Effect of casting rate and concrete temperature on formwork pressure of self-consolidating concrete

An experimental program was undertaken to determine the effect of casting rate and concrete temperature on formwork lateral pressure that can be developed by self-consolidating concrete (SCC). Concrete mixtures prepared with initial temperatures varying from 10 to 30^∘C or with high early-strength cement and set-accelerating admixture were evaluated. The concrete was placed either continuously at casting rates varying between 5 and 25 m /h or by stopping the placement for some predetermined periods of time. Test results show that the increase in casting rate from 5 to 25 m/h can lead to 15% increase in initial formwork pressure; however, no significant effect on the rate of pressure drop with time is observed. The variations in fresh concrete temperature have limited effect on the initial lateral pressure; however, the rate of pressure drop was significantly increased with the increase in temperature. The time for the cancellation of pressure is directly affected by the concrete temperature and materials in use, and occurs shortly after the end of the dormant period of cement hydration.     

A Physical Model for the Prediction of Lateral Stress Exerted by Self-Compacting Concrete on Formwork

In this paper, a model is proposed to describe the evolutions of the lateral stress exerted by self compacting concrete (SCC) on a formwork during and after casting. The predictions of the model are compared to pressure drop measurements after the end of casting carried out on real formwork simultaneously with measurements of the evolution of the apparent yield stress of the cast concrete. Then, the predictions of the model during the casting phase are compared to results from the literature and show that the proposed model is able to explain and predict the experimental observations and the quantitative evolutions.     

Effect of mineral admixtures on formwork pressure of self-consolidating concrete

Owing to enhanced filling ability, self-consolidating concrete offers accelerated casting and superior quality control during construction. However, its high fluidity and high placement rate increase the lateral pressure on the formwork, necessitating an extensive supporting system to retain fresh mixtures in a desired shape. Current recommendations of formwork design for self-consolidating concrete adopt the concept of hydrostatic pressure, even though the measured pressure could be less than the recommended level. This study shows that mineral admixtures such as processed clays can appreciably lessen the formwork lateral pressure. In addition, the correlation between the formwork pressure response and the loss of slump flow is derived, providing an approximate method to estimate the reduction in formwork pressure.     

Regression models to predict SCC pressure exerted on formworks containing vertical and transverse reinforcing bars

The use of self-consolidating concrete (SCC) containing recycled concrete aggregate (RCA) considerably increased in sustainable structural applications and civil engineering works. However, current literature and construction practices are not clear regarding the influence of RCA additions and presence of steel reinforcement on formwork pressure exerted by the plastic concrete. This paper reports experimental data obtained from 32 SCC mixtures possessing different stability levels and cast in 1.6-m high formwork containing various combinations of vertical and transverse steel bars. Test results have shown that mixtures incorporating recycled aggregates exhibited reduced initial maximum pressure, given the higher RCA surface roughness that promotes internal friction and material build-up at rest. The decrease in pressure was particularly accentuated in presence of steel bars, suggesting that the reinforcement cage confines the plastic concrete and carries part of its load. The transverse steel was around 1.5-times more influential than vertical steel in reducing the formwork pressure. The rates of pressure drop over time were not altered because of steel, implying that pressure decay is governed by the concrete intrinsic properties such as thixotropy, RCA friction, and cement hydration. Special emphasis was placed to develop regression models and examine suitability of existing ones to predict lateral pressure of RCA-modified SCC cast in formworks containing reinforcing bars.     

高能同步辐射光源(HEPS)氦低温传输系统模拟与计算

使用EcosimPro商业软件和关联式编程两种方法,对HEPS氦低温传输系统进行了模拟和计算,研究了管道内径和管道粗糙度对管道压降的影响。结果表明流动压降随着管道内径的增大而减小,但降低的幅度越来越小,当管道直径从30.8 mm提高至56.8 mm,压降仅降低了约510 Pa;流动压降随粗糙度的增加而增加,但管径越大压降增加越小,当管径为30.8 mm时,选用电抛光管压降下降了954 Pa;管道的漏热量和降温复温时间随管径的增大而提升。通过计算分析最终选择主干来流管道公称直径为DN25,内径为30.8 mm的普通金属管,分支来流管道公称直径为DN10,内径为14.6 mm的普通金属管,主干回流管道公称直径为DN50,内径为56.8 mm的普通金属管,分支回气管道公称直径为DN20,内径为23.8 mm的普通金属管,满足工程需求。     

Field validation of models for predicting lateral form pressure exerted by SCC

One obstacle to increased use of self-consolidating concrete (SCC) in cast-in-place applications is a fear of high, lateral formwork pressure. The literature has repeatedly shown that pressures far lower than hydrostatic can be obtained, but still a widespread concern of high pressure exists. Several models for predicting the lateral pressure when casting SCC have been developed. A number of experts representing these models gathered in Stockholm, Sweden to carry out a field evaluation. Eight instrumented wall elements with various geometries were cast with SCC using different mix designs as well as various casting rates. Pressure transducers were used to determine the lateral pressure. All necessary parameters for the respective models were characterized simultaneously by the participants using samples taken from the same batch. Results show a wide range of form pressures, and the evaluation of the ten included models reveals that all of them predict the lateral form pressure satisfactorily.     

