The role of 0–2 mm fine recycled concrete aggregate on the compressive and splitting tensile strengths of recycled concrete aggregate concrete

The aim of this study is to investigate the role of 0–2 mm fine aggregate on the compressive and splitting tensile strengths of recycled concrete aggregate (RCA) concrete with normal and high strengths. Normal coarse and fine aggregates were substituted with the same grading of RCAs in two normal and high strength concrete mixtures. In addition, to keep the same slump value for all mixes, additional water or superplasticizer were used in the RCA concretes. The compressive and splitting tensile strengths were measured at 3, 7 and 28 days. Test results show that coarse and fine RCAs, which were achieved from a parent concrete with 30 MPa compressive strength, have about 11.5 and 3.5 times higher water absorption than normal coarse and fine aggregates, respectively. The density of RCAs was about 20% less than normal aggregates, and, hence, the density of RCA concrete was about 8–13.5% less than normal aggregate concrete. The use of RCA instead of normal aggregates reduced the compressive and splitting tensile strengths in both normal and high strength concrete. The reduction in the splitting tensile strength was more pronounced than for the compressive strength. However, both strengths could be improved by incorporating silica fume and/or normal fine aggregates of 0–2 mm size in the RCA concrete mixture. The positive effect of the contribution of normal sand of 0–2 mm in RCA concrete is more pronounced in the compressive strength of a normal strength concrete and in the splitting tensile strength of high strength concrete. In addition, some equation predictions of the splitting tensile strength from compressive strength are recommended for both normal and RCA concretes.     

Steel–concrete bond strength of lightweight self-consolidating concrete

The bond behavior of lightweight self-consolidating concrete (LWSCC) must be understood in order to use this type of high performance concrete in structural members. The objective of this research program is to assess the bond behavior of reinforcing steel bars embedded in LWSCC members. Three different classes of LWSCC mixtures were developed with two different types of lightweight aggregates. In addition, one normal weight SCC (NWSCC) was developed and used as a control mixture. A total of twenty four pullout tests were conducted on deformed reinforcing bars with an embedded length of either 100 or 200 mm and the load-slip responses, failure modes and bond strengths of LWSCC and NWSCC were compared. Based on the results of this study, the bond strength of deformed bars for LWSCCs are found to be less (between 16 and 38%) as compared with NWSCC. Under the conditions of equivalent workability properties and compressive strength, bond slip properties were shown to be significantly influenced by the type of lightweight aggregate used. In this study, the use of expanded shale in the production of LWSCC significantly enhanced the pullout strength when compared with lightweight slag aggregate.     

Capillary absorption in concrete and the Lucas–Washburn equation

Capillary absorption kinetics of concrete–ethylene glycol system was studied with respect to concrete matrix porosity and liquid viscosity. Porosity of specimens was altered by air-entraining agents and superplasticizers. Liquid which doesn’t react with cement gel was chosen for the experiment in order to study the reasons for deviation from Lucas–Washburn equation observed in concrete–water system. Viscosity of ethylene glycol changes from ～23 to 2 mPa s in the temperature range from 20 to 100 °C. The values of the capillary coefficient were determined at 20, 60 and 100 °C using Neutron Radiography and were found to be in the range from ～1.5 to 4.9 mm h^?1/2. The results show that the Lucas–Washburn equation in concrete–ethylene glycol system is valid only for ～25 h, which indicates that swelling and rehydration of cement gel are not the main reasons for deviation observed in concrete–water system.     

Predicting the service-life of concrete structures – Limitations of simplified models

North American civil infrastructure systems are deteriorating. Roads, bridges, overpasses, marine and airport facilities are all impacted. The primary causes of premature concrete deterioration are harsh climatic conditions and chemical attacks, particularly exposure to winter de-icing salts and seawater. Given the growing concern for concrete degradation, numerous computer-assisted tools have been developed to assist engineers in the prediction of the service-life of structures. Many of these models are based on simplified equations that significantly restrict the scope of their application. The limitations of these approaches for the design of new construction and rehabilitation of existing structures are discussed. The theoretical assumptions at the basis of these models are first reviewed. Special attention is paid to the consequences of these simplifying assumptions on the reliability of the models. The difficulties of using these simplified models for the treatment of actual structures exposed to natural conditions are then discussed.     

Effect of confinement level,aspect ratio and concrete strength on the cyclic stress–strain behavior of FRP-confined concrete prisms

Behavior of rectangular concrete columns confined with FRP composites depends on several parameters, including unconfined concrete strength, confinement level, aspect ratio of cross-section (defined as the depth/width of the cross-section), and the sharpness of the section corners. For modeling the cyclic stress–strain behavior of FRP-confined rectangular concrete columns, effect of column parameters on the cyclic behavior of these columns should be examined properly. In this paper, effects of unconfined concrete strength, confinement level and the aspect ratio of cross-section are studied. The test database includes 10 prisms from recent study of authors and 18 prisms from a new experiment. Results of tests show that some aspects of cyclic behavior of FRP-confined concrete prisms such as envelope curve and stress deterioration are unaffected by the considered parameters. Results also indicate that the plastic strain decreases as the unconfined concrete strength increases, but it is independent of the aspect ratio and the confinement level. While the reloading path in all specimens was almost linear, the unloading path was highly nonlinear and was affected by unconfined concrete strength.     

