Effects of Time and Temperature on the Viscoelastic Properties of Chinese Fir Wood

The effects of time and temperature on dynamic viscoelastic properties of Chinese fir (Cunninghamia lanceolata [Lamb.] Hook) were investigated using dynamic mechanical analysis in this study. The isothermal tests were applied to the small clear specimens with a moisture content of about 0.6% at constant temperatures ranging from 25 to 200°C for 550 min at atmospheric pressure. Changes in storage modulus and loss tangent with heating time were examined. The results indicated that heating time mainly resulted in thermal softening, thermal degradation of wood, and the reduction of wood stiffness. At more than 60°C, the reduction in storage modulus was accelerated generally as the wood was subjected to a higher temperature or longer heating time. At constant temperatures of 140 and 160°C, a relaxation phenomenon was observed with a slight change in weight, which could be attributed to the relocation of lignin molecules. At the temperature range of 140 to 180°C, the higher the heating temperatures, the earlier the tanδ peak appeared. It is suggested that the wood thermal softening occurs at higher temperatures with shorter heating times or at lower temperatures with longer heating times. At temperatures of 180 and 200°C, the loss of amorphous polysaccharides due to thermal degradation is considered to be the main factor affecting wood viscoelasticity.     

Effect of high temperature or fire on heavy weight concrete properties

Temperature plays an important role in the use of concrete for shielding nuclear reactors. In the present work, the effect of different durations (1, 2 and 3 h) of high temperatures (250, 500, 750 and 950 °C) on the physical, mechanical and radiation properties of heavy concrete was studied. The effect of fire fitting systems on concrete properties was investigated. Results showed that ilmenite concrete had the highest density, modulus of elasticity and lowest absorption percent, and it had also higher values of compressive, tensile, bending and bonding strengths than gravel or baryte concrete. Ilmenite concrete showed the highest attenuation of transmitted gamma rays. Firing (heating) exposure time was inversely proportional to mechanical properties of all types of concrete. Ilmenite concrete was more resistant to elevated temperature. Foam or air proved to be better than water as a cooling system in concrete structure exposed to high temperature because water leads to a big damage in concrete properties.     

Stress-strain-behaviour of concrete at extremely low temperature

The application of prestressed concrete for the construction of storage vessels for liquefied natural gas (LNG) requires concise knowledge of the behaviour of concrete at extremely low temperatures. The tests reported herein show that the strength of concrete increases with the decrease of temperature. This increase is the more pronounced the greater the moisture content of concrete at test. While at room temperature concrete exhibits a ductile behaviour, the stress-strain-behaviour is changed at low temperatures towards increasing brittleness. A relation between the compressive strength, the strain at maximum stress and the thermal strain was found in the tests as a function of temperature.     

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.     

Compressive behavior of fiber reinforced high-performance concrete subjected to elevated temperatures

In this paper, the effects of elevated temperatures on the compressive strength stress–strain relationship (stiffness) and energy absorption capacities (toughness) of concretes are presented. High-performance concretes (HPCs) were prepared in three series, with different cementitious material constitutions using plain ordinary Portland cement (PC), with and without metakaolin (MK) and silica fume (SF) separate replacements. Each series comprised a concrete mix, prepared without any fibers, and concrete mixes reinforced with either or both steel fibers and polypropylene (PP) fibers. The results showed that after exposure to 600 and 800 °C, the concrete mixes retained, respectively, 45% and 23% of their compressive strength, on average. The results also show that after the concrete was exposed to the elevated temperatures, the loss of stiffness was much quicker than the loss in compressive strength, but the loss of energy absorption capacity was relatively slower. A 20% replacement of the cement by MK resulted in a higher compressive strength but a lower specific toughness, as compared with the concrete prepared with 10% replacement of cement by SF. The MK concrete also showed quicker losses in the compressive strength, elastic modulus and energy absorption capacity after exposure to the elevated temperatures. Steel fibers approximately doubled the energy absorption capacity of the unheated concrete. They were effective in minimizing the degradation of compressive strength for the concrete after exposure to the elevated temperatures. The steel-fiber-reinforced concretes also showed the highest energy absorption capacity after the high-temperature exposure, although they suffered a quick loss of this capacity. In comparison, using PP fibers reduced the energy absorption capacity of the concrete after exposure to 800 °C, although it had a minor beneficial effect on the energy absorption capacity of the concrete before heating.     

