Compression Zone Research Articles

Effects of U-wrap anchored doubly FRP strengthened reinforced concrete beams on ductility and serviceability improvements

This paper presents a paradigm shift from the classical thinking that CFRP strengthening increases ultimate capacity only. It addresses a new strengthening technique change that yields significant improvements in ductility and serviceability. A series of eight reinforced concrete beams were cast and tested under four-point bending until failure. The beams were divided into two groups: the control group comprising two beams without any strengthening, and one beam reinforced with one tension layer only. The examined group consisting of four beams having double FRP layers in compression and a single layer in tension, with different anchorage configurations. Additionally, one beam in the examined group had a lower concrete strength. The results demonstrated that the doubly strengthened beams exhibited larger post-cracking stiffness and load capacity compared to the control beams. The anchoring of tension layers and incorporation of compression FRP layers significantly increased ductility of the beams. The utilization of U-wrap anchors successfully shifted the failure mode from sheet debonding to FRP rupture and FRP buckling in the compression zone, effectively maximizing the benefits of the FRP reinforcement. Moreover, the addition of compression FRP layers improved the load distribution and reduced the critical compressive stresses.

Research on the mechanical model of anchorage resistance in deeply filled jointed rock masses

The calculation model of anchorage resistance is the core issue in the study of anchorage theory in jointed rock masses. Due to the complex geological environment of deep rock masses, the anchorage mechanism of soft rock filled in joints is even more complicated and has not yet been clearly defined. Based on the theory of structural mechanics, considering the restraining stress in the plastic extrusion deformation zone of the bolt as a rectangular distribution pattern is more in line with the interaction state between the bolt and the surrounding medium when the rod is at its limit load. Taking into account the influence of the joint filling part, the value of restraining stress is revised to include a formula expression that factors in the strength of the filling material(σcj) and the degree of filling(Δ).Develop a mechanical model of anchorage resistance for filled joint soft rock considering large deformation conditions due to turning angles. Further, conduct shear tests on anchored filled joint rock bodies under various σcj−Δ combination patterns with constant normal stiffness (CNS) boundary conditions to validate and analyze the model. The research results indicate that under the influence of deep rock mass mechanical boundaries, the ratio of the shear deformation section length to the bolt diameter differs from existing research results, with specimens l/D under different σcj−Δ combinations typically ranging between 0.7 and 1.5 times.The displacement of the bolt at turning angle θ increases with the increase of Δ and decreases with the decrease of σcj, where Δ is the main factor affecting the degree of tensile-bending deformation of the bolt. As Δ increases, the calculated contribution rates of bolt shear resistance under three different σcj conditions increase from 9.8% to 34.8%, 40.3%, and 39.4%, respectively. This indicates that an increase in Δ can reduce the increment of normal stress, further decrease the overall stress and compressive deformation zone’s normal constraint stress in the rock mass, leading to a gradual increase in the degree of tensile-bending deformation of the bolt and an increase in resistance contribution.For the specimens with three different values of σcj, the ratio of shear force to axial force providing resistance on the joint surface decreased from a maximum of 2.83 to 0.72, 1.04, and 2.01, respectively. This indicates that when σcj is relatively large, tensile-bending deformation is more likely to occur, mobilizing the axial force of the bolt to provide the main resistance. Further systematic analysis was conducted on the length of the shear deformation section of the bolt and the distribution characteristics of the anchor resistance under condition β=30°∼90°. The reasonableness of the model was verified through the comparison of model calculation results with experimental data. Finally, a series of recommendations were proposed for preventing failure of the anchored system in filled jointed rock masses in actual engineering projects.

