Structural Aggregate Concrete Research Articles

Frost resistance of recycled aggregate concrete subjected to coupling sustained compressive load and freeze-thaw cycles

Recycled aggregate concrete (RAC) structures situated in cold regions are required to withstand the combined effects of mechanical load and environmental factors throughout their operational lifespan. The influence of sustained load introduces complexity to the degradation mechanism of RAC. This research examines the frost resistance of RAC across varying levels of recycled coarse aggregate (RCA) replacement (0, 50 %, 100 %) and sustained compressive loading (0, 0.3 fc, 0.5 fc). The frost resistance was quantified by the indices of macroscopic morphology change, mass loss, relative dynamic modulus of elasticity, and mechanical properties. Results show that 0.3 fc can effectively improve the frost resistance of RAC, a trend that becomes more pronounced with increasing replacement rates of RCA. Excessive stress (0.5 fc) was found to be unfavorable to the frost resistance of RAC and increase the compressive strength variability of RAC after freeze-thaw cycles. Analysis of meso/micro-structure characteristics using optical and scanning electron microscopes, along with the mercury intrusion porosimetry, reveals that optimal compressive loading (0.3 fc) enhances mortar integrity, reduces porosity, and improves compactness, thereby bolstering RAC’s frost resistance.

Comparative eco-efficiency assessment of 3D-printed recycled aggregate concrete structure for mid-rise residential buildings

3D-printed concrete (3DPC) is an emerging construction technique that extrudes concrete to automatically fabricate structural elements. While prior studies indicate the environmental benefits of 3DPC, limitations include a lack of project-level inventory data and assessment of whole buildings. This study provided a comprehensive framework through a life cycle assessment (LCA) comparison by using 3DPC structures (3DPCS) with project-specific data and conventional masonry structures (CMS) for six-storey residential buildings sharing similar architecture designs. Findings showed that 3DPC reduced cost by 16.9 % due to a 64.1 % decrease in workforce requirements. Environmental impacts were lower by up to 72.5 % for 3DPCS compared to CMS, primarily due to the low-cement-content concrete mixture incorporating recycled aggregates. The research further identified opportunities to enhance the economic and sustainability advantages of 3DPCS versus traditional approaches by increasing the proportion of 3D-printed components and the utilization of 3D printers across different buildings. This empirical research validated 3DPCS sustainability benefits and provided strategies for further promotion of digital concrete techniques in urban and rural construction.

Optimal Replacement Ratio of Recycled Concrete Aggregate Balancing Mechanical Performance with Sustainability: A Review

Significant construction and demolition waste (CDW) is produced by many useless concrete buildings, bridges, airports, highways, railways, industrial mining, etc. The rising need for new construction has increased the use of natural materials, impacting the ecosystem and incurring high costs from mining natural aggregates (NA) and processing CDW. The concept and implementation of recycled aggregate concrete (RAC) offer a sustainable solution for the concrete industry. Crushed concrete, made from recycled concrete, can be used instead of natural aggregates in structural concrete. This sustainable byproduct, recycled concrete aggregate (RCA), has the potential to replace natural aggregate. This paper examines the benefits of RAC from economic, social, environmental, and technological perspectives and discusses the replacement ratio (RR)—the weight percentage of natural aggregate replaced by recycled aggregate—which is crucial to RAC performance. A collection of used data on mechanical properties and economic performance, national specifications, standards, and guidelines is reviewed to determine the optimal replacement ratio for RCA, which was found to be 20%. Finally, we discuss the challenges and future of using RAC in structural concrete.

