Lattice modeling of chloride diffusion in sound and cracked concrete

Reinforced concrete structures are frequently exposed to aggressive environmental conditions. Most notably, chloride ions from sea water or de-icing salts are potentially harmful since they promote corrosion of steel reinforcement. Concrete cover of sufficient quality and depth can ensure protection of the steel reinforcement. However, it is necessary to study the effects of material heterogeneity and cracking on chloride ingress in concrete. This is done herein by proposing a three-dimensional lattice model capable of simulating chloride transport in saturated sound and cracked concrete. Means of computationally determining transport properties of individual phases in heterogeneous concrete (aggregate, mortar, and interface), knowing the concrete composition and its averaged transport properties, are presented and discussed. Based on numerical experimentation and available literature, a relation between the effective diffusion coefficient of cracked lattice elements and the crack width was adopted. The proposed model is coupled with a lattice fracture model to enable simulation of chloride ingress in cracked concrete. The model was validated on data from the literature, showing good agreement with experimental results.     

Use of recycled concrete aggregate in high-strength concrete

The results of a test programme to study the use of recycled concrete aggregate (RCA) in high-strength, 50 N/mm² or greater, concrete are described. The effects of coarse RCA content on the ceiling strength, bulk engineering and durability properties of such concretes have been established. The results showed that up to 30% coarse RCA had no effect on concrete strength, but therafter there was a gradual reduction as the RCA content increased. A method of accommodating the effects of high RCA content, involving simple adjustment to water/cement ratio of the mix is given. It is shown that high-strength RCA concrete will have equivalent engineering and durability performance to concrete made with natural aggregates, for corresponding 28-day design strengths. The practical implications of the study for concrete construction are discussed.

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Résumé Sont décrits ici les résultats d’une série d’essais destinés à étudier l’utilisation de granulats provenant du recyclage d’éléments en béton (RCA) dans des bétons de haute résistance (50 MPa et plus). Les effets de la teneur en gros granulats recyclés sur la résistance des plafonds et des batiments, ainsi que les propriétés de tels bétons ont été établis. Les résultats ont montré qu’une teneur allant jusqu’à 30% en gros granulats recyclés n’a pas d’effet sur la résistance du béton, mais qu’au dessus de 30%, la résistance diminue progressivement à mesure que la teneur en gros granulats recyclés augmente. Une méthode visant à accommoder les effets dus à une forte proportion de RCA, nécessitant un simple ajustement du rapport eau/ciment dans le mélange, est proposée. Il est prouvé que le béton RCA de haute résistance aura des qualités de résistance et de durabilité équivalentes à celles de bétons constitués de granulats naturels, pour les résistances mécaniques à 28 jours prévues. Les implications pratiques de l’étude sur la réalisation d’ouvrages en béton sont présentées.

     

Properties of laterite aggregate concrete

The paper presents the results of tests conducted on fresh and hardened concrete using laterite as aggregate. Several mixes of laterite aggregate concrete were made with varying watercement and aggregate-cement ratios to study the properties like workability, compressive, flexural, tensile strength and Modulus of elasticity. The tests indicate that the workability decreases with increasing aggregate-water ratio. The compressive strength of laterite aggregate concrete is considerably lower than that of gravel or crushed granite aggregate concrete, while the average ratio of cylinder to cube strength compared favourably with that for normal aggregate concrete, for the range of aggregate and water-cement ratios covered in this investigation.     

Physio-chemical reactions in recycle aggregate concrete

Concrete waste constitutes the major proportion of construction waste at about 50% of the total waste generated. An effective way to reduce concrete waste is to reuse it as recycled aggregate (RA) for the production of recycled aggregate concrete (RAC). This paper studies the physio-chemical reactions of cement paste around aggregate for normal aggregate concrete (NAC) and RAC mixed with normal mixing approach (NMA) and two-stage mixing approach (TSMA) by differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). Four kinds of physio-chemical reactions have been recorded from the concrete samples, including the dehydration of C(3)S(2)H(3), iron-substituted ettringite, dehydroxylation of CH and development of C(6)S(3)H at about 90 degrees C, 135 degrees C, 441 degrees C and 570 degrees C, respectively. From the DSC results, it is confirmed that the concrete samples with RA substitution have generated less amount of strength enhancement chemical products when compared to those without RA substitution. However, the results from the TSMA are found improving the RAC quality. The pre-mix procedure of the TSMA can effectively develop some strength enhancing chemical products including, C(3)S(2)H(3), ettringite, CH and C(6)S(3)H, which shows that RAC made from the TSMA can improve the hydration processes.     

