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Highlights • The refractory with 51% of alumina showed an improved behavior for all temperatures. • At 1200 °C a recovery in the mechanical strength was observed due to CA 2 formation. • The sintering process effect is dependent on the CaO/Al 2 O 3 ratio of the matrix. Abstract The influence of different temperatures on the chemical and mechanical behavior of three types of refractory concretes is studied. These materials were formulated using cements and alumina aggregates of different qualities. The alumina content considered is 51, 71 and 90 wt.%. The methodology implemented included: X-ray diffraction (XRD), thermogravimetric analysis (TGA), digital scanning calorimetry (DSC) and compression tests. After 7 days of curing, specimens were exposed to room temperature, 150, 300, 600, 1000 and 1200 °C and tested. The comparative analysis of the stress strain curves showed a quasi-brittle behavior, linear at the beginning and then non-linear until fracture. The concretes exhibited differences in the mechanical behavior and properties in function of the content of Al 2 O 3 and CaO. For all refractories, a mechanical strength drop was observed as the temperature increased and at 1200 °C a recovery in the mechanical strength was noticed. The refractory with 51 wt.% of alumina showed an improved mechanical behavior for all temperature ranges when compared with to the others. The specimens with 90 wt.% of alumina had the least degraded mechanical behavior with the increase in temperature. The results indicate that different refractories with different mechanical properties can be used together as a lining material and applied in layers. [ABSTRACT FROM AUTHOR]

This paper reports the experimental results of the investigation on the microstructure of Dendrocalamus giganteus bamboo laminae and the influence on its mechanical properties. The laminae were extracted from the central culm sector and from specific wall thickness positions in order to identify the fiber volume fraction, porosity and other matrix constituents, due to the functionally gradient characteristic. Samples were prepared from each set of laminae, both for microanalysis and for mechanical testing, under proposed configurations. The microscale morphology was observed and characterized both qualitatively and quantitatively through transmitted and reflected light microscopy, scanning electron microscopy and X-ray microtomography. An original method was proposed to quantify porosity through a humidity variation test. Tension and compression tests were carried out and their results were discussed with respect to the bamboo microstructure. Additionally the modes of failure were registered and analyzed. Results show that voids constitute an expressive portion of the culm. The mechanical properties are dictated by the fiber volume fraction when the specimens were tested under tensile load and by the density variation when tested under compressive load. [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]

• FRC post peak instability in Barcelona tests is influenced by the testing frame stiffness. • FRC with low modulus fiber shows high instability in Barcelona tests. • Three-point bending tests are correlated with barcelona tests with an equivalence between TCOD and CMOD. • The use of a circunferenical extensometer can reduce the post-peak instability in Barcelona tests. The application of fiber reinforced shotcrete as primary system support in underground mining applications faces major quality control problems due to the complex setup and sophisticated equipment necessary to obtain the mechanical parameters from bending tests. Considering the advantages presented by the double punch test (also known as Barcelona), the present work investigated the influence of the machine stiffness and system control methodology (using the actuator LVDT and circumferential extensometer under closed loop) on the post-peak behavior of FRC of the Barcelona test and its correlation with three-point bending tests. While machines with higher stiffness significantly reduced the post-crack instabilities and mitigated the correlation errors, the circumferential extensometer control successfully limited the machine influence on the post-crack properties of FRC. [ABSTRACT FROM AUTHOR]

