Mechanical properties of steel fibre reinforced concrete exposed at high temperatures

Steel fibre reinforced concrete (SFRC) is an advanced cementitious composite where fibres can act as a profitable replacement for diffused reinforcement, like welded steel mesh, especially for thin cross sections. In this case fire becomes a very important condition in the design. Previous experimental research has shown the benefits in fire resistance of steel fibres, when structural elements are bent. A careful mechanical characterization of a SFRC used for prefabrication after thermal cycles at high temperature is here presented. Three different tests are considered: four point bending, uniaxial compression and fixed-end uniaxial tension. In the paper the decay of peak and post-cracking strengths versus temperature increase for uniaxial compression, uniaxial tension and bending are discussed.     

Mechanical properties of lightweight concrete made with coal ashes after exposure to elevated temperatures

This study investigated the thermal resistance of lightweight concrete with recycled coal bottom ash and fly ash. Specimens were exposed to temperatures up to 800 °C then cooled to room temperature before conducting experiments. Compressive strength test, FF-RC test, TG analysis, and XRD analysis were performed to analyze the physicochemical effects of coal ashes on the thermal resistance of concrete. Test results indicated that both bottom ash and fly ash were associated with a substantial increase in the residual strength of thermal exposed concretes. The results were attributed to the surface interlocking effect and the smaller amount of SiO₂ for bottom ash. For fly ash, the formation of pozzolanic C-S-H gel and tobermorite retained water at high temperatures, and the consumption of Ca(OH)₂ lowered stress from rapid recrystallization after exposure to 600 °C. It was concluded that the incorporation of coal ashes allows for lightweight concrete with good thermal resistance.     

Mechanical properties of steel fibre reinforced lightweight concrete with pumice stone or expanded clay aggregates

This paper presents basic information on the mechanical properties of steel fibre-reinforced light-weight concrete, manufactured using pumice stone or expanded clay aggregates. Results are presented for standard compressive tests and indirect tensile tests (splitting tests on cylinder specimens and flexure tests on prismatic beams using a three-point loading arrangement) under monotonically increasing or cyclically varying loads. The influence of steel fibres and aggregate types on modulus of elasticity, compressive and tensile strength and post-peak behaviour is evaluated. Test results show that compressive strength does not change for pumice stone aggregates, while an increase is observed for expanded clay; tensile strength and fracture toughness are significantly improved for both pumice stone and expanded clay. The results also show that with both expanded clay and pumice stone lightweight aggregates a suitable content of fibres allows one to obtain performances comparable with those expected from normal weight concrete, the important advantage of lower structural weight being maintained.     

Mechanical properties of photocatalytic white concrete subjected to high temperatures

The paper describes the consequences of progressive damage in architectural high performance concrete when exposed to different heating treatments. Specimens were tested for uniaxial compressive, direct, and indirect tensile strengths at ambient conditions approximately one day after the exposure to the high temperature. Modifications in the microstructure, porosity, and pore size distribution of the fire deteriorated specimens were identified using scanning electron microscopy and mercury intrusion porosimetry techniques. Test results revealed no significant variations in the mechanical strengths for specimens exposed to temperatures up to 250 °C. Per contra, significant damage was observed for higher temperature, 500 °C and 750 °C, treatments, similar to that of ordinary concrete made with similar aggregates. Based on X-ray diffraction analysis, photocatalytic properties of the concrete were lost at 750 °C.     

