Chloride penetration into concrete structures in the marine atmosphere zone – Relationship between deposition of chlorides on the wet candle and chlorides accumulated into concrete

The relationship between chlorides from marine aerosol and chlorides accumulated into concrete is discussed in this paper. The experimental programme comprised an environmental characterisation, with climatic and chloride deposition data, and a study of chloride penetration into concrete based on natural exposure of specimens in a marine atmosphere zone. Results show that salt concentration in marine aerosol strongly decreases in the first meters from the sea. Chlorides present in the atmosphere can be studied using the wet candle method and correlated with chlorides accumulated into concrete. This relationship can be represented by the equation $C_{\text{tot}}=C_0+k_{\text{d}}·\sqrt{D_{\text{ac}}}$, where k_d is a coefficient which depends on concrete and environmental characteristics, C_tot is the average total amount of chlorides accumulated into concrete, C₀ is the chloride content in concrete before exposure and D_ac is the accumulated dry deposition of chlorides.     

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.     

Durability of concrete structures in marine atmosphere zones – The use of chloride deposition rate on the wet candle as an environmental indicator

Durability of concrete structures under marine environments has been studied for a long time. This work was focused on marine atmosphere zone and studied the deposition of chlorides on wet candle devices and its relation with chlorides accumulated into concrete. Concrete specimens with three different mixtures were exposed at places located at four different distances from the sea. Periodically, chloride profiles were obtained and analysed taking into account environmental data. Results of numerical extrapolations show that chloride deposition rate on the wet candle can be used as an environmental indicator, helping to preview the expectancy of service life of concrete structures or suggesting minimum concrete cover thicknesses for a required service life. Regarding the studied region, service life decreases between 30% and 60% were observed when changing chloride deposition from 120 mg/m² day to 500 mg/m² day, which shows that chloride deposition plays an important role as an environmental indicator on service-life analysis of concrete structures in marine atmosphere zone.     

Evaluation of non-linear behavior of timber–concrete composite structures using FE model

This article describes a numerical model that was developed for the analysis of composite timber–concrete beams. This model presents a simplified methodology for determining the effective bending stiffness of the timber–concrete composite structure. It is based on previous work done usually referred to in some non-normative literature by γ-method. The implemented methodology assumes some simplifications, as for instance, linear elastic behavior of all components, constant stiffness of the connection and sinusoidal loading. For comparison purposes, the work benefits from an experimental program in which full-scale beams were tested in bending and timber–concrete connections were tested in shear. The FE model has shown the ability to overcome the simplifications of the Eurocode, namely the variation of shear force along the beam axis. The numerical model is capable of detecting and quantifying the influence of the non-linear behavior of the connections on the composite structure. Different parameters are analyzed and, for instance, the ductility behavior of the timber–concrete connection could be more important than the maximum strength, which is an interesting result. By comparing theoretical predictions with test results, it is clear that the numerical model used in this work is a very interesting method when compared with the usual design models, such as that of Annex B of Eurocode 5 (EN 1995-1-1). The influence of the connections behavior on the ultimate load of the composite structure is very important and the described approach proved to give good predictions.     

Simple dynamic models of elastic–plastic structures under impact

The dynamic behaviour of elastic–plastic structures is much more difficult to predict in comparison with the corresponding quasi-static one because of complicated loading conditions and the effect of inertia. If the quasi-static structural and interface behaviour of a structure is specified, by assuming that its dynamic deformation mode is broadly similar to the quasi-static one and the material is strain-rate insensitive, two simple mechanical models, namely the stick model and non-stick model, are proposed in this paper to predict the dynamic response of the structure under a rigid-projectile impact. Each model contains two lumped masses and two inelastic springs. While spring

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represents the highly non-linear and inelastic mechanical property of the interface between the target structure and the striker, spring

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represents the elastic–plastic behaviour of the structure itself, with hardening, perfectly plastic or softening in the plastic range. With the complicated deformation history involving loading, unloading and reversed loading being taken into account, the dynamic response can be predicted by these models. A number of numerical examples are given to demonstrate the effects of the mass ratio, energy ratio, structural stiffness/local rigidity and the hardening/softening factor on the maximum and final deformations of the simple models. Finally, the validity of the models proposed for real structures is verified by the impact tests on simply supported beams.     

Asymptotic behavior of robust estimators in partially linear models with missing responses: the effect of estimating the missing probability on the simplified marginal estimators

In this paper, we consider a semiparametric partially linear regression model where missing data occur in the response. We derive the asymptotic behavior of the robust estimators for the regression parameter and of the weighted simplified location estimator introduced in Bianco et al. (Comput. Stat. Data Anal. 54:546–564, 2010a). For the latter, consistency results and the asymptotic distribution are derived when the missing probability is known and also when it is estimated.     

