The suitability of crushed building and demolition waste as a raw material for the production of calcium silicate products has been determined. Therefore calcium silicate bricks have been produced by replacing natural sand with crushed building and demolition waste of different sources. The mechanical properties of the bricks made with these wastes are comparable or in some cases even better than those of bricks with natural sand. In particular the green shear strength of the bricks is higher. The amount of quartz and reactive SiO2 in the waste materials is high enough for the formation of tobermorite and hydrogarnet as cementitious materials between the grains. A negative aspect is the appearance of brown stains on the surface of the bricks when the waste materials are slightly contaminated with organic substances. This risk can be reduced by including a washing process, in addition to crushing of the waste.
This research explores the relationship between permeability and crack width in cracked, steel fiber--reinforced concrete. In addition, it inspects the influence of steel fiber reinforcement on concrete permeability. The feedback--controlled splitting tension test (also known as the Brazilian test) is used to induce cracks of up to 500 microns (0.02in) in concrete specimens without reinforcement, and with steel fiber reinforcement volumes of both 0.5% and 1%. The cracks relax after induced cracking. The steel fibers decrease permeability of specimens with relaxed cracks larger than 100 microns. Keywords: permeability, fiber-reinforced concrete, steel fibers 1 NSF Center for Advanced Cement--Based Materials, Northwestern University, 2145 Sheridan Rd., Evanston, IL, 60208--4400, USA 2 Saint Gobain Technical Fabrics, P. Box 728, St. Catharines, Ontario, L2R-6Y3, Canada 3 Department of Industrial Engineering and Management Science, Northwestern University, 2145 Sheridan Rd, Evansto...
This paper presents the development of new methods for measuring aggregate resistance to polishing, and degradation. These are important properties that influence pavement skid resistance and resistance to distresses under traffic loading. The aggregate imaging system is used to measure aggregate surface texture after different polishing time intervals in the micro-Deval test. Mathematical functions are then used to describe the initial texture, rate of polishing, and final texture. Aggregate abrasion is quantified by the percent difference in surface angularity before and after the micro-Deval test, while aggregate breakage is described by the weight loss in this test. The efficacy of the new methods is demonstrated through the analysis of aggregates from different sources that exhibit a wide range of properties. In addition, the new methods are verified by comparing the measured aggregate properties to the degradation of these aggregates in hot mix asphalt mixtures due to compaction forces.
An experimental campaign supports a model named TransChlor for bringing liquid water moved by capillary suction and chloride ions into concrete. The principal objective of the experimental campaign is to represent conventional concretes under real conditions of a local microclimate. Capillarity tests were performed on specimens of three different types of concrete dried at different levels of relative humidity at low temperatures (to -20°C) to represent the effect of a real climate in winter. This article shows that low temperatures decrease water absorption by capillarity. The use of TransChlor to simulate liquid water and chloride ion penetration into concrete is presented with an emphasis on the faster transport mode of chloride ions by convection when water is in contact with concrete.
Proportioning of concrete mixes is carried out in accordance with specified code information, specifications, and past experiences. Typically, concrete mix companies use different mix designs that are used to establish tried and tested datasets. Thus, a model can be developed based on existing datasets to estimate the concrete strength of a given mix proportioning and avoid costly tests and adjustments. Inherent uncertainties encountered in the model can be handled with fuzzy based methods, which are capable of incorporating information obtained from expert knowledge and datasets. In this paper, the use of adaptive neuro-fuzzy inferencing system is proposed to train a fuzzy model and estimate concrete strength. The efficiency of the proposed method is verified using actual concrete mix proportioning datasets reported in the literature, and the corresponding coefficient of determination r(2) range from 0.970-0.999. Further, sensitivity analysis is carried out to highlight the impact of different mix constituents on the estimate concrete strength.
This paper presents a variability study of several engineering properties of tire-derived aggregate based on a comprehensive literature survey of experimental test programs. The dry compacted unit weight, cohesion intercept, friction angle, constrained modulus, and modified secondary compression index were evaluated and compared to the engineering parameter variability of natural soils. A series of regression analyses were performed to investigate the presence and significance of scale-dependency. The results of the variability analysis indicate that unit weight has the lowest value of coefficient of variation COV whereas the shear strength parameters, constrained modulus, and compression index have COV values that are substantially higher. Regression analyses indicated that unit weight and constrained modulus showed the greatest sensitivity to changes in maximum tire particle size. A nonstatistical investigation was used to further investigate the variability and scale-dependency of the shear strength parameters. Using Mohr-Coulomb failure criterion and assuming that cohesion is negligible, the analysis showed a scale-independent relationship which is consistent with the statistical findings for cohesion and friction angle.