Flexural performance of GFRP-sheathed cold-formed steel composite panel filled with lightweight phosphogypsum

To develop lightweight, high-strength and environmentally friendly prefabricated panels, this paper proposed a novel GFRP plate-sheathed cold-formed steel (CFS) composite panel (GFRP-CFS panel) filled with lightweight phosphogypsum (LPG) and investigated its flexural performance experimentally, numerically and theoretically. Firstly, nine full-scale specimens were subjected to four-point bending test. The effects of the number, web height, thickness and arrangement of the longitudinal CFS studs, the number of lateral CFS studs, and the support connection on the flexural performance were discussed. Results indicate that the failure mode of the GFRP-CFS panels was debonding of GFRP plate in the compression zone at midspan and local buckling of CFS below the loading point. The strain of GFRP plate always lagged behind that of the CFS stud at the corresponding position, and the built-up cross section of the panel no longer conformed to the plane-section assumption. In addition, a nonlinear finite element modelling (FEM) analysis based on DIANA software was conducted to validate the test results. The load-deflection response, strain response and failure mode from the FEM analyses were in good agreement with the test results. Furthermore, based on the theoretical analysis considering the bond-slip effect between GFRP and CFS, a closed-form solution for the GFRP utilization coefficient was derived. The effect of different variables on the GFRP utilization coefficient was analyzed through parametric analysis. Finally, the calculation method for the flexural strength and ultimate deflection of GFRP-CFS panels were established, and the predicted results were in good agreement with test results. Therefore, the developed GFRP-CFS panels have excellent potential for prefabricated floor and wall applications.

Electromechanical response of smart ultra-high-performance concrete beams under flexure: Effects of filler type and electrode spacing

This study assessed the electrical resistance of smart ultra-high-performance concrete types (S–UHPCs) in the compressive and tensile zones of the flexural specimens subjected to four-point bending tests. The investigated S–UHPCs included a normal ultra-high-performance concrete (UHPC) (M1) integrated with fine steel slag aggregates (FSSAs) (M2) and a UHPC composed of multiwall carbon nanotubes and FSSAs (M3). The electrical resistance of these S–UHPCs remained relatively stable up to the limit of proportionality followed by a significant decrease. The suitable interelectrode distances for electrical resistance change measurements of the S-UHPC beam (100 × 100 × 400 mm3) were found to be 30:50:30 mm. Moreover, M2 demonstrated the highest stress sensitivity coefficient in the deflection hardening region (SSCMOR) at both tensile and compressive zones, with values of −4.29 and −1.20 %/MPa, respectively. The SSCMOR for M2 in the flexural–tensile zone was 3.58 times greater than that in the flexural–compressive zone.

Fluvial geomorphic parameters of the Shuiluo River Catchment and their tectonic implications, SE Tibetan Plateau

The Shuiluo River Catchment (SRC) is the front zone of the southeast compression and uplift of the Tibetan Plateau, with intense tectonic activity. In the basin, a series of regional large NW–SE trending active faults are developed. Studying clearly the geomorphic evolution of the SRC is conducive to further understanding the uplift and expansion mechanism of the SE edge of Tibetan Plateau. Our research was based on geographic information system, numerical analysis tool, and digital elevation model data, to extract six geomorpic parameters (hypsometric integral, asymmetry factor, basin shape ratio, valley floor width–valley height ratio, normalized channel steepness index and index of relative active tectonics) in SRC. After eliminating the impacts of climate, catchments area, and glacier, the geomorphic evolution of the SRC is mainly affected by geological structure and differential tectonic uplift movement; in the upstream and midstream (upper part), the shape of valleys and stream longitudinal profile shapes are affected by lithology; affected by geological structure and tectonic uplift, the tectonic activity in the midstream and downstream is relatively strong, and the intensity of activity in the downstream is stronger than that in the midstream, which may suggest that the faults’ activity in the downstream is stronger; the index of relative active tectonics values of the SRC are consistent with the regional seismic intensity, field-work and low-temperature thermochronology which indicates it is reasonable to use the fluvial geomorphic parameters to study the regional geomorphic evolution. The morphological parameters we extracted show different values in different regions of SRC, which may be the result of differential uplift in the southeastern of the Tibetan Plateau.