Air voids reduction phenomena of asphalt concrete – A continuum approach

Asphalt concrete used in flexible highway pavement construction has 5–8 percent air voids immediately after laying of the roadway. Constitutive laws for asphalt concrete developed till now have modelled the mix as a linear elastic or viscoelastic material and have not taken into account the effect of void concentration on the mechanical behaviour of the material. In this paper, the theory of linear elastic material with voids is used to model asphalt concrete under isothermal conditions. Two cases of void reduction behaviour are studied, one in which the void volume reduces asymptotically under a constant load and the other in which it reaches the refusal air void content. The model is used to predict the creep behaviour under constant compressive stress as well as to obtain the hysteretic stress-strain behaviour. Solutions for the case of uniaxial deformation are derived and the strains are simulated for a constant compressive stress. Use of the air voids reduction measure as a possible damage parameter is also examined. This revised version was published online in July 2006 with corrections to the Cover Date.     

Welding simulation of complex structures – possibilities and limits

The possibilities of predicting welding distortions are extensive. The boundary conditions used in industrial production play an important role in choosing the right strategy. Not only the right abstraction of the welding process is essential for correct and useful results, the clamping conditions and pre-tack welding are also very important. This article reviews experiments and FEM calculations of welded complex structures of industrial relevance. The examined structure comes from a railway vehicle and contains u-profiles with a sheet thickness of 4 mm. The review starts with the explanation of the researched structure and shows different welding situations, like unclamped and clamped manufacturing. Then the FE model with several weld seams is explained and the used boundary conditions are shown. Finally, the measured and calculated distortions are compared and discussed.     

Image analysis: a tool for the characterisation of hydration of cement in concrete – metrological aspects of magnification on measurement

This study aims at seeking what magnification is optimal in view to evaluate the hydration state of cement-based materials. When observing surfaces of flat polished sections of cement paste and mortar under SEM in BSEI mode at 100×, 200×, 400×, 600×, 1000× magnifications, an image analysis procedure, derived from the entropy maximisation, is applied to evaluate the area fraction of the anhydrous remnant cement grains A_A (NH) at a given maturity.Results show that:

• •200× is sufficient to correctly determine A_A (NH) since higher magnifications induce marginal variations of A_A (NH),
• •by calculating the covariance, 200× is also a representative field to describe the dispersion of the anhydrous phase.

The number of fields necessary to obtain a mean value of A_A (NH) with a defined accuracy at 200× is specified according to statistical laws.     

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.     

Predicting the flexural load–deflection response of steel fibre reinforced concrete from strain, crack-width, fibre pull-out and distribution data

A semi-analytical model is presented, based on conventional principles of mechanics, to predict the flexure behaviour of steel fibre reinforced concrete. The model uses a stress-block approach to represent the stresses that develop at a cracked section by three discrete stress zones: (a) a compressive zone; (b) an uncracked tensile zone; and (c) a cracked tensile zone. It is further shown that the stress-block, and hence flexural behaviour, is a function of five principal parameters: compressive stress–strain relation; tensile stress–strain relation; fibre pull-out behaviour; the number and distribution of fibres across the cracked section in terms of their positions, orientations and embedment lengths; and the strain/crack-width profile in relation to the deflection of the beam. An experimental investigation was undertaken on both cast and sprayed specimens to obtain relationships for use in the model. The results of the study showed a reasonable agreement between the model predictions and experimental results. However, the accuracy of the model is probably unacceptable for it to be currently used in design. A subsequent analysis highlighted the single fibre pull-out test and the sensitivity of the strain analysis tests as being the main cause of the discrepancies.     

Size effects in concrete fracture – Part II: Analysis of test results

This paper presents an analysis of the extensive experimental program aimed at assessing the influence of maximum aggregate size and specimen size on the fracture properties of concrete. Concrete specimens used were prepared with varying aggregate sizes of 4.75, 9.5, 19, 38, and 76mm. Approximately 250 specimens varying in dimension and maximum aggregate size were tested to accomplish the objectives of the study. Every specimen was subjected to the quasi-static cyclic loading at a rate of 0.125mm/min (0.005in./min) leading to a controlled crack growth. The test results were presented in the form of load-crack mouth opening displacement curves, compliance data, surface measured crack length and crack trajectories as well as calculated crack length, critical energy release rate, and fracture toughness (G ₁). There is a well pronounced general trend observed: G ₁ increases with crack length (R-curve behavior). For geometrically similar specimens, where the shape and all dimensionless parameters are the same, the R-curve for the larger specimens is noticeably higher than that for the smaller ones. For a fixed specimen size, G ₁ increases with an increase in the aggregate size (fracture surface roughness). For the same maximum aggregate size specimens, the apparent toughness increases with specimen size. It was clear that the rate of increase in G ₁, with respect to an increase of the dimensionless crack length (the crack length normalized by the specimen width), increases with both specimen size and maximum aggregate size increase. The crack trajectory deviates from the rectilinear path more in the specimens with larger aggregate sizes. Fracture surfaces in concrete with larger aggregate size exhibit higher roughness than that for smaller aggregate sizes. For completely similar specimens, the crack tortuosity is greater for the larger size specimens. The crack path is random, i.e., there are no two identical specimens that exhibit the same fracture path, however, there are distinct and well reproducible statistical features of crack trajectories in similar specimens. Bridging and other forms of crack face interactions that are the most probable causes of high toughness, were more pronounced in the specimens with larger maximum size aggregates.     

