Influence of curing regimes on compressive strength of ultra high performance concrete

The present paper is aimed to identify an efficient curing regime for ultra high performance concrete (UHPC), to achieve a target compressive strength more than 150 MPa, using indigenous materials. The thermal regime plays a vital role due to the limited fineness of ingredients and low water/binder ratio. By activation of the reaction kinetics, the effectiveness of the binder is enhanced which leads to improvements in mechanical as well as durability properties. The curing cycle employed are ambient air curing, water curing and hot air curing. The specimens were exposed to thermal regime at (90°C/150°C/200°C) for duration of 24, 48 or 72 hours at the age of 3rd and 7th day followed with air curing or water curing till 28 days. The results showed a marked difference in compressive strength ranging from 217 to 142 MPa with change in curing regimes. The samples when thermally cured at the age of 3rd and 7th day produced an average ultimate strength of 217–152 MPa and 196–150 MPa, respectively.     

热固性粘结炸药固化特性测试方法的研究

为了准确表征固化后期热固性粘结炸药(PBX)的固化特性,以确定PBX的固化终点时间。采用力学实验机测试获得PBX在不同固化时间的力-位移曲线,确定50N加载力为其最大载荷加载。结果表明:固化测试头的直径为10mm,加载速率为1mm/min时,测试获得的力-位移曲线较为平直,测试后样品结构完整,可进行下一次测试。力-位移测试方法获得的固化特性数据偏差远远小于硬度计法,可作为热固性PBX固化特性准确的测试研究方法。     

Effects of different curing regimes on the compressive strength properties of self compacting concrete incorporating fly ash and silica fume

This paper presents the effect of air curing, water curing and steam curing on the compressive strength of Self Compacting Concrete (SCC). For experimental study, SCC is produced with using silica fume (SF) instead of cement by weight, by the ratios of 5%, 10% and 15%, and fly ash (FA) with the ratios of 25%, 40% and 55%. It is observed that mineral admixtures have positive effects on the self settlement properties. The highest compressive strength was observed in the concrete specimens with using 15% SF and for 28 days water curing. Air curing caused compressive strength losses in all groups. Relative strengths of concretes with mineral admixtures were determined higher than concretes without admixtures at steam curing conditions.     

Examining the effects of site selection criteria for evaluating the effectiveness of traffic safety countermeasures

The primary objective of this paper is to describe how site selection effects can influence the safety effectiveness of treatments. More specifically, the goal is to quantify the bias for the safety effectiveness of a treatment as a function of different entry criteria as well as other factors associated with crash data, and propose a new method to minimize this bias when a control group is not available. The study objective was accomplished using simulated data. The proposed method documented in this paper was compared to the four most common types of before–after studies: the Naïve, using a control group (CG), the empirical Bayes (EB) method based on the method of moment (EB_MM), and the EB method based on a control group (EB_CG). Five scenarios were examined: a direct comparison of the methods, different dispersion parameter values of the Negative Binomial model, different sample sizes, different values of the index of safety effectiveness (θ$\theta$), and different levels of uncertainty associated with the index. Based on the simulated scenarios (also supported theoretically), the study results showed that higher entry criteria, larger values of the safety effectiveness, and smaller dispersion parameter values will cause a larger selection bias. Furthermore, among all methods evaluated, the Naïve and the EB_MM methods are both significantly affected by the selection bias. Using a control group, or the EB_CG, can mutually eliminate the site selection bias, as long as the characteristics of the control group (truncated data for the CG method or the non-truncated sample population for the EB_CG method) are exactly the same as for the treatment group. In practice, finding datasets for the control group with the exact same characteristics as for the treatment group may not always be feasible. To overcome this problem, the method proposed in this study can be used to adjust the Naïve estimator of the index of safety effectiveness, even when the mean and dispersion parameter are not properly estimated.     

The effectiveness of coincidence site lattice criteria in predicting creep cavitation resistance

The coincidence site lattice (CSL) concept is often used in microstructural characterization by researchers studying grain boundary engineering as a method for improving the performance of polycrystalline materials. It is assumed that a high degree of shared lattice sites in the boundary between two grains will result in improved mechanical properties. For practical application of the CSL concept to experimental results, a maximum deviation from ideal CSL orientation relationships must be defined to distinguish potential CSL boundaries from random boundaries that are not likely to exhibit “special” properties. Several different maximum deviation criteria have been proposed in the literature. In this study, four of these criteria are investigated for their effectiveness in predicting the creep cavitation resistance of boundaries of different CSL character in three model alloys: pure Cu, Cu-Bi, and Cu-Sb. Bi and Sb strongly segregate to Cu grain boundaries and are detrimental to creep life. The experimental observations are compared to simulation results for a non-textured polycrystal. It is observed that only boundaries related to cubic annealing twins (Σ3 and Σ9) exhibit special resistance to creep cavitation, that boundaries with Σ > 3 are affected by the presence of segregants, and that the fraction of non-Σ(3,9) boundaries tracks closely with what would be expected from a random polycrystal. It is shown that more restrictive criteria result in more reliable characterization of the fraction of cavitation-resistant boundaries only because they exclude more non-Σ(3,9) boundaries from the analysis.     

