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The study and utilization of fully recycled aggregate concrete (FRAC), in which coarse and fine aggregates are completely replaced by recycled aggregates, are of great significance in improving the recycling rate of construction waste, reducing the carbon emission of construction materials, and alleviating the ecological degradation problems currently faced. In this paper, investigations were carried out to study the effects of steel fiber (0.5%, 1.0%, and 1.5%) and polypropylene fiber (0.9 kg/m3, 1.2 kg/m3 and 1.5 kg/m3) on the properties of FRAC, including compressive strength, splitting tensile strength, the splitting tensile load–displacement curve, the tensile toughness index, flexural strength, the load–deflection curve, and the flexural toughness index. The results show that the compressive strength, splitting tensile strength, and flexural strength of fiber-reinforced FRAC were remarkably enhanced compared with those of ordinary FRAC, and the maximum increase was 56.9%, 113.3%, and 217.0%, respectively. Overall, the enhancement effect of hybrid steel–polypropylene fiber is more significant than single-mixed fiber. Moreover, the enhancement of the crack resistance, tensile toughness, and flexural toughness obtained by adding steel fiber to the FRAC is more significant than that obtained by adding polypropylene fiber. Furthermore, adding polypropylene fiber alone and mixing it with steel fiber showed different FRAC splitting tensile and flexural properties. [ABSTRACT FROM AUTHOR]

This paper provides a state-of-the-art report on the up-to-date research on the emerging 3D concrete printing technology from the concrete materials perspective. It reviews the recent research focused on understanding and characterizing the rheological necessities of the concrete printing process and discusses how the researchers are tailoring compatible mix proportions for the 3D concrete printing process by using eco-friendly binders, waste aggregates, chemical admixtures, and nano-additives. This paper systematically evaluates anisotropic behavior in the mechanical properties of printed concrete and establishes an order for anisotropic behavior in the compressive, flexural, and tensile strengths along three different axes (X, Y, and Z axes) of printed concrete. It evaluates the ratio of flexural strength to the compressive strength of printed concrete along the above three axes. This article explains the influence of variation of printing process parameters on the mechanical properties and discusses reinforcement approaches used for increasing structural performance. The microstructure at the interface of adjacent layers and also at the interface of the reinforcement-cement matrix is discussed. The recent research on the durability performance of printed concrete is critically discussed and future research needs for 3D concrete printing are identified in this paper. [ABSTRACT FROM AUTHOR]

Professionals in industries are making progress in creating predictive techniques for evaluating critical characteristics and reactions of engineered materials. The objective of this investigation is to determine the optimal settings for a 3D printer made of acrylonitrile butadiene styrene (ABS) in terms of its conflicting responses (flexural strength (FS), tensile strength (TS), average surface roughness (Ra), print time (T), and energy consumption (E)). Layer thickness (LT), printing speed (PS), and infill density (ID) are all quantifiable characteristics that were chosen. For the experimental methods of the prediction models, twenty samples were created using a full central composite design (CCD). The models were verified by proving that the experimental results were consistent with the predictions using validation trial tests, and the significance of the performance parameters was confirmed using analysis of variance (ANOVA). The most crucial element in obtaining the desired Ra and T was LT, whereas ID was the most crucial in attaining the desired mechanical characteristics. Numerical multi-objective optimization was used to achieve the following parameters: LT = 0.27 mm, ID = 84 percent, and PS = 51.1 mm/s; FS = 58.01 MPa; TS = 35.8 MPa; lowest Ra = 8.01 m; lowest T = 58 min; and E = 0.21 kwh. Manufacturers and practitioners may profit from using the produced numerically optimized model to forecast the necessary surface quality for different aspects before undertaking trials. [ABSTRACT FROM AUTHOR]

Recently, 3D concrete printing has progressed rapidly in the construction industry. However, this technique still contains several factors that influence the buildability and mechanical properties of the printed concrete. Therefore, this study investigated the effects of the nozzle speed, the interlayer interval time, the rotations per minute (RPMs) of the screw in the 3D printing device, and the presence of lateral supports on the buildability of 3D concrete printing. In addition, this paper presents the results of the mechanical properties, including the compressive, splitting tensile, and flexural tensile strengths of 3D printed concrete. The buildability of 3D printed structures was improved with an extended interlayer interval time of up to 300 s. The printing processes were interrupted because of tearing of concrete filaments, which was related to excessive RPMs of the mixing screw. The test results also showed that a lateral support with a wide contact surface could improve the resistance to buckling failure for 3D printed structures. The test results of the mechanical properties of the 3D printed concrete specimens indicated that the compressive, splitting tensile, and flexural tensile strengths significantly depended on the bonding behavior at the interlayers of the printed specimens. In addition, although metal laths were expected to improve the tensile strength of the printed specimens, they adversely affected the tensile performance due to weak bonding between the reinforcements and concrete filaments. [ABSTRACT FROM AUTHOR]

