Crystalline hydrophilic additives are increasingly used as efficient methods for reducing water permeability in concrete. Their effectiveness in hindering water penetration has been proven in different cementitious materials, although scarce information has been reported concerning their action mechanism. In the present work, the efficacy of a hydrophilic blended crystalline mix (Krystaline Add1) as a water-reducing additive has been confirmed. Furthermore, an extended study about how the presence of the additive influences both the fresh state and the hardened state properties is presented. Finally, characterization techniques such as Mercury Intrusion Porosimetry (MIP), X-ray Powder Diffraction (XRD) and Back-Scattered Scanning Electron Microscopy (BSEM) with Energy Dispersive X-ray analysis (EDAX) have been used for deducing the mechanism of the additive. No significant deleterious influence on the concrete properties due to the addition of the additive has been detected. In fact, the additive seems to have provided a positive influence on the concrete given that a slight reduction in the w/c ratio for similar consistency has been detected, with the subsequent improvement of the compressive strength values. Its effectiveness as a water permeability reducing additive has shown encouraging results having reduced the water permeability by approximately 50% during testing. The action mechanism of the studied additive seems to be related to hydration reactions in the presence of water, producing new solid amorphous phases in the concrete bulk.

Fiber reinforcement of concrete is an effective technique of providing ductility to concrete, increasing its flexural residual strength while reducing its potential for cracking due to drying shrinkage. There are currently a wide variety of industrial fibers on the market. Recycled steel fibers (RSF) from tires could offer a viable substitute of industrialized fibers in a more sustainable and eco-friendly way. However, mistrust exists among users, based on fear that the recycling process will reduce the performance, coupled with the difficulty of characterization of the geometry of the RSF, as a consequence of the size variability introduced by the recycling process. This work compares the behavior of RSF from tires compared with industrialized steel or polypropylene fibers, evaluating the fresh state, compressive strength, flexural residual strength, and drying behavior. The concept of Equivalent Fiber Length (EFL) is also defined to help the statistical geometrical characterization of the RSF. A microstructural analysis was carried out to evaluate the integration of the fiber in the matrix, as well as the possible presence of contaminants. The conclusion is reached that the addition of RSF has a similar effect to that of industrialized fibers on concrete’s properties when added at the same percentage.

The determination of thermal conductivity of cement-based materials is relevant from the perspective of buildings’ energy efficiency. The absence of unified tests for its measurement in mortars and concrete results in a heterogeneity of the data available in the literature. This work’s purpose is to determine the relevant influence from a a statistical viewpoint that three factors; degree of saturation, measuring time and use of a conductive paste, have in the measurement of the conductivity using the hot-wire needle probe method in two concretes with different thermal behavior: standard-weight concrete and lightweight concrete. The results obtained allow for the establishment of recommendations for future researchers on the minimum information to be included in their reports of thermal conductivity of cement-based materials by the needle probe method, the need to treat outliers, the most favorable saturation conditions and measuring times, as well as the possible benefits of using conductive pastes.