Showing papers in "Materials and Structures in 2002"

Strength of short concrete columns confined with CFRP sheets

Abstract: The confinement of concrete columns provided by carbon fibre reinforced plastic (CFRP) sheets can be an efficient technique for their structural strengthening. The principal advantages of this technique are the high strength-to-weight ratio, good fatigue properties, non-corroding characteristics of the CFRP, and the facility of its application.

Pull-out behaviour of hooked steel fibres

Abstract: A programme of research is described that has investigated the fracture of steel fibre reinforced sprayed concrete under flexural load, with the aim of developing a stress-profile model to predict flexural behaviour in the form of a load-deflection response. This paper reports the work associated with establishing the pull-out characteristics of hooked end fibres. The effects of matrix strength, fibre embedment length and orientation are described, together with the interaction of these parameters. The relationships established can be used to model the tensile response of a beam at varying crack width and hence form a key part of the stress-profile for predicting residual flexural strength, which is an essential requirement of a much needed design rationale for steel fibre concrete.

Influence of industrially produced recycled aggregates on flow properties of concrete

Abstract: This research aims at evaluating the main risks for the durability of concrete made of industrially produced recycled aggregates called Recycled Aggregate Concrete (RAC). A characterisation of recycled aggregates is performed and their peculiarities are highlighted. A comparison between the behaviour of RAC and that of ordinary natural aggregate concrete is carried out. The influence of both the composition and the curing conditions is discussed. The durability study is focused on the assessment of parameters representing the porous structure and concrete characteristics. Because of the high total water/cement ratio of RAC, their flow properties control their durability. It is established that RAC are characterised by significantly higher water absorption and air permeability. The diffusion of the carbon dioxide is faster, too. That leads to a weaker resistance of RAC to environmental attacks. Since the main durability problems are caused by the fine recycled fraction, its use needs to be restricted. Another way to increase RAC durability seems to be the extended curing in wet environment.

Efficiency of lightweight aggregates for internal curing of high strength concrete to eliminate autogenous shrinkage

Abstract: High Strength Concretes (HSC) with extremely low water to binder (w/b) ratios are characterized by high cracking sensitivity which is a consequence of increased autogenous shrinkage. The major reason for autogenous shrinkage—self-desiccation—cannot be eliminated by traditional curing methods. The application of the concept of internal curing by means of saturated lightweight aggregate was applied and shown to be effective in eliminating autogenous shrinkage. The present paper describes an approach to optimize the size and porosity of the light-weight aggregate to obtain effective internal curing with a minimum content of such aggregate.

Effect of aggregates on drying shrinkage microcracking in cement-based composites

Abstract: In this paper the effect of the aggregate content on drying shrinkage microcacking in cement-based composites is investigated Experiments were performed on cement-based composites containing spherical glass particles as aggregates It was found that the use of these model particles is an excellent tool to study the basic mechanisms of drying shrinkage microcracking in cement-based composites The microcracks were made visible by means of an impregnation technique in combination with optical microscopy Crack-patterns were quantified by using image analysis software The effect of aggregate size and quantity on the defined crack-parameters is reported in this paper Cracking was recorded on two stages of drying, which allowed to elucidate the long-term evolution of drying shrinkage microcracking

Comparison of particle packing models for proportioning concrete constitutents for minimum voids ratio

Abstract: This paper reports the findings of a study of four particle packing models used to proportion the mix constituents (solid particles) of concrete to produce a minimum voids ratio (or maximum packing density). The models have been compared using laboratory tests and published data. The basic mathematics of the models is discussed, particularly how each model defines the particle size distribution of the solid particles. The models have been applied to both the aggregate (sand and gravel) and the cement phase (PC, PFA, GGBS and limestone fines) and the estimated voids ratio compared with that measured in the laboratory. It was found that the models give broadly the same output and suggest similar combinations of materials to give the minimum voids ratio. Using the materials considered it was found that the largest improvement in voids ratio was achieved with the aggregate phase. The particle sizes of the cements considered here were similar and, as a result, only small improvements in voids ratio could be achieved. It was noted that proportioning concrete mix constituents to minimise voids ratio did tend to produce a harsher mix than normal. However, using the mix suitability factor, proposed by Day (1999), reduced this problem. There are some detail differences between the models suggesting further refinements could be carried out and a modification to one of the models is provided.

