Pull-out Tests Research Articles

Large deformation simulation of uprooting of trees with complex root system architectures using material point method with embedded truss elements

Abstract Background and aims Urban trees in coastal cities like Hong Kong may suffer from an uprooting failure when subjected to extreme winds. A proper numerical model for tree uprooting simulation can help to select tree species or soil types that better resist uprooting failure. However, modeling tree uprooting is challenging as it is a cross-disciplinary problem involving complex root system architectures (RSAs) and large deformation of both roots and soils. This study aims to develop a hybrid numerical model that combines truss elements and material point method (MPM) to simulate the entire large-deformation uprooting process of trees with complex RSAs. Methods The tree uprooting model is developed by coupling truss elements in finite element method (FEM) with MPM. Laboratory pull-out tests using artificial roots and real root cuttings are adopted to validate the developed model. A comparative study is performed to investigate the difference between using complex and simplified RSAs in tree uprooting simulations. Results The developed model provides consistent predictions of peak load, critical displacement and failure mode when compared with results from laboratory tests. Moreover, the comparative study shows that the uprooting resistance obtained with a complex RSA is higher than that with a simplified RSA. The difference varies with the soil and root mechanical properties. Conclusion The developed hybrid model offers a novel way for simulating an entire tree uprooting process involving large deformations and complex RSAs. The study shows that using a simplified RSA to approximate the complex RSA might result in misleading failure modes.

Optimizing superelastic shape-memory alloy fibers for enhancing the pullout performance in engineered cementitious composites

Abstract This study explores the effect of integrated superelastic shape-memory alloy fibers (SMAFs) on the mechanical performance of engineered cementitious composites (ECCs). Various SMAF configurations – linear-shaped SMAFs (LS-SMAFs), hook-shaped SMAFs (HS-SMAFs), and indented-shaped SMAFs (IS-SMAFs) – with diameters of 0.8 and 1.0 mm were incorporated into ECC matrices, and surface texturization was achieved through abrasive paper treatment. Their mechanical properties were assessed through single fiber pullout tests on ECC mixtures containing 1.5 and 2.0% polyvinyl alcohol (PVA), subjected to both monotonic and cyclic loading conditions. Qualitative analysis, employing scanning electron microscopy, demonstrated that the IS-SMAF configuration provided superior mechanical interlocking and fiber–matrix adhesion, with a distinct flag shape observed during tensile testing. Quantitative data indicated that IS-SMAFs significantly improved the tensile strength and pullout resistance, with slip distances of ≥5 mm and average pullout loads ranging from 263 to 403 N. LS-SMAFs demonstrated better performance compared to HS-SMAFs and LS-SMAFs in terms of tensile and pullout characteristics. Additionally, ECCs with increased PVA content exhibited enhanced withdrawal performance. Thermogravimetry analysis and X-ray diffraction provided insights into the high-temperature stability and crystalline structure of the composites. These results underscore the effectiveness of IS-SMAFs in enhancing ECC properties, offering significant implications for the development and optimization of high-performance composite materials in civil engineering applications.

Innovative high-strength screw connections for additive manufactured thermoplastic components

AbstractComponents that are additively manufactured by material extrusion (MEX) are exposed to complex challenges due to their layered structure. Anisotropy and the risk of delamination must be given special consideration, especially when they are exposed to high loads. In case high-loaded components have additionally to be connected to other parts or assemblies via screw connections, the connection area in the MEX-manufactured component is a limiting factor for usability. This article presents a novel method in which a preheated metallic threaded sleeve with internal and external threads is inserted into the component in order to create a high-strength threaded anchoring. This prevents pre-damage such as delamination in advance. As part of preliminary investigations, a selection of relevant parameters was first made. An open parameter test rig was developed with which the threaded sleeves could be applied into test specimens with high repeat accuracy. The effectiveness and the increase in the achievable pull-out forces were demonstrated by means of pull-out tests. It was also shown that the heat input via the outer thread flanks during application has a positive influence on the overall strength of the component.