This thesis presents a continuum model for asphalt concrete incorporating non- linear viscoelasticity, viscoplasticity, mechanically-induced damage and moisture- induced damage. The Schapery single-integral viscoelastic model describes the nonlinear viscoelastic response. The viscoplastic model of Perzyna models the time- dependent permanent deformations, using a Drucker-Prager yield surface which is modified to depend on the third deviatoric stress invariant to include more complex dependence on state of stress. Mechanically-induced damage is modeled using continuum damage mechanics, using the same modified Drucker-Prager law to determine damage onset and growth. A novel moisture damage model is proposed, modeling moisture-induced damage using continuum damage mechanics; adhesive moisture- induced damage to the asphalt mastic-aggregate bond and moisture-induced cohesive damage to the asphalt mastic itself are treated separately. The analytical model is implemented numerically for three-dimensional and plane strain finite element analyses, and a series of simulations is presented to show the performance of the model and its implementation. Sensitivity studies are conducted for all model parameters and results due to various simulations corresponding to laboratory tests are presented. In addition to the continuum model, results are presented for a micromechanical model using the nonlinear-viscoelastic-viscoplastic-damage model for asphalt mastic and a linear elastic model for aggregates. Initial results are encouraging, showing the strength and stiffness of the mix as well as the failure mode varying with moisture loading. These initial results are provided as a an example of the model's robustness and suitability for modeling asphalt concrete at the mix scale.
Current adhesion theories fail to explain completely the stripping phenomenon. Quantification of stripping potential prior to mix design, and based on variables described in current adhesion theories, remains difficult. Current laboratory evaluation procedures are not completely reliable and fail to predict, with accuracy, the stripping susceptibility of various asphalt concrete mixes. This paper evaluates the existing adhesion theories and stripping mechanisms suggested in the literature. Factors contributing to the lack of success in predicting stripping susceptibility of asphalt concrete in the laboratory are also discussed. An earlier comprehensive investigation of the stripping phenomenon revealed serious deficiencies in the current approach to the problem. First, the interaction between the different components of the asphalt-concrete mixture under a wide temperature range is neglected. It is proven that differential thermal contraction, as a result of the large difference in the coefficients of thermal contraction, is an important factor. This paper addresses another deficiency, which is the lack of a precise description of the location where debonding associated with moisture occurs. Observations made during the experimental investigation revealed that initial separation in the compacted mix takes place between the asphalt matrix (mix of asphalt cement and fine material) and coated aggregate particles.
Current distress analysis practices and material specifications associated with low temperatures, including recent developments by the Strategic Highway Research Program, do not address the potential for localized damage associated with thermal incompatibility of asphalt concrete components. The analytical approach used to explain transverse surface cracking assumes a homogeneous pavement material. Although this approach produced satisfactory results for the prediction of low temperature cracking, it may obscure other forms of damage related to exposure to extreme low temperatures. This paper discusses localized forms of damage related to the vast difference in the coefficients of thermal contraction of asphalt concrete components (binders and mineral aggregates). Microscopic examination of asphalt concrete samples exposed to low temperatures revealed the presence of hairline cracks within the asphalt matrix. The examination also revealed deterioration of the bond at the aggregate/binder interface. The observed damage patterns explain the results of mechanical testing conducted using indirect tensile strength and three point bending procedure. Analysis demonstrates the potential impact of the observed forms of damage on pavement performance, it also explains results published by other researchers.
The pseudo-strain concept based on Schapery’s extended nonlinear elastic-viscoelastic correspondence principle is demonstrated by a considerable database to be able to characterize both microdamage and microdamage healing during the damage process. A change in pseudo stiffness can be used to quantify microdamage and healing during the fatigue test. Pseudo stiffness decreases consistently with increasing number of loading cycles, indicating that microdamage occurs during the fatigue test. The significant recovery of pseudo stiffness after rest periods demonstrates a healing effect due to rest periods. Dissipated pseudo-strain energy is a strong and consistent quantifier of damage and healing. High levels of cumulative dissipated pseudo-strain energy are consistent with high levels of fatigue damage, whereas low levels of cumulative pseudo-strain energy are associated with fatigue damage resistance. Pseudo stiffness is the chord slope of the stress-pseudo strain hysteresis loop. Since linear viscoelastic-based time effects are eliminated in this approach, pseudo stiffness and/or pseudo-strain energy are superior indicators of damage than either stiffness or total dissipated strain energy. The effects of rest periods on fatigue life extension due to healing of microcracks are significant. It has been demonstrated that longer rest periods result in more healing, and in turn in greater fatigue life.
