Static and dynamic elastic moduli of carbon fibres

Conclusions -- The mean static and dynamic elastic moduli of a number of carbon fibres have been determined. It has been shown that the static elastic modulus exceeds the dynamic elastic modulus by 40–100 GPa.-- The basic reasons for the divergence between experimental values of the elastic modulus of the very same carbon fibres, both in different tests and also in identical test methods, have been analyzed and discussed.Translated from Khimicheskie Volokna, No. 4, pp. 34–36, July–August, 1991.     

Dynamic strength of polymer modified and fiber-reinforced concretes

For evaluating the performance of concrete construction such as foundation piles, a knowledge of the dynamic properties of concrete is also required. Some results of a study of concrete under dynamic impact loading are given in this paper. ø 100×200 mm cylindrical specimens were cast from plain and polymer cement concrete; some of the specimens were reinforced by polypropylene fibers also. The experimental details of the Hopkinson split-bar method used are described. The dynamic strength of concrete obtained was 40–45 % higher than the static strength. Compared to normal concrete, polymer cement concrete showed 30–35 % higher dynamic strength, and significantly higher energy transmission capacity.     

The thermal conductivity of fibre-reinforced concrete

The effect of copper and steel fibre inclusions on the thermal conductivity of mortar and concrete is investigated. The experimental technique is based on the conventional steady-state method using desiccated specimens. The results indicate that copper fibres significantly increase thermal conductivity while steel fibres have a lesser effect. Vibration of the fresh concrete, during specimen manufacture, produces some fibre alignment. As a result, it is found that theoretical methods based on the assumption of a random fibre distribution under-estimate the experimental values.     

聚氨酯涂层加固混凝土梁的力学性能研究

通过对混凝土试件进行试验研究,在应变率为0.000 3 s^-1和0.065 s^-1的加载条件下进行三点弯曲试验,模拟静态荷载和冲击荷载,评估聚氨酯(PU)涂层在静态荷载和冲击荷载作用下增强混凝土试件力学响应的有效性。结果表明,PU涂层(在混凝土试件的正面或反面)的应用对提高混凝土结构的延性具有积极作用,最大应变和累积应变能随着PU涂层厚度的增加近似线性增加,反面涂覆的方法在静态荷载作用下对混凝土结构的加固效率更高,采用正面涂覆的方法可增强混凝土结构的抗冲击性,通过增加涂层厚度能有效提高加固能力。PU涂层在试件的最终破坏过程中不会脱胶,这意味着材料具有良好的附着力,能有效减弱试件的破碎效应。     

回收轮胎钢纤维再生骨料混凝土基本力学性能试验研究

混凝土的基本力学性能与破坏形态是反映试件在不同受力状态下承载能力与韧性的重要指标。为了研究回收轮胎钢纤维(RTSF)再生骨料混凝土的基本力学性能,试验设计了8组不同种类的混凝土试件。通过坍落度、含气量、立方体抗压、劈裂抗拉与抗折试验,系统的探究了RTSF体积掺量(0.25%、0.5%、0.75%和1.0%)和再生骨料取代率(质量分数分别为50%、75%和100%)对混凝土基本力学性能以及破坏形态的影响。研究表明:随着再生骨料取代率的升高,混凝土拌合物坍落度减小、含气量增大,各项力学性能均产生不同程度的降低;RTSF能够有效提高再生骨料混凝土的基本力学性能,且试件的破坏形态随RTSF掺量的增加呈现出明显的延性破坏特征。综合各项指标,当再生骨料取代率为50%时,RTSF体积掺量为0.5%的RTSF再生骨料混凝土力学性能最佳。其试件立方体抗压强度(28 d)较普通混凝土仅降低1.0%,而劈裂抗拉强度与抗折强度较普通混凝土分别提高9.6%和12.5%。此外其弯曲韧度指数I₅、I₁₀和I₂₀分别为普通混凝土的2.7倍、3.8倍和4.8倍。     

Effect of temperature and age on the relationship between dynamic and static elastic modulus of concrete

This study investigates the effects of cement type, curing temperature, and age on the relationships between dynamic and static elastic moduli or compressive strength. Based on the investigation, new relationship equations are proposed. The impact-echo method is used to measure the resonant frequency of specimens from which the dynamic elastic modulus is calculated. Types I and V cement concrete specimens with water-cement ratios of 0.40 and 0.50 are cured isothermally at 10, 23, and 50 °C and tested at 1, 3, 7, and 28 days.Cement type and age do not have a significant influence on the relationship between dynamic and static elastic moduli, but the ratio of static to dynamic elastic modulus approaches 1 as temperature increases. The initial chord elastic modulus, which is measured at low strain level, is similar to the dynamic elastic modulus. The relationship between dynamic elastic modulus and compressive strength has the same tendency as the relationship between dynamic and static elastic moduli for various cement types, temperatures, and ages.     

