Prof. Neha (Research Scholar), Dr. Sudisht Mishra and Dr. Rajeev Goel, review and explore the advantages and disadvantages of recycled concrete aggregate so that further implications for improvement and enhancements level can be identified.
A review was carried out on the recycled concrete aggregate obtained from the construction and demolition waste. This research is on the properties of RCA, its effects on the properties of concrete as a material, and its impact on the strength of structural members on a very large scale. In general, RCA is used for secondary purposes due to adhering mortar to recycled concrete aggregate. RCA possesses a higher porous and higher absorption with lowering the strength criteria in respect of mechanical and durability as compared to conventional aggregate.
At present, the availability of RCA has improved in quantity due to the demolition of some old concrete structures being dismantled for recycling into new structures. RCA is a concrete aggregate that is produced from the wastes of construction or demolition that either replaces the conventional coarse aggregates completely or partially. A majority of the demolished materials is generally concrete and is sometimes dumped on land, resulting in damage to the availability of land and blocking passage for groundwater recharging. It cannot be reused for other purposes. Recycled Coarse Aggregate is not homogeneous compared with the dynamic properties.
It can be said that it is a simple method to recycle dismantled concrete as it goes through methods of breaking, crushing, or removing materials like steel reinforced from the concrete that is already existing, to a material that has a specific quality and size. Sustainability is provided by recycling concrete in many ways. The process of recycling concrete decreases the quantity of material for landfills. RCA reduces the requirements of virgin aggregates. That eventually reduces the impact of the process from natural mines of aggregate extraction. This process is useful as RCA absorbs carbon dioxide in a large amount from the surroundings. It is useful as the concrete becomes aggregate and it is then possible to remove or recycle any embedded metals.
Physical Properties of RCA Concrete
It is observed that hardened concrete is generally equivalent to the natural rocks that are not impacted by the grade of the original concrete. RCA consists of original aggregates as well as a hydrated paste of cement and fine aggregate. This paste helps in reducing the specific gravity as well as increases the absorbency compared with the virgin aggregates. Higher the absorbency of RCA higher the permeability. The physical characteristics of the concrete are durability, strength, stability of volume, impermeability, and so on. The main physical properties of RCA are porosity, density, and the absorption of water by the recycled aggregate, the gradation, and shape of aggregate, and the resisting capacity of crushing or abrasion of aggregate.
Porosity, Density, Water Absorption
The adhered bond in aggregate is the most important factor that affects the properties such as porosity, density, and absorption of water of RCA. The density is lower in the case of RCA than the natural aggregate. It is because of the lower density of the adhesive bond as compared to that of the underlying rock. The density may vary depending on some specific aggregate in question. The relative density in the case of RCA that is in the dry state saturated surface is nearly 7-9% lesser than the natural aggregate. The density of RCA and natural aggregate (NA) differs from each other at a certain percentage. The adhered bond is lightweight compared with the aggregate of the same volume which is responsible for the decrease in density.
The water absorption and the porosity are the characteristics of RCA that are related to each other that are also allotted to residual mortar. As per the view of Fardin et al. (2022), the Natural Aggregate has lower porosity that causes the lower absorption of water. However, the adhered mortar density on RCA consists of higher porosity which in turn permits the aggregate for holding more water in its pores compared to that of NA.
Characteristics such as porosity, density, and water absorption are useful in determining a properly concreted mixture. These are the properties that can limit the capacity of absorption of aggregates by not more than 5% in the case of structural concrete. So, the RCA proportion can be limited to a concrete mixture.
Gradation and Shape
The shape of aggregate can influence the ability to work on the concrete. The process of producing RCA as well as the crusher type is used in the procedure which is very much influential at the time of shaping the RCA. Natural aggregate has an angular shape along with smooth sides. The RCA that are plant-produced is blurry in texture. RCA is rounded, spherical in shape, and seemed to be improved for workability. The residual mortar of RCA helps in smoothing out the edges of original aggregates that are hard. This helps the new mortar flow better on all sides of the aggregate. There are some consequences of the shape as well as strength of aggregate on the workability.
The standards of concrete aggregate can define a span of gradation that lies for being accepted as the aggregate for the structural concrete. The gradation curve of RCA is always in its specific range. It is the factor of indication of the acceptable gradation without any adjustment only by applying the standards.
Crushing and Abrasion
Crushing and abrasion are the tests for measuring the durability or longevity of aggregate materials by themselves. RCA has a crushing and abrasion value higher than the NA that means at the time of containment and crushing of aggregate by some steel balls in case of abrasion test the RCA contains fine particles. RCA contains a residual mortar to be easily broken off at the zone of interfacial transition which is a very weak zone of concrete. At the time of loading the residual mortar can be broken off from the RCA.
The crushing and abrasion test illustrates how weak the adhered mortar is. This layer is to be expected as broken off, and it can also be forecasted that the layer of adhered mortar may create a poor connection within the concrete.