Behavior and design approach of ultra-high-performance concrete slender columns

The existing literature concerning the behavior of slender columns made of reinforced ultra-high-performance concrete (UHPC) is severely limited. Furthermore, current design standards lack specific guidelines for reinforced UHPC (R–UHPC) slender columns. Based on this, a comprehensive experimental investigation was conducted on four columns with 2 % steel fiber content subjected to compression to explore the influence of eccentricity ratios (e/h0 = 0, 0.15, 0.3, and 0.6) on the compression behavior of the R–UHPC slender columns. The results revealed that all R–UHPC slender columns experienced failure due to compressive zone UHPC crushing, with no observed concrete spalling. Moreover, a detailed parametric study was conducted on 3000 R–UHPC slender columns using a validated finite element model. By analyzing the parameters and employing the moment magnification method, a statistically derived design equation for effective flexural stiffness (EI) was formulated. The results indicated that EI decreased with increasing eccentricity ratio and increased with rising slenderness ratio. Additionally, calculation formulas for determining cross-section strength were proposed for R–UHPC slender columns, accounting for the contribution of tensile zone UHPC to the overall section strength. Finally, the proposed design approach was validated by comparing predicted results with available experimental data.

Analysis of failure mechanism of local compression in concrete cone foundation based on FEA

The ANSYS software is used for finite element numerical simulation to analyze the failure mechanism of local compression in concrete cone foundations in conjunction with previous experimental experience. The results reveal that the “hooping action” of surrounding concrete and the arch structure under the compression zone causes the increase of local compression in the concrete cone foundation. According to the findings of the analyses, the failure of local compression in concrete cone foundations is caused by the combined action of top shear force, middle splitting force, and top splitting force. The results also demonstrate that the failure mechanism of local compression in concrete cone foundations may be explained more rationally by integrating the theories of hoop strengthening and shear strength. These results serve as a basis for further study into the calculation method of cone foundation local compression.

BEARING CAPACITY OF BEAMS UNDER SHORT-TERM LOADS USING THE DIAGRAM OF CONCRETE DEFORMATION IN THE APPEARANCE OF THE FIFTH DEGREE POLYNOM

The theoretical determination of the load-bearing capacity of reinforced concrete beams under short-term loads using the concrete deformation diagram in the form of a fifth degree polynomial and its comparison with the results of experimental studies is considered. In the calculations of reinforced concrete structures using the deformation model of sections, the bending moments are calculated relative to the neutral line, which is located at a distance from the extreme fiber of the compressed concrete and limits its height. In the DBN of Ukraine, it is proposed to describe the concrete deformation diagram by a polynomial of the fifth degree or by a fractional-rational dependence, which was proposed and substantiated by the State Research Institute of Building Structures. Under these conditions, the theoretical bearing capacity of the reinforced concrete beam was obtained, which was also found experimentally. The authors conducted experimental studies of the load-bearing capacity of three series of beams of different types (three identical samples in each series): Series I - beams made of ordinary concrete; II series - steel-reinforced concrete beams; III series - beams of combined section, in which the lower (stretched) height zone is made of steel fiber concrete, and the upper (compressed) zone is made of ordinary concrete. Fiber, the total volume of which was 1%, was uniformly added to the composition of the concrete mass for the II and III series of samples during mixing. A comparison of the theoretical bearing capacity of a reinforced concrete beam with its value obtained experimentally shows that the theoretical value is quite significantly (1.36 times) higher than the experimental value.

Superposition principle of impact on the working environment of actuating elements for site preparation machines