Calculation of the emergency regimes of cryostats

Analytical expressions are obtained that allow one to evaluate the dependence between the time of pressure growth in cryostats containing cryogenic fluids and the thermal energy entering the cryostat. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 70, No. 3, pp. 413–419, May–June, 1997.     

多面体低聚倍半硅氧烷/氰酸酯杂化树脂固化动力学与固化工艺研究

采用非等温差示扫描量热法测试了不同升温速率下氰酸酯及氰酸酯/多面体低聚倍半硅氧烷(POSS)复合材料的固化过程,分析了不同升温速率下,POSS对树脂体系固化行为的影响.运用Kissinger法和Flynn-Wall-Ozawa法对杂化树脂固化反应活化能进行了计算.结果表明,POSS对氰酸酯树脂固化具有催化作用,能显著降低树脂固化温度,含10%POSS的杂化体系固化温度可降至212℃;两种不同模型计算的活化能分别为83.30kJ/mol和85.68kJ/mol,与纯氰酸酯相比,杂化树脂的固化活化能和反应级数均有所增大.     

Testing carbon sequestration site monitor instruments using a controlled carbon dioxide release facility

Two laser-based instruments for carbon sequestration site monitoring have been developed and tested at a controlled carbon dioxide (CO(2)) release facility. The first instrument uses a temperature tunable distributed feedback (DFB) diode laser capable of accessing the 2.0027-2.0042 microm spectral region that contains three CO(2) absorption lines and is used for aboveground atmospheric CO(2) concentration measurements. The second instrument also uses a temperature tunable DFB diode laser capable of accessing the 2.0032-2.0055 mum spectral region that contains five CO(2) absorption lines for underground CO(2) soil gas concentration measurements. The performance of these instruments for carbon sequestration site monitoring was studied using a newly developed controlled CO(2) release facility. A 0.3 ton CO(2)/day injection experiment was performed from 3-10 August 2007. The aboveground differential absorption instrument measured an average atmospheric CO(2) concentration of 618 parts per million (ppm) over the CO(2) injection site compared with an average background atmospheric CO(2) concentration of 448 ppm demonstrating this instrument's capability for carbon sequestration site monitoring. The underground differential absorption instrument measured a CO(2) soil gas concentration of 100,000 ppm during the CO(2) injection, a factor of 25 greater than the measured background CO(2) soil gas concentration of 4000 ppm demonstrating this instrument's capability for carbon sequestration site monitoring.     

Selection of the annealing regimes for irradiated diamonds

A diamond graphitization curve is plotted by the experimental data for T < 2000 K. A formula is derived for the calculation of the maximum pressure to provide for an optimum diamond annealing at a preset temperature and duration of the treatment.     

超声振动对快速固化环氧树脂体系固化动力学特性的影响

通过非等温差示扫描量热法,结合黏度测试和傅里叶红外光谱分析,研究了不同超声波振动条件下环氧树脂体系的固化特性。基于Flynn-Wall-Ozawa/FWO、Kissinger-Akahira-Sunose/KAS和Boswell积分型动力学模型,计算了不同超声波振动下环氧树脂体系的活化能。结合Malek最大概然函数法,得到了超声振动下树脂体系的固化反应动力学方程,并与实测固化度对比进行了验证。研究表明,超声振动振幅越大,树脂体系黏度降低越明显,较小的超声波振幅振动下树脂体系活化能增大,而振幅增大后活化能有明显的降低。固化物的红外光谱分析表明,随着超声振幅的增大,羟基吸收峰减弱,表明超声效应加速了胺基加成反应或者羟基醚化反应。超声振动条件下的树脂固化反应模型符合自催化模型形式,但超声振动并不能改变树脂体系的固化反应机制。以上研究结果对设计和优化碳纤维增强树脂复合材料超声振动辅助树脂传递模塑成型(RTM)工艺具有一定的指导意义。     

Determination of siphon operating regimes

Necessary and sufficient conditions are found for continuous and periodic liquid motion in systems containing siphons.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 52, No. 6, pp. 916–920, June, 1987.     

The effect of curing period and curing delay on concrete in hot weather

The effect of curing period and of delay in carrying out curing by the wet burlap method on some properties of concrete with different mix proportions (cement content, water content) in hot weather was studied. The results show that a minimum of 3 days curing was sufficient for rich mixes, while a longer period was required for leaner mixes (min. 7 days). The delay of curing had a harmful effect on concrete and the first day of delay caused the largest effect. Curing after delaying increased the compressive strength of concrete, but it did not recover the reduction in strength caused by the curing delay.     