A two-dimensional non-linear finite-element analysis micro-modeling approach to simulate unreinforced masonry beams in bending is extended to include a retrofit with a thin layer of ductile fiber-reinforced cement-based material referred to as engineered cementitious composite (ECC). The retrofit method is one that has been demonstrated to add significant ductility to unreinforced masonry infill walls under in-plane cyclic loading and is further expected to enhance out-of-plane bending resistance. The objective of the research is to identify and propose a modeling approach for this complex system of four materials and three different types of interface using basic material properties and established model parameters for future analyses of the retrofit system in structural applications. Of the two geometric models investigated, a simplified approach using expanded brick units with zero-thickness mortar elements is recommended and validated. Brick-mortar interface opening, cracking of the ECC layer below the mortar joints, and failure of the ECC were captured well. The simulated response is found to be particularly sensitive to the adopted constitutive model of the ECC. Research areas for enhancing the ability of the adopted modeling approaches in predicting the response of this complex system, are identified. DOI: 10.1061/(ASCE)MT.1943-5533.0000412. (C) 2012 American Society of Civil Engineers.
Concrete is good in compression but week in tension that is, concrete is a brittle material. So, in order to improve the tensile properties, short fibers are used. Effects of steel fibers on flexural performance of reinforced concrete (RC) beams are the main objectives of this study. The hooked-end steel fibers with the dimensions of 0.75 mm in diameter, 50 mm in length and with the aspect ratio of 67 were used in this study. Initially the optimum percentage addition of steel fibers in concrete was determined. In order to accomplish this task, several concrete prisms and cubes with the same mix proportioning of concrete and different volume fractions of steel fiber (0.5 %, 1%, 1.5 %, and 2 %) were prepared. Then, by determining the flexural and compressive strength of samples, it was concluded that the optimum volume fraction was 1 % ( 78.5???? ??3). In the next step,the flexural behavior of RC beams with the addition of steel fibers with lower and higher compressive strength of concrete was considered. The study was conducted on two types of concrete with different grades of 30 and 50. For each grade of concrete, two beams were cast which steel fiber was included in one of the beams, with the addition of 1% volume fraction, and the other beam was considered as control beam. The overall dimensions of the beams were 170 mm in height, 120 mm in width, and 2400 mm in length. The beams were tested under four-point loading test. The results showed that addition of steel fibers in concrete increases the first crackingload, ultimate load, stiffness and ductility of the concrete beams. Furthermore, the addition of steel fibers has more effect on the properties of RC beams with higher concrete grade compared to lower grade.
Steel fiber reinforced concrete (SFRC) is widely applied in the construction industry. Numerical elastoplastic analysis of the macroscopic behavior is complex. This typically involves a piecewise linear failure curve including corner singularities. This paper presents a single smooth biaxial failure curve for SFRC based on a semianalytical approximation. Convexity of the proposed model is guaranteed so that numerical problems are avoided. The model has sufficient flexibility to closely match experimental results. The failure curve is also suitable for modeling plain concrete under biaxial loading. Since this model is capable of simulating the failure states in all stress regimes with a single envelope, the elastoplastic formulation is very concise and simple. The finite element implementation is developed to demonstrate the conciseness and the effectiveness of the model. The computed results display good agreement with published experimental data.
The Boltzmann-Matano methodology was used to calculate the nonsteady-state chloride diffusion coefficients for blended cement concrete, which incorporated fly ash and ground granulated blast furnance slag. It was found that the chloride diffusion coefficients depend on time and concentration/depth and can be expressed as linear functions of the Boltzmann variable. Furthermore, there was a marked reduction in the chloride diffusion coefficients when the slag content and curing time were increased. The optimum replacement was 65% slag.
The results of a metallographic study of gray cast-iron water pipes are reported. These pipes were installed between 1885 and 1973 in eight different water systems. Each pipe had been extracted during scheduled maintenance or failure repairs to provide data for a larger study to produce a methodology for determining the residual life of gray cast-iron pipes. This metallographic study was conducted to determine the causes of variations in the mechanical properties of these pipes. Pit-cast and spun-cast pipes were found to have distinctly different types of graphite flakes, flake sizes, and metallic matrices. These differences were directly responsible for the variations in the mechanical properties between the two types of pipes, with the larger flake sizes of the pit-cast pipes, in particular, producing weaker material. Examples of anomalous pipes that did not have the standard appearance of either type of manufacture were also found and the reasons for their appearance identified. The results of the study show that the metallurgy of the pipes may be a major contributing factor, along with external forces such as corrosion or poor installation practices. Metallographic analysis therefore can assist water utilities managers in making decisions on repairs, rehabilitation, and replacement.
