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.     

Residual strength of hybrid-fiber-reinforced high-strength concrete after exposure to high temperatures

Residual strengths of high-strength concrete (HSC) and hybrid-fiber-reinforced high-strength concrete (HFRHSC) after exposure to high temperatures were investigated in the paper. The results showed that normal HSC is prone to spalling after exposure to high temperatures, and its first spalling occurs when the temperature approaches 400 °C. For HSC reinforced by high melting point fibers, the first spalling occurs when the temperature reaches to approximately 800 °C, while there is no spalling during exposing to high temperatures for HSC reinforced by polypropylene (PP) fiber with a low melting point. Mixing high melting point fiber (i.e., carbon or steel fiber) with low melting point fiber (i.e., PP fiber) HSC greatly improves the properties of HSC after exposure to high temperatures.     

Physicochemical, mineralogical, and morphological characteristics of concrete exposed to elevated temperatures

Concrete cubes prepared from ordinary Portland cement (OPC) of known chemical, mineralogical, and physical performance characteristics and fired to various temperature regimes up to 1000 °C in steps of 100 °C for a constant period of 5 h have been studied by using X-ray diffraction (XRD) and DTA/TGA to establish the effect of elevated temperatures on the mineralogical changes occurring in the hydrated phases of concrete. The changes in physical state of concrete were studied by measuring ultrasonic pulse velocity (UPV) and consequent deterioration in the compressive strength with increase in temperature. Scanning electron microscopy (SEM) studies showed distinct morphological changes corresponding to deterioration of concrete exposed to higher temperatures.     

Effect of high temperatures on high performance steel fibre reinforced concrete

After being subjected to different elevated heating temperatures, ranging between 105 °C and 1200 °C, the compressive strength, flexural strength, elastic modulus and porosity of concrete reinforced with 1% steel fibre (SFRC) and changes of colour to the heated concrete have been investigated.The results show a loss of concrete strength with increased maximum heating temperature and with increased initial saturation percentage before firing. For maximum exposure temperatures below 400 °C, the loss in compressive strength was relatively small. Significant further reductions in compressive strength are observed, as maximum temperature increases, for all concretes heated to temperatures exceeding 400 °C. High performance concretes (HPC) start to suffer a greater compressive strength loss than normal strength concrete (NSC) at maximum exposure temperatures of 600 °C. It is suggested that HPC suffers both chemical decomposition and pore-structure coarsening of the hardened cement paste when C-S-H starts to decompose at this high temperature. Strengths for all mixes reached minimum values at 1000 or 1100 °C. No evidence of spalling was encountered. When steel fibres are incorporated, at 1%, an improvement of fire resistance and crack [F.M. Lea, Cement research: retrospect and prospect. Proc. 4th Int. Symp. On the Chemistry of Cement, pp. 5-8 (Washington, DC, 1960).] resistance as characterized by the residual strengths were observed. Mechanical strength results indicated that SFRC performs better than non-SFRC for maximum exposure temperatures below 1000 °C, even though the residual strength was very low for all mixes at this high temperature. The variations with colour, which occured, are associated with maximum temperatures of exposure.     

Zero-eccentricity direct-tension testing of thermally damaged cement-based materials

This study presents some results on direct-tension strength of two cementitious mortars using a test set-up specifically designed to virtually eliminate any load eccentricity. The tests were conducted on cement mortars with and without condensed silica fume after being exposed to high temperature (200, 300, 400 and 500 °C). Direct-tension tests were also carried out at room temperature (20 °C) for reference. The specimens were exposed to high temperature and were then allowed to cool to room temperature before testing up to failure. The strength values measured in this study exhibit a trend that is similar to that exhibited by the compressive strength cited in the literature. The results show that mortar specimens exhibited a small increase in strength at temperatures up to 200 °C for regular mortar and up to 230 °C for silica fume mortar. At temperatures above 200/230 °C, the residual tensile strength of the mortar decreases significantly and rapidly. Adding silica fume to the cement mortar increases the resistance to high temperature.     

