Parent Concrete Research Articles

Feasibility of utilizing recycled coarse aggregates in commercial concrete production

This study addresses a critical gap in circular construction practices by assessing the use of high-quality Recycled Coarse Aggregates (RCA) from end-of-life concrete on an industrial scale. Unlike previous studies, which predominantly relied on theoretical mix designs or laboratory-level experiments, this research focuses on real-world applicability, employing commercially produced RCA and conventional production methods in industrial settings to identify upscaling challenges. Advanced Dry Recovery technology is utilized to produce high-quality RCA for both ready-mix and prefab concrete production. To ensure practical relevance, the research examines three water-to-cement ratios for ready-mix concrete and three strength classes for prefab concrete, all prepared and cast in a commercial setting using standard industrial practices. The results show that by selecting the appropriate application for RCA, there is potential for concrete companies to produce mixes using 100% RCA that meet standard requirements in terms of fresh, mechanical, and durability properties without the need for extra treatments or specific mixing methods, particularly when the water absorption of RCA is less than 4%. Achieving optimal performance requires adjustments in the mix design, specifically by considering the effective water-to-cement ratio. Additionally, the study underscores the impact of the parent concrete's properties on the RCA quality. This research not only demonstrates the feasibility of employing RCA in industrial-scale concrete production along with its associated challenges but also highlights the potential for enhancing circularity in the construction industry through large-scale adoption of RCA, thereby contributing to sustainable and circular construction practices.

Potential for Recycling Metakaolin/Slag-Based Geopolymer Concrete of Various Strength Levels in Freeze-Thaw Conditions.

Geopolymer concrete (GPC) represents an innovative green and low-carbon construction material, offering a viable alternative to ordinary Portland cement concrete (OPC) in building applications. However, existing studies tend to overlook the recyclability aspect of GPC for future use. Various structural applications necessitate the use of concrete with distinct strength characteristics. The recyclability of the parent concrete is influenced by these varying strengths. This study examined the recycling potential of GPC across a spectrum of strength grades (40, 60, 80, and 100 MPa, marked as C40, C60, C80, and C100) when subjected to freeze-thaw conditions. Recycling 5-16 mm recycled geopolymer coarse aggregate (RGAs) from GPC prepared from 5 to 16 mm natural coarse aggregates (NAs). The cementitious material comprised 60% metakaolin and 40% slag, with natural gravel serving as the NAs, and the alkali activator consisting of sodium hydroxide solution and sodium silicate solution. The strength of the GPC was modulated by altering the Na/Al ratio. After 350 freeze-thaw cycles, the GPC specimens underwent crushing, washing, and sieving to produce RGAs. Subsequently, their physical properties (apparent density, water absorption, crushing index, and attached mortar content and microstructure (microhardness, SEM, and XRD) were thoroughly examined. The findings indicated that GPC with strength grades of C100, C80, and C60 were capable of enduring 350 freeze-thaw cycles, in contrast to C40, which did not withstand these conditions. RGAs derived from GPC of strength grades C100 and C80 complied with the criteria for Class II recycled aggregates, whereas RGAs produced from GPC of strength grade C60 aligned with the Class III level. A higher-strength grade in the parent concrete correlated with enhanced performance characteristics in the resulting recycled aggregates.

Comparative Study of the Crushing Strengths of Recycled Coarse Aggregates Concretes and Natural Aggregates Concretes

Recycled Coarse Aggregate (RCA) is a recycled concrete material from construction sites, which is being reused for fresh concrete production. The use of this material reduces quarrying of Natural Coarse Aggregate (NCA). Properties of RCA are variable depending on the properties of parent concrete. In a fast-growing economy, the generation of construction waste has increased considerably, and social and environmental concerns on recycling the waste have accordingly increased. Construction Engineers have resorted to recycling these demolition wastes to maintain a friendly green environment. This study compared the crushing strengths of Recycled Coarse Aggregate concrete with Natural Aggregate concretes. The RCA was obtained from demolished drains and culverts while the Natural Coarse Granite Aggregate was from Akamkpa, Cross River State, Nigeria. Using RCA and NCA of maximum size 20mm, concrete cubes were prepared for compressive strength tests. Other properties of the aggregates were determined. Three mix proportions of 1:1½:3, 1:2:4 and 1:3:6 at water cement ratio of 0.6 were utilized and the specimens were tested at curing ages of 7, 14, 21 and 28 days. Results indicated that RCA produced concretes with strengths lower than those using NCA. RCA concretes also had slumps lower than those of NCA concretes.

