Compressive strength development of concrete with different curing time and temperature

In this experimental and analytic research, the strength development for various curing histories was investigated with particular regard to the influences of curing time points with given temperatures. For this purpose, four different points of curing time were considered with an individual interval of 24 h. Two different temperatures of 5°C and 40°C were applied for the selective intervals, whereas the rest period days were under the reference curing condition of 20°C. A new model for the strength prediction was suggested based on the rate constant model. In this model, the equivalent ages introduced in the Saul and Arrhenius models were modified to show the effects of curing temperature at different ages. Test results show that the concrete subjected to a high temperature at an early age attains higher early-age strength but eventually attains lower later-age strength. The concrete subjected to a low temperature at an early age leads to lower early-age strength but almost the same later-age strength. Moreover, the proposed model showed better agreement with the test results than the existing models.     

A kinetic study of the effect of three catalytic systems on the curing of an unsaturated polyester resin

We studied by differential scanning calorimetry (DSC) the influence of three tertiary amines used as promoters on the curing kinetics of unsaturated polyester resins catalyzed with an organic peroxide. The kinetic study was made by means of the analysis of isothermal experiments and we used a kinetic model that does not presuppose knowledge of the experimental rate equation f(α), which relates the reaction rate dα/dt at constant temperature with the degree of conversion α through a rate constant k according to dα/dt = kf(α). It is thus possible to predict the time, temperature, and degree of curing without needing to know f(α). This model makes a linear relation between the logarithm of time needed to reach a given degree of conversion with the inverse of the inverse of the curing temperature according to the expression ln t = A+E/RT for a constant α, where A is a constant, R is the universal gas constant, and E the activation energy. The proposed model has been used to calculate the activation energies for each degree of conversion according to the type and amount of promoter used. From the relation between the time and the curing temperature for a given conversion, it is possible to predict values of curing time for different temperatures. We thus simulated curing kinetics by using the proposed model and compared them with those obtained experimentally by DSC. © 1994 John Wiley & Sons, Inc.     

Early-age tensile properties of structural epoxy adhesives subjected to low-temperature curing

The early-age mechanical property development of structural adhesives during low temperature curing is critical for the outdoor construction of engineering structures, such as bridges or buildings. Construction of these structures is also carried out during winter time at low curing temperatures. Experimental investigations showed that the development of the tensile properties of a commercial structural epoxy adhesive strongly depended on the curing temperature. Lower curing temperatures significantly decelerate the process and consequently the rate of development of mechanical properties. At 0 °C, curing was inhibited or did not initiate at all. Tensile strength and stiffness developed at the same rate, although the former was slightly delayed compared to the latter. Significant development of the mechanical properties began only after the onset of material vitrification. This was in contrast to the development of the glass transition temperature, which increased particularly before vitrification. A proposed analytical model predicted the development of mechanical properties well, particularly under low isothermal and cyclic temperature conditions.     

Curing kinetics of epoxy‐urethane copolymers

The cure of the epoxy resin diglycidyl ether of bisphenol A (Araldyt GY9527) with a mixture of cycloaliphatic amines (Distraltec) was studied, and the focus was on the effect of the copolymerization with a commercial polyurethane (PU) elastomer (Desmocap 12). A simplified phenomenological model was proposed to represent the copolymerization reaction. It considered the effect of the temperature and the concentration of the elastomer on the reaction rate, and it was simple enough to be included in models of processing conditions. A nonlinear regression analysis of the experimental conversion data obtained from differential scanning calorimetry was utilized to find the best fitting parameters to Kamal's equation for the chemically controlled part of the reaction (short times) under isothermal and constant heating‐rate conditions. The Rabinowitch approach together with the Addam–Gibbs theory was utilized to introduce the effect of diffusion control at the end of the reaction on the overall constant for the reaction rate. The Di Benedetto equation was used to predict the conversion at which vitrification takes place for each run. Experimental results for conversions higher than this critical conversion were utilized to obtain information about the diffusion kinetic constant using a nonlinear regression analysis as previously. The overall model obtained was used to calculate a calorimetric conversion and reaction rate as functions of time, which was in excellent agreement with the experimental results. The addition of PU elastomers affected the values of the activation energies of the chemically and diffusion controlled parts of the reaction, as well as the final conversion reached by the epoxy–amine system. The proposed model allowed prediction of all the observed features using parameters that were independent of the temperature of the curing reaction. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 79: 1771–1779, 2001     

