Optimización del tiempo de desencofrado en piezas verticales de hormigón autocompactantedesarrollo de metodología y estudio experimental

Abstract

The construction industry has been changing and evolving as well as the technological advances that aim at increasing the freedom of construction designs at its most. The self-compacting concrete, due to its characteristics is easily adapted to modern requirements, since it is able to flow and consolidate under its own weight, with no need of any method of compacting, and at the same time, it is cohesive enough to fill all the spaces of any size and shape, without segregation and bleeding. These characteristics, apart from being useful in the construction of complex structures or when the density of reinforcement is very high, they also allow to increase the rate of placement, reducing the construction time. However, these advantages lead to the necessity of modifying the criteria considered in the design of vertical formworks compared to the ones used in conventional concrete. The cost of formworks can be up to 60% of the cost of the concrete structure. The way to optimize this cost would depend on two factors: the minimum time needed to remove the formwork, so that it can be reused and consequently increase its competitiveness; and the correct selection of its panels, which depends on the lateral pressure caused by concrete, which they have to be able to support. The demoulding time is affected by the development of the concrete resistance, since the formwork will be removed once the concrete is able to support itself and the corresponding efforts of the striking. In this thesis, the research has been focused on the development of a demoulding methodology, with its respective analysis and its experimental validation in situ. In order to apply this methodology it is necessary to determine two parameters: resistance and temperature. Thus, the study of the different existing methodologies in the literature to determine the resistance of concrete at early ages has been deepened. Consequently, an experimental campaign has been made with the elaboration of about ten beams and a hundred cylindrical specimens of concrete to study which of these methodologies best fits to the methodology described above. Once that experimental campaign was completed, the demoulding methodology was validated in two real constructions placed in Montevideo, with different mixtures and at different times of the year. On the other hand, as previously explained, the design of vertical formworks also includes a correct selection of the panels which depend on the lateral pressure that they have to support. Therefore, overrating the pressure would lead to thicker panels and higher cost. The most conservative solution to determine the lateral pressure is to consider the hydrostatic distribution of a liquid with the same density of concrete, whose use would produce robust formworks and at a very high cost. What is more, the economical aspect is limited by safety since underestimating the lateral pressure can lead to bad quality pieces or, even worse, to faulty formworks. From the bibliography , about 130 experimental data records, obtained by different authors, have been compiled. This data has enabled, firstly, to detect the areas where a major experimental effort is necessary. For this reason, an experimentalcampaign to study the influence of the temperature and the presence of reinforcement and its quantity on the formwork striking times and the value of the lateral pressure exerted by the SCC has been made. Secondly, this data has allowed to evaluate the precision of the different experimental models described to predict the maximum lateral pressure, regarding the level of control in the construction and the rate of placement of the SCC into the formwork. Finally, a statistical model based on confidence intervals of simple linear regressions,which allows to obtain very good results of the predicted maximum lateral pressure in situ was developed. It is intended that all the work conducted in this thesis allowing the development of tools to optimize the minimum demoulding time and the improving of the knowledge of the lateral pressure exerted by this type new type of concretes.