高温作用下不同掺量玄武岩纤维混凝土力学性能研究

为了研究高温作用下玄武岩纤维混凝土的力学性能,分析不同温度作用下的混凝土(玄武岩纤维掺量分别为0、0.2%和0.4%)的物理变化,并结合单轴压缩试验,进一步研究高温对混凝土力学性能的影响。结果表明:基准混凝土与玄武岩纤维混凝土随着温度的升高,其烧失量均逐渐增加,玄武岩纤维的掺加对高温作用下混凝土水分消散阻止作用较小;表面所产生裂纹数逐渐增加,基准混凝土所产生的裂纹无论是数量还是长度与宽度均为最大。可见,玄武岩纤维在混凝土高温时所起作用主要为减少混凝土爆裂现象的产生;抗压强度均表现为先升高后降低的趋势,其中相同温度时,玄武岩纤维混凝土强度始终高于基准混凝土,且随着纤维掺量的增加而增加;由于玄武岩纤维具有较好的阻裂效果,因此当试样破坏时,玄武岩纤维混凝土破坏程度小于基准混凝土。玄武岩纤维在高温作用下主要作用为减少爆裂现象对水分蒸发的影响较小。     

Osmotic Dehydration of Apple Cylinders: III. Continuous Medium Flow Microwave Heating Conditions

Continuous flow osmotic drying permits a better exchange of moisture and solids between the food particle and osmotic solution than the batch process. Osmotic drying has been well studied by several researchers mostly in the batch mode. Microwave heating has been traditionally recognized to provide rapid heating conditions. Its role in the finish drying of food products has also been recognized. In this study, the effects of process temperature, solution concentration on moisture loss (ML), solids gain (SG), and mass transport coefficients (k_m and k_s) were evaluated and compared under microwave, assisted osmotic dehydration (MWOD) versus continuous flow osmotic dehydration (CFOD). Apple cylinders (2 cm diameter, 2 cm height) were subjected to continuous flow osmotic solution at different concentrations (30, 40, 50, and 60°Brix sucrose) and temperatures (40, 50, and 60°C). Similar treatments were also given with samples subjected to microwave heating. Results obtained showed that solids gain by the samples was always lower when carried out under microwave heating, while the moisture loss was increased. The greater moisture loss strongly counteracted solids gain in MWOD and thus the overall ratio of ML/SG was higher in MWOD than in CFOD.     

Thermal degradation of high nitrile barrier polymers

Effects of absorbed moisture on degradation behavior of high nitrile barrier polymers were monitored using thermogravimetric analysis techniques. Non-modified and 10 percent rubber modified samples were heated isothermally at nitrile processing temperatures (200°C to 240°C) in air and nitrogen environments. Degradation was evaluated in terms of weight loss as a function of heating time and sample coloration. It was determined that complete removal of moisture, as well as high moisture concentration, contribute to increased degradation at the temperatures evaluated. Moisture levels in the range of 0.15 to 0.5 percent were found to minimize degradation. Heating environment, time, temperature, and rubber modification were also found to influence thermal stability.     

Self-consolidating concrete subjected to high temperature: Mechanical and physicochemical properties

This paper presents an experimental study on the performance of self-consolidating concrete (SCC) subjected to high temperature. Two SCC mixtures and one vibrated concrete were tested. These concrete mixes were developed in the French National Project B@P. Mechanical and microstructural properties were studied at ambient temperature and after heating. We studied compressive strength, flexural strength, bulk modulus of elasticity, porosity and permeability. For each test, the specimens 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 specimen, the temperature was held constant at the target temperature for 1 h before cooling. In addition, the specimen mass was measured before and after heating in order to determine the loss of water during the test. The results allowed us to analyze the degradation of SCC and vibrated concretes due to heating.     