Bond properties between corroded steel bars and recycled aggregate concrete after high-temperature exposure

Increasingly used reinforced recycled aggregate concrete (RAC) structures may suffer from steel corrosion and high-temperature exposure. The bond behaviors between corroded steel bars and RAC after high-temperature exposure are important in the high-temperature/fire damage assessment of aging RAC structures. In this study, pull-out tests were carried out on specimens of corroded steel and natural aggregate concrete (NAC) or RAC after high-temperature exposure. The specimens were exposed to 20, 100, 200, 300, 400, 500, 600, 700 and 800 °C with steel bar in the specimens corroded with corrosion ratios of 0.00 %, 1.05 %, 3.07 %, 5.34 %, 7.50 %. The failure modes, bond-slip curves and bond strength were studied. Effects of corrosion/temperature on bond strength and their difference when using different aggregates (RAC and NAC) were analyzed. The test results indicated that the bond strength of most specimens decreased with increasing high-temperature exposure, and the bond strength degradation of NAC was slightly higher than that of RAC. The bond strength first increased and then decreased as the corrosion ratio increased, and the bond strength of NAC is higher than that of RAC under high corrosion conditions. The failure modes and bond strength also varied with and without stirrups. A bond-slip constitutive model of corroded steel bar and RAC without stirrups after high temperature was established introducing temperature and corrosion influence coefficients.

Exploring seismic resilience in frame structures through advanced numerical modeling and dynamic nonlinear analysis of recycled aggregate concrete

This study developed an advanced three-dimensional spatial model to examine the behavior of Recycled Aggregate Concrete (RAC) frame structures using sophisticated nonlinear beam-column fiber elements. It thoroughly investigates the dynamic properties such as natural frequency, vibration modes, and stiffness of RAC structures and conducts a detailed nonlinear dynamic analysis focusing on acceleration, displacement, drift, and hysteresis curves. The seismic resilience of RAC structures, especially inter-story drift under various seismic intensities, was evaluated and compared with Natural Aggregate Concrete (NAC) structures by adjusting the concrete material model parameters in the finite element framework. The accuracy of the numerical model was validated against shaking table tests, showing that RAC and NAC structures comply with structural damage criteria across seismic phases from 0.066 g to 1.170 g. The findings suggest RAC structures are as seismically resilient as NAC ones, making them suitable for earthquake-prone areas.

Fire and postfire compressive strength of recycled aggregate concrete made with ceramic stoneware

This study investigated the possibilities of reusing ceramic stoneware (CS) waste, a particular type of tiles ceramic waste (TCW), provided by a treatment plant in Castellon (east Spain), as recycled aggregate in structural concrete, and its influence on concrete behaviour when exposed to elevated temperatures (200 °C, 400 °C, 600 °C, 800 °C; compressive strength determined in hot and after air and water cooling). Although all samples exhibited similar strength values at room temperature, their thermal conductivity reduced with increasing CS contents. Strength values progressively dropped with rising temperatures, with the highest results recorded in hot, followed by air and, finally, by water cooling. Smaller differences between the strength registered in hot and after cooling were generally recorded in the recycled aggregate concretes (RAC) than in the reference sample. The concrete prepared with 100 vol% CS gravel was the only one to generally provide better residual strength after air-cooling than in hot.

Experimental study and modelling of flexural behaviour of continuous fibre-reinforced lightweight aggregate concrete one-way slab at ambient and elevated temperatures

An experimental study on the flexural behaviour of six fibre-reinforced lightweight aggregate concrete (FRLWAC) one-way slab strips under ambient and elevated temperatures is presented in this paper. Two slabs, one simply-supported and one continuous, were loaded to failure at ambient temperature as references for the fire tests, while another simply-supported and three continuous slabs with varying degrees of design moment redistribution were loaded and exposed to elevated temperatures until failure. The ambient tests showed that the steel fibre addition prevents premature flexural-compression failure commonly observed in lightweight aggregate concrete structures. The rotations of FRLWAC plastic hinges were concentrated in a single crack under ambient temperature and a few cracks under fire exposure. The fire tests revealed significant moment redistribution towards the hogging moment area in the continuous slabs due to restrained thermal-induced deflection. Furthermore, the reinforcement configuration that catered for moment redistribution towards the hogging area was found to enhance the fire resistance of continuous FRLWAC slabs. A fibre-reinforced concrete (FRC) plastic hinge model, based on the rigid-body rotation concept, was proposed and validated. This model predicts the moment-rotation behaviour of FRC hinges at ambient temperature and the fire resistance of FRLWAC one-way slabs with good accuracy.