Active and non-active porosity in concrete Part I: Experimental evidence

Abstract: The findings of an experimental study are described, documenting and quantifying the influence of concrete porosity on its mechanical properties in saturated and dry states Concrete porosity consists of capillary and entrained air, and described respectively as active and non-active, depending on its influence on mechanical properties with changing moisture state It was found that the active (capillary) porosity influence on mechanical properties is more pronounced with decreasing moisture The influence of non-active (entrained-air) porosity on concrete mechanical properties is invariant of moisture state

Simplified modelling of calcium leaching of concrete in various environments

Abstract: Deterioration of concrete structures subjected to aggressive water is often characterized by the leaching of calcium from hydrates. Since calcium leaching is a nonlinear phenomenon, which is influenced by a wide range of parameters, the residual service life of the structures affected by such deterioration cannot be predicted satisfactorily on the basis of experimental results alone. In order to provide more fundamental information on this phenomenon, a numerical model was developed. The model considers the thermodynamic equilibrium between the pore solution and the various hydrated phases. In addition, the deterioration mechanism is treated from a macroscopic point of view. This model has been first validated on cement paste samples leached by deionised water. Second, extension to other kinds of environment and materials was done by considering modification of the chemical equilibrium between pore solution, and solid hydrates.

Restrained shrinkage cracking: the role of shrinkage reducing admixtures and specimen geometry

Abstract: Early-age cracking may occur in concrete elements if shrink-age is prevented by the surrounding structure. The risk of early-age shrinkage cracking in any given structure is influenced by many factors including the magnitude of shrinkage, rate of shrinkage, degree of stress relaxation, degree of structural restraint, and rate at which material properties develop. In addition to the aforementioned factors, this paper highlights the fact that shrinkage cracking is also influenced by geometry. This paper compares two series of experiments to better understand the role of specimen geometry. In the first series ring specimens of varying size were prepared to undergo the same rate of shrink-age and maximum strain development. Although the maximum residual tensile stress that developed was nearly identical for all geometries, the age at which cracking occurred varied with specimen thickness. In the second series of experiments the combined effect of moisture gradients and specimen geometry was investigated. In these experiments specimen thickness was varied while the surface area exposed to drying was maintained constant. The age of cracking was measured, as was the development of a moisture gradient. Again the influence of specimen size/geometry was apparent with smaller specimens cracking at an earlier age. This paper provides an explanation for this geometry dependence through the use of fracture mechanics concepts.

Drying, moisture distribution, and shrinkage of cement-based materials

Abstract: The aim of this study was to develop a new method to determine hygral diffusion, film, and shrinkage coefficients of cement-based materials. These coeffcients are required for the numerical simulation of shrinkage strain and total deformation of concrete elements and structures using the finite element method. Both an experimental approach to determine the time-dependent relative humidity in the pore system of concrete and a numerical method to determine material coefficients on the basis of experimental data are described in this paper. The hygral diffusion coefficient can be expressed as function of moisture content and as function of relative humidity. An experiment is carried out with sliced specimens measuring 150×100×3 mm. Each specimen is prepared by piling up 11 slices and sealing the outer surfaces with aluminum sheet. The distribution of relative humidity is estimated by measuring the shrinkage strain on each slice at arbitrary drying times. An inverse analysis is then used to obtain the diffusion coefficient from the measured relative humidity distribution. A numerical approach based on the weighted residual method and on a nonlinear least squares method is proposed on the basis of the experimental results.

Multiscale modeling of fluid transport in heterogeneous materials using discrete Boltzmann methods

Abstract: The lattice Boltzmann method is a promising approach for modeling single and multicomponent fluid flow in complex geometries like porous materials. Here, we review some of our previous work and discuss some recent developments concerning fluid flow in multiple pore size materials. After presenting some simple test cases to validate the model, results from large scale simulations of single and multi-component fluid flow through digitized Fontainebleau sandstone, generated by X-Ray microtomography, are given. Reasonably good agreement was found when compared to experimentally determined values of permeability for similar rocks. Finally, modification of the lattice Boltzmann equations, to describe flow in microporous materials, is described. The potential for modeling flows in other microstructures of interest to concrete technology will be discussed.