• Experimental tests showed that the knurled fiber presented an improved thermomechanical behavior up to 1000 °C. • The FE model can extrapolate investigations to structures in practice and save enormous laboratory time. • The constitutive model implemented in commercial software showed the accuracy of 7% with the experimental round panel test. Stainless steel fibers are widely used as reinforcement in the refractories due to advantages such as high thermal compatibility, cost-effective on account of long life and effective resistance to corrosion at high temperatures. The purpose of this research is to investigate the thermomechanical behavior of alumina based refractory concrete reinforced with different shapes of stainless steel fibers. Straight, wavy and knurled shaped stainless-steel fibers were selected for the study. These refractory composites are extensively procured for the industrial applications. A research program of experimental test and numerical simulations was carried out. Three-point bending, fiber pull-out, direct tensile and compression tests were performed at temperatures ranging from 25 to 1200 °C to obtain the mechanical residual values. The experimental test results allowed the development of the material's constitutive model that was implemented in the finite element program ABAQUS. The constitutive model was validated through thermomechanical simulations of the round panel test. The flexural results showed the behavior of the reinforcement with knurled fibers was superior for all temperatures due to its higher bond with the matrix. The experimental tests and the numerical simulations showed that the reinforcement benefits the pre-peak and post-peak behavior of the refractory concretes. The influence of the fibers on the shape of the softening curve allows the prediction of the fracture behavior of the composites. [ABSTRACT FROM AUTHOR]

UHPC is a composite material with residual capacity that presents permanent strains and stiffness degradation, resulting in nonlinear behavior. Constitutive models that couple plasticity and damage, as the Lubliner criteria combined with isotropic damage theory, can capture such behavior. However, UHPC is influenced by fiber orientation and distribution, resulting in an orthotropic material, which limits the use of isotropic models to represent phenomena such as strain localization. Thus, this paper presents the calibration of a constitutive model based on experimental tests and a strategy for modeling the heterogeneous behavior of UHPC based on statistical properties' distribution. For modeling, the uniaxial and damage evolution curves must be calibrated to suitably represent the behavior of the material in compression and tension. Monotonic tests are conducted for the calibration of uniaxial models, and cyclic tests are used to determine the evolution of experimental damage. Parameters were defined to calibrate Birtel and Mark's damage evolution model for UHPC. These models are then used in material scale numerical models considering the material variability for UHPC's mechanical properties to determine the CDP parameters. The parameters are then validated by comparing the numerical and experimental results on an unreinforced UHPC beam. Finally, large-scale beams were modeled to verify the calibrated technique for real-sized specimens. A heterogeneous modeling strategy was used for the finite element models both on the material and structural scales. The results for the numerical models showed that the calibrated uniaxial and damage models resulted in adequate behavior, reflected not only in the load-displacement curves through the load capacities and yield loads for the modeled beams but also in the localization of the failure on the numerical models. For load capacities, the beam models presented a variation of approximately 15 % and 5 % of the experimentally verified load capacity. • Uniaxial cyclic tests provided damage evolution laws for UHPC. • The paper presents a calibration of damage evolution laws for UHPC in tension and compression. • The calibrated constitutive model was validated against the results of a tested beam. [ABSTRACT FROM AUTHOR]

Highlights • The behavior of concretes with polypropylene and sisal fibers was investigated. • Both fibers could provide similar residual strength, if at equivalent dosages. • Evaluation of fiber-matrix interface showed a similar behavior for both fibers. • The use of low alkaline matrix resulted in no degradation after ageing tests. Abstract This investigation aims to compare the mechanical behavior of concretes reinforced with polypropylene and sisal fibers. Both fibers were 51 mm long and were incorporated in fractions of 3, 6 and 10 kg/m3 into the matrices. The composites were tested under three-point monotonic and cyclic flexural loads. Pullout tests were performed to study the fiber-matrix interaction. It was observed that the sisal fiber could provide the same level of residual strength as the polypropylene fiber, as long as the equivalence of the dosages of the fibers is taken into account. The composites were classified according to fib Model Code 2010 based on parameters obtained from flexural tests. The durability of the composites was studied by assessing the flexural behavior after wetting and drying cycles. Concrete reinforced with both fibers did not present mechanical losses after the cycles. [ABSTRACT FROM AUTHOR]

