Influence of bonding on the tensile creep behavior of paper in a cyclic humidity environment

It has been shown, as paper structure is improved through increased bonding (by increasing relative bonded area or specific bond strength), a fully efficient loaded structure can be achieved. Once fully efficient, further improvements in bonding become redundant and have no effect on some paper deformation behaviors; deformation is dictated only by the characteristics of the fibers. Although previous work had shown this was true for elastic modulus and short time duration stress-strain behavior, it has only recently been shown to be true for constant humidity tensile creep behavior. In this study, the goal was to ascertain if cyclic humidity tensile creep behavior (known as accelerated creep) would follow the same trend. To accomplish this, sheets were made at differing levels of relative bonded area and specific bond strength. This was done by applying two different wet pressing levels (to alter relative bonded area) and using bonder and debonder (to change specific bond strength). It was found that accelerated creep behavior of paper sheets is no different than constant humidity creep behavior; changing bonding does not influence accelerated creep if the sheet has a fully efficient loaded structure. If the sheet structure is inefficiently loaded (there is no redundancy in bonding), accelerated creep will be affected by bonding. However, it is proposed that the only reason accelerated creep is influenced by bonding when inefficiently loaded is because constant humidity creep behavior determines the accelerated behavior and it is influenced, in this case, by bonding.     

Early-age stress analysis of a concrete diaphragm wall through tensile creep modeling

This paper reports a recent large-scale experimental investigation on early-age stress evolution in a deep underground concrete diaphragm wall. To evaluate the early-age stress induced by hydration temperature rise, autogeneous shrinkage and reinforcement restraint, both laboratory tests and in situ large-scale model wall test are performed. The laboratory tests include concrete adiabatic temperature rise, autogeneous shrinkage and restraint test. The in situ model wall simulates continuous and sliding design options for the external and inner layers with thermal and strain sensors installed in the inner layer. The restraint test results are interpreted via tensile creep modeling and an algorithm is conceived to calibrate the concrete tensile creep law. With the identified creep law, a thermomechanical analysis is performed on the model wall to calculate the concrete temperature and stress evolution at early age. The identified tensile creep law is furthermore validated by the numerical results and in situ measurements. Furthermore, the early-age stress analysis is performed on the full-scale diaphragm wall. Comments on the concrete tensile creep law and the diaphragm wall design option are given in the end.     

Tensile basic creep versus compressive basic creep at early ages: comparison between normal strength concrete and a very high strength fibre reinforced concrete

The paper presents experimental results concerning the comparison of tensile and compressive basic creep behaviours at early ages of two different concretes: a normal strength concrete (NSC) and a very high strength fibre reinforced concrete (HPFRC). This research project has been done in the context of a bilateral collaboration between Polytechnique Montreal and IFSTTAR. Observations on the HPFRC showed specific compressive creep similar to the specific tensile creep. Moreover, the specific creep curves obtained under compressive and tensile loading had always positive values, i.e. they were in same direction of the applied load on specimens. Measurements made on the NSC revealed specific compressive creep with positive values (in the loading direction). However, specific tensile creep presented negative values (opposite direction of loading) for a long period. A physical explanation based on the existence of two mechanisms with opposite effect is proposed to describe these basic creep results. The first mechanism is a coupling between the microcracking process and the water transfers that lead to additional self-drying shrinkage; the second mechanism is the self-healing of concrete induced by the microcracking.     

Experiments on concrete under uniaxial impact tensile loading

A problem of practical importance for designing of structural elements is discussed in this paper—the behaviour of concrete subjected to uniaxial impact tensile loading. The “Split Hopkinson Bar” technique was adopted for testing concrete in uniaxial tension at stress rates between 2 and 60 N/mm²/ms. A remarkable increase in tensile strength was observed due to high stress rate. The ratio of impact and static tensile strength varied between 1.33 and 2.34 for various concrete mixes. The influence of maximum aggregate size, water-cement ratio, cement content, cement type and quality, specimen humidity, static compressive strength and loading/casting direction upon the uniaxial impact tensile strength was studied. The high stress rate resulted in an increase of the modulus of elasticity of concrete in uniaxial tension. An explanation for the observed phenomena is suggested.     

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.     

Three-dimensional creep measurements in young concrete

Creep tests are reported in which sealed concrete specimens were loaded at the early age of 1 day and maintained under load for a further 61 days. Three systems of compressive stress were applied, uniaxial, equal biaxial and hydrostatic, all principal stresses being less than 50 percent of the uniaxial strength. It was found that Poisson's ratio for total strain (elastic+creep) remained sensibly constant throughout the test and was little affected by the system of loading. In general, the characteristics of creep and creep recovery under the multiaxial stress systems are the same as those observed in older concrete. A further test with stresses of 70–100 percent of the uniaxial strength resulted in increased unit strains, even under hydrostatic stress.     

