Degree of hydration based Kelvin model for the basic creep of early age concrete

Based on simple but fundamental physical observations, a simple Kelvin model with degree of hydration based stiffnesses and viscosity is developed for the simulation of the visco-elastic behaviour of early age concrete, including instantaneous deformation and basic creep. The validity of the model is verified by means of creep tests under constant or varying stresses. A good agreement with the experimental creep results is noticed. With the newly developed degree of hydration based Kelvin model the fundamental nature of the degree of hydration for the early age creep behaviour is illustrated once again.

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Résumé Sur la base d'observations physiques simples mais fondemantales, l'auteur a établi un modèle Kelvin simple dont les paramètres sont basés sur le degré d'hydratation et qui permet la simulation du comportement visco-élastique du béton jeune, comprenant la déformation instantanée et le fluage de base. Plusieurs essais de fluage sous contrainte constante ou variable ont permis d'évaluer le modèle et de constater la bonne concordance entre les résultats des essais et le modèle en question. Ces résultats montrent une fois de plus l'importance du degré d'hydratation pour le fluage de base du béton jeune.

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Open pore description of mechanical properties of ceramics

A dependence of Young's modulus of elasticity on open porosity in ceramics is derived from an open-porosity model, which in the literature, is applied to salinity conductivity and fluid permeability in rocks. A random distribution of grain and pore size is assumed. The relation developed,E(p)=E _o(1–"p)^m, whereE is the modulus of elasticity of the porous ceramic,E _o is the theoretical elastic modulus,p is the porosity andm is an exponent dependent on the tortuosity of the structure of the ceramic, adequately describes the dependence of the modulus of elasticity on porosity. The model is applied to the experimental data from several ceramics such as alumina, silicon nitride, silicon carbide, uranium oxide, rare-earth oxides, and YBa₂Cu₃O_7– superconductor, and the value ofm is obtained for each case. We have shown thatm has a value of nearly 2 for sintered ceramics, unless sintering aids or hot pressing have been used during fabrication of the ceramic. Such additional procedures approximately double the magnitude ofm. On joint appointment with: Materials Laboratory, Physics Department, University of the West Indies, Mona, Kingston 7, Jamaica,     

Investigation on mechanical properties of young concrete

Young concrete usually refers to the concrete with an age less than 7 days. Due to the progress of hydration, the mechanical properties of young concrete are quite different from those of mature concrete. This investigation is aimed at understanding the mechanical properties of young concrete under both uniaxial compression and tension. The uniaxial compression and uniaxial tension tests have been conducted on the concrete specimens at ages of 12 hours, 18 hours, 24 hours, 48 hours, 72 hours, and 168 hours. By utilizing the circumferential control and adaptive control, the complete stress-strain (deformation) curves have been obtained for young concrete under either uniaxial compression or unixial tension. The experimental results show that the behavior of young concrete is quite ductile until about 3 days. The effect of incorporating metakaolin into concrete mix has also been studied in this research. It is found that the metakaolin can significantly enhance the mechanical properties of young concrete.     

Testing system for determining the mechanical behaviour of early age concrete under restrained and free uniaxial shrinkage

A modified uniaxial restrained shrinkage test was developed, characterized by complete automation and high accuracy of measurements. The system developed was such that tensile stresses remained constant throughout the cross-section, excluding any premature failure of specimen. This testing arrangement enables separation of creep strain from shrinkage by means of simultaneous testing of twin specimens, one under restrained shrinkage, and the other under free shrinkage. A variety of mechanical characteristics of the concrete (individual components of strain, shrinkage stresses, moduli of elasticity, creep coefficient and tensile strength) may be determined in one test. Results using this testing arrangement are presented for concrete cured in sealed conditions for 1 day and then exposed to drying at 30° C/40% RH.

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Resume On a mis au point un essai de retrait empêché uniaxial modifié caractérisé par une totale automatisation et une grande précision de mesures. Le système mis au point était tel que les contraintes de traction restent constantes tout le long de la section transversale, excluant toute rupture prématurée de l'éprouvette. Ce genre d'essai permet de distinguer la déformation de fluage du retrait au moyen d'essais simultanés de deux éprouvettes jumelles, l'une sous retrait empêché, l'autre sous retrait libre. Un certain nombre de caractéristiques mécaniques du béton (composants individuels de déformation, contraintes de retrait, modules d'élasticité, coefficient de fluage et résistance en traction) peuvent être déterminées en un seul essai. Les résultats obtenus avec cette technique portent sur un béton conservé dans des conditions d'étanchéité pendant un jour et exposé ensuite au séchage à 30° C/40%RH.

