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Showing papers in "Aci Materials Journal in 2005"


Journal ArticleDOI
TL;DR: In this paper, the effect of corrosion attack on mechanical properties of reinforcement is investigated through physical tests on bars with simulated and real corrosion damage and through a simple numerical model, which is related principally to the variability of attack along the length of the bar.
Abstract: Corrosion of embedded reinforcement is the most prevalent form of degradation of reinforced concrete structures, and may impair structural capacity through loss of bar section, loss of bond between reinforcement and concrete as a result of longitudinal cracking, or loss of concrete cross section. The effect of corrosion attack on mechanical properties of reinforcement is investigated through physical tests on bars with simulated and real corrosion damage and through a simple numerical model. Bars subjected to local or pitting attack may suffer a relatively modest loss of strength but a significant loss of ductility, and this is related principally to the variability of attack along the length of the bar. The numerical model supplements experimental work through a parametric study on the influence of steel characteristics. Finally, guidelines on assessment are suggested that are derived from results reported in the paper and from elsewhere in the published literature.

327 citations


Journal ArticleDOI
TL;DR: In this article, the authors used semi-adiabatic calorimeter tests on 13 different concrete mixtures and with heat of hydration data from 20 different cement types to provide a convenient, indirect means of characterizing the formation of hydrated products by measuring the heat released during hydration.
Abstract: Models are used to characterize the behavior of concrete exposed to in-place conditions. These models need to include methods to quantify the heat of hydration of cementitious materials. This article presents the formulation of a general hydration model for cementitious materials. The authors note that the degree of hydration characterizes the formation of hydration products as hydration progresses over time, and each concrete mixture has a unique degree of hydration development. The authors used semi-adiabatic calorimeter tests on 13 different concrete mixtures and with heat of hydration data from 20 different cement types to provide a convenient, indirect means of characterizing the formation of hydration products by measuring the heat released during hydration. Their hydration model incorporates the effect of following variables: cement chemical composition, cement fineness, supplementary cementing materials (Class F fly ash, Class C fly ash, and ground-granulated blast-furnace (GGBF) slag cement), mixture proportions, and concrete properties (density, thermal conductivity, and specific heat). The authors conclude that this model provides a reasonable and accurate representation of the heat of hydration development under different curing temperatures.

309 citations


Journal ArticleDOI
TL;DR: In this article, the flexural toughness properties of hybrid fiber-reinforced concrete were investigated and synergistic effects between fibers were identified, if present, and some hybrid composites demonstrated some synergy between fibers.
Abstract: In fiber-reinforced concrete (FRC), fibers can be effective in arresting cracks at both macro and micro levels. Most of the FRC used today involves the use of a single fiber type, which implies that any fiber can provide reinforcement only at one level and within a limited range of strain or crack opening. For an optimal response, various types of fibers may be combined to produce hybrid fiber-reinforced concrete (HyFRC). The influence was quantified of various hybrid fiber combinations on fresh properties of concrete (that is, workability) and on hardened properties such as compressive strength. The objectives of the present study, however, were to investigate the flexural toughness properties of hybrid fiber-reinforced concrete and to identify synergistic effects between fibers, if present. The study found that some hybrid composites demonstrated some synergy between fibers.

139 citations


Journal ArticleDOI
TL;DR: In this paper, the authors used a 150 m-long experimental pumping circuit to carry out a number of full-scale pumping tests in order to investigate the impact of shear on steel-concrete interfaces.
Abstract: Pumping is a technique used for the transport of freshly-mixed concrete, allowing the supply of fresh material in the formwork without the use of any bucket or conveying belt This article reports on a study on pressure and flow during concrete pumping The authors used a 150 m-long experimental pumping circuit to carry out a number of full-scale pumping tests In addition, the fresh concrete rheological and steel-concrete interface behaviors were measured in the laboratory with a rheometer equipped with accessories The authors showed that, in many cases, concrete is not sheared in a pipe during pumping The concrete goes ahead by slip, due to a grout layer that forms close to the pipe wall The losses of pressure are due to the deformations by shear located in this layer The characterize this steel-concrete interface, the authors developed the coaxial cylinder tribometer to measure the yield stress and the viscous constant of the interface The results were analyzed and further validation was acquired on two real job sites The authors note that a complete set of tools (guide, laboratory devices, and software) is now available to apply this method in practice