Use of fine aggregate matrix for computational modeling of low temperature fracture of asphalt concrete

In this study, a discrete element computational model is applied to simulate the fracture behavior of asphalt mixtures at low temperatures. In this model, coarse aggregates are explicitly represented by rigid spherical particles. The bonds that connect these particles represent the fine aggregate matrix (FAM), which is defined as the combination of asphalt binder and fine aggregates. The bending beam rheometer (BBR) tests are performed to determine the strength and Young’s modulus of FAM at low temperatures. The model is then used to simulate the semi-circular bend (SCB) tests on the mixtures. The model is verified by a series of BBR and SCB tests on both conventional and graphite nano-platelet modified asphalt materials. The comparison between the experimental and simulated results indicates that the peak load capacity of the SCB specimens is primarily governed by the tensile strength of the FAM. However, in order to capture the entire load–displacement curve of the SCB specimens, one needs to employ a softening constitutive model of the FAM, which requires the information on its fracture energy. Several experimental methods for measuring the fracture energy of FAM are discussed for future prediction of the complete load–displacement response of asphalt mixtures at low temperatures.     

Carbonation behaviour of recycled aggregate concrete

This paper reviews the effect of incorporating recycled aggregates, sourced from construction and demolition waste, on the carbonation behaviour of concrete. It identifies various influencing aspects related to the use of recycled aggregates, such as replacement level, size and origin, as well as the influence of curing conditions, use of chemical admixtures and additions, on carbonation over a long period of time. A statistical analysis on the effect of introducing increasing amounts of recycled aggregates on the carbonation depth and coefficient of accelerated carbonation is presented. This paper also presents the use of existing methodologies to estimate the required accelerated carbonation resistance of a reinforced recycled aggregate concrete exposed to natural carbonation conditions with the use of accelerated carbonation tests. Results show clear increasing carbonation depths with increasing replacement levels when recycled aggregate concrete mixes are made with a similar mix design to that of the control natural aggregate concrete. The relationship between the compressive strength and coefficients of accelerated carbonation is similar between the control concrete and the recycled aggregate concrete mixes.     

Assessing relationships among properties of demolished concrete, recycled aggregate and recycled aggregate concrete using regression analysis

Recycled demolished concrete (DC) as recycled aggregate (RA) and recycled aggregate concrete (RAC) is generally suitable for most construction applications. Low-grade applications, including sub-base and roadwork, have been implemented in many countries; however, higher-grade activities are rarely considered. This paper examines relationships among DC characteristics, properties of their RA and strength of their RAC using regression analysis. Ten samples collected from demolition sites are examined. The results show strong correlation among the DC samples, properties of RA and RAC. It should be highlighted that inferior quality of DC will lower the quality of RA and thus their RAC. Prediction of RAC strength is also formulated from the DC characteristics and the RA properties. From that, the RAC performance from DC and RA can be estimated. In addition, RAC design requirements can also be developed at the initial stage of concrete demolition. Recommendations are also given to improve the future concreting practice.     

Recycled aggregate concrete as structural material

The use of recycled aggregates in concrete opens a whole new range of possibilities in the reuse of materials in the building industry. The utilisation of recycled aggregates is a good solution to the problem of an excess of waste material, provided that the desired final product quality is reached. The studies on the use of recycled aggregates have been going on for 50 years. In fact, none of the results showed that recycled aggregates are unsuitable for structural use. However, some hypothetical problems related to durability aspects resulted in recycled aggregates being employed practically only as base filler for road construction. This paper focuses on the possibility of the use of recycled aggregate concrete as a structural material. For that purpose an experimental study of the shear behaviour and strength of beams made with recycled aggregate concrete was studied. Twelve beam specimens with the same compression strength, four concrete mixtures using different percentages of recycled coarse aggregates (0%, 25%, 50% and 100%) and three different transverse reinforcement arrangements were cast and tested up to failure. Analytical predictions of the experimental results were carried out using a numerical model based on the modified compression field theory and simplified models such as those proposed by Cladera & Mari, the Canadian standard CSA and the Eurocode-2. The results obtained indicate that a substitution of less than 25% of coarse aggregate, scarcely affects the shear capacity of RC beams, provided that all measures related to dosage and durability aspects have been adopted.     

