Predicting the tensile creep of concrete

Over a four year period, six phases of testing were performed to observe the influence of age at loading, applied stress level, mix composition and relative humidity on the tensile creep of concrete. From these investigations it was possible to develop a model which allowed the prediction of tensile creep based on a knowledge of the compressive strength of the concrete (determined at the age of loading), the applied stress level and the relative humidity. Subsequently, this model was validated using the results from three independent investigations. Compressive creep as well as tensile creep was also obtained. This allowed a comparison of compressive creep with tensile creep and illustrated that on the basis of equal stresses, tensile creep is on average between 2 and 3 times greater than compressive creep (the maximum ratio is in excess of 8). For this investigation, however, on the basis of stress/strength ratio the difference between tensile and compressive creep is less significant. Considering a simply supported flexural reinforced concrete element, the investigation suggests that it is unwise to consider actual compressive creep equal to actual tensile creep as is often the case in design practice.     

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.     

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.     

Comparison between high strength concrete and normal strength concrete subjected to high temperature

Two normal strength concretes and three high strength concretes, with 28-day compressive strengths of 28, 47, 76, 79 and 94 MPa respectively, were used to compare the effect of high temperatures on high strength concrete and normal strength concrete. After being heated to a series of maximum temperatures at 400, 600, 800, 1000 and 1200°C, and maintained for 1 hour, their compressive strengths and tensile splitting strengths were determined. The pore size distribution of hardened cement paste in high strength concrete and normal strength concrete was also investigated. Results show that high strength concrete lost its mechanical strength in a manner similar to or slightly better than that of NSC. The range between 400 and 800°C was critical to the strength loss of concrete with a large percentage of loss of strength. Microstructural study carried out revealed that high temperatures have a coarsening effect on the microstructure of both of high strength concrete and normal strength concrete.     

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.     

Microcracking of concrete under compression and its influence on tensile strength

A new parameter representing damage of concrete due to compressive stresses is investigated. Very important tensile strength losses are obtained after compressive loading over 40% of the compressive strength. Microcracking of concrete takes place under such load leading to tensile strength losses depending also on time under load, water content and type of aggregate. Under maximum compressive load a tensile strength reduction of about 50% is obtained.     

Modeling basic creep of concrete since setting time

Modeling the early age evolution of concrete properties is necessary to predict the early age behavior of structures. In case of restrained shrinkage or application of prestress load [1], creep plays an important role in the determination of the effective stress. The difficulty lies in the fact that the modeling of creep must be based on experimental data at early age and this data must be obtained automatically because the hardening process of the concrete takes place rapidly during the first hours and also the first days. This paper presents a new methodology to model basic creep in compression since setting. Two kinds of tests are used: classical loadings and repeated minute-scale-duration loadings. The classical test is used to characterize the creep function for one age at loading and the repeated minute-scale-duration loadings test is used to define two ageing factors for the creep function. A new model based on the physical mechanisms and the two ageing factors is presented. A comparison with the Model Code 2010 is done and an advanced way to consider ageing with the Model Code 2010 is presented.     

Size effect for normal strength concrete cylinders subjected to axial impact

This paper describes a multi-national collaborative study that included both numerical and experimental efforts carried out on geometrically similar normal strength concrete (NSC) cylinders of different sizes subjected to axial impact. A parallel study on high strength concrete (HSC), that was carried out earlier, was described in a previous paper (International Journal of Impact Engineering, Elsevier Ltd. 2003; 28: 1001–1016). This study investigated the size effect phenomenon for NSC cylinders under short duration time-dependent compressive loads. Unlike the study on HSC that used soft impact only, this study on NSC used both soft and hard impact. Results from the tests and simulations showed the existence of a size effect in NSC cylinders under both, hard and soft, impact loading. This time-dependent size effect is different from the known static size effect.     

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.     

Elastic moduli of a lean and a pavement quality concrete under uniaxial tension and compression

Results are given of strain measurements taken during a limited proramme of tests on a lean concrete and a pavement quality concrete to compare the elastic moduli under direct tension and compression. For each type of concrete differences of less than 10% were found between the tensile and compressive tangent moduli. In tension tests deviation from linearity, indicating commencement of microcracking, occurred at a lower percentage of the ultimate tensile strength for the concrete having the higher cement content. Modulus values were higher for the lean concrete than for the richer pavement quality concrete.     

