Use Of Recycled Concrete Research Articles

Load Transfer Mechanism and Bond–Slip Behavior of Recycled Concrete-Encased Steel (RCES) Subjected to Cyclic Loading

RCES composite structures, which combine the advantages of high bearing- and good anti-seismic capacity of ordinary steel reinforced concrete (SRC) with the characteristics of energy-saving and environmental protection of recycled concrete, are beneficial to promote the use of recycled concrete. However, the bond–slip behavior and load transfer mechanism are essential issues of RCES composite structures. This paper primarily focused on the load transfer mechanism and bond–slip behavior of RCES composite structures subjected to cyclic loading. A total of 14 specimens, which were designed with different replacement ratios of recycled concrete, cover thicknesses of H-shaped steel, transverse reinforcement ratios, and recycled concrete strengths, were tested to investigate the load transfer mechanism and interface damage. The results indicate that the whole loading procedure can be divided into four phases and four limit points. The bonding shear damage of a specimen develops rapidly, and most damage happened during 0~0.2 mm slip. The bonding stress values of chemical bonding stress τca, friction resistance stress τf, and mechanical biting stress τm were calculated. Moreover, in order to reflect the influence of cyclic loading, a degradation factor ξ was proposed. The mean values of chemical bonding stress τca: friction resistance stress τf: mechanical biting stress τm are 1:0.187:0.696, in which the chemical bonding stress is the largest, friction resistance stress is the second, and the mechanical biting stress is the least.

Preparation Technology and Properties of Carbon-Reinforced Recycled Aggregate Concrete and Mortar

Crushing the demolished waste concrete as aggregate to prepare fresh concrete may cause aggregate internal damage leading to strength reduction. To counter this issue, in the current study, the recycled aggregate is mixed with nano-calcium oxide in a vacuum reactor, and then a high concentration of carbon dioxide is introduced to fill the internal damages of the recycled aggregate. A layer of hydrophilic alkaline substance is formed on the surface of recycled aggregate to absorb carbon dioxide, which improves the strength of recycled aggregate particles and the bonding performance of aggregate-cement interface area. The test results show that the mechanical properties of recycled aggregate particles are improved after vacuum carbon strengthening, and the vacuum carbon strengthening effect of recycled aggregate with large particle size is more obvious. Compression strength of concrete and mortar prepared by vacuum carbon-reinforced aggregate is observed to be 13.6% and 30% higher than ordinary recycled concrete, respectively. Also, the proposed recycled aggregate improvement method manifests a 21.1% reduction in carbon emissions. These findings offer crucial parameters for the engineering use of recycled concrete, promote new ideas to reduce carbon emissions caused by fresh concrete production, and provide an important reference for the sustainable recycling of waste concrete.

Barriers to the use of recycled concrete from the perspective of executing companies and possible solution approaches - case study Germany and Switzerland

The objectives of the study are the identification of obstacles to the use of recycled concrete and the empirical examination of the effectiveness of selected economic policy measures to increase the use of recycled concrete in building construction. Combined item-based and factorial survey of Swiss and German executing companies in the construction and concrete value chain was carried out. Results indicate that the consideration of recycled materials in the public tender increases the likelihood of tender application using recycled concrete. The reimbursement of costs when using recycled materials also has a positive, whereas the recognition of recycled materials by a label has a negative effect. Lack of governmental support in its various manifestations and market related factors such as low demand and supply for recycled concrete have been identified as central obstacles to the use recycled concrete in Germany from the perspective of executing companies. In context of promoting sustainable construction, the results point to the need to consider recycled materials in the public tender, stronger governmental support and further training and qualification measures on the subject of recycled concrete.

Numerical simulation of mechanical properties of recycled concrete based on characterization of interfacial transition zone characteristics

Recycled concrete is the recycling of concrete waste materials. Making full use of recycled concrete can effectively reduce the consumption of natural energy and protect the environment. The interface transition zone of recycled concrete is complex and changeable, which leads to the instability of mechanical properties of recycled concrete. This paper summarizes the numerical simulation of recycled concrete and its interface transition zone, analyses the existing problems, and prospects the future development trend.