The recovery of container glass (cullet) for recycling can often lead to the generation of surplus quantities of the material. The pozzolanicity of finely ground glass cullet (GGC) can be exploited by using it as a cement component in concrete. This paper examines two of the chemical reactions most significant to this mode of use, namely, the pozzolanic reaction and the potentially deleterious alkali-silica reaction. Although the presence of GGC has little effect on the reactions of portland cement, larger quantities of calcium silicate hydrate gel are formed, which are thought to contain quantities of sodium. Blends containing the largest quantities of calcium silicate hydrate gel also displayed the highest strengths. Expansion due to alkali-silica reaction of mortars containing GGC is reduced relative to controls. This can be understood when it is considered that the pozzolanic and alkali-silica reactions are chemically identical. The fine cullet particles have a high surface area and, hence, favor the rapid pozzolanic rate of reaction over the slower alkali-silica reaction rate.
The lamellate structure of clay and the extrusion forming process cause terra cotta ceramic's anisotropic elastic behavior at low strain and may induce gradients of macroscopic mechanical properties. Among other techniques, strain measurements obtained through digital image correlations were used to locally characterize transverse isotropic elastic behavior. Kinematic field measurements on small specimens were also compared with fields derived from three-dimensional finite element computations in order to check the consistency of the local characterization procedure.
Passive site stabilization is a new technology proposed for nondisruptive mitigation of liquefaction risk at developed sites susceptible to liquefaction. It is based on the concept of slowly injecting colloidal silica at the edge of a site with subsequent delivery to the target location using natural or augmented groundwater flow. Colloidal silica is an aqueous dispersion of silica nanoparticles that can be made to gel by adjusting the pH or salt concentration of the dispersion. It stabilizes liquefiable soils by cementing individual grains together in addition to reducing the hydraulic conductivity of the formation. Centrifuge modeling was used to investigate the effect of colloidal silica treatment on the liquefaction and deformation resistance of loose, liquefiable sands during centrifuge in-flight shaking. Loose sand was successfully saturated with colloidal silica grout and subsequently subjected to two shaking events to evaluate the response of the treated sand layer. The treated soil did not liquefy during either shaking event. In addition, a box model was used to investigate the ability to uniformly deliver colloidal silica to loose sands using low-head injection wells. Five injection and two extraction wells were used to deliver stabilizer in a fairly uniform pattern to the loose sand formation. The results of the box model testing will be used to design future centrifuge model tests modeling other delivery methods of the grout.
Premature deterioration of concrete structures has created awareness and concern about the durability of concrete. Concrete mixtures used in the construction of residential basement walls and foundations have a high water to cement (w/c) ratio (w/c>0.6) and low cement content (<280 kg/m3). The result is friable concrete with a highly porous surface layer and high potential for cracking. The defects have a direct impact on the durability of concrete. This experimental study examines the effects of three parameters-mix design, formwork, and consolidation-on the quality of the surface of high w/c concrete. The fresh concrete is characterized using its rheological properties-in particular, its yield stress and plastic viscosity. Pulse velocity, pull-off strength, and compressive strength were measured to evaluate the quality and the mechanical properties of the hardened concrete. The durability of the hardened concrete was evaluated by measuring its surface transport properties-namely, its air permeability and sorptivity. The results show that it is possible to correlate the rheological properties of fresh concrete to the mechanical and permeation properties of the hardened concrete.
