International Concrete Abstracts Portal

Structural conception of bridges is strictly related to the construction methods involved. A wide application of precasting technologies of reduced dimension elements assembled on site is today the most common choice. During construction, static schemes and strength characteristics of the elements generally change. The connection of different structural elements in subsequent stages, the introduction of delayed restraints and the use of provisional supports modify the structural configuration. Such transient phases can influence significantly the final behaviour of the bridge. In such a way the designer, acting during the constructive stages, can force the structural scheme towards the solution that optimizes elements functionality. In this paper a study about the influence of delayed strains due to creep on the structural conception of prestressed bridges is considered. The effects of additional restraints and support removal on the final solution are evaluated and discussed. The study permits characterizing the phenomena, to define possible simplifications, useful for developing simplified models in the hypothesis of linear viscoelasticity and homogeneous structures. These models allows the use of elastic solutions by means of the redistribution functions. A case study of an arch-portal bridge is presented. During construction a fundamental role is played by the removal of temporary piers after the introduction of delayed restraints, with a change of the static scheme from the continuous beam behaviour to the arch one.

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.

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.

Differential shrinkage - shrinkage difference between an old concrete substrate and a new-cast concrete overlay - causes stresses of substantial magnitude in repaired concrete structures. Consequently, it is important to determine normal stresses in the overlay and in the substrate as well as shear stresses in the interface. The literature on differential shrinkage problems goes back to the 1950s. Most theories use Bernoulli’s hypothesis as an important assumption. Bernoulli’s hypothesis states that plane sections remain plane after bending. It facilitates the computations. In this paper, Swedish laboratory tests on overlaid concrete beams have been reconsidered to test the validity of Bernoulli’s hypothesis. Strain measurements across the beam depth support Bernoulli’s hypothesis. Still the ultimate and generally accepted theory for computing stresses and strains in composite concrete structures subjected to differential shrinkage is missing but this paper shows that Bernoulli’s hypothesis may constitute one of the foundation-stones in such a theory.

Concrete cracks when the tensile stress exceeds the tensile strength. Tensile stress arises from restrained contraction. The main sources of contraction are early age effects, temperature drop and drying shrinkage. Creep offers significant relief to early-age contractions, but only a little to shrinkage. Tensile strength reduces under sustained stress, so the duration of the contraction is important. A new graph is presented to show that although cracking can occur at any age, it is most likely either during the first 3-10 days or after some years. This is because the creep relief provides a margin for temperature variations and some shrinkage to occur without exceeding the critical tensile stress. However this margin diminishes with time. A new flow chart is then presented which shows what controls are necessary at each of these two key stages. This differentiates between avoiding cracking and controlling cracking, an important distinction that is often confused in the approach to design. The case study of a feature wall where this was not appreciated is described.