Static and dynamic behavior of single‐edge notched glass fabric composites

This article describes the results of a series of tests conducted to support the development of a semiempirical model, for predicting the mechanical degradation of a satin weave glass fabric composite, with through‐thickness damage in the form of a crack‐like edge‐centered notch. Test specimens were subjected to loading in both static three‐point bending and dynamic mechanical thermal analysis (DMTA) tests. The effect of both notch length and displacement rate, on the static and that of notch length on the dynamic behavior of the materials tested, was evaluated and compared with respective predictions, as derived from the application of the residual property model. A satisfactory agreement between the experimental results and theoretical predictions was observed in all cases. Scanning electron microscopy was applied to observe the failure topographical characteristics of the DMTA specimens that were off‐plane loaded. POLYM. COMPOS. 27:177–183, 2006. © 2006 Society of Plastics Engineers.     

Mesoscale modelling of dynamic tensile behaviour of fibre reinforced concrete with spiral fibres

This paper performs numerical simulations of dynamic splitting tensile tests to study the dynamic properties of FRC materials. A two-dimensional mesoscale model is developed with consideration of the fibres, aggregates, and cement mortar. The FRC models with hooked-end fibres and newly developed spiral fibres are developed to investigate the effect of fibre shape on the dynamic properties of FRC material, such as the dynamic increase factor (DIF), the energy absorption capacity and the crack opening velocity. Accuracy of the numerical models for two types of FRC materials with 1% fibre content is verified with the experimental results. The effect of fibre content on the dynamic properties of the two FRC materials is also investigated. Numerical results demonstrate that the proposed mesoscale model can reliably simulate the dynamic splitting tests of FRC materials. They also demonstrate the effectiveness of the spiral FRC in resisting the dynamic tensile loads as compared to the conventional hooked-end FRC.     

Estimating the lifetime of tennis strings

Conclusions -- The static and dynamic strength characteristics of experimental specimens of tennis string have been studied; these were prepared on a basis of nylon complex yarn with a polyamide coating.-- It was shown that the experimental strings have a high strength, good fatigue characteristics, in particular high resistance to abrasion, which permits one to increase their lifetime.-- Play tests of experimental strings in rackets which were performed on open and closed tennis courts have supported an elevated lifetime for these strings as compared with serially produced nylon string, and correspond to the level of foreign analogs.Translated from Khimicheskie Volokna, No. 6, pp. 50–51, November–December, 1990.     

Effect of short fibers on residual permeability and mechanical properties of hybrid fibre reinforced high strength concrete after heat exposition

Mechanical and permeability performance of fibre reinforced high strength concrete after heat exposition were evaluated in the experimental study. Cylindrical concrete specimens were exposed to heat with the rate of 10 °C/min of up to 400 °C. In order to study the effect of short fibres on residual performance of heated high strength concrete, polypropylene and steel fibres had been added into the concrete mix. The melting and vaporization of its fibre constituents were found to be responsible for the significant reduction in residual properties of polypropylene fibre reinforced high strength concrete. In terms of non-destructive measurement, UPV test was proposed as a promising initial inspection method for fire damaged concrete structure. Furthermore, the effect of hybrid fibre on the residual properties of heated fibre reinforced high strength concrete was also presented.     

An integrated approach for modelling the tensile behaviour of steel fibre reinforced self-compacting concrete

The present work resumes the experimental and numerical research carried out for the development of a numerical tool able of simulating the tensile behaviour of steel fibre reinforced self-compacting concrete (SFRSCC). SFRSCC is assumed as a two phase material, where the nonlinear material behaviour of SCC matrix is modelled by a 3D smeared crack model, and steel fibres are assumed as embedded short cables distributed within the SCC matrix according to a Monte Carlo method. The internal forces in the steel fibres are obtained from the stress–slip laws derived from the executed fibre pullout tests. The performance of this numerical strategy was appraised by simulating the tensile tests carried out. The numerical simulations showed a good agreement with the experimental results.     

