Recycled Coarse Aggregate Research Articles

The Mechanical Properties and Microstructure of Tailing Recycled Aggregate Concrete.

The aim of this study was to develop sustainable concrete by recycling concrete aggregates from steel waste and construction waste (iron ore tailings (IOTs) and recycled coarse aggregates (RCAs)) to replace silica sand and natural coarse aggregates. In experimental testing, the compressive strength, peak strain, elastic modulus, energy dissipated under compression, and compressive stress-strain curve were analyzed. Microscopically, scanning electron microscopy and energy-dispersive spectrometry were employed to investigate the microstructural characteristics of the interfacial transition zone (ITZ), and the results were compared with the ITZs of natural aggregate concrete and recycled aggregate concrete (RAC). In addition, the pore structure of concrete was determined by nuclear magnetic resonance. The results revealed that an appropriate IOT content can improve the ITZ and compactness of RAC, as well as optimize the mechanical and deformation properties of RAC. However, due to the presence of a smaller number of microcracks on the surface of IOT particles, excessive IOTs could reduce the integrity of the matrix structure and weaken the strength of concrete. According to the research, replacing silica sand with 30% IOTs led to a reduction in the porosity and microcracking which resulted in a much denser microstructure.

Multiscale analysis of recycled coarse aggregate concrete under the synergistic action of KH560 and PVA fibers

Considering the long-term working performance of the waste concrete building elements, this study used 3-Glycidoxypropyltrimethoxysilane (KH560) and Polyvinyl alcohol (PVA) fibers to modify the recycled concrete (RC) with the aim of improving the mechanical and long-term properties of RC. In this study, the mechanical properties of recycled coarse aggregate (RCA) concrete before and after modification were systematically investigated. The macroscale results showed that KH560 compensates for the reduced compressive strength of RCA concrete due to PVA fibers. The synergistic effect of PVA and KH560 substantially increased the flexural and shear strength of RCA concrete, with a maximum increase of 22.6% in flexural strength when the RCA substitution rate was 80%. The results of three microscopic tests (SEM / FTIR/XRD) revealed that KH560 increased the content of cement gel, which then optimized the internal pore structure of RCA concrete. The chemical reaction between KH560 and cement produced SiKH560-OKH560-SiCSH. At the nano-scale, computational analysis methods in atomistic modeling techniques were employed to model PVA fibers, cement and KH560, and the dynamic and static simulation results of the model were combined to discover a consistent SiKH560-OKH560-SiCSH chemical bond between KH560/CSH and both old and new CSH, Additionally, the findings revealed the coexistence hydrogen and ionic bonds. KH560 improves the interaction between PVA fibers and cement by forming numerous hydrogen bonds, and resolves the weak interface between the new and old cement through stable chemical bonds.

Evaluation of urea hydrolysis for MICP technique applied in recycled aggregate: Concentration of urea and bacterial spores

Aggregate is an indispensable raw material for concrete. Microbial induced carbonate precipitation (MICP) is an environmentally friendly aggregate modification method, which could modify recycled aggregate (RA) to replace natural aggregate for the preparation of concrete in the future. The urea hydrolysis is the key to modified RA by MICP. Biological calcium carbonate is generated during mineralization reaction in modification solution to fill the weak part of aggregate, which will enhance the performance of RA. The bacterial solution and urea solution, as the sources of CO32- in biological CaCO3, jointly affect the modification effect of RA. In this paper, the different ratios of bacterial solution concentration (×108 cells/mL) to urea concentration (mg urea/mL) are used for exploring the improvement effect and the cooperative enhancement mechanism of recycled coarse aggregate (4.75 mm-9.5 mm). The results show that the bacterial mineralization efficiency is most improved when bacterial solution concentration/urea concentration (B/U) is in the optimal interval of 5–6 because 1 mg urea provides a moderate number of bacteria during the modification period. In the interval, the precipitation amount of biological CaCO3 and effective CaCO3 that could modify RA is larger, and the pores of the attached mortar of RA are better filled and blocked. Therefore, the macro performance of RA is greatly improved in the internal. When the B/U is outside the optimal interval, the modification effect is poor because the amount of biological CaCO3 and effective CaCO3 is too less to fill pores or too more to enter pores.

