Materials and Structures最新文献

This paper re-examines common notions and conventions regarding the compressive strength of concrete in general and of the uniaxial compressive strength of concrete in particular. A distinction is introduced between the strength of the specimen and the strength of the concrete as a material, and the commonly measured and adopted strength is shown to be the specimen’s strength, wrongly interpreted as the material’s strength. the two major damage modes of concrete specimens (with the formation of either longitudinal cracks or shear bands) are discussed. Such failure modes are wrongly considered as features of concrete behavior in uniaxial compression, but this is not the case. Longitudinal cracking is due to lateral expansion (Poisson’s effect) and occurs at a relatively low applied load in absence of friction at specimen’s top and bottom boundaries. Shear failure (accompanied by the formation of an inclined shear band) is related to the shear envelope parameters that are related to the concrete mixture, but the applied ultimate pressure is not the concrete uniaxial compressive strength. Hence, though caused by applied compressive loading, these failure modes are little/hardly related to the concrete material intended as the ultimate uniaxial stress (strength) corresponding to a maximum value of the uniaxial compressive strain. Using the shear envelope parameters has been proven to yield a very good prediction of the applied compressive loading of the specimen in the limit state, as a demonstration that the applied pressure at specimen’s failure resulting from the formation of inclined fracture bands is the specimen’s failure strength, and not the material’s compressive strength! Reasons are given against the existence of a uniaxial compressive strength failure for concrete, and a piece of evidence in this direction is provided by concrete specimens subjected to pure hydrostatic compression, that do not fail at all. The entire issue requires, therefore, a deep revisiting and re-thinking, to provide correct measures for representing concrete response under compression in analysis and design.

Ultra-high-performance fiber-reinforced concrete (UHPFRC) is applied to joint nodes with its excellent mechanical properties, which helps to improve the force transfer performance of UHPFRC structures. The strength of the connections is dependent on the adhesion and friction between the connected materials in the bridge design procedure. This research aims to identify the adhesion performance between UHPFRC and UHPFRC under different interfacial roughening methods. To this end, the maximum tensile stress and the load–displacement curves of UHPFRC wet joints treated by high-pressure water jet roughening, uniform plastic formwork roughening, embedded wire mesh roughening, manual mechanical roughening, and epoxy resin were obtained via direct tension tests. The test results indicate that the bond strength of UHPFRC wet joints can reach 22.36%-68.06% of the tensile strength after different interfacial treatments, among which the roughening methods using high-pressure water jet significantly improve the bond performance of UHPFRC wet joints, followed by the roughing method of uniform plastic formwork and embedded steel wire mesh. Physical roughening treatment has less effect on the stiffness of UHPFRC wet joints and exhibits a typical brittle failure mode. A tensile constitutive model in the elastic phase of the UHPFRC wet joint interface and the simplified interfacial tensile stress-relative displacement model were proposed. Finally, the performance of the interfacial adhesion parameters was appraised by finite element modeling. The finite element analysis showed a good agreement with the experimental results.

The setting behavior of ultra-high performance concrete (UHPC) is demonstrably different from that of conventional concrete; thus, tools and guidance extending beyond common test methods such as Vicat and penetration are needed. While UHPC is known for its enhanced mechanical and durability properties, due to the low water and high cementitious contents, UHPC-class materials are prone to early-age autogenous shrinkage. Recognizing that UHPCs are commonly supplied to construction sites as prebagged, proprietary mixes with unknown constituents, and that accurate determination of setting time is crucial in determining the early-age autogenous shrinkage of UHPC-class materials as well as for scheduling construction operations and quality control actions, this study explores alternate test methods such as isothermal calorimetry (ASTM C1679), semi-adiabatic calorimetry (ASTM C1753), autogenous shrinkage (ASTM C1698), chemical shrinkage (ASTM C1608), and dual ring test (American Association of State Highway and Transportation Officials (AASHTO T 363) to evaluate the setting behavior of UHPCs. Setting times obtained using the alternate test methods aligned well with each other and were found to be different than the setting times indicated through standard test methods. Discussion and guidance on the applicability and the use of alternate test methods to determine the setting time of UHPCs for various laboratory and field applications are provided.

This paper investigates the durability performance of mortars with varying replacement levels of dolostone or limestone filler (0–30% by mass) and the stability of mortars with dolostone filler for 2 years. Compressive strength, total porosity, capillary water absorption, and chloride migration coefficients were determined. Results show that compressive strength decreases and the total porosity increases with increasing filler content due to a dilution effect, regardless of the filler composition. The capillary water absorption and the chloride migration coefficients rise significantly for mortars with 20–30% filler. However, the dolostone filler cements have lower chloride coefficients than those with limestone blended cement. Volumetric stability assessments reveal no significant expansion, and XRD and FT-IR analyses suggest the formation of hydrotalcite-like phases.