Recycled coarse aggregate from parent concrete with supplementary cementitious materials under freeze-thaw environment: Recyclability, environment and economic evaluation

In order to improve the utilization efficiency of waste concrete in cold regions, this paper studied the properties of parent concrete (PC) and its recycled coarse aggregate (RCA) based on ordinary Portland cement (OPC), fly ash (FA), silica fume (SF), and ground granulated blast-furnace slag (GGBS) combination optimization in freeze-thaw (F-T) environment. The influence of supplementary cementitious materials (SCMs) on the frost resistance of PC was analyzed by compressive strength and relative dynamic elastic modulus. The recyclability potential of RCA was evaluated by the apparent density, water absorption, crushing index and attached mortar content. In addition, the phase composition and microstructure of RCA were analyzed by XRD and SEM, respectively, and the degradation mechanism of PC during F-T process was analyzed. Finally, the environment and economic benefits from cradle to cradle is analyzed. The results showed that PC containing SCMs had good frost resistance. The PC with a binder system of 75 % OPC +15 % FA + 10 % SF had the smallest F-T damage at 0.068, and its RCA quality could be upgraded from Class III to II after mechanical grinding. This combination of cementitious materials reduced harmful pores in concrete and inhibits the development of cracks during F-T cycles. In addition, it had a carbon emission 18 % lower than that of traditional concrete, with cost-effectiveness of 1.054. This research can provide a low-carbon and economic strategy for the preparation and recycling of waste concrete in cold regions.

Development of self-consolidating concretes based on recycled aggregates: Effect of parent concrete strength

Self-consolidating concrete is a recent type of highly fluid concrete. The high paste concentration ensures excellent rheological behavior. Concrete includes can be selected with durability aims in mind, considering the severe conditions to which the concrete will be exposed. However, the irregularity of recycled aggregates (initial properties unknown from the old concrete) leads to a need for more knowledge of durability indicators. This research is an experimental evaluation of the characteristics of SCC made with recycled aggregates and sand whose initial parent concrete strength was specified. Three ranges of parent concrete strength were chosen (20 to 30 MPa, 35 to 45 MPa, and above 50 MPa). The main objective is to study the effect of introducing recycled concrete aggregates on the mechanical behavior and durability of SCC (total immersion in water and capillary absorption). The test specimens were analyzed after 28 days of curing. The findings demonstrate increased durability and mechanical behavior were achieved using recycled aggregates with high strength in the parent concrete. However, the mechanical strength and durability of SCCs are reduced when recycled concrete aggregates with low parent concrete strength (20 to 30 MPa) are used in the mix.

Effect of the strength grade of parent concrete on the performance of recycled aggregate treated by cement-fly ash slurry under prolonged soaking duration