A COMMERCIALLY VIABLE SOLAR WOOD DRYING KILN SYSTEM

The purpose of the study was to create a totally passive solar wood drying kiln that would dry lumber to 9% moisture content in a reasonable amount of time. A series of modifications led to a kiln design that dried freshly-cut lumber to 8% in a 29-day period with no case hardening or cracking. Air speed, internal and external temperatures and relative humidity levels were measured at 5-minute intervals. The average temperature inside the kiln was 12% higher with relative humidity levels 19% lower than outside the kiln. It is hypothesized that the daily cycles of heating and cooling permitted the interior moisture of the wood to reach the surface through diffusion, thus lessening stress and speeding drying of the lumber.     

Modeling and simulation of drying characteristics on flexible filamentous particles in rotary dryers

Experiments were conducted to demonstrate the effects of the drum wall temperature on the heat and mass transfer in rotary dryers. The drying characteristics of flexible filamentous particles in rotary dryers were further explored. In addition, the inlet and outlet temperatures and moisture contents of granular particles were measured. As a result, the good agreement between the simulations and experiments verified the rationale and feasibility of the numerical method. Therefore, the approach was adopted to evaluate the temperature and moisture content of wet granular particles in a rotary dryer in different conditions, for instance, drum wall temperature and rotational speed. The results revealed that the higher drum wall temperature led to hotter particles with lower outlet moisture content. Conversely, the higher rotational speed resulted in cooler particles with higher outlet moisture content due to the decrease of residence time in the rotary dryer.     

The influence of pyrolysis conditions on the subsequent gasification of lignocellulosic chars

Prune pit chars prepared by pyrolysis at heating rates of 1 and 15°C/min to 500, 700 and 900°C were subsequently gasified by exposure to CO₂ at 900°C for various lengths of time. Gasification rate was found to be dependent on the conditions during pyrolysis: slow heating below 500°C and prolonged exposure to high temperatures (~900°C) during pyrolysis in an inert atmosphere lead to lower rate gasification. Despite differences in gasification rate, the pore structure developed for a given mass loss due to the gasification reactions was apparently independent of the char preparation conditions. Pore volume in the gasified char (expressed on an absolute basis) passed through a maximum at 40–50% burnoff, apparently due to mass loss from the exterior of the particles.     

Effect of aggregate and water to cement ratio on concrete properties at elevated temperature

Properties of concrete during and after fire exposure are of significant importance for serviceability and rehabilitation of buildings. This article presents an experimental investigation on the effects of elevated temperature on physical and mechanical properties of concrete made using two types of locally available coarse aggregates (crushed and wadi aggregates) and water‐to‐cement (w/c) ratios of 0.50 and 0.70. Temperature range from 200 °C to 1000 °C was used with intervals of 200 °C. Test results indicate that the weight of concrete reduced with increase in temperature. This reduction was quite sharp beyond 800 °C. Minor spalling was observed in concrete with Wadi aggregates at temperatures beyond 800 °C. The results also reveal that relative strength of concrete decreased as exposure temperature increased. The effect of high temperatures on the strength of concrete was more pronounced in concrete with Wadi aggregates. w/c ratio had insignificant effect on weight loss after exposure to elevated temperatures, but it increased the rate of strength degradation irrespective of aggregate type used. Comparison of results with Eurocode (EC‐2) and American Concrete Institute (ACI) standards indicate that the concrete with both aggregate types can satisfy the limits of siliceous aggregates set by ACI, but concrete made with Wadi aggregates with w/c ratio of 0.50 failed to satisfy limits of EC‐2. Copyright © 2015 John Wiley & Sons, Ltd.     