     

Microstructure and mechanical properties of accelerated sprayed concrete

Considering the different hydration processes of concrete without accelerator, sprayed concrete with low-alkali accelerator not only presents short setting times and high early-age mechanical properties but also yields different hydration products. This study presents an analysis of the mechanical properties of concrete with and without accelerator and sprayed concrete with three water–binder (w/b) ratios and four dosages of fly ash (FA) after different curing ages. It also examines the setting time, mineral composition, thermogravimetric–differential scanning calorimetry curves and microscopic images of cement pastes with different accelerator amounts. Furthermore, the setting time and microstructure of accelerated sprayed concrete with different w/b ratios and FA contents are examined. Results show that the retarded action of gypsum disappears in the accelerated cement–accelerator–water system. C₃A is quickly hydrated to form calcium aluminate hydrate (CAH) crystals, and a mesh structure is formed by ettringite, albite and CAH. A large amount of hydration heat improves the hydration rate of the cement clinker mineral and the resulting density, thereby improving mechanical properties at early curing ages. The setting times of the pastes increase with increasing w/b ratio and FA dosage. Thus, the hydration level, microstructure and morphology of the hydration products also change. Models of mechanical properties as functions of w/b, FA and curing age, as well as the relationship between compressive strength and splitting tensile strength, are established.     

Predicting the mechanical properties of ordinary concrete and nano-silica concrete using micromechanical methods

By combining several materials with specific mechanical properties, new materials with unknown mechanical properties are obtained. Various experiments are required to determine the mechanical properties of the produced composite materials. Since conducting experiment processes is costly and time-consuming, comprehensive studies have been conducted in recent years to solve the problem. Fortunately, it is possible to easily predict the mechanical properties of composite materials without the need to construct them, by inspecting their constituent’s properties using micromechanical methods. Although various micromechanical methods have been presented so far, few of them yielded precise predictions of the properties of composite materials. Therefore, selecting a method suitable to predict the properties of composite materials is of much importance. In this study, some micromechanical approaches, including Hirsch, Hansen, Bache, Cavento, Mori–Tanaka, Eshelby, self-consistent, effective interface and double-inclusion models, were employed for the estimation of elasticity modulus and Poisson’s ratio of ordinary and nanomaterial concretes. The results obtained from the micromechanical methods were compared to those obtained from experimental tests. The obtained numerical results showed that Bache’s model is the most accurate micromechanics model for predicting the elastic mechanical properties of ordinary and nanomaterial concretes.     

A review of the hardened mechanical properties of self-compacting concrete

Data from more than 70 recent studies on the hardened mechanical properties of self-compacting concrete (SCC) have been analysed and correlated to produce comparisons with the properties of equivalent strength normally vibrated concrete (NVC).The significant scatter obtained in much of the data is a consequence of the wide range of materials and mixes used for SCC, but clear relationships have been obtained between cylinder and cube compressive strength, tensile and compressive strengths, and elastic modulus and compressive strength. It is also clear that limestone powder, a common addition to SCC mixes, makes a substantial contribution to strength gain.Bond strength of SCC to reinforcing and prestressing steel is similar to or higher than that of normally vibrated concrete. Variation of in situ properties in structural elements cast with SCC is similar to that with NVC, and the performance of the structural elements is largely as predicted by the measured material properties.The analysis has shown that sufficient data have been obtained to give confidence in the general behaviour of SCC, and future studies need only be focused on specific or confirmatory data for particular applications.     

Effect of steel fibres on mechanical properties of high-strength concrete

Steel fibre reinforced concrete (SFRC) became in the recent decades a very popular and attractive material in structural engineering because of its good mechanical performance. The most important advantages are hindrance of macrocracks’ development, delay in microcracks’ propagation to macroscopic level and the improved ductility after microcracks’ formation. SFRC is also tough and demonstrates high residual strengths after appearing of the first crack. This paper deals with a role of steel fibres having different configuration in combination with steel bar reinforcement. It reports on results of an experimental research program that was focused on the influence of steel fibre types and amounts on flexural tensile strength, fracture behaviour and workability of steel bar reinforced high-strength concrete beams. In the frame of the research different bar reinforcements (2∅6 mm and 2∅12 mm) and three types of fibres’ configurations (two straight with end hooks with different ultimate tensile strength and one corrugated) were used. Three different fibre contents were applied. Experiments show that for all selected fibre contents a more ductile behaviour and higher load levels in the post-cracking range were obtained. The study forms a basis for selection of suitable fibre types and contents for their most efficient combination with regular steel bar reinforcement.     