The detrimental effects of quarrying natural resources have contributed to large-scale environmental pollution. Utilizing recycled concrete aggregate (RCA) obtained from construction and demolition waste as a replacement for natural aggregates has become an effective solution for environmental sustainability. However, the inevitably diminished characteristics of the adhered mortar and the existence of an additional interfacial transition zone (ITZ) have negatively suppressed the widespread use of these materials in production lines. The present study aims to comprehensively investigate the effects of a cement-fly ash slurry with a mass ratio of 50% of the total weight of RCA on the microstructure and engineering properties of RCA derived from three different parent concretes with strength grades of 20, 30, and 60 MPa. The RCAs were soaked for 24 h, 48 h, and 72 h to evaluate the long-term effects of the pozzolanic slurry. A sequence of laboratory tests was conducted, including mercury intrusion porosimetry (MIP) test, thermal gravimetric–differential thermal analysis (TG-DTA), X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses, and measurements of water absorption and crushing value. The MIP results demonstrated a significant increase in medium capillary and gel pores, and a reduction in large capillary pores in the RCA with an increase in soaking time. TG-DTA results indicated an increase in Ca(OH)2 content during the first 24 h and 48 h of treatment. However, after 72 h of treatment, the Ca(OH)2 content of the treated RCA decreased in all cases. XRD analysis revealed the appearance of a few additional ettringite peaks among the treated RCAs in all cases. SEM images demonstrated the presence of additional hydration products after treatment, which filled the pores and improved the surface texture. Furthermore, after treatment, the crushing value of all treated RCA decreased by 6.0–27.1%, whereas the water absorption decreased by 12.0–30.9% compared to the untreated RCA. Based on several experimental results, the formation of hydration products and the improvement mechanism of RCA from three different parent concretes by slurry treatment with different soaking durations were discussed. In conclusion, a prolonged impregnation time of 72 h yielded a dominant performance, effectively intensifying their inferior properties, regardless of the initial strength grade of parent concrete.

Effect of reactive coarse recycled aggregate with different expansion history on alkali silica reactivity and mechanical properties of new concrete

AbstractRecycled concrete aggregates (RCA) provide an environmentally friendly solution by reducing the depletion of natural resources. This study investigates the impact of the expansion history of RCA on its reactivity with alkalis in newly produced concrete. Parent concrete samples were collected from the RILEM AAR‐4 curing cabinet (60°C and >90% RH) to produce coarse RCA with four different expansion levels (0.00%, 0.05%, 0.12%, and 0.20%). The results show that the expansion levels of the new concrete mixtures decreased as the ASR expansion history of parent concrete increased. The 20‐week compressive strength and elastic modulus of the reactive aggregate concrete containing no mineral admixture and exposed to AAR‐4 test conditions were lower than those of their water‐cured counterparts. However, no decrease was observed in the elastic modulus of concrete mixtures that showed an expansion level lower than 0.03% (the maximum expansion for nonreactive aggregate as recommended by the RILEM AAR‐4 standard).

Assessing the potential of recycled fine aggregate from freeze-thawed parent concrete for structural concrete

Recycled fine aggregate (RFA) generated from waste concrete, especially in harsh environment, can be considered as an alternative to natural sand. The yield rate, gradation and properties of RFA from natural aggregate concrete with the target strength of C40 as parent concrete (PC) every 200 freeze–thaw (FT) cycles are investigated. To more accurately evaluate the application potential of RFA, the mechanical properties and durability of recycled fine aggregate concrete (RFAC) is further studied. The results showed that as the FT cycles of PC increased, the yield rate of RFA decreases and the grading curve of RFA meets the requirements of Class II aggregate. The limit FT cycles of PC in Class II and III RFA are 148 and 450, respectively. For the compressive strength of RFAC that meets the design requirements, the FT cycles of PC are no more than 530. Based on 50 years of RFAC in Class D and Class E environments, the limit FT cycles of PC are 663 and 200, respectively. The limit FT cycles of PC are 221 based on 50 years of RFAC service in cold regions. Through the FT cycles of PC, the Class of RFA and the mechanical and durability of RFAC can be directly predicted. This provides a theoretical and data support for improving the utilization rate of waste concrete in FT environment.