Freeze-thaw, chloride penetration and carbonation resistance of natural and recycled aggregate concrete containing rice husk ash as replacement of cement

This study investigated the effect of using rice husk ash (RHA) as replacement of cement (i.e., replacement ratios of 0%, 10%, 20% and 30%) on the durability of natural aggregate concrete (NAC) and recycled aggregate concrete (RAC). We examined the freeze-thaw resistance, chloride penetration resistance, and carbonation resistance of NAC and RAC (with 50% recycled coarse aggregate replacement ratio). It was found that during the freeze-thaw process, both NAC and RAC showed an overall weight increase, which was further enhanced by the addition of RHA. Meanwhile, the compressive strength of NAC was reduced more obviously than that of RAC after 100 freeze-thaw cycles. A replacement of 10% cement with RHA resulted in larger reduction in strength loss for NAC (by 36%) compared to RAC (by 4%). As the RHA replacement ratio increased, the chloride penetration resistance of RACs was improved more rapidly than that of NACs, and the difference in the total charge passed between NACs and RACs also decreased considerably. In addition, an increase in the RHA replacement ratio led to an increase in the carbonation coefficient of NAC. Conversely, in the case of RAC, the carbonation coefficient displayed an initial decrease at a 10% RHA replacement ratio, followed by a subsequent upward trend, indicating a positive impact of RHA on preventing carbonation for RAC.

WATER ABSORPTION AND SPECIFIC GRAVITY OF RECYCLED CONCRETE AGGREGATE FROM DEMOLISHING WASTE OF NAWABSHAH CITY

Concrete has been proved to be a leading construction material all over the world. Because of the demand for new structures, new construction is increasing day by day. This rapid demand of structures has increased the requirement of concrete. New structures are being constructed and old structures are being demolished. This results in the generation of construction waste on a large scale. The throwing away of this huge mass of construction waste has happened to a major social and environmental issue all around the world, mostly in developing countries. Therefore, a proper utilization of demolished waste is important in order to deal with this waste and have an alternate material for utilization in concrete. Therefore, experimental research has been conducted to utilize this waste as recycled coarse aggregates to partially replace natural coarse aggregates. Old concrete was collected from 5 locations in Nawabshah. After collecting all the material we used 5 samples from each source, so we used 25 samples of recycled aggregates. 25 samples of recycled coarse aggregate are tested for water absorption and specific gravity to understand its properties and compare to the natural coarse aggregates. The water absorption of recycled coarse aggregate is greater than the water absorption of natural coarse aggregates. Among these five sites of Nawabshah city, New Naka area’s recycled coarse aggregates have greater water absorption which is 5.8%. The specific gravity of recycled coarse aggregate is lower than the specific gravity of natural coarse aggregates. Among these five sites of Nawabshah city, New Naka & Mehran Hotel area’s have lesser specific gravity which is 2.237. So, these aggregates can be used in new constructions, but it is proposed to be initially utilized in low load areas.

Determination of the effectiveness of porcelain fine waste to enhance the performance of geopolymer concrete with recycled waste using central composite design and Taguchi method

This study aims to evaluate the effectiveness of incorporating bone china fine aggregate (BCA) in geopolymer concrete (GPC) with recycled coarse aggregate (RCA). A total of 25 experiments were conducted to optimize the mix design of GPC with coarse RCA and fine BCA. The suitability of a response surface model using the central composite design approach and the Taguchi method with an L25 orthogonal array was assessed. Both methods provide valuable insights and recommendations for achieving ideal mix proportions to enhance strength. The resulting model, with a higher coefficient of determination, successfully predicted the mechanical properties of GPC in fresh and hardened states. The findings suggest that GPC with up to 50% RCA and up to 100% BCA demonstrates optimal performance in terms of hardened mechanical properties. Furthermore, a strong correlation was observed between the predicted and actual values, indicating the reliability of the model.

Research on Capillary Water Absorption Characteristics of Modified Recycled Concrete under Different Freeze–Thaw Environments

Recycled coarse aggregate is processed through the second crushing, which causes some internal damage, resulting in its physical indicators being far worse than natural coarse aggregate; its durability is relatively poor, and in the northern region, the soil contains a large number of acidic salt ions from the erosion of concrete, resulting in a decline in its durability. In this test, concrete was made from the single and composite immersion of recycled coarse aggregate using 5% water glass and 8% silane solution and subjected to a rapid freeze–thaw test in water, 3.5% NaCl solution, and 5% Na2SO4 solution, followed by a capillary water absorption test. The study was conducted to test the durability of recycled concrete, establish the initial capillary water absorption prediction model under freeze–thaw in different media, and analyze the internal structure of the RAC group after freeze–thaw using SEM. The test results showed that the composite-modified water absorption decreased the most, which can effectively improve the durability of recycled concrete, and the chloride salt caused the greatest erosion of recycled concrete and had the least clear water. The predictive model has high accuracy and can be used as a reference for capillary water absorption experiments on recycled concrete.

