Effect of curing methods on strength and durability of concrete under hot weather conditions

This paper reports the results of a research study conducted to evaluate the effect of curing methods on the mechanical properties of ordinary Portland cement (OPC) and Silica Fume Cement (SFC) concretes. Slab and beam specimens were prepared and cured by covering them with wet burlap or applying a curing compound under field conditions. Four types of curing compounds, namely water-, acrylic-, and bitumen-based and coal tar epoxy, were applied on the concrete specimens. The curing compounds were applied immediately after casting or after an initial period of burlap curing. The effect of the selected curing regime on the properties of OPC and SFC concrete specimens was evaluated by measuring compressive strength, water-absorption and chloride permeability. The strength and durability characteristics of both OPC and SFC concrete specimens cured by applying the selected curing compounds were similar or better than that of concrete specimens cured by covering with wet burlap. Though no significant change in strength could be noted due to the curing methodology; however, its effect was noticeable on the durability. The best performance was shown by concrete specimens cured by applying the bitumen-based curing compound followed by those cured by applying coal tar epoxy, acrylic-based or water-based curing compound. The initial period of water curing, prior to the application of the curing compound, was also noted to be beneficial in increasing the durability of concrete.     

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.     

Comprehensive study of concrete creep,shrinkage, and water content evolution under sealed and drying conditions

A comprehensive study of the time‐dependent behaviour of concrete, enhanced by measuring the evolution of the respective mass water content, is presented. Compressive creep as well as shrinkage are investigated on sealed and unsealed specimens in terms of the concrete age at loading of 2, 7, and 28 days, respectively, at a loading level of 30% of the compressive strength at loading. In addition, on the basis of the collected measurement data, the impact of load application on the moisture content is studied and the Pickett effect is re‐examined. The obtained set of material data consists of basic and drying creep strains for different concrete ages at loading, autogenous shrinkage strains as well as combined autogenous and drying shrinkage strains, the evolution of the mass water content, the water desorption isotherm, and the time‐dependent evolution of Young's modulus and the compressive strength. It provides a consistent set of material data for a particular concrete grade and will be available for calibrating and validating concrete models.     

Deformation behavior of cement treated demolition waste with recycled masonry and concrete subjected to drying and temperature change

Cement treated materials are widely used as road bases in pavements. Shrinkage of these materials due to moisture and temperature changes is a critical issue for determining shrinkage cracking in pavements. This paper presents the influence of four mixture variables (masonry content, cement content, water content and degree of compaction) on drying shrinkage and coefficient of thermal expansion (CTE) of cement treated demolition waste with recycled masonry and concrete (CTM_iG_r). The experimental results showed that the masonry content was the dominating factor affecting dry shrinkage and CTE of CTM_iG_r. Increasing the masonry content can not only lead to an obvious decrease of dry shrinkage of CTM_iG_r, but also a low CTE level. Dry shrinkage of CTM_iG_r increased as the increase of cement content as well as degree of compaction and water content. The CTE of CTM_iG_r was between 7.58 × 10⁻⁶/°C and 10.22 × 10⁻⁶/°C, which was mainly determined by the masonry and cement content.     

Specific surface area of aggregate and its relation to concrete drying shrinkage

This paper deals with the influence of aggregate properties on the shrinkage of concrete during drying. The drying shrinkage strains of concretes with various types of aggregates were measured and their influences on the fundamental properties of the different types of aggregates were investigated. Furthermore, the specific surface areas (SSAs) of aggregates were obtained by the BET method using both nitrogen (N₂) and water vapour (H₂O). The SSAs determined by using H₂O exhibited higher values than those by using N₂. The drying shrinkage strains of concretes increased with the H₂O SSAs of the aggregates used. Our results suggest that the SSA determined by using H₂O is an effective index for evaluating the influence of the aggregate type on the drying shrinkage of concrete.     

Incorporating phase change materials in concrete pavement to melt snow and ice

This paper discusses the use of phase change materials (PCM) in concrete pavement as a method to store energy which can be used as a heat source during cooling events to melt ice/snow. The experimental program includes: (1) use of low-temperature differential scanning calorimetry to evaluate thermal properties of PCM, and (2) use of large-scale concrete slabs containing PCM to evaluate the ability of the PCM concrete to melt snow on the surface of the concrete pavement. The temperature in the concrete slabs and the snow melting rate were monitored as quantitative measurements of the efficiency of the PCM in the concrete. In addition, time-lapse images were taken. Two approaches were used to incorporate PCM in concrete: placing the PCM in lightweight aggregate (LWA) which was then mixed into the concrete, and placing the PCM in embedded metal pipes embedded in the slab during concrete casting. In this study, paraffin oil was use as a PCM that is effective in releasing heat near the freezing temperature of PCM when the PCM undergoes a phase transformation from liquid to solid. The heat released during the phase transformation can melt ice and snow on the concrete pavement surface. The results indicate that incorporating PCM in concrete pavement is not only feasible, but also practical.     