Previous investigations have primarily focused on the use of circular basalt fiber reinforced polymer (BFRP) bars in concrete compression members, neglecting the application of these lightweight composites in different structural sections. This study aims to address this research gap by examining the structural efficiency of square concrete compressive members reinforced with various BFRP sections. A total of eight samples were cast, including seven with BFRP angle sections, BFRP plate sections, BFRP tubes, and BFRP circular rebars, and one control sample with conventional steel rebars as the main reinforcement and stainless steel rebars as lateral reinforcement. All samples had a square cross-section of 200 mm width and 1200 mm height, except for the BFRP tube compressive member, which had a cross-section of 180 mm × 180 mm and the same height. The experimental results demonstrated that failure patterns were influenced by the reinforcement material, reinforcement section, and vertical spacing of stainless-steel stirrups. The sample with steel reinforcement exhibited the highest strength of 1451 kN. The ultimate strength reductions for samples with BFRP angle sections, BFRP plate sections, BFRP tubes, and BFRP circular rebars were 22%, 10.3%, 49.6%, and 10%, respectively. The study found that the vertical spacing of ties significantly impacted the load–deflection behavior. For angle section BFRP rebars, increasing the tie spacing from 50 to 100 mm resulted in a 9.50% increase in strength and a 22.20% increase in axial deflection. In contrast, for plate section BFRP rebars, increasing the tie spacing led to a 14.2% decrease in load and a 5.7% decrease in deflection. Members reinforced with BFRP circular rebars showed a 4% decrease in strength and a 13.4% increase in deflection. A finite element analysis (FEA) model was developed to evaluate the structural efficiency of members reinforced with BFRP sections and to conduct a parametric investigation. The concrete damaged plasticity (CDP) model was employed in the FEA to simulate concrete behavior. The proposed FEA model showed discrepancies of 7.2% for the ultimate load and 5.5% for the equivalent deflection and accurately captured crack patterns. Additionally, to compare with theoretical computations, predictions from three international codes were calculated to highlight differences between experimental measurements, FEA results, and theoretical predictions.

Textile Reinforced Concrete is a cementitious composite with non-metallic distributed reinforcement, which could exhibit strain-hardening response under uniaxial tensile loading. The tensile behaviour must be characterized properly in order to obtain reliable and appropriate input for structural design. However, it is seen that the end conditions of the test specimen could affect the response, which is discussed in this paper by considering two extreme types of boundary conditions, with the end rotation during testing either being negated or permitted. The strains and displacements were monitored using Digital Image Correlation, along with axial extensometers. The data indicate that fixed ends result in unsymmetric cracking and non-uniform strain distribution across the lateral section of the test specimen whereas rotating ends lead to more uniform cracking and strain distributions. It was further found that the mean width of the strain localization zone is larger for rotating ends. On the other hand, ultimate stress and strain, as well as the average crack opening are comparable for both the end conditions, The maximum crack widths were in the order of 0.2 mm for about 0.8% nominal strain (i.e., close to failure), for the specimens considered. The analysis suggests that rotating end conditions be used to obtain more unambiguous response.

This paper addresses the growing use of bio-based materials in Europe, thanks to their low embodied energy and carbon sequestration. Despite favorable hygrothermal and acoustic properties, the inherent challenge lies in the low mechanical properties of biobased concrete. This study presents an innovative approach to strengthen hemp concrete through natural FRCM (Fabric Reinforced Cementitious Matrix), using two distinct reinforcement techniques. Firstly, a bending reinforcement consists of applying natural FRCM as the outer skins of a composite sandwich, with hemp concrete as the core. The effect of textile layers and pre-impregnation on the FRCM mechanical properties within the composite sandwich is evaluated. Secondly, compressive reinforcement entails confining hemp concrete specimens with FRCM. The results show a significant improvement in the mechanical properties of hemp concrete, with bending and compressive reinforcement leading to increases in the mechanical strength up to 17,530% and 258%, respectively. Configurations involving mineral-impregnated fabric (PM FRCM) demonstrate superior mechanical reinforcement since it allows a better interphase bond between fabric and cementitious matrix. Different failure modes are observed between reference (non-reinforced) specimens and reinforced specimens, with bending reinforced specimens exhibiting shear failure and debonding at the interface of the composite sandwich, while reference specimens fail in bending. Moreover, compressive reinforced specimens undergo crushing of hemp concrete after tensioning rupture of the fabric, while reference specimens present angular shear path in the middle of the specimens. The results underscore the promise of FRCM in mechanically reinforcing bio-based concrete, opening new opportunities for their expanded use in the construction industry.

Ensuring the reliability of the joint of prefabricated composite columns is crucial for the overall performance of assembled steel–concrete composite structures. Hybrid connected prefabricated composite columns, utilizing the steel pipe connection with post-tensioning connection, was proposed to enhance the joint integrity. Horizontal low-cyclic repeated loading test was performed on one cast-in-place specimen and three precast specimens with varying steel pipes. The precast specimens exhibited higher load carrying capacity, stiffness and ductility properties along with comparable energy consumption capacity, compared to the cast-in-place specimen. The impact of parameters such as sectional dimension and anchorage length of the steel pipe, thickness of cover plates, and post-tensioning stress, was also examined based on the experimental results and corresponding finite element analysis.