Optimization of eco-friendly concrete with recycled coarse aggregates and rubber particles as sustainable industrial byproducts for construction practices

In this technology era, sustainable construction practices have become quite imperative. The exploration of alternative materials to reduce the environmental footprint is of paramount importance. This research paper delves into an exhaustive investigation concerning the utilization of recycled coarse aggregates (RCA) and rubber particles (RP) in concrete. It contributes to the growing body of knowledge aimed at fostering sustainable development in the construction industry by reducing waste, promoting recycling, and mitigating the environmental footprint of building materials. The objective of the study is to evaluate the potential benefits and limitations associated with incorporating these materials, thereby providing a sustainable alternative to conventional concrete. In this research, construction and demolition waste were recycled and used as RCA as a fractional switch of natural coarse aggregate (NCA) from 0% to 100%, with an increment of 20% replacement of NCA in concrete. The RP received from discarded tires generated as automobile industry waste were used as a volumetric fractional substitution of sand in concrete from 0% to 20%, with a 5% increment. No pre-treatment for RCA and RP was carried out before their utilization in concrete. A total of 26 mixes, including control concrete without NCA and RP, with a design strength of 40 MPa, were prepared and tested. Concrete mixes were examined for workability, density, mechanical, and durability properties. It was found that the concrete with 60% RCA and 10% RP showed satisfactory results in evaluation with the strength parameters of control concrete, as the compressive strength obtained for this concrete mix is 40.18 MPa, similar to the control mix. The optimization for RCA and RP was conducted using Response Surface Methodology (RSM). The major concern observed was a rise in water absorption with an increase in the percentage replacement of NCA and natural sand by RCA and RP. Findings from the investigation illustrate a promising prospect for the use of RCA and RP in concrete applications, displaying competent mechanical properties and enhanced durability under certain conditions, offering a viable option for environmentally friendly construction practices. However, the research also sheds light on some constraints and challenges, such as the variability in the quality of RCA and the necessity for meticulous quality control to ensure the reliability and consistency of the end product. It is discerned that further refinement in processing techniques and quality assurance measures is pivotal for mainstream adoption of RCA and RP in concrete construction.

Influence of the Original Concrete Strength and Initial Moisture Condition on the Properties Improvement of Recycled Coarse Aggregate via Accelerated Carbonation Reactions.

The physical and mechanical properties of recycled coarse aggregate (RCA) are worse than those of natural coarse aggregate (NCA), and the overall performance of recycled concrete prepared from RCA is worse than that of natural aggregate concrete. Treatment of RCA by CO2-accelerated carbonation effectively improves the macroscopic properties of RCA. The degree of influence of raw material factors, i.e., the original concrete strength (OCS) and initial moisture content (IMC) of RCA, on the carbonation of RCAs is very complex. Herein, an accelerated carbonation experiment for RCA with different material factors as variables was carried out to explore the influence of the abovementioned factors on the physical properties of carbonated recycled coarse aggregate (CRCA). By analyzing the microstructure of the RCA with the best modification effect before and after carbonation, the carbonation modification mechanism of the RCA was revealed. The physical performance indexes, including the apparent density, water absorption and carbonation rate, of the dried RCA with an OCS of C40 and C50 were significantly improved. The research results can provide basic data and theoretical support for promoting the popularization and application of RCA and recycled concrete in practical engineering.

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.