Deformation behaviour of self-compacting concrete under tensile loading

The deformation behaviour of self-compacting concretes of the compressive strength classes C30/37, C45/55 and C60/75 according to the European Standard EN 206-1 under sustained tensile loading was investigated up to 2.5 years. The long-term tensile strength was estimated to be 69% of the short-term tensile strength, determined at an age of 28 days. Load-free shrinkage has been measured on companion specimens. The usual way to determine creep is to subtract the measured shrinkage strain and the elastic or initial strain from the measured total strain. In doing so, a phenomenon was discovered which is called stress-induced shrinkage. It turned out that the drying shrinkage was larger for loaded specimens than for load-free specimens. Similar results have been found earlier. It seemed that the stress-induced shrinkage tends to increase with decreasing compressive strength. An important practical consequence is arising from stress-induced shrinkage: structural elements subjected to tensile␣load, tend to dry and to shrink faster than one␣would expect based on the assumption of load-free shrinkage. In the case of sustained deformation, this would raise the risk of cracking and would have a negative effect on the durability of a concrete structure. An erratum to this article can be found at     

钢管混凝土轴心受压构件的徐变预测模型及其徐变性能分析

为研究混凝土徐变对钢管混凝土轴心受压构件长期受力性能的影响,考虑构件截面内力重分布,建立了钢管混凝土轴心受压构件截面应力和应变以徐变系数为参数的随混凝土龄期变化关系的理论模型,结合已有试验数据和国内外常用12种混凝土徐变预测模型对该模型进行验证,并找到了适用于钢管混凝土轴心受压构件的徐变预测模型--Huo模型;在此基础上,计算并分析了钢管混凝土轴心受压构件混凝土龄期为10000 d的截面应力和应变;通过对混凝土强度等级、环境年平均相对湿度、初始加载龄期、含钢率、构件长度、截面应力水平等因素的不同取值,分析了各因素对钢管混凝土轴心受压构件徐变性能的影响程度及规律。结果表明:当钢管混凝土轴心受压构件的轴力不大于其极限承载力的60%时,随着加载龄期的增长,钢管截面应力逐渐增大,最大变化量达61.4%,而混凝土截面应力逐渐减小,最大变化量达26.2%;加载初期构件应变增长迅速,1000 d以后应变增长速度减慢,构件最终应变是初始应变的1.61倍;在轴压比相同的条件下,钢管混凝土轴心受压构件的徐变应变终值随着混凝土强度等级的提高而逐渐增大,随着含钢率的增大显著减小,随着初始加载龄期、环境年平均相对湿度、构件长度的增大而逐渐减小,轴压比不大于0.6时,其徐变应变终值随轴压比增长。研究成果可为钢管混凝土轴心受压构件在正常使用阶段徐变计算以及徐变变形控制提供依据。     

Creep behavior and manufacturing parameters of wood flour filled polypropylene composites

The influence of wood flour content, coupling agent and stress loading level on the creep behavior of wood flour–polypropylene composites was investigated. Maleated polypropylene (MAPP; Epolene G-3003™) was used as the coupling agent to treat the wood flour used as reinforcing filler for polypropylene composite. The tensile strength and modulus of various wood flour–polypropylene composites (WPCs), manufactured using the melt blending, extrusion, and palletizing methods, were measured before performing the creep test. The residual tensile strength, creep strain, and fractional deflection of the resultant wood flour–polypropylene composites were measured by means of the creep test. It was shown that the tensile strength decreased with increasing wood flour level in the composites. The creep strain also decreased as the wood flour level increased. The presence of the coupling agent increased the tensile strength of the wood flour–polypropylene composites, compared with the specimens made of pure polypropylene. For those composites containing the coupling agent, the creep deflection was significantly lower than those made without any coupling agent. The creep strains of the WPC specimens observed during the creep test fitted perfectly with the four-element burger creep model. Further investigation is required of the effects of combined mechanical and environmental loading in varying proportions.     