Influence of freezing-thawing cycles on strength of fiber recycled concrete column

In order to reduce the influence of freezing-thawing cycle on the recycled concrete and improve the use of recycled concrete in cold regions, this paper systematically explored the influence of different variables such as the mix ratio of recycled aggregate, the content of basalt fiber and freezing-thawing cycle on the mechanical properties of recycled concrete. The results show that adding basalt wire can improve the bearing capacity and ductility of the recycled concrete, but the mechanical properties of the recycled concrete are greatly reduced after the freeze-thaw cycle.

Attitude towards sustainability, study contents and the use of recycled concrete in building construction - case study Germany and Switzerland

This study examines the attitudes towards sustainability among students of architecture, civil and environmental engineering and analyses the perceived coverage of sustainability and recycled concrete related contents. For this purpose, a combined item-based and discrete choice survey of Swiss and German students of architecture, civil and environmental engineering was carried out. Results indicate that sustainability and recycled concrete related contents are covered to a higher extent in Switzerland - however, students in Switzerland are characterised by a lower willingness to pay for sustainability related attributes. In context of promoting sustainable construction in Germany, the results point to the need to increase the coverage of theoretical and practical sustainability and recycled concrete related study contents.

ТЕХНОЛОГИЯ ПЕРЕРАБОТКИ БЕТОННОГО ЛОМА С ЦЕЛЬЮ ПОЛУЧЕНИЯ САМОУПЛОТНЯЮЩЕГОСЯ БЕТОНА

The article presents the recycling technology of reinforced concrete formed during the renovation program in Moscow city. It is shown that the part of concrete can be recycled to obtain raw materials for self-compacting concrete use, including products of recycling. The products include recycled concrete aggregate 5-10 mm fraction and the filler. It is indicated that the efficient concept of recycling process is sorting of various types of products and removal of contaminants. The sequence of technological operations for dismantling buildings, transportation and recycling of concrete, storing and quality control of the resulting product has been developed. Technological methods that allow to obtain self-compacting concrete mixtures with the use of recycled concrete in concrete plant are suggested.

Analysis of Deformative-Strength Characteristics of Concretes, Manufactured with Addition of Coarse Aggregate, Obtained from Concrete Rubbles

In the construction practice, various concretes are used which, depending on the purpose, meet different requirements in terms of reliability, resistance to external environmental factors and other equally important parameters. In this article, as a coarse aggregate, crushed stone obtained in the process of crushing concrete waste from fragments of destroyed buildings and structures is considered. The results of experimental studies by various scientists have shown that the use of concrete rubble as aggregate is a promising direction. Thus, the economic effect of the use of crushed concrete as a coarse aggregate for the production of various concretes can be very significant for the construction industry as a whole. The results of the experiments presented in this article show that the use of recycled concrete in the form of crushed stone is very necessary, since an increase in the volume of construction waste creates a huge environmental problem for all countries and its partial solution is connected, including with the scientific research presented here. Keywords: crushed stone from concrete waste, coarse aggregate, concrete, strength, compression, tension, bending, modulus of elasticity.

Recycled concrete aggregate mixed with crumb rubber under elevated temperature

The use of recycled concrete is a significant step to reduce the need for natural resources and the demand on landfill sites for disposing the waste. Previous studies have mainly focused on its behaviours under ambient temperature although the characteristics of recycled concrete after high temperature exposures is important in many practical applications such as fire resistance. Also, the effect of elevated temperature on the mechanical and porosity behaviours of recycled concrete aggregate (RCA) containing rubber for pavement base/subbase applications has never been studied. In this study, the effects of high-temperature environment on the mechanical properties of recycled concrete aggregate (RCA) mixed with crumb rubber (CR) in a range of 0.5–2% were investigated both experimentally and theoretically. Unconfined compression strength (UCS) and X-ray micro-tomograph tests were undertaken on the mixed samples which were heated to different temperatures ranging from 20 °C to 600 °C. It was observed from the experimental results that UCS increased under a higher temperature up to 300 °C. At an ambient temperature of 20 °C, RCA samples without any addition of CR exhibited the highest UCS value while under high temperature, RCA samples with 1% CR had the highest UCS value. The strength of RCA mixed with crumb rubber showed an increasing trend under a temperature higher than 150 °C, which can be attributed to the melting processes of the crumb rubber. According to the X-ray micro-tomograph tests, the inclusion of rubber led to a decreasing porosity. It is interesting to note that porosity rises when temperature increased from 20 °C to 150 °C. This may be due to the restructuring of the shape of rubber particles into spheres as well as the excessive pore water forces generated from water evaporation. Also, when the temperature increased from 150 °C to 450 °C, the porosity decreased since the melted rubber particles flowed into the voids and filled more pores. The outcomes of this study can provide practical guidance on the application of recycling concrete.