Practical prediction of creep and shrinkage of high strength concrete

Model for practical prediction of creep and shrinkage of normal strength concrete, developed previously, is extended to high strength concrete. It is found that only a minor adjustment for the concrete strength effect is needed in the formulas for drying creep. The formulas for basic creep and shrinkage need no adjustment. The prediction model is compared with test data for creep and shrinkage obtained recently by Ngab, Nilson and Slate, and by Collepardi, Corradi and Valente, and a satisfactory agreement is demonstrated. The coefficient of variation of the deviations from test data is not larger than that for the normal cient of variation of the deviations from test data is not larger than that for the normal strength range. However, the existing data are rather limited and further testing is desirable.     

钢纤维及陶粒掺量对轻质混凝土基本力学性能的影响

制备了3种强度等级共13组配合比的钢纤维增强轻质混凝土（Steel Fibre Reinforced Lightweight Aggregate Concrete,SFRLAC）,测量了立方体抗压强度、劈裂抗拉强度和轴心抗压强度,得到了SFRLAC轴心受压应力-应变曲线。试验结果表明,钢纤维能小幅度提高轻质混凝土（Lightweight Concrete,LC）的抗压强度,随陶粒比率（Haydite Ratio,V_h）的增大,抗压强度降低,且强度等级越低,降幅越大。钢纤维能显著提高LC的劈裂抗拉强度,钢纤维对低强度等级LC劈裂抗拉强度的贡献优于对高强度等级LC的贡献。低强度等级SFRLAC （LC30和LC40）的劈裂抗拉强度受V_h的影响较大,而高强度等级SFRLAC （LC50）与之相反。当V_h达到80%时,V_h不再是影响SFRLAC劈裂抗拉强度的主要因素,而钢纤维的增强效应显著。试块的破坏形态表明钢纤维能改善LC的塑性。V_h对抗拉强度的降低效应远大于对抗压强度的降低效应。建立了SFRLAC轴心抗压强度与立方体抗压强度的关系式。SFRLAC应力-应变曲线综合体现了钢纤维的增强效应和陶粒的削减作用,陶粒降低LC的峰值应力和韧性,钢纤维主要提高LC的韧性。     

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...     

Identification of damage mechanisms in concrete under high level creep by the acoustic emission technique

Many researchers have proposed hypotheses concerning the physical mechanisms that govern creep and among them the development of microcracks is well recognized. For high load levels, microcracking may initiates at the moment of load application and begins to grow to form a time-dependent crack path. An experimental investigation is proposed here in order to provide interesting insight into the coupling between creep and damage with specimens loaded in flexure. The acoustic emission (AE) technique is used as a tool to provide information on the pertinence of the physical hypothesis that microcracks appear during creep. An original test is performed to accelerate the creep phenomenon by submitting concrete beams to desiccation after a basic creep period. The results show a good proportionality between the creep deformation and the AE activity and thus the efficiency of acoustic measurements for the estimation of the state of damage. In addition, an unsupervised pattern recognition analysis is used as a tool for the classification of the monitored AE signatures. The cluster analysis shows two clusters during basic creep and three clusters during desiccation creep indicating different damage mechanisms.     

Creep and cracking of concrete hinges: insight from centric and eccentric compression experiments

Existing design guidelines for concrete hinges consider bending-induced tensile cracking, but the structural behavior is oversimplified to be time-independent. This is the motivation to study creep and bending-induced tensile cracking of initially monolithic concrete hinges systematically. Material tests on plain concrete specimens and structural tests on marginally reinforced concrete hinges are performed. The experiments characterize material and structural creep under centric compression as well as bending-induced tensile cracking and the interaction between creep and cracking of concrete hinges. As for the latter two aims, three nominally identical concrete hinges are subjected to short-term and to longer-term eccentric compression tests. Obtained material and structural creep functions referring to centric compression are found to be very similar. The structural creep activity under eccentric compression is significantly larger because of the interaction between creep and cracking, i.e. bending-induced cracks progressively open and propagate under sustained eccentric loading. As for concrete hinges in frame-like integral bridge construction, it is concluded (i) that realistic simulation of variable loads requires consideration of the here-studied time-dependent behavior and (ii) that permanent compressive normal forces shall be limited by 45% of the ultimate load carrying capacity, in order to avoid damage of concrete hinges under sustained loading.     