Bond of FRP bars in fine‐grained alkali‐activated concrete

AbstractAlkali‐activated cement (AAC) is an alternative binder with a promising potential to replace ordinary Portland cement (OPC) and mitigate its environmental issues. The use of fiber‐reinforced polymer (FRP) reinforcements with AAC concrete enables the development of corrosion‐resistant, environmentally friendly reinforced concrete structures. Bond behavior is critical in reinforced concrete structures and must be thoroughly studied for such new alternative materials. This study employs pullout tests to investigate the bond behavior between FRP bars and fine‐grained AAC concrete. Three fine‐grained AAC concretes with low to high strength, glass and carbon FRP bars and wrapped, milled, and smooth surface treatments were examined. The effect of bar casting position was also investigated. The compressive strength showed a significant influence on the bond strength. An average bond strength of approximately 18 MPa was observed for both glass and carbon FRP bars when used with 65 MPa concrete. Both the glass and carbon FRP bars with wrapping showed a lower bond strength than their milled FRP bars counterparts. The carbon bars without surface preparation (smooth bars) resulted in a much lower bond strength, around 4 MPa. In terms of casting positions, the bars cast in the middle section of the concrete block showed a higher bond strength than those at the bottom and top.

A novel personalized homogenous finite element model to predict the pull-out strength of cancellous bone screws

BackgroundOrthopedic surgeries often involve the insertion of bone screws for various fixation systems. The risk of postoperative screw loosening is usually assessed through experimental or finite element (FE) evaluations of the screw pull-out strength. FE simulations are based on either personalized complex but accurate heterogeneous modeling or non-personalized simple but relatively less accurate homogeneous modeling. This study aimed to develop and validate a novel personalized computed tomography (CT)-based homogeneous FE simulation approach to predict the pull-out force of cancellous bone screws.MethodsTwenty FE simulations of L1-L5 vertebral screw pull-out tests were conducted, i.e., 10 heterogeneous and 10 homogenous models. Screws were inserted into the lower-middle region of vertebrae. In our novel homogeneous model, the region around approximately twice the diameter of the screw was used as a bone material reference volume. Subsequently, the overall material property of this region was homogeneously attributed to the entire vertebra, and pull-out simulations were conducted.ResultsThe mean error of the predicted pull-out forces by our novel homogenous simulations was ~ 7.9% with respect to our heterogeneous model. When solely the cancellous bone was involved during the pull-out process (i.e., for L1, L2, and L3 vertebral bodies whose cortical bone in the inferior region is thin), the novel homogenous model yielded small mean error of < 6.0%. This error, however, increased to ~ 11% when the screw got involved to the cortical bone (for L4 and L5 vertebrae whose cortical bone in the inferior region is thick).ConclusionThe proposed personalized CT-based homogenous model was highly accurate in estimating the pull-out force especially when only the cancellous bone was involved with the screw.

Investigation of the bond between bamboo and ribbed steel reinforcements and the surrounding concrete by pull-out test

Traditional housing did not use expensive materials like steel reinforcement, yet some have outlived sandcrete or mud-brick housing. One means to build at a low cost is to supplement high-cost materials with low-cost types, which have comparable strength. This study compares the bond between bamboo (Bambusa vulgaris and Bambusa vittata) reinforcement as well as ribbed steel enforcement and the surrounding concrete by pull-out test. Bamboo specimens were split and cut into the dimensions of 600 mm × 20 mm ×10 mm. Sixty (60) samples of each of the two varieties were prepared, thirty (30) were coated with tar oil as waterproof agent to a length of 200 mm and embedded into fresh concrete samples and allowed to cure, while another 30 did not receive any coating. Thirty (30) specimens of 12.5 mm diameter of ribbed steel rods of length 60 mm were used as control Materials for the pull-out test were also embedded in the centre of 150 mm concrete cubes and left to cure for 28 days. The load required to pull the steel rods from concrete (5.49±0.22 kN) was greater than those for both coated Bambusa vulgaris and Bambusa vittata (1.67±0.11 kN – 1.67±0.12 kN) and uncoated Bambusa vulgaris and Bambusa vulgaris vittata (2.04±0.14 kN– 2.14±0.19 kN) bamboo. The differences were significant (p&lt;0.05). The pull-out was also conducted in conformity with the Bureau of Indian Standard (IS) 2770: 1997.

