Abstract

Sustainability is a primary concern that needs to be addressed since infrastructure development requires significant natural resources. Using Recycled Concrete Aggregates (RCA) for road construction has numerous benefits in saving natural resources and the environment. In the present investigation, the demolition waste is being used for road construction, partially/fully replacing natural aggregates. The Cement Treated Recycled Concrete Aggregate (CTRCA) specimens were prepared at 3%, 5%, and 7% cement with various blends of RCA and Natural Coarse Aggregates (NCA) ranging from 0 to 100%. The strength characteristics in terms of Unconfined Compressive Strength (UCS), Flexural Strength (FS), Elastic Modulus, Indirect Tensile Strength (ITS) and durability tests were conducted on cured samples. Microstructural analysis using Scanning Electronic Microscope (SEM) revealed that the pores and cracks in the old mortar have a detrimental influence on the mechanical properties of CTRCA mixes. However, Energy Dispersive Spectroscopy (EDS) and durability tests have shown positive results. The Response Surface Method (RSM) was utilised to optimise the RCA and cement content in CTRCA mixes. The research resulted in the maximum possible RCA of up to 70% with a cement content of 5.8%, which met the Indian Road Congress (IRC) specifications for Cement Treated Bases (CTB).

Introduction

The demolition waste generated from old structures is inevitable due to economic development and technological advancement. More than half of global demolition waste comes from China, India, and the United States of America, with concrete waste accounting for up to 67% of total demolition waste (Chung and Lo 2003, Akhtar and Sarmah 2018). Every year, more than 6 billion tonnes of concrete is produced worldwide (Ä et al. 2012). This waste is disposed of in dumping yards, water bodies, drain trenches, and untilled grounds around human habitations.

Furthermore, accumulating demolition waste in landfills tends to inhabit vacant land that could have been used for other purposes. These waste materials have become vital resources increasingly in road construction due to the scarcity of natural resources. Recycling aggregates from construction and demolition waste is the sole solution to conserve traditional aggregates and safeguard the environment. The potential application of RCA in base and sub-base pavement courses will solve disposal problems and conserve natural resources (Leite et al. 2011, Arulrajah et al. 2014). It is necessary to characterise RCA before its use in pavements. A crucial factor influencing the performance of pavements is the particle size distribution. Different crushing techniques and the concrete’s strength are responsible for the variances in the particle size distribution of RCA (Aydilek.pdf n.d., Arulrajah et al. 2012, Gabr and Cameron 2012). The density of RCA is lower than NCA because of the attached mortar content on its surface (Tang et al. 2018).

Conclusions

In this study, the optimisation and characterisation of the CTRCA mixes were carried out. The RSM was employed to study the effect of individual and combined factors on CTRCA mixtures. Based on test results, RCA can be used in cement-treated base and sub-base courses in Pavements. The following conclusions are drawn based on the present investigation in the laboratory.

. The mix with NCA could get the required strength in UCS, FS, ITS and Elastic Modulus at 5 per cent Cement content after seven days of curing. It also passes the durability test.

. The CTRCA mix with 30 per cent NCA and 70 per cent RCA could get the required strength with 5.8 per cent cement after seven days of curing in terms of UCS, FS, ITS and Elastic Modulus. All the samples satisfy the durability test.

Abstract

     Bioenergy with carbon capture and storage (BECCS) is a carbon dioxide removal (CDR) solution necessary to achieve net-zero-carbon-emissions goals. While the BECCS potential from large industrial emitters has been quantified, the BECCS potential of small emitters, such as biogas facilities, has not been investigated. Moreover, most BECCS solutions rely on the expected availability of large geological storage capacity for future CDR implementation, although the deployment of CO2 transport and storage supply chains is still a barrier for geological carbon storage ambitions. An alternative opportunity for permanent sequestration of CO2 is concrete, in which captured CO2 can be permanently fixed through carbon dioxide mineralization technologies. We describe and discuss this solution by quantifying the potential of a European bioenergy with carbon capture, utilization, and storage (BECCUS) supply chain, which relies on biogenic CO2 from biogas facilities as a CO2 source, and on carbon dioxide mineralization in concrete as a permanent CO2 sink. This solution is available today, can be adopted seamlessly, and does not need economies of scale for its deployment. We find that European biogas facilities produce 24 Mtons of biogenic CO2 per year, of which 4 Mtons of CO2 per year are emitted from facilities already upgrading biogas into bio-methane.