Experimental study on seismic behavior of recycled aggregate concrete infilled profile steel structures

Experimental study on seismic behavior of recycled aggregate concrete infilled profile steel structures

Investigation on the response of steel fiber reinforced lightweight aggregate concrete slab under sequential impact loading

The current study investigates the response of steel fiber-reinforced lightweight concrete slabs under sequential impact loading. The potential damages caused by impact loads which are a threat to many structures and the use of fiber to mitigate them was evaluated experimentally. Moreover, a comprehensive parametric study was conducted using the finite element method to evaluate the effect of influential factors such as fiber content, slab geometry, and energy of impact. The results show that impact loads can cause significant damage to lightweight aggregate concrete slabs, including cracking and spalling. The use of steel fibers in lightweight aggregate concrete can improve its stiffness, energy absorption, overall stability, and prevent crack propagation resulting in the reduction of the damage risk under impact loads, compared to specimen without fiber. The incorporation of fibers resulted in a significant rise in the number of impacts required for failure. In contrast to the control slab, which failed completely after three impacts, lightweight concrete slabs containing steel fibers at volume fraction of 0.5%, 1%, and 1.5% endured 87, 710, and 1213 impacts before failure, respectively. In addition, adding steel fibers causes a more consistent response in terms of stiffness as the frequencies of lightweight concrete slabs containing steel fibers at volume fraction of 0.5%, 1%, and 1.5% remained unchanged until 34%, 17%, and 40% of the total number of impacts resulting in failure. Moreover, the utilization of fibers preserves structural integrity and localizes crack formation. Overall, the current study provides valuable insights into the response of light weight aggregate concrete structures under impact loading and the potential benefits of using steel fiber and lightweight aggregate in concrete.

An Innovative Approach for Evaluating the Quality of Recycled Concrete Aggregate Mixes

The objective of this research is to develop an innovative methodology for evaluating the quality of coarse aggregate mixes that incorporate recycled concrete aggregate for use in structural concrete. The proposed approach consists of three steps: measuring the packing density, quantifying the adhered mortar, and characterizing the mechanical resistance of the aggregate mixes containing both natural and recycled concrete aggregate. Comprehensive practical recommendations for each step are discussed in detail to facilitate the broader future utilization of recycled concrete aggregate in structural concrete. The mechanical resistance of aggregate mixes is significantly influenced by both packing density and the quantity of adhered mortar. Predictive models are used to illustrate the mechanical resistance of aggregate mixes based on packing density and adhered mortar amount, recognizing the direct correlation between these properties.

Optimizing Coal Ash as a Sustainable Substitute of Cement and Aggregate in Structural Concrete

The manufacture of concrete for constructing the structures like highways, bridges, and buildings requires large amounts of cement and aggregates. This high concrete production depletes natural resources like sand and gravel for the construction industry. It also negatively impacts the environment due to cement usage. This study looked at using waste materials like coal bottom ash (CBA) and fly ash (FA) in concrete as substitutes for some of the typical aggregates and cement. The goal was to reduce the environmental impact and preserve natural resources. The study made concrete with regular Portland cement, sand, 10mm coarse aggregate, locally available CBA, and FA. 15% fly ash was selected as an optimal level from initial testing. The CBA was used to replace 0-35% of the fine aggregate sand. Test cubes and cylinders were made with different mixes. Compressive strength, tensile strength, carbonation, and sulfate attack tests were done after curing. Results showed 25% CBA improved the concrete’s mechanical performance. The compressive and tensile strengths increased but not above conventional concrete. This is because CBA needs more moisture for full hydration over longer curing times. Also, the concrete’s durability improved in terms of resistance to carbonation and sulfate attack.

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.

A robust carbonation depth model in recycled aggregate concrete (RAC) using neural network

Carbonation depth involves complex physical process and interactions of multiple variables and is thus extremely complicated to predict in concrete structures. It is imperative to quantify this depth due to its vital role in the corrosion of rebars in recycled aggregate concrete (RAC). This paper developed a novel carbonation depth prediction model from a large database of 445 experimental results using artificial neural network (ANN). The relative importance and effect of the independent parameters in the carbonation depth are identified using Garson index and parametric analysis, respectively. Among all the architectures considered, the N 8-10-1 having 10 nodes in the hidden layer provided the best prediction in good agreement with experimental results. The model demonstrated superior performance relative to existing carbonation depth equations in the literature. Despite the presence of fuzziness in the data, the effect of each variable in the development of carbonation is explored in great detail. The model proposed here can provide a robust prediction of carbonation depth that can be used as a basis for assessing the structural health of recycled aggregate concrete structures.