In order to study the workability and mechanical performance of high fluidity recycled concrete, parallel tests were carried out to prepare high fluidity recycled concrete by changing the amount of recycled aggregate (20%, 30%, 40%, 50% and 60%) and the sand ratio (0.37, 0.38, 0.39 and 0.40). The fluidity, compression strength, tensile strength and flexural strength of each mix were tested. The results show that the slump of a recycled concrete mixture is 120 mm when the content of recycled aggregate is less than 30%, and the mechanical strength satisfies the requirement of the high fluidity ordinary concrete. As the ratio of sand increases, the fluidity of the recycled concrete also improved. The best sand ratio is not consistent with the strength index. When the sand ratio is between 0.37 and 0.40 and the content of recycled aggregate is not more than 40%, the concrete of C60 can be prepared. Both the fluidity and the strength index can meet the design requirements and can be used in the practical engineering. [ABSTRACT FROM AUTHOR]

Yeast-like fungi such as Candida albicans (C. albicans) are the primary pathogenic microorganism in the oral cavity of denture wearers. The research available so far, conducted according to a protocol based on the exposure of specimens to a C. albicans suspension and their cutting with water cooling, shows that hard polymethyl methacrylate (PMMA) prosthetic materials are not only surface colonized, but also penetrated by microorganisms in a short time. This justifies the hypothesis that exposure to a suspension of the C. albicans strain causes the changes in mechanical properties due to surface colonization and/or penetration of the samples. In the current study, the chosen mechanical properties (flexural strength, flexural modulus, tensile strength, impact strength, ball indentation hardness, and surface Vickers hardness at 300 g load) of the PMMA denture base material Vertex RS (Vertex-Dental, The Netherlands) exposed for 30, 60, and 90 days to a suspension of C. albicans were investigated. The potential penetration of yeast was examined on the fractured surfaces (interior of specimens) to eliminate the risk of the contamination of samples during cutting. There was no influence on the flexural strength, flexural modulus, tensile strength, impact strength, or ball indentation hardness, but a significant decrease in surface hardness was registered. Microscopic observations did not confirm the penetration of C. albicans. On the surface, blastospores and pseudohyphae were observed in crystallized structures and in traces after grinding, which indicates that in clinical conditions, it is not penetration but the deterioration of surface quality, which may lead to the formation of microareas that are difficult to disinfect, causing rapid recolonization. [ABSTRACT FROM AUTHOR]

The fractionation of diatomaceous earth (DE) using sedimentation made it possible to obtain separate unbroken diatom fractions from broken or agglomerated bodies with a range of particle sizes. The produced filler was used to prepare polylactide (PLA)/diatomaceous earth biocomposite samples containing different particle sizes, which were subjected to mechanical testing (tensile strength, flexural strength, impact strength), colloidal testing (contact angle, color change test, SEM/EDS), and thermal testing (TGA, DSC, DMA). Modification of the PLA containing the smallest particle size with diatomaceous earth (Fraction 5) resulted in a higher impact strength compared to both the pure PLA and the PLA/DE composite that contained base diatomaceous earth. Furthermore, the melt flow rate was improved by more than 80 and 60% for the composite modified with fractionated diatomaceous earth (Fraction 4) compared to pure PLA and base diatomaceous earth, respectively. The elasticity of the composite was also improved from 3.3 GPa for pure polylactide to 4.4 GPa for the system containing the smallest diatomaceous earth particles (Fraction 5). [ABSTRACT FROM AUTHOR]