注浆成型结合真空发泡法制备梯度多孔氮化硅陶瓷

通过注浆成型结合真空发泡制备出具有明显梯度结构的多孔氮化硅陶瓷。该工艺在浆料中加入发泡剂, 然后注入底部为石膏的模具中, 抽真空保压并恒温保存, 浆料在低压下会形成孔径梯度结构。研究发现, 气压对样品的梯度结构有较明显影响, 随着气压由80 kPa降低到9 kPa, 样品气孔率由59.01%提高到80.85%, 当气压为80 kPa时, 样品无宏观梯度结构, 随着气压的降低, 梯度结构趋于明显。压汞法测试表明, 9 kPa气压制备的样品孔径分布比较分散, 具有0.5、20、30、40、60、100和200 μm等多种气孔孔径值。该工艺具有简易、廉价, 孔径分布和气孔率可调的优点, 可实现大尺寸、复杂形貌材料的制备, 为梯度多孔材料的制备方法提供了新思路。     

Experimental investigation of two-phase flow characteristics of HFC-134a through short-tube orifices

The two-phase flow characteristics of HFC-134a, including flow pattern, mass flow rate, pressure distribution and temperature distribution through short-tube orifices are experimentally investigated. Short tube diameters ranging between 0.605 and 1.2 mm with length-to-diameter ratios ranging between 8.3 and 33 are used in the experiments. The test runs are performed at upstream pressure ranging between 900 and 1300 kPa, downstream pressure ranging between 300 and 400 kPa, and degree of subcooling ranging between 1 and 12 °C. Two groups of short-tube orifices are used in the experiment. The first is used to visualise the flow pattern. The second is used to measure temperature and pressure distributions along the tube. The results from the present experiment show that metastable flow and choked flow phenomena exist inside the short-tube orifices over the whole range of experimental conditions. The metastable liquid flow region increases with increasing degree of subcooling and upstream pressure. The mass flow rate is directly proportional to upstream pressure and degree of subcooling. The results of pressure distribution inside the short-tube orifices indicate that accelerational pressure drop at the inlet and outlet has a significant effect on the total pressure drop across the short-tube orifice.     

多层喷射共沉积制备 SiCP/Al-8.5Fe-1.3V-1.7Si复合材料

采用多层喷射沉积工艺制备SiC_P/Al-8.5Fe-1.3V-1.7Si复合材料, 研究了雾化及沉积工艺参数对沉积坯状态及SiC颗粒捕获的影响。结果表明, 液流直径大、雾化气体压力小、喷射高度小会导致沉积坯组织恶化, 反之则造成收得率低、致密度低。雾化器扫描不均匀则会造成沉积坯形状不均匀, 而且会由于热量集中导致显微组织恶化。SiC颗粒输送压力的提高有利于SiC颗粒的捕获以及颗粒的均匀分布。多层喷射沉积制备SiC_P/Al-8.5Fe-1.3V-1.7Si的优化工艺参数为: 液流直径3.6 mm, 雾化气体压力0.8 MPa, 喷射高度200 mm, SiC 颗粒输送压力0.5 MPa。 沉积坯存在两种SiC-Al界面: 晶态Si界面层与非晶态SiO₂界面层。     

R22在矩形微细管内凝结的气液两相流动压降实验研究

实验研究了R22在当量直径为0.952 mm的水平不锈钢矩形管内凝结的气液两相流动压降。实验时的饱和温度为40-50oC、质量流速为200-800 kg/m2 s、干度为0-1。研究结果表明,实验段的压降占总压降95%以上,而出口处流通断面扩张压降所占比例很小可以忽略。R22的压力梯度随质量流速和干度的增大而增大,在较高干度区增大趋势更加明显。随饱和温度的增大压降减小。与R22相比,相同实验工况下R152a的凝结压降小于R22的。     

Measurement systems for determining formwork pressure of highly-flowable concrete

