Reinforcing strategies for three-dimensional printing (3DP) of cementitious materials (mostly mortars) have been extensively studied in recent years. Among various reinforcement strategies available for 3DP of cementitious materials, the use of fibers is frequently mentioned as a promising approach to enhance their mechanical performance. This work aims to evaluate the influence of four types of fibers (polyvinyl alcohol [PVA], nylon, rayon, and basalt) on the flowability and flexural strength of mortars used in 3DP. The flexural behavior of 3DP beams was compared with that of cast specimens, and the digital image correlation (DIC) technique was used to evaluate the development of the cracks. The fiber orientation in the reference (cast) and 3DP samples was examined using optical microscopy. The results revealed that, among four types of fibers used, the PVA fibers were most effective in increasing the flexural strength of both the cast and 3DP specimens. In addition, the results show that all fibers preferentially aligned parallel to the printing direction. 3DP specimens with filaments aligned in the direction perpendicular to the direction of the applied load showed superior flexural strength when compared to the cast specimens.

Ultra-high-performance concrete (UHPC) is considered a sophisticated concrete construction solution for infrastructure and other structures because of its premium mechanical traits and superior durability. Fibers have a special effect on the properties of UHPC, especially as this type of concrete suffers from high autogenous shrinkage due to its high cementitious content, so the properties and volume fraction of fibers are more important in UHPC. This study will describe previous related works on the mechanical behavior of UHPC specimens reinforced with micro- and nanoscale fibers, and compare of the behavior of UHPC reinforced with microfibers to that reinforced with nanofibers. The compressive strength, flexural behavior, and durability aspects of UHPC reinforced with nanoand/or microscale variable types of fibers were studied to highlight the issues and make a new direction for other authors.

Inconsistencies in standards and codes result in confusion, increased costs, and do not promote the efficient use of concrete. In addition to inconsistencies, the lack of science-based approaches and data used for defining criteria in these standards and codes can limit the reliability and trust of these requirements. A review of industry documents indicates that inconsistencies and lack of science-based approaches exist across many documents, both throughout the industry and within ACI, relating to the corrosion of steel reinforcement embedded in concrete. This paper proposes to address five key issues to promote science-based standardization of requirements necessary for reinforced concrete systems exposed to corrosive conditions. These five issues include the need for: 1) standardization of chloride testing methods and requirements; 2) standardization of chloride reporting units; 3) standardization of terminology for specifying chlorides in cementitious systems; 4) standardization of exposure classifications for corrosive conditions; and 5) standardization of allowable chloride limits. This paper presents current inconsistencies in guide documents and codes for each of the items listed previously and then proposes an approach to standardize each using either available data and/ or a scientifically based approach. Recommendations for testing, reporting, definition of exposure classifications, and allowable chloride limits are then proposed. It is hoped that the systematic approach used herein will lead to standardization and consistency, less confusion, and will promote the efficient use of durable and economical concrete.

Carbon-based nanomaterials such as graphene oxide sheetreinforced cementitious composites have attracted extensive interest owing to their improved post-fire mechanical properties. However, the role of graphene in anti-thermal detriment is still unclear. In the current study, the mechanical characteristics, pore structure, and interface evolution of graphene-toughened cementbased materials under high temperatures are investigated. Scanning electron microscope analysis showed that graphene implanted in the cement matrix had out-of-plane deformation at elevated temperature. The deformation caused the evolution of the interface between graphene and the cement-based material with respect to temperature. Correspondingly, the toughening effect of graphene on cement-based materials decreased first and then increased. The reinforced domain of graphene switched from mesopores to capillary pores when the temperature was beyond 400°C, contributing to the enhanced reinforcement efficiency of the cement mortar. The interfacial evolution process with an in-depth analysis based on multiple scales would benefit from optimizing the design of graphene composites at high temperatures.

A reliable compressive stress-strain model was established for concrete with varying densities reinforced with either steel fibers alone, or a combination of steel fibers and micro-synthetic fibers. Moreover, a simple equation was presented to determine the compressive toughness index of fiber-reinforced concrete in a straightforward manner. The fiber reinforcing index was introduced to explain the effect of various parameter conditions of fibers on the enhancement of the concrete properties under compression. Numerical and regression analyses were performed to derive equations to determine the key parameter associated with the slope at the pre- and post-peak branches and compressive toughness index through extensive parametric studies. The proposed models are promising tools to accurately predict the stress-strain relationships of fiber-reinforced concrete with different densities, resulting in less-scattered values between experiments and predictions, and reasonably assess the efficiency of fiber reinforcements in enhancing the compressive response of concrete.