Problem. The analysis of the ripper working elements designs showed that their improvement perspective direction is using vibration and impact actuating elements. They work by the principle of dynamic energy accumulation with its subsequent use. However, there are currently no approaches to determining the tip rational parameters with the dynamic cutting edge. Goal. The main hypothesis formation and justification of the superposition principle impact on the working environment of actuating elements for ripper-pick. Methodology. Analytical methods of studying the soil destruction process by site preparation machines equipped tip with a dynamic cutting edge were used. Results. The presented model that describes the soil destruction process by the tip with the dynamic cutting edge has been developed. It combines the work of static force, determined by the traction parameters of the base machine, and the work of dynamic forces, determined by the physical and mechanical conditions of the soil destruction process. As a result, we obtained an equation for determining the force to overcome soil cutting resistance by the tip with the dynamic cutting edge. Originality. We propose to apply the superposition principle of effecting several factors on the working environment simultaneously in the soil destruction process by ripper-pick, such as static and dynamic load, due to the possibility of energy accumulation with subsequent use. This would create the high-speed power pulse on the cutting edge to ensure the preliminary formation of the compressed zone in the massif, with decreases in the power intensity of the static load for complete separation of the soil element. Practical value. The presented hypothesis will allow the technological basis development of such systems creation and application to develop fundamentally new technical solutions, to create effective, safe and durable working elements.

Structural concrete measurements: New distributed approach for standard specimens

Concrete structures are widely used in construction because of their many advantages, including high corrosion resistance, durability, low operating costs, and the ability to design any geometry. However, they also have some disadvantages, such as their high dead weight and susceptibility to cracking. Although this material is widely used globally, new types of concrete, such as high-strength or lightweight aggregate-based, are still being developed. This requires constant control and testing in both laboratory and field conditions. Concrete is a heterogeneous material due to the influence of aggregate grains and local discontinuities, voids or pores. Given these characteristics, it is clear that most conventional diagnostic solutions, such as spot foil strain gauges or extensometers, are unable to accurately analyse these localised effects as they average local strains over the measurement base. In contrast, distributed fibre optic sensing (DFOS) enables strain readings to be taken with millimetre geometric resolution, significantly extending the structural assessment capability. The article describes research on standard concrete cylinders equipped with surface-mounted optical fibres for simultaneous measurements of distributed strains in both the longitudinal direction (compression zone) and circumference direction (tension zone). The novelty of the proposed approach, apart from the way of arranging the sensing fibres on the specimens, lies mainly in the new diagnostic approach that allow (1) the evaluation of the homogeneity of the concrete using statistical parameters, (2) the identification of microcracking and (3) the identification of near-to-failure state based on Poisson’s ratio analysis. Twenty cylindrical specimens were laboratory tested using reference techniques to validate the performance of the DFOS-based system and to provide data for statistical evaluation. The results indicate the possibility of detecting the approaching damage even without knowledge of the material properties of the specimen or the initial stress level at the moment of starting the measurements. The discussion also covered future research directions and potential improvements.

Bambusa balcooa bamboo-reinforced concrete beams: experimental and FEM investigation for energy-efficient pavement construction.

This study explores the viability of using Bambusa bambos, sourced from Madhya Pradesh, India, as a reinforcement material in continuously reinforced concrete pavement (CRCP) construction, aiming to assess its potential as a sustainable alternative to traditional steel reinforcement. The research encompasses a comprehensive evaluation of physical and mechanical properties, including tensile, compressive, and bending strengths, and a detailed microstructural analysis using scanning electron microscopy (SEM) and Fourier-transform infrared spectroscopy (FTIR) of Bambusa bambos. The study involved finite element analyses that modeled bamboo-reinforced concrete (BRC) beams, exploring the impact of horizontal and vertical placements of bamboo strips on flexural behavior under bending loads. The analysis aided in observing compressive and tensile stresses generated in concrete and bamboo, with specific FEA results indicating that beams with vertically aligned bamboo strips in both the compression (compressive stress of 16.90MPa for beam B1) and tension zones (tensile stress of 7.22MPa for beam B1) withstand flexural stresses effectively. Additionally, the multi-criteria decision-making approach using the TOPSIS method to rank different beam designs. Key findings obtained from FEA indicate that the vertical alignment of bamboo strips in both the compression and tension zones of the beams is optimally effective in handling flexural stresses.