Possibilities of Using Recycled Concrete and Silica Binders for Development of Sound Absorbing Concrete

With development and reconstruction of traffic routes there has recently been an increased need for noise reduction measures, out of which the most effective one appears to be the construction of noise barriers that prevent sound propagation from its source and (in case of sound absorbing barriers) absorb sound and thus reduce the noise load near the sound source (around a traffic route). The manufacturing of noise barriers often enables the use of secondary raw materials and thus to reduce the energy and material costs of their production as well as decrease the environmental impact. An interesting option is the use of recycled concrete that is suitable for production of concrete with open structure to be used in the absorber layer of a sound barrier and can even be used in the structural layer as well. The paper describes the possibilities of using recycled concrete in the production of sound absorbing barriers with a high degree of sound absorption.

Prediction of the mechanical properties of structural recycled concrete using multivariable regression and genetic programming

This study focuses on the prediction of some of the most important properties of structural recycled concrete (compressive strength, modulus of elasticity and splitting tensile strength) taking into account, not only the recycled percentage and the quality of the recycled aggregates used, but also the production method. For said purpose, a database has been developed with 1831 mixes obtained from 81 papers.Firstly, in this manner, these properties have been compared with those of conventional concrete. Then, the need to adapt the prediction code expressions (adjusted for conventional concretes) was analyzed to take into account the use of recycled concrete, developing, if finally necessary, the correction coefficients which allow engineers to predict the recycled properties with the same approximation degree as in conventional concretes. These correction coefficients have been adjusted using multivariable regression, and have been analyzed using different statistical indexes.Lastly, specific expressions used to predict these properties in structural recycled concretes have been optimized. Two different tools have been used to develop these expressions: multivariable regression and genetic programming. The proposed expressions have been analysed using statistical parameters which have been compared with those obtained using the expressions proposed by other authors. In this regard, and finally, the best prediction expressions for the modulus of elasticity and the splitting tensile strength of structural recycled concretes have been proposed.

Development of Aggregate Avoidance Index for Evaluating Recycled Aggregate Concrete

Early strength gain in concrete is typically associated with transition from the plastic to the semisolid state, and later to the solid state. When tensile failure occurs during the plastic and semisolid states, the paste, being the weak link in the matrix, provides the path for cracking as the aggregates are avoided. As the paste begins to strengthen in concrete with virgin aggregates, more fracture may occur through the aggregates. However, in recycled aggregate concrete, a possible enhancement of the interfacial transition zone (ITZ) at later ages may redirect the fracture path around some recycled aggregate particles. This research investigated mechanical and rheological properties of concrete with various proportions of recycled coarse aggregate ranging from 0% to 100% by weight. Gradations were as received from the crushing process but limited to a maximum size of 1.5 in. (38.1 mm) and minimum size of 0.19 in. (4.75 mm). Samples with higher percentages of recycled aggregate exhibited improved mechanical properties and freeze–thaw durability. A counting of the fractured aggregates in the exposed faces of cracked flexural beams suggested an enhancement of the ITZ in re cycled aggregates. For elimination of counting error, an aggregate avoidance index method was used to evaluate the fractured faces. Results indicated lower aggregate avoidance indexes for higher recycled aggregate content, a validation of the enhancement of the ITZ, possibly by the absorptive properties of the recycled aggregates. This validated, beneficial property of recycled concrete aggregates can be used to promote sustainable use of recycled concrete.