Influence of recycled glass content and curing conditions on the properties of self-compacting concrete after exposure to elevated temperatures

The aim of this paper is to assess the performance of self-compacting glass concrete (SCGC) after exposure to four elevated temperatures of 300 °C, 500 °C, 600 °C and 800 °C. The influence of curing conditions on the high temperature performance of SCGC was also investigated. For each curing regime, five SCGC mixtures were prepared with recycled glass (RG) which was used to replace natural fine aggregate at the level of 0%, 25%, 50%, 75% and 100%. After exposure to the elevated temperatures, concrete mass loss, density, water porosity, ultrasonic pulse velocity (UPV) and water sorptivity were determined and then a compressive strength test was conducted. The test results indicate that regardless of the exposure temperature, all the water cured specimens had higher residual strengths and mass losses while the water porosity and water sorptivity values were lower as compared to the corresponding air cured specimens. The incorporation of RG in the concrete mixes helped to maintain the concrete properties after the high temperature exposure due to the melting and resolidification of the recycled glass in the concrete matrix.     

A structural experimental technique to characterize the viscoelastic behavior of concrete under restrained deformations

An innovative variable restraint frame is proposed to characterize the viscoelastic behavior of concrete under tensile stresses induced by restraints to shrinkage deformations (mainly due to drying). Two concrete specimens with the same cross section are used, subjected to equal thermal and moisture conditions: one is made of plain concrete, to assess the “free” deformations due to shrinkage and temperature; the other is reinforced with two steel threaded rods, which induce a manually controlled axial restraint to shrinkage. The restrained specimen is installed on a reaction frame, being stretched in force control mode. The concrete and the rods are instrumented with strain gauges and temperature sensors, which allow separation of the different components of concrete strains with the aid of equations based on equilibrium and compatibility conditions. This permits identifying the elastic and tensile creep concrete strains, as well as the concrete tensile stresses induced by the restrained shrinkage. The device also allows assessing the concrete modulus of elasticity during the test and remains operational even upon concrete cracking, features of great interest for the intended material characterization.     

A comparison of the finite element and control volume numerical solution techniques applied to timber drying problems below the boiling point

Drying is a process which involves heat and mass transfer both inside the porous material, where a phase change in moisture occurs from the liquid to the gaseous state, and in the external boundary layer of the convected hot dry air, which heats the porous medium. The equations which govern this process consist of three tightly coupled, highly non-linear partial differential equations for the unknown system variables of moisture content, temperature and pressure. Due to the inherently complex boundary conditions and intricate physical geometries in any practical drying problem, an analytical solution is not possible. In order to obtain a transient drying solution it is necessary to resort to a numerical technique. The numerical solution techniques which were employed in this research were the finite element method and the control volume method. The transient numerical results were compared and contrasted for two timber drying problems, first, at a dry bulb temperature of 50°C, and secondly, at 80°C, both cases being below the boiling point of water.     

Determination of the kinetic curves of drying of blocks of adsorbents-dessicants with different cross sections of channels and of a granulated adsorbent on a change in the flow velocity

The kinetics of convective drying (regeneration) of different types of adsorbents-dessicants with different geometric parameters has been studied. It is shown that adsorbents of a “channel” (cellular) type have a long constant-rate stage of drying, whereas a granulated polydisperse adsorbent is mainly dried at a variable rate. Adsorbents having a maximum water yielding capacity have been revealed. It has been established that the velocity of a drying gas flow substantially influences the process of moisture extraction from all types of adsorbents. In order to describe the kinetic curves of the given phenomenon, a model of a relaxation kinetic equation that quite satisfactorily describes the kinetics of drying of the adsorbents studied was used. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 82, No. 1, pp. 65–68, January–February, 2009.     