Highlights • Steel fibers are more efficient than PP in improving the crack growth resistance. • Hybrid composites (PP and steel) are a good alternative for lower fiber volume fractions. • Steel fibers are more efficient in increasing the strength and result in a lower rate of stiffness degradation. • The addition of fibers in concrete can delay the strain development in the rebar. Abstract This article presents the results of an experimental investigation on the mechanical behavior of a self-consolidating concrete reinforced with steel, polypropylene and hybrid fibers. Hooked end steel fibers with different lengths and diameters were used as reinforcement in fiber volume fractions of 0.50%, 1.00% and 2.00%. Polypropylene fibers were used as reinforcement in volume fractions of 0.33%, 0.66% and 1.10%. Hybrid composites (HyFRC) were also developed to study the synergy effects of both fibers. The hybrid fiber self-consolidating concrete was produced with the addition of 0.50% of hooked-end steel fibers and 0.66% of polypropylene fibers. Pre-notched SCC prims where tested under monotonic and cyclic three-point bending. The tests where controlled by the crack mouth opening displacement in order to have a better analysis of the post cracking regime. The addition of polypropylene, hooked-end and hybrid fibers was effective in improving the post-peak strength of the composites, even though the use of steel fibers promoted higher values of flexural residual stress due to its geometrical and material properties. The same behavior was observed for the fracture parameters results with an improvement of the crack growth resistance for the analyzed composites. Finally, the effect of the fiber reinforcement on the flexural behavior was studied through structural tests on fiber reinforced self-consolidating concrete beams. Both hooked-end steel fiber and hybrid composites enhanced the resistance at yielding and promoted a lower rate of stiffness degradation, while the polypropylene fiber reinforcement presented an improvement in the ductile behavior of the structural composite. The present work gives an important contribution on the structural and cyclic behavior of steel, PP and hybrid fiber reinforced concrete. The paper makes a comparison on the mechanics of the different composite systems aiming its application in structural elements. [ABSTRACT FROM AUTHOR]

The present work reports the results of an experimental program on the mechanical characterization of Reinforced Concrete (RC) beams externally strengthened/repaired by natural sisal Fabric-Reinforced Cementitious Matrix (FRCM) composite. For such, undamaged and pre-cracked RC beams were U-wrapped with a sisal FRCM composite and then tested under bending load. The results showed significant increases in shear strength, up to 25 % for beams with stirrups and 75 % for beams without them. The strengthened/repaired beams also presented stirrups yielding delay during the tests. When compared with other synthetic FRCM strengthening systems from the literature, the enhanced mechanical results of the present work were mechanically equivalent. The cracking pattern and debonding of the sisal FRCM wrap system were measured by the Digital Image Correlation (DIC) technique. The wrap application reduced the crack propagation and debonding behavior was not detected. Finally, the experimental results were confronted with an analytical approach guided by ACI 549.4R-20 and considered sufficiently accurate. • Sisal FRCM composite provided significant shear strengthening for the RC beams. • The sisal FRCM strengthening gains were comparable with the synthetic FRCM. • Experimental results and the analytical approach were sufficiently accurate. [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]

• Carboxylated styrene butadiene rubber (XSBR) improves natural fiber mechanical performance. • XSBR polymer present higher interaction to cellulose with higher crystallinity. • Polymer creates a chemical bonding between fiber and matrix by anchor points. • XSBR seems to be an efficient treatment in order to produce high performance composites. Carboxylated styrene butadiene rubber (XSBR) coating was applied on several vegetable fibers and its effect on physical–chemical and mechanical performance were evaluated. Curauá, jute and sisal fibers were used in this study. The influence of coating treatment was evaluated by water absorption capacity test, scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and thermogravimetry (TGA). Pullout tests were performed in order to evaluate the fiber–matrix bonding. Fiber embedment lengths of 25 mm was used. A free calcium hydroxide cement mortar was used as matrix. Furthermore, direct tensile tests were performed in the treated and virgin natural fibers. The treatment creates a polymer layer surrounding the fiber, reducing the water absorption capability. The results indicate an increase of the tensile strength of all studied fibers. The interaction between the polymer and natural fibers may be ruled by the cellulose amount of fibers and their crystallinity. An increase in maximum load was observed during fiber pullout, as a consequence of the increase of chemical bonding between treated fibers and the cementitious matrix. [ABSTRACT FROM AUTHOR]