Eurocode 2 design of recycled aggregate concrete elements under compression: Evaluation through model uncertainties from tests on columns

AbstractRecycled aggregate concrete contributes to the recovery of construction and demolition waste, which is a growing societal concern. The technical feasibility of recycled aggregate concrete has been extensively tested and validated and the revisions of codes are already including clauses for recycled aggregate concrete specification and design. However, there are still doubts regarding its structural behavior. This article assesses the suitability of Eurocode 2 design clauses for concrete elements under compression when used for recycled aggregate concrete design. This is done through the analysis of model uncertainties that are estimated from a database collected from laboratory experiments on columns. Both the current version of Eurocode 2 (with and without taking confinement into account) and the upcoming (second generation) Eurocode 2 are considered. A database was collected with clearly defined criteria and includes coarse recycled aggregates produced from concrete waste, since columns made with other types of recycled aggregates are scarcely tested and do not represent the current status for structural recycled aggregate concrete design and production. The statistics of the model uncertainty for natural and recycled concrete columns were compared and it was observed that the mean model uncertainty tends to decrease slightly when recycled aggregates are used. However, a preliminary assessment of whether a partial factor for recycled aggregate concrete compression design should be used concluded that this partial factor is not necessary to ensure reliable design and the design of columns may be carried out using the same formulae used for conventional concrete. The format proposed may be used for other resistance models, provided that the partial factor is calibrated for each resistance model.

Compressive behavior of FRP-confined normal concrete or seawater sea-sand concrete containing FRP-bar recycled coarse aggregates

Fiber-reinforced polymer (FRP) waste has become an environmental concern owing to the non-biodegradable nature and widespread use of FRP composites. Cutting the disposed waste of FRP bars into small pieces (referred to as FRP-bar recycled aggregate or FRP-RA) and replacing natural coarse aggregate in concrete with these pieces is considered a promising solution for recycling FRP waste. Owing to its excellent corrosion resistance, FRP-RA can be used as a raw material for the production of seawater sea-sand concrete (SSC). This study investigated the axial compressive behavior of FRP-confined normal concrete (NC) or SSC containing FRP-RA. The FRP-RA was cut from glass FRP (GFRP) bars of different diameters. Axial compression tests were conducted on 96 specimens. The specimens were designed with the following parameters: the type of concrete (NC and SSC), natural coarse aggregate replacement ratio (0%, 33%, 66%, and 100% by volume), thickness (1-layer and 2-layer), and type (carbon FRP and GFRP) of the FRP jacket. The test results indicated that the presence of the FRP-RA reduced the compressive strength and elastic modulus of the concrete. FRP confinement significantly enhanced the compressive strength and ultimate axial strain of the concrete with FRP-RA. No discernible difference was observed between SSC and NC with respect to FRP-RA. Finally, a widely adopted stress-strain model for FRP-confined NC was first evaluated using the test results, and a modified stress-strain model was then proposed for more accurate predictions.

Effect of accelerated carbonation of fully recycled aggregates on fracture behaviour of concrete

Accelerated carbonation of recycled aggregates (RAs) enables fast carbon sequestration while improving engineering properties of prepared concrete. This paper presents a systematic experimental study on the fracture behaviour of concrete with carbonated recycled coarse and fine aggregates (RCA and RFA), in terms of fracture parameters and fracture process zone (FPZ). The relevant influencing mechanism was investigated by relating the macroscopic fracture responses to microstructural features. Results indicate that carbonated RFA mainly enhanced the initial fracture toughness of concrete, while carbonated RCA primarily increased its unstable fracture toughness. The enhancement of fracture energy was closely associated with carbonated RCA and RFA. The utilization of RFA resulted in the looseness and higher porosity of mortar matrix, which reduced the crack initiation resistance of concrete, mainly affecting its initial cracking stage. The incorporation of RCA reduced crack bifurcation and deflection of concrete, leading to a smaller width of FPZ, which made the initial descending segment of the tension softening curve (TSC) steeper and its tail segment shorter. The carbonation treatment of RAs improved the densification of the adhered old mortar and reduced the porosity of prepared concrete, rising the cracking resistance and inducing a wider FPZ. As a result, the initial descending segment of TSC became flatter and its tail segment tended to be longer.

Study on the mechanical properties and deterioration mechanism of recycled aggregate concrete for low-grade highway pavements

This study is motivated by the imperative for sustainable construction practices, specifically focusing on recycling construction waste into low-grade highway pavements to enhance environmental sustainability and resource efficiency. The investigation explores the workability, mechanical properties, and interfacial transition zones (ITZs) of recycled aggregate concrete (RAC), emphasizing the incorporation of recycled coarse aggregate (RCA) from construction waste. The primary objective is to delineate the optimal mixture proportions of RAC that effectively balance resource utilization with the technical requisites for low-grade highway pavements. Through rigorous orthogonal testing and analysis, a mixture was identified that achieves a high RCA replacement rate while preserving workability and mechanical properties. Additionally, nanoindentation tests were employed to probe the nanomechanical properties of the ITZs in RAC, aiming to elucidate the underlying degradation mechanisms. Results reveal that the porous and rough surface of RCA significantly impacts the properties of RAC, with a marked decrease in elastic modulus and an increase in ITZ width identified as primary contributors to mechanical deterioration. Consequently, this study not only provides crucial insights for the application of RAC in low-grade highway pavements but also sets the stage for future research aimed at enhancing the sustainability and performance of recycled construction materials.