This paper offers recommendations regarding test set-ups and measurement systems that can be used for laboratory evaluation and field testing of lateral pressure exerted by flowable concrete and self-consolidating concrete (SCC). Test results indicate that pressure sensors placed flush with the inner surface of the formwork can be used to assess the lateral pressure exerted by plastic concrete. The pore-water pressure resulting from the fluid phase of concrete can be evaluated using pore-water pressure sensors similar to the ones employed in soil mechanics. The sensors can be attached at different heights onto rigid formwork system to monitor changes in pressure with time.Sono-tubes made of cardboard are found not to be suitable for monitoring concrete pressure variations because of their flexibility which leads to erroneous values. A pressure column was developed to evaluate lateral pressure and its variations with time exerted by fresh SCC for heights of up to 10 m. Another system consisting of strain gages welded onto steel anchored bars inserted in the formwork was tested and compared to the pressure transducers system.     

The influence of inlet velocity profile on three-dimensional three-generation bifurcating flows

 The influence of inlet velocity profile on the three-dimensional three-generation bifurcating flow has been numerically studied using a CFD code based on finite volume method. The bifurcating airways simulated the branches of human lung. The axial length and cross-sectional diameter of the three-generation airway are taken from the anatomic data of the 5th–7th generation airway of an averaged height man. The curvature and bifurcating angle of each junction are taken as 2.265 diameter of the next generation and 70^∘, respectively, from physiological consideration. Computations are carried out for eight Reynolds numbers ranging from 200 to 1600, each under uniform and parabolic inlet velocity profiles, to consider the relations between the entry flow patterns and the overall flow characteristics including mainstream flow pattern, secondary flow vortices, asymmetrical flow partition and pressure drop. The mass flow ratio between the medial and lateral branch, and the total pressure drop are closely related to the entry flow patterns for larger Reynolds numbers. Received: 1 October 2001 / Accepted: 12 July 2002 Support given by the Research Grants Council of the Government of the HKSAR under Grant No. PolyU 5166/01E and by the Hong Kong Polytechnic University under Central Research Grant No. A-PD75 is gratefully acknowledged.     

水平微圆管内R22和R410a凝结压降的实验研究

R410a是一种被广泛看好的R22替代物,研究R410a的凝结压降特性对开发适用此种制冷工质的凝结换热设备有重要意义。搭建了微细尺度凝结压降实验台,实验研究了R22和R410a在内径为0.941mm水平不锈钢圆管内饱和温度为40℃、质量流速为200-1000 kg/m^2.s、干度为0.2-0.8时的凝结压降特性。实验结果表明：凝结压降随着质量流速的增大而增大,在较高干度时更加明显。与R22相比,R410a的凝结压降在较低干度和质量流速时与R22相当,在较高干度和较高质量流速时明显低于R22。     

Influence of LiAl-layered double hydroxides with 3D micro-nano structures on the properties of calcium sulphoaluminate cement clinker

The effect of LiAl-layered double hydroxides (LiAl-LDHs) with 3D micro-nano structures on the early properties and hydration process of calcium sulphoaluminate cement (CSAC) clinker was investigated. Three LiAl layered double hydroxides (LiAl-LDHs) with different particle size were prepared through a facile solvothermal method. CSAC clinker incorporating LiAl-LDHs with constant water to cement radio were made and tested. The results indicate that a higher content of LiAl-LDHs resulted in a faster hydration rate, a shorter setting time and a higher early compressive strength. Besides, with the decrease of the particle size of LiAl-LDHs, the hydration rate was accelerated with corresponding increase in compressive strength. However, the influence of particle size of LiAl-LDHs on the setting time of CSAC clinker is not significant. Moreover, the addition of LiAl-LDHs did not result in a new phase formed, but increased the quantity of hydration products providing higher compressive strength and shorter setting time.     

Evaluation of cement grouts for embedding anchors under water

Approximately 110 pull-out tests were conducted on grouted anchors cast in the laboratory to investigate the effects of the rheological and mechanical properties of cement grouts, the initial free drop distance of grout, as well as the age of testing on the behavior of anchorages cast in dry and submerged conditions. The anchors consisted of 19-mm, conventional steel bars with an embedment length fixed at five times the bar diameter. Two different casting procedures corresponding to initial free drop of the grout in water of 20 and 200 mm were investigated. The cement grouts used in this investigation incorporated either a Type 10 or a blended silica fume cement and a fixed water-binder ratio of 0.40. The grouts incorporated various concentrations of a high-range water reducer, a rheology-modifying admixture, and silica fume. The rheology-modifying admixture and high-range water reducer were jointly incorporated to enhance both fluidity and washout resistance. The test results indicate that properly designed grouts can be easily cast into place, and yet be cohesive enough to resist the washout of cementitious materials. The spread in load-carrying capacity between anchor bars cast in dry versus submerged conditions can decrease when cement grouts having a greater washout resistance level are used which can be secured by incorporating a rheology-modifying admixture and silica fume. The bond strength is shown to increase when incorporating silica fume regardless of the casting condition.     