Development of lightweight concrete with high resistance to water and chloride-ion penetration

This paper presents an experimental study to evaluate the influence of coarse lightweight aggregate (LWA), fine LWA and the quality of the paste matrix on water absorption and permeability, and resistance to chloride-ion penetration in concrete. The results indicate that incorporation of pre-soaked coarse LWA in concrete increases water sorptivity and permeability slightly compared to normal weight concrete (NWC) of similar water-to-cementitious materials ratio (w/cm). Furthermore, resistance of the sand-lightweight concrete (LWC) to water permeability and chloride-ion penetration decreases with an increase in porosity of the coarse LWA. The use of fine LWA including a crushed fraction <1.18 mm reduced resistance of the all-LWC to water and chloride-ion penetration compared with the sand-LWC which has the same coarse LWA. Overall, the quality of the paste matrix was dominant in controlling the transport properties of the concrete, regardless of porosity of the aggregates used. With low w/cm and silica fume, low unit weight LWC (～1300 kg/m³) was produced with a higher resistance to water and chloride-ion penetration compared with NWC and LWC of higher unit weights.     

Mechanical properties of a TiAl-based alloy at room and high temperatures

Intermetallic titanium aluminides have attracted considerable industrial interest for aero and automotive applications owing to their specific strength. These alloys are candidates for high-temperature applications such as turbine blades and turbocharger turbine wheels. In this work, the mechanical behaviour of an as-cast Ti–47Al–3Cr–3Nb (at.-%) alloy was studied. Four-point bending tests were performed to determine the mechanical properties of the alloy at both room and high temperatures. Microhardness and Young’s modulus were evaluated by dynamic indentation tests at room temperature. Analyses of the as-cast alloy showed that, with the selected production process conditions, it has a duplex microstructure. Tests revealed that, while at room temperature the alloy is brittle, between 700 and 900°C it exhibits good ductility and mechanical properties.     

Mechanical characteristics of self-compacting concretes with different filler materials,exposed to elevated temperatures

In this paper, the studies concern the influence that different fillers have on the properties of SCC of different strength classes when exposed to high temperatures. A total of six different SCC and two conventional concrete mixtures were produced. The specimens produced are placed at the age of 180 days in an electrical furnace which is capable of reaching 300°C at half an hour and 600°C at 70 min. The maximum temperature is maintained for an hour. Then the specimens are let to cool down in the furnace. The hardened properties measured after fire exposures are the compressive strength, splitting tensile strength, water capillary absorption and the ultrasonic pulse velocity. Explosive spalling occurred in most cases when specimens of higher strength class are exposed to high temperatures. The spalling tendency is increased for specimens of higher strength class C30/37 irrespective of the mixture type (SCC or NC) and the type of filler used.     

Cellular thermoplastic fibre reinforced composite (CellFRC): A new class of lightweight material with high impact properties

In many high performance composite applications, specific properties could be more valuable than absolute ones and each reduction of density in composite structures could represent a fundamental goal. Cellular lightweight fibre reinforced composites (CellFRC) were prepared embedding gas bubbles of controlled size within a thermoplastic matrix reinforced with continuous fibres. A semicrystalline high performance polymer, poly(ethylene 2,6-naphthalate), and glass fibre fabrics were used for the preparation of both conventional and CellFRC structures. Pores were induced after the composite was first saturated with CO₂ and then foamed by using an “in situ” foaming/shaping technology based on compression moulding with adjustable mould cavities. The presence of micro- or submicro-sized cells in the new CellFRC reduced the apparent density of the structure and led to significant improvements of its impact properties. Both structural and functional performances were further improved through the use of a platelet-like nanofiller (expanded graphite) dispersed into the matrix.     

Mechanical properties of nanoparticle chain aggregates by combined AFM and SEM: isolated aggregates and networks

Mechanical properties of nanoparticle chain aggregates (NCA) including tensile strength and Young's modulus were measured using an instrument incorporating an AFM tip under SEM imaging. The NCA were studied individually and as network films. Carbon NCA were made by laser ablation of graphite, and SnO2 NCA were made by oxidation of a tin compound. The films were deformable and showed elastic behavior. NCA serve as reinforcing fillers in rubber and films of SnO2 NCA for trace gas detection.     