Nano graphite platelets (NGPs) belong to the carbon family and have a huge impact on the construction industry. NGPs are used as multi-functional fillers and have the potential to develop reinforcing within cementitious composites. In this paper, NGPs were incorporated in cementitious composites to investigate the effects of NGPs on the fresh, mechanical, durability, and microstructural properties of concrete. Five mixes were prepared with intrusion of NGPs (0%, 0.5%, 1.5%, 3%, and 5% by weight of cement). The properties studied involved workability, air content, hardened density, compressive strength, tensile strength, flexural strength, sorptivity, ultrasonic pulse velocity (UPV), water absorption, and external sulfate attack. The workability and percent air content decrease by 22.5% and 33.8%, respectively, for concrete with 5% NGPs compared to the control mix. The specimens containing 5% of NGPs revealed the hardened density, compressive, tensile, and flexural strength to increase by 11.4%, 38.5%, 31.6%, and 44.34%, respectively, compared to the control mix. The results revealed that the incorporation of 5%NGPs in cementitious composites reduces the sorptivity and water absorption by 32.2% and 73.9%, respectively, whereas, it increases the UPV value by 7.5% compared to the control mix. Furthermore, the incorporation of NGPs provided better resistance against external sulfate attacks. SEM–EDX spectroscopy was carried out to investigate its microstructural analysis. [ABSTRACT FROM AUTHOR]

Orthogonal experiments were designed for hybrid fiber rubber concrete (HFRC). The mechanical properties of HFRC were tested and compared with ordinary concrete. The effects of basalt fiber volume ratio (VBF), PVA fiber volume ratio (VPF) and rubber volume ratio (VR) on the compressive strength, splitting tensile strength and flexural strength of HFRC were analyzed. The results show that the strength of HFRC is the best when the volume ratio of basalt fiber is 0.3%, the volume ratio of PVA fiber is 0.2% and the volume ratio of rubber is 5%. Basalt fiber has the greatest influence on the strength of HFRC. The strength of HFRC mixed with hybrid fiber is greatly improved, which reflects the good fiber "positive hybrid effect". With the increase of rubber volume ratio, the strength of HFRC decreases gradually. With the help of SEM and EDS, the toughening and cracking resistance mechanism of the fiber to HFRC was analyzed. Finally, the strength of HFRC was predicted by model. [ABSTRACT FROM AUTHOR]

The article discusses the influence of the post-process on the mechanical properties of elements produced with the use of the mask stereolithography (mSLA) method. Printed samples were subjected to the following post-process steps: Washing and post-curing, at various times. Then, static tensile and static bending tests were carried out, as well as Shore D hardness measurements for the inner and surface part of the sample, as well as profilographometric analysis of the surface. The post-curing time has been found to strongly affect the tensile and bending strength of printouts, and to improve their surface quality. Washing has an ambiguous effect on the strength of the printouts, but, in the end, it was found that extended washing slightly reduces the strength. Washing significantly affects the quality of the printout surface. A washing time that is too short results in a surface that strongly resembles the printing process, with high roughness. Increasing the washing time to 10 min lowers the roughness by one order of magnitude. Post-curing has also been shown to be beneficial for the cured sample with the application of shielding water. This approach results in an improvement in the flexural strength of the printouts. In general, the obtained research results indicate that, for printouts with cross-sectional dimensions of several mm, the optimal washing time is no more than 10 min and the post-curing time is at least 30 min. [ABSTRACT FROM AUTHOR]

Mechanical properties and data analysis for the prediction of different mechanical properties of geopolymer concrete (GPC) were investigated. A relatively large amount of test data from 126 past works was collected, analyzed, and correlation between different mechanical properties and compressive strength was investigated. Equations were proposed for the properties of splitting tensile strength, flexural strength, modulus of elasticity, Poisson's ratio, and strain corresponding to peak compressive strength. The proposed equations were found accurate and can be used to prepare a state-of-art report on GPC. Based on data analysis, it was found that there is a chance to apply some past proposed equations for predicting different mechanical properties. CEB-FIP equations for the prediction of splitting tensile strength and strain corresponding to peak compressive stress were found to be accurate, while ACI 318 equations for splitting tensile and elastic modulus overestimates test data for GPC of low compressive strength. [ABSTRACT FROM AUTHOR]

In this study, the effect of wood powder and nanoclay particle content on composites' mechanical behavior made with polyethylene matrix has been investigated. The wood flour as a reinforcer made of wood powder was at levels of 30, 40, and 50 wt.%, and additional reinforcement with nanoclay at 0, 1, 3, and 5 wt.%. Furthermore, to make a composite matrix, high-density polyethylene was used at levels of 70, 60, and 50% by weight. Wood-plastic composite (WPC) specimens were manufactured in injection molding. After preparing the specimens, tensile and bending tests were performed on samples. The mechanical properties such as tensile and flexural strength and flexural modulus were measured. Results showed that nanoclay particle content increases flexural modulus, flexural strength, modulus of elasticity, and tensile strength. The experimental test results show that Young's moduli increased with the volume of wood flour. The biggest modulus of elasticity was achieved in the samples having 50 wt.% of wood powder. Furthermore, the highest value of tensile strength was achieved at the level of 30 wt.%. The highest flexural strength was for the sample containing 50% wood powder by weight. Additionally, a numerical model was made utilizing the Abaqus software using the finite element method (FEM). Comparing the numerical and experimental results, it was found that they are compatible in the linear-elastic and plastic state of the material. There are no crucial differences between experiment and FEM. [ABSTRACT FROM AUTHOR]

