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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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.     

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.

     

Assessment of mechanical properties of concrete incorporating carbonated recycled concrete aggregates

An accelerated carbonation technique was employed to strengthen the quality of recycled concrete aggregates (RCAs) in this study. The properties of the carbonated RCAs and their influence on the mechanical properties of new concrete were then evaluated. Two types of RCAs, an old type of RCAs sourced from demolished old buildings and a new type of RCAs derived from a designed concrete mixture, were used. The chosen RCAs were firstly carbonated for 24 h in a carbonation chamber with a 100% CO₂ concentration at a pressure level of 0.1 Bar and 5.0 Bar, respectively. The experimental results showed that the properties of RCAs were improved after the carbonation treatment. This resulted in performance enhancement of the new concrete prepared with the carbonated RCAs, especially an obvious increase of the mechanical strengths for the concrete prepared with the 100% carbonated new RCAs. Moreover, the replacement percentage of natural aggregates by the carbonated RCAs can be increased to 60% with an insignificant reduction in the mechanical properties of the new concrete.     

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.     

On the mechanical properties of polymer Portland cement concrete

Abstract

In this paper, epoxy which is the most common type of polymer used by civil engineers, and polyester, which is cheaper than most other types of polymers, were utilized to produce Polymer Portland Cement Concrete (PPCC). A rich concrete mix was designed and then five different ratios of cement (0, 20, 40, 60 and 100%) were replaced by either epoxy or polyester, rendering five different mixes for each type of polymer. Three types of samples were cast, compacted and then cured for 28 days. The three types of samples are standard beams of size 150×150×600 mm, standard cubes and standard cylinders. One third of the specimens were tested at room temperature, while the rest of the specimens were heated in an oven for 24 hours. Specimens were heated to two temperature stations of 80°C and 150°C. Samples received for flexure, compression, split tension, and direct shear tests, using a specially manufactured apparatus that was used before and found to be effective. The variations of compressive strength, split tensile strength, direct shear strength, and flexural strength with different variables such as temperature, percentage of polymer and type of polymer were determined and assessed.     

UHMWPE纤维混凝土动态压缩力学性能研究

试验研究了一种捻制超高分子量聚乙烯(UHMWPE)纤维增强的新型纤维混凝土动态压缩力学性能。研制了4种纤维体积掺量(0.3%、0.5%、0.7%、1.0%)的C70等级纤维混凝土,采用Φ100 mm分离式霍普金森压杆进行冲击压缩试验,研究了纤维混凝土在140~255 s~(-1)应变率下的动态压缩力学性能。试验结果表明:UHMWPE纤维混凝土抗压强度、峰值应变和弹性模量具有明显的应变率敏感性;纤维混凝土抗压强度应变率敏感性弱于素混凝土,但其弹性模量应变率敏感性强于素混凝土;动态强度增长因子与应变率对数呈线性关系,具体关系与纤维掺量相关。     

Stress curves and mechanical properties of high performance concrete

Abstract

In this research, high performance concrete (HPC) was designed by the minimum void ratio method, and slag and silica fumes partially replaced cement, as well as fly ash replacing about 15% of sand. Stress curves for compressive, splitting and flexure strengths of HPC specimens were measured and indicated the experimental concretes had better pastes to void ratios than control batches ratio N=V_p /V_v =1.3. The result indicates that pozzolanic materials provide not only a chemical strength effect, but also a physical packing effect. The compressive stress curves may keep growing as the concrete ages.     

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.     

Tensile behaviour of early age concrete: New methods of investigation

The assessment of the tensile properties of early-age concrete is essential for reducing the risk of cracking due to restrained shrinkage. The tensile strain capacity of concrete, which was defined as a measure of the ability of the material to withstand deformation without cracking, is useful but few data can be found in available literature and the measure of the displacements of concrete is sometimes questionable. New direct tensile testing apparatus and experimental procedure were designed to provide reliable data on concrete specimens. The measure of displacements was deduced from digital image correlation. They enabled determining a stress–strain relationship of concrete before cracking. The results showed a very rapid increase of strength from the end of setting. The evolution of the tensile strain capacity showed a minimum corresponding to the period that includes the setting time and early hardening, thus this is a critical stage for plastic shrinkage cracking. Even if the values are closely linked to the boundary conditions and experimental procedure, the effect of aggregate type could be investigated.     

Monitoring of concrete at very early age using stiff SOFO sensor

The SOFO system is based on low-coherence interferometry in single-mode fibres and allows the measurement of deformations in civil structures built with classical civil engineering materials (concrete, steel and wood). It has been successfully tested in different types of structures such as bridges, dams, tunnels and piles. In order to monitor behaviour of concrete at the very early age a stiff SOFO sensor has been developed. Using standard SOFO sensor it is possible to measure deformation of concrete at the very early age (thermal swelling and shrinkage). However, by coupling it with a stiff sensor, it is possible to determine the hardening time of concrete and to measure initial stress in the rebars. The stiff sensor and the results obtained using its first prototype are presented in this paper.     

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.     

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.     

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.