Internal corrosion characteristics of endogenous sulfate introduced by recycled aggregates in recycled aggregates concrete: Insights into the macro-mechanical and meso-mechanical properties

The old cement paste adhering to recycled aggregates (RA) carries some sulfate corrosion media because of the external sulfate attack occurred in parent concrete. These sulfate corrosion media would be introduced into recycled aggregates concrete (RAC) that is prepared using RA. For studying the effects of the internal sulfate corrosion media on RAC, different Na2SO4 solutions (0%, 1%, 5% and 8%) were used to soak RA for 120 days. The effects of the soaked RA on macro-mechanical properties and meso-mechanical properties of RAC were explored. It was found that the endogenous sulfate ions diffused from RA into the new cement paste to react with cement hydration products (such as Ca(OH)2) to generate corrosion products, ettringite and gypsum. The moderate corrosion products could fill pores and cracks at interfacial transition zone (ITZ) to enhance the early compressive strength and flexural strength of RAC. However, as the erosion level of RA and curing time of concrete increased, much corrosion products caused the obvious deterioration in strength of RAC and microhardness of ITZ, as well as the increase of thickness of ITZ. For easing the comprehensive evaluation of microhardness and thickness of ITZ, a new parameter (Δζ) was proposed. The smaller the Δζ, the higher the quality of ITZ.

Performance assessment of recycled waste materials for use in unbound pavement Layers: A case study from Turkey

Construction and demolition wastes (CDW) have become uncontrollable due to population expansion, fast urbanization, ongoing technological advancements, and changing building materials throughout time, as well as the storage of these wastes and the environmental degradation they create. In addition, recently, the depletion of natural resources and rising expense of waste disposal have increased the importance of recycling and reuse of waste. Recycling and reuse of CDW can reduce the rate of depletion of natural aggregate resources, landfill area and carbon footprint. However, the lack of knowledge about parent concrete's qualities, such as compressive strength, is one of the most significant barriers to using these waste materials. In addition, further research is needed on the use of high-strength building waste that was devastated by earthquakes and floods as aggregates. The study aimed to investigate the quality of waste concrete and assess the usability of mixtures prepared using recycled aggregates obtained from waste concrete with a predetermined strength of 35–40 MPa in combination with crushed limestone aggregate in granular road base and sub-base layers. The engineering performance of RCA will be evaluated, along with a comparison to natural aggregate, and its compliance with the limit values given in the Turkey Highways Technical Specifications (THTS) will be examined. 5 different mixes were created by replacing RCA with LA at 0%, 25%, 50%, 50%, 75%, and 100%. The strength, durability and hydraulic properties of the prepared mixtures were investigated. It was concluded that the use of RCA had no negative effects on the mixture's properties, with the exception of permeability, and even outperformed natural aggregate in terms of soaked and unsoaked CBR, CBR strength after a freeze–thaw cycle, and modulus of elasticity. Additionally, it was established that RCA satisfies the standards set in the THTS for the materials to be used in granular road base and sub-base layers.

Disparity in research findings on parent concrete strength effects on recycled aggregate quality as a challenge in aggregate recycling

Although the idea behind recycled concrete aggregates (RCA) is not particularly novel, its potential for reuse must be verified by carefully evaluating the properties of RCA produced from various sources. Previous research has demonstrated that the mechanical qualities of the parent concrete (PC), which may originate from a number of sources, can have a substantial impact on the characteristics of RCAs. But the conclusions of these literary works were inconsistent. Researchers and construction stakeholders in regions where there have been few or no studies on the topic such as Ethiopia may find it challenging to choose which of these concepts to use in their practical applications as a result of these comparable contradictions in the literature. Therefore, this research conducted new physical and mechanical property tests on recycled aggregates derived from various strengths of parent concrete laboratory cubes and concrete remains found from demolished buildings in Ethiopia in an effort to explain the considerable range of variation in results in the literature and to investigate the aspects of recycled aggregates that have not yet been explored. Parent concretes with 28-day cube compressive strengths of 25, 37, 45, and 67 MPa were produced in the laboratory, and demolished concrete from real-life structures aged 5 and 23 years with cube compressive strengths of 25 and 37 MPa, respectively, was utilized. The study's findings showed that the qualities of recycled aggregates were favorably associated with an increase in the strength of parent concrete. Recycled concrete aggregates' resistance to abrasion, impact, and crushing resistance, specific gravity, and water absorption all improved with an increase in the strength of parent concrete, although mortar content climbed. As a result, it is anticipated that this research will assist researchers and construction stakeholders in better understanding how recycled aggregate properties are impacted by variations in parent concrete strength.