Variation in mechanical properties of natural and recycled aggregate concrete as related to the strength of their binding mortar

Experimental work was performed to study the effect of binding mortar strength on the mechanical properties of recycled natural aggregate concrete mixes as well as reference corresponding natural aggregate concrete mixes. The moduli of elasticity of both NAC and RAC were found to be higher than that of corresponding mortar by about 40% and 10% respectively, for all compressive strengths investigated. It was possible to reach compressive strength for RAC of 53.5 MPa. The ratios of compressive strength of NAC or RAC to that of mortar varied between (1.05–1.56) and (1.02–1.26) respectively, these ratios decreased with the increase in compressive strength. Also from the results of compressive strength, it was found that the ratios cylinder/cube compressive strengths of RAC and mortar were smaller than those of NAC. The ranges of values obtained were (0.71–0.84) and (0.69–0.75) for RAC and mortar respectively, while for NAC this ratio ranged between (0.81–0.92), these values were obtained for compressive strengths ranging between 15 to 55 MPa. It was found that it is better to relate the cylinder/cube strength ratio to the modulus of elasticity of the concrete or mortar rather than to its compressive strength. The flexural strength showed an opposite trend, the ratios of NAC and RAC to that of mortar ranged between (0.72–0.95)% and (0.61–0.80)% respectively. These ratios increased with the decrease in compressive strength of mortars. On the other hand, the splitting tensile strength of NAC was higher than that of RAC and mortar for all strength levels investigated. The ratio of NAC to mortar splitting tensile strength ranged between (1.13–1.69), while this ratio for RAC ranged between (0.87–1.36). Finally, several regressions were developed that can relate the mechanical properties of the three materials investigated.     

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.     

Fictitious degree of hydration method for the basic creep of early age concrete

At the Magnel Laboratory for Concrete Research, University of Ghent, Belgium, an extensive experimental programme was set up in order to study the basic creep behaviour of early age concrete. Tests were carried out for a loading age ranging from 12 hours to 14 days, 2ith two different stress levels: 20% and 40% of the compressive strength at the age of loading. Three different cement types were considered: one Portland cement and two blast furnace slag cements. Based on the experimental results a new fundamental model is proposed. The basic creep evolution is completely related to the evolution of the degree of hydration. Time is no longer an explicit parameter. The stress-strain non-linearity of the basic creep is correlated with the stress-strain non-linearity of the instantaneous deformation at loading. Furthermore, based on the fundamental basic creep model and following the principles of the equivalent time method a new degree of hydration based formulation for the early age basic creep under varying stresses is developed: the fictitious degree of hydration method. Simulations of experimental results show that this new method provides a good alternative for the traditional superposition method.

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Résumé Au laboratoire Magnel de Recherche sur le Béton (Université de Gand, Belgique), on a réalisé un vaste programme de recherche expérimentale concernant le fluage de base du béton en phase de durcissement. On a exécuté des essais de fluage à partir d'un age variant de 12 heures à 14 jours, et ceci à deux niveaux de compression: 20% et 40% de la résistance en compression à l'age de la mise en charge. Un ciment Portland et deux ciments de haut fourneau ont été pris en considération. Sur la base des résultats expérimentaux, on a établi un nouveau modèle fondamental non linéaire. Ce modèle décrit la relation entre l'évolution du fluage de base du béton jeune et l'évolution du degré d'hydratation. Le temps n'intervient plus d'une manière explicite. En plus, il y a une corrélation entre la non-linéarité du fluage de base et celle de la déformation instantanée. Ensuite, sur la base de ce nouveau modèle et par analogie avec la méthode du temps équivalent, on a établi une nouvelle méthode: celle du degré d'hydratation fictif. Plusieurs essais de fluage sous contrainte variable ont permis de constater que la nouvelle méthode est une alternative valable pour la méthode de superposition.

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Editorial Note Prof. Dr. Ir. Luc Taewe is a RILEM Senior Member. Prof. Dr. Ir. Geert De Schutter is a RILEM Affiliate Member. He participates in the work of RILEM TC EAS: ‘Early age shrinkage induced stresses and cracking in cementitious systems’.     

Drying creep of concrete: constitutive model and new experiments separating its mechanisms

A new experimental method which allows the direct separation of the components of drying creep due to microcracking and stress-induced shrinkage is developed, demonstrated and validated. The basic idea is to compare the curvature creep of beams subjected to the same bending moment but very different axial forces. The results confirm that drying creep has two different sources: microcracking and stress-induced shrinkage. The latter increases continuously, whereas the former first increases and then decreases. The test results are fitted using a finite element model. The results validate the present model for drying creep. The microcracking is described by an established model, and the free (unrestrained) shrinkage of a material element is shown to depend approximately linearly on the humidity drop.