Mechanical performance and anisotropic analysis of rubberised 3D-printed concrete incorporating PP fibre.

The research investigates the effects of substituting sand with rubber particles derived from waste tyres-up to 40% by volume-and the inclusion of polypropylene (PP) fibres. Unlike steel fibres, which can cause operational challenges and surface irregularities in the printing process, PP fibres' flexibility integrates well within the concrete matrix. This integration ensures smooth extrusion and a high-quality surface finish, enhancing the printability of the concrete. The study's findings reveal that including rubber particles and PP fibres impacts the concrete's properties, showing a general decline in compressive and flexural strengths as the rubber content increases. Nevertheless, the PP fibre-enhanced mixtures maintain sufficient structural strength, demonstrating an anisotropic compressive strength above 30 MPa and a flexural strength of 4 MPa. These results underscore the feasibility of using rubberised 3D-printed concrete with PP fibres in sustainable construction practices, aligning with standards (ACI 318:2018) and contributing to eco-friendly and innovative construction methodologies.

Effects of eccentric loading on performance of concrete columns reinforced with glass fiber-reinforced polymer bars.

Glass fiber-reinforced polymer (GFRP) reinforcements are superior to traditional steel bars in concrete structures, particularly in vertical elements like columns, and offer significant advantages over conventional steel bars when subjected to axial and eccentric loadings. However, there is limited experimental and numerical research on the behavior of GFRP-reinforced concrete (RC) columns under eccentric loading having different spacing of stirrups. In this study, six specimens were cast under three different values of eccentricities (25 mm, 50 mm, and 75 mm) with two groups of stirrups spacing (50 mm and 100 mm). The experimental results showed that by increasing the eccentricity value, there was a reduction in the load-carrying capacity of the specimens. The finite element ABAQUS software was used for the numerical investigation of this study. The results from the finite element analysis (FEA) were close to the experimental results and within the acceptable range. The maximum difference between the experimental and FEA results was 3.61% for the axial load and 12.06% for the deformation.

Effects of width-height ratio and roof-floor strength on the mechanical characteristics of cemented gangue backfill pier-column.

Cemented gangue backfill pier-column (CGBP) which was made of coal gangue, fly ash, cement, and water is the supporting component of the goaf in partial backfill mining or constructional backfill mining for controlling the surface subsidence of coal mining. The width-height ratio and roof-floor strength directly affect the bearing capacity of CGBP under axial compression, which is essential for the design of CGBP. Herein, the effect of width-height ratio (1:3-1:1) on the mechanical characteristics of CGBP with different curing ages under uniaxial compression was system studied through experiment, and the damage process was analyzed by ultrasonic equipment and DIC. Based on the experimental results and discrete element theory, a three-phase numerical model for CGBP was established, which considered the real aggregate shape and distribution and the mechanical characteristics of each phase. Then, the effects of the end friction coefficient and the strength ratio of roof-CGBP-floor combination on the strength and failure characteristics of CGBP (large width-height ratio: 1:1-4:1) were investigated. The results show that CGBP shows the width-height ratio effect obviously and the strength and ductility increase with the increase of the width-height ratio, and the width-height ratio effect increases with the increase of curing age and strength ratio. The end friction constraint is the main reason for the width-height ratio effect, and the higher the friction coefficient is, the larger the width-height ratio effect shows, and the width-height ratio effect disappears without end friction constraint. The increase of the width-height ratio of CGBP and the strength ratio of the roof-CGBP-floor combination increases the strength of the combination. Whether the strength of the combination is greater than that of CGBP may have a roof-floor strength threshold or a strength ratio threshold, which are between 31.44-54.11 MPa and 3.75-6.44, respectively. When the strength of the roof and floor is different, the strength of the combination is mainly controlled by the weak carrier and increases with the increase of the strength of the weak carrier. The peak strain energy of CGBP and combination increases with the increase of end friction coefficient, width-height ratio, and strength of roof and floor. The experimental and simulation results can be used to guide the design of CGBP in constructional backfill mining or partial backfill mining.