For use in fire resistance calculations, the relevant thermal properties of high strength concrete were determined as a function of temperature. These properties included the thermal conductivity, specific heat, thermal expansion and mass loss, of plain and steel fibre-reinforced concrete made of siliceous and carbonate aggregate. The thermal properties are presented in equations that express the values of these properties as a function of temperature in the temperature range between 0°C and 1000°C. The effect of temperature on thermal conductivity, thermal expansion, specific heat and mass loss of HSC is discussed. Test data indicate that the type of aggregate has significant influence on the thermal properties of HSC, while the presence of steel fibre-reinforcement has very little influence on the thermal properties of HSC. Dans le cas de leur utilisation dans le calcul de la résistance au feu, les propriétés thermiques pertinentes du béton haute résistance ont été déterminées en fonction de la température. Ces propriétés incluent la conductivité thermique, la chaleur massique, la dilatation thermique et la perte massique du béton ordinaire ou armé de fibres d'acier et fabriqué avec un granulat siliceux ou carbonaté. Les propriétés thermiques sont présentées dans des équations qui en expriment les valeurs en fonction de la température, dans la plage de températures qui va de 0 à 1000 °C. L'effet de la température sur la conductivité thermique, la dilatation thermique, la chaleur massique et la perte massique du béton haute résistance est traité. Les données d'essai indiquent que le type de granulat a une influence importante sur les propriétés thermiques du béton haute résistance, tandis que la présence du renforcement de fibres d'acier n'a que très peu d'influence sur ces propriétés. RES
An extensive laboratory experimental program was conducted to develop high-performance fibrous latex-modified concrete (LMC) and microsilica concrete (MSC) overlay mixtures based on target performance characteristics. The designed LMC and MSC overlay normal weight mixtures have the desired workability, 28-day f(')c and f(r) greater than 41 and 4.5 MPa, respectively, drying shrinkage less than 600 mu epsilon at 90 days, low permeability, and adequate hardened air-void parameters. In comparison, the LMC showed lower shrinkage and permeability values than the MSC. The fibrous LMC and MSC mixtures experienced toughness indices I-30 values in the range of 8.78-16.42 and had lower shrinkage values than identical mixtures without fibers. In addition, the values of residual strength factors reflect the same fact that synthetic and steel fibers were very effective in recovering the load in postcracking range. The entire overlay types showed high bond strengths as a result of the excellent surface preparation using water-jet blasting. The careful implementation of the right procedures of mixing, placing, finishing, and curing significantly assisted in producing crack-free overlay applications with high bond strengths.
A two‐stage freezing model is proposed for concrete at a near‐saturated state. The first‐stage involves a mechanism considered by Powers, while the second‐stage is based on a theory due to Everett. The volumetric consequences of this model checked by dilatometry do not perfectly materialize, since the pore air is not compressible enough as assumed in Powers' theory. Experiments qualitatively confirm the expected abrupt dependence of frost damage on degree of saturation. The role of air entrainment in minimizing the frost damage in the field is explained primarily by lowering the rate of saturation and partly by its retardation of the second‐stage freezing by increasing the compressibility of the gas phase. Experiments also confirm that the dilatometric expansion on freezing is a measure of significant damage as predicted by the proposed model.
In concrete pavements, steel dowels are exposed to a particularly aggressive environment that leads to depassivation and greatly reduces the corrosion initiation stage. Aggressive agents such as chlorides and CO2 have easy access to the dowels through pavement joints and, consequently, the corrosion performance of the system depends largely on the properties of the steel dowel being used. This study investigates the corrosion performance of several types of steel dowels embedded in concrete and subjected to accelerated corrosion by exposure to 3.5% NaCl solution for 18 months . Seven types of dowels were tested: bare carbon steel, stainless steel clad, grout-filled hollow stainless steel, microcomposite steel, carbon steel coated with bendable epoxy, and carbon steel coated with nonbendable epoxies. Half-cell potential, polarization resistance, visual inspections, and microscopic investigations by scanning electron microscopy were carried out to evaluate their corrosion performance. Results show that microcomposite steel dowels exhibit greater resistance to corrosion propagation than carbon steel dowels, but lesser than stainless clad and stainless hollow bars. In epoxy-coated bars, corrosion occurred at a few localized defective areas, generally at holidays and edges of bar ends. No significant difference was observed between nonbendable and bendable epoxy-coated dowels.
Meso-scale constitutive models of frost-damaged concrete are developed in this study through numerical simulation using a two-dimensional Rigid Body Spring Model (RBSM). The aim of the simulation is to predict the macro behavior of frost-damaged concrete subjected to mechanical loading. The models also clarify the difference in failure behavior of concrete with and without frost damage. Zero strength elements and the concept of meso-scale plastic tensile strain are introduced into the normal RBSM springs to consider the experimentally observed cracking and plastic deformation caused by frost damage. The difference in the effect of frost damage on compression and tension behavior as found in the experiments is clearly predicted. Finally, analysis of a notched beam subjected to bending after different degrees of frost damage is carried out. The resulting load-deflection curves agree well with those obtained in the experiments. These good correlations confirm the applicability of the meso-scale model for predicting the macro behavior of frost-damaged concrete.
The effects of high temperature on the strength and stress-strain relationship of high strength concrete (HSC) were investigated. Stress-strain curve tests were conducted at various temperatures (20, 100, 200, 400, 600, and 800degreesC) for four types of HSC. The variables considered in the experimental study included concrete strength, type of aggregate, and the addition of steel fibers. Results from stress-strain curve tests show that plain HSC exhibits brittle properties below 600degreesC, and ductility above 600degreesC. HSC with steel fibers exhibits ductility for temperatures over 400degreesC. The compressive strength of HSC decreases by about a quarter of its room temperature strength within the range of 100-400degreesC. The strength further decreases with the increase of temperature and reaches about a quarter of its initial strength at 800degreesC. The strain at peak loading increases with temperature, from 0.003 at room temperature to 0.02 at 800degreesC. Further, the increase in strains for carbonate aggregate HSC is larger than that for siliceous aggregate HSC.
