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Synopsis: 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., SCOPUS: cp.p, info:eu-repo/semantics/published

Synopsis: 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., SCOPUS: cp.p, info:eu-repo/semantics/published