Axial compressive behavior of steel reinforced GGBS-RFBP-FA ternary composite geopolymer recycled fireclay brick aggregates concrete columns

In this study, the axial compressive behaviors of steel reinforced ternary composite geopolymer recycled fireclay brick aggregates concrete columns were investigated. The ternary binder material used in this study was produced by the combination of industrial by-products (i.e., granulated blast furnace slag (GGBS), recycled fireclay brick powder (RFBP), and fly ash (FA)) and alkali-activated solution. A total of 60 square steel reinforced composite columns (with a 200-mm side length and 600-mm height) were tested under axial compression. The tested variables included the mix proportion of ternary composite geopolymer concrete with recycled aggregates containing recycled fireclay brick aggregates (GRA-RFBAC), the recycled coarse aggregate replacement ratios (0%, 30%, 50%, 70%, 100%), the longitudinal reinforcement ratios (1.13%, 2.01%), and the hoop stirrup reinforcement ratios (0.81%, 1.62%). Each group contained three identical samples. The ductility, failure mode, and axial load-bearing capacity of the columns were recorded and analyzed. The experimental results revealed that the damage progression and patterns of steel reinforced GRA-RFBAC columns were similar to those of ordinary Portland cement based recycled concrete columns. The ultimate bearing capacity of steel reinforced GRA-RFBAC columns decreased with an increasing replacement ratio of recycled coarse aggregate and increased with an increasing reinforcement ratio of longitudinal and hoop stirrups. Meanwhile, the stress-strain model of steel reinforced GRA-RFBAC columns under axial compression was proposed based on the experimental results. The present study suggests an efficient and environmental-friendly compression member.

Transforming waste into sustainable building materials: Properties and environmental impacts of Geopolymer concrete with recycled concrete aggregates

<p>Industrial by-products such as cementitious materials or aggregates have the potential to mitigate the adverse environmental effects associated with traditional cement manufacture. Geopolymer Concrete (GPC) is more eco-friendly than conventional concrete because it does not require cement. GPC with Recycled concrete aggregates (RCA), Ground granulated blast furnace slag (GGBS), and Fly Ash (FA) reduce raw material use and create sustainable infrastructure. GPC compounds increase workability, slump value above standard concrete, and reduce the amount of water usage. The study examines GPC mechanical properties, durability, and environmental properties with different RCA content. The M30 concrete mix design is established by trial and error utilizing a 0.45 water/binder ratio. GPC with 0%, 10%, 20%, 30%, 40%, and 50% recycled coarse aggregate replaced natural aggregate(NA) by mass. GPC with 50% GGBS provides an early strength of 96% of normal compressive strength on day seven. The compressive, split tension, and flexural strengths exhibit significant improvement with up to a 40% substitution of NA with RA. These results highlight GPC's potential as a sustainable alternative in the construction sector.</p>

Preparation of recycled sand from WRSG and compressive and flexural behavior of recycled aggregate concrete

The use of both recycled coarse aggregates (RCAs) and recycled sand (RS) derived from weathered residual soil of granite (WRSG) into concrete has the potential to greatly enhance the recycling of construction and demolition waste. However, the characterization of RS from WRSG and the compressive and flexural performance in fresh concrete containing RCAs and RS have not been thoroughly investigated. In this study, clay content, fineness modulus, chemical compositions, mineral compositions, and pore structure of RS from WRSG were tested. On this basis, the optimized preparation parameters of RS were suggested. The compressive behavior, flexural behavior, and cement hydration degree of recycled aggregate concrete (RAC) simultaneously containing RS and RCAs were investigated comprehensively. A stereological model was proposed to explain the results related to cement hydration. The results showed that: (a) the optimized preparation could substantially lower the clay content of RS; (b) RS was more porous than natural sand (NS), resulting in a higher water absorption during mixing; (c) the compressive strength of concrete containing RS developed faster than the concrete with NS; (d) at day 90, the compressive and flexural strength of the concrete containing RS were not less than those of the concrete with NS; and (e) RS was shown to have a greater influence on the hydration degree of cement paste than RCAs, due to RS significantly reducing the average value of inter-aggregate spacing in concrete, making the cement paste more susceptible to the internal curing effect induced by the water in aggregate pores.