Development of early age shrinkage stresses in reinforced concrete bridge decks

This paper describes the instrumentation and data analysis of a reinforced concrete bridge deck constructed on 3-span continuous steel girders in Evansville, West Virginia. An instrumentation system consisting of 232 sensors is developed and implemented specifically to measure strains and temperature in concrete deck, strains in longitudinal and transverse rebars, the overall contraction and expansion of concrete deck, and crack openings. Data from all sensors are automatically collected every 30 minutes starting at the time of placing the concrete deck. Measured strain and temperature time-histories were used to calculate the stresses, which were processed to attenuate the thermal effects due to daily temperature changes and isolate the drying shrinkage component. The results indicated that most of concrete shrinkage occurs during the first three days. Under the constraining effects from stay-in-place forms and reinforcement, early age shrinkage leads to elevated longitudinal stress, which is the main factor responsible for crack initiation.     

Comparative evaluation of early age toughness parameters in fiber reinforced concrete

Early age strength development is a major consideration for design and construction processes such as the shotcrete mixtures used for tunneling applications. Adding the fibers to high strength concrete helps in resisting potential early age thermal and shrinkage cracking in addition to maintaining long-term strength. The post cracking tensile strength is one of the critical safety parameters to insure a safe level of ground support. Results of several bending tests on early-age fiber reinforced concrete are presented as load–deflection responses. A strain softening response is used to model the behavior of different types of fiber reinforced concrete and simulate the experimental flexural response. Closed form equations for moment–curvature response of a rectangular beam in conjunction with crack localization rules are utilized. As a result, the stress distribution that considers a shifting neutral axis can be simulated which provides a more accurate representation of the residual strength of the fiber cement composites. The analysis is performed to evaluate effects of age and fiber type on back calculated tensile stress strain response, along with experimental and simulated flexural load–deflection curves. The back-calculated tensile post cracking strengths are compared and correlated with the corresponding parameters used by ASTM, JCI, and RILEM methods and scale factors for the elastic methods are proposed which are in-line with the current fib Model Code. Caution must be exercised in application of results from the standard test methods due to the overestimation of the residual strength parameters that are based on elastic approaches.     

Residual mechanical properties of heated concrete incorporating different pozzolanic materials

This paper aims at showing the effect of high temperatures on mechanical properties of concretes in which Portland cement (PC) has been partially replaced by pozzolanic materials. Three types of pozzolanic material, one natural pozzolana and two lignite fly ashes (one of low and one of high lime content) were used for the replacement. Concrete specimens were tested at four temperature levels of 200, 400, 600 and 800°C without any imposed load. The specimens for each of the chosen temperatures were heated under the same heating regime, so a comparison of their behaviour during heat exposure concerning their mechanical properties was possible. Tests of compressive strength, splitting tensile strength and modulus of elasticity were carried out on specimens cooled slowly to room temperature 1 day, 7 days, 1 month and 3 months after heating. Based on the results of this experimental work it can be said that concretes with pozzolanic materials added to the mixer in partial replacement of PC are more sensitive to exposure to fire than conventional concretes. A relatively greater drop in the strength of the concretes with pozzolanic materials was found especially in the temperature area of 200°C when they were compared with conventional concrete made from PC of Greek type (10% insoluble residue). This particular sensitivity was attributed to heat deformations of hardened pastes, as all the other testing parameters concerning consistency and rate of heating remained the same for all specimens. It is suggested that this is caused by the escape of the non-evaporable water that exists as combined water in hardened pastes of pozzolanic binding agents. These pastes are richer in compounds such as calcium aluminates, calcium aluminate sulfates and tobermorite gel (C-S-H) which are decomposed between 110 and 150°C.