Explosive spalling and residual mechanical properties of fiber-toughened high-performance concrete subjected to high temperatures

In this paper, an experimental investigation was conducted to explore the relationship between explosive spalling occurrence and residual mechanical properties of fiber-toughened high-performance concrete exposed to high temperatures. The residual mechanical properties measured include compressive strength, tensile splitting strength, and fracture energy. A series of concretes were prepared using OPC (ordinary Portland cement) and crushed limestone. Steel fiber, polypropylene fiber, and hybrid fiber (polypropylene fiber and steel fiber) were added to enhance fracture energy of the concretes. After exposure to high temperatures ranged from 200 to 800 °C, the residual mechanical properties of fiber-toughened high-performance concrete were investigated. For fiber concrete, although residual strength was decreased by exposure to high temperatures over 400 °C, residual fracture energy was significantly higher than that before heating. Incorporating hybrid fiber seems to be a promising way to enhance resistance of concrete to explosive spalling.     

Influence of fine ceramic aggregates on the residual properties of concrete subjected to elevated temperature

The residual properties of concrete subjected to elevated temperature are of importance to assess the stability of the structure. This paper investigates the performance of concrete containing white ware ceramic sand exposed to elevated temperature. Concrete mixes containing 0%, 50%, and 100% ceramic sand were prepared. The specimen were exposed to elevated temperatures of 200°C, 500°C, and 800°C for a duration of 60 minutes. Their residual mechanical properties (compressive strength, split tensile strength), ultra sonic pulse velocity, and mass change for different cooling regimes were investigated and compared among specimen. The results showed that incorporation of ceramic sand in concrete mixes improved the resistance against elevated temperature of hardened concrete.     

Effect of stretching temperature on structure and properties of polyethylene terephthalate (lavsan) fibre

Conclusions 1. With an increase in the stretching temperature up to a definite limit (170°C) the tensile strength of PETP and other fibres from crystalline polymers increases. However, at higher temperatures (230°C) the strength diminishes. This is evidently due to a reduction in the density of the intercrystallite regions of the fibrils, in which there is greater probability of polymer failure originating. Such behaviour of fibre made from PETP at elevated stretching temperatures is evidently associated both with the polymer structure and with its low molecular weight.2. For the preparation of a PETP fibre with high mechanical strength high orientation is a necessary but not sufficient condition; the fibre must be strengthened at these temperatures in order to form dense intercrystallite regions in the fibre fibrils together with a high degree of orientation.May we express our gratitude to Yu. A. Zubov and V. I. Selkhova (L. Ya. Karpov Physico-Chemical Scientific Research Institute) for the x-ray determination of the structure of the fibres investigated and also for the interest shown in discussions on the present.All-Union Synthetic Fibre Research Institute. Translated from Khimicheskie Volokna, No. 3, pp. 43–45, May–June, 1969.     

High pressure crystallization of random propylene-ethylene copolymers: α-γ Phase diagram

Two random propylene copolymers with low ethylene content synthesized by Ziegler-Natta catalysts were used is this study to investigate the formation of γ-crystal phase during isothermal crystallization at high pressures. At atmospheric pressure these copolymers crystallize in a mixture of α- and γ-crystals. The content of the γ-phase in the copolymer crystals increased with increasing defect content, crystallization temperature and pressure. Wide-angle X-ray diffraction studies showed that crystallization of these copolymers at pressures above 88 MPa and temperature above 142 °C leads to formation of pure γ-phase. The equilibrium melting temperature of the γ-phase has been determined as a function of defect content and crystallization pressure. Temperature-pressure-composition α-γ phase diagram of isotactic polypropylene was constructed based on the Gibbs free energy approach. This diagram enabled the extrapolation of the equilibrium melting temperatures of both phases for defect free isotactic polypropylene. They were found to be 186.9°C for the α-phase and 189.9°C for the γ-phase.     

Effect of elevated temperatures on the residual mechanical properties of high-performance mortar

The effect of high temperature on the mechanical properties of high-strength mortar was investigated. Specimens were heated up to elevated temperatures (300, 600, 900 °C ) without loading. After being exposed to these oven temperatures, the residual modulus of elasticity, flexural strength and compressive strength of the specimens were determined. The effect of the rate of heating, duration of exposure to maximum temperature and the role of graphite powder, which is known as a high-temperature refractory material on the behavior of the mortar specimens, were observed. Temperatures up to 600 °C resulted in considerable losses in mechanical properties, and at 900 °C, specimens lost almost all of their strength. Higher rate of heating and exposure to the maximum temperature for a shorter period of time resulted in higher residual properties. The useful effect of graphite addition on the residual compressive strength and modulus of elasticity of the mortar specimens as percentages of initial values for each of the heating cycles are clearly observed.     