Application practices of bituminous hot-poured crack sealants, typically used to effect repairs on pavements, were monitored in the field and heating conditions were reproduced in the laboratory. From viscometry, thermogravimetry, and tensile testing measurements, it was found that sealants degrade upon heating at recommended installation temperatures. Consequently, given the level of product degradation observed from laboratory evaluations, it is not expected that a consistent low temperature sealant performance could be obtained in the field. These conclusions help explain the number of premature sealant failures that exist despite the fact that these materials may be in accordance with the requirements in ASTM D 3405.
Publicación ISI Email : firstname.lastname@example.org; email@example.com; firstname.lastname@example.org; email@example.com An experimental program is reported in which the mechanical properties of high-damping natural rubber were measured. Rubber specimens were tested using a custom-designed mechanical apparatus that is equipped with a temperature chamber as well as displacement and force transducers. Each specimen was subjected to a wide range of carefully controlled dynamic loading tests. Variables studied include the strain, strain rate, and temperature of the material. Results from laboratory testing were used to formulate a fuzzy model of mechanical behavior for shear of the rubber compound. Instead of a mathematical representation of the high-damping rubber material, a Takagi-Sugeno-Kang fuzzy inference system is used to relate three input variables with the shear stress. Parameters of seven membership functions are determined through the use of an (adaptive neurofuzzy inference system). Experimental data applied for training and checking of the model are concatenated sets. The resulting fuzzy inference system is shown graphically and analytically to represent accurately the behavior of the high-damping rubber while minimizing computational requirements.
The effects of accelerated heat aging on three polyvinyl chloride (PVC) roofing membranes was determined using thermogravimetry (TG), dynamic mechanical thermal analysis (DMA), and tensile testing. The heat aging was done at 100°C and 130°C. In general, it was found that the degradation of the reinforced PVC roofing membranes was proportional to the aging temperature and exposure time. Furthermore, it was found that these thermoanalytical techniques can be used to not only record the changes incurred by the samples due to heat aging, but also to provide a ranking of the heat stability of the various materials. The order of stability, using these techniques, was determined to be VI < V3 < V2, which corresponded with the observed order of stability from actual records obtained on roofs. The results obtained demonstrate that TG and DMA, in conjunction with tensile testing, provide a viable, routine method of evaluating the quality of single- ply PVC-based roofing membranes.
Internal curing has emerged over the last decade as an approach to counteract the negative effects associated with self-desiccation in low water-to-cement ratio (w/c) mixtures. Specifically, much of the early research on internal curing focused on the reduction of autogenous shrinkage. Recent work has demonstrated, however, that internal curing can also be beneficial in reducing drying-shrinkage cracking, reducing the propensity for thermal cracking, reducing fluid absorption, and reducing ion diffusion in concrete. However, several aspects of internal curing still require closer examination. One of these aspects is the application of internal curing for mixtures with a wider range of water-to-cement ratios. This paper describes results from experiments that investigated the potential use of internal curing in mortar systems with w/c ratios of 0.30, 0.36, 0.42, and 0.45 that were cured under sealed conditions, in terms of water absorption and electrical conductivity. Test results show that internal curing reduces the water absorption in all the systems. Similarly, results obtained on electrical conductivity at late ages (1 year) also show a benefit. Care needs to be taken to analyze electrical conductivity results at early ages because of the increased amount of fluid resulting from the inclusion of the prewetted lightweight aggregate. DOI: 10.1061/(ASCE)MT.1943-5533.0000377. (C) 2012 American Society of Civil Engineers.
In order to prolong the service life of reinforced concrete structures in a severe marine environment, a novel alloy steel (00Cr10MoV) was developed. The passivation behavior of 00Cr10MoV steel in a simulated concrete pore solution (SCPS) was studied by electrochemical impedance spectroscopy (EIS). Furthermore, the influence of chloride concentrations and passive film on the pitting corrosion resistance of 00Cr10MoV steel was investigated using cyclic potentiodynamic polarization (CPP) and scanning electron microscopy (SEM)/energy dispersive X-ray spectroscopy (EDS). For comparison, conventional low-carbon steel (20MnSiV) and 2304 duplex stainless steel were also investigated. The results indicate that the pitting corrosion resistance of passivated 00Cr10MoV steel is significantly higher than that of passivated 20MnSiV steel. The formation of a protective passive film and the generation of dense Cr-bearing corrosion products of 00Cr10MoV steel are responsible for its high pitting corrosion resistance.
