International Concrete Abstracts Portal

Integral abutment bridges are becoming popular among a number of transportation agencies owing to the benefits, arising from elimination of expensive joints, installation, and reduced maintenance cost. Unlike framed structures, in addition to the effects of creep, shrinkage, and temperature, integral bridges are also subjected to the soil¬substructure-superstructure interaction. The analysis of these bridges requires realistic modeling that can include the time-dependent material behavior. Statical indeterminacy in the structure introduces time-dependent variations in the redundant forces. An analytical model is developed in which the redundant forces in the integral abutment bridges are derived considering the time-dependent effects of creep and shrinkage. The analysis includes nonlinearity due to cracking of the concrete, as well as the time dependent deformations of composite cross section due to creep, shrinkage and temperature. American Concrete Institute (ACI) and American Association of State Highway and Transportation Officials (AASHTO) approaches are considered in modeling the time dependent material behavior. Age-adjusted effective modulus method with relaxation procedure is used to include the creep behavior of concrete. The partial restraint provided by the abutment-pile-soil system is modeled using discrete spring stiffness for translational and rotational degrees of freedom. The effects of creep and shrinkage on the service life are illustrated and the results from the analytical model are compared with the published field test data of a two-span continuous integral abutment bridge.

This research focuses on deviations from the linear viscoelastic behavior of concrete occuring at high stress levels (from 0.5 f’c to 0.7 f’c), at early age loading (1 to 2 days) and in case of unloading implying strain reversal. A large series of creep tests was performed on high strength concrete specimens undergoing creep under constant stress, followed by a period of recording of the creep recovery after complete unloading. Some specimens were heat cured before loading. Some nonlinear effects at very early age have been observed. After unloading, experimental data show that the creep recovery deviates strongly from the numerical predictions obtained by the application of the principle of superposition but seems to conform rather well to the recovery model proposed by Yue and Taerwe3. This model was then applied, through a step-by-step approach, for the time-dependent structural analysis of a precast composite prestressed bridge deck with 26 m span. The application of the recovery model yielded computed strains which are in good agreement with in situ measured strains, and in better agreement than the strains computed by the application of the principle of superposition. This enhanced approach was then used to optimize the phases of construction of this kind of structure. Thanks to this research, the age at transfer of prestress could be significantly reduced.

This paper presents the results of a 10-year instrumentation and monitoring program on the North Halawa Valley Viaduct, a major prestressed box girder viaduct on the Island of Oahu, Hawaii. The long-term monitoring program was initiated in 1994 during construction of the long-span post-tensioned box-girder viaduct. Over 200 electrical strain, displacement, temperature and load sensors were installed in one unit of the structure and have been monitored continuously since. These instruments monitor vertical deflections, span shortening, prestress loss, longitudinal strains and temperature in the box-girder concrete. The long-term response of this structure is presented and compared with the initial predictions made during the design process. Modified material properties based on short-term shrinkage and creep tests were incorporated into the long-term prediction model to produce significantly improved comparisons. A procedure is proposed for prediction of upper and lower bounds for the long-term response of long-span prestressed concrete bridges. This improved prediction model is applied to the other five units making up the NHVV to verify its performance as a design tool. The results of this study were then incorporated into the development of an instrumentation system for the planned Kealakaha Bridge on the Island of Hawaii. Application of the prediction model is demonstrated using shrinkage and creep data determined from short-term tests performed on the concrete mixture proposed for this new long-span box-girder bridge structure.

The long-term service behavior of modern reinforced or prestressed concrete structures, whose final static configuration is frequently the result of a complex sequence of phases of loading and restraint conditions, are influenced largely by creep. Creep substantially modifies the initial stress and strain patterns, increasing the load induced deformations, relaxing the stresses due to imposed strains, either artificially introduced or due to natural causes, and activating the delayed restraints. The resulting influences on serviceability and durability are twofold, creep acting both positively and negatively on the long-term response of the structure. The paper shows that use of the four fundamental theorems of the theory of linear viscoelasticity for aging materials, and the related fundamental functions, offers a reliable and rational approach to estimate these effects. Extremely compact formulations are obtained, which are particularly helpful in the preliminary design, as well as in the control of the output of the final detailed numerical investigations and safety checks, and suitable for codes and technical guidance documents. Particular attention is dedicated to the problem of change of static system.

The study included A3 – General Paving (21 MPa at 28 days), A4 – General Bridge Deck (28 MPa at 28 days), and A5 – General Prestress (35 MPa at 28 days) concrete mixtures approved by the Virginia Department of Transportation (VDOT). The study also included a lightweight, high strength concrete mixture (LTHSC) used in the prestressed beams of the Chickahominy River Bridge, and a high strength (HSC) concrete mixture used in the prestressed beams of the Pinner’s Point Bridge. For the A3, A4, and A5 portland cement concrete mixtures, the CEB 90 model appears to be the best predictor. However, there is little difference in prediction capabilities between the CEB 90, GL2000 and B3 models. For mixtures containing supplemental cementitious materials, slag and fly ash, the GL2000 model appears to be the best predictor. For the LTHSC concrete mixture, the CEB-C90 model appears to be the best early age predictor, while the Bazant B3 model appears to be the best predictor a later ages. And for the HSC concrete mixture, the Gardner/Lockman model appears to be the best predictor.