The Performance Evaluation and Application of Recycled Concrete in Constructions

In recent years, with the urbanization in our country speeding up, the phenomena of wasting concrete due to the demolition and renovation are very common. For our country, resources are very scarce. How to reuse the waste concrete, so as to keep the sustainable utilization of building resources, is a topic worthy of deep thinking. In order to realize the full use of recycled concrete, this paper points out that the use of recycled concrete is the key to keep the sustainable utilization of natural resources. Based on this, it discusses the work performance, mechanical properties, durability properties, shear properties, seismic performance of recycled concrete and its performance evaluation.

Influence of Natural Coarse Aggregate Type on the Transport Properties of Recycled Concrete

The use of recycled concrete represents an environmentally friendly solution for minimizing the impact of construction and demolition wastes. In many countries, the incorporation of recycled coarse aggregate (RCA) is a common practice because the maximum contents of RCA are usually limited to approximately 25% of the total coarse aggregate content. The incorporation of higher volumes of RCA is a field of discussion, primarily regarding the durable behavior of concrete. This study analyzes different transport properties of concretes with compressive strength of 20–50 MPa prepared with variable contents of RCA (0, 25, and 75%). Eight types of RCA were obtained from concretes incorporating four different natural coarse aggregates: granitic, basaltic, quartzitic crushed stones, and siliceous river gravel. Capillary absorption, water penetration, and chloride diffusion tests were conducted. The variation of transport properties with concrete compressive strength and the effect of RCA content on the variability of transport properties were analyzed. According to the results, the durable behavior of recycled concretes is different according to the transport mechanisms to which they are exposed and this behavior can match that of concretes made with natural coarse aggregates.

Experimental Research on Behavior of Composite Beams of Steel-Reinforced Recycled Concrete

The recycled aggregate is dealt with in three ways, the first way is to keep the recycled aggregate intact; the second way is to wrap the recycled aggregate in cement slurry of Water-cement ratio 0.55 for 30 minutes, and then place it at room temperature for 7 days; the third way is to immerse the recycled aggregate in water for 24 hours. Three kinds of different recycled aggregate is separately used for mixing of recycled concrete. The strength and slump of recycled concrete with different recycled coarse aggregate contents are tested. Experiments show that the recycled concrete produced by using the recycled aggregate immersed in water and selecting a suitable mix has the higher strength and slump，which can be industrialized. And then, the recycled concrete with 40% recycled coarse aggregate contents is used in the steel-reinforced recycled concrete beams, which are tested, the failure process of steel-reinforced recycled concrete beams,section strains and maximum load are acquired. Research shows the steel-reinforced recycled concrete beams conform to plane hypothesis before the load reachs 80% the limit load, the flexural bearing capacity of the steel-reinforced recycled concrete beams can meet the demand of Technichal Specification of Steel-Reinforced Concrete Structures. The use of recycled concrete in the composite structures is proposed.

Carbon footprint of sludge treatment reed beds

Sludge treatment reed beds (STRBs) performance has become a popular solution to dewater and stabilize surplus sludge from wastewater treatment systems because of the low investment, operational and maintenance costs of the systems. The environmental impact associated with this technology has, however, to be assessed in order to evaluate the appropriateness of the technology. The goal of this study is to determine the carbon footprint of STRBs. To this end, a Life Cycle Assessment was performed using methane and nitrous oxide emissions quantified in two full-scale STRBs located in Northern and Southern Europe (Denmark and Spain). Methane emissions ranged between 1000 and 3700mg CH4/m2d, while nitrous oxides ranged between 200 and 750mg N2O/m2d. Thus, greenhouse gas emissions correspond to 0.09 and 0.25kg CO2eq/m2d for the Danish and Spanish system, respectively. The treatment of 1 ton of sludge (wet weigh) in the Spanish system corresponds to 2.1kg CO2eq, while in the Danish system it corresponds to 1.0kg CO2eq. The contribution of greenhouse gas emissions in the carbon footprint is insignificant in comparison with raw materials and energy consumption. However, the results put forward the relevance of STRB design in the environmental impact of the technology. Thus, the excavation of basins or the use of recycled concrete are recommended to guarantee a low carbon footprint of this technology.