Mechanical and microstructural properties of calcined diatomite powder modified high strength mortars at ambient and high temperatures

In this experimental work, mechanical and microstructural properties of calcined diatomite powder (CDP) modified high strength mortars (HSMs) at ambient and high temperatures were researched. In the HSM mixtures, Portland cement was replaced with CDP at ratios of 0%, 5%, 10%, 15% and 20%. Several experiments regarding the hardened properties were carried out at 14, 28, 56 and 90 days at the ambient temperature of 25 °C. HSMs were heated up to high temperatures of 400 °C to 1000 °C at the age of 56 days, separately. Then, high temperature resistances of HSMs were quantified in terms of the residual hardened properties and the changes in the microstructural properties after heating. The residual performance was found to be higher in the HSMs modified with CDP than in the reference HSM (R-HSM) which was made 0% CDP. Based on the test results, it was seen that the HSM modified with 15% CDP (15CDP-HSM) exhibited the highest mechanical properties at ambient and high temperatures.     

Recommendation of RILEM TC 200-HTC: Mechanical concrete properties at high temperatures—modelling and application

Mechanical concrete properties at high temperatures depend on many parameters. The main parameters are the specimen type and the test conditions. The report describes the test parameters and test procedures for relaxation tests in the range of 20 to 750°C. TC Membership: The draft of this document has been prepared by the following 16 Committee full members representing 12 countries. Chairman: U. Schneider, Austria; Secretary: R. Felicetti, Italy. Members: G. Debicki, France; U. Diederichs, Germany; J.-M. Franssen, Belgium; U.-M. Jumppanen, Finland; G.A. Khoury, UK; S. Leonivich, Republic Belarus; A. Millard, France; W. A. Morris, UK; L. T. Phan, USA; P. Pimienta, France; P. Rodrigues, Portugal; E. Schlangen, The Netherlands; P. Schwesinger, Germany; Y. Zaytsev, Russian Federation.     

高温后高延性混凝土的抗压性能及微观结构

为研究高温后高延性混凝土(HDC)的抗压性能,考虑温度、冷却及养护方式、静置时间三个因素设计了49组立方体试件,并测试其抗压强度。通过5组棱柱体试件的超声回弹试验,探讨HDC抗压强度、经历最高温度与超声波速和回弹值的相互关系。结果表明:温度对抗压强度影响较大,随着温度升高,抗压强度降低;温度低于200℃时,冷却方式的影响不可忽略;温度为400℃时,静置时间对自然冷却试件也有较大影响。超声回弹试验表明, HDC抗压强度与超声波速和回弹值有良好的相关性;通过回归分析,建立了高温后HDC测强曲线及推定其经历最高温度的公式。借助XRD、 SEM和热重-差热分析(TG-DSC)等试验,揭示了高温对HDC力学性能影响的微观机制。     

Formation of chloroaluminates in calcium aluminate cements cured at high temperatures and exposed to chloride solutions

The formation of hexagonal chloroaluminates in mortar specimens pre-cured at 20, 40 and 60°C for two weeks and stored in a 0.5 M NaCl solution for up to 255 days has been studied. The appearance of this phase as a function of time has been monitored by X-ray diffraction. In addition, its microstructure has been observed by means of backscattering electron microscopy. The chemical composition was studied by X-ray microanalysis. The formation of chloroaluminate phases in reinforced concrete is related to the immobilization of chloride ions penetrating through the concrete to the reinforcement. Thus the formation of stable chloroaluminates could lower the risk of corrosion. In order to check this point, corrosion rate measurements were performed throughout the experiment. In spite of the high capacity of aluminous hydrates to react with chloride ions to form chloroaluminates, the remaining chloride ions in the pore solution leads over time to reinforcement corrosion. The presence of hexagonal phases in the cement paste ensure a better resistance against the penetration of chloride ions than when cubic phases are present. This effect was attributed to the denser microstructure exhibited, by samples containing the hexagonal phases. This revised version was published online in November 2006 with corrections to the Cover Date.     

