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Freezing and thawing of water in saturated concrete induces stresses, cracks, spalling, and eventually allows chlorides, sulphates and other ions to penetrate through the porous space of concrete causing the corrosion and ultimately structural failure. These detrimental factors are limited if concrete has the ability to repel water. Water repellant concrete was achieved through internal and external surface applications. Hydrophobicity, over-and super-hydrophobicity are demonstrated by the ability of a surface to repel water and are characterized by contact angles. In cementitious composites internal hydrophobization can be achieved through the addition of superhydrophobic admixtures to fresh concrete resulting in an air void system with small, well-dispersed air bubbles to provide superior resistance to freezing and thawing. Hydrophobic, over-and super-hydrophobic surface coatings were achieved by the design of hierarchical surfaces tailoring the fiber content, mixture proportion, and superhydrophobic emulsions. Contact angle tests were used to characterize the developed coatings. The use of internal hydrophobization improves freezing and thawing resistance of fiber-reinforced composites as demonstrated by a durability factor of 100 through as many as 700 accelerated (-50°C to 20°C) cycles in 5% NaCl solution.

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Ultrahigh performance concrete (UHPC), which is characterized by dense microstructure and strain hardening behavior, provides exceptional durability and a new level of structural response to modern structures. However, the design of the UHPC matrix often requires the use of high quantities of supplementary cementitious materials, such as silica fume, which can significantly increase the cost and elevate the production expenses associated with silica fume handling. This paper demonstrates that a fiber-reinforced composite with properties similar to conventional UHPC can be realized with very low quantities of silica fume, such as 1% by mass of cementitious materials. The proposed UHPC is based on reference Type I cement or Type V Portland cement with very low C3A (<1%) that also complies with Class H oil well cement specification, silica fume, small quantities of Al2O3 nanofibers, and high-density polyethylene or polyvinyl alcohol macro fibers. Previous research has demonstrated that nanofibers act as a seeding agent to promote the formation of compact and nanoreinforced calcium silicate hydrate (C-S-H) clusters within the interparticle and nanofiber spaces, providing a nanoreinforcing effect. This approach produces a denser and stronger matrix. This research expands upon this principle by adding synthetic fibers to ultrahigh strength cement-based composites to form a material with properties approaching that of UHPC. It is indicated that the developed material provides improved strain hardening and compressive strength at the level of 160 MPa.