Introduction

     Reaching net-zero greenhouse gas (GHG) emissions by 2050 requires a rapid decarbonization of European economies (European Commission, 2018), as well as the deployment of carbon dioxide removal (CDR) solutions to offset unavoidable emissions (European Commission, 2021). About 10% (500 Mton CO2eq per year) of current European GHG emissions are estimated to be hard-to-decarbonize and will likely need some degree of CDR (European Commission, 2021). Many net-zero-GHG emissions (hereafter simply net-zero) scenarios rely on CDR using bioenergy with carbon capture and storage (BECCS) (Scott and Geden, 2018; Rogelj et al., 2019). BECCS consists in deploying biomass to produce bioenergy, capturing, and transporting the resulting CO2, and permanently storing it in underground geological formations (Bui et al., 2018; Fuss et al., 2018). Because sustainable biomass is a limited resource (Rosa et al., 2021b), there is a growing consensus that the societal value of using biomass for CDR (i.e., BECCS) will exceed the value of using biomass for bio-energy production only (Sandalow et al., 2021a; Fajardy et al., 2021).

Conclusions

     A portfolio of solutions will be required to mitigate climate change. We introduce biogas-concrete BECCUS supply chains that permanently store biogenic CO2 in recycled concrete aggregates through carbon dioxide mineralization technologies. Biogas-concrete BECCUS supply chains combine carbon utilization with permanent sequestration and have the potential to permanently remove CO2 from the atmosphere while generating a marketable product, namely concrete, thus implementing a circular economy model. BECCUS supply chains represent a CDR solution available for small enterprises, such as biogas facilities, to mitigate their carbon emissions without the need of economies of scale and complex supply chains to transport and store CO2. We find that biogas-concrete BECCUS supply chains can permanently store 8 Mtons of CO2 per year in recycled concrete aggregates in Europe. On the one hand, this is a relatively small fraction of the CO2 that will likely need to be removed in Europe to reach net-zero GHG emissions (500 Mtons of CO2eq per year); in other words, the limited CO2 storage potential in recycled concrete aggregates is the bottleneck of BECCUS supply chains. On the other hand, short CO2 source-sink transportation distances represent a promising and efficient CDR opportunity for small emitters. This suggests that BECCUS supply chains should be deployed whenever possible and will have to be complemented with geological CO2 storage (or BECCS) to achieve European ambitious climate targets.

Abstract

     This paper studies the fresh and mechanical properties of high-strength concrete (HSC) by incorporating recycled concrete aggregates (RCA) of varying sizes and concentrations. The recycled aggregate concrete (RAC) was prepared by partially replacing RCA with natural coarse aggregate (NCA) at 0%, 15%, 30%, and 45%, with aggregate sizes ranging from 5 to 12 and 12–20-mm. Fresh concrete properties, such as slump, Kelly ball, compacting factor, K-slump, and fresh density, were tested to determine the influence of RCA size and concentration. In addition, the mechanical properties were studied through the execution of compressive, split-tensile, and stress-strain tests. The test results revealed that increasing the RCA concentration declines the fresh and hardened properties of HSC. In the fresh concrete experimentation, the 12–20 mm aggregate size RAC mixes exhibited greater workability than the 5–12 mm aggregate mixes. On the contrary, 5–12 mm aggregate mixes RAC had higher compressive and split-tensile strength and a higher modulus of elasticity than 12–20 mm aggregate mixes concrete. When it comes to sustainability, the study found that the smaller size range of RAC produces inferior embodied CO2 (eCO2) and provides a cost-effective and sustainable solution for the construction industry.

Introduction

     Over the last decades, the construction community has faced several challenges, including a scarcity of natural resources and excessive energy consumption that directly impacts environmental pollution. These challenges include using natural resources and energy, the generation of greenhouse gases (CO2), and the growth of building and demolition waste (Datta et al., 2022, Vo et al., 2021). Recycling the demolition waste is the most promising option for reducing the building industry’s adverse environmental effects in the construction sector. This attention is increasingly growing since it was projected that the world has to reckon with the problems of disposing of 3 billion tons of demolished concrete waste annually until 2012 (Akhtar et al., 2018). In recent years, Zhang et al. (2020) reported that the global accumulative carbonation of concrete waste might produce roughly 3.0 Bt CO2 between 2018 and 2035. As a result, using recycled concrete aggregate (RCA) made from waste concrete in engineering construction can help protect the environment while increasing the residual value of waste concrete. In addition, these RCAs have gained widespread acceptance from the construction research community as alternative building structure materials in recent decades.