Effect of strain rate and temperature on the performance of epoxy mortar

The behavior of epoxy mortar was studied under various curing conditions, temperature and strain rate. The effect of aggregate size and distribution on the mechanical properties of epoxy mortar was also studied. Epoxy mortar with a uniform fine sand was cured at various temperatures to determine the optimum curing condition. The strain rate was varied between 0.01 to 6 percent strain per minute and the testing temperature between 22°C and 80°C. The strength, modulus, and compressive strain-strain relationship of polymer mortar are influenced by the curing method, testing temperature, and strain rate to varying degrees. The influence of test variables on the mechanical properties of epoxy mortar are quantified. Compared to the uniformly graded fine aggregate fillers the gap-graded aggregates produced polymer mortar with better mechanical properties. The compressive modulus and splitting tensile strength of epoxy mortar are related to their compressive strength. A new nonlinear constitutive model is proposed to predict the complete compressive stress-strain behavior of epoxy mortar. The constitutive relationship parameters are also related to the testing temperature and logarithmic strain rate.     

固化温度对缓凝黏合剂固化时间和强度的影响

为研究固化温度对缓凝黏合剂固化时间和强度的影响,将缓凝黏合剂在固化温度分别为25、45、65、85 ℃的条件下养护。通过邵氏硬度试验研究了3种缓凝黏合剂在不同固化温度条件下的固化速率,测试了缓凝黏合剂在不同固化温度条件下的拉伸剪切强度、抗折强度和抗压强度。结果表明,固化温度越高,缓凝黏合剂固化时间越短,以25 ℃为基准,45 ℃条件下的固化时间缩短约50 %,45 ℃以上,温度每升高20 ℃,固化时间缩短约10 %;以25 ℃条件下的强度为基准,缓凝黏合剂在不同固化温度条件下拉伸剪切强度的变化率在6 %内,抗折强度的增加率在6 %~45 %,抗压强度的增加率在9 %~50 %。     

铝土矿选尾矿制备碱激发胶凝材料的性能

以煅烧铝土矿选尾矿、水玻璃为主要原料制备碱激发胶凝材料,研究了其在不同养护条件下的强度发展规律及耐热性能。结果表明:虽然提高养护温度可显著提高砂浆强度,但长时间高温养护会引起强度倒缩;封闭恒温潮湿空气养护可使砂浆强度稳定发展,但开放条件下养护因失水而使强度大幅度降低;在400～500℃的高温下灼烧1h,其强度没有明显下降。     

Long‐term development of thermophysical and mechanical properties of cold‐curing structural adhesives due to post‐curing

The long‐term changes in the thermophysical and mechanical properties of a cold‐curing structural epoxy adhesive were investigated by accelerating the curing reaction by post‐curing at elevated temperatures. Experimental data concerning the glass transition temperature for periods of up to 7 years and tensile strength and stiffness measurements could be extrapolated for a period of up to 17 years. An existing model for the long‐term development of concrete properties was modified for the prediction of the long‐term mechanical properties of adhesives. The applicability of the acceleration procedure and the new model was confirmed by several verification procedures. Structural adhesives exhibit significant increases in glass transition temperature, strength and stiffness over the long term provided that joints are adequately sealed and protected from humidity and UV radiation. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

电伴热预养护条件对混凝土受冻临界强度的影响

电伴热预养护是一种保证预拌混凝土冬期施工养护温度和强度增长的简单高效的方法。本文采用7 d恒负温(-5 ℃、-10 ℃、-15 ℃)一次冻结转标准养护的试验,研究电伴热预养护不同温度和时间对一种高坍落度C30普通混凝土抗压强度的影响。依据混凝土受冻临界强度的定义,确定基于电伴热预养护条件下的混凝土受冻临界强度值及其合理的预养护时间。结果表明:与标准养护相比,经电伴热高温预养护的混凝土抗压强度均得到了提高,但电伴热预养护温度宜控制在30 ℃,较高的预养护温度下强度发展速率和R_-7+28(负温养护7 d再转标养28 d的抗压强度)值反而降低;当预养护温度为30 ℃,硬化温度不低于-15 ℃时,合理的预养护时间在36～48 h之间;恒负温(-5 ℃、-10 ℃、-15 ℃)硬化条件下,采用电伴热预养护的混凝土受冻临界强度的范围是6.6～17.8 MPa,为混凝土立方体抗压强度标准值的22.0%～59.3%。研究旨在比较电伴热预养护制度对普通混凝土力学性能的影响,进而指导相关工程应用。     

Resin‐transfer molding of natural fiber–reinforced plastic. I. Kinetic study of an unsaturated polyester resin containing an inhibitor and various promoters