This research investigates the mechanical and microstructural evolution of a metakaolin-based geopolymer (GP) subjected to combined temperature and pressure curing conditions. The study's methodology encompassed measuring compressive strengths through both ultrasonic cement analysis (UCA) and destructive uniaxial testing. To evaluate thermal stability, pore size distribution, and morphology, the study employed thermogravimetric analysis (TGA), mercury intrusion porosimetry (MIP), and environmental scanning electron microscopy (ESEM), respectively. The findings indicate that a curing temperature of 90 °C encourages the development of regular nanoporosity, while higher temperatures contribute to increased porosity. Conversely, curing pressure was found to either promote or inhibit the material's reorganization, with pressures of 20 MPa facilitating and those above 40 MPa preventing this process. An enhancement in the refinement of macropores to micropores was observed across all pressure levels. The optimal curing conditions, in terms of both mechanical and morphological characteristics, were identified as 50 °C and 20 MPa for a minimum duration of one day, which yielded a 177 % improvement in compressive strength compared to curing at 21°C under atmospheric pressure. This study significantly advances the understanding of pressurized thermal curing processes for the rapid setting of geopolymers, presenting new avenues for diverse applications. • The study evaluates the microstructural development of a MK-based GP under temperature and pressure curing conditions. • Curing GP at RT (21°C) results in delayed setting during the initial stages due to issues in achieving gel homogenization. • Elevated temperatures, when coupled with pressure, positively affect geopolymerization kinetics. • Exceeding 100°C in curing can cause early cracks due to water evaporation. [ABSTRACT FROM AUTHOR]

Highlights • Well dispersed CNTs were obtained on cement particles in a non-aqueous media of isopropanol. • Mechanical performance of cement paste composites was enhanced in presence of well dispersed CNTs. • The cement paste composites with up to 0.10% of CNTs recorded higher fracture energy, flexural and tensile behaviour. • The direct tensile tests show an improvement of 45% in the presence of 0.10% of CNTs. • Cement paste with addition of 0.05% of CNTs recorded a gain of 90% in fracture energy with respect to the reference. Abstract Carbon nanotubes (CNTs) can be incorporated into cement-based materials in order to nucleate hydration products, enhance mechanical behavior, control micro crack formation and reduce deformation. Given that, the present paper investigates the mechanical behavior of a cement paste system reinforced with 0.05% and 0.10% of CNTs dispersed in a non-aqueous media of isopropanol on the surface of cement particles. Even though compromising the consistency and affecting the setting time, if well dispersed and properly bonded to the cement hydration products, the CNTs can act as nucleation site of cement hydration products and improve the mechanical properties of a cement paste system. Three-point bending tests in notched specimens and direct tensile tests were carried out using the manufactured material. The presence of CNTs on the cement paste resulted in a 90% of gain in fracture energy, 46% of flexural strength, and 47% tensile strength with addition of up to 0.10% of CNTs. Through SEM observation it is possible to affirm that the dispersion process was effective and CNTs acted as nucleation sites of cement hydration products, allowing its use as a reinforcement material. [ABSTRACT FROM AUTHOR]

• A direct tensile test set-up was developed for brittle specimens. • Nanocellulose changed calcium silicate hydrate and calcium carbonate contents. • Micro and nanocellulose promoted enhancement of bending and tensile strengths. • Micro and nanocellulose had no effect on compressive strength and early-age properties. When nanomaterials are added to cementitious materials, their microstructure and mechanical properties are affected. The most targeted effect of their use is the improvement of the composites' mechanical performance. The present investigation brings the characterization of a cement paste reinforced with two different cellulosic inclusions: microcrystalline cellulose (MCC) and nanofibrillated cellulose (NFC), added in the contents of 0.050%, 0.075%, and 0.100%, in relation to cement weight. First, the morphology, crystallinity, surface area, and thermal decomposition of MCC and NFC were analyzed. Then, the composites' microstructure was characterized by thermogravimetric analysis, X-ray diffraction, and porosity measurements. This analysis was made considering the development of hydration between 3 and 28 days. Furthermore, their mechanical performance was investigated by compressive, flexural, and direct tensile tests at 28 days of hydration. Despite the differences regarding the morphologies and physical aspects between MCC and NFC, their inclusions resulted in positive effects on the flexural and tensile strengths. The cement pastes reinforced with 0.075% of MCC and NFC had their tensile strength enhanced by over 70% compared to the reference. Nevertheless, TGA and XRD analyses showed that the addition of MCC and NFC decreased the formation of all hydration products, except the calcium carbonate. Therefore, the combination of mechanical and microstructure analyses indicates that the tensile strength improvement obtained with the MCC and NFC addition was most likely due to their reinforcing capability. [ABSTRACT FROM AUTHOR]