This paper reports a systematic study conducted during application of pumping concrete for distances up to 2.432 km at an actual construction site. Admixture design for maintaining flowable concretes through 12 h, while setting within 20 h and developing stripping strength in 30 h was critical. Full-scale field trials for checking field behavior of concrete, pump, their interactions and manpower training are synergistic; weather plays a critical role. The mixes are to be adjusted in response to change in ambient temperatures and differences between production and placement temperatures. A distance-dependent mixture design approach was used for optimizations that lead to the development of a distance-paste-binder curve.
The importance of sufficient masonry mortar joint-bond strength when a structure is subjected to in-plane and out-of-plane loads has been emphasized by several researchers. However, masonry unit/mortar bond strength is difficult to predict, and performing mechanical tests on existing masonry buildings to determine masonry flexural bond and shear bond strengths is generally not practical, such that predictive expressions relating the masonry flexural bond and shear bond strengths to other masonry properties are desirable. Although relationships between brick/mortar bond and compressive strength have been investigated previously by researchers located in many different parts of the world, most of these studies were laboratory-based and did not include testing of existing masonry buildings within their scope. The writers aimed to characterize the material properties of New Zealand unreinforced clay brick masonry (URM) buildings that were generally built between 1880 and 1930, with in situ testing and sample extraction performed on six heritage buildings. Masonry compression, bond wrench, and shear bond tests were undertaken. The experimental results indicate that the masonry flexural bond strength and bed-joint cohesion can be satisfactorily related to the mortar compressive strength.
This paper reviews the methods adopted to produce high-performance concrete (HPC) and ultrahigh-performance concrete (UHPC). The chronological development of these concretes in terms of their constituents, mixture proportions, mixing protocols, and particle packing models from selected literature are presented. The paper highlights the earliest techniques that were used to obtain cementitious materials with high strength and durability, including pressure mixing and heat curing. The paper also covers the work done on HPC and UHPC since the late 1990s and summarizes the current state of the art. Numerous mixture proportions to attain target compressive strengths between 100 and 200 MPa are presented. Higher compressive strengths are achieved with denser mixtures (with practically achievable maximum particle packing densities, i.e., interparticle pores are minimized). In other words, particle packing density is a major attribute in the achievement of low porosity, flowability, durability, and reduced defects in concrete. Therefore, models, theories, and trial methods to achieve a higher packing density in concrete are presented.
The infiltration characteristics of two different hydrophobic materials were investigated in this study. The materials used in this study were fine and coarse recycled concrete aggregates (RCA). One-dimensional (1D) vertical infiltration column tests were carried out on fine RCA over coarse RCA, fine RCA over coarse RCA with oil, and fine RCA over coarse RCA with wax. The column apparatus were instrumented with a tensiometer-transducer system, data acquisition system, soil moisture sensor, and electronic weighing balance. The comprehensive instruments were used to measure pore-water pressure, water content, and outflow rate of water instantaneously and automatically. The experimental results indicated that the water flowed faster within the layer of coarse RCA with oil and wax as compared to that within the layer of coarse RCA without oil and wax. This could be attributed to the hydrophobicity of each solid particle of coarse RCA that was coated with oil and wax.
The addition of nanomodifiers is an effective method to improve the performance of cementitious materials. However, the strengthening mechanism is still not fully understood. Molecular dynamics (MD) simulation is an effective research tool that makes it possible to understand the genome of cement-based materials and, thus, their interactions with nanomodifiers at the nanometer scale. Therefore, MD would serve as an aid to design composite materials toward improved performance, reduced material cost, and enhanced ecoefficiency. This article briefly overviews the progress in MD simulations of nanomodified cement-based materials over the past five years. It mainly includes the application of MD simulation methods in understanding the structure, properties, and microstructure evolution of C–S–H toughened by aluminum phase doping, graphene oxide, polymers, and new nanomaterials. In particular, the research on the ion curing ability and transmission properties of the toughened composite materials are reviewed. Challenges related to nanomodified concrete, as revealed by MD simulations, are also discussed.
Two types of steel, 20MnCr5 and S275JR, are compared in this paper based on numerical prediction of their fracture behavior. The comparison was made using the numerically determined J-integral, an important fracture mechanics parameter. For that reason, a numerical algorithm that calculates the J-integral as a measure of crack driving force was developed. As an input, the algorithm uses results from the finite element analysis conducted on numerical models of two types of specimens usually used in experimental fracture investigations, single-edge notched bend (SENB) and disc compact type (DCT). The J-integral results obtained are plotted versus specimen crack growth size (a) for a range of specimens' initial crack sizes (a/W=0.25, 0.5, 0.75).