Effect of crystal seeding on the hydration of calcium phosphate cement

In this paper, the effect of crystal seeding on the hydration of calcium phosphate cement (CPC) has been carefully investigated. The setting time of the CPC slurry not containing any crystal seeds was 150 min, while the setting time for the specimen containing 5 wt% low crystallinity hydroxyapatite used as a crystal seed was 7 min. This improvement in the setting time was due to HAP serving as a substrate for heterogeneous nucleation which accelerated nucleation. In addition, the compressive strength of the specimen containing the crystal seeding was deduced and we report values different from those previously reported in the literature. The calorimetric curve indicated that crystal seeding could reduce the induction period. A.c. impedance spectroscopy revealed that at the beginning of hydration, the rate of reaction increased and also that the mean diameter and porosity decreased as the seed content increased. At the end of the hydration reaction the situation was changed with the mean diameter and porosity in the sample without any seeds being a minimum, which indicated that the compressive strength was a maximum. This result could be explained by the dissolution and reprecipitation of small hydration products produced by the high rate of reaction produced by the introduction of the crystal seeds.     

Characteristics of vibro-fluidization for fine powder under reduced pressure

A cohesive powder (Geldart group C) was fluidized under reduced pressure (P = 0:5 - 10 kPa) with vibration. The fluidized powder was composed of glass beads 6 μm in diameter. The bed pressure drop was measured by decreasing gas velocity and the flow patterns in the bed were observed. A slanting particle flow, which was not observed at atmospheric pressure in a previous study, appeared at a lower pressure than about P = 20 kPa and with a larger vibration strength than the critical vibration strength, A_cr. Under the above conditions, the pressure drop curve changed abnormally due to the occurrence of this slanting particle flow. On the other hand, when the vibration strength was smaller than A_cr, a typical pressure drop curve was obtained. In light of these results, the interrelation between the slanting particle flow and the change in the pressure drop curve was examined.     

浸轧压力对STF-Kevlar织物高速冲击性能影响研究

为研究浸轧压力对剪切增稠液体(STF)增强Kevlar织物高速冲击性能的影响,分别采用50 kPa, 100kPa, 200kPa的压力浸轧STF-Kevlar织物,并使用钛合金模拟叶片弹体开展打靶试验。研究结果表明:STF表现出明显的剪切增稠现象,增稠范围为169～1 500 s^-1,增稠比为56.4;Kevlar织物浸渍STF后,二氧化硅(SiO₂)纳米粒子均匀附着在纤维表面;浸轧压力的增加使STF-Kevlar织物的质量增加率降低;STF-Kevlar织物的能量吸收较纯Kevlar织物高29.4%,但其弹道性能指数(BPI)低于纯Kevlar;浸轧后STF-Kevlar织物的能量吸收高于纯Kevlar织物,但低于未浸轧STF-Kevlar织物;在100 kPa的浸轧压力下,STF-Kevlar织物的单位面密度吸收的能量最高;STF-Kevlar织物能量吸收的变化趋势与织物极限变形高度的变化一致。     

Uniform coating of CVD diamond on metallic wire substrates

Diamond was coated onto wire substrates of various transition metals (Mo, W or Ti) of 0.5 mm diameter by the microwave plasma CVD method from a gas mixture of the CO–H₂ system. The CVD conditions for a uniform diamond coating were microwave power, 750–1100 W; total pressure, 2000 Pa; total flow rate, 200 ml min-1; CO concentration, 5 vol%; treatment time, 5 h. The wire substrates were mounted vertically or horizontally on a pyrophyllite susceptor, which was placed parallel to the irradiation direction of microwave power. Homogeneous and fine-grained diamond film was prepared on the whole surface of horizontal W wire substrate with a wire height of 2 mm from the susceptor. To obtain a dense diamond coating, the height has to be as low as possible in the plasma region, where the plasma density is higher at lower substrate temperature. Low pressure and high microwave power were suited for fine-grained coating. Diamond deposition rate was found to be more dependent on pressure than substrate temperature. As the pressure increased, a glassy carbon film was formed instead of diamond. This revised version was published online in November 2006 with corrections to the Cover Date.