Constitutive Model for Equivalent Stress-Plastic Strain Curves Including Full-Range Strain Hardening Behavior of High-Strength Steel at Elevated Temperatures.

High-strength steel has been increasingly applied to engineering structures and inevitably faces fire risks. The equivalent stress-plastic strain (σeq- εeqp) curves of steel at elevated temperatures are indispensable if a refined finite element model is used to investigate the response of steel members and structures under fire. If the tensile deformation of steel is considerable, the σeq- εeqp curves at elevated temperatures are required to consider the strain-hardening behavior during the post-necking phase. However, there is little research on the topic. Based on the engineering stress-strain curves of Q890 high-strength steel in a uniaxial tension experiment at elevated temperatures, the σeq-εeqp curves before necking are determined using theoretical formulations. An inverse method based on finite element analysis is used to determine the σeq- εeqp curves during the post-necking phase. The characteristics of σeq-εeqp curves, including the full-range strain hardening behavior at different temperatures, are discussed. An equivalent stress-plastic strain model of Q890 steel at elevated temperature is proposed, which is consistent with the σeq-εeqp curves. The constitutive model is further verified by comparing the finite element analysis and test results.

A study on the influences of a hygrothermal environment on the compressive strength and failure criteria of asphalt mixtures based on true triaxial tests.

Based on a high-pressure servo static and dynamic true triaxial test machine (tawz-500/300), uniaxial and true triaxial tests of AC-25C asphalt mixtures under different heat moisture coupling treatments were performed, and the triaxial compressive strength value was determined by mathematical treatment. The test results show that in the range of 20-60 °C, the uniaxial and triaxial compressive strength of the AC-25C asphalt mixture decreases with increasing drying and soaking environment temperature. In the temperature range of 20-40 °C, the drying temperature sensitivity and soaking temperature sensitivity of the asphalt mixture with compressive strength and failure strain value as indices are the maximum. The increase in the intermediate principal stress can improve the triaxial compressive strength, and the increase reaches the maximum when the environmental treatment temperature is 40 °C. The maximum stress ratio is in the range of σ2/σ3 = 0.25 to σ2/σ3 = 0.5. The failure forms of uniaxial tension and triaxial tension are mainly caused by tensile stress. The influence of temperature, humidity and stress ratio on triaxial failure strength is analysed. The relationship between the failure strength and temperature coefficient k is established using a variety of failure criteria, which can provide an experimental and theoretical basis for the mechanical analysis of asphalt mixtures under complex stress states.

Performance of Different Concrete Types Exposed to Elevated Temperatures: A Review.

Concrete is a heterogeneous material that consists of cement, aggregates, and water as basic constituents. Several cementitious materials and additives are added with different volumetric ratios to improve the strength and durability requirements of concrete. Consequently, performance of concrete when exposed to elevated temperature is greatly affected by the concrete type. Moreover, post-fire properties of concrete are influenced by the constituents of each concrete type. Heating rate, days of curing, type of curing, cooling method, and constituents of the mix are some of the factors that impact the post-fire behavior of concrete structures. In this paper, an extensive review was conducted and focused on the effect of concrete constituents on the overall behavior of concrete when exposed to elevated temperature. It was evident that utilizing fibers can improve the tensile capacity of concrete after exposure to higher temperatures. However, there is an increased risk of spalling due to the induced internal stresses. In addition, supplementary cementitious materials such as metakaolin and silica fume enhanced concrete strength, the latter proving to be the most effective. In terms of the heating process, it was clear that several constituents, such as silica fume or fly ash, that decrease absorption affect overall workability, increase the compressive strength of concrete, and can yield an increase in the strength of concrete at 200 °C. Most of the concrete types show a moderate and steady decrease in the strength up until 400 °C. However, the decrease is more rapid until the concrete reaches 800 °C or 1000 °C at which it spalls or cannot take any applied load. This review highlighted the need for more research and codes' provisions to account for different types of concrete constituents and advanced construction materials technology.

Photocatalytic concrete for degrading organic dyes in water.