Investigation of punching shear performance in concrete slabs reinforced with GFRP and synthetic fibers: An experimental study

The punching shear resistance mechanism of reinforced concrete slabs with glass fiber-reinforced polymers (GFRP-RC) is weaker than that of steel-RC slabs. The reduced shear strength and modulus of elasticity of GFRP reinforcement which led to greater tensile strains of the reinforcement and shallower compressive zone reduce the contribution of concrete in compression, aggregate interlocking, and dowel action to the punching shear mechanism. This condition results in reduced capacity and a higher likelihood of catastrophic failure compared to their steel-RC counterparts. To address this weakness of GFRP-RC, this paper aims to examine the punching shear performance of GFRP-RC slabs and explore the potential improvements through the incorporation of synthetic fibers. A total of 11 two-way slabs having a dimension of 1000 × 1000 mm with a thickness of 80 mm were tested under punching shear load and the effect of several types of fiber (polypropylene (PP) and (polyvinyl alcohol (PVA)), fiber content (0, 0.5%), reinforcement ratio, and reinforcement type (steel and GFRP) was investigated. The slabs’ behavior was assessed by considering crack width, deflection performance, the mode of failure, and the overall load-bearing capacity. The findings indicated that the influence of fibers was more noticeable in GFRP-RC slabs compared with steel-RC counterparts. Moreover, the fibers were found to effectively reduce deflections and crack widths. The results particularly showed the superior performance of PVA fibers in improving the capacity of the two-way slabs, whereby the addition of 0.5% PVA fibers led to the punching shear capacity increase of 44–51%. The findings of this study offer a better understanding of the punching shear behavior of slabs reinforced with GFRP reinforcement and synthetic fibers.

Experimental Investigation of Ductility in GFRP RC Beams by Confining the Compression Zone

Nowadays, building structures in corrosive environments requires some considerations. Being lightweight, high tensile strength, and corrosion resistance are the features that make fiber-reinforced plastic (FRP) bars an alternative component for longitudinal steel reinforcement of concrete. On the other hand, the linear elastic behavior of FRP bars, alongside the brittle behavior of concrete, makes brittle members without considerable ductility. In this paper, the effect of compression region confinement with CFRP sheets on the FRP-reinforced concrete beams was experimentally investigated. Eight GFRP reinforced beams with 2 m length, including one reference beam and seven confined beams, were constructed and tested under a four-point bending test. Based on the type of confinement, specimens are categorized into four groups. Flexural behavior improvements, including load carry capacity, energy dissipation capacity, and ductility, were observed in at least one specimen of each confined group. According to the results, the specimen that was spirally confined with a 30 mm ribbon width and angle of 10° had the best total energy absorption up to about 110% improvement in comparison to the unconfined specimen. On the other hand, vertically confined specimens with 50 mm ribbon width showed the highest improvement in ductility indices and load carrying capacity up to 60% and 11% in comparison to unconfined specimens, respectively. Due to concrete compression zone fractures in flexural failure mode, the over-reinforce method is considered the design philosophy. Results indicate that regardless of the confinement type (discrete vertical, discrete spiral, or continuous spiral confinement), there is an optimal amount for width, blank space between ribbons, and depth of confinement to achieve the best flexural behavior.

Observed Characterization of Multi‑level Retaining Structure for Deep Excavation of Subway Station

Traditional support structures cannot meet the complex conditions of different excavation depths and areas in underground transportation hubs. On the basis of fully considering the spatial position relationship of foundation pit groups, this article proposes a multilevel retaining system that meets the requirements of multilevel foundation pit excavation. The evolution law of the support structure during the excavation process of the inner pit was explored using on-site monitoring and numerical simulation methods. The results indicate that the excavation of the inner pit reduces the passive earth pressure, and the deformation of the outer support structure can be effectively suppressed by setting a retaining structure or a bottom slab in the bench zone. The excavation of the inner pit causes significant vertical deformation of the support structure adjacent to the foundation pit, while the impact on the structure far away from the foundation pit is relatively small. According to the contact force chain and soil pressure between the two rows of support structure, the soil in this area is divided into a “relaxation zone” and a “compression zone.” The evolution mechanism of earth pressure in the case of mutual-effect failure between two rows of piles is revealed. This paper addresses the deformation properties of multilevel support structures as well as the mechanism of earth pressure evolution between structures.