Rebar corrosion and bond in concrete with recycled glass granules

Waste glass granules are considered a cost-effective alternative material to replace aggregates in concrete. However, the bond degradation of corroded concrete containing recycled glass granules has not been explored. This study investigated the effect of glass granules on rebar corrosion and the bond loss between rebars and concrete subjected to accelerating corrosion. The compressive strength and pore structure of the concrete were first evaluated, and the corrosion rate and pull-out testing of rebar in concrete were conducted after corrosion. The results show that, despite reducing the initial bond strength before corrosion, a high replacement of recycled glass granules improved the concrete's resistance to corrosion due to the optimized microstructure of concrete. In addition, the compressive strength of the concrete dominated the initial bond strength, which reduced with increasing the corrosion period. However, this negative effect of corrosion was mitigated by increasing the recycled glass granules, followed by reduced bond strength loss and a lower rebar corrosion rate.

A Murine Model of Non-Wear-Particle-Induced Aseptic Loosening

Background: The current murine models of peri-implant osseointegration failure are associated with wear particles. However, the current clinical osseointegration failure is not associated with wear particles. Here, we develop a murine model of osseointegration failure not associated with wear particles and validate it by comparing the cellular composition of interfacial tissues with human samples collected during total joint arthroplasty revision for aseptic loosening. Materials and Methods: Thirty-two 16-week-old female C57BL/6 mice underwent implantation with a press-fitted roughened titanium implant (Control, n = 11) to induce normal osseointegration and a press-fitted smooth polymethylmethacrylate implant (PMMA, n = 11), a loosely fitted smooth titanium implant (Smooth-Ti, n = 5) or a loosely fitted roughened titanium implant (Overdrill, n = 5) to induce osseointegration failure. Pullout testing was used to determine the strength of the bone–implant interface (n = 6 of each for Control and PMMA groups) at 2 weeks after implantation. Histology (n = 2/group) and immunofluorescence (n = 3/group) were used to determine the cellular composition of bone–implant interfacial tissue, and this was compared with two human samples. Results: Osseointegration failure was confirmed with grossly loosening implants and the presence of fibrous tissue identified via histology. The maximum pullout load in the PMMA group was 87% lower than in the Control group (2.8 ± 0.6 N vs. 21 ± 1.5 N, p &lt; 0.001). With immunofluorescence, abundant fibroblasts (PDGFRα+ TCF4+ and PDGFRα+ Pu1+) were observed in osseointegration failure groups and the human samples, but not in controls. Interestingly, CD146+PDGFRα+ and LepR+PDGFRα+ mesenchymal progenitors, osteoblasts (OPN+), vascular endothelium (EMCN+) cells were observed in all groups, indicating dynamic osteogenic activity. Macrophages, only M2, were observed in conditions producing fibrous tissue. Conclusions: In this newly developed non-wear-particle-related murine osseointegration failure model, the cellular composition of human and murine interfacial tissue implicates specific populations of fibroblasts in fibrous tissue formation and implies that these cells may derive from mesenchymal stem cells.

Numerical modeling of the transfer length of 17.8 mm (0.7 in) diameter prestressing strands in pretensioned members

The bond between concrete and reinforcing steel bars in reinforced concrete structures is crucial. Prestressing strands with a diameter of 17.8 mm (0.7 in) transfer higher compression per unit area than smaller strands, improving moment capacity and shear strength and extending the girder span. However, limited research on the bond performance of these 17.8 mm diameter prestressing strands has restricted their widespread use despite their advantages. This study presents a methodology to determine the transfer length of pretensioned members numerically. The influence of pretension force and bond strength on a 17.8 mm diameter strand was assessed through the validation of a finite element model using experimental results from a standardized pretensioned pull-out test. Analytical calculations using different code formulas were also conducted, encompassing strands of varying diameters. Our finite element analysis suggests that a 50.8 mm spacing between adjacent 17.8 mm diameter strands is applicable, ensuring there is no stress overlap in the transfer zones, which can lead to concrete cracking and reduced structural integrity. Existing code formulas, such as ACI 318–19 and AASHTO LRFD 9th edition, require longer transfer lengths when compared with the numerical results. The fib MC 2010 and EC2 formulas offer closer results to numerical results of smaller diameter strands but underestimate the transfer length for the 17.8 mm diameter strand. Therefore, improvements to code formulations may be necessary for more accurate transfer length predictions for this specific strand size.