The determination of the permeability of concrete to oxygen by the Cembureau method—a recommendation

A committee was set up in 1981 by the Cembureau Working Party on the Quality of Concrete with the following aims and objectives:

How to monitor the modulus of elasticity of concrete,automatically since the earliest age?

Monitoring the evolution of an early age set of parameters on concrete is necessary to predict the early age behaviour of structures. The difficulty lies in the fact that this monitoring must be automatic because the concrete hardening process takes place over a long period after the casting. This paper presents a new methodology and an apparatus, specifically designed at IFSTTAR, to monitor the hardening process of a concrete. Mainly, the Young’s modulus can be monitored in compression. Measurements start soon after having cast the concrete and the sample temperature is completely controlled so that the concrete maturity is well mastered. The performances of this apparatus, obtained on an ordinary concrete, are compared to more classical measurements using an extensometer mounted on the sample just after the setting time and to ultrasonic measurements. In these cases, the temperatures were not controlled and results have to be expressed in equivalent time. A comparison with another method developed and used at ULB by using the same concrete, in the frame of a joined cooperation between our two laboratories is achieved. This test set up is based on the so called Temperature Stress Testing Machine (TSTM). This device has been specifically designed with a control of the concrete maturity by the use of a dummy specimen only submitted to free deformations (thermal, shrinkage). The TSTM allows compressive and tensile testing starting just after the setting time. In addition, concrete properties, such as compressive and tensile strength, have been characterized at early age. These values have been used for the design of the loading histories applied in the automatic tests. The heat released by the cement hydration has also been measured in order to express the results on a maturity scale.     

Integrating the customers’ perceived risks and benefits into the triple-channel retailing

Multiple channel retailing and channel selection strategy have become key issues for many corporations to place themselves at the heart of a new era of retailing. This paper studies a triple-channel system in which a manufacturer operates a conventional channel, a direct online channel, as well as increases its online presence with an online shopping platform. It also takes the heterogeneity of consumers’ price sensitivity and channel preferences into account. It highlights that the perceived risks and perceived benefits are aggregated into an inhibitor or activator entering the customers’ acceptance of triple channels and further affecting their purchasing decisions. Three cases are discussed to investigate the demand distributions, the profit behaviours, the optimal pricing strategies and the channel selection strategies. Theoretical analysis is further validated by a case study with the aid of agent-based modelling. Sensitivity analysis demonstrates that two acceptance indices – respectively, regarded as the consumers’ willingness to tolerate the perceived risks of the online channel and approve the perceived benefits from the third-party online platform – significantly govern the total profit in two stylised scenarios. This study schematically characterises the structure of demand distributions, helps the corporation to discern consumers’ channel choices and then focus its marketing efforts towards more profitable customers and strategic channel structures. Furthermore, some implications are outlined for the optimal supply chain designation decision.     

An Investigation into the Relationship Between Maintenance and Finance

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Shrinkage stress in concrete under dry–wet cycles: an example with concrete column

This paper focuses on the simulation of shrinkage stress in concrete structures under dry–wet environments. In the modeling, an integrative model for autogenous and drying shrinkage predictions of concrete under dry–wet cycles is introduced first. Second, a model taking both cement hydration and moisture diffusion into account synchronously is used to calculate the distribution of interior humidity in concrete. Using the above two models, the distributions of shrinkage strain and stress in concrete columns made by normal and high strength concrete respectively under dry–wet cycles are calculated. The model results show that shrinkage gradient along the radial direction of the column from the center to outer surface increases with age as the outer circumference suffers to dry. The maximum and minimum shrinkage occur at the outer surface and the center of the column, respectively, under drying condition. As wetting starts, the shrinkage strain decreases with increase of interior humidity. The closer to the wetting face, the higher the humidity and the lower the shrinkage strain, as well as the lower the shrinkage stress. As results of the dry–wet cycles acting on the outer circumference of the column, cyclic stress status is developed within the area close to the outer surface of the column. The depth of the influencing zone of dry–wet cyclic action is influenced by concrete strength and dry–wet regime. For low strength concrete, relatively deeper influencing zone is expected compared with that of high strength concrete. The models are verified by concrete-steel composite ring tests and a good agreement between model and test results is found.     

Mix design and strength of soil–cement concrete based on the effective water concept

Saturated surface-dry condition of fine aggregate has been well defined in mortar and concrete production as the dry mass percentage moisture content where aggregate particles are saturated. However in soil–cement concrete construction, no mix design based on the saturated surface-dry condition of soil was attempted partly because the control of soil moisture on site is difficult. We have proposed the definition and testing methods of the saturated surface-dry condition of soil in the previous papers. In this paper, we introduce the concept of effective unit water to the soil–cement concrete mix design and propose a mix design method for soil-sand-cement systems. Strength and density of the soil–cement concretes were determined and discussed. A number of mixes were proportioned with accurate control of composition (C/W, soil/sand ratio, free water, etc.) and consistency. It was found that strength can be determined by C/W for a wide variety of clayey soils in various soil/sand ratios except for a soil with very high content of organic material. A set of nomograms for C/W versus strength and soil/sand versus strength were presented for future proportioning.     