Improvement compressive strength of concrete in different curing media by Al2O3 nanoparticles

In the present work, the effect of curing medium on microstructure together with physical, mechanical and thermal properties of concrete containing Al₂O₃ nanoparticles has been investigated. Portland cement was partially replaced by Al₂O₃ nanoparticles with the average particle size of 15 nm and the specimens were cured in water and saturated limewater for specific ages. The results indicate that Al₂O₃ nanoparticles up to maximum of 2.0% produces concrete with improved compressive strength and setting time when the specimens cured in saturated limewater. The optimum level of replacement for cured specimens in water is 1.0 wt%. Although the limewater reduces the strength of concrete without nanoparticles when it is compared with the specimens cured in water, curing the specimens bearing nanoparticles in saturated limewater results in more strengthening gel formation around Al₂O₃ nanoparticles causes more rapid setting time together with high strength. Accelerated peak appearance in conduction calorimetry tests, more weight loss in thermogravimetric analysis and more rapid appearance of peaks related to hydrated products in X-ray diffraction results, all indicate that Al₂O₃ nanoparticles could improve mechanical and physical properties of the specimens.     

Prediction of Interfacial Cracking due to Differential Drying Shrinkage of Concrete in Precast Shell Pier Cap

In a precast shell pier cap, cracking at the interface between the precast shell and the cast-in-place concrete may happen due to differences between the drying shrinkage of the inner and the outer concrete. The objective of this study is to establish a prediction method for interfacial cracking that will consider the real mechanism of differential drying shrinkage and creep. The main parameters used in the analysis were determined from experiments for a concrete mix that is applied to the manufacturing of pier caps. The variation of internal relative humidity over time was first calculated based on the nonlinear moisture diffusion; cracking analysis then followed. Prediction of the initiation of interface cracks and the increase of their width over time was performed. It was found that additional reinforcement across the interface is very effective at reducing crack width.     

The effects of Cr2O3 nanoparticles on strength assessments and water permeability of concrete in different curing media

In this study, the effect of limewater on strength assessments and percentage of water absorption of concrete incorporating Cr₂O₃ nanoparticles has been investigated. Portland cement was partially replaced by Cr₂O₃ nanoparticles with the average particle size of 15 nm and the specimens were cured in water and saturated limewater for specific ages. The results indicate that Cr₂O₃ nanoparticles up to 2.0 wt% could produce concrete with improved strength and water permeability when the specimens cured in saturated limewater while this content is 1.0 wt% for the specimens cured in tap water. Although the limewater reduces the strength of concrete without nanoparticles when compared with the specimens cured in water, curing the specimens bearing Cr₂O₃ nanoparticles in saturated limewater results in more strengthening gel formation around nanoparticles causing improved permeability together with high strength. In addition, Cr₂O₃ nanoparticles are able to act as nanofillers and recover the pore structure of the specimens by decreasing harmful pores. Accelerated peak appearance in conduction calorimetry tests, more weight loss in thermogravimetric analysis and more rapid appearance of peaks related to hydrated products in X-ray diffraction results, all indicate that Cr₂O₃ nanoparticles could improve mechanical and physical properties of the specimens.     

电子封装用环氧树脂固化温度与应变的三维有限元模拟

环氧树脂因具有许多优异的性能而被广泛用作电子封装材料,然而环氧树脂在固化过程中产生的内应力会对封装产品的性能产生严重影响。针对一种用于电子封装的环氧树脂,通过实验分析了其固化动力学、密度、导热系数、玻璃化转变温度、弹性模量、化学收缩应变和热应变等性能参数,建立了固化过程中的数学模型。通过ABAQUS建立三维有限元模型,采用顺序耦合分析方式,分步进行传热分析和应力应变分析,模拟环氧树脂固化过程中的温度场、固化度场和应力应变场。最后采用光纤布拉格光栅（FBG）监测环氧树脂在固化过程中内部的温度和应变变化,并与模拟进行对比,结果表明本文所建立的有限元模型具有较高的可靠性。      

Optimization of a headspace solid-phase microextraction-gas chromatography–mass spectrometry procedure for odor compounds from polyolefin resin used in plastic food packaging

This research investigated the best headspace solid-phase microextraction (HS-SPME) extraction conditions and used the response surface Box–Behnken trial design to optimize the parameters for the analysis of volatile compounds in polyolefin resin, which is made from powdered polyolefin resin and additives by heating processing. The results showed that when the extraction temperature was 64°C, the extraction time was 34 min, the sample weight was 2.0 g, the testing matrix was air, and the fiber type was 50/30 μm DVB/CAR on PDMS, and the number of volatile compounds in the resin pellets was the highest. The performance characteristics of the optimized method were also determined, and they showed excellent linear ranges, recovery, repeatability, detection, and quantification limits. In the resin pellets and powdered polyolefin resin, the alkane content was the highest, at 585.346 and 581.789 mg/kg, respectively. In the additives, the benzene content was the highest at 39.495 mg/kg. Compounds with odor activity values (OAVs, ratio of concentration to odor threshold) above 1.0 are considered key aroma-active compounds. In total, 135 volatile compounds were identified, and 19 of them had an OAV above 1.0, and 4-methyl-octane and o-xylene were identified as the key odorants in the resin pellets. Only one odorant, 4-methyl-octane, was detected in the powdered polyolefin resin. In the additives, 18 volatile compounds were identified as key aroma-active compounds, such as octanal, nonanal, hexanol, octanol, heptanol, and decanol. The significance of this research was to furnish data to support the further study of odor abatement from food packaging materials.     