Postfire full stress–strain response of fire‐damaged concrete

This paper reports experimental data establishing the postfire full stress–strain response of fire‐affected concrete. Such data are useful in situations when redesign of fire‐damaged concrete elements is considered. Heating was carried out to various temperatures in the range 217–470°C. Cooling was carried out either by quenching or in air. The postfire strain at ultimate stress significantly increased after heating to temperatures higher than 320°C. Quenching seems to aggravate the loss in compressive strength and further increase the strain at ultimate stress. Quenching involved spraying the heated concrete with tap water for 5 min. It is evident that knowledge of maximum temperature of exposure alone is not sufficient for estimation of the postfire stress–strain relationship. Other characteristics of exposure such as method of cooling are also important in evaluating the modification in the structural behaviour of fire‐affected concrete. Copyright © 2005 John Wiley & Sons, Ltd.     

Postfiring stress‐strain hysteresis of concrete subjected to various heating and cooling regimes

This paper reports an experimental investigation involving 90 siliceous aggregate concrete cores (75 mm dia. 175 mm long), which were subjected to 18 different heating and cooling regimes in the temperature range (217°C–470°C). The cores were heated to the point when the centre of the core reached the same temperature as the outer surface, the point of uniformity. Subsequently, the cores were either taken out of the furnace to cool or soaked at the test temperature for 2 h. Cooling was either by spraying the heated core with water for 5 min or by air‐cooling in a controlled environment of 20°C and 65% RH. The extent of damage and internal fracture were assessed using the stiffness damage test (SDT) and the ultrasonic pulse velocity (UPV) as well as petrography methods. Both prolonged duration of thermal exposure and rapid cooling by water quenching resulted in a further loss of stiffness of fire damaged concrete cores. Extending the duration of thermal exposure by 2 h from the point of uniformity resulted in an approximately 10% further reduction in the residual chord modulus (E_c). However, spraying hot concrete with water after reaching the point of uniformity caused more reduction in stiffness than a 2 h extended duration of thermal exposure to temperatures below 320°C. The damage index (DI), which represents the dissipated strain energy in a hysteresis loop, showed a similar effect. The damage index of cores subjected to water quenching was significantly higher than the damage index of cores soaked at temperatures below 320°C. The residual plastic strain (PS), which is measured at the end of a hysteresis loop, showed very similar variation to that of the damage index. The ultrasonic pulse velocity (UPV) test method confirmed that spraying hot concrete with water from the point of uniformity was more damaging than a 2 h extended duration of thermal exposure to temperatures below 320°C. The extended duration of thermal exposure did not seem to affect the degree of non‐linearity of the stress‐strain response of fire affected concrete while water quenching resulted in a more concave response. The results are explained by identifying the various damage mechanisms associated with the various heating and cooling regimes. Copyright © 2002 John Wiley & Sons, Ltd.     

内构件固定床反应器中不同水分煤的热解特性

通过在有无内构件(传热板和中心集气管)固定床反应器中研究不同水分含量煤的热解特性,考察了两反应器中煤料的升温特性、热解产物分布、焦油品质以及气体产物组成和半焦热值。结果表明,内构件可以强化传热和调节热解产物在反应器内的流动,相对无内构件反应器,有内构件反应器的反应时间缩短近一半。在有内构件反应器中,当煤水分增加,导致煤热解反应要求的时间延长,焦油中轻质组分(沸点低于360℃)含量明显升高,焦油收率先增加后降低,热解水和热解气产率升高,而无内构件反应器的热解产物无明显差异。当加热温度900℃时,煤水分从0.41%(本文中无特殊说明的均为质量分数) 增加至11.68%,焦油产率从9.21%增长到10.74%;当煤水分增加到15.93%,焦油产量下降到10.26%。两反应器气体平均组成随水分增加的变化趋势相似,气体热值均随水分增加呈下降趋势。     

Synthesis gas and char production from Mongolian coals in the continuous devolatilization process