The empirical study of gradation of crushed sand concrete properties using six sigma DMAIC methodology

Fine aggregate plays a key role as a filler material in concrete’s fresh and hardened properties. Generally, in Portland cement concrete aggregate occupies 70-85% by weight and 60-70% by mass. In Pakistan, Lawrence pur the sand as the only sand that meets the ASTM standards. But due to heavy transportation costs, it is not cost-effective to use in most parts of the country. The tests were carried out in accordance with ASTM standards, and the concrete mix designed with three water-cement ratios of 0.40,0.45, and 0.50 called for a mixing procedure. At 28 days of curing, an M30 concrete mix comprising five control mixes including parent concrete was evaluated and compared to parent concrete. This experimental work aimed to enhance the fresh and hardened properties of concrete such as the workability of concrete mix, compressive, and flexural strengths by using locally available Jhelum River sand gradation crushed sand (passing sieve #4, the waste product of Sargodha coarse aggregate), s a fine aggregate material. Using six sigma DMAIC Methodology 9 defects in the Jhelum River sand and control mix have been identified and corrective measures were taken to improve the quality of the concrete mix. The results derived from this experimental work show that adding 50% crushed sand in Jhelum River sand increases compressive strength by 27% and flexural strength by 20% which makes it according to ASTM Standards. The use of crushed sand as an alternative to natural river sand efficient and safe material. This control mix also reduces the construction cost by 10% in comparison to lawerancepur sand.

Humidity field and moisture transfer of concrete with different pre-saturated recycled sand

The water absorption/desorption process of two kinds of recycled sand with parent concrete strength grade C20 and C50 respectively was tested. The influence of the pre-saturated recycled sand on the internal relative humidity development of concrete with strength grades of C30, C50, and C80 was tested. A critical water loss Wc of each concrete with pre-saturated recycled sand was calculated. The theoretical calculation model of concrete humidity considering the effect of pre-saturated recycled sand was established. The results show that the desorption process of recycled sand is closely related to its parent concrete strength grade (pore structure). The recycled sand with a weaker parent concrete induces a greater water release during the decline of ambient humidity. The use of recycled sand can prolong the humidity saturation period and improve the final humidity level of concrete. The effects are related to the pore size of the recycled sand and the new casting concrete. The recycled sand with a weaker parent concrete has stronger effect on the humidity of concrete. For the same recycled sand, it has a better effect on the humidity of high-strength concrete. The use of recycled sand can significantly increase the critical water loss of concrete. The introduction of pre-saturated recycled sand will increase the diffusion coefficient of concrete.

Non-destructive screening methodology based on handheld XRF for the classification of concrete: cement type-driven separation

The utilization of locally available concrete waste for producing recycled concrete aggregates is recognized as one of the most sustainable ways of satisfying the growing demand for concrete production. However, the quality of concrete waste depends on its origin and it may significantly differ from one concrete structure to another. Knowing the chemical composition of the parent concrete is crucial for determining or verifying the origin of the raw materials. For this reason, pre-demolition concrete waste streams need to be characterized and classified. Therefore, a new non-destructive method for determining the cement and aggregate type in hardened concrete using handheld X-ray fluorescence (hXRF) analyser is presented in this paper. The method was tested on different raw powders and on concretes containing different types of cements including CEM I 42.5 N (Portland cement), CEM II/B-V 42.5 N (Portland-fly ash cement), CEM III/B 42.5 N (GGBFS cement). Combined desktop XRF and Energy-dispersive X-ray Spectroscopy (EDS) measurements were used for the purpose of validation. The results revealed that the curing of concrete affects the results: a dried concrete surface condition was optimal for measurements since it limits the impact of the concrete surface moisture and efflorescence on characteristic element oxides, such as CaO. The effective measurement duration was 30 s. A CEM III/B 42.5 N (GGBFS)-based concrete surface was distinguished from other concretes using Al2O3, MgO and Fe2O3 as characteristic oxides. The inner layers of concrete were rich in SiO2, the oxide characteristic for the aggregate composition tested in this study. This shows that hXRF is suitable for use in concrete, provided that the concrete surface is dried and the characteristic elements are defined to ensure a distinction between different cement and aggregate types. Direct adoption of such characterization, however, requires field testing across a wide range of concrete compositions and in situ conditions.