[Display omitted] • Effective modification of sisal fibers using XSBR emulsion was developed. • The polymeric layer in sisal fiber reduces water absorption by 80%, and also minimizes fiber alkaline degradation. • Effective polymeric coating transforms sisal structure, leading to substantial changes in the fiber's mechanical properties. • Polymer-coated sisal fibers presented enhanced adhesion and stability in cement-based matrices. The performance of sisal fibers in cement-based matrices is highly dependent on the interface efficiency between the fiber and the matrix. This behavior in turn is directly related to two main factors: the sisal fiber swelling tendency and fiber alkaline degradation. To enhance these properties and consequently the fiber–matrix adhesion, this study proposed the use of carboxylate styrene-butadiene rubber latex (XSBR) as a coating for sisal fibers. Different XSBR polymers and emulsions concentrations were adopted. The effects of this treatment on various properties of the fibers such as physical, chemical, and mechanical properties were studied. The physical properties of the modified fibers were investigated by means of scanning electron microscopy (SEM) and atomic force microscope (AFM) analyses. Also, changes in the moisture absorption capacity of the coated fibers were evaluated. To study the chemical influence of the polymer coating in the modified fibers, thermogravimetric analysis (TGA) and Fourier transform infrared spectroscopy (FTIR) techniques were adopted. Direct tensile and pullout tests were also assessed in natural and coated sisal fibers, to mechanically evaluate the modification in terms of fiber strength and adherence, respectively. The results evidenced the formation of a new polymeric layer around the fiber, responsible to reduce the water absorption tendency of the natural fiber. The increase in the fiber–matrix adhesion obtained in the pullout evaluation was attributed to the rougher superficial aspect of the modified fibers and increased chemical interaction between the polymer and the cementitious matrix. Moreover, it was proven that the polymeric layer reduced the fiber's alkaline degradation susceptibility resulting in a more stable sisal fiber. [ABSTRACT FROM AUTHOR]

During the life cycle of an oil well, the annular cement sheath will be exposed to load combinations that can result in failure. To enhance the structural integrity and ductility of the cement paste, and to improve the resistance to tensile crack propagation, fiber reinforcement can be used. In the present work, the effects of different polyvinyl alcohol (PVA) fiber concentrations on the rheological and mechanical properties of a class G oil well cement paste was investigated. The study focuses on impacts of fibers on the cement paste viscosity, its confined and unconfined stress–strain responses, and resistance to shear and bending loads. An increase in the effective viscosity of the cement paste with increasing fiber content was observed, although the impact was minor for the smallest (semi-dilute) fiber concentration. All cement paste specimens showed a modest frictional strengthening with increasing confining pressure, with the fiber reinforced samples exhibiting improved post-peak load-bearing capacity compared to the base formulation with no fiber additive. Fibers were found to significantly improve cement paste resistance to shear and bending loads, to enhance the toughness and to arrest tensile crack growth. Despite all the benefits already known about adding fibers, for application in the oil and gas industry, it is essential to evaluate the rheology of the mixture that will directly impact pumpability and placement. In this work, flow curves were measured only for concentrations ranging from 0.1% to 0.5% since the other cases approached the concentrated suspension regime with the fluid behaving like a plug. Still, the results elucidate the potential benefits of adding even a relatively small concentration of high-aspect-ratio PVA fibers to oil well cement pastes, particularly for enhancing early-age cement's shear and bending strength. • Effects of adding high-aspect ratio fibers to class G cement paste. • Increasing fiber concentration results in modest viscosification of slurry. • Fibers improve post-peak load-carrying capacity of cement. • PVA fibers improve the plastic behavior of the material under triaxial loading. [ABSTRACT FROM AUTHOR]