Effect of curing parameters on CO2 curing of concrete blocks containing recycled aggregates

In this study, a CO₂ curing process was adopted in order to promote rapid strength development of concrete blocks containing recycled aggregates. The influence of several factors associated with the curing conditions on the curing degree and compressive strength of the concrete blocks were investigated, including curing time, temperature, relative humidity, pressure and post-water curing after the pressurized CO₂ curing (PCC) process. In addition a flow-through CO₂ curing (FCC) method at ambient pressure was also used. The results of the PCC experiments showed that, considerable curing degree and compressive strength were attained during the first 2 h of CO₂ curing, and a prolonged curing time yielded slower gains. The variations of temperature from 20 °C to 80 °C and relative humidity from 50% to 80% had limited impacts on PCC; but the effects of CO₂ gas pressure on the curing degree and compressive strength were more pronounced. The post-water curing after pressurized CO₂ curing allowed the concrete blocks to attain further strength gain but its effectiveness was inversely proportional to the CO₂ curing degree already attained. The FCC experimental results indicated that although a lower curing degree and slower strength development at the early age were observed, after 24 h of curing duration, they were comparable to those obtained by the PCC method. To assess the thermal stability of the concrete blocks, the optimum CO₂ curing regime was adopted for preparing the concrete blocks with recycled aggregates, and the CO₂ cured specimens exhibited better fire resistance than the water-cured ones at 800 °C.     

Influence of curing schedule on the integrity of geopolymers

The curing at ambient and controlled relative humidity (RH) with mild heating (40–60°C) of a metakaolinite-based geopolymer of molar ratios Si/Al and Na/Al of 2 and 1 respectively was studied. To obtain these geopolymers in a crack-free state, rapid drying during curing should be avoided. Curing at a lower RH (e.g. 30%) is preferable to that at a higher RH (e.g. 70%). Curing in an RH oven does not offer any advantage over curing at ambient followed by mild heating (40–60°C) in sealed containers. The compositions of the geopolymers were then varied somewhat to see the effect on open porosity for the same curing schedule. The compositions with Si/Al = 1.86 and Na/Al < 0.8 had an increased tendency to crack, probably due to the larger water loss during curing. The lowest open porosity of <1% was obtained for a geopolymer of composition Si/Al = 2.14 and Na/Al = 0.87, but this developed some cracks after 1 year. However, the geopolymer of composition, Si/Al = 2, Na/Al = 1, on which most of the work was carried out when cured correctly did not crack even after 1 year.     

Thermal regimes of relativistic magnetron oscillators

Experiments with relativistic magnetron oscillators (RMOs) using high-current electron accelerators allowed a level of output power from several hundred megawatts to several gigawatts to be reached at an efficiency of 10–30%. The operation principle of RMOs is identical to that of classical devices, but the determination of working regimes and calculation of output characteristics require taking into account relativistic corrections. In addition, a high cathode-anode voltage and high current in RMOs lead to some specific effects related to the damage of anode units as a result of thermal shocks. RMO operation in a periodic pulsed (pulse train) regime, which is of the most practical interest, imposes stringent requirements on the working life of anode units. The principles of choice of the material and design for the anode units of RMOs are formulated. Calculations of the limiting thermal regimes of RMO operation depending on the electron beam power flux density and the off/duty ratio of heating pulses are presented. Methods of increasing the working life of anode units are proposed.     

Thermal regimes of anisotropic-thermoelement thermal converters

A computational procedure is developed for the thermal regime of anisotropic-thermoelement thermal converters of cavity and plane types used to investigate heat fluxes and energy parameters of radiation. Heat losses from different elements of the thermal-converter structure are analyzed and the parameters of a battery of anisotropic thermoelements are optimized, providing maximum sensitivity at minimum measurement errors. The results obtained were used in developing a fundamentally new cavity-type filter pyrheliometer and commercially produced automatic absolute radiometers. Translated from Inzhenerno-Fisicheskii Zhurnal, Vol. 73, No. 2, pp. 447–453, March–April, 2000.     

环境试验设备实用检测技巧

本文主要讲述了环境试验箱检测中接线的改进及低气压的密封技术.     

A small-strain model to simulate the curing of thermosets

This contribution presents a newly developed phenomenological model to describe the curing process of thermosets undergoing small strain deformations. The governing equations are derived from a number of physical and chemical presuppositions and details of the numerical implementation within the finite element method are given. The curing of thermosets is a very complex process involving a series of chemical reactions which result in the conversion of liquid low molecular weight monomer mixtures into highly cross-linked solid macromolecular structures. This phase transition from a viscous fluid to a viscoelastic solid can be modelled by a constitutive relation which is based on a temporal evolution of shear modulus and relaxation time. Some numerical examples demonstrate the capability of the model to correctly represent the evolution of elastic and inelastic material properties as well as the volume shrinkage taking place during the curing process.