126 citations


Journal ArticleDOI
TL;DR: In this article, the effect of fly ash content on the properties of MPC was studied with two dead burnt magnesia materials with different MgO contents and fineness, and the results demonstrated that fly ash does improve the bonding and compressive strength of MPCs even at very early ages.
Abstract: To understand the mechanism of fly ash in a novel magnesia phosphate cement (MPC), the microstructure and properties were investigated in the present study. Reference specimens without the incorporation of fly ash were also investigated for comparison purposes. The effect of fly ash content on the properties of MPC was studied with two dead burnt magnesia materials with different MgO contents and fineness. The results demonstrated that fly ash does improve the bonding and compressive strength of MPC, even at very early ages. Fly ash content of 30 to 50% has the best improving effect on MPC, despite the two types of magnesia. Due to the difference of MgO content and particle fineness of two dead burnt magnesia materials, MPC mortars with finer magnesia revealed higher compressive strength. The incorporation of fly ash does not retard the setting reaction of MPC, but it does reduce its total heat evolution. The hydrates and microstructure of MPC paste were examined by an x-ray diffractometer, scanning electron microscopy-energy dispersive x-ray analysis (SEM-EDX), and Fourier transform infrared (FTIR) spectroscopy. These techniques revealed that the products formed in MPC paste were crystal magnesium potassium phosphate hexahydrate and amorphous species. The particles of fly ash fill the voids of MPC paste and strongly bond together with hydrates of MPC. The microanalysis showed that the strengthening of fly ash to the cement might come from physical and chemical effects.

114 citations


Journal ArticleDOI
TL;DR: In this paper, the effect of fiber properties on the plastic shrinkage cracking characteristics of concrete during the first 24 hours after mixing has been investigated, and the authors conclude that the volume fraction and diameter of fiber reinforcement are the two most influential parameters in controlling plastic shrinkages of concrete.
Abstract: Plastic shrinkage cracks in concrete become critical weak points for aggressive substances to penetrate into the internal portion of concrete leading to the acceleration of concrete deterioration. Thus, controlling plastic shrinkage cracking (which usually occurs in the first few hours after the concrete is placed) is vital for developing more durable and longer-lasting structures at a minimum life-cycle cost. This article reports on a study undertaken to evaluate the effect of a number of fibers on the plastic shrinkage cracking characteristics of concrete during the first 24 h after mixing. The study included four synthetic fibers: polypropylene, polyvinyl acetate (PVA), high-density polyethylene (HDPE), and carbon, as well as flexible metallic fibers (FMF), at volume fractions varied from 0.05 to 0.4%. The research explored a range of fiber properties including diameter, length, cross section, form, bond strength, and elastic modulus. Prismatic concrete specimens freshly cast on top of a grooved and hard concrete substrate were investigated under adverse environmental conditions, namely, high temperature, low relative humidity, and high volume and velocity of airflow. The authors conclude that the volume fraction and diameter of fiber reinforcement are the two most influential parameters in controlling plastic shrinkage cracking of concrete. The addition of any fiber with a diameter smaller than 40 microns, with an aspect ratio above 200, in volume fractions in the range of 0.2 to 0.4% (or exceeding that) should, for all practical purposes, eliminate plastic shrinkage cracking in concrete.

104 citations


Journal ArticleDOI
TL;DR: In this article, a self-consolidating lightweight concrete is designed using a combination of the least void volume for a binary aggregate mixture, excessive paste theory, and ACI standard practice for selecting proportions for structural lightweight concrete.
Abstract: This paper deals with the design and properties of self-consolidating lightweight concrete Self-consolidating lightweight concrete is designed using a combination of the least void volume for a binary aggregate mixture, excessive paste theory, and ACI standard practice for selecting proportions for structural lightweight concrete Glass powders and ASTM Class F fly ash are added to produce excessive paste to increase the flowability and segregation resistance of the concrete The five designed concrete mixtures exhibit good flowability and segregation resistance Compared with the fly ash, the use of ground glass powder decreases setting times and increases chloride migration resistance, the strength and drying shrinkage of the concrete The designed self-consolidating lightweight concretes have good freezing-and-thawing resistance When expanded shale or clay is used as coarse aggregate, the concrete containing glass powder does not exhibit deleterious expansion, even if alkali-reactive sand is used as fine aggregate of the concrete