Active and non-active porosity in concrete. Part II: Evaluation of existing models

Abstract: Mechanical, semi-empirical, and theoretical models suitable for describing concrete mechanical properties (modulus of elasticity, bulk modulus, and ultrasonic pulse velocity) are evaluated. Extensive experimental data is generated from saturated and dry concrete specimens with varying volume of active and non-active pores. Experimetnal data established that active and non-active pores appear at different aspect ratios furthermore, active pores change shape with moisture content. The experimental data is compared to those obtained from these three model categories. Models are evaluated based on their capability in evaluating the effect of pores during dry and saturated states. One model, capable of representing pores at various aspect ratios, was successful in describing mechanical properties in dry and saturated states.

Determination of the coefficient of thermal expansion of high performance concrete from initial setting

Abstract: Autogenous shrinkage, which is a consequence of the absolute volume contraction resulting from cement hydration, occurs in any concrete but its effect is particularly amplified in high performance concrete in which it can be as large as drying shrinkage. Autogenous shrinkage can be directly measured in concrete samples under isothermal conditions but from a practical standpoint the experimental procedure is not always possible. On the other hand, it can be evaluated after having taken into account volumetric variations due to the release in heat during cement hydration. To separate the thermal effect from autogenous shrinkage, it is necessary to know at any moment the evolution of the coefficient of thermal expansion of the concrete from initial setting. A simple method to determine the coefficient of thermal expansion at early ages is proposed in this paper. It consists in submitting concrete samples instrumented with vibrating wire extensometers to thermal shocks. The response of the concrete sample to this shock results in a nearly instantaneous deformation, which is measured by the sensor. These deformations, as well as the temperature signal, are used to calculate the coefficient of thermal expansion. By repeating this experiment at various ages, it is possible to follow the variation in the coefficient of thermal expansion of the concrete over time.

Correlations in the behaviour of fibre reinforced shotcrete beam and panel specimens

Abstract: Satisfactory structural performance of Fibre Reinforced Shotcrete (FRS) in applications such as tunnel linings is dependent on an ability to support load after cracking. At present, both beam and panel specimens are used to measure the post-crack flexural capacity of this material. Although these tests have been widely used to develop improved fibres and FRS mix designs, the relationship between performance data produced in beam and panel tests is unclear. This investigation was therefore undertaken to examine possible correlations in behaviour between beam and panel specimens and determine which was the most appropriate type of test for a given FRS application.

Influence of concrete relative humidity on the amplitude of Ground-Penetrating radar (GPR) signal

Abstract: In order to improve radar technique efficiency and to offer innovative applications as non-destructive evaluation tools for civil engineering structures, the authors studied the influence of concrete moisture on GPR sounding. The presence of water in concrete leads to modifications of its electromagnetic properties and thus to specific effects upon waveform characteristics. In particular, the paper focuses on amplitude variations during processes which consume free water (hydration, drying). Radar measurements were carried out on laboratory samples using a commercial ground-coupled antenna of 1.5 GHz central frequency, while relative humidity profiles of the concrete were provided by specific sensors placed at various heights through the concrete samples. An increase in the amplitude of signals propagating through the samples was observed during hydration and drying. Moreover, radargrams recorded during the test also show that the direct wave propagating at the surface of the samples, is ereatly influenced by the evolution of concrete moisture. Concrete hygrometric measurements performed at the same time provide quantitative information on the effect of moisture reduction on radar signals.

Filling ability and plastic settlement of self-compacting concrete

Abstract: The use of self-compacting concrete (SCC) facilitates the placing of concrete by eliminating the need for compaction by vibration. Given the highly flowable nature of such concrete, care is required to ensure excellent filling ability and adequate stability. This is especially important in deep structural members and wall clements where concrete can block the flow, segregate and exhibit bleeding and settlement which can result in local defects that can reduce mechanical properties, durability and quality of surface finish.