Conclusions

     In this paper, the size and concentration effect of RCA on the production of HSC has been studied. Eight concrete mixes with different RCA concentration levels with an interval of 15% were considered. The obtained experimental results of the rheological and mechanical properties are concluded as follows:

• All the mixes had a satisfactory slump result greater than 150 mm, which led to the high workability of concrete. Concrete mixtures of the 12–20 mm aggregate size range have a larger slump, compacting factor, ball penetration, and K-slump value than the 5–12 mm size range concrete. In addition, the high density and workability of mixes decrease with an increasing concentration of RCA replacement level.

• The decrease in the split-tensile strength of RAC with the increase of the RCA concentration is relatively smaller than compressive strength and MOE. The 5–12 mm coarse aggregate size enhances the strength and MOE of the mixes compared to the 12–20 mm aggregate size. In terms of sustainability assessment, the cost and eCO2 emission decrease with RCA incorporation, leading to a low-cost and environment-friendly concrete production with better strength.

1- Introduction

2- Microstructure of RAC

3- Defects in RAC microstructure

4-  Enhancement treatments

5- Conclusions

References

Abstract

Recycled concrete aggregate (RCA) differ from natural aggregate as the former contains attached mortar. Given the recycling process of RCA, that is, by crushing, the microstructure presents several disadvantages, such as porosity, micro-cracks, as well as weak interface transition zones (ITZs), which will also damage the mechanical properties and durability of recycled aggregate concrete (RAC). According to current literature, the microstructure of RAC is mainly affected by ITZs and the porosity of RCA. This paper summarises the principles, advantages and disadvantages of the methods for improving the microstructure of RCA from four aspects, such as removal of attached mortar in the RCA, surface coating on RCA, different mixing methods and CaCO3 precipitation. Methods are also discussed from the aspects of environment, RCA particle size and application, which provide readers with a reference for improving the microstructure of RCA.

Introduction

Since the beginning of the 21st century, with the rapid development of urbanisation and industrialisation, the construction industry has consumed large amounts of natural resources and produced large amounts of construction and demolition waste [1]. The impact on the environment according to Oikonomou [2] is ‘‘Take 50% of raw materials from nature, consume 40% of total energy, and create 50% of total waste”; and construction became the industry with the largest impact on the environment [3]. Concrete is widely used in the construction industry because of its low price, wide application and the ability to take any shape [3,4]. It is a composite material prepared from a certain proportion of cementitious materials, water, aggregates and admixtures [5]. The construction industry produces about 25 billion tons of concrete annually worldwide [6]. Developing countries, such as China and India, which are among the top 10, are the biggest consumers of concrete [3,5].

Based on the fine powder and the activation characteristics of iron tailings, the basic mechanical properties and deterioration mechanism of the recycled concrete with tailings before and after carbonization were macroscopically and microscopically analyzed via rapid carbonization tests. The macroscopic results showed that the cubic compressive strength increased first and then decreased with the tailings addition at different dosages, with a peak value of 30%. The values of the splitting tensile strength and axial compressive strength fluctuated differently, and the tailings content ranged from 20% to 40% at the peak point before and after carbonization. The peak strain and elastic modulus of recycled concrete were reduced to different extents, and the longer the carbonization age was, the greater the reduction range was. Simultaneously, the carbonization age also increased the brittleness of recycled concrete, causing the descending section of the constitutive curve to greatly fluctuate. Microscopically, the influence of tailings on the matrix structure of the recycled concrete was verified by energy dispersive spectrometer (EDS) and scanning electron microscope (SEM) analysis. The results proved that the appropriate tailings content could improve the interface transition zone (ITZ) and the compactness of the concrete, also optimize the mechanical properties of the recycled concrete. However, excessive tailings could reduce the integrity of the matrix structure, increase the number of harmful pores. According to the research, when the tailings content was 20%e40%, the recycled concrete had high mechanical properties and reasonable microstructure. This study provided a theoretical basis for the popularization and application of the recycled concrete with tailings.

