SP-246CD This CD-ROM is a collection of papers prepared for a session held at the ACI 2007 Fall Convention in Puerto Rico on the effects of shrinkage and creep of concrete. The papers are organized into four groups: 1) design, construction, and behavior of bridge structures; 2) effect of concrete shrinkage and creep on the design and construction of tall buildings; 3) deflection and cracking serviceability of slabs, beams, and walls; and 4) other problems and basic questions.

The paper presents aspects of an investigation made upon the cracking states induced by early age restrained shrinkage of concrete, at the piers and abutments of the bridges from the new Transylvania Motorway, Romania. The National Building Research Institute [INCERC] Cluj-Napoca Branch and Technical University of Cluj-Napoca performed the investigation at the request of the general contractor Bechtel International Inc. Ltd. At approximately one month age of concrete, cracks width range between 0.05-0.5 mm (0.002-0.02 in) were observed. The paper emphasizes the analyses, interpretations and analytical predictions for the future progress of the cracking states, based on the data acquired from the technical documentation supplied by general contractor, and monitoring of the crack widths. The good match of theoretical results with the real registered data led to the conclusion that cracking states are caused mainly by restrained volume contraction within the first week. Thus, repair measures could be done in time and the normal progress of the works continued.

An extensive experimental research has been carried out to extend the Belgian technique of steel-concrete prebent beams to Very High Performance Concrete (VHPC: self-compacting C80/95 with silica fume). Besides concerns of mixture proportions optimization with respect to rheological requirements for ensuring a perfect concreting of the thin enough VHPC flange, a major issue of the programme consisted in validating a design method for the prediction of stress redistribution due to concrete creep within the cross-section. Two 13 m-long beams were made and monitored during several months after the VHPC casting. Two months after the VHPC casting, dead loads, representative of upper deck, rails and equipments weight, were applied. The experiments realised on the beams allow drawing the following conclusions: • the evolution of the deflection appears very limited when concrete age is beyond 2 months - corresponding to the period where a significant part of creep and shrinkage has already taken place - what confirms VHPC interest; • the feasability to optimize a prebent beam with a VHPC is demonstrated. Practically, the design is always controlled by the serviceability limit state for this kind of structures, due to the following criteria; • control of the concrete delayed effects by limiting the stress during the prestressing, which implies to have accurate data on creep and mechanical properties of the concrete, particularly at early age; • control of cracking by keeping compression under permanent loads.Detailed results of the global response of one the beams as well as the validation of the computation method for taking creep into account are analyzed in conjunction with deflection and strain measurements.

The formulation of creep problems in concrete structures, using linear viscoelasticity leads to integral equations that, generally, can not be solved in closed form. Methods of analysis developed to overcome this problem include the AAEM algebraic method, the method based on the theorems of linear viscoelasticity, or methods that assume simplified creep models. But when the structural problem is complex, involving non-homogeneities, construction steps, changes in static scheme, complex geometry, high prestressing, different materials, a computational approach based on the numerical solution of the systems of equations is required. Because of the form of the viscoelastic law, in which the present strain is a functional of the whole stress history, the stress history of the structure needs to be stored during calculation. Furthermore the numerical methods necessary to solve the integral equations are not immediately compatible with linear and non-linear finite element analysis and the substitution of integral equations with rate-type approximate laws, seemed to be necessary to allow the use of FEM solvers. In this paper a set of structural problems is described from the point of view of mathematical and computational formulation, and the general method based on coupling integral equations with the equilibrium method, and then with finite element method, is shown. The proposed formulation allows to perform the step-by-step analysis of any kind of viscoelastic structure, without regard to its complexity, and the computational load, in terms of memory requirements, appears to be no longer prohibitive.

It is well known that creep and shrinkage of concrete cause long term deflections of continuous prestressed concrete bridges. A large amount of research has been conducted in this area. While the bridges continue to deform with time, the moment distributions of the bridges remain essentially unchanged under the influence of creep if the bridge temperature is uniform. The change of stresses is caused by prestress losses due to the creep, shrinkage of concrete and the relaxation of steel. However, rate of creep of concrete increases with temperature. The seasonal and daily temperature variations of a prestressed concrete deck not only induce thermal stresses but also significant stress redistribution due to non-homogeneous creep rates in the concrete. The paper presents some recent findings from Steady State analyses. The provision in the latest European code EN1992 (EC2) for thermal creep is discussed. The amount of moment redistribution will be quantified against a range of thermal creep rates. The results show that stresses could be significantly underestimated if temperature and thermal creep are not considered in design. The advancement of EC2 as well as its limitation in modelling the long term behaviour of prestressed concrete bridges are also discussed. The paper also addresses the problems that can be created when the structural continuity is created for the first time due to the remedial works after the bridge has been in service many years. Under these circumstances creep can have a major effect on both serviceability and safety.