Sustainability assessment of recycled aggregate concrete structures: A critical view on the current state‐of‐knowledge and practice

AbstractThe environmental impacts of activities such as raw material extraction, construction of infrastructure, and demolition, place construction as one of the sectors that exert the highest pressures on the environment, society, and economy. Some of the major environmental impacts for which the construction industry is responsible are mineral resource depletion, greenhouse gas emissions, and waste generation. Among the different strategies that exist to decrease such impacts, recycling demolition waste into recycled concrete aggregates has been considered a promising alternative. As such, at present, the literature dealing with the impact assessment of recycled aggregate concrete structures is very extensive. Therefore, the objective of this article is to present a critical view of the state‐of‐the‐art in terms of sustainability assessment of recycled aggregate concrete structures, taking a holistic perspective by considering environmental, social, and economic impacts.

Study on mechanical properties of stud connectors in steel-lightweight aggregate concrete composite structures

To systematically study the factors affecting the shear bearing capacity of lightweight aggregate concrete stud connectors, six groups of 12 push-out specimens were made to investigate the concrete type, stud diameter, concrete strength and stirrup spacing. The effect of stirrup spacing on the shear performance of lightweight aggregate concrete stud connectors was analyzed in detail by using finite element modeling analysis. The load-slip curve equation and the shear bearing capacity equation with stirrup ratio factor were proposed for the lightweight aggregate concrete stud connectors through the experimental value and the finite element analysis value, respectively. The test results show that the shear bearing capacity of lightweight aggregate concrete stud connectors increases with increasing stud diameter, with stud diameter having a greater influence on stud connector shear bearing capacity. The shear bearing capacity of lightweight aggregate concrete stud connectors increases with increasing concrete strength, and the influence of concrete strength on the bearing capacity is low. The shear bearing capacity of lightweight aggregate concrete stud connectors decreases with increasing stirrup spacing, and the influence of stirrup spacing on the bearing capacity is lower. The theoretical shear stiffness calculated by the load-slip curve formula and the shear bearing capacity calculation formula proposed in this paper are more consistent with the test shear stiffness when the cut line slope is set to 0.2 mm. This research is critical for the promotion and use of lightweight aggregate concrete stud connectors in bridge engineering.

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.

Chemically treated plastic replacing fine aggregate in structural concrete

Plastic wastes (PW) create chronic environmental problems mainly because it is not biodegradable. The utilization of PW as a fractional substitution of fine aggregate in concrete production is a sustainable application. However, the smooth surface of plastic weakens the cement-plastic bond and consequently lessens the mechanical properties of concrete. This research improves the cement-plastic bond by increasing the hydrophilicity and coarseness of polyethylene terephthalate (PET) through chemical treatment with 5wt% polystyrene (P.S.) and 5wt% PKHH-phenoxy resin. Twenty eight mixtures with different w/c ratio; 0.4, 0.45, 0.5, and 0.6 and different percentage of sand substitution; 5%, 20%, and 50% are cast. Tests are conducted to determine the behavior of fresh and hardened concrete in terms of workability, water absorption, density, compressive strength, flexural strength, splitting tensile strength, and durability. Results show that the proposed chemical treatment rises the cement-plastic bond. The compressive, splitting tensile, and flexural strength were improved by 45%, 35%, and 6% respectively compared to corresponding untreated plastic recycled concrete. The highest mechanical properties are achieved for mixes with 5% plastic. Nevertheless, a 20% replacement of sand with treated plastic can be used effectively to produce structural concrete.