Introduction

Tailings and construction waste are inevitable products of accelerated urbanization construction and rapid economic development. Due to the immaturity of regeneration technology, a large number of tailings ponds and construction waste treatment plants exist, which not only occupy land, waste resources, and pollute the environment but also pose a substantial threat to the property and personal safety of the surrounding residents (Li al., 2018; Kox et al., 2019; Lu et al., 2018). Currently, making recycled concrete from tailings and construction waste is the most effective approach. However, at present, most studies worldwide have been focused on concrete with tailings or recycled aggregate concrete (RAC). In these investigations, the effects of aggregates, such as tailings, and natural coarse aggregates (NCA) on dam concrete were compared, their mechanical properties and frost resistance were discussed. The studies demonstrated the feasibility of using tailings as the aggregate to completely replace an ordinary aggregate in dam concrete (Lv et al., 2019). Cheng et al. (2016) studied the mechanical and chemical activation of iron tailings and the performance of concrete after the partial replacement of cement. The results showed that the compressive strength of concrete decreased gradually with the replacement rate. Shettima et al. (2016) used iron tailings to replace river sand in concrete, and studied its compressive strength, splitting tensile strength, elastic modulus and durability, the results showed that the strength and elastic modulus of the modified concrete were higher than that of ordinary concrete due to the addition of tailings.

1. Introduction

2. Experiment descriptions

3. Results and discussions

4. Conclusions

Acknowledgment

References

Abstract

This paper evaluates the feasibility of preparing self-compacting concrete (SCC) by incorporating waste concrete recycling materials (WCRMs). Some or all of the natural materials were substituted with WCRMs, such as recycled coarse aggregate (RCA), recycled fine aggregate (RFA) and recycled powder (RP). Nine SCC mixtures were produced, including eight self-compacting recycled concrete mixes and one control mix without WCRMs. The SCC mixes were designed with four different RCA replacement rates (0%, 25%, 50% and 100%) along with a constant amount of RFA (10%) and varying percentages of RP (10% and 20%). The water/binder ratio was designed as 0.4 for all the SCC mixtures. The workability properties (e.g., flowability, viscosity and passing ability) and mechanical properties (e.g., compressive, splitting tensile and flexural strengths) of self-compacting recycled concrete were investigated. The findings indicate that the substitution ratio of WCRMs will affect the workability and mechanical behavior of SCC. This study demonstrates that the decline in the workability and mechanical behavior of self-compacting recycled concrete results from the cooperation of WCRMs. When natural materials are substituted and the amount of WCRMs is limited, the workability of self-compacting recycled concrete can satisfy the European standard EFNARC (2005). Meanwhile, this study also finds that the selfcompacting recycled concrete can achieve acceptable mechanical performance when compared to the ordinary SCC without WCRMs.

Introduction

In recent decades, the construction industry has faced a series of problems, from a shortage of resources, and energy to environmental pollution. These problems include the consumption of natural resources and energy, emission of greenhouse gas (CO2) and the creation of construction and demolition waste (CDW). Recycling CDW is the most promising solution for reducing the adverse environmental impacts in landfills caused by the construction industry. In 2015, 3.9 billion tons of CDW were produced in China, nearly 30% of which was waste concrete (Fu, 2016; Xiao, 2018). Therefore, utilizing waste concrete recycling materials (WCRMs) in engineering will contribute to protecting the environment and capturing the residual value of waste concrete. The WCRMs include recycled coarse aggregate (RCA), recycled fine aggregate (RFA) and recycled powder (RP). Recycled aggregate (RA), consisting of RCA and RFA, refers to aggregate with a diameter of less than 31.5 mm manufactured by crushing waste concrete. RP is a fine particle produced by collecting dust or finely grinding the CDW (Zhu et al., 2019). In recent years, these WCRMs have become widely accepted as alternative recycled materials that can be used in the construction industry.