High strain-rate compression and tension behaviour of an epoxy bi-component adhesive

The dynamic behaviour of the bonded joints is influenced by the dynamic mechanical properties of the material of adherends and adhesives. The literature contains plenty of information about the dynamic mechanical properties of many structural materials (especially metals), obtained through different test types. Conversely, the study of the dynamic mechanical properties of the adhesives is not so common. The purpose of this work is to assess the dynamic mechanical behaviour of an epoxy bi-component adhesive for structural bonding. In particular, the study has been focussed on the influence of the strain-rate on the tensile and compressive strength of specimens made of adhesive. The experimental tests have been performed with a hydraulic universal testing machine and a tensile-compression Hopkinson bar. The results of the tests show that the adhesive strength increases substantially by increasing the strain-rate. The Cowper–Symonds and Johnson–Cook models of strain-rate dependence have been used to fit the experimental data with unsatisfactory results, thus also a poly-linear fit has been adopted.     

Fibre Surface Energy Characterization

A new technique has been developed to study the surface energy characteristics of small diameter fibres. This method is tensiometric and based on wetting properties in a two phase liquid system. Two types of carbon fibre have been investigated under both static and dynamic conditions in immersion and emersion.

The high strength fibre, which was found to have a polymeric coating, gave reproducible results consistent with typical values for a polymer. However, the high modulus fibre could not be characterized due to considerable scatter in the experimental results. It is suggested that the source of this phenomenon is the existence of surface energy gradients which leads to hysteresis effects.     

往复荷载下含孔洞缺陷衬砌混凝土的力学性能试验研究

通过添加不同含量的发泡聚苯乙烯(EPS)颗粒来预制目标孔隙率以模拟混凝土的不同孔洞缺陷,制备了不同孔隙率的C25和C30两种强度等级的混凝土试件,开展单调及往复荷载下含孔洞缺陷混凝土力学性能的试验研究,分析了混凝土试件破坏形态、强度、应变、弹性模量等随孔隙率的变化规律,探讨不同孔洞缺陷对混凝土力学性能的影响。试验结果表明:单调及往复荷载下,无预制孔洞缺陷的混凝土试件均表现为脆性破坏特征。但随着孔隙率的增加,混凝土试件的强度明显降低,应力-应变曲线逐渐趋于鱼肚状分布,试件由脆性向延性破坏转变,逐渐呈现多裂缝扩展特征,特别是对于往复加载情况。两种加载方式和两种强度等级条件下,试件主要力学参数随着预制孔隙率的增加均表现出一致的变化规律。峰值应力和弹性模量随孔隙率的上升呈指数下降,而峰值应变和应变极值呈线性增大。往复荷载下试件力学性能受孔隙率的影响程度均大于单调加载情况,而且,这种影响随混凝土强度的提高而减小。在往复加载过程中,孔隙率越大,峰值应力前试件的刚度比值越大,而峰后的刚度退化也愈严重。对于相同的孔隙率,混凝土强度等级越高,峰值应力前的刚度增长率及峰后的刚度退化程度越小。     

Reproportioning of steel fibre reinforced concrete mixes and their impact resistance

In this experimental investigation, a practical rapid method of proportioning steel fibre reinforced concrete (SFRC) mixes is developed and validated. The basis for developing this is to use the reproportioning method, which has already been developed for proportioning normal density cement concrete mixes, for SFRC mixes. Based on the results of the trial mix, two SFRC mixes having 28 day target strength of 30 and 50 MPa are designed using this technique and examined regarding its validation. In addition, the impact resistance of these reproportioned Plain Concrete (PC) and SFRC is studied at 7 and 28 days. It is observed that the SFRC has developed significant impact resistance even for a small addition of steel fibres. Pulse velocity test is conducted at different ages to assess the quality of concrete. It is found that all concrete specimens could be classified under good quality.     