Recycling of Macro-Synthetic Fiber-Reinforced Concrete and Properties of New Concretes with Recycled Aggregate and Recovered Fibers

The study aims to investigate the feasibility of using recycled aggregate (RA) and recovered fibers (RFs) obtained from recycling polypropylene fiber-reinforced concrete (PPFRC) in new concrete production. The mechanical properties were compared between a parent PPFRC, polypropylene fiber-reinforced recycled aggregate concrete (PPRAC), and recovered polypropylene fiber concrete (Re-PPRFC). All concretes were designed to have the same compressive strength and slump. The parent concrete was produced with 3 and 9 kg/m3 of polypropylene fibers. After recycling, the RA and RF were collected, and new concretes with RA and RF, PPRAC and Re-PPRFC, respectively, were produced with the same fiber content as the parent concretes. Both the compressive and flexural tensile strength (pre- and post-cracking) were characterized and the stress–strain relations derived accordingly. The results obtained for the different concretes were compared, proving that the RA and RF obtained by PPFRC recycling can benefit the design-oriented properties (workability and mechanical performance) of new concretes.

Pre-demolition concrete waste stream identification: Classification framework

Demand for high quality recycled concrete aggregates (RCA) to offset the use of primary materials is significantly rising due to circular economy goals and high-value reuse of concrete. The quality of RCA significantly affects their availability for new concrete production due to the variability of parent concrete streams. The optimization of recycling procedures is under development to improve the quality of RCA, however, the costs and energy efficiency of such processes are of practical concern. With this in mind, this paper presents a new framework for reducing the variability of RCA quality by identifying concrete members before their demolition. The goal of identifying demolished concrete members from a structure is to provide groups of concrete members with similar mechanical and chemical properties through a systematic classification of the structural members. The quality assessment of concrete structures and their mechanical and chemical (composition, contamination) properties prior to demolition is generally recognized as challenging due to the absence of guidelines and the lack of easy-to-use in situ characterization techniques. This paper proposes experimental approaches that can non-destructively determine the properties of concrete structures, with a major emphasis on the measurement of the chemical composition of concrete before demolition. Characteristic quality indicators to classify concrete members are first proposed and can be instrumental in setting up future studies. A new method is proposed for in situ chemical composition testing of existing concrete structures; assuming that no records about the parent concrete are available. Next, the challenging parameters for in situ, non-destructive measurements are outlined. The practical application of the proposed method and its uptake in industry can potentially unlock a huge potential for optimized material recovery and contribute greatly to a fully circular construction industry.

Model validation of hollow core slab with overlay using Response-2000

Hollow Core Slab (HCS) is a precast slab with prestressing system and voids along the span of the slab which serves to reduce its self-weight. For HCS to be able to increase its flexural and shear capacity, an overlay is used on the interface along the length of HCS. Overlay serves to create a continuous surface level and increase the load capacity of the slab. The advantage of using an overlay system, namely strength and serviceability, is available after the element is fully composite against the parent concrete. An analytical study will be carried out using Response-2000 version 1.0.5 program related to flexural behavior using two research configurations. The first configuration is HCS with overlay, with an overlay thickness of 50 mm. The second configuration is HCS without using an overlay. Therefore, the second specimen is referred to as a control specimen. The evaluation of the analysis results was reviewed based on the load-displacement curve, cracking load, displacement at cracking load, peak load, displacement at peak load, and cracking mode. The final result of this study is a form of validation to ensure the flexural behavior of the specimen in analytical modeling is consistent with the specimen in experimental studies.