1- Introduction

2- Materials and methods

3- Experimental results and discussion

4- Conclusions

References

Abstract

This paper aims to develop and evaluate the performance of steel fiber-reinforced concrete made with recycled concrete aggregates (RCA) and desert dune sand. Different fresh and hardened properties of RCA concrete mixtures with and without steel fibers were evaluated and compared with those of a mixture made with natural coarse aggregates (NA). Test parameters included the RCA replacement percentage and steel fibers (SF) volume fraction (vf). Test results showed that the substitution of 30% of NA with RCA in plain concrete mixes did not reduce the design cylinder compressive strength (f′c), whereas the use of higher percentages of RCA replacement compromised the fresh and hardened properties of the concrete. The addition of SF significantly reduced the adverse effects caused by the inclusions of RCA in the mixes. RCA-based concrete mixtures having 70 and 100% RCA replacements could be produced with f′c values comparable to that of a NA-based concrete, when minimum vf values of 1, and 2% were added, respectively. The RCA replacement increased the water absorption and sorptivity but decreased the ultrasonic pulse velocity, bulk resistivity, and abrasion resistance. The addition of SF improved the hardened properties of RCA-based concrete mixes to an extent of exceeding some of those of the NA-based counterpart, as in the case of abrasion resistance. Analytical regression models were introduced to correlate different concrete properties to the 28-day cylinder compressive strength.

Introduction

Concrete is one of the most consumed construction materials. It uses significant amount of non-renewable natural resources in the production of its components, aggregates and cement. To sustain the continuous population growth and construction development, more concrete will be required. Consequently, the increasing demand for aggregates raises concerns of the depletion of their current sources and the availability of new sources. At the end of its service life, a concrete structure is demolished, creating massive amounts of construction and demolition waste (CDW) [1]. The concrete waste is mostly disposed of in landfills, instigating serious environmental hazards. The limited space for landfills to contain CDW and the scarcity of natural resources for natural aggregates (NA) promote the concept of reusing this waste by manufacturing RCA as a potential substitute to NA. This promising approach is considered an innovative means to promote the use of sustainable materials in construction. The usage of concrete made with RCA has been limited to nonstructural applications such as roadway subbase [2,3]. This is because RCA has lower strength and elastic modulus and higher water absorption and porosity compared to NA [4,5]. As a result, the incorporation of RCA in concrete mixtures causes a decrease in its mechanical performance compared to its conventional counterpart [6–8]. Çakır [9] studied the effect of using a 100% RCA replacement on the concrete compressive strength. It was concluded that the compressive strength of concrete with a 100% replacement of RCA decreased by approximately 24% compared to that of concrete made of NA.

1- Introduction

2- Materials and Testing

3- Testing

4- Results and discussion

5- Conclusions

References

Abstract

Recycled Concrete Aggregate (RCA) is considered to be a potential substitute for natural aggregates in asphalt mixtures. Despite some contradictory results achieved by researchers, it is believed that RCA treatment can improve quality of recycled asphalt mixes considerably. In this research, a two-stage treatment was applied on coarse RCA materials in order to improve their properties. The treatment was consisted of first soaking RCAs in Hydrochloric Acid (HCl); second impregnating the treated RCAs into Calcium Metasilicate (CM). The pores of RCAs were filled with CM particles. Substituting virgin aggregates with different amounts of coarse RCA materials into HMA resulted in increased tensile properties of mixes as tested in indirect tensile fatigue test (ITFT). The treatments resulted in reduced moisture sensitivity of mixes. The improvements were mostly attributed to the reduced water absorption of RCA materials. Moreover, morphological characteristics of the treated coarse RCA materials were determined using Scanning Electron Microscopy (SEM) photography.

Introduction

Over the past decades, the growing amounts of Construction and Demolition Waste materials (CDW) have been a great concern for the environmental protection agencies [1]. Unlike the proper actions of recycling these materials and also re-using in different applications, the general trend is to use virgin mineral aggregates for constructing roads and other civil engineering projects. This trend will certainly have adverse effects on the environment [2]. However, lack of adequate landfill areas for damping CDW materials, and the detrimental environmental issues have conducted highway agencies to perform research works so that to find alternative solutions for recycling CDW materials [3]. In fact, road construction projects, that require huge amounts of aggregates are proper sites for using these recycled materials [4]. Production of mineral aggregates in quarries, not only have negative impacts on the environment, but it results in emission of large amounts of CO2 gases during the processing phases [5].