Lattice modelling of size effect in concrete strength

This paper uses a recently improved lattice network model to study the size effect in the strength of plain concrete structures. The several improvements made to the lattice network model are: (i) tension softening of the matrix phase is included in the material modelling; (ii) the structural response is modelled by incrementing the deformation rather than the load. This eliminates the need for introducing arbitrary scaling parameters in the beam element failure criteria and; (iii) a square rather than a triangular lattice beam network is found to be adequate for modelling concrete, thus greatly reducing the computational time.The improved square lattice network has been used to simulate the complete load-deformation response of notched three-point bend beams of different sizes with a view to checking the validity of several size effect models available in the literature. Lattice simulation was found to identify microcracking, crack branching, crack tortuosity and bridging, thus allowing the fracture process to be followed until complete failure. The improved lattice model predicted smooth structural response curves in excellent agreement with test results.The simulated nominal strengths also correlated very well with the test results, apart from that for the smallest beams (depth 38.1 mm). However, even in the relatively broad range of sizes (1:8) of the test beams, there was no clear evidence that one size effect model is superior to the other. In fact, rather surprisingly the test data would appear to be equally well described by all the available size effect models. The lattice simulations however indicated a trend which is better predicted by the multifractal scaling model.     

Restrained shrinkage cracking of recycled aggregate concrete

The increase in drying shrinkage and decrease in tensile properties of concrete proportioned with recycled concrete aggregate (RCA) can result in a high risk of cracking under restrained conditions. However, the reduction of the modulus of elasticity of such concrete, can lead to greater stress relaxation and reduction in cracking potential. An experimental program was undertaken to evaluate the effect of using RCA at high substitution rates of 50 and 100% (by vol.) on the cracking potential under restrained conditions. Four different types of coarse RCA, two binder types, and water-to-cementitious materials ratio (w/cm) of 0.37 and 0.40 were considered in the study. Mechanical properties, drying shrinkage, and cracking potential using the ring test were investigated. Test results indicated no cracking up to 35 days in the case of the reference mixture and the concrete prepared with 50% RCA replacement. The 28-day stress rate of such mixtures were limited to 0.12 MPa/day. Depending on the RCA type, the incorporation of 100% coarse RCA in a binary system made with 0.40 w/cm increased the 35-day cracking potential to up to 74%, with values of stress rate ranging from 0.25 to 0.34 MPa/day. The mixtures proportioned with 100% RCA developed tensile creep coefficient of 0.34–0.78 at the time of cracking compared to 0.34–0.36 for the reference concrete at the same age. However, greater elastic concrete strain and lower tensile strength resulted in reduced time to cracking at 100% RCA replacement, which was 9.0–11.0 days.     

The role of aggregate in the fracture of concrete

The fracture of concrete is reviewed. Tests on notched and unnotched beams made from concretes containing aggregates of different sizes and types are described. A simple model is proposed to elucidate and formalize the role of the aggregate particles in a brittle matrix. It is concluded that aggregate particles cause cracks to form in a concrete matrix at a lower stress than that at which the matrix would crack if it contained no aggregate. It is also concluded that aggregate particles impede the extension of matrix cracks.     

Durability of recycled aggregate concrete prepared with carbonated recycled concrete aggregates

The mechanical properties of recycled aggregate concrete (RAC) incorporating carbonated recycled concrete aggregates (RCAs) have previously been reported. However, the durability of RAC prepared with carbonated RCAs remains to be accessed. In this study, the durability properties of RAC prepared with non-carbonated RCAs and carbonated RCAs, in terms of deformation (drying shrinkage), water absorption and permeability (bulk electrical conductivity, gas and chloride ion permeability), are presented. The experimental results indicated that: (i) the incorporation of the carbonated RCAs in RAC not only helped to reduce the water absorption of RAC, but also reduced its permeability; (ii) when 100% carbonated NRCAs was used, the improvement extent of impermeability was 15.1%, 36.4% and 42.4% for bulk electrical conductivity, chloride ion permeability and gas permeability, respectively. Comparing the results of the mechanical and durability properties, the CO₂ curing treatment of RCAs had a greater beneficial impact on the durability properties of the RAC; and (iii) there was a good correlation between the water absorption of RAC and its permeability indicators. The water absorption value of RAC may be used as a criterion of the durability of RAC.     