Early age creep and stress relaxation of concrete containing blended cements

Abstract: The main objective is to investigate key properties that influence the stress development in concrete at early ages and the effect of using blended cements. Mineral additives and amount by weight of total binder used in the blended cements are fly ash (25%), ground granulated blast furnace slag (25%), and silica fume (10%). The properties investigated include tensile creep, elastic modulus, split tensile strength, and autogenous shrinkage. The relaxation modulus used for stress prediction was obtained from the creep data fitted using a log-power creep function. These findings show that tensile creep and stress relaxation are important properties of Portland cement concrete. These properties however are reduced in concretes containing blended cements. Blended cements affect the early age strength and elastic modulus moderately but significantly alter the autogenous deformation. Water/cement ratio (w/c), type and dosage of mineral additives were found to influence the magnitude of autogenous deformation. This deformation was found to be significant in low water-cement ratio concretes and should be included in early age stress calculations.

Long-term moisture transport in high performance concrete

Abstract: Moisture is decisive for a large number of binding and transport processes in high performance concrete affecting the durability, shrinkage and performance in various environments. An experimental study on the moisture transport properties of 20 concrete mixes was made during seven years, with type of binder, additives and waterbinder ratio as parameters. An upside-down glass cup method was used to obtain the steady-state flow through concrete discs.

Size effect of test specimens on tensile splitting strength of concrete: general relation

Abstract: A broad experimental program was carried out on cubes, cylinders and prisms (loaded transversally or longitudinally) of sizes usually used in laboratory testing. For casting, concrete mixes of various compositions were used. The 1600 results obtained were evaluated by mathematical statistical methods. The volume of test specimens was substituted either by the size of the corresponding fracture area or by the size of the highly stressed volume (HSV) in their loaded cross-section. The absolute values of the tensile splitting strength were converted to relative values (depending on the basic size of the fracture area or on the highly stressed volume). For mathematical expression of the relationships between these quantities, an exponential function was used. This relation enables to convert the test results obtained on specimens of different sizes and shapes to the strength of the basic size specimen. This is important for the test results obtained on samples taken from finished structures, as these samples are usually of different sizes.

Use of recycled glass as a raw material in the manufacture of Portland cement

Abstract: Scrap glass is a solid waste from daily recycling. Most of the waste glass is sodium-lime-silicate glass which has, more or less, similar chemical compositions to clay, a raw material in cement manufacturing. Therefore, we utilize the solid waste in cement raw mix by replacing part of the clayey component. In this study, the effects of the glass in cement raw mix on clinker burning were investigated. The experimental results show that the addition of the glass into cement raw mix (1) results in the formation of more liquid phase between 950°C to 1250°C compared with conventional raw meals; (2) decreases C3S content in the clinker; and (3) increases NC8A3 content, which leads to flash setting and poor strength development of the cement. Therefore, it is necessary to increase the SG value [SG=SO3•100%/(1.292 K2O+0.85 Na2O)] of the clinker when the glass is present in the raw mix.

Predicting the performance of concrete structures exposed to chemically aggressive environment—Field validation

Abstract: The behavior of two series of concrete slabs exposed to sulfate-bearing soils was investigated by a numerical model called STADIUM. In addition to the diffusion of ions and moisture, the model also accounts for the effects of dissolution/precipitation reactions on the transport mechanisms. The simulations yielded by the model were compared to the actual degradation of the slabs after 8 years of exposure. The microstructural alterations of concrete resulting from the penetration of magnesium, chloride and sulfate ions were studied by backscatter mode scanning electron microscope observations and energy-dispersive X-ray analyses. The comparison of both series of data indicates that the model can reliably predict the various features of the microstructural alterations of concrete.

Modeling of structural performances under coupled environmental and weather actions

Abstract: The authors propose a so-called life-span simulator that can predict concrete structural behaviors under arbitrary external forces and environmental conditions. In order to realize this kind of technology, two computational systems have been developed; one is a thermohygro system that covers microscopic phenomena in C-S-H gel and capillary pores, and the other one is a structural analysis system, which deals with macroscopic stress and deformational field. In this paper, the unification of mechanics and thermo-dynamics of materials and structures has been made with the ion transport of chloride, CO2 and O2 dissolution. This proposed integrated system can be used for simultaneous overall evaluation of structural and material performances without distinguishing between structure and durability.