Properties evaluation of concrete made with recycled coarse aggregate modified by graphene oxide

Previous application of graphene oxide (GO) in recycled aggregate concrete (RAC) is the direct replacement of cement or binder to achieve performance improvement. To develop a more efficient recycled aggregate (RA) modification technique and improve the properties of the corresponding RAC, this paper performed GO solution pre-spraying and pre-soaking treatment on RA. The modification efficiency of GO was evaluated by compressive strength, interfacial transition zone (ITZ) properties, and chloride ion permeability of RAC. The results show that the pre-spraying and pre-soaking methods improved the compressive strength of RAC at different ages by 20%–28.6 % and 8.3%–30.8 %, respectively. The GO-modified RAC also exhibited lower electric flux in the chloride ion permeability tests. However, no significant effect was observed on the elastic modulus of RAC. The modified RAC showed lower total porosity and harmful pore content. Meanwhile, the GO-modified ITZ between old and new mortar had denser hydration products. As a result, the elastic modulus and hardness of the ITZ increased by 27.8%–65.8 % and 9.8%–107 %, respectively, while the width of the ITZ decreased by 15.4%–30.8 %. The good filling and nucleation effect of GO were credited with the enhancement mechanism of RA by pre-spraying and pre-soaking methods.

Circular recycled aggregate concrete-filled stainless steel tube stub columns after exposure to fire: Experiments, simulations, and design

This paper presents experimental and numerical investigations into the post-fire cross-section compressive behaviour and resistances of circular recycled aggregate concrete-filled stainless steel tube (RACFSST) stub columns. Twelve stub column specimens, fabricated from concretes with three recycled coarse aggregate replacement ratios (0%, 35% and 70%), were tested after exposure to the ISO-834 standard fire for 0 min (i.e. at ambient temperature), 15 min, 30 min and 45 min. The experimental investigation included heating and cooling of specimens, cylinder tests and post-fire tensile coupon tests and stub column tests. The test results, including load–end shortening curves, failure loads and failure modes, were fully reported and the initial compressive stiffnesses and confinement effect were discussed. The numerical investigation was subsequently conducted, where thermal and mechanical finite element models were developed and validated against the test results and then used to perform parametric studies to generate further numerical data. Given the absence of design codes for stainless steel–recycled aggregate concrete composite structures after exposure to fire, the relevant design rules for circular natural aggregate concrete-filled carbon steel tube stub columns at ambient temperature, as set out in the European code, Australia/New Zealand standard and American specification, were assessed, using post-fire material properties, for their applicability to circular RACFSST stub columns after exposure to fire, based on the test and numerical data. The assessment results generally revealed that the European code and the Australian/New Zealand standard led to an acceptable level of design accuracy and consistency, while the American specification resulted in rather conservative and scattered post-fire cross-section compression resistance predictions.

Post-fire behaviour and resistances of square recycled aggregate concrete-filled stainless steel tube stub columns

Experimental and numerical studies on the cross-section compressive behaviour and residual resistances of square recycled aggregate concrete-filled stainless steel tube (RACFSST) stub columns after exposure to fire are reported in this paper. An experimental programme was firstly carried out on twelve stub column specimens with three recycled coarse aggregate replacement ratios (0%, 35% and 70%) after exposure to the ISO-834 standard fire for 0 min (i.e. at ambient temperature), 15 min, 30 min and 45 min. The test results, including load–end shortening curves, failure loads and failure modes, were presented, with the initial compressive stiffness and confinement effect analysed. The experimental programme was followed by a numerical modelling programme, where thermal and mechanical finite element models were developed and validated against the test results and afterwards used to conduct parametric studies to generate additional numerical data over a wide range of cross-section dimensions. Based on the test and numerical data, the relevant design rules for square natural aggregate concrete-filled carbon steel tube stub columns at ambient temperature, as specified in the European code, Australia/New Zealand standard and American specification, were evaluated, using post-fire material properties, for their applicability to square RACFSST stub columns after exposure to fire. The evaluation results generally revealed that the European code and Australian/New Zealand standard led to a good level of design accuracy, while the American specification resulted in slightly conservative post-fire cross-section compression resistance predictions.