Application of 3D-DIC to characterize the effect of aggregate size and volume on non-uniform shrinkage strain distribution in concrete

To elucidate the effect of aggregate size and volume on the non-uniform strain distribution in concrete, drying shrinkage of mortar and concretes were determined with 3D digital image correlation (3D-DIC). The distribution of shrinkage displacements and strains in mortar and concrete were analyzed. The results show that 3D-DIC makes it possible to measure non-uniform displacement distributions initiated by shrinkage in mortar and concrete. The non-uniformity became more remarkable with drying time. The presence of aggregates larger than 5 mm in concrete have locally changed the displacement and strain fields. Aggregates within 5–25 mm make non-uniform strain of concrete more fluctuant, especially when the aggregate size is larger than 10 mm. The maximum and minimum principal strain distributions became more heterogeneous with decreasing volume of aggregates.     

Fractional rate of change of swim-bladder volume is reliably related to absolute depth during vertical displacements in teleost fish

Fish must orient in three dimensions as they navigate through space, but it is unknown whether they are assisted by a sense of depth. In principle, depth can be estimated directly from hydrostatic pressure, but although teleost fish are exquisitely sensitive to changes in pressure, they appear unable to measure absolute pressure. Teleosts sense changes in pressure via changes in the volume of their gas-filled swim-bladder, but because the amount of gas it contains is varied to regulate buoyancy, this cannot act as a long-term steady reference for inferring absolute pressure. In consequence, it is generally thought that teleosts are unable to sense depth using hydrostatic pressure. Here, we overturn this received wisdom by showing from a theoretical physical perspective that absolute depth could be estimated during fast, steady vertical displacements by combining a measurement of vertical speed with a measurement of the fractional rate of change of swim-bladder volume. This mechanism works even if the amount of gas in the swim-bladder varies, provided that this variation occurs over much longer time scales than changes in volume during displacements. There is therefore no a priori physical justification for assuming that teleost fish cannot sense absolute depth by using hydrostatic pressure cues.     

三个国标中甲醛测定方法的比较和探讨

对GB 18582-2001、GB 18583-2001、GB/T 15516-1995三个国家标准中甲醛含量测定方法进行了比较,指出了其异同点,并提出了笔者对标准的一些理解和看法。     

Quantification of evolving moisture profiles in concrete samples subjected to temperature gradient by means of rapid neutron tomography: Influence of boundary conditions,hygro-thermal loading history and spalling mitigation additives

Concrete has a tendency to spall, that is, to eject layers when subjected to high temperatures. This is an erratic phenomenon, and our understanding of the underlying physical process is still limited. A driving process is moisture transfer, whose experimental investigation has so far mostly been limited to macroscopic or point-wise observations, limiting both our understanding and the validation of the proposed models. In this paper, a non-contact technique, neutron imaging, is used to extract a the full-field distribution of moisture in 3D and in real time, while the concrete is heated at high temperatures. This reveals a number of processes often underestimated or ignored in the traditional experimental approaches reported in the literature. Notably, the effect on the evolving moisture profiles of varying heating rates for multiple insulation techniques as well the strong influence of the addition of spalling-mitigating additives is presented. The first ever example of neutron tomography of a spalled sample is also reported, and some preliminary analyses of the effect that moisture clog formation and heating rates have on it are revealed.     

Maximum likelihood estimation of change point from stationary to nonstationary in autoregressive models using dynamic linear model

Change point estimation is a useful concept in time series models that could be applied in several fields such as financing, quality control. It helps to decrease costs of decision making and production by monitoring stock market and production lines, respectively. In this paper, the maximum likelihood technique is developed to estimate change point at which the stationary AR(1) model changes to a nonstationary process. Filtering and smoothing of dynamic linear model are used to estimate unknown parameters after change point. We also assume that correlation exists between samples' statistics. Simulation results show the effectiveness of the proposed estimators to estimate the change point of stationary. In addition based on Shewhart control chart, filtering has a better accuracy in comparison to smoothing. A real example is provided to illustrate the application.