In this study an autocatalytic model was used to describe the cure of a polyester system containing various promoters and an inhibitor. The effect of the initiator concentration was investigated. Isothermal DSC measurements were used to determine the kinetic parameters for the curing reaction. The rate of curing increased with increasing initiator concentration. The parameters were found to be temperature dependent. The nonlinear regression analysis showed that by fixing one parameter at a constant value the temperature dependency of the other parameters was described by simple relationships. The model was then compared to the experimental data. The reaction rate could be predicted fairly well in a wide range of temperatures. These results will be used to model the cure of this resin in a resin transfer‐molding (RTM) process. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 2553–2561, 2003     

Cure Kinetic of Poly (alkyltetrasulfide) Using a Rheological Method

The kinetics of the curing of poly (alkyltetrasulfide) was investigated rheologically by evaluating the viscoelastic material functions such as elastic storage modulus, and viscous loss modulus, during the curing process, isothermally. The isothermal kinetic reaction was described using three phenomenological equation based on the Kamal and Sourour model, gel times at various temperatures and a kinetic model developed by Hsich and co-workers. The rate of the reaction was found to be second order. The temperature dependence of the cross-linking rate constant was described by an Arrhenius plot with an apparent activation energy equal to 34 kJ/mol, using the temperature dependence of gel time, which is in reasonable agreement with the value obtained from the Kamal and Sourour and the Hsich models.     

NR硫化返原过程的动力学研究

采用依据ＮＲ硫黄硫化返原机理建立起来的硫化返原动力学模型,对普通硫黄硫化体系和半有效硫化体系的ＮＲ在不同温度下的硫化曲线进行了计算机非线性拟合,从而求出了各步反庆的速率常数,并进上步得到多硫键、双硫键和单硫键密度之和及交联密度随硫化时间变化的关系。结果表明：不同温度下两种体系采用该模型得到的模拟曲线与实际数据相吻合,求出的同一体系速率常数与硫化温度的关系符合Ａｒｒｈｅｎｉｕｓ方程。两种体系多硫键的     

A variable reaction order model for prediction of curing kinetics of thermosetting polymers

Based on the study of the curing reaction of an fluorinated aliphatic cyanate ester resin using an isothermal Differential Scanning Calorimetric (DSC) method, a new kinetics model with variable reaction order was proposed to describe the curing of thermosetting resins in both the chemical controlled and diffusion controlled regions. α-T_g relationship was used to calculate the diffusion controlled reaction rate constant during cure. Then the chemical and diffusion rate constants were combined in the Simon-Gillham equation to model the overall rate constant for the entire reaction. By using the overall rate constant combined with the variable reaction order, a simple model can be built up for prediction of curing kinetics of thermosetting polymer in both the chemical and the diffusion controlled regions. The new model is simple, easy to use and has very good agreement with the experimental results.     

The effect of excessive steam curing on Portland composite cement concrete

Steam curing at atmospheric pressure is an important technique for obtaining high early strength values in precast concrete production. Cement type, as well as curing period and temperature, is an important parameter in the steam-curing process. PC42.5 is the type of cement that is most commonly used in Turkish precast concrete plants. Its behavior is well known. Nowadays, the production of composite cements is becoming more popular every other day due to its advantages. The object of this study was to determine the properties of this relatively new binder comparatively with conventional PC42.5 under steam curing. For this purpose, 15-cm concrete cubes were prepared with a water/cement ratio (W/C) of 0.44 and were subjected to steam curing for five different curing periods of 4, 8, 16, 24 and 36 h under curing temperatures of 65 and 85 °C. Cement dosage was kept constant (400 kg/m³) for all specimens. The variation of compressive strength values and maturity for each condition has been presented comparatively within this study. Test results indicated that Portland composite cement (PKC/A42.5) can be used in place of PC42.5 for steam curing at atmospheric pressure in precast concrete production. However, in case of early high strength demand for early demolding purposes, curing temperature should be increased to 85 °C for PKC/A42.5 cement concretes.     

Effect of strength and age on the stress-strain curves of concrete specimens

Many researchers have rigorously studied the nonlinear behavior of the stress-strain relationship of concrete using mathematical curves. Currently, most empirical expressions for the stress-strain relationship, however, have been focused on hardened concrete and are unable to completely represent the behavior of concrete at an early age. A broad understanding of the behavior of concrete from early age to old age is important in evaluating the durability and service life of concrete structures.In this paper, the effect of five different strength levels and various ages from 12 h to 28 days on the compressive stress-strain curve was observed experimentally and analytically. Tests were carried out on ∅ 100×200 mm cylindrical specimens cured in a standard moist room at a temperature of 20±3 °C. An analytical expression of the stress-strain curve with strength and age was developed using regression analyses on experimental results. For verification of the proposed model equation, the equation was compared to the experimental data and existing model equations.The result shows that the proposed model equation is not only compatible with the experimental data but also describes satisfactorily the effect of strength and age on the stress-strain curve.     

