited. The purpose of this study is to investigate cracking behavior and crack width of transversely prestressed concrete decks in box girder bridges, which have been frequently adopted in common practice for the design of deck slabs in box girder bridges. For this purpose, full-scale box girder segments were fabricated and tested. Major test variables include magnitude of precompressive stresses and prestressing steel ratios. The crack widths at the bottom surface of deck slabs were measured automatically according to the increase of applied loads. Strains of prestressing strands and nonprestressed reinforcing bars were also measured during the loading process. Bond characteristics of post-tensioned prestressing strands were identified from the relation between prestressing steel strains and nonprestressed ordinary reinforcing bar strains under the same loading conditions. The bond effectiveness of prestressing steel was then obtained from these test data. The effective circumferential perimeter of multiple strands in a duct that is bonded to concrete is derived and used to determine the crack width. A realistic formula for predicting crack width of prestressed concrete deck slabs was derived in terms of effective combined reinforcement ratio and the reinforcing bar stress after decompression. The proposed equation and test data agrees very well. It is seen, however, that the existing crack width formulas for general flexural beams show large deviations from the present test data of box girders and are not directly applicable to the top slabs of box girders due to different geometries. The proposed equation may be a good base for the realistic analysis/design of prestres
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