Effects of bonding enhancers on shear stress and bonding strength of modified asphalt binders under different aging and moisture conditions

Insufficient bonding within pavement material causes widespread propagation of traffic-induced stresses, leading to accelerated damage accumulation and significant reduction in the service life of flexible pavements. This research aimed to examine the effects of using bonding enhancers, specifically Polyalkylene Glycol-based (PLG) and Alkylamines-based (ALM), on modified asphalt binders to improve the bonding characteristics. These additives were utilized at different concentration levels, i.e., 0.5 % and 1.0 %, based on the mass of asphalt binders, with the aim of enhancing the bonding performance between the materials within the asphalt mixture composite. The effectiveness of the modified asphalt binders was compared to the conventional ones through laboratory tests, which included shear-compression and pullout tests. These tests were conducted to assess the bonding performance of the modified asphalt binders by preparing different sets of sandwich samples using concrete cube substrates. A sodium carbonate solution was used as a medium to accelerate the effect of moisture damage on samples during the conditioning process. Additionally, the bonding performance of the asphalt binders was evaluated through a series of pullout and shear-compression tests under designated moisture and aging conditions. The dynamic shear rheological (DSR) test results of Rolling Thin Film Oven (RTFO) and Pressure Aging Vessel (PAV)-aged binders incorporating bonding enhancers showed superior resistance to rutting and fatigue failure. Significant improvements in stress and shear strength were observed under moisture and aging conditions, highlighting improvements in the bonding performance of the modified asphalt binders. The incorporation of additives has positively contributed to the pavement performance, despite only a modest increase in bonding performance, tensile strength, and shear stress resistance.

Indoor tests of sensor-enabled piezoelectric geocable–geogrid composite structure for slope rehabilitation and monitoring

Sensor-enabled piezoelectric geocables were combined with a geogrid to acquire a sensor-enabled piezoelectric geogrid (SPGG) based on the impedance–strain relationship. Tension, pullout, and straight shear tests were conducted on this SPGG configuration. The tension test results indicated that the tensile strain–normalized impedance curves were exponential in form within the first 7 % of strain and the rate of shift in impedance was independent of the tension loading rate. An excellent correspondence between the peak strength and impedance inflection point was observed in the results of the pullout and straight shear tests. Additional validation of the proposed SPGG was conducted through a collapse test of the reinforced soil slope model. The results indicated that the SPGG-obtained strains were similar to actual strain gauge measurements but provided a larger measurement range and that the SPGG was able to sense real-time vibrations during the slope collapse using a voltage analysis, confirming that the proposed SPGG can simultaneously provide soil reinforcement, strain monitoring of reinforcement materials, and vibration sensing. This research is expected to inform the development of a dynamic and static monitoring, large range, and accurate method for monitoring the conditions of reinforced soil over their entire lifecycles.

Nitric acid-modified amorphous alloy fiber and its effects on mechanical properties of ultra-high performance concrete (UHPC)

The traditional steel fiber UHPC has the phenomenon of low strength and poor durability, which can not give full play to the practical value of UHPC. Nevertheless, amorphous alloy fibres possess notable strength, flexibility and corrosion resistance. The incorporation of amorphous alloy fibres into ultra-high performance concrete (UHPC) has been demonstrated to enhance the strength and toughness of UHPC. However, the surface of amorphous alloy fibres is characterised by a smooth and hydrophobic quality, which presents a challenge in terms of the interface bond with the cement matrix. In order to address this issue, three concentrations of nitric acid (HNO3, 2 mol/L, 4 mol/L and 6 mol/L) were employed to modify the surface of amorphous alloy fibres, and then the amorphous alloy fibres were incorporated into the cement matrix for fiber pull-out test, compression, flexural strength and impact testing. The changes in the physical and chemical properties of the amorphous alloy fibres before and after modification were analysed, and the effects on the interface bond strength and dynamic and static mechanical properties of UHPC were studied by means of SEM. The results demonstrate that the adhesion strength of the modified amorphous alloy fibre with 4 mol/L HNO3 is the most robust, with an increase of 18.7 % in compressive strength, 13.8 % in flexural strength, and 5.4 % in impact resistance. The method has been demonstrated to be effective in strengthening and toughening the cement matrix with HNO3-modified amorphous alloy fibres.