New photoimageable dielectric insulating copolyester thin films: Synthesis and characterization

In this article, we describe the synthesis and characterization of a new family of photoimageable dielectric insulating polymer films. Four different photoimageable thin films have been prepared from all-aromatic and aromatic/aliphatic copolyesters, which exhibit good photospeed (10–180 s, 15.5 mW/cm² intensity), resolution and line width (10 μm), thermal stability (330–400°C), adhesion on different substrates, mechanical strength, and reasonable glass transition temperature (120–150°C). One feature of the new photoimageable copolyester is the formation of a low dielectric constant film (2.5 at 1 kHz, 25°C) upon curing at temperatures up to 280°C. The low dielectric constant is a result of foaming arising from evolution of by-products during curing. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 63: 1199–1211, 1997     

Effects of TMPTMA and silane on the compressive strength of low‐temperature cured acrylic polymer concrete

This study addresses the effects of additives on the compressive strength of low‐temperature cured acrylic polymer concrete (PC). Three curing temperatures (0°C, ?10°C, and ?20°C) and five ages (6, 12, 24, 72, and 168 h) with two different types of additives [trimethylolpropane trimethacrylate (TMPTMA) and silane] were investigated. As a result, the compressive strength tended to decrease as the curing temperature decreased. The compressive strengths at 24 h were approximately 90% of those at 168 h at both curing temperatures of 0°C and ?20°C, indicating that the rate of early age strength development was quite high even at a very low curing temperature range. The results of two‐way variance analysis revealed that silane had a greater impact on the compressive strength than TMPTMA. About 13%–23% strength improvements with a 168‐h compressive strength of over 80 MPa could be obtained at ?20°C by adding silane. Furthermore, this study proposed optimum mixture proportions of acrylic PC that generate a working life of 50–70 minutes with a compressive strength of 80 MPa at subzero temperatures. The findings of this study are expected to be effectively used in field applications of acrylic PC, especially in the cold regions during winter season. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40939.     

Enhanced thermodynamic analysis coupled with temperature-dependent microstructures of cement hydrates

This paper describes a computational method for simulating concrete performance. The method is enhanced to take into account the influence of the curing temperature history on the early-age development process. In order to investigate the effect of temperature on hydration, micro-pore structure development, and moisture profile in concrete, the authors carried out systematic sensitivity analyses. These analyses were used to create a numerical model that takes into account the temperature-dependent intrinsic porosity of hydrates and the available space for hydrate precipitation. For example, at high curing temperatures, the intrinsic porosity decreases and the available space for hydration increases. Experiments verified that the proposed method accurately predicts hydration processes, microstructure formation, and relative humidity for different mix proportions under various temperatures. Compared with the conventional model, the new model offers greater overall computational accuracy for low W/C and high temperature curing.     

Residual stress-strain relationship for concrete after exposure to high temperatures

An experimental research is performed on the complete compressive stress-strain relationship for concrete after heating to temperatures of 100-800 °C. All concrete specimens are ?15 cm × 30 cm standard cylinders, made with siliceous aggregate. The heated specimens are tested at 1 month after they are cooled to room temperature. From the results of 108 specimens with two original unheated strengths, a single equation for the complete stress-strain curves of heated concrete is developed to consider the shape varying with temperature. Through the regression analysis, the relationships of the mechanical properties with temperature are proposed to fit the test results, including the residual compressive strength, peak strain and elastic modulus. Compared with the experimental curves, the proposed equation is shown to be applicable to unheated and heated concrete for different temperatures. In addition, the split-cylinder tests of 54 specimens are also carried out to study the relationship of splitting tensile strength with temperature.     

养护温度对UHPC水化及力学性能影响研究

热养护能够提高超高性能混凝土(UHPC)的早期力学性能,促使UHPC水化,但对UHPC长期性能的发展尚未有系统深入的研究。通过力学性能测试、SEM和XRD研究了五种养护温度(20 ℃标养、90 ℃蒸汽养护、120 ℃干热养护、200 ℃干热养护和250 ℃干热养护)对UHPC长期力学性能发展以及水化的影响。结果表明,热养护能够促进UHPC早期水化,显著提升UHPC的早期抗压强度,特别是养护温度超过200 ℃时,能够生成明显的硬硅钙石提高UHPC的早期抗压强度。不同养护温度对于UHPC后期抗压强度的影响则不尽相同,90 ℃蒸养和120 ℃干热养护后UHPC试件的后期抗压强度仍然能持续增长,而200 ℃和250 ℃干热养护后试件的后期抗压强度则出现显著降低的趋势,后期抗压强度的变化主要取决于UHPC早期水化后内部剩余的未水化颗粒的含量。