The research project SeLCo was completed – 'Service life behaviour of concrete structures: a mul
About Project
Concrete is a composite material whose stress history begins at its early ages. Soon after concrete casting important exothermic cement hydration reactions take place, ensuring the development of an internal porous structure responsible for the material strength growth. Simultaneously, chemical and physical processes driven by early cement hydration and by concrete interactions with the surrounding environment are responsible for load-independent volumetric changes, of thermal and autogenous/drying shrinkage origins. Under partial or total restraints these volumetric changes lead to an initial stress state build-up that may either limit concrete ability to carry further tensile stresses, or generate cracks, during service life operating conditions.
In spite of the growing attention devoted by recent design codes (like Eurocode 2) to service life behaviour of concrete structures, the know-how on this subject is still far from being consolidated in civil engineering practice. This is due to the enormous variations that may occur on the concrete compositions and on the in-situ environmental conditions, which makes rather difficult to predict the actual shrinkage deformations based on the simplified rules provided in codes. Furthermore, concrete performance as related to shrinkage is still insufficiently known in what concerns to stress relaxation under restrained conditions, and to the role of humidity on the development of local shrinkage deformations. Prediction of concrete actual stresses and cracking since casting and throughout service life, with due account for major influences such as shrinkage, early age thermal deformations and evolution of the mechanical properties, is therefore a crucial issue. The present project aimed to contribute to overcome this knowledge insufficiency, providing as a final output a more realistic numerical simulation framework to predict concrete stresses under service life conditions, allowing economical savings as far as malfunction avoidance is concerned.
Two of the main phenomena involved in the load-independent volumetric changes of concrete are: (i) expansion/contraction associated to temperature variations (heat of hydration included); (ii) expansion/contraction associated to changes in the moisture conditions within the pore structure of the cementitious concrete matrix (both due to internal water consumption in chemical reactions, and to evaporation towards the surrounding environment).
A recently concluded research project headed by the same Principal Investigator (POCI/ECM/56458/2004 – Early age concrete: Behavioural prediction) allowed the research team to get substantial know-how on volumetric changes (i), related to cement hydration heat release. It is considered that the experience of the research team, the thermo-mechanical framework developed so far and the lab equipments acquired within that project are cornerstones to the proposal of the present research project. To cope for the concrete volumetric changes (ii), within the present proposal an extensive experimental research focused on the evolution of moisture distribution inside concrete, and its relation with shrinkage deformations, is considered strategic for fulfilment of the project objectives (Tasks 1, 2 and 3). Besides, a variable axial restraint testing device (developed within Task 4) was used to characterize the tensile behaviour of concrete induced by shrinkage (autogenous or drying), using specimens with a dog-bone configuration (Task 5). Tensile stress relaxation due to concrete creep, as well as post-cracking behaviour, were taken in due account on this testing device.
After this experimental campaign, a numerical framework was implemented (Task 6), upgrading the existing thermo- mechanical numerical model developed by the research team during the concluded project referenced above. The upgraded numerical model, to was implemented on a Finite Element platform, has to solve a thermal problem (to predict the distribution of temperatures generated by the cement hydration heat release), and to evaluate the relative humidity distribution inside concrete by solving a moisture problem (an essential improvement of the present project). The output of these thermal and moisture models allows the evaluation of the corresponding concrete deformations. Then, a mechanical model takes these deformations into account to finally predict concrete stresses, with due allowance to phenomena like strength growth (aging), creep and cracking. Due to the explicit modelling of temperature, moisture and stress fields, this multiphysics approach renders a thermo-hygro-mechanical numerical framework, an essential outcome of the present project.
Finally, Task 7 was focused on the numerical simulation of real structures, appropriately monitored, to provide an in-depth validation of the developed numerical tools, and a thorough discussion concerning service life concrete performance.