• Alkali treatment improves the mechanical performance of sisal fibers. • Stretch of the fibers during treatment procedure results in enhanced fiber strength. • Treated sisal fibers present an improved bond with cement-based matrix. • Alkali-treated sisal fibers are more stable in alkaline aggressive conditions. This work aims to evaluate the effect of alkali treatment on sisal fibers to improve the fiber properties and the effectiveness of their use as reinforcement in cement-based matrices. Alkaline solution concentration (1, 5 and 10 wt% NaOH) and post-treatment protocol (use of distilled water or acetic acid) influence were evaluated. The physical, chemical, thermal, wettability and surface characteristics were analyzed. Also, single-fiber direct tensile and pullout tests were performed. Results showed an increase in cellulose proportion with a reduction in hemicellulose and lignin content upon alkali treatment. Consequently, enhanced thermal stability and a decrease on the water uptake tendency were reached. Additionally, improved tensile properties, as well as an enhanced bond with the cementitious matrix, were found for alkali-treated fibers. Also, fibers became more stable when subjected to alkaline aggressive conditions. Thus, the alkali-treated sisal fiber seems to be an adequate and sustainable alternative as reinforcement for cement-based matrices. [ABSTRACT FROM AUTHOR]

• GPR can be used to investigate fiber density in FRC specimens. • GPR with high frequency antenna can be used to assess weak zones of FRC elements. • Digital and processed images of fiber distribution correlated well with GPR traces. • GPR cannot quantitatively characterize the orientations of fibers in concrete. The present work focuses on the steel fiber distribution in self-consolidating concrete short beams. The objective is to assess the feasibility of the ground penetrating radar (GPR) in evaluating the homogeneity of fibers within concrete materials, for use in testing laboratories to study the modes of placing and compaction employed for preparing specimens during quality control or material development. Self-consolidating concrete beams were cast, tested, and evaluated. A GPR survey employing a concrete scanning system with a shielded 1.2 GHz center frequency was performed on self-consolidating casted short beams just before a monotonic four-point bending test was carried out. After failure, a representative short beam was cut and the resulting slices were photographed to obtain the position and distribution of the steel fibers. Results were processed using dedicated software and an integrated analysis was accomplished. The digital and processed images correlated well with the GPR mappings in a single radar trace mode. In particular, the GPR amplitude provides information on the steel fiber density that was used as reinforcement in concrete specimens. [ABSTRACT FROM AUTHOR]

• Young's modulus and Poisson's coefficient decrease with increasing confinement. • Increase in ductility and strength was observed for higher confinement. • DVC shows that cracks tend to occur in ITZ. • Aggregates displace less within the cement paste when increasing confinement. Considering that structures destined for civil construction are commonly subjected to confinement loadings, triaxial testing is already standard for the characterization of cementitious materials, and novel methods to validate or explain the macro mechanical responses are needed. Since the macro behavior is linked to the behavior of the meso and microstructure of the material, the development of analysis methods at these scales is required. MicroCT technique provides 3D images in mesoscale that can bring relevant materials information when coupled with mechanical tests. This study presents the results of ex-situ microCT tests on cementitious materials subjected to uniaxial and triaxial loading. Cement pastes and concretes were tested in triaxial compression under 0, 10 and 20 MPa confining pressures. The macro mechanical responses could then be correlated with the material meso behavior through microCT images. For instance more significant meso structural changes were noticed with an increasing level of confinement. The digital volume correlation technique was applied to obtain the strains when a regular concrete was subjected to uniaxial and triaxial tests. Information such as the strain levels of aggregates within the cementitious matrix and the cracking pattern were obtained. For example, it was found that the aggregates present higher relative displacement within the cement paste when there is no confinement. [ABSTRACT FROM AUTHOR]