91 citations


Journal ArticleDOI
TL;DR: In this article, the pultruded fabric-cement components were found to have relatively high tensile strength and ductility, exhibiting strain hardening behavior even for fabrics with low modulus of elasticity.
Abstract: The use of reinforcement in thin cement-based elements is essential to improve the tensile and flexural performance. The reinforcements can be either short fibers or continuous reinforcements, in a fabric form. Practical use of fabric-cement composites requires an industrial, cost-effective production process. The objective of this study was to develop the pultrusion technique as an industrial, cost-effective production method of prefabricated thin-sheet fabric-reinforced cement composites. Woven fabrics made from low-modulus polyethylene and glass meshes were used to produce the pultruded cement composites. The influence of fabric type, cell opening, application of pressure during casting, and cement-based matrix modification were examined. The tensile strength and ductility of the pultruded fabric-cement components were found to be relatively high, exhibiting strain hardening behavior even for fabrics with low modulus of elasticity. The best performance was achieved from glass fabric composites with a high content of fly ash. The mechanical properties were significantly affected by the matrix formulation, rheology of the matrix, and the intensity of the pressure applied after the pultrusion process. The promising combination of fabric reinforcement in cement composite products using the pultrusion process is expected to lead to a new class of high-performance fabric-cement composite materials.

84 citations


Journal ArticleDOI
TL;DR: In this paper, an alkali-silica reaction (ASR) study was carried out on beam specimens at the Laboratoire Central des Ponts et Chaussees (LCPC), with Electricite de France (EDF) as a partner.
Abstract: This article reports on an alkali-silica reaction (ASR) study that was carried out on beam specimens at the Laboratoire Central des Ponts et Chaussees (LCPC), with Electricite de France (EDF) as a partner. In the study, three-dimensional deformations were measured on five 3 m-long plain or reinforced concrete beams undergoing partial drying over a period of 14 months. The authors discuss the effect of a moisture gradient over the depth of the beams and the influence of reinforcement on the development of ASR-induced expansions. Tests on companion specimens (cylinders and prisms) were carried out to measure the material expansion and mechanical characteristics, including longitudinal strains and beam deflections, transverse strains, vertical strains, and cracking. The authors make three main conclusions: the anisotropy of ASR-induced expansions has been verified with vertical swellings twice as large as horizontal ones; significant ASR expansions (0.10%) can occur without an external water supply and can lead to significant structural degradation; and an increase in ASR expansions with an external water supply has been quantified by this work. The authors note that once models are validated in this framework, they will be used to assess the dimensional stability and the residual bearing capacity of real ASR-affected structures (e.g., bridges and dams).

82 citations


Journal ArticleDOI
TL;DR: In this article, a study of strain sensing in carbon fiber reinforced cement, as enabled by piezoresistivity, is presented, which is characterized by the gauge factor, defined as the fractional change in electrical resistance per unit strain.
Abstract: The ability of a structural material to sense its own strain (without attached or embedded sensors) is a positive attribute of smart structures. Specific applications include structural vibration control, traffic monitoring, weighting, room occupancy monitoring, and building security. This article reports on a study of strain sensing in carbon fiber reinforced cement, as enabled by piezoresistivity. This type of strain sensing is characterized by the gauge factor, which is defined as the fractional change in electrical resistance per unit strain. This study involved simultaneous measurement of the piezoresistive behavior in the longitudinal and transverse directions for each specimen. Results showed that, under uniaxial compression, the gauge factor in both longitudinal and transverse directions decrease in magnitude with increasing specimen size from 13 to 51 mm, due to a slight decrease in the degree of preferred orientation of the 5 mm-long fibers. The gauge factor in both directions also decreases in magnitude as the fiber content increases beyond the percolation threshold.

66 citations


Journal ArticleDOI
TL;DR: In this paper, the authors present the results of an experimental study on the shrinkage for up to approximately 2 years of high-strength lightweight concrete exposed to a dry environment compared with that of the normal-weight concrete.
Abstract: This article presents the results of an experimental study on the shrinkage for up to approximately 2 years of high-strength lightweight concrete exposed to a dry environment compared with that of the normalweight concrete. The effects of the coarse aggregate density, steel fiber content, and silica fume were also studied and are discussed. The authors also discuss the risk of potential drying-shrinkage cracking of the concrete, the flexural tensile strength, and the modulus of elasticity. Normalweight sand was used for all of the concrete mixtures. Results showed that the shrinkage of the normalweight concrete with granite aggregate was higher than that of the corresponding lightweight concretes (with equivalent mixture proportioning) in the first 6 months. The shrinkage decreased with a decrease in the aggregate density that corresponded to an increase in the aggregate porosity and water absorption. After 1 year, the shrinkage of the lightweight concretes was somewhat higher than that of the normalweight concrete with one exception. The incorporation of 5% silica fume reduced the shrinkage of the concrete significantly, and its effect on the lightweight concrete was more substantial than that on the normalweight concrete.