High temperature behaviour of self-consolidating concrete: Microstructure and physicochemical properties

This paper presents an experimental study on the properties of self-compacting concrete (SCC) subjected to high temperature. Two SCC mixtures and one vibrated concrete mixture were tested. These concrete mixtures come from the French National Project B@P. The specimens of each concrete mixture were heated at a rate of 1 °C/min up to different temperatures (150, 300, 450 and 600 °C). In order to ensure a uniform temperature throughout the specimens, the temperature was held constant at the maximum temperature for 1 h before cooling. Mechanical properties at ambient temperature and residual mechanical properties after heating have already been determined. In this paper, the physicochemical properties and the microstuctural characteristics are presented. Thermogravimetric analysis, thermodifferential analysis, X-ray diffraction and SEM observations were used. The aim of these studies was in particular to explain the observed residual compressive strength increase between 150 and 300 °C.     

Post-fire residual mechanical properties of concrete made with recycled concrete coarse aggregates

This paper presents results of an experimental study on the residual mechanical performance of concrete produced with recycled coarse aggregates, after being subjected to high temperatures. Four different concrete compositions were prepared: a reference concrete made with natural coarse aggregates and three concrete mixes with replacement rates of 20%, 50% and 100% of natural coarse aggregates by recycled concrete coarse aggregates. Specimens were exposed for a period of 1 h to temperatures of 400 °C, 600 °C and 800 °C, after being heated in accordance with ISO 834 time–temperature curve. After cooling down to ambient temperature, the following basic mechanical properties were then evaluated and compared with reference values obtained prior to thermal exposure: (i) compressive strength; (ii) tensile splitting strength; and (iii) elasticity modulus. Results obtained show that there are no significant differences in the thermal response and post-fire mechanical behaviour of concrete made with recycled coarse aggregates, when compared to conventional concrete.     

Tacticity distribution of isotactic polypropylene prepared with heterogeneous Ziegler-Natta catalyst. 1. Fractionation of polypropylene

Two polypropylene samples, one with relatively low isotacticity and the other with high isotacticity were fractionated using a series of solvents and temperatures. For both samples 4-9 fractions were collected and characterised with differential calorimetry, size exclusion chromatography and ¹³C NMR spectroscopy. The collected fractions showed typical characteristics of a fractionation based on isotacticity, but also similarities to results from temperature rising elution fractionation (TREF), even though a separate controlled crystallisation step was not used. The melting temperatures of the fractions were found to increase linearly as a function of the meso diad fraction. A calibration, which can be used to convert DSC melting curves to wt% curves of isotacticity, was constructed for the temperature range 108-165 °C. The calibration enables quick analysis of samples in polypropylene manufacturing processes.     

Crystal morphology and tensile properties of LLDPE containing PP fibers as obtained via dynamic packing injection molding

The fibrillated linear low-density polyethylene (LLDPE)/isotactic polypropylene (iPP) fiber blends were subjected to dynamic packing injection molding (DPIM), in which the prolonged shear was exerted on the melt during solidification stage. Transcrystallization of LLDPE on PP fibers, with stacked lamellae parallel to each other and aligned approximately perpendicular to the long axial of the fibers, has been achieved for the first time in DPIM due to the prolonged shear. PP fibers were found to align parallel to the flow direction along thickness up to the oriented zone of sample prepared by DPIM. The presence of oriented PP fibers enhanced the orientation of LLDPE that developed row-nucleated type morphology. The molding temperatures were changed between 160 °C and 200 °C to investigate the effect of molding temperature on the crystalline microstructure of the blends. As increasing the molding temperature from 160 °C to 200 °C, the partial melting of PP fiber was changed to complete melting, resulting in a dramatic change of the crystal morphology and the mechanical properties as well.     

Thermal and mechanical properties of fiber reinforced high performance self-consolidating concrete at elevated temperatures

This paper presents the effect of temperature on thermal and mechanical properties of self-consolidating concrete (SCC) and fiber reinforced SCC (FRSCC). For thermal properties specific heat, thermal conductivity, and thermal expansion were measured, whereas for mechanical properties compressive strength, tensile strength and elastic modulus were measured in the temperature range of 20–800 °C. Four SCC mixes, plain SCC, steel, polypropylene, and hybrid fiber reinforced SCC were considered in the test program. Data from mechanical property tests show that the presence of steel fibers enhances high temperature splitting tensile strength and elastic modulus of SCC. Also the thermal expansion of FRSCC is slightly higher than that of SCC in 20–1000 °C range. Data generated from these tests was utilized to develop simplified relations for expressing thermal and mechanical properties of SCC and FRSCC as a function of temperature.     