This study designed an eco-friendly pervious concrete (PC) product using waste glass cullet (WGC) and recycled concrete aggregate (RCA) by dry-mixed compaction technique. The mechanical properties, water permeability behavior and related pore structure characteristics, thermal conductivity of the PCs were determined. The experimental results showed that the use of silica fume in the cement paste was effective to compensate for the low compressive strength of the PCs due to the use of narrowed graded aggregates without the incorporation of fine particles (less than 2.36 mm). Although the incorporation of recycled aggregates (i.e. WGC and RCA) into the PCs led to reductions in compressive strength, the water permeability of the PCs was improved, especially for the PCs prepared with WGC, as the use of WGC was conducive to improving the water permeability due to the negligible water absorption nature and smooth surface of glass cullet. Another encouraging result indicated that the PCs comprising 50% WGC as the fine aggregate and 50% RCA as the coarse aggregates could achieve satisfactory strength and permeability, which could largely meet the requirement of the standard (JIS A 5371) for permeable pedestrian pavers. The appropriate blending of the selected size of WGC (2.36e5 mm) and RCA (5 e10 mm) to produce a desirable pore structure in the PCs were responsible for the good performance. In addition, the low thermal conductivity of the produced PCs provided an opportunity to use the PC as partition block materials for saving energy consumption of buildings.

Introduction

The use of pervious concrete (PC) is a unique and effective means to address several important environmental issues, including improving the recharge of groundwater, reducing stormwater runoff, promoting tree survival by providing air and water, improving water quality and reducing the heat island effect, etc. (Tennis et al., 2004). Especially, in Hong Kong, heavy rainfall could happen commonly (about 2400 mm per annum of rainfall are recorded on average) (DSD, 2018). How to manage the heavy rainfall becomes a crucial issue for Hong Kong due to the densely built up urban areas and the mountainous terrain which further exacerbates the risk of flooding. Hence, it is necessary to undertake flood and waterlogging mitigation measures to improve the flow capacity. The U.S. Environmental Protection Agency identified PC pavement as one of the effective Best Management Practices for stormwater management as PC has sufficient void space to allow rapid percolation of water through the pavement (Tennis et al., 2004; DEP, 2006). Therefore, PC pavement has been extensively applied in the parking lots, drainage layers in exterior areas, residential street parking lanes, walkways/sidewalks, bike paths and pavements with minimal heavy truck traffic. Usually, PC consists of Portland cement (supplementary cementitious materials sometimes are used), aggregates and water. The difference from conventional concrete is that PC contains no fine aggregates and a lesser amount of cement paste is used to fill the voids between the aggregate particles. As a result, open cells (pores) are formed and the induced high porosity of PC leads to good internal drainage and infiltration.

The brittleness and low fracture toughness of autoclaved aerated concrete (AAC) could be improved by fibres, but the deterioration of its thermal insulation performance are commonly observed. In this study, different combinations of recycled wood fibre and rubber powder were added in AAC to improve its performance and reduce the negative environmental impacts of solid waste. In addition to the fluidity of slurry, the physical, mechanical and thermal properties of AAC were systematically investigated, together with the phase components and microstructure images. AAC with relative high mechanical strengths and excellent thermal insulation performances could be obtained because of the reinforcement of wood fibre and the gas introduced by rubber powder. The scanning electron microscopy (SEM) and X-ray diffraction (XRD) analyses revealed that the positive effects of wood fibre and rubber powder were the results of physical interactions.

Introduction

Recently, energy conservation with respect to buildings has become a growing concern (Ko
cí et al., 2013). In China, 20.6% of the total energy and 19.0% of the total carbon emission have been consumed by building in 2016 (China Building Energy Saving Technic Association, 2019). A series of energy codes for buildings has implemented to effectively reduce building energy consumption. To achieve the goal of 65% building energy conservation, the internal-external insulation measure and wall materials with good thermal insulation performance are adopted (Wang et al., 2019). Autoclaved aerated concrete (AAC) has been reported to be the only wall material meets the Chinese national standard requirement of the energy conservation for buildings (He et al., 2018). AAC is usually the mixture of quartz powder (or fly ash), cement, lime, gypsum, water and small quantities of aluminum powder, which is hardened and strengthened under pressurized steam (Israngkura Na Ayudhya, 2016; Schreiner et al., 2018). Compared with ordinary concrete, AAC has the more excellent thermal insulation properties and the lower density (Asadi et al., 2018; Ma et al., 2016; Yuan et al., 2017). However, AAC products also have some drawbacks, such as high brittleness and low mechanical strength, which limits their commercial applications (Cong et al., 2016; Ferretti et al., 2015; Koudelka et al., 2015). Moreover, recent trends have focused on utilizing industrial wastes to produce highperformance AAC, that is, AAC with high volume stability, high mechanical strength, and excellent thermal insulation properties (El-Didamony et al., 2019). Studies have shown that adding fibre was a simple and effective way to obtain AAC with relatively high volume stability and mechanical strength (Bonakdar et al., 2013; Laukaitis et al., 2012; Pehlivanlı et al., 2016).

