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 paper illustrates how stress relaxation can be used to obtain valuable information regarding the behavior of concrete at early ages. Five concrete mixtures were investigated using a so-called discretized restrained shrinkage (DRS) testing device, allowing the determination (from the time of casting) of the increase in load induced by autogenous shrinkage and the evaluation of the different strain components (free shrinkage, elastic strain, creep). Test results indicate that the stress due to early-age restrained autogenous shrinkage is quite variable, in good part due to the variation in the relaxation capacity of the mixtures. Both the relaxation ratio, defined as the stress generated divided by the theoretical stress, and the relative relaxation, defined as the absolute value of stress relaxation divided by the average applied stress, can be used to illustrate and analyze the variation of the relaxation phenomena as a function of the type of mixture tested.

Creep and shrinkage data for two high strength lightweight aggregate concretes were collected over a two-year period. The concretes, with unit weight of 1922 kg/m3 (120 pcf), were developed using expanded slate as coarse aggregate. Strengths of 55.2 MPa (8,000-psi) and 69.0 MPa (10,000-psi) were obtained at 56 days. Creep specimens were loaded to 40 or 60 percent of the initial compressive strength at 16 or 24 hours after casting. Based on this preliminary study, AASHTO-LRFD creep estimates of high strength, lightweight aggregate concrete were within 20% accuracy for ages later than one month. ACI-209 estimated creep of the 55.2 MPa lightweight concrete and shrinkage of the 69.0 MPa concrete within 20% accuracy, but greatly underestimated shrinkage of the 55.2 MPa mix. When compared with normal weight, high strength concrete of similar strength and similar cement paste content from previous research, the 69.0 MPa lightweight mix experienced lower total strain after two years.

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.

Many structural problems involving creep in concrete structures can be solved in very compact closed forms through the fundamental theorems of linear viscoelasticity of aging materials. This general approach requires the knowledge of three basic functions: the compliance function J, derived directly from the creep prediction models available in the literature and in technical guidance documents, and the relaxation (R) and redistribution () functions, that can be calculated from J. This paper presents an interactive web site for quick automatic calculation of these three basic functions, with reference to the principal creep models presently considered by international civil engineering societies. Starting from the approach suggested by Bazant for the numerical solution of the fundamental Volterra integral equation relating R to J, identically applied to derive ~ from J, a complete procedure has been developed, including the user interface necessary for setting input data and handling output results. The immediate availability of the basic functions allows extended comparisons of the outputs of the different models and evaluation of the influence that the selection of a particular model has on the assessment of structures. The web site has a flexible architecture and will be progressively extended to include calculation of other functions of interest for the creep analysis of structures, e.g. the aging coefficient X of the age-adjusted-effective-modulus-method, and the reduced relaxation functions R* that extend the field of application of the fundamental theorems to the analysis of heterogeneous structures, such as e.g. cable-stayed bridges.