Since the advent of photocatalytic degradation technology, it has brought new vitality to the environmental governance and the response to the energy crisis. Photocatalysts harvest optical energy to drive chemical reactions, which means people can use solar energy to complete some resource-consuming activities by photocatalysts, such as environmental governance. In recent years, researchers have tried to combine photocatalyst TiO2 with building materials to purify urban air and obtained good results. One of the important functions of photocatalysts is to degrade organic pollutants in water through light energy, but this technology has not been reported in the practical application areas. To extend this technology to practical application areas, photocatalytic concrete for degrading pollutants in waters was proposed and demonstrated for the first time in this paper. The photocatalytic concrete proposed based on the K-g-C3N4 shows a strong ability to degrade the organic dyes. According to the experiment results, the angle of light source plays an important role in the process of photocatalytic degradation, while waters with pH value of 6.5-8.5 hardly influenced the degradation of organic dyes. When the angle of light source is advantageous for photocatalytic concrete to absorb more visible light, more organic dyes will be degraded by photocatalytic concrete. The degradation rate of methylene blue could reach about 80% in ½ hour under desirable conditions and is satisfied compared with that of reported works. This study implicates that photocatalytic concrete can effectively degrade organic dyes in water. The influences of changes in the water environment hardly affect the degradation of organic pollutants, which means photocatalytic concrete can be widely used in green infrastructures to achieve urban sewage treatment.

Geopolymer Concrete with Lightweight Fine Aggregate: Material Performance and Structural Application.

Limited information and data are available on the material and structural performance of GC incorporating lightweight fine aggregate. In this research, three types of lightweight fine materials were utilized to partially replace sand volume of GC. These lightweight materials were rubber, vermiculite, or lightweight expanded clay aggregate (LECA) and they were used in contents of 20%, 40%, 60%, and 100%. The variables were applied to better investigate the efficiency of each lightweight material in GC and to recommend GC mixes for structural applications. The concrete workability, compressive strength, indirect tensile strength, freezing and thawing performance, and impact resistance were measured in this study. In addition, three reinforced concrete slabs were made from selected mixes with similar compressive strength of 32 MPa and then tested under a 4-point bending loading regime. The results showed that using LECA as sand replacement in GC increased its compressive strength at all ages and all replacement ratios. Compared with the control GC mix, using 60% LECA increased the compressive strength by up to 44%, 39%, and 27%, respectively at 3, 7, and 28 days. The slabs test showed that partial or full replacement of GC sand adversely affected the shear resistance of concrete and caused premature failure of slabs. The slab strength and deflection capacities decreased by 9% and 30%, respectively when using rubber, and by 23% and 59%, respectively when using LECA, compared with control GC slab. The results indicated the applicability of GC mix with 60% LECA in structures subjected to axial loads. However, rubber would be the best lightweight material to recommend for resisting impact and flexural loads.

Dynamic Tensile Behavior of Steel HRB500E Reinforcing Bar at Low, Medium, and High Strain Rates.

The strain rate effect of engineering materials should be considered in the assessment of the performance of reinforced concrete (RC) structures under extreme dynamic loads such as blast and impact. However, the strain rate behavior of 500 MPa-grade anti-earthquake hot-rolled high-strength ribbed bar (HRB500E), used in critical RC members to improve the anti-earthquake performance, has not been investigated and reported in the open literature. That restricts its application in RC structures subjected to extreme dynamic loads. In this paper, dynamic tensile tests of HRB500E steel were conducted using an electromechanical universal testing machine and a servo-hydraulic high-speed testing machine. The stress-strain curves at strain rates ranging from 0.00025 to 550 s-1 were obtained where HRB500E steel was found significantly sensitive to strain rate. Existing formulations to evaluate the dynamic increase factor for yield stress (DIFy) are found to be not suitable for HRB500E steel, thus the widely used Cowper-Symonds and Malvar models for predicting the DIFy were modified based on the test results. Furthermore, the parameter of the Mander material model for describing engineering stress-strain relationship was also calibrated. Finally, the Johnson-Cook and proposed constitutive models for the true stress-strain relationship were examined. The proposed constitutive model can provide better prediction accuracy for yield stress than the Johnson-Cook model.