Study on the Mechanical Performance of RC Beams under Load Reinforced by a Thin Layer of Reactive Powder Concrete on Four Sides

To repair reinforced concrete beams efficiently in a limited building space, the four-sided application of a reinforcing thin layer of reactive powder concrete (“RPCTL”) was proposed to improve the bending capacity of the members. Static flexural tests of one comparison beam and five reinforced beams were completed on a four-point centralized loading device. Changes in deflection, cracks, stresses, and damage characteristics of the specimens were measured under various levels of loading. The test results showed that the damage patterns of the reinforced specimens were dominated by the yielding of longitudinal tensile reinforcement at the bottoms of the beams and the crushing of the cementitious material in the top compression zones of the beams. The cracking load greatly increased by 1.42 to 7.12 times, and the ultimate bearing capacity increased by 0.29 to 1.41 times. The distribution characteristics and dynamic changes in the displacement, stress, and damage of the specimens were dynamically simulated by finite element software. The effects of reinforcement and initial load-holding level on the reinforcement effect were investigated. A bending capacity calculation formula for RPCTL reinforcement technology is proposed that aligns with the test results and can provide a reference for the design of RPCTL reinforcement.

Mechanism of vertical displacement beneath shallow compression zone in coastal area

Previous studies showed that there is a discrepancy between total subsidence (measured on the ground surface) and shallow compression (measured within several hundred meters beneath the ground surface) in Taiwan. This difference is referred to as deep displacement in this study. The variations of deep displacement are opposite to those of ground surface displacement and groundwater level within the depth of several hundred meters. The mechanism is unknown and requires further investigation. This study adopts two kinds of geodetic observation data and separates land subsidence into shallow compression and deep displacement to investigate the mechanism of deep displacement. A tectonically active coastal area in Taiwan is selected as the study area. The assessment results show that the associated variations are likely due to cyclic hydraulic loading and unloading. The variations of deep displacement are opposite to those of ground surface displacement. This study proposes that these variations are due to hydraulic expansion and contraction. The mechanism is demonstrated using a hydromechanical model. The results of a cross-correlation analysis show that hydraulic expansion and contraction occur at certain depths. The study results provide important information on the mechanism of deep displacement that can be used in tectonophysical and land subsidence investigations.Graphical

Shear strength model of FRP-reinforced concrete beams without shear reinforcement

The current design codes for evaluating the shear strength of concrete beams reinforced with fiber-reinforced polymer (FRP) bars predominantly rely on a semi-empirical approach through test data calibration without a robust theoretical background. This approach may lead to either overestimation or underestimation, resulting in significant scattering in strength predictions. This study proposed a modified approach to enhance the existing unified shear-strength model developed for steel-reinforced concrete (RC) beams based on the compression zone failure mechanism, for predicting the shear strength of FRP-RC beams without shear reinforcement. The proposed model integrates strain and stress approaches, accounting for the distinctive characteristics associated with FRP reinforcement. Additionally, the overall shear resistance capacity of the FRP-RC beam is considered as the sum of contributions from the intact concrete of the compression zone and the aggregate interlock mechanism. The reliability of the proposed model is verified by comparing its predictions with a comprehensive database comprising 264 FRP-RC beam specimens without shear reinforcement reported by 41 research groups. Furthermore, the accuracy of the proposed model was assessed through a comparative analysis with existing evaluation methods, encompassing machine learning-based models, empirical models, theoretical models, and design codes. The results indicate that the proposed model satisfactorily predicts the shear strength of FRP-RC beams within a wide range of design parameters. In comparison with machine learning-based models, the proposed theoretical model exhibits a similar level of accuracy in terms of the mean shear strength ratio and coefficient of determination (R2) values but with less scattering, indicated by lower coefficient of variation (COV) and root mean square error (RMSE) values. In addition, the proposed model outperforms existing empirical and theoretical models, as well as current design codes, by significantly enhancing prediction accuracy, demonstrated by lower COV, RMSE, and higher R2 values. Finally, a parametric study was conducted to investigate the effects of the key parameters on the shear strength of FRP-RC beams using both the proposed model and representative existing evaluation methods.