Sustainable and Low-Cost Hemp FRP Composite Confinement of B-Waste Concrete

Each year, massive amount of construction waste is generated that needs proper attention in terms of its disposal without deteriorating surrounding environment. A significant portion of this waste comprises bricks. Besides, large number of new construction works are resulting in the depletion of natural resources rapidly. Intuitively, a sustainable solution demands to consume this construction waste in the best way possible. This study targeted brick waste as a potential material to be used as a partial replacement of natural aggregates in structural concrete. It has been known that the concrete constructed with recycled brick aggregates possesses substandard mechanical properties. Traditionally, synthetic FRPs are known to strengthen recycled aggregate concrete. However, recognizing high costs associated with them, this study proposed the use of natural hemp fiber ropes to strengthen recycled aggregate concrete constructed with brick aggregates. To assess the efficacy of hemp ropes in strengthening mechanical properties of the concrete with coarse aggregates partially replaced with recycled brick aggregates (B-waste), an experimental framework was conducted. Sixteen cylindrical specimens were tested in two groups depending upon the concrete strength. Within each group, 2 specimens each were strengthened with 1, 2, and 3 layers of hemp fiber ropes. Axial monotonic compressive loading was applied to each specimen. Results revealed that hemp fiber ropes significantly improved ultimate compressive strength and the corresponding strain. A substantial improvement in axial ductility was observed. For the sake of performance-based non-linear modelling, accurate constitutive modelling at material level is necessary. For this purpose, several existing analytical stress-strain models were tested in this study to predict ultimate confined compressive strength and strain. It was found that several models predicted confined compressive strengths with reasonable accuracy. However, very few models were able to predict confined peak strain with good accuracy.

Assessment of the engineering properties of biomass recycled aggregate concrete developed from coconut shells

This study examines the potential of using biomass recycled aggregates developed from coconut shells to replace aggregates in structural concrete. Compared with natural aggregate (NA), coconut shell has many disadvantages as coarse aggregate in concrete due to the differences in structure, particle shape, and density from NA. Therefore, the contribution of this study is to improve the possibility of using agricultural byproducts in structural concrete. The main experimental work is to clarify the effect of using crushed coconut shell aggregate (CCSA) and synthetic biomass recycled aggregate (SBRA), which are developed based on coconut shell, to replace NA with comparable mechanical properties, shrinkage properties, thermal conductivity, microstructure of concrete. The results indicated that the mechanical strength of concrete with SBRA in 3d and 28d achieved at least 90 % and 80 % strength of that of natural aggregate concrete (NAC), respectively. In addition, compared with NAC, crushed coconut shell aggregate concrete (CCSAC) and synthetic biomass recycled aggregate concrete (SBRAC) show significant improvements in thermal conductivity, showing excellent building energy saving potential. Compared with CCSA, the cement paste matrix surrounding the SBRA can effectively prevent water loss and reduce the thickness of interfacial transition zone (ITZ) to less than 10 mm, thus ensuring the volume stability of concrete; The lower water-cement ratio and carbonized SBRA (C-SBRA) had positive effects on improving the properties of CCSAC and SBRAC, respectively.

Reliability‐based recommendations for EN1992 carbonation cover design of concrete with coarse recycled concrete aggregates

AbstractResearch argues in favor of using recycled aggregates in structural concrete. However, lack of specific clauses in standards for recycled aggregate concrete design hinders the use of recycled aggregate concrete by the industry. Recycled aggregate concrete‐specific design clauses are hindered because conventional research must be complemented with experiments directed towards stochastic modeling, so that reliability concepts may evaluate whether the probabilities of failure of design comply with the expectations of Society or whether specific design rules are needed. This article compares the within‐batch variability of carbonation under accelerated conditions of four natural aggregate concrete and four analogue recycled aggregate concrete mixes and analyses the reliability of Eurocode 2 cover design for carbonation exposure. In general, the incorporation of coarse recycled concrete aggregates worsened the mean durability behavior of concrete, including resistance to carbonation. However, the variability of carbonation depth was not meaningfully affected by the recycled aggregates used (coarse recycled aggregates produced from concrete waste) and the reliability of concrete cover design only decreased due to the effect of the recycled aggregates on the mean resistance to carbonation. A 5 mm increase of concrete cover was found to more than offset the detrimental influence of full incorporation of recycled aggregates on the reliability of concrete cover design.