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The behavior of steel fiber reinforced concrete (SFRC) is highly dependent on mix design, where the properties of its components, such as cementitious matrix and fibers, impact the physical and mechanical properties of the composite. Incorporating steel fibers into concrete modifies its physical properties, potentially leading to fiber segregation and resulting in a nonuniform distribution of the fibers within the material. In experimental studies, an increase in fiber volume has been found to change the failure mode of SFRC beams from shear to flexure, and numerical investigations have shown that randomness in fiber distribution can significantly influence the behavior of the composite. Therefore, this study aims to develop a numerical model that considers variations in material properties to simulate the change in the failure mode of SFRC beams due to variations in fiber volume. The failure and yielding parameters were initially established and integrated into the concrete damaged plasticity (CDP). Subsequently, the response of both steel and concrete to tension or compression stresses was ascertained. Finally, the appropriate meshes and finite elements were chosen for the simulation. In the final approach, two methods were employed to introduce randomness into the beam. One divided the beam into different vertical segments: 30, 45, and 90, and the other combined vertical and horizontal segments: 30 vertical with 4 horizontal, 45 vertical with 5 horizontal, and 90 vertical with 10 horizontal. A script was developed in Python to automatically insert the properties. The numerical model successfully captured the change in failure mode from shear to flexure resulting from fiber volume increase. Vertical and horizontal segments increased toughness due to crack deviation, better predicting cracking pattern and loading capacity for the highest fiber contents. [ABSTRACT FROM AUTHOR]

• Sandwich panels with natural fiber reinforced composite skins and AAC core are a sustainable alternative to construction materials; • Sandwich panels' bonding between FRC and AAC demonstrated a predominant composite failure and a satisfactory adhesion in the short and long terms; • The unidirectional curauá fiber reinforcement is capable of bridging the cracks and increasing the energy absorption capacity of the composite skin layers; • Subjected to cyclic loading, the panels exhibited a composite response with gradual energy loss and adequate ductility. The mechanical response of sandwich panels with two layers of curauá fiber cement composites and a core layer of autoclaved aerated concrete (AAC) was investigated. Each layer consisted of long unidirectional curauá fibers and a cementitious matrix. For durability purposes, 50% of the Portland cement was replaced with pozzolanic materials to reduce the content of calcium hydroxide in the matrix. The performance of sandwich panels with 350 mm × 60 mm × 90 mm (length × width × thickness) was evaluated through monotonic and cyclic four-point bending tests to obtain the load–deflection response, flexural strength, and toughness. Monotonic bending tests were performed on each component of the panels – i.e., composites and AAC blocks. Pull-off tests were performed to evaluate the adhesion between the skin layers and the core, while optical and scanning electron microscopes were used to observe the interface's topography. The results revealed the externally bonded layers' efficiency, which provided a higher deflection capacity to the material and increased its flexural strength. The AAC blocks revealed a more ductile response when assisted by the composite layers in the sandwich structure. Under cyclic bending test conditions, the sandwich panels exhibited a satisfactory post-peak ductility so that the energy was not abruptly lost but gradually released throughout its deflection-softening behavior. [ABSTRACT FROM AUTHOR]

The construction industry demand for environmentally friendly and energy-efficient solutions has driven researchers to matching non-traditional materials, as natural resources, with modern building technologies. The present work investigated the mechanical behavior and cracking mechanisms of a cementitious composite reinforced with natural curauá unidirectional fabric towards its use as reinforcement for a structural concrete element. The experimental program included direct tension, bending and cyclic tests in cement-based composites reinforced with curauá natural fabrics. Different specimen scales were tested under direct tensile loading in order to study the cracking mechanisms and strain sensitivity to size effect. A structural beam externally reinforced with this newly developed composite was tested under bending and an analytical model was proposed to corroborate the experimental tests. The proposed model gave very accurate prediction of the moment-curvature. The composite used as a structural reinforcement behaved efficiently, resulting in a rebar yielding delay, providing to the beam a higher ductility and a flexural strength gain of about 16%. • Natural fiber composites can be used as a structural reinforcement in R/C elements. • The use of natural fiber composites can increase ductility and flexural strength in structural concrete. • An analytical model was proposed and resulted in an accurate prediction of the moment-curvature. • The use of the natural fiber composites resulted in a delay of the strain development in the rebars. [ABSTRACT FROM AUTHOR]