Flexural Behavior of RCC Beam Using Confined Reinforcement with Helical Stirrups

In the present condition the cost of construction goes on increasing due to usage of steel. In order to improve the tensile strength in concrete and reducing the usage of steel, a new type of reinforcement is essential for making an economical building. The main objective of our project is to reduce the usage of steel by providing helical stirrups in the reinforcement recommended as per IS 456:2000. By this method of reinforcement, we can achieve flexure in reinforced concrete beams which opposes conventional method of reducing the neutral axis depth of the beam. The under reinforced section of the beam can be achieved by this method of helical stirrups in the reinforcement. Steel is introduced in the tension zone to take the tensile stress, but as strength of concrete is ignored in tension zone with respect to compression zone. The objective of this research is to investigate the behavior of singly, doubly and confined reinforced beams. Ultimate load, deflection load, compressive stresses and tensile stresses were compared with the controlled plain reinforced helical stirrups beams. Single and double reinforced concrete beams were cast and tested for this purpose of investigation. The results obtained have indicated that enhanced the strength of concrete.

Experimental and theoretical evaluation of inclined screws in glued laminated bamboo and timber-concrete composite beams

In the timber-concrete composite beam, the timber beam in tension tends to fail in a brittle manner, which results in insufficient utilization of material strength in the compression zone and significant deformation problems. To improve the mechanical performance of the weak sections in the composite beam, a novel type of glued laminated bamboo and timber (GLBT)-concrete composite beam is proposed in which the bamboo layers are placed in the tension and compression zones of the beam. In this paper, thirty-six shear experiments were conducted to investigate the shear behavior of inclined screws in GLBT-concrete composite beams. The test results showed that the failure modes of diagonally positioned screws in GLBT-concrete composite beams involved pull-out failure of the screw and cone expulsion failure of the concrete. The pure tensile-shear loaded screws experienced the pull-out failure and single-hinge bending failure. For bamboo thicknesses of 30, 60, and 300 mm, the shear capacity of diagonally positioned screws increased by 27.9 %, 34.7 %, and 40.7 %, respectively, compared to the screws in pure wood members. Besides, the shear stiffness was improved by 40.4 %, 78.1 %, and 88.7 %, respectively. It was found that as bamboo thickness increased, the shear capacity and stiffness of diagonally positioned screws gradually improved, although the rate of increase gradually diminished. Among the four types of embedding methods, the ±45° diagonal screw connectors exhibited the highest shear stiffness, while the 135° unidirectional inclined screws (pure compressive-shear screws) had the lowest shear capacity and stiffness. This paper presented a theoretical model for estimating the shear capacity of inclined screw connectors in GLBT-concrete composite beams, which considered the various stress distributions of the screw connectors embedded in bamboo and timber layers.

Durability of concrete subjected to acidic exposure under flexural loading

The durability of concrete under combined acidic exposure and flexural loading was studied. Concrete specimens were made and subjected to up to 168 days of exposure to flexural loading (stress levels of 0, 0.35 and 0.50 of the 28 day flexural strength) and an acidic environment. Concrete properties such as neutralisation depth, mass change, compressive strength and flexural strength were measured. The compositions of different samples were determined by means of X-ray diffraction and chemical analysis and the microstructure of samples was examined using scanning electron microscopy. The results showed that, with an increase in stress level from 0 to 0.50, the performance of the concrete deteriorated gradually and the deterioration in the tension zone was greater than that in the compression zone. The concrete performance improved within 28 days but deteriorated dramatically afterwards. The acidic solution reacted with the cement paste, leading to the formation of harmful gypsum and the decomposition and dissolution of cement hydration products. An increase in stress level increased the porosity of the concrete, accelerating the rate of acid erosion, which led to a deterioration of concrete performance.