International Concrete Abstracts Portal

The purpose of this study was to conduct a sensitivity study on shrinkage prediction of concrete utilizing the ACI 209 (1), GL 2000 (2), B3 (3), and CEB MC 90-99 (4) models. The sensitivity of a prediction model is function of different parameters utilized in the equations describing the model. The influence of changing input parameters on shrinkage prediction was investigated. The study reveals that the change of relative humidity will result in similar sensitivity for different shrinkage models. The autogenous shrinkage component in the CEB MC 90-99 was found to be most sensitive to strength change at 28 days. The GL 2000 was found most sensitive to cement type while the B3 model was found to be most sensitive to specimen size and type of curing. In general the B3 was the most sensitive model while the ACI was the least sensitive.

The Australian Standard for Concrete Structures, AS3600-2001 (1), is currently under review, with the intention to expand its scope to include concrete with characteristic compressive strengths up to 100 MPa. The procedures in the existing Standard for the estimation of the deformation characteristics of concrete, including the tensile strength, the elastic modulus, the creep coefficient and the shrinkage strain, are not applicable for high strength concrete. To provide reasonable agreement with the test data for Australian high strength concretes, new models for predicting the instantaneous and time-dependent material characteristics had to be developed. In this paper, the new models that have been adopted for inclusion in the next edition of AS3600 (due for release in 2006) are presented.

Concrete shrinks. Steel doesn’t, and the resistance of reinforcement to shrinkage causes deflection of slabs and beams and shortening of columns and walls. A simple visualization is given, and used to derive formulae for analysis. Current methods of calculating shrinkage curvature and deflection in reinforced sections are examined and compared, concluding that the ACI method appears realistic while the UK and European methods significantly over-estimate the deflection. Restraint by differential contraction between an insitu concrete overlay and an older substrate produces tension in the overlay and curvature and deflection of the composite unit. A method for calculating this is given, and the resulting effects are found to be significant in certain circumstances. The method is extended to consider shrinkage in insitu slabs in steel-concrete composite construction. The deflection from shrinkage is found to be approaching span/750, and cracking in the slabs is predicted in some cases. External restraint to contraction induces tensile stresses, and a rational approach to providing sufficient reinforcement to control cracking in direct tension is given. It is particularly relevant to elements which need to be water-resisting, and a case study of a basement is presented. The reinforcement needed to control cracking reliably is found to exceed most current US, UK and European recommendations.

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.