Microdroplet Pull-out Testing: Significance of Fiber Fracture Results

Fiber reinforced composites are used in structural materials that required light weight and stiffness. The properties of the fibers or matrix are important, but the interfacial properties have a significant impact on the properties of fiber reinforced composite. In this study, the interfacial shear strength (IFSS) was measured using acrylic resin and epoxy resin as base materials. The chemical composition of acrylic and epoxy matrix materials was analyzed to predict the effects on IFSS. Additionally, IFSS was measured through a microdroplet pull-out test. The reliability of the experimental results was enhanced by applying a statistical analysis to IFSS results. In the case of epoxy, GF/epoxy exhibited higher IFSS to twice and half times than CF/epoxy specimens. It means that the surface treatment of the fibers has a significant impact on the interface. In the case of acrylic, IFSS could be measured for GF. But in the case of CF, IFSS was too low to get accurate results of IFSS. Through this research, methods to improve the accuracy of composite interfacial strength measurement experiments were examined, and the study suggested the need for standardized criteria to evaluate composite interfacial adhesion.

Explainable Tuned Machine Learning Models for Assessing the Impact of Corrosion on Bond Strength in Concrete

This study mainly aims to evaluate the bond strength of corroded reinforcements in reinforced concrete members. In this regard, a comprehensive dataset containing a total of 285 specimens was collected from previous experiments. All collected specimens, including normal concrete, were subjected to pull-out tests to ensure consistent results. The features evaluated are associated with both concrete and rebar characteristics, corrosion rate, and duration. Six machine learning (ML) models were used to assess the dataset: Decision Tree, Random Forest, Light Gradient-Boosting Machine, Gradient Boosting, Extreme Gradient Boosting, and Extra Tree. Hyperparameter tuning was conducted using grid search to optimize model performance and enhance predictive accuracy. Additionally, the Shapley Values technique was utilized to interpret the significance of the features on bond strength.The results show that Extreme Gradient Boosting and Extra tree methods outperformed the other models, with score of 0.9 each and RSME of 2.21 and 1.87, respectively. Furthermore, tuned models resulted in more accurate performance than the default models. Evaluating the significance of studied features indicated that the elevated levels of corrosion were associated with a negative impact on bond strength. In addition, the corrosion rate is considered to be the most influential factor affecting the bond strength.

Experimental and theoretical investigation on bond performance between surface-modified bamboo scrimber bars and concrete

With the aim of facilitating the adoption of eco-friendly bamboo scrimber bars as an alternative to the widely used steel bars in concrete structures, center pull-out tests were conducted on 75 specimens to investigate the bond performance between bamboo scrimber bars and concrete. Two innovative modification methods were employed, including the sand coating modification method with varying particle sizes and a newly proposed method involving wrapping with basalt fiber bundles, with a focus on the impact of bundles spacing on bond strength. Additionally, other influence factors, including the concrete compressive strength, development length, and equivalent diameter of bamboo scrimber bars, were investigated. The test results show that compared to unmodified bamboo scrimber bars, those modified with sand and basalt fiber bundles show increases in bond strength of 5.50-5.96 times and 4.82-6.15 times, respectively. The bond strength was most significantly influenced by the development length, which decreases with increasing development length. Besides, specimens with development length less than 75 mm mainly exhibit pull-out failure, while those with development length equal to 75 mm primarily occur concrete splitting failure. Furthermore, a bond strength prediction formula and a bond stress-slip constitutive model for the interface between bamboo scrimber bars modified with sand coating and concrete were established, which show great agreement with the test results.