Effect of parent concrete strength on recycled concrete performance

The reuse of concrete waste as a secondary aggregate could be an efficient solution for sustainable development and long-term environmental protection. However, the variable quality of waste concrete, especially with various compressive strengths, can have a negative effect on the final compressive strength of recycled concrete. In this approach, the major goal of this research is to study the effect of parent concrete qualities on the performance of recycled concrete. To accomplish this task, three grades of different compressive strengths (10 to 15) MPa, (20 to 25) MPa, and (30 to 40) MPa have been analyzed in an experimental test program, in which an unknown compressive strength is introduced as well. The experimental mix use 40% of secondary aggregates (both course and fine) and 60% of natural aggregates. This led to the decreasing of the compressive strength of the test concrete between 14% and 23.7% compared to the normal concrete. This loss was improved by adding an amount of cement equivalent to 4% of the weight of the recycled aggregate used. The achieved results prove that the strength properties of the parent concrete have a limited effect on the compressive strength of the recycled concrete. Additionally, low compressive strength parent concrete, when crushed, generates a high amount of fine aggregate and large percentage of recycled coarse aggregates with less attached mortar, and presents the same compressive strength as an excellent parent concrete.

Comprehensive Evaluation of the Sustainability of Waste Concrete towards Structural Concrete Application in Freeze-Thaw Environment

To promote the in-situ and structural application of waste concrete in cold regions, the sustainable application potential of waste concrete in a freeze-thaw (F-T) environment was comprehensively evaluated from three aspects of performance, environmental load, and economic benefit. The recycled aggregate concrete (RAC) was produced by recycled coarse aggregate (RCA), which was obtained from the crushing of natural aggregate concrete (NAC) after every F-T 150 cycles until F-T failure. The effects of F-T damage of parent concrete on the physical properties of RCA and mechanical and frost resistance of RAC under 35% flexural stress were studied. Besides, the sustainability of NAC and RAC was compared and analyzed by emergy theory. The results suggested that the physical properties of RCA deteriorated gradually with the accumulation of F-T damage to parent concrete. The RCA obtained from parent concrete that suffered F-T damage could be used as coarse aggregate for structural concrete when F-T damage is smaller than 0.367. The F-T damage of parent concrete had an adverse effect on the mechanical properties and frost resistance of RAC. The frost resistance of RAC obtained from parent concrete with larger F-T damage was worse. The RAC prepared from parent concrete without F-T failure can serve 50 years in cold regions, while that with F-T failure can only serve 30 years. The F-T damage microelements were dispersed in the adhesive mortar of RCA and transferred to RAC, resulting in the reduction of the mechanical properties and frost resistance of RAC. Emergy analysis showed that the reuse of waste concrete after F-T failure required higher economic input, higher environment load, lower output efficiency, and sustainability. The performance, environmental load and economic benefit of RAC prepared by using waste concrete after F-T failure were inferior to that of waste concrete without F-T failure. Waste concrete after F-T failure is not recommended to be used as coarse aggregate for structural concrete.

Multi-recycling of polypropylene fibre reinforced concrete: Influence of recycled aggregate properties on new concrete

• Recycling of polypropylene fibre reinforced concrete was investigated. • Three generations of recycled aggregate concrete were produced, tested and recycled. • The content of fibres recovered by recycling and embedded in aggregate was quantified. • Results show the benefits of using recovered and embedded fibres in new concrete. Herein, an investigation of multi-generational recyclability of polypropylene fibre reinforced concrete (PPFRC) was performed. The parent concretes were produced with 0 and 6 kg/m 3 of polypropylene fibres. After recycling, the obtained coarse recycled concrete aggregates (RCAs) were used in new concrete with 0 and 6 kg/m 3 of polypropylene fibres, repeating for three generations always with the same quantities of fibres. Properties of RCA, the mechanical properties of recycled aggregate concrete, the recovery rate of polypropylene fibres and the content of fibres embedded within the RCA were measured. The results of the study show that RCA obtained by PPFRC recycling offers significant benefits to new concrete production. This is achieved through the recovered fibres reintroduced into the new concrete, as well as through the fibres embedded in the recycled aggregates, leading to increased residual tensile strength.