International Concrete Abstracts Portal

Previously, a significant research effort was made to analyse the long-term behaviour of a kind of composite steel-concrete, prebent and prestressed beam used extensively in Belgium as simply supported railway bridge decks. They are trough type with U shaped cross section. Parallel with that, an extensive experimental research has been carried out to extend the Belgian technique of steel-concrete prebent beams to Very High Performance Concrete (VHPC). The main advantage of using VHPC instead of the C50 concrete currently used is to decrease the prestressing losses of the system thanks to a significant decrease of the creep deformations. One of the motivations behind this research was to develop realistic models to assess the feasability of constructing continuous railway bridges by connecting such simple decks over intermediate supports. For railway bridge decks, the condition of no cracking under load is paramount. This implies to devise a form of in situ prestressing of the connection between the simply supported decks. A structural analysis program was developed that could cope with multiple phases of construction and loading for such a highly heterogeneous structure. The structural analysis is performed within the framework of beam-type displacement-based finite element analysis. A step-by-step time-dependent analysis is based on the algorithm of superposition. The creep and shrinkage model is the CEB-FIP MC 90 model, which was found to best reproduce the strains recorded in the laboratory creep and shrinkage tests for the C50/60 concrete of the singly supported decks and for the C80/95 VHPC. Creep recovery effects occurring at unloading have been accommodated in the step-by-step time-dependent analysis with the two-function method proposed by Yue and Taerwe. The purpose of this paper is to present a design optimization study of the construction phases of a continuous bridge with prebent and prestressed composite decks. The most influential parameters on the long-term behavior of the continuous bridge are analyzed.

Differential shortening caused by creep and shrinkage of reinforced concrete columns and shear walls affects the serviceability of high-rise buildings. For structures up to 30 stories or 400 ft (120 m) high, the effects of creep and shrinkage are usually ignored without serious consequences. For reinforced concrete buildings beyond 30 stories, and for shorter buildings of hybrid or mixed construction, ignoring the effects of creep and shrinkage may create several undesirable conditions in the serviceability of the structure. Owners of high-rise concrete buildings are aware of the potential for undesirable behaviors in service in both structural and in nonstructural elements from the effects of differential shortening of columns and shear walls. Examples include sloping floors; cracking of structural members and interior partitions; buckled elevator guide rails, misaligned elevator stops relative to floors, and damage to façade elements and plumbing risers. To minimize these behaviors, the structural engineer is challenged to predict, design for, and adjust for differential shortening in each of the structural components during construction, as well as forecast future behaviors. The structural design process and related construction requirements are discussed and illustrated within.

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.

An extensive experimental study 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). In comparison to the present constructions in Belgium, the main advantage of using VHPC instead of C50 concrete is to decrease the prestressing losses of the system thanks to a significant decrease of the creep deformations, together with the possibility to optimize the beam weight and its serviceability domain. As one research step was the casting of concrete around the bottom flange of two 13m-span steel girders, it was necessary to use a self compacting concrete with a suitable maximum aggregate size due to the very dense network of wires, sensors, ribbed stirrups and steel square ribs. In addition, the VHPC had to develop a very high compressive strength at early age to remove the prebending loads applied on the steel girders 48 hours after casting. The autogenous shrinkage development was examined both under standard isothermal (20°C) and realistic temperature (same as within the beams) conditions. Mechanical characterization, Young’s modulus, creep and shrinkage tests in standard (20°C, 50% RH) and variable (same as within the beams) ambient conditions were performed, so that a correct analysis of the structural behavior of the beams could be done. Due to the efforts of mixture proportions optimization, it could be demonstrated that VHPC delayed deformations were reduced by about a factor 2 in comparison with currently used C50 concrete, which made the use of prebending significantly more efficient. The purpose of this paper is to report on these experimental investigations and to present the computation method which was used to predict the time-dependent evolution of VHPC creep and shrinkage based on European model codes for the creep and shrinkage and on the recovery model proposed by Yue & Taerwe for the creep recovery.

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.