Hybrid synthetic/natural fiber-reinforced strain-hardening magnesia-based composites

Synthetic fiber-reinforced strain-hardening magnesia (MgO)-based composites (SHMC) are an emerging group of fiber-reinforced cementitious composites (FRCC) with ultra-high ductility and CO2 sequestration potential. However, the size of SHMC members is limited by the inadequate carbonation depth of the MgO matrix. In this work, hybrid synthetic/natural fiber-reinforced SHMC are developed containing different contents of PVA fibers (0-2 vol.%) and sisal fibers (0-4 vol.%). The new Hybrid-SHMC were experimentally studied on micro-scale and composite scale. On the micro-scale, the effect of cement matrix carbonation on the PVA/sisal fiber-to-matrix interfacial bonds and the matrix moduli were studied by single-fiber pullout test and local nano-indentation. On the composite scale, the effect of fiber dosage on the compressive/tensile behavior and carbonation profile of SHMC were studied. The effects of the carbonation-induced micromechanical enhancements on the composite behaviors were validated by a micromechanics-based fiber-bridging model. The results demonstrated that the new hybrid SHMC could achieve ultra-high tensile ductility (>2.5%) and uniform carbonation simultaneously (≥ 0.25g CO2/g MgO at different depths). In Hybrid-SHMC, the PVA fibers mainly contributed to the mechanical crack-bridging and thus strain-hardening, while sisal fibers with porous microstructure improved the carbonation depth and thus the fiber-matrix interfacial bonds, composite compressive, and tensile performances.

Different Wire Surface Treatments on Adhesion Efficacy of Orthodontic Fixed Retainer: An In Vitro Study.

This study assesses the impact of surface treatment with sandblasting and Z-primer on the adhesion efficacy of fixed lingual retainers. Dead soft stainless steel wire 0.016 × 0.022-inch (n = 120) was treated by different techniques and classified into four groups equally (n = 30) according to surface treatment. Group I wire without treatment, group II wire treated with sandblasting, group III wire treated with Z-primer alone, and group IV wire treated with sandblasting with Z-primer. The stainless steel wire (n = 40) was bonded to 80 extracted premolars in pairs mounted in acrylic. Other stainless steel wires (n = 80) are embedded into acrylic blocks. All groups were divided into two subgroups according to thermocycling teeth samples were assessed by shear bond strength (SBS) A stereomicroscope was used to calculate the adhesive remnant index (ARI), while the acrylic block was by pull-out test. Finally, data were analyzed by IBM-SPSS (V 27, 2020). Mann-Whitney U-test; Kruskal-Wallis H-test and, two-way ANOVA were utilized to assess for SBS and pull-out. Kruskal-Wallis H-test showed a non-significant difference in ARI between all groups, while in two-way mixed ANOVA demonstrated a significant difference in SBS between group III (sandblasting/Z-primer) vs group I and group IV Z-primer (p = 0.028) and control (p = 0.016), and a significant difference between group II sandblasting vs both group I and group IV Z-primer (p = 0.024) and control (p = 0.014). The two-way mixed ANOVA tests showed a significant difference in pull-out between sandblasting/Z-primer vs Z-primer (p = 0.012). Using of mixed surface treatment for fixed retainer as sandblasting with Z-primer is considered as the best method to increase adhesion efficacy between wire and composite and improve the quality of orthodontics fixation when compared with single treatment (sandblasting alone or Z prime). On the other hand, the sue of sandblasting alone for fixed retainer surface treatment is better than Z-primer alone but both treatments are better than fixed retainer without treatment. Developed and examined new and traditional techniques used to treat the surface of wire used as a retainer after orthodontics treatment to improve patients' treatment and life quality and decrease the chance of relapse. How to cite this article: Naji SM, Mohammad MH, Enan ET, et al. Different Wire Surface Treatments on Adhesion Efficacy of Orthodontic Fixed Retainer: An In Vitro Study. J Contemp Dent Pract 2024;25(7):677-683.

Data-Driven Optimised XGBoost for Predicting the Performance of Axial Load Bearing Capacity of Fully Cementitious Grouted Rock Bolting Systems