Degree of hydration based prediction of early age basic creep and creep recovery of blended concrete

An accurate estimation of the early-age creep behavior is not only required to successfully control the early age cracking of concrete, but also to analyse the vertical and differential deformations of super high-rise buildings during construction. The fictitious degree of hydration model was developed to study basic creep behavior of hardening concrete, however, nowadays more complex binder systems are applied, consisting of several different types of powders, requiring further validation of the applicability of this creep model. The compressive basic creep and creep recovery of concrete based on ternary blends including Portland cement, blast furnace slag, and fly ash is experimentally studied. The tests are conducted at different ages of loading at early age under varying stress level. It is shown that the fictitious degree of hydration method can be successfully applied to ternary blends, even simplifying the hydration process to one overall reaction, considering only one degree of hydration.     

考虑不同应力水平影响的混凝土徐变预测模型修正

为开展混凝土徐变预测模型修正研究,对不同应力水平下的混凝土进行了徐变试验和数值模拟,并进行了相互印证。通过数值模拟,研究了拉压应力、周期应力、双轴应力下的徐变特性,针对徐变预测模型CEB/FIP 1990估算精度低的现状,基于不同应力水平下混凝土徐变特性,结合工程实际给出了便于运用的徐变系数修正值。研究结果表明,采用的数值模拟方法为混凝土徐变预测模型的修正提供了一条新思路。     

Short-term tensile creep and shrinkage of ultra-high performance concrete

The tensile creep and free shrinkage deformations of ultra-high performance concrete (UHPC) were examined through short-term testing to assess the influences of stress/strength ratio, steel fiber reinforcement, and thermal treatment. The use of fibers and the application of thermal treatment decreased 14-day drying shrinkage by more than 57% and by 82%, respectively. Increasing the stress-to-strength ratio from 40% to 60% increased the tensile creep coefficient by 44% and the specific creep by 11%, at 14 days of loading. Incorporating short steel fibers at 2% by volume decreased the tensile creep coefficient by 10% and the specific creep by 40%, at 14 days. Also, subjecting UHPC to a 48-h thermal treatment at 90 °C, after initial curing, decreased its tensile creep coefficient by 73% and the specific creep by 77% at 7 days, as compared to ordinarily cured companion mixes. Comparison of tensile creep behavior to published reports on compressive creep in UHPC reveal that these phenomena differ fundamentally and that further evaluation is necessary to better understand the underlying mechanisms of tensile creep in UHPC. Results from this study also showed that the effects of both thermal treatment and fiber reinforcement were more pronounced in tensile creep behavior than tensile strength results of different UHPC mixes. This emphasizes the importance of conducting tensile creep testing to predict long-term tensile performance.     

Time-dependent mechanisms in fracture of paper

A hypothesis for the time-dependent mechanisms underlying the mechanical behavior of commercial bond paper, a fibrous material, is proposed based on an examination of the fracture morphology. Machine-made bond paper has preferred directions due to the dominant fiber orientation, and the mechanisms differ depending on the orientation of the applied load with the fiber direction. Information is obtained from tensile increasing load and creep fracture tests on the relative role of intrafiber mechanisms and those occurring in the interfiber bonds. Mechanisms consistent with the data include fiber pullout and fiber swing initiated by high ambient humidity induced release of interfiber bond compressive residual stresses. In low relative humidity, intrafiber mechanisms apparently play a larger role. The mechanisms deduced are consistent with those previously hypothesized by the author as the cause of moisture-accelerated creep of paper.     

自密实混凝土拉压徐变比较试验研究

对32根梁式自密实混凝土试件进行三点弯曲持续加载,对测得梁体受拉区和受压区应变数据进行累加,得到自密实混凝土受拉徐变的发展特征曲线,并得到自密实混凝土梁拉压徐变比;研究了自密实混凝土养护龄期、水胶比和粉煤灰掺量对拉压徐变度比值的影响。通过试验结果的分析得到:无论是开放状态还是密封状态下,自密实混凝土拉伸徐变都略小于压缩徐变,除拉压基本徐变比随粉煤灰掺量的增大略微增加外,拉伸徐变与压缩徐变的比值随着养护龄期、水胶比和粉煤灰掺量的增大而减小;最后拟合得到拉压徐变比值的计算式,为工程应用提供一定的参考。     

Room temperature creep and its effect on flow stress in a X70 pipeline steel

This paper studied the phenomenon of room creep deformation and its effect on tensile property of a X70 pipeline steel under stress-control loading pattern using round tensile test specimen. Significant time-dependent deformation under constant load was observed in the steel at room temperature, and the deformation is not only dependent on loading stress rate but also dependent on the loading process. It is also found that the loading-unloading-reloading process reduces the subsequent creep strain, while the occurrence of room temperature creep obviously enhances the subsequent yielding strength and the flow stresses.     

