Diseño de hormigones de altas prestaciones para la mejora de su resistencia y durabilidad

Resumo

Concrete is currently the most significant material in the world of construction due to its use in numerous applications and the research studies about it currently carried out worldwide. Concrete technology has shown a marked technological advance in recent decades. The appearance of new types of cements, highly reactive additions, structural fibres and latest generation additives have allowed the development of special concretes with improved performance that adapt to current requirements and demands. Among the different types of concrete, ultra-high performance concrete (UHPC) stands out, which in addition to improving the mechanical properties of concrete, also enhances its durability. Such concrete, through the use of aggregates of smaller diameters than traditional ones, additions that can reach nanometric sizes and the use of superplasticizer additives, manages to modify the microstructure of its matrix, making it more homogeneous, denser and with less porosity. This composite material is the focus of this research, which attempts to broaden the understanding of UHPC at a microstructural level. The main objective of the study is to optimize the microstructure of the UHPC, which might be translated into the improvement of its mechanical properties and durability. In addition, the mentioned improvements will be explained based on the results of an experimental campaign. To achieve the proposed objectives, the investigation has been divided into four steps: mixture design, assessment of the mechanical properties, microstructural characterization and durability evaluation. The research considered a mixture as reference, which was termed CTRL, where several additions, such as: metakaolin (MK), silica fume (SF) and two types of nanosilica, 50 and 200 m2/g of specific surface (NS1 and NS2, respectively) were added. Besides, a group of specimens was manufactured including steel fibres 13 mm in length and 0.20 mm in diameter. The specimens were subjected to compressive strength tests, differential thermal and thermogravimetric analysis (DTA-TG) and mercury intrusion porosimetry (MIP), electrical resistivity, chloride migration, accelerated chloride penetration and accelerated carbonation. Compressive strength tests showed that the mixes with additions increased their strength with respect to CTRL formulation, up to 13% at 7 days of curing age, up to 16% at 28 days and up to 14% at 91 days of age. The specimens that included steel fibres (Vf = 0.89%), increased their compressive strength at 7 and 28 days, at least 20%, with respect to the analogous specimens without fibres. DTA-TG tests showed that the formulations with additions, especially those that included NS1 and NSC (combination of NS1 and NS2), increased the amount of C-S-H gel with respect to CTRL specimens both at 28 and 91 days, as well as the C-S-H gel/portlandite ratio. MIP test showed a reduction in total porosity with the use of additions, reaching values of 5.91% at 28 days in the case of the 4MK.4SF+2NS1 mixture. The refinement of the porous network was reflected in the reduction of the percentage of large capillaries and in the increase of the percentage of gel capillaries, especially with the use of NS1 and NSC. Lastly, the durability tests showed that the use of additions substantially improves the durable behaviour of UHPCs, especially if nanoadditions are included. After analysing the results obtained, it was concluded that both the distribution of the porous network and the formation of different hydrated products in UHPC mixtures are related to their mechanical resistance and durability. The work carried out can be used as a starting point for future research on the use of nanoadditions in new types of UHPC.