This work deals with the flexural performance of recycled concrete subjected to increasing loads up to failure. For this purpose, eight reinforced concrete beams were made with recycled coarse aggregates using two different water to cement ratios (0.50 and 0.65) and four replacement percentages (0%, 20%, 50% and 100%). Firstly, the basic concrete properties were determined (mechanical strengths and modulus of elasticity) and then, beam specimens were loaded up to failure using a four-point bending test at 28 days. As a result, bending moments, deflections, strains and curvatures were obtained at different load levels (cracking, service, yielding and ultimate state conditions), and also, the crack pattern. On the basis of these results, it can be noted that service, yielding and ultimate state of recycled concrete exhibits, in general, a similar trend to that of conventional concrete. However, the cracking behaviour shows differences between recycled and conventional concrete. Finally, code-based expressions were used to calculate bending moments and deflections under flexural load, taking into account the different content of recycled coarse aggregate.

Introduction

In order to promote the sustainability of concrete, efforts have been made to address some of the environmental problems associated with concrete waste. In line with this, numerous researches have been conducted to analyse the structural performance of recycled concrete [1–13]. However, its use in real building and civil engineering applications requires more full scale studies, to assess the load-deformation response of recycled concrete that leads to good agreement on structural design. Regarding flexural performance of structural recycled concrete, different investigations have been carried out [4,8,9,12]. However, the number of studies on concretes with high replacement percentages is scarce and additionally, some contradicting conclusions have been detected. While some authors [1,9,10,13] have found that recycled concrete beams show higher deflections, and cracking moments lower or similar to those of the conventional, others [6,8] have reported no significant difference between recycled and conventional concrete in terms of flexural performance. On the basis of the literature review, it can be seen that the codebased procedures for conventional concrete can also be applied to recycled concrete predictions for flexural behaviour [2,6,9], although some differences have been detected. In line with this, the increase in recycled coarse aggregate results in the reduction of concrete stiffness [9], which is consistent with the increased beam deflections of recycled concrete. Most authors [2,8–10] point out similar yielding and ultimate behaviour, but some [7] have found little differences in cracking behaviour, in terms of crack spacing and pattern. In general, it can be said that recycled concretes are able to fulfill strength and serviceability requirements similarly to conventional reinforced concrete. However, in order to encourage its use as structural concrete, it is important to be able to design reinforced concrete members made with recycled coarse aggregate, using existing design methods [6]. Accordingly, recent publications [2,9] have indicated that more full-scale research should be done to stablish a good agreement on its flexural performance and increase the results database for structural recycled concrete. This would lead to concrete properties, service-load and ultimate–load behaviour being predicted with the same approximation degree as conventional concrete. Furthermore, with the aim of carrying out an accurate structural design, some concrete parameters have to be considered such as, tension stiffening, strength capacity or cracking behaviour. Tension stiffening is known as the concrete contribution after cracking and has significant importance in structural design. This parameter is required to accurately design concrete structures as customary serviceability conditions occur after cracking.

Recycled aggregates recovered from abandoned buildings or demolished concrete structures were reused to produce concrete columns as new building components. A quasi-static test on 10 recycled aggregate concrete flled steel tubular (RACFST) columns was carried out. The mechanical properties of RACFST columns in the study include failure mode, hysteretic loops, skeleton curves, stifness deterioration curves and energy dissipation capacity. RACFST columns with diferent recycled aggregate replacement ratios were tested under constant axial load and cyclic lateral load. The seismic performance of the columns was analyzed. The experimental results indicate that concrete adding recycled aggregates does not reduce the lateral stifness of the columns, and their energy dissipation capacity is nearly as good as that of natural aggregate concrete flled steel tubular columns. The usage of recycled aggregate concrete has little infuence on the lateral load-bearing capacity and the stifness deterioration of RACFST columns. The current calculation for natural aggregate concrete flled steel tubular columns to estimate the lateral load-bearing capacity of RACFST columns tends to be relatively conservative. The reuse of recycled aggregates for load-bearing components of building structures especially for concrete flled steel tubular columns is feasible.