Pine cone fiber/clay hybrid composite: Mechanical and thermal properties

This paper deals with the mechanical and thermal properties of clay and pine cone fibers reinforced polypropylene hybrid composite at a total weight percent of 30. To enhance charges wettability within the polymer, a coupling agent was added and a mercerization treatment was carried out to the fibers. Tensile, torsional, hardness tests were conducted for these composites to evaluate the impact of hybrid charges. The tensile properties results indicate that the Young’s modulus has increased for whole systems reaching a gain of 80%, while tensile strength remained stable with the use of both charges. For torsional and hardness characterizations of such hybrid composite, an increase in the torsional resistance is noted with clay addition when hardness properties were in decrease at high clay loading. Thermal degradation decreases with the addition of fibers which is normal with the addition of a low degradation temperature charge.     

Influence of elevated temperatures on the mechanical properties of blended cement concretes prepared with limestone and siliceous aggregates

The investigation performed was aimed at showing the influence of high temperatures on the mechanical properties and properties that affect the measurement by non-destructive methods (rebound hammer and pulse velocity) of concrete containing various levels (10% and 30%) of pozzolanic materials. Three types of Pozzolans, one natural pozzolan and two lignite fly ashes (one of low and the other of high lime content) were used for cement replacement. Two series of mixtures were prepared using limestone and siliceous aggregates. The W/b and the cementitius material content were maintained constant for all the mixtures. Concrete specimens were tested at 100, 300, 600 and 750 °C for 2 h without any imposed load, and under the same heating regime. At the age of 3 years, tests of compressive strength, modulus of elasticity, rebound hummer and pulse velocity were come out. Results indicate that the residual properties of concrete strongly depend on the aggregates' and the binder type. Relationships between strength of concrete as well as rebound and pulse velocity versus heating temperatures are established. The above results are evaluated to establish a direct relationship between non-destructive measurements and compressive strength of concrete exposed to fire.     

碳纳米管增强2024铝基复合材料的力学性能及断裂特性

为了研究碳纳米管对铝基复合材料性能的影响,采用冷等静压、热挤压方法制备了质量分数1.0%的多壁碳纳米管增强2024Al基复合材料.采用扫描电镜、透射电镜和拉伸试验对复合材料的显微组织进行了观察和分析,并对其力学性能进行了测试.结果表明,碳纳米管均匀地分布在复合材料中,碳纳米管和铝基体的界面结合良好,没有发现界面产物Al4C3的形成;复合材料的断口上存在大量的撕裂棱,韧窝,并涉及碳纳米管的拔出或拔断与桥接,与2024Al基体材料相比,复合材料的硬度、弹性模量和抗拉强度显著提高,同时复合材料的延伸率却并不下降.碳纳米管的加入可以显著提高铝基复合材料的力学性能.     

Mechanical properties and failure mechanisms of composite laminates with classical fabric stacking patterns

In the design of composite materials, the properties and failure modes/mechanisms are always of the main concern. In this work, the mechanical properties and failure mechanisms of composite laminates with classical fabric stacking patterns ([(0, 90)]₈ and [(0, 90)/(±?45)]₄) were systematically investigated through mechanical experiments and FEM (finite element method) numerical simulations. The results show that the tensile modulus and bending modulus of the laminates were reduced by 22.2% and 37% after partially changing the stacking angle to?±?45°, but corresponding elongation and bending displacement were increased by 8.8% and 11.7%, respectively. Bending failure mode changes from complete fracture to partial fracture. Meanwhile, the delamination damage and tow peeling from the matrix increase significantly. FEM simulations on tensile and bending processes of the composites indicate that the?±?45° stacking angle leads to the change of the axial stress direction from S_X (0°) to S_Y (±?45°), which is difficult to be observed from mechanical experiments. The FEM simulation provides a cost effective and efficient way for the structural visual optimization design and failure prediction of the actual composite materials.