[Display omitted] • SHGC can be successfully manufactured with 2% of short PVA and PE fiber reinforcements. • Na-based composites show improved performances under both regular and elevated temperature conditions. • The residual results present inferior performance than the quasi in-situ ones. • PVA fibers show increased thermomechanical efficiency, opposed to the melting occurring for PE at 200 °C. Interest in geopolymers (GP) has been continuously increasing due to their comparable-to-concrete mechanical properties and enhanced durability characteristics. Despite their chemical stability at high temperatures, the brittle nature of geopolymers may prevent their use in applications requiring inelastic deformability, e.g., strengthening layers or elements subjected to dynamic loading. Strain-hardening geopolymer composites (SHGC) made with short high-performance fibers appear as a promising new class of materials that can yield a quasi-ductile tensile behavior under increasing loading. This article assesses distinct types of metakaolin-based SHGC with respect to the temperature effects on their mechanical performance. In particular, the efficiency of Na and K alkali solutions is compared when combined with short fibers made of polyvinyl alcohol (PVA) and ultra-high molecular weight polyethylene (PE). Flexural properties were obtained for all material variations after exposure to a temperature of 100 °C or 200 °C in cooled down state (residual) and in hot state (quasi in-situ). Additionally, thermogravimetry (TGA), mercury intrusion porosimetry (MIP), dilatometry, and environmental scannning electron microscopy (ESEM) techniques were used. At room temperature, NaGP-based composites showed higher flexural strength due to the superior properties of the matrix and crack-bridging performance of the fibers. All quasi in-situ tested specimens demonstrated higher losses in strength and ductility when compared to the residual ones. This can be traced back to higher deformability of the fiber when in hot state. As opposed to PVA, the PE fibers yielded a higher thermal sensitivity in terms of crack-bridging capacity due to their lower melting temperature. [ABSTRACT FROM AUTHOR]

[Display omitted] • SHGC can be successfully manufactured with 2% of short PVA and PE fiber reinforcements. • Na-based composites show improved performances under both regular and elevated temperature conditions. • The residual results present inferior performance than the quasi in-situ ones. • PVA fibers show increased thermomechanical efficiency, opposed to the melting occurring for PE at 200 °C. Interest in geopolymers (GP) has been continuously increasing due to their comparable-to-concrete mechanical properties and enhanced durability characteristics. Despite their chemical stability at high temperatures, the brittle nature of geopolymers may prevent their use in applications requiring inelastic deformability, e.g., strengthening layers or elements subjected to dynamic loading. Strain-hardening geopolymer composites (SHGC) made with short high-performance fibers appear as a promising new class of materials that can yield a quasi-ductile tensile behavior under increasing loading. This article assesses distinct types of metakaolin-based SHGC with respect to the temperature effects on their mechanical performance. In particular, the efficiency of Na and K alkali solutions is compared when combined with short fibers made of polyvinyl alcohol (PVA) and ultra-high molecular weight polyethylene (PE). Flexural properties were obtained for all material variations after exposure to a temperature of 100 °C or 200 °C in cooled down state (residual) and in hot state (quasi in-situ). Additionally, thermogravimetry (TGA), mercury intrusion porosimetry (MIP), dilatometry, and environmental scannning electron microscopy (ESEM) techniques were used. At room temperature, NaGP-based composites showed higher flexural strength due to the superior properties of the matrix and crack-bridging performance of the fibers. All quasi in-situ tested specimens demonstrated higher losses in strength and ductility when compared to the residual ones. This can be traced back to higher deformability of the fiber when in hot state. As opposed to PVA, the PE fibers yielded a higher thermal sensitivity in terms of crack-bridging capacity due to their lower melting temperature. [ABSTRACT FROM AUTHOR]