Corrosion of steel fibre reinforced concrete from the cracks

The corrosion of steel fibres in the cracked section has been under investigation by many researchers since the last 15 years. It is reported widely that in case of steel fibres reinforced concrete (SFRC), corrosion is less active as compared with steel bars. In the cracked section, the durability of the material depends on the performance of the bridging capacity of the fibres embedded in the concrete. The corrosion of the fibres not only could produce the spalling of concrete but it could also reduce the sectional area of the fibres, turning the durability of structures in danger. This study focuses on those two aspects of fibre corrosion. The tests were performed on cracked SFRC samples with 0.5-mm crack mouth openings (CMOs) exposed to marine-like environment for 1 year. The results confirm the small sensitivity of SFRC to corrosion. Surprisingly, they made appear an increase of the flexural strength after corrosion. The factors affecting the corrosion of the fibres and the reasons for the increase in flexural strength after corrosion are discussed.     

Comparative evaluation of steel mesh, steel fibre and high-performance polypropylene fibre reinforced shotcrete in panel test

In this study, experimental investigations were performed on steel mesh (SM), steel fibre (SF) and high-performance polypropylene fibre (HPPF) reinforced shotcrete (HPPFRS) panels to evaluate performance characteristics such as toughness, flexural ductility, energy absorption and load capacity. The panel tests, in accordance with European specification for sprayed concrete (EFNARC), were made on 18 prismatic specimens having the same mix designs and were cured for 28 days but reinforced with various fibres. In addition, the rebound characteristics of these mixes were determined to compare the actual in situ fibre contents.Test results show that all reinforcements, including HPPFs that are low-modulus fibres, greatly improved the flexural ductility, toughness, and load-carrying capacity of the brittle matrix. It was seen that there was a positive synergy effect between steel and polypropylene fibre in hybrid fibre usage from a performance point of view. According to results, it can be concluded that a hybrid polypropylene-SF can be used alternatively instead of SM and monosteel fibre as a reinforcement in shotcrete applications to get better efficiency in mechanical properties of composite.     

Modified Alumina Cement with High Service Properties

This scientific paper gives consideration to the information on the contemporary structure of binary systems and three-component CaO-Al_2O_3-ZrO_2 system required to select rational parameters for the synthesis of cementing agents.The production process of experimental clinker that has a Ca_7Al_6ZrO_(18) composition has been described.The data of X-ray phase analysis and petrographic analysis of the experimental composition of special cement have been given and the reasons for availability of inappropriate phases have been analyzed.The processes of hydration hardening of the cement stone have been studied and the data of differential thermal analysis were used for the establishment of the hydration mechanism for basic crystal phases in the cement clinker composition.The differences in the hydration processes of experimental special cement were analyzed in comparison with known mechanisms.A possibility of the exploitation of special binder in contact with ZrO_2(CaO-stabilized)was tested using the specimens of fine concrete that were subjected to thermal treatment in different modes.The physical and mechanical properties of specimens and the mineralogical composition of concretes were given based on the data of X-ray analysis and the petrographic analysis.The cement additive was selected and its amount in the concrete composition was defined to slow-down the hardening and the mixture plasticization time.The relationships of physical and mechanical characteristics of concrete specimens were defined experimentally as a function of granulometric composition of the filler and thermal treatment temperature.The investigation data of corrosion resistance obtained using the static method were used to find out if experimental concretes can operate in contact with the steel-smelting alkaline slag.     

Compressive behavior of post-heated circular CFST short columns externally strengthened with CFRP sheets

Due to the increasing usage of concrete-filled steel tube (CFST) members in structural engineering, there will be more chances of fire hazards on these structures in the near future. Externally bonding carbon fiber reinforced polymer (CFRP) composites has emerged as a popular method for repairing damaged steel-reinforced concrete members. However, limited research is available to evaluate the behavior of CFRP strengthened CFST members. This paper presents the results of an experimental investigation on the compressive behavior of the post-heated circular CFST short columns externally strengthened with CFRP sheets. A total of twenty-one specimens are tested to investigate the influence of temperature and the number of CFRP layers on the mechanical behavior of repaired specimens. The ultimate strength is obtained from monotone static tests. The extensometer technique based on gauge strain is used to measure strains of electric gauges glued to the external surface of specimens. The results indicate that the increase in the number of CFRP layers leads to a significant change in the mechanical properties of post-heated CFST columns. Furthermore, it is shown that increases in the number of CFRP layers remarkably enhance the ultimate strength and initial stiffness of specimens subjected to the same heat treatments, while deteriorate the ductility. Based on extensive experimental analysis, simplified formulae are proposed to estimate the compressive ultimate strength of all specimens tested, providing reasonably good correlation with the experimental results. Besides, the proposed formulae are compared with some existing empirical models, and validity of the proposed formulae is evaluated.