Journal ArticleDOI
TL;DR: In this paper, the authors presented four shrinkage and creep prediction models: the ACI 209 model, the CEB 90 model, B3 model, and GL 2000 model.
Abstract: Shrinkage and creep of concrete cause deflections and affect the stress distribution in reinforced concrete (RC) structures and within concrete elements. This article presents four shrinkage and creep prediction models: the ACI 209 model, the CEB 90 model, the B3 model, and the GL 2000 model. The shrinkage and creep values determined by each of these models are compared against the RILEM experimental data bank. Five statistical methods are used to determine the order of performance of each model. The authors conclude that the B3 and the GL 2000 are best performing models for shrinkage strain prediction, although the B3 tends to underestimate and the GL 2000 is an overestimating model. The CEB 90, B3, and GL 2000 models are considered the best models to predict creep; all three models tend to have underestimating characteristics. The authors caution that careful selection and interpretation of data and the statistical methods used can influence the conclusions on the performance of model prediction. This information should prove useful for those discussions taking place within international associations on the proper prediction models to be considered in practical design situations.

Journal ArticleDOI
TL;DR: In this paper, a study was conducted to optimize the design of high-strength, high-volume fly ash concretes, where eight concrete mixtures were prepared using the same batch of ordinary portland cement (OPC) and ground low-lime fly ash.
Abstract: The objective of this study was to optimize the design of high-strength, high-volume fly ash concretes. Eight concrete mixtures were prepared using the same batch of ordinary portland cement (OPC) and ground low-lime fly ash. The aggregate grading used in the mixtures of concrete, water-binder ratio, and the maximum particle size of aggregate were kept constant in all concretes, however, the partial replacement of cement by fly ash was varied from 0% to 70% OPC concrete, in steps of 10%. The replacement was on a one-to-one weight basis. At 28 days, there was little reduction in compressive strength up to 40% cement replacement by ground fly ash; then a significant decrease was recorded for the further fly ash dosages. At 56 and 120 days, however, the compressive strength up to 40% cement replacement by fly ash was found to be almost identical to that of the no fly ash concrete, and for one year it was even higher. Beyond 40% replacement, the compressive strength decreased significantly. It was shown that the brittleness index increases substantially with increasing compressive strength of concrete. The results of the rapid chloride penetration tests revealed that high volume ground fly ash concrete had better resistance to the penetration of chloride ions. The mortar phases of these concretes were also prepared. As the dosage of fly ash increased, 2, 7, and 28-day compressive strengths of the mortars decreased. However, at later ages at 56, 120, and 365 days, nearly 40% cement replacement by ground fly ash, the compressive strength of mortar with fly ash was about equal to that of mortar without fly ash. The pozzolanic effectiveness ratio increased with increasing curing time and fly ash content.

Journal ArticleDOI
TL;DR: In this article, the authors report on an experimental study of the development of the compressive strength and elastic moduli of the Portland cement pastes with three different water-cement ratios (0.35, 0.5 and 0.6) cured under an isothermal condition (25 deg C).
Abstract: Reliable information about the early-age properties of cementitious materials is vital in order to assure high quality and avoid problems in performance throughout the life of a concrete structure. This article describes the effect of the connectivity of the solid phase in the microstructure on the mechanical behavior of cement pastes. The authors report on an experimental study of the development of the compressive strength and elastic moduli of the Portland cement pastes with three different water-cement ratios (0.35, 0.5, and 0.6) cured under an isothermal condition (25 deg C.). In addition, the cement hydration model HYMOSTRUC3D was used to simulate the formation of the microstructure. The authors conclude that the percolation threshold of the solid phase in the microstructure of a cement paste is closely related to the w/c (water to cement ratio) that is used. The solid phase percolated earlier, and at a lower degree of hydration, in a cement paste with a lower w/c than in a paste with a higher w/c. The specific effective contact area is a microstructure parameter that describes the degree of interparticle bonding among the hydrating particles. The specific effective contact area appears to be directly related to both the measured compressive strength and the elastic moduli.