Devolatilization of Mongolian coal (Baganuur coal (BC), Shievee Ovoo coal (SOC), and Shievee Ovoo dried coal (SOC-D)) was investigated by using bench-sized fixed-bed and rotary kiln-type reactors. Devolatilization was assessed by comparing the coal’s type and dry basis, temperature, gaseous flux, tar formation/generation, devolatilization rate, char yield, heating value, and the components of the raw coal and char. In the fixed bed reactor, higher temperatures increased the rate of devolatilization but decreased char production. BC showed higher rates of devolatilization and char yields than SOC or SOC-D. Each coal showed inversely proportional devolatilization and char yields, though the relation was not maintained between the different coal samples because of their different contents of inherent moisture, ash, fixed carbon, and volatile matter. Higher temperatures led to the formation of less tar, though with more diverse components that had higher boiling points. The coal gas produced from all three samples contained more hydrogen and less carbon dioxide at higher temperatures. Cracking by multiple functional groups, steam gasification of char or volatiles, and reforming of light hydrocarbon gas increased with increasing temperature, resulting in more hydrogen. The water gas shift (WGS) reaction decreased with increasing temperature, reducing the concentration of carbon dioxide. BC and SOC, with retained inherent moisture, produced substantially higher amounts of hydrogen at high temperature, indicating that hydrogen production occurred under high-temperature steam. The continuous supply of steam from coal in the rotary kiln reactor allowed further exploration of coal gas production. Coal gas mainly comprising syngas was generated at 700–800 °C under a steam atmosphere, with production greatest at 800 °C. These results suggest that clean char and high value-added syngas can be produced simultaneously through the devolatilization of coal at lower temperature at atmospheric pressure than the entrained-bed type gasification temperature of 1,300–1,600 °C.     

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.     

Osmotic Dehydration of Apple Cylinders: III. Continuous Medium Flow Microwave Heating Conditions

Continuous flow osmotic drying permits a better exchange of moisture and solids between the food particle and osmotic solution than the batch process. Osmotic drying has been well studied by several researchers mostly in the batch mode. Microwave heating has been traditionally recognized to provide rapid heating conditions. Its role in the finish drying of food products has also been recognized. In this study, the effects of process temperature, solution concentration on moisture loss (ML), solids gain (SG), and mass transport coefficients (k _m and k _s ) were evaluated and compared under microwave, assisted osmotic dehydration (MWOD) versus continuous flow osmotic dehydration (CFOD). Apple cylinders (2 cm diameter, 2 cm height) were subjected to continuous flow osmotic solution at different concentrations (30, 40, 50, and 60°Brix sucrose) and temperatures (40, 50, and 60°C). Similar treatments were also given with samples subjected to microwave heating. Results obtained showed that solids gain by the samples was always lower when carried out under microwave heating, while the moisture loss was increased. The greater moisture loss strongly counteracted solids gain in MWOD and thus the overall ratio of ML/SG was higher in MWOD than in CFOD.     

CONDUCTIVE DRYlNG CHARACTERISTICS OF GELATINIZED RICE STARCH

The main objective of the experiment in thispaper is to investigate the influence of the ratio of the thickness of the plate and that of the film, and initial condi- tions on the conductive drying characteristics of thin films of gelatinized rice starch on a hot plate under constant heat and mass transfer to the surroundings. The plate was. supported on one arm of a sensitive balance which measured and recorded the weight loss due to drying. Tbennocouples situated at the upper and lower surfaces of the plate recorded the respective temperatures. The gelatinized starch was prepared by cooking a slurry of rice starchin a heated and agitated vessel at slightly above the gelatinization temperatureof the starch 70C. The film was applied after the plate was heated to its initial temperature which ranged from 70C to 119C. The weight 10s and the temperature of the upper and lower surfaces of the plate were recorded. The m3isture content of the dried sample was determined by measuring the weight loss after heating it in an oven at 90C for 24 hours.The dimensionless average moisture content was fitted to the drying rate eauation of the form The dimensionless temperature of tbe filmplate interface data was fitted to the following equation It was found that the critlcal moisture content Xc increases when both the intial temperature level and the ratio of the thickness of the plate to that of the film are raised. The dimensionless avenge moisture content profiles against dimensionless time and the dimensionless drying rate ploti against the dimensionless moisture content are not affected at all by this ratio. There was an initial cooling period fol lowed by a continuous rue in temperature. The dimensionless temperature is independent of the ratio of the thickness of rbe plate to that of the film. The dimen