This article investigates the application of eXtreme gradient boosting (XGBoost) and hybrid metaheuristics optimisation techniques to predict the axial load bearing capacity of fully grouted rock bolting systems. For this purpose, a comprehensive dataset of 72 pull-out tests was built, considering various influential parameters such as three water-to-grout (W/G) ratios, five ranges of curing time (CT), three different grout admixtures with two different fly ash (FA) contents, and two different diameter confinements (DCs). Additionally, to find out the effect of the mechanical behaviour of grouts on the performance of fully grouted rock bolting systems, seventy-two uniaxial compression strength (UCS) samples were cast and tested simultaneously with pull-out samples. The UCS samples were prepared with the same details as the pull-out samples to avoid any inconsistency. The results highlight that peak load values generally increase with longer curing times, lower W/G, and higher UCS and DC values. The main novelty of this paper lies in its data-driven approach, using various XGBoost models. This method offers a time-, cost-, and labour-efficient alternative to traditional experimental methods for predicting rock bolt performance. For this purpose, after building the dataset and dividing it randomly into two training and testing datasets, five different XGBoost models were developed: a standalone XGBoost model and four hybrid models incorporating Harris hawk optimisation (HHO), the jellyfish search optimiser (JSO), the dragonfly algorithm (DA), and the firefly algorithm (FA). These models were subsequently evaluated for their ability to predict peak load values. The results demonstrate that all models effectively predicted peak load values, but the XGBoost-JSO hybrid model demonstrated superior performance, achieving the highest R-squared coefficients of 0.987 and 0.988 for the training and testing datasets, respectively. Sensitivity analysis revealed that UCS values were the most influential parameter, while FA content had the least impact on the maximum peak load values of fully cementitious grouted rock bolts.

Mechanical Properties of Fully-Grouted Bolts Support Based on Compression Tests of Anchored Rock Mass

The mechanical properties of fully-grouted bolt support are critical for the safety of support engineering works. To study the influences of factors including the bolt length and diameter, strength of the rock, and fracture angle on the mechanical properties of fully-grouted bolt support, compression tests were conducted on an anchored rock mass, considering the shortcomings of pullout tests on bolts. The discrete element software PFC2D (4.0) was adopted for numerical simulation and analysis from two aspects, namely, the stress distribution and anchorage force supplied by such bolts. The research found that by increasing the bolt diameter and length as well as the strength of the rock, the maximum anchorage force of bolts increases. Whereas the stress distribution of all bolts increases at first and then decreases along the bolts, and there is only one peak on the stress distribution curves, which also gradually shifts to a greater depth. In a fractured rock mass, the maximum anchorage force of bolts decreases, then increases (and is minimized at a fracture angle of 45°) with the decrease in fracture angle. The influence of fractures with different angles on the stress distribution of bolts is mainly reflected in the fracture zone. The bolt stress decreases abruptly in the zone with a fracture angle of 90°, forming a valley. The bolt stress increases suddenly in the zones with fracture angles of 60° and 45°, thus forming peaks. The bolt stress does not increase or decrease suddenly in the zone with a fracture angle of 30°. Therefore, it necessitates consideration of the influences of fractures on the anchorage force and the selection of bolts of appropriate size during anchorage design. After installation, the bolt stress should be monitored for stability and early warning of anchored rock mass according to changes in the stress provided.

Enhancing mechanical properties and crack resistance of high-strength SHCC/ECC for durable transportation through ethylene-vinyl acetate polymer modification

Strain-hardening cementitious composites (SHCC) possess excellent ductility and toughness, making them suitable for reinforcement and repair projects of transit infrastructures, such as tunnel linings, bridges, and highways. Especially, polyethylene (PE) fiber is admixed to SHCC for fabricating high-strength PE-SHCC. However, the hydrophobic nature of PE fibers results in relatively weak interfacial frictional bond strength with cementitious matrix, ultimately leading to larger crack widths and wider gaps. This poses durability and safety concerns for the practical applications of PE-SHCC in transit infrastructure. In this study, ethylene-vinyl acetate (EVA) dispersible polymer is selected to address this problem. The effect of admixing EVA on mechanical and direct tensile properties of PE-SHCC is investigated. The compressive strength, first crack strength and matrix fracture toughness of PE-SHCC tended to decrease after EVA modification, however, the tensile strength and tensile strain increased significantly with doping. After that, the single fiber pullout test combined with flaw size/distribution analysis is performed to explore the improvement mechanism of PE-SHCC after EVA modification. It was found that the addition of EVA could significantly improve the interfacial transition zone (ITZ), increase the interfacial friction between PE fibers and matrix, and optimise the defect distribution within the matrix. The recommended dosage of EVA is 3–6 % comprehensive consideration of the tensile strength, strain energy, crack width, and mechanical strengths of the EVA-modified PE-SHCC. The enhancement of mechanical strengths and crack resistance in PE-SHCC contributes to the development of transit infrastructure towards greater reliability, durability, and resilience.