UHPFRC tensile creep at early age

Ultra high performance fibre reinforced concrete (UHPFRC) early age viscoelastic behaviour under tension was investigated. The tests results showed a high creep potential due to the high volume paste (88%). This result is of major importance because the viscoelastic properties contribute to mitigating the high early age stresses generated under restrained shrinkage. This beneficial effect was reflected by the increased linear-relationship between tensile creep and shrinkage. As expected, UHPFRC tensile creep behaviour was also sensitive to the loading level. Above 35% of the tensile strength at the loading age, the material exhibited viscoplastic behaviour. A Maxwell chain model was applied to predict the early age UHPFRC tensile creep and confirms the induced non-linear response.

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Résumé  Le comportement viscoélastique en traction au jeune age du Béton Fibré Ultra Performant (BFUP) a été étudié. Les résultats d’essais ont montré que le matériau est caractérisé par un potentiel élevé de fluage attribué au grand volume de pate contenu dans le BFUP (88%). Ces résultats sont d’une importance majeure du fait que les propriétés viscoélastiques contribuent à atténuer les autocontraintes susceptibles de se produire au jeune age sous certaines conditions d’entrave. Cet effet bénéfique est reflété par la relation linéaire croissante entre le fluage en traction et le retrait. Comme attendu, le comportement viscoélastique du BFUP est sensible aux niveaux de sollicitation. Au-delà de 35% de la résistance à la traction à l’age de sollicitation, le matériau présente un comportement viscoplastique. Le modèle de Maxwell généralisé a été appliqué pour prédire le fluage en traction au jeune age, son application confirme la réponse non-linéaire observée.

     

Assessment of Pre‐Stress Losses in Instrumented Pre‐Stressed Concrete Beams Using Stochastic Analysis

Abstract: In this study, stochastic analyses of pre‐stress losses in two pre‐stressed concrete beams are carried out, and the results of the analyses are compared with those obtained from the laboratory experimental investigations. The deterministic models given in ACI 209 (with modelling error) are used for predicting the pre‐stress losses. The compressive strength of concrete, water–cement ratio, coarse aggregate–cement ratio, fine aggregate–cement ratio, cement content, initial stress in steel and relative humidity are considered as random variables. Monte Carlo simulation technique is used to determine the statistical properties of pre‐stress losses at different times. From the results obtained, it is noted that while the relaxation strain can be assumed to follow a normal distribution, the frequency distributions of creep and shrinkage strains are not uni‐modal. A relation for estimating total losses is also proposed in the article. The stochastic analysis approach presented in the article would be useful in the assessment of pre‐stress losses in existing pre‐stressed concrete members in a more rational way.     

等强度下混凝土组分对徐变性能的影响

采用自制的徐变加载装置,研究了聚乙烯醇(PVA)纤维、双掺粉煤灰和矿渣以及减缩剂对7d等强度混凝土徐变性能的影响规律,结合与混凝土同水胶比浆体的化合结合水量分析了其影响机理.结果表明,混凝土徐变系数发展较快,加载100d左右趋于稳定;减缩剂和双掺矿物掺合料均明显降低了混凝土的徐变系数,以掺减缩剂效果更好,450d值仅为...     

高强高性能混凝土三轴拉压压力学性能试验研究

采用大型动静真三轴伺服液压试验系统,对单轴压强度为90.6 MPa的高强高性能混凝土进行三轴拉压压等比例试验研究,获得了各应力比下试件的破坏模式、多轴强度及应力-应变曲线。试验结果表明:高强高性能混凝土三轴拉压压应力状态下的破坏为典型的受拉破坏;最大主应力方向的极限强度远低于其单轴压强度,中间主应力效应不明显;拉应力的存在对最大主应力方向应力-应变曲线影响十分明显,呈现出明显的线性特征。基于试验结果提出了适用于高强高性能混凝土的强度准则,为高强高性能混凝土本构关系的建立提供了试验和理论依据。     

无粘结预应力高性能粉煤灰混凝土桥梁收缩与徐变变形试验研究

结合洛湛线的建设,以8根不同掺量的高性能粉煤灰混凝土无粘结预应力桥梁的收缩、徐变试验为基础,研究了不同掺量高性能粉煤灰混凝土在荷载长期作用下收缩、徐变性能及其上拱随时间的变化规律,探讨了温度、湿度等环境因素对不同掺量高性能粉煤灰混凝土收缩、徐变的影响。300多天的实验观测结果表明:高性能粉煤灰掺量20%～40%混凝土梁不但具有良好的工作性能和力学性能,而且长期性能良好,与同强度的未掺高性能粉煤灰的梁相比,其后期强度和抗压弹性模量增大,收缩徐变减小,具有良好的社会和经济效益。