An optimised preconditioning procedure for gas permeability measurement

Abstract: To study the permeability of cementitious materials; a procedure of drying out moisture before the actual measurement of this property, normally called preconditioning, is an unavoidable necessity. In this study, the preconditioning procedure developed by the authors earlier, is used to carry out the measurement of permeability to gases of 15 concrete compositions in order to compare the different steps proposed by these procedures: these concretes have been studied in terms of durability in a “BHP2000” project. These 15 compositions include 4 ordinary concretes, 5 medium strength concretes, 4 HPCs and 2 VHPCs. We have compared the preconditioning temperatures for their possible effects on the measured value of permeability as well as its adequacy in drying out moisture during a short period of time. Also, in addition to be practicality of the procedures, the coverage of the whole range of degrees of saturation by the permeability results is given due importance. These considerations have culminated in a optimised preconditioning procedure which is not only simple and flexible but also guarantees adequate information about the permeation characteristics of the material under study, for a whole range of concrete qualities.

Fundamental study of early age concrete behaviour as a basis for durable concrete structures.

Abstract: An overview is given of a global research program performed at Ghent University, concerning early age concrete. More specifically the carly age thermal cracking due to the heat of hydration in hardening massive concrete elements has been focused. A fundamental approach was followed, based on the degree of hydration. By means of the degree of hydration, a fundamental and very accurate description of early age concrete behaviour can be realised. The resulting material laws can be applied within a further study of the durability of massive concrete structures in marine environment. In this way, the fundamental study of early age concrete behaviour can serve as a basis for further durability improvement of concrete structures.

Properties of early age concrete-Experiments and modelling

Abstract: For the simulation of temperature fields and restraint stresses in hardening concrete structures, the properties of early age concrete has to be known. This report is an extract of a research project which dealt with the thermal and mechanical properties, stress-strain curve, creep and relaxation and autogenous shrinkage of early age concrete. Here, only the mechanical short and long-term properties of early age concrete will be dealt with. In addition to the experimental results, models will be presented.

Stress-strain behavior of concrete columns confined with hybrid composite materials

Abstract: A new type of concrete columns was developed at the University of Alabama in Huntsville for new construction to achieve more durable and economical structures. The columns are made of concrete cores encased in a PVC tube reinforced with fiber reinforced polymer (FRP). The PVC tubes are externally reinforced with continuous impregnated fibers in the form of hoops at different spacings. The PVC acts as formwork and a protective jacket, while the FRP hoops provide confinement to the concrete so that the ultimate compressive strength and ductility of concrete columns can be significantly increased. The volume of fibers used in this hybrid column system is very modest compared to other existing confinement methods such as FRP tubes and FRP jackets. This paper discusses the stress-strain behavior of these new composite concrete cylinders under axial compression loading. Test variables include the type of fiber, volume of fiber, and the spacing between the FRP hoops. A theoretical analysis was performed to predict the ultimate strength, failure strain and the entire stress-strain curve of concrete confined with PVC-FRP tubes. Test results show that the external confinement of concrete columns by PVC-FRP tubes results in enhancing compressive strength, ductility and energy absorption capacity. A comparison between experimental and analytical results indicates that the models provide satisfactory predictions of ultimate compressive strength, failure strain and stress-strain response.

Explanation of size effect in concrete fracture using non-uniform energy distribution

Abstract: A local fracture energy model originally proposed to explain the influence of fracture process zone (FPZ) on fracture energy of cementitious materials is further developed in this study. By assuming a bilinear distribution for the fracture energy distribution, the ligament-dependent fracture energyG f is obtained. The analytical expression ofG f contains two important prameters: the intrinsic size-independent fracture energyG F and a reference ligament sizea l * which determines the intersection of the two linear fracture energy functions. It is shown that the ligament-dependentG f approaches the size-independentG F asymptotically. As a result,G F can be determined from the ligament-dependentG f results. It is also found that while the reference ligament sizea l * is influenced by the specimen geometry, size and loading conditions, the derived fracture energyG F is virtually constant. The present local fracture energy distribution model is also discussed and compared with the original local fracture energy model.