Journal ArticleDOI
TL;DR: In this article, the critical chloride threshold values of five uncoated steel reinforcement types (ASTM A 706, ASTM A 615, microcomposite, stainless steel 304, and stainless steel [SS] 316LN) with as-received and polished surface conditions were quantitatively determined using the accelerated chloride threshold test procedure.
Abstract: Deicing and anti-icing salts and seawater are the main sources of chloride ions that cause the corrosion of steel reinforcement embedded in reinforced concrete (RC) bridge and marine structures. This article reports on a study undertaken to evaluate the influence of the steel reinforcement surface condition on the corrosion performance. In the study, the critical chloride threshold values of five uncoated steel reinforcement types (ASTM A 706, ASTM A 615, microcomposite, stainless steel 304, and stainless steel [SS] 316LN) with as-received and polished surface conditions were quantitatively determined using the accelerated chloride threshold (ACT) test procedure. Micrographs of the surfaces (characterizing the mill scale and surface topography) for all steel reinforcement types were obtained using both optical and scanning electron microscopy (SEM). Results showed that the mean critical chloride threshold values increased with the complete removal of the as-received surface and with surface polishing for the ASTM A 706, microcomposite, and stainless steel 304 reinforcements. In addition the mean critical chloride threshold values decreased with the complete removal of the as-received surface and with surface polishing of the ASTM A 615 and SS316LN steels. The authors conclude that removal of the mill scale on the microcomposite steel reinforcement showed a significant improvement in the critical chloride threshold and removal of this mill scale may be economically justified.

Journal ArticleDOI
TL;DR: In this paper, the authors present a literature review and describe the experimental 148 m-closed-loop pumping circuit that was built near Roissy, France, where over 68 concrete loads were pumped, including a number of which were subject to blockage.
Abstract: The concrete pump is often used in commercial construction and is particularly helpful where access is difficult (for example, in tunnels). However, concrete pumping remains to date an empirical, trial-and-error process, involving frequent troubleshooting on construction sites. This article describes a joint research project about concrete pumping. The authors present a literature review then describe the experimental 148 m-closed-loop pumping circuit that was built near Roissy, France. In the study, over 68 concrete loads were pumped, including a number of which were subject to blockage. Results demonstrated four common types of blockages at different stages. The authors focus on the most common type, which happens at priming and is due to coarse aggregate particles, which tend to leave the concrete front and to flow through the grout section, eventually forming a dense plug ahead of the flow. The authors also describe the development of a bleeding test (using a modified air meter) that provides a simple and cheap site test that can alert construction crews to the likelihood of an aggregate blockage.

Journal ArticleDOI
TL;DR: In this article, an algorithm is developed to simulate the three-dimensional distribution of cement particles between aggregates in concrete, and the authors investigate the characteristics of ITZ microstructure in a statistical manner.
Abstract: Interfacial transition zone (ITZ) constitutes the weakest link in the normal concrete matrix and significantly affects the properties of the concrete. To quantitatively evaluate the effect of ITZ on the physical and mechanical behavior of concrete, it is imperative to characterize the microstructure of ITZ. The intention of this paper is to investigate the characteristics of ITZ microstructure in a statistical manner In this paper an algorithm is developed to simulate the three-dimensional distribution of cement particles between aggregates in concrete.

Journal ArticleDOI
TL;DR: In this article, the effectiveness of a nonlinear ultrasonic testing method was compared with established ASTM testing methods in detecting early damage in concrete. But, the authors concluded that the nonlinear method shows considerable promise as a sound, nondestructive evaluation technique for detecting early damages in concrete, and concluded that this method generally increased with increasing water to cement (w/c) ratio.
Abstract: Many types of damage or deterioration in concrete cause microcracking. Damage usually progressed from evenly distributed microcracks, to major cracks, to eventual failure of the concrete. This article reports on a study undertaken to compare the effectiveness of a nonlinear ultrasonic testing method with established ASTM testing methods in detecting early damage in concrete. In this study, geometrically identical concrete specimens were evaluated with nonlinear ultrasonic techniques. When the fundamental ultrasonic frequency interacts with a material, harmonics are generated. As damage increases, the magnitude of the nonlinear interaction increases, causing a greater portion of the fundamental frequency to be converted to higher harmonics. The authors included specimens of three distinct water-cement ratios which were compressed at regular load increments to induce damage. At each load increment, no cracking was visually observed. However, with increases in damage to the specimens, increases in harmonic ratios were measured. These increases were markedly larger as compared with changes noted with other nondestructive ASTM tests performed. The authors conclude that the nonlinear ultrasonic method shows considerable promise as a sound, nondestructive evaluation technique for detecting early damage in concrete. Sensitivity of this method generally increased with increasing water to cement (w/c) ratio.

