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Creep in pre-cracked fiber reinforced cementitious composites has become an important topic of study recently. This is due to the fact that under serviceability limit state the concrete may crack and its stability will depend on the fiber and fiber-matrix interface properties. The time dependent behaviour of FRC and long term stability of cracks under sustained bending loads are still poorly understood. This work seeks to explore the use of steel and PP fibers in order to define their influence on creep, by analysing the crack opening displacement rate in FRC specimens. For the creep tests, the specimens were pre-cracked to 0.5 mm, and then tested under constant load during 45 days. The constant load applied was calculated based on the residual stress found for 0.5 mm of crack opening, which corresponds to 30% of its residual load. In order to better understand the related mechanisms, creep tests were also carried on a fiber pullout configuration. Analysing the creep tests results, it was verified that the COD rate is an interesting tool to evaluate the long-term behaviour of the cracked FRC and to define a stability criterion. In addition, it was found that concrete incorporating macro synthetic fibers presents higher creep deformations and higher creep rate than concrete reinforced with steel fibers. This can be explained by the different fiber-matrix bond characteristics. Finally, the residual properties of creep tested specimens were determined by monotonic flexural tests performed in the FRC specimens after the creep tests.

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Today's demand for environmentally friendly and energy-efficient solutions to the construction industry has driven researchers to match natural resources with traditional technics to develop new building technologies. However, there are literature limitations about the correlation of fiber-matrix interface with the failure of natural fibers in mortar plates, which hinders the advances in understanding the mechanical properties of these composites on a structural scale. The present work investigated the mechanical behavior and fractography of cement-based composites reinforced by natural piassava and jute fibers. The experimental program included flexural tests and scanning electron microscopy analyzes. The developed composite material under flexural tests demonstrated a flexuralsoftening behavior, reaching up to 5.7 MPa, with a considerable residual strength ruled by toughness. The fractography analyses presented the fibers' structure after mechanical tests and how effective its interaction with the matrix was. The piassava fibers demonstrated significant adherence when favorably oriented, while jute fibers (used as twisted yarn) provided voids in the composite by its partial matrix-covered filaments.

Besides the strength enhancement and strain improvement (strain-hardening behavior), the use of natural fibers as reinforcement in cement-based matrices can also be highlighted as an economical and eco-friendly alternative for the future of the construction industry. In the present work, cement-based composites reinforced by natural sisal fibers were produced and tested under direct tensile loading. The Portland cement was partially replaced by pozzolans (metakaolin -MK and fly ash -FA), aiming to produce a calcium hydroxide-free matrix to ensure the durability of the fibers. The natural sisal fibers were used in a 5% volume fraction (in mass), divided into three layers. The mechanical properties of composite plates were compared to other literature results and demonstrated to be compatible with recent research. The crack pattern was analyzed by Digital Image Correlation (DIC) for a better understanding of their failure mechanisms. The material presented a tensile strength increase after the first crack formation, marked by multiple cracking partners from this point. Finally, a comparison between direct (LVDTs) and indirect (DIC) methods of strain measurement was done and demonstrated minor results for the DIC, approximately 84% of those obtained from the LVDTs.

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