C60高性能混凝土高温后红外检测及CT图像分析

对掺0%和0.2%聚丙烯纤维的C60高性能混凝土进行了模拟火灾高温试验,测试了高温前后混凝土的轴心抗压强度,采用红外热成像检测技术,研究了高温后混凝土的红外热像图谱.建立了高性能混凝土红外热像平均温升与受火温度和轴心抗压强度比的关系;对常温、300 ℃、400 ℃、500 ℃高温后高性能混凝土试件进行了CT图像扫描试验,分析纤维对混凝土内部裂纹产生和扩展的影响.     

Carbon nanotube dispersion and exfoliation in polypropylene and structure and properties of the resulting composites

Nitric acid treated single and multi wall carbon nanotubes (SWNT and MWNT) have been dispersed in polypropylene using maleic anhydride grafted polypropylene (MA-g-PP) and butanol/xylene solvent mixture. SWNT exfoliation was characterized by Raman and UV-vis-NIR spectroscopies. Evidence for hydrogen bonding between maleic anhydride grafted polypropylene and nitric acid treated nanotubes was obtained using infrared spectroscopy. Polypropylene/carbon nanotube composites were melt-spun into fibers. Dynamic mechanical studies show that for fibers containing 0.1 wt% SWNT, storage modulus increased by 5 GPa at −140 °C and by about 1 GPa at 100 °C, suggesting temperature dependent interfacial strength. The crystallization behavior has been monitored using differential scanning calorimetry and optical microscopy. Control fibers exhibited 27% shrinkage at 160 °C, while the shrinkage in the composite fibers was less than 5%. Fibers heat-treated to 170 °C show very narrow polypropylene melting peak (peak width about 1 °C).     

Strengthening of polypropylene fibre

Conclusions It has been shown that elongation of polypropylene fibre at a temperature above the melting point of the unoriented polymer is accompanied not only by an increase in structure orientation, but also by formation of a denser structure, which ensures a high fibre strength.All-Union Scientific Research Institute for Synthetic Fibres (VNIIV). Translated from Khimicheskie Volokna, No. 5, pp. 12–13, September–October, 1969.     

Retained properties of concrete exposed to high temperatures: Size effect

Concrete as a construction material is likely exposed to high temperatures during fire. The retained properties of concrete after such exposures are still of great importance in terms of the serviceability of structures. This paper presents the effects of high temperatures on the physical, mechanical, and microstructural properties of concrete. Specimens with different sizes were exposed to high temperatures ranging from 200 to 1200^°C. The compressive strength, splitting tensile strength, ultrasonic pulse velocity, and rebound numbers of the specimens were determined. The microstructures of the specimens were examined by scanning electron microscope (SEM) analyses. The test results indicated that the retained compressive strength of concrete considerably decreased with increase in temperature. The effect of specimen size on the retained compressive strength was not pronounced. The retained splitting tensile strength of concrete remarkably reduced as the temperature was increased. The specimen size played an important role on the retained splitting tensile strength of concrete up to 400^°C. The test results revealed that ultrasonic pulse velocity (UPV) test can be successfully used in order to check the uniformity of fire‐damaged structures. The rebound numbers decreased with increase in exposure temperature. SEM studies on specimens exposed to 800^°C revealed significant changes in the microstructure of the concrete. Copyright © 2009 John Wiley & Sons, Ltd.     

Aluminosilicate concretes bonded with water glass

Summary A composition was developed for aluminosilicate concrete with a water-glass bond with a density of 1.25–1.30 g/cm³ and with a setting accelerator-Portland cement.By reducing the density of the glass and eliminating the addition of sodium silicofluoride, the RUL is increased by 300° C compared with ordinary concrete used at present. The concrete possesses a high strength over the entire temperature range, a high abrasion resistance and excellent spalling resistance.The technical properties of the fireclay concrete suggest that this material can be used at up to 1300° C, and the aluminous at up to 1450° C in place of piece aluminosilicate goods.To solve the problem of the reliable and mass use of the recommended concretes in heat exchangers and in other parts of rotary cement furnaces it is necessary to carry out extra tests with the concretes on a bigger scale.