• Sustainable composites reinforced with natural fibers can be used as shear reinforcement in R/C structures. • The reinforcement with natural fibers for shear strength is comparable to conventional reinforcement materials. • The used composite did not show any signs of delamination during shear loading. • A new shear model is proposed using experimental data from Iosipescu tests. The use of fabric reinforced cementitious matrix for shear strengthening of concrete structures has been widely investigated and its field of application is not an uncommon practice. However, the majority of previous works has been limited to synthetic or metallic fabrics. Thus, the present study seeks to investigate the shear mechanical behavior of a cementitious composite reinforced with natural curauá fabrics and its application as a shear structural reinforcement. The experimental program includes Iosipescu tests and the use of Digital Image Correlation technique for material characterization and flexural tests for strengthened structural beams. The developed composite material under shear tests demonstrated a shear strain-hardening behavior with well-defined stages. The beam externally reinforced with the composite presented an improved ultimate strength of 28%. Analytical model predictions were compared to experimental results and a good agreement was achieved. [ABSTRACT FROM AUTHOR]

• Composites reinforced with saturated sisal fibers resulted in a denser interface with smaller crack width. • The fiber–matrix interface plays a major role during creep tests in pre-cracked specimens. • During creep tests composites reinforced with saturated sisal fibers showed lower crack widths. • Composites reinforced with natural humidity fibers presented higher first crack strength but larger crack widths in static and long term tests. In the present paper, the creep behavior of sisal fiber cement based composites was investigated. The composites were manufactured using a matrix with low calcium hydroxide content, obtained by partially replacing the cement by fly ash and metakaolin. Two types of composites were developed, each with three layers of long and unidirectional sisal fibers, varying the condition of the fibers in saturated with water and with natural humidity. The mechanical behavior was evaluated through direct tension and four-point flexural tests. The composites reinforced with saturated fibers showed higher strain capacity in tension due to the properties of the fiber–matrix interface and lower ultimate bending strength. Cracking mechanisms were studied through photographs obtained during the tests, as well as analyzes by a stereoscopic microscope. All composites presented strain/deflection hardening behavior with the formation of multiple cracks. Tensile and flexural creep tests were performed on pre-cracked composites in order to study time-dependent responses. The evolution of cracks was monitored throughout the creep test with a stereoscopic microscope. The composites showed a reduction in strength and strain capacity after the creep tensile and flexural test, which resulted in a decrease in toughness of 36.9% and 48.5% for the composites with saturated fibers and natural humidity, respectively. In addition, for the flexural creep test, the composites with saturated fibers showed a decrease in toughness of 48.2% while the composite with natural humidity had a decrease of 35.3% for the load equivalent to 50% of their strength. [ABSTRACT FROM AUTHOR]

• Half-hooked fibers presented energy dissipation capacity similar to hooked fibers. • Optimal fiber content was found to be 0.75% for the investigated composites. • Half-hooked fibers had more pullout sensitivity to the presence of fibers in the matrix. • Embedment length had no conclusive influence on the pullout load. The work in hand reports an experimental work on the pullout behavior of hooked-end, half-hooked and straight steel fibers from a High-Strength Fiber-Reinforced Self-Compacting Concrete (HSFRSCC). The influence of three matrix fiber contents on pullout was investigated and compared to an unreinforced control matrix. It was found that half-hooked fibers had the best capacity during the friction regime and can dissipate more energy than hooked-end fibers. Also, the presence of fibers in the matrix had the most beneficial effect on the composite with a fiber content of 0.75%, explained by the improved confinement of the reinforced matrix, fiber to fiber interlock, and better control of splitting cracks for fiber with mechanical anchorage. Further increase in fiber content (V f = 1%) had a deleterious effect reducing the bond performance. In General, the half-hooked fibers were the only ones presenting better dissipation capacity than the unreinforced matrix. There was no evidence that the increase in embedment length increases the pullout load. [ABSTRACT FROM AUTHOR]

Mineral-impregnated, carbon fiber composites (MCF) are a promising alternative to conventional concrete reinforcements. For the efficient industrial production of MCF, sufficient processing time for the impregnation suspension must be ensured. In the present investigation, a metakaolin-made geopolymer (GP) has been developed and tested for this purpose. The impregnation of carbon-fiber yarns was performed continuously and automated. Subsequently, the MCF were heat-treated at 75 °C to accelerate the reaction processes. The mechanical performance of MCF gradually increased in the advancement of the curing process from 2 to 8 h, which is attributed to the greater extent of geopolymerization. In such extended curing, thermogravimetric and microscopic analysis showed indeed a more "reacted" microstructure but also a higher content of voids. After heating for 8 h, the tensile strength and Young's modulus of MCF reached 2960 MPa and 259 GPa, respectively, when related to the yarn cross-sectional area. [ABSTRACT FROM AUTHOR]