Journal ArticleDOI
TL;DR: In this article, the effect of local high-volume fly ash (HVFA) incorporating a mid-range water reducer on compressive strength of concrete and on chloride penetration and steel corrosion under tropical climate conditions was investigated.
Abstract: This study investigated the effect of local high-volume fly ash (HVFA) incorporating a mid-range water reducer on compressive strength of concrete and on chloride penetration and steel corrosion under tropical climate conditions. The results of electrically accelerated corrosion tests were compared with normal tests. Half-cell potential measurements and visual examination indicated the effectiveness of the corrosion reduction, independent of compressive strengths. A mid-range water reducer improved the 28-day strength by 50 to 75%, compared with 8 to 36% of concrete with 0 to 35% replacement. It also yielded lower chloride permeability and corrosion risk. HVFA concrete with a replacement of 50 to 65% was ranked at greater than 90% probability of no steel corrosion, and the corrosion rank improved from a severe-moderate level to slight or no corrosion at all. Fly ash content, water reducer, and water-binder ratio (w/b), respectively, were three parameters that influenced corrosion risk, while w/b was the most important factor for normal concrete.

Journal ArticleDOI
TL;DR: In this paper, a modified ACI drop-weight impact test for concrete is proposed, which is applied to polypropylene fiber-reinforced concrete (PPFRC).
Abstract: This article reports on a study of a modified ACI drop-weight impact test for concrete. The industry standard, the ACI Committee 544’s repeated drop-weight impact test, is often criticized for large variations within the results. The authors of this paper identifed the sources of these large variations and developed modifications to the ACI test. The proposed modifications were evaluated by conducting impact resistance tests on 40 specimens from two batches of polypropylene fiber-reinforced concrete (PPFRC). The impact resistance of PPFRC specimens tested with the current ACI test exhibited large coefficients of variation (COV) of 58.6% and 50.2% for the first-crack and the ultimate impact resistance, respectively. The corresponding COV for PPFRC specimens tested according to the modified technique were 39.4% and 35.2%, indicating that the reliability of the results was significantly improved. Using the current ACI test, the minimum number of replications needed per each concrete mixture to obtain an error below 10% was 41, compared to 20 specimens for the modified test. The authors conclude that although such a large number of specimens is still not good enough for practical and economical reasons, the reduction presents a good step forward on the development of a standard impact test.

Journal ArticleDOI
TL;DR: In this paper, the effects of cement hydration and carbonation on the strength, porosity, and elastic modulus of carbonated concrete specimens were identified using the three-layer inclusion model of composite materials theory.
Abstract: Three zones of full carbonation, partial carbonation, and noncarbonation were identified to investigate the changes in the mechanical properties of carbonated concrete. The porosity of each zone was measured using the mercury intrusion porosimetery (MIP) method. A three-layer inclusion model of composite materials was adopted to calculate the effective elastic modulus. The results of this study indicate carbonation reduces the porosity of concrete, hence compressive strength and elastic modulus increases with an increase in the degree of carbonation, but conversely for ductility. The effects of cement hydration and carbonation on the strength, porosity, and elastic modulus of concrete were identified. The effective elastic modulus Eg calculated using the three-layer inclusion model of composite materials theory could accurately predict the elastic modulus of carbonated concrete specimens.

Journal ArticleDOI
TL;DR: In this article, the authors investigated the fatigue strength of steel fiber-reinforced concrete (SFRC) containing mixed fibers and found that the distribution of fatigue life of SFRC can be modeled by two-parameter Weibull distribution.
Abstract: The fatigue strength of steel fiber-reinforced concrete (SFRC) subjected to flexural loading is highly important in the design of bridges and pavement slabs because the flexural stresses in such structures are critical. The present study investigates the fatigue strength of SFRC containing mixed fibers. An experimental investigation was conducted to obtain the fatigue lives of SFRC at various stress levels. Two-hundred and sixty-nine SFRC beam specimens, 500 x 100 x 100 mm, were tested under four-point flexural fatigue loading to obtain the fatigue lives of SFRC specimens. One-hundred and eight complimentary static flexural tests were also performed to obtain the static flexural strength prior to fatigue testing. The specimens incorporated 1.0, 1.5, and 2.0% volume fractions of corrugated mixed steel fibers of 0.6 x 2.0 x 25 mm and 0.6 x 2.0 x 50 mm in different proportions. The test data was used to generate S-N relationships, and equations were proposed to predict the flexural fatigue strength of SFRC. It was found that the distribution of fatigue life of SFRC can be modeled by two-parameter Weibull distribution. This distribution was utilized to incorporate the failure probabilities into the fatigue life of SFRC. The fatigue lives corresponding to different failure probabilities were calculated and the data therein obtained was used to generate Pf-S-N diagrams.