Challenges for composites into the next millennium — a reinforcement perspective

This paper summarises the current level of technology within the manufacturing processes of filament winding, fibre placement, pultrusion and advanced textile preforming. It also examines the current problems within each of these manufacturing techniques and the areas of predicted future development.     

Patch repairs on reinforced concrete structures – Model investigations on the required size and practical consequences

For various reasons, world-wide numerous concrete structures have to be repaired due to corrosion problems of the steel reinforcement. In some cases, frequent use of road salt in past winters, not foreseen during the planning of older traffic structures, has led to damage; in other cases the causes lay in deficient planning and execution, as for example

• •irregular, insufficient concrete cover of the reinforcement;
• •unsuitable concrete mixture proportions for outdoor structures;
• •insufficient concrete curing, subsequently bad concrete quality in the concrete cover.

The model investigations described in this paper relate to construction practice, in which local damages due to reinforcement corrosion, e.g., spalls and cracks, are repaired solely in the area of visible surface damage. When damage is dealt with in this way, the carbonated or chloride contaminated concrete is often not removed completely, to avoid stability problems in the structure. In consequence, even after the repair measure there are areas of the reinforcement where there is no guarantee of sufficient protection against corrosion and a high corrosion risk remains. The reinforcement in the repaired area may even accelerate corrosion in unrepaired areas adjacent to it.     

Iontophoresis from a micropipet into a porous medium depends on the ζ-potential of the medium

Iontophoresis uses electricity to deliver solutes into living tissue. Often, iontophoretic ejections from micropipets into brain tissue are confined to millisecond pulses for highly localized delivery, but longer pulses are common. As hippocampal tissue has a ζ-potential of approximately -22 mV, we hypothesized that, in the presence of the electric field resulting from the iontophoretic current, electroosmotic flow in the tissue would carry solutes considerably farther than diffusion alone. A steady state solution to this mass transport problem predicts a spherically symmetrical solute concentration profile with the characteristic distance of the profile depending on the ζ-potential of the medium, the current density at the tip, the tip size, and the solute electrophoretic mobility and diffusion coefficient. Of course, the ζ-potential of the tissue is defined by immobilized components of the extracellular matrix as well as cell-surface functional groups. As such, it cannot be changed at will. Therefore, the effect of the ζ-potential of the porous medium on ejections is examined using poly(acrylamide-co-acrylic acid) hydrogels with various magnitudes of ζ-potential, including that similar to hippocampal brain tissue. We demonstrated that nearly neutral fluorescent dextran (3 and 70 kD) solute penetration distance in the hydrogels and OHSCs depends on the magnitude of the applied current, solute properties, and, in the case of the hydrogels, the ζ-potential of the matrix. Steady state solute ejection profiles in gels and cultures of hippocampus can be predicted semiquantitatively.     

Serviceability Limit State criteria based on steel–concrete bond loss for corroded reinforced concrete in chloride environment

This paper will focus on the study of reinforced concrete beams stored in a chloride environment for a period of 14–23 years under service loading. According to the experimental results, a Serviceability Limit State (SLS) criteria is proposed based on an excessive steel–concrete bond reduction. Corrosion of reinforcement in chloride environment leads to a specific local steel cross-section loss as well as a steel–concrete bond loss. Experimental results have shown that, in the first stage of corrosion propagation period, the deflection is more sensitive to chloride-induced corrosion than the ultimate capacity due to the effect of the tension steel–concrete bond loss even if both are correlated. Given this high sensibility of the bending stiffness to corrosion pitting attacks, it appears that a Serviceability Limit State (SLS) criteria based on excessive deflection of structural members is an adequate factor for SLS assessment. Later in corrosion propagation period, when the bond is already significantly reduced, only the ultimate capacity is affected by the steel cross-section loss. This does not affect the serviceability, because pitting attacks are very localised with an insignificant influence on the global deflection. Then, once the steel–concrete bond is lost in critical parts of the beams (high bending moment areas), pitting corrosion propagation does not affect anymore serviceability (stiffness reduction, bending or corrosion cracks patterns) but still leads to an ultimate capacity reduction, which is not acceptable. As a result, excessive steel–concrete debonding can be considered as the SLS criteria.     

A stochastic damage model for evaluating the internal deterioration of concrete due to freeze–thaw action

This paper presents a stochastic damage model for evaluating the internal deterioration of concrete due to freeze–thaw action, which involves great uncertainty and randomness. In this model, the structural element of concrete is discretized into infinite microelements, whose lifetimes are assumed to be independent random variables. Then expressions for the mean and variance of the damage of concrete are analytically derived. To calibrate the model parameters, a series of freeze–thaw tests in water on non-air-entrained concrete were conducted and back-calculation analyses were performed on the test results of dynamic modulus. The reliability of the proposed stochastic damage model is further validated through comparisons with the results of 80 other existing test specimens. The present model offers a theoretical basis for exploring the statistical aspect of concrete behavior during freeze–thaw.