Introduction

The rapid development of urbanization in China results in a large amount of demolished building materials, in which concrete is the most. However, the recycled aggregates recovered from abandoned buildings or demolished concrete structures can be reused to produce recycled concrete if properly treated such as crushing, washing, sieving and grading. The aggregate made from abandoned buildings or demolished concrete structures can be used to replace some or all of the coarse aggregate in concrete mixture to form recycled aggregate concrete (RAC). Up to now, extensive researches have been carried out on the mechanical properties of RAC, including the study of recycled concrete members and structures. It was shown that RAC could be used mainly in the structures with lower stress or force level because of the characteristic limitations of recycled aggregate itself [1, 2]. However, thanks to the combination of RAC and concrete flled steel tubular (CFST) structures, which has been put forward by researchers and instantly aroused an interest in practice [3], the method of flling the steel tube with recycled aggregate concrete or demolished concrete lumps partially or wholly not only reduces the whole weight of buildings, but also decreases the price of concrete members [4]. As the recycled aggregate has a positive infuence on the ductility of RAC structures [5], it is desired to apply on the buildings with high seismic resistance requirements or the areas with frequent destructive earthquakes. Meanwhile, recycled aggregate concrete is considered to provide high and obvious economic and social benefts and promising application prospect [6]. The basic mechanism of RAC filled steel tubular (RACFST) members is similar to the situation of natural aggregate concrete flled steel tubular members, and tests for the material have shown that the Poisson’s ratio of RAC is nearly the same as that of natural aggregate concrete [7], which tends to indicate that the steel tube can be applied on confning RAC as it does for natural aggregate concrete. A series of investigations on the mechanical property, durability and structural performance of RACFST columns have been carried out in the past 15 years after initially studied in the late 90’s [8]. The related studies are ranged on the mechanical performance of RACFST columns, especially under axial or eccentric compression [9–12], the shrinkage and creep of the core concrete [13], the working mechanism of steel tube confnement [14, 15], the energy dissipation capacity of RACFST columns [16, 17], and so on.

According to the World Commission on Environment and Development (WCED), ‘‘sustainable development is a development that meets the needs of the present without compromising the ability of future generations to meet their own needs” [1]. When considering the total amount of natural resources in a large scale, in Europe for example, the exploitation of resources might not be as significant as in a smaller scale, in Cyprus for instance, where the increased exploitation may reduce the resources to a crucial level, due to country’s small size, compromising the needs of the following generations. There is a significant research activity regarding the mechanical performance of recycled aggregate concrete (RAC) which shows that the strength of RAC is adequate for use as structural concrete. On the contrary, the replacement percentage of natural aggregates with recycled aggregates and the durability properties of RAC are still under investigation, since a wide variability in the results is reported [2]. In fact, the durability properties are still under examination and so the RCA are mainly used for secondary level construction activities, as road base and landfilling materials [3,4]. The microstructure of RAC is much more complicated than that of conventional concrete since it includes two kinds of interfacial transition zones, one between the RCA and the new mortar and a second one between the RCA and the adhered mortar. The old mortar includes many micro-cracks, formed during RCA production, and has high porosity [5], thus it becomes the weakest link in RAC and its strength is the upper limit of the strength of concrete. As the mortar-aggregate bond strength increases, the concrete strength also increases [6,7]. RAC is weaker than Natural Aggregate (NA) concrete and its failure occurs through the aggregates themselves (including the old interfacial transition zone (ITZ)) instead of the new ITZ [8,9] similarly to the high strength concrete [3,5]. The presence of the adhered mortar is significant since recycled aggregates consist of 65–70% by volume of natural coarse aggregate and 30–35% by volume of old cement paste [7]. In order to enhance the properties of RCA, it is fundamental to elaborate a treatment method that is capable to remove the adhered mortar at such level that diminishes the negative effects. In general, RCA are of lower quality than NA. RCA have higher water absorption values and lower densities. The water absorption of RCA ranges from 3 to 15% [3,7]. The presence of the mortar lowers the density values of the recycled aggregates (2200–2400 kg/ m3 ) [7]. Furthermore, the resistance to fragmentation is lower and their texture is more porous, rough and irregular due to crushing of the old concrete. The lower quality aggregates have an immediate effect on the quality of the hardened concrete. The increase of the replacement ratio of RCA decreases the compressive strength of RAC. Generally, the compressive strength can be 10–25% lower than conventional for 100% replacement. Etxeberria et al. reported losses of 20–25% for 100% replacement, maintaining the same effective w/c ratio (0.50) and the same cement content [8].