Journal ArticleDOI
TL;DR: In this article, the influence of concentration and nominal size of coarse aggregate on the development of lateral and pore water pressures of self-consolidating concrete (SCC) is investigated.
Abstract: This article reports on a study undertaken to determine the influence of concentration and nominal size of coarse aggregate on the development of lateral and pore water pressures of self-consolidating concrete (SCC). The study evaluated 9 mixtures prepared with sand-total aggregate ratios (S/A) varying between 1.0 and 0.30. An experimental column measuring 2800 mm in height and 200 mm in diameter was used to determine lateral pressure and pore water pressure during the plastic stage of cement hydration. Results show that lateral pressure is significantly influenced by the S/A value. The pore water pressure affects the development of lateral pressure of SCC in the plastic stage. The authors conclude that the lateral pressure developed by the plastic concrete is directly related to internal friction resulting from the coarse aggregate concentration. The greater the degree of increase in internal friction, which corresponds to mixtures made with relatively low S/A values, the lesser the magnitude of initial lateral pressure becomes and the faster the drop in pressure.

Journal ArticleDOI
TL;DR: In this paper, the effect of binder type and content on variations in the lateral pressure of self-consolidating concrete was investigated and the influence of thixotropy, determined from concrete and concrete-equivalentmortar (CEM) mixtures, on the variations of lateral pressure development was also investigated.
Abstract: Self-consolidating concrete (SCC) is a high-performance concrete that flows readily under its own weight and achieves good consolidation with a minimum degree of segregation. This article reports on a study undertaken to determine the effect of binder type and content on variations in the lateral pressure of SCC. The mixtures were prepared with five binder types incorporated at various contents varying from 400 to 550 kg/m3. The influence of thixotropy, determined from concrete and concrete-equivalent-mortar (CEM) mixtures, on the variations of lateral pressure development was also investigated. Results showed that, for a given binder content, the initial lateral pressure and rate of pressure drop with time are significantly affected by the binder type in use. Self-consolidating concrete made with 450 kg/m3 of Type 10 CSA cement (GU) and no supplementary cementitious materials exhibited the highest initial pressure, corresponding to 98% of hydrostatic pressure. Test results also indicate that the rate of pressure drop following casting is dependent on the degree of increase in cohesion. The increase in the degree of thixotropy of SCC and CEM can lead to lower initial pressure. The increase in thixotropy determined from concrete mixtures is highly affected by internal friction resulting from the presence of coarse aggregate. This can overshadow the development of cohesion resulting from the binder phase that controls the rate of pressure drop with time. Thixotropy evaluated using CEM mixtures is more adequate to assess the decrease in lateral pressure development with time than thixotropy determined from SCC mixtures.

Journal ArticleDOI
TL;DR: In this paper, the authors investigated the size effect phenomenon on both normal-and high-strength concrete cylinders subjected to static and dynamic compression and concluded that both tests and numerical models proved the existence of a size effect in parameters other than strength such as the modulus of elasticity and the strain at maximum stress.
Abstract: Structural concrete can be subjected to high loading rates, such as those associated with impact incidents (vehicle collision into structures). This article reports on a study in which the size-effect phenomenon was investigated numerically and experimentally on both normal- and high-strength concrete cylinders subjected to static and dynamic compression. Parallel pretest and post-test computational simulations were used to perform numerical tests of the same specimens and to explore the role of the time dimension on the physical phenomena that contribute to the size effect. The authors discuss the static test results, static test data and size-effect law, modulus of elasticity, strain at maximum stress, dynamic test results, modes of failure, strength criteria, and the loading rate effect. The authors conclude that both tests and numerical models proved the existence of a size effect in parameters other than strength such as the modulus of elasticity and the strain at maximum stress. The article includes numerous high-speed photographs that enable the detection of many modes of failure of concrete specimens under dynamic tests.

Journal ArticleDOI
TL;DR: In this paper, an automated linear traverse system for determination of air-void parameters in hardened concrete is described, where an image analysis system is used for acquisition of surface images through a microscope, there is a methodology for detection and identification of entrained air voids, and a computer-controlled two-dimensional motorized actuator for bringing the sampling points in line with the microscope.
Abstract: Air-void against are used to improve freezing and thawing resistance of concrete. Depending on the size of aggregates used in the concrete mixture, the volume of air required for optimum resistance against frost damage varies from 4.5% to 7% by volume of concrete. This article describes the development of an automated linear traverse system for determination of air-void parameters in hardened concrete. The image analysis system is used for acquisition of surface images through a microscope, there is a methodology for detection and identification of entrained air voids, and a computer-controlled two-dimensional motorized actuator for bringing the sampling points in line with the microscope. In addition to the sample preparation procedures involved in ASTM C 457, the automated technique requires an additional step that includes pigmenting of the sections to create color contrast between the voids and the concrete matrix. The authors report that, following the sample preparation steps, the process requires approximately 15 min for the complete determination of air-void parameters along a 2642 mm (104 in.) traverse length. The system automatically analyzes the data and reports on air-void parameters in terms of air content, specific surface, spacing factor, void frequency, and other parameters of importance. The authors report on their evaluation of the system with a series of experiments that involved nine concrete specimens with various levels of entrained air contents.