An experimental and simulation study on mechanical properties and cracking behavior of zinc film on 2Cr13 steel is carried out. The three-point bend with the acoustic emission technique and uniaxial tension tests were innovatively conducted to study the film fracture toughness and the critical thickness of zinc film at different strains. Meanwhile, simulations were made to analyze the influence of different factors on the dimensionless energy release rate of a surface crack and the stress field around the crack tip with the extended finite-element method. Results indicate that the film will not crack until the film thickness reaches the critical thickness, which decreases with the increase of tensile strain. The elastic modulus ratio, the thickness ratio, and the crack spacing have great impacts on the crack energy release rate and substrate stresses around the crack tip. Also, if the substrate stiffness is greater than the film stiffness for a given film thickness, the substrate thickness and the crack spacing are greater when the crack propagation is in a steady state.
In order to reveal shear deformation mechanisms of architectural non-crimp fabric (NCF) composites, detailed
shear stiffness-nonlinearities for a typical coated warp-knitted NCF composite under shear-tension coupling
loading were studied from experimental investigation and FEA models. Shear-tension coupling effects on shear
stiffness of the NCF composite were investigated for different stress states. Its shear stiffness shows significant
nonlinear characteristics which are highly tensile stress level (TSL) related, and therefore a critical TSL value was
obtained for the studied material. Moreover, a new method for determination of shear stiffness was proposed,
and its feasibility was verified by FEA simulation using UMAT. In addition, differences in shear stiffness and
micro-deformation mechanisms between NCF and plain-woven fabric (PWF) composites were analyzed specifically.
Apparently, the yarn-to-yarn compression on the yarns could contribute to the different stiffness variations
between these two composites.
Plastic shrinkage cracking in restrained cement pastes reinforced with wollastonite particles of micro and submicron sizes was studied using a quantitative two-dimensional (2D) cracking experiment. A series of blended paste mixes with portland cement and different grades of wollastonite fibers were developed and tested under low vacuum conditions. Testing parameters included four grades of wollastonite with aspect ratio in the range of 3:1-20:1 and average particle size ranging from 33 to 2,000 μm at 15% cement replacement. Wollastonite beneficially altered the shrinkage cracking morphology by arresting crack growth, wherein crack lengths and widths were reduced by a factor of two, and the area by a factor of three when compared with the control specimens. However, the initial evaporation rate, early age diffusivity, and cumulative moisture loss increased. Influence of the microfibers in controlling early age drying were related to the porosity of the microstructure using mercury intrusion porosimetry (MIP).
In quantifying the morphological features of aggregate, X-ray computed tomography (CT) is one of the most popularly used approaches for capturing aggregate images that is considered accurate and reliable. In this study, however, a three-dimensional (3D) optical scanner was also employed to characterize aggregate morphology in addition to X-ray CT. The major objective is to validate the accuracy of the optical scanner-based image analysis method through a comparative study of the optical scanning and X-ray CT results. Four types of aggregate particles were selected, and their aggregate images were obtained through those two methods. Second, the scanned aggregate images were saved in STereoLithography (STL) files and analyzed. Then the differences in the morphological features of aggregate obtained from these two methods were quantified and evaluated. Through this study, it was found that even though there were relatively large differences in special aggregate particles (flat or elongated aggregates), the 3D optical scanner was comparable with the X-ray CT in the overall quality of the morphological characteristics of aggregate. Furthermore, differences between the two scanning technologies are dependent on particle shape and surface features. Aggregate particles with sharp angles or rich surface features may induce larger differences.
This paper develops a three-dimensional (3D) microscopic model to investigate the mechanical response of cement mortar with random defects at high strain rates. Ellipsoids with randomness in size, shape, and spatial distribution are used to simulate the defects in mortar matrix. First, we propose the steps to generate the ellipsoid. Second, a "take and place" algorithm is employed to generate a model of cement mortar composed of defects. The mapping algorithm is used to generate a finite-element grid. In finite-element modeling, the material model is used in an advanced general-purpose multiphysics simulation software package to simulate the nonlinear behavior of mortar matrix with strain rate effects. Numerical simulations of the specimen under static loadings agree well with test observations, which reveal that the proposed 3D microscopic model and finite-element analytical approach can give reliable predictions. Finally, numerical studies are conducted, focusing on the effect of the defects on the dynamic responses of cement mortar. It is demonstrated that the defects have effects both on the dynamic behavior and failure pattern under high strain rate loading.