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This paper seeks to optimize the mechanical and durability properties of ultra-high performance concrete (UHPC). To meet this objective, concrete specimens were manufactured by using 1100 kg/m3 of binder, water/binder ratio 0.20, silica sand and last generation of superplasticizer. Silica fume, metakaolin and two types of nano silica were used for improving the performances of the concrete. Additional mixtures included 13 mm long OL steel fibers. Compressive strength, electrical resistivity, mercury intrusion porosimetry tests, and differential and thermogravimetric thermal analysis were carried out. The binary combination of nano silica and metakaolin, and the ternary combination of nano silica with metakaolin and silica fume, led to the best performances of the UHPC, both mechanical and durable performances.

This paper seeks to optimize the mechanical and durability properties of ultra-high performance concrete (UHPC). To meet this objective, concrete specimens were manufactured by using 1,100 kg/m3 of binder, water/binder ratio 0.20, silica sand and last generation of superplasticizer. Silica fume, metakaolin and two types of nano silica were used for improving the performances of the concrete. Additional mixtures included 13mm long OL steel fibers. Compressive strength, electrical resistivity, mercury intrusion porosimetry tests and differential and thermogravimetric thermal analysis were carried out. The binary combination of nano silica and metakaolin, and the ternary combination of nano silica with metakaolin and silica fume, led to the best performances of the UHPC, both mechanical and durable performances.

Resumen-Research developed in the last decades has allowed significant advances in the concrete technology, achieving remarkable improvements in its performancemainly related to its microstructure. In order to relate the behaviour under compressive stresses of high performance concrete (HPC) and the changes in its microstructure, an experimental campaign was developed with specimens of dimensions 160x40x40 mm 3. A control mixture without additions and a series of mix designs with partial substitution of cement by of silica fume and metakaolin in different proportions were produced. A low water-to-binder ratio of 0.20 was used, which forced the use of a superplasticizer additive based on modified polycarboxylates, which improved the workability of the mixtures. Silica sand with 98% of SiO2 content and an average particle size of 0.75 mm was used. The specimens were subjected to compressive strength tests at 2, 7, 28 and 91 days, and complementary tests of electric resistivity, mercury intrusion porosimetry (MIP) and differential thermalthermogravimetric analysis (DTA-TG) at 28 days. The results of the tests showed that the increase in compressive strength with the use of additions was directly related to the reduction in total porosity and the increase in the proportion of total CSH gel / total portlandite, shown by the MIP and DTA-TG tests, respectively. Mixtures with additions showed reductions of the porosity by up to 31.7% compared with the control mixture, while increasing the proportion of CSH gel/ total portlandite by up to 17.1%. The use of small particles of mineral additions significantly increased the electric resistivity of HPC specimens as evidenced in the present study. This is explained by the reduction in the percentage of large capillary pores and the relative increase of small capillaries, which could enhance the durability of the concrete elements.

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