Journal ArticleDOI
TL;DR: In this article, a new technique of the neural identification of the compression strength of concrete on the basis of nondestructive tests is described, and the neural training and testing processes were carried out on a data set created on a basis of non-destructive tests.
Abstract: In this article, a new technique of the neural identification of the compression strength of concrete on the basis of nondestructive tests is described. The neural training and testing processes were carried out on a data set created on the basis of nondestructive tests. The data set covers a concrete compression strength range from 24 to 105 MPa after 28 days of curing. The methodology of the neural identification of the compression strength of concrete is presented. The results presented herein demonstrate that the identification of compressive strength of concrete by neural networks on the basis of such nondestructive methods as ultrasonic, sclerometric, and pull-out is a viable technique.

Journal ArticleDOI
TL;DR: In this paper, the effects of rheological parameters and density differences between lightweight coarse aggregates (LWA) and mortar matrix on the stability of fresh lightweight aggregate concrete (LWAC) under vibration were studied.
Abstract: The effects of rheological parameters and density differences between lightweight coarse aggregates (LWA) and mortar matrix on the stability of fresh lightweight aggregate concrete (LWAC) under vibration were studied. Thestability of fresh concrete decreased with its yield stress or plastic viscosity. Tests using LWA of three different densities showed that a LWAC with a smaller density difference between the LWA and mortar matrix had better stability. Air entrainment in concrete reduced the density difference between a given LWA and the mortar matrix, and also reduced the plastic viscosity and the LWA content for a fixed material proportion. The former tends to increase stability, and the latter tends to reduce stability. The results showed that the stability of concrete was reduced with an increase in entrained air content. This indicated that the latter had a more significant effect on the stability of concrete.

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TL;DR: In this article, the relationship between reflection loss and the compressive strength of a portland cement mortar under different isothermal curing conditions was investigated and the established correlations between the two studied parameters were verified by applying alternating curing temperatures to the mortar.
Abstract: Among the various factors that influence the strength gain of cementitious materials at early age, curing temperature can be regarded as an important parameter due to its great effect on the hydration kinetics of portlandcement. The temperature effects on the relationship between the compressive strength of paste, mortar, or concrete and the reflection loss obtained with ultrasonic shear-wave reflection measurements were observed. In the study, three different constant curing temperatures (15, 25, and 35 °C) were used for a given paste or mortar. The temperature effects were observed by comparing the established relationship between reflection loss and compressive strength for portland cement mortar under different isothermal curing conditions. Additionally, the established correlations between the two studied parameters were verified by applying alternating curing temperatures to the mortar. As a third step, the temperature dependence of the relationship between the two studied parameters was further verified by observing concrete cured under various conditions. The study shows that for a given material, the relationship between the reflection loss and the compressive strength is independent of curing temperature.

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TL;DR: In this paper, the behavior of cellulose fiber-reinforced paste, mortar, and concrete is investigated and the relationship among fiber dispersion, mechanical performance, and rheology is investigated.
Abstract: Fiber-reinforced concrete is a fast-growing technology. This research considers the use of cellulose fibers as an inexpensive alternative to synthetic fibers. This study characterizes the behavior of cellulose fiber-reinforced paste, mortar, and concrete and investigates rheological, dispersion, and mechanical properties of the materials. The addition of cellulose fibers to a cementitious material stiffens the matrix. This stiffening limits the maximum usable fiber volume. At fiber volumes used, fibers offer little improvement in flexural properties. Optical and scanning electron microscopy are used to inspect fiber dispersion. A technique is developed to locate fibers in hardened specimens. Correlations among fiber dispersion, mechanical performance, and rheology are investigated. Restrained ring shrinkage tests are performed on paste and mortar. Fibers are found to reduce the width of shrinkage cracks in both materials. Fiber dispersion, matrix rheology, and mechanical performance cannot be correlated. Fibers disperse well under normal mixing conditions, regardless of matrix rheological properties.