• Humidity variations barely altered the mechanical properties of jute fiber. • Sisal fiber presented better adhesion under the proposed humidity levels. • Humidity effect on fibers can be assessed by their cellulose crystallinity index. • Sisal fiber presented better adhesion under the proposed humidity levels. The aim of this study is to determine how the mechanical behavior of natural fibers is affected by the environmental and internal relative humidity, especially on the fiber-matrix interface. For that, curauá, jute and sisal fibers were selected. The experimental program commenced determining the fibers' mechanical properties under tensile stresses, followed by X-ray diffraction measurement, thermogravimetric analysis, infra-red spectroscopy and dynamic water vapor sorption analysis, as well as a microscopic examination of fibers' rupture under tensile stresses. In addition, a 2D finite element analysis was carried out to assist the interpretation of the mechanical behavior of the pull-out tests. The results indicate that the absorption capacity of each fiber can be correlated to the amount of hemicellulose and cellulose cristalinity, and the fibers' swelling process was influenced by each microstructure and hemicellulose content. Results from the mechanical tests showed a strength enhancement when the fibers were submitted to low levels of relative humidity, while a drastic decrease in strength was observed along with higher levels. The development of finite element models helped understanding the geometric particularities of each fibers. From the results it is possible to state a unique correlation between each fiber and its respective moisture content. It is believed that this data can help the use of such fibers as reinforcement of cement-based systems and their durability under a wider range of relative humidity levels. [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]

[Display omitted] • Post-cracking tensile strength, toughness and pull-out resistance were improved using treated fabrics. • SEM-EDS analysis suggests pozzolanic reaction of NS along the treated sisal yarns. • The peeling of yarns promoted a strain hardening response in the composites. • The combination P + NS resulted in superior pull-out and toughness performance. This work presents an experimental investigation focused on the mechanical properties of a Textile Reinforced Concrete fabricated with bi-directional sisal fabrics treated with three different procedures. The treatments consisted of immersion in a suspension of nano-silica, mechanical peeling and mechanical peeling followed by immersion in a suspension of nano-silica. Results indicated that the treatments on the fabric influenced the cracking process of the composites and were capable to improve their post-cracking tensile strength and the yarn-matrix bonding. The combination of nano-silica and peeling treatments proved to be more effective than the treatments applied independently. [ABSTRACT FROM AUTHOR]

• Both K- and Na-based formulations are used for SHGC manufacturing. • Na-based SHGC achieves improved mechanical responses, with an enhanced strain capacity. • K-based SHGC do not present a typical strain-hardening curve when in presence of fine-grained sand. • When compared to typical SHCC, the developed SHGC shows a mechanically improved cracking behavior. • SHGC is expected to be used in a wide range of applications. Strain-hardening cementitious composites (SHCC) are known by their high deformation capacity due to a multiple cracking tolerance under increasing tensile loading. While SHCC have been well investigated, the design of geopolymer (GP) composites in the same context is not sufficiently understood, mainly due to varying precursor materials used in previous studies. In this work, two metakaolin-based GP mixtures were produced, using K and Na as alkali sources. The incorporation of fine-grained sand was extensively investigated to evaluate its compatibility with both strain-hardening geopolymer composites (SHGC) reinforced with short polyvinyl-alcohol (PVA) fibers. Their mechanical properties were analyzed by means of compression, bending, and tension tests. Digital Image Correlation (DIC) was used to evaluate the fracture mechanisms under uniaxial tension. Particle size distribution (PSD), X-ray diffraction (XRD), Thermogravimetric analysis (TGA) and mercury intrusion porosimetry (MIP) were conducted. Additionally, single-fiber pullout tests with subsequent Environmental Scanning Electron Microscopy (ESEM) were carried out. NaGP composites, reinforced with fine-grained sand, yielded improved mechanical responses, reaching a strain capacity of approximately 4.5%, with enhanced multiple cracking, as opposed to the lower performance observed for KGP composites, that did not present a typical strain-hardening curve when in presence of fine-grained sand, but a strain-softening one. When compared to typical SHCC responses, Na-based SHGC showed a mechanically improved multiple cracking behavior, indicating the potential of this material for a wide range of applications, such as dynamic loading and requirements of long-term durability. [ABSTRACT FROM AUTHOR]