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.     

Creep and shrinkage prediction model for analysis and design of concrete structures— model B3

RILEM Draft Recommendation107-GCS Guidelines for the Formulation of Creep and Shrinkage Prediction Models

Creep and shrinkage prediction model for analysis and design of concrete structures— model B₃     

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.     

Crack development in concrete structures due to imposed strains—Part II: Parametric study of a wall fully restrained at the base

Crack development due to imposed strains in concrete walls fully restrained at the base is studied in order to improve control of cracking. In this second paper the influence of reinforcement and concrete properties as well as geometry on crack width and crack spacing is studied. In a first paper a two-dimensional FE-method is described, with closing forces in cracks concentrated to spring elements. Temperatures changes are used as load and the calculations are performed stepwise with opening of nodes and implementation of spring elements. It is shown that the two-dimensional behaviour of the wall only gives about half the crack widths compared to a one-dimensional bar with the same percentage of reinforcement. The reason is that the restraint along the base will effectively facilitate distribution of cracking along the wall. The two-dimensional analysis shows that the crack widths are limited also by low reinforcement ratios. For the same reinforcement ratio the crack width will increase with tensile strength of the concrete. The geometry of the wall has very little influence on the cracking behaviour unless the wall is very short. Bond stiffness and bar diameter have a limited effect on the crack width in the wall.     

Waste glass as a supplementary cementitious material in concrete – Critical review of treatment methods

Concrete is the most widely used construction material and, for the most part, is produced using non-renewable natural resources and energy intensive processes which emit greenhouse gasses. There exists an opportunity to improve the sustainability of this industry by further exploring the use of alternative materials. As an aggregate, glass bottle waste has faced specific challenges including bond, ASR gel production, and strength degradation of the concrete. This paper reviews the literature pertaining to incorporating waste glass into concrete as a supplementary cementing material. Pozzolanic properties of waste glass as an SCM and ASR are related to particle size and percent addition. Lithium additives control ASR expansion; however, the mechanism of this control has yet to be defined.     

Obsolescence – A review of the literature

Technology progress and fierce competitiveness between manufacturers creates intense pressues to innovatively develop and sell new products. These products could be household or industrial items, such as telephones, computers, machines, robots, unmanned aerial vehicle (UAV), motors, industrial processes, electronic devices, tools, and spare parts in general. The technological progress implies the use of the word “obsolescence.” The new products have higher performance metrics compared to the older units, such as reliability, resilience, memory capacity, improved material, precision, artificial intelligence, lower energy consumption, ergonomics, and safety. Therefore, obsolescence became a paradox in our daily life and industry. This paper presents a literature review of the main published works on obsolescence (1976–2020). Its typologies, consequences and replacement strategies are illustrated.     

Some remarks on the theory of SQUID structures. I. Topology of SQUID structures—A unified picture

A topological analysis of different rf SQUID structures is performed. A simple unified picture of these devices is obtained by considering the topological properties of the magnetic force lines around superconducting (or resistive) SQUID structures containing one weak link. Due to the periodic collapse of the superconducting order parameter in the weak link the normal space around these structures can change its connectivity. A number of rf SQUID structures are analyzed from the point of view of simple topological homotopy theory. The rf SQUID is then viewed as a device which realize a topological switching between two different classes of simple closed loops of the magnetic force lines. This switching (through the weak superconductivity region) is modulated by the low-frequency closed loops of the magnetic field and results in the well-known periodic response of the rf SQUID. The surfaces surrounding the rf excitation coil of the rf SQUID are analyzed from the point of view of homology theory. The screening currents flowing on these surfaces are represented as one-dimensional closed loops (cycles). It is shown that the SQUID can work only if the surface of the structure has at least one class of so-called nonbounding cycles, which are all cut by the weak link. A number of known SQUID structures are analyzed from the point of view of homotopy and homology theory and some new SQUID structures are discussed.