Recycled concrete as an aggregate for concrete—a review

The present state of knowledge on the use of recycled concrete as an aggregate in new concrete is reviewed and suggestions made as to what further work is necessary before a proper assessment of the material can be made. Where crushed uncontaminated concrete is used the properties of the material as an aggregate and the basic engineering characteristics of the concrete made with it are well established. Much less is known about the type and quantity of impurities which could occur in crushed concrete from general building rubble and the effect these would have on concrete made using such crushed concrete as aggregate.

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Résumé On fait le point sur l'état présent des connaissances relatives à l'utilisation du béton recyclé comme granulat du béton nouveau. Et l'on présente quelques suggestions sur les travaux à accomplir avant qu'une évaluation correcte de ce matériau, puisse être faite. Lorsqu'il est utilisé sans impuretés, les propriétés du béton concassé en tant que granulat et les caractéristiques techniques de base du béton confectionné avec ce matériau sont bien établies. Mais on en sait beaucoup moins sur les types et quantités d'impuretés qui se rencontrent dans le béton concassé à partir d'un matériau de démolition global, et sur les effets qui peuvent en résulter pour le béton confectionné avec ces débris.

     

石屑应用于混凝土中的探索性试验

本文将石屑和水洗河砂按比例配制组成混合砂应用于混凝土中,通过试验调整和试用,混凝土工作性和强度良好。     

Short and long-term behavior of structural concrete with recycled concrete aggregate

Recycling concrete construction waste is a promising way towards sustainable construction. Coarse recycled concrete aggregates have been widely studied in recent years, however only few data have been reported on the use of fine recycled aggregates. Moreover, a lack of reliable data on long-term properties of recycled aggregate concrete has to be pointed out.In this paper the effects of both fine and coarse recycled concrete aggregates on short and long-term mechanical and physical properties of new structural concrete are investigated. The studied concrete mixes have been designed by adjusting and selecting the content and grain size distribution of concrete waste with the goal to obtain medium–high compressive strength with high content of recycled aggregates (ranging from 27% to 63.5% of total amount of aggregates).Time-dependent properties, such as shrinkage and creep, combined with porosity measurements and mechanical investigations are reported as fundamental features to assess structural concrete behavior.     

Utilisation of steel slag as an aggregate in concrete

This paper aims to investigate the possibility of utilizing steel slags produced in Croatian plants as a concrete aggregate. Aggregate properties were determined on coarse slag fractions (4–8, 8–16 mm) according to the relevant European Standards. Considering the obtained results, slags were specified in accordance with the classes as given in the main European standard for aggregates, whereupon these classes were compared to the Croatian regulation requirements. The obtained results proved that coarse slag fractions can be suitable for concrete application. Therefore, concrete mixtures were prepared with coarse slag fractions whose hardened state properties (compressive and flexural strength, static modulus of elasticity, volume changes and corrosion susceptibility) were then compared with the properties of reference concrete made of commonly used natural aggregate materials, namely dolomite. According to the obtained test results it can be concluded that the observed slags can be a good substitute for natural aggregate materials.     

Limestone aggregate concrete,usefulness and durability

Inland and on sea coasts, crushed limestone aggregate has been used in France and elsewhere for structural concrete when gravel aggregates were not available. Crushed limestone fillers have, since the oil crisis, been allowed as partial constituent of cement; they are now currently used as additions in concrete mixtures, as partial replacement of Portland cement. Limestone aggregate concretes have not been considered inferior to gravel aggregate concretes, particularly in durability. However, recently some cases of alkali-silica deleterious reactivity and one case of sulphate attack, have questioned their good performance status.