This study investigated fundamental mechanical properties of a basalt fiber reinforced engineered cementitious composite (BF-ECC) with different volume fractions of basalt fiber (BF), water–binder ratio (W/B) and fly ash (FA) content. The compressive strength, splitting tensile strength, flexural strength and static modulus of BF-ECC were studied at 3, 28 and 56 days, respectively, to explore their development along the ages. Furthermore, the scanning electron microscopy (SEM) analysis was conducted to evaluate the microstructure of BF-ECC. Experiment results demonstrated that bond quality between the BF and the matrix is good, which leads to a significant increase in the flexural strength and splitting tensile strength. The pozzolanic effect of FA obviously improved the splitting tensile and flexural strength of BF-ECC after 56 days of curing, and the appropriate content of the FA content in the BF-ECC ranges from 50% to 60%. [ABSTRACT FROM AUTHOR]

High-performance concrete has low tensile strength and brittle failure. In order to improve these properties of unreinforced concrete, the effects of adding recycled polypropylene fibers on the mechanical properties of concrete were investigated. The polypropylene fibers used were made from recycled plastic packaging for environmental reasons (long degradation time). The compressive, flexural and split tensile strengths after 1, 7, 14 and 28 days were tested. Moreover, the initial and final binding times were determined. This experimental work has included three different contents (0.5, 1.0 and 1.5 wt.% of cement) for two types of recycled polypropylene fibers. The addition of fibers improves the properties of concrete. The highest values of mechanical properties were obtained for concrete with 1.0% of polypropylene fibers for each type of fiber. The obtained effect of an increase in mechanical properties with the addition of recycled fibers compared to unreinforced concrete is unexpected and unparalleled for polypropylene fiber-reinforced concrete (69.7% and 39.4% increase in compressive strength for green polypropylene fiber (PPG) and white polypropylene fiber (PPW) respectively, 276.0% and 162.4% increase in flexural strength for PPG and PPW respectively, and 269.4% and 254.2% increase in split tensile strength for PPG and PPW respectively). [ABSTRACT FROM AUTHOR]

To study the effect of redispersible polymer emulsion powder on the mechanical properties of carbon fiber-reinforced polymer concrete (CFRPC), the compressive, flexural, and splitting tests of CFRPC specimens with different polymer–cement ratios (polymer–cement mass ratios) were performed in this study. The modification effect of emulsion powder on CFRPC was analyzed from the perspectives of the strength and deformation properties of the specimens. The results show that the static properties of CFRPC increased first and then decreased with the increase of the polymer–cement ratio, in which the splitting tensile strength had the most significant increase; the flexural strength took second place and the compressive strength had a slight increase. When the polymer–cement ratio was 8%, the flexural and splitting tensile strength of the CFRPC specimens increased significantly by 36% and 61%, respectively. According to electron microscopy images, adding emulsion powder can effectively improve the structure of fiber–matrix transition zones and enhance the bond property between fibers and the matrix. [ABSTRACT FROM AUTHOR]

Multi-walled carbon nanotubes (MWCNTs) have great potential to improve the strength and microstructure of traditional cement-based materials. In this research, different aspect ratios of MWCNTs (F-type and L-type) were dispersed into water using surfactants, and then incorporated into reactive powder concrete (RPC) for improving mechanical and microstructure properties. With the addition of 0.025 wt.% F-MWCNTs, the 28 days compressive strength and initial-cracking flexural strength increased by 7.2% and 36%, respectively. Moreover, the first-cracking tensile strengths of the composites containing L-MWCNTs were improved by 16%. Energy absorption capability indices were formulated based on tensile load–displacement curves, and results showed that the energy absorption capabilities of RPC at initial cracking improved as a result of the incorporation of MWCNTs. Furthermore, microscopic analysis indicated that MWCNTs decelerate crack development at the nanoscale and improve the initial-cracking tensile strength of RPC. [ABSTRACT FROM AUTHOR]