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Resume Cette étude fait partie d'une recherche sur l'effet de la température sur les propriétés mécaniques de béton dans lequel le ciment Portland a été remplacé au cours du mixage par des matériaux pouzzolaniques. Trois types de ces matériaux, soit deux cendres volantes et une pouzzolane naturelle, ont été utilisés afin de remplacer partiellement le ciment Portland dans le béton. Les éprouvettes de béton non modifié ont été soumises à différentes températures: 200, 400, 600 et 800°C. La résistance en compression, en traction, et le module d'élasticité ont été mesurés à un jour, sept jours, un mois et trois mois après ;'essai thermique. Les bétons renfermant des matériaux pouzzolaniques ont manifesté une forte diminution de la résistance et de l'élasticité. On peut dire que le remplacement du ciment Portland par des matériaux pouzzolaniques ne contribue pas à l'augmentation de la résistance du béton au feu, tout au moins dans le cas de températures peu élevées.

     

Anisotropic mechanical properties of extrusion-based 3D printed layered concrete

Journal of Materials Science - Extrusion-based 3D printed concrete is a promising material and processing technique for use in the construction industry. In this study, 3D printed specimens were...     

Abrasion resistance and mechanical properties of high-volume fly ash concrete

The abrasion resistance and mechanical properties of concrete containing high-volume fly ash (HVFA) were investigated. Sand (fine aggregate) was replaced with 35, 45, and 55% of Class F fly ash by mass. The water to cement ratio and the workability of mixtures were maintained constant at 0.46 and 55 ± 5 mm respectively. Properties examined were compressive strength, splitting tensile strength, flexural strength, modulus of elasticity and abrasion resistance expressed as depth of wear. Test results indicated that replacement of sand with fly ash enhanced the 28-day compressive strength by 25–41%, splitting tensile strength by 12–21%, flexural strength by 14–17%, and modulus of elasticity by 18–23% depending upon the fly ash content, and showed continuous improvement in mechanical properties up to the ages of 365 days. Replacing fly ash with sand significantly improved the abrasion resistance of concrete at all ages. Strong correlation exists between the abrasion resistance and each of the mechanical properties investigated.     

Effects of fly ash fineness on the mechanical properties of concrete

The present study reviews the effects of fly ash fineness on the compressive and splitting tensile strength of the concretes. A fly ash of lignite origin with Blaine fineness of 2351?cm²/g was ground in a ball mill. As a consequence of the grinding process, fly ashes with fineness of 3849?cm²/g and 5239?cm²/g were obtained. Fly ashes with three different fineness were used instead of cement of 0%, 5%, 10%, and 15% and ten different types of concrete mixture were produced. In the concrete mixtures, the dosage of binder and water/cement ratio were fixed at 350?kg/m³ and 0.50, respectively. Slump values for the concretes were adjusted to be 100 ± 20?mm. Cubic samples were cast with edges of 100?mm. The specimens were cured in water at 20°C. At the end of curing process, compressive and splitting tensile strengths of the concrete samples were determined at 7, 28, 56, 90, 120 and 180?days. It was observed that compressive and splitting tensile strength of the concretes was affected by fineness of fly ash in short-and long-terms. It was found that compressive and tensile strength of the concretes increased as fly ash fineness increased. It was concluded that Blaine fineness value should be above 3849?cm²/g fineness of fly ash to have positive impact on mechanical properties of concrete. The effects of fly ash fineness on the compressive and splitting tensile strength of the concretes were remarkably seen in the fly ash with FAC code with fineness of 5235?cm²/g.     

A humidity-adjusted maturity function for the early age strength prediction of concrete

The early age strength development of concrete is determined by a proposed maturity method, which considers not only the curing temperature history at the core of the concrete specimens, but also the relative humidity of the environment. The humidity factor is incorporated with the original rate constant model to form a new maturity function for the prediction of concrete strength development. In order to calibrate this humidity-adjusted rate constant, compression tests were conducted on concrete cylinders cured at different conditions of temperature and humidity. By comparing the ratio of rate constants of concrete cylinders cured at the same temperature but different humidities, the humidity factor could be quantified. Verification programs (including concrete cylinders cured at the programmed temperature and relative humidity, submerged in water baths and cured outdoors, and air-dried outdoors) were used to determine the applicability of the prediction model. Experimental results show that this new maturity function is able to predict the in-place strength development of ordinary concrete cylinders at early age with a maximum difference of 10%.     

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’.