Other Properties of RCA Concrete
The total power/strength of RCA concrete is influenced by the recycled aggregate. There are influential factors including water-cement ratio, coarse aggregate percentage, and amount of adhered mortar that can affect the compressive strength. Reducing the compressive strength of RCA concrete helps in increasing water absorption. Since the aggregate can store more water, the water is released into a new mortar for continuing the feeding of the cement for a long time which may improve the strength. RCA lowers the compressive strength for the high value of water absorption. In case of the application of structural concrete, strength tests can be performed for ensuring the strength of concrete, and the RCA has the ability to produce, and confirm the RCA fraction that is acceptable in case there is a need for any changes in the water-cement ratio for producing the concrete of the required strength.
Review of the Durability of RCA Concrete
Durability is a sort of ability for lasting for a long time without any absence of significant deterioration. A material that is durable is useful for the environment by protecting the resources and by reducing the wastes and the impacts on the environment for repairing and replacing. Concrete can resist weathering actions, abrasion, and chemical attacks to maintain the required properties. The durability of different concrete is different as per the exposure and properties that are desired. The factors that determine the exact durability are the concrete ingredients, their interactions, proportion, and so on.
Factors related to durability of concrete:
High humidity as well as rain: Having little or no content, the concrete is resistant to deterioration because of the rotting or rusting in hot, humid climates. Moisture can be entered into a building through the joints of concrete elements. It is observed that when the moisture enters it cannot damage the concrete. Moisture may rot the insulation, wood framing, and sheathing. It erodes the metal framing and the attachments of metal.
Inedible: Insects and vermin do not destroy the concrete as it is inedible. The soft materials are inedible; however, it still gives the path to the insects. Vermin and insects are not bored due to the hardness of concrete.
Resistance of UVR: The ultraviolet rays of solar radiation cannot harm the concrete. The usage of color pigment in the concrete keeps the colors in the elements for a long period that fades due to the effect of solar rays.
Abrasion Resistance: Concrete has the power of resisting the abrasive effects when the weather is ordinary. Particles of moving water, and floating ice cubes are examples of severe erosion, and abrasion.
Resistance to Freezing: The most destructive factors for weather are freezing, and thawing when the concrete becomes wet, specifically when the deicing chemicals are present in the concrete. The causes of deterioration are freezing of the water, the expansion of the paste, the particles of the aggregate, or both. A proper microscopic system of air bubbles can be obtained with the addition of air when the concrete is a high resisting factor while thawing, and freezing. The air bubbles are microscopic within the accommodation of concrete when the water is expanded to an ice cube so that it generates internal pressure.
The durability feature of concrete contains RCA that can be evaluated by a comprehensive investigation that also involves nine control mixtures. Durability properties are water absorption, acid attack, absorptivity, and chloride permeability, these are to be determined.
Mechanical Properties of RCA Concrete
RCA is produced from demolition and construction wastes. It either completely or partially replaces conventional aggregates of neutral. It conserves the resources that are natural and decreases the space that is required for the disposal of landfills. RCA has several properties: physical, mechanical, and chemical.
- Compressive strength: Compressive strength defines the capacity of one structure or material to resist loads that have the tendency to decrease the size. As per the view of Shang et al. (2022), the recycled concrete aggregate has a compressive strength between 40 to 51 Mpa. The compressive strength ensures the quality of the product. The compressive strength of aggregate concrete is determined weekly, monthly, and quarterly. The compressive strength of the RCA concrete is less than the concrete that is constructed from similar proportions. The overall amount of depletion in strength turns on several parameters like the grade of concrete, w/c ration, replacement ratio, and processing of aggregate.
- Flexural and tensile strength: The strength of flexural is the quantity of the strength of slabs and concrete beams. The tensile strength is less than the flexural strength. The strength of flexural is measured by failure because of tensile and compressive stresses. The strength of flexural is the measurement of a concrete beam that is unreinforced to withstand failure. As per the view of Xu et al. (2022), the depletion in flexural strength and splitting of RCA concrete is approximately 6-12% and 3-14% respectively.
- Bond strength: Bond strength defines the strength of the concrete that holds more than one atom together. As per the view of Zhao et al. (2021), recycled concrete has a bond strength that is approximately equal to 20Mpa strength. For large strength, it behaves disproportional, which means the strength of higher concrete is very low. This bond strength comes from adhesion and friction.
- Elastic modulus: Elastic modulus is one type of measurement of the rigidity of a particular material. The units of these properties are the unit of pressure and which is described as the division between stress and strain. The elastic modulus of the aggregate concrete is constant in case of lower stress levels and starts reducing at a higher level of stress. As per the view of Wu et al. (2018), the value of elastic modulus is in the range of 10-35 Gpa. In this case, it is mandatory to conduct unloading and loading to decrease error and increase accuracy. The main reason for this lower elastic modulus is a higher segment of hardened paste. The elastic modulus is one type of critical parameter to design the structures.
- Toughness: Toughness defines the quality and state of being inviolable. It refers to the ability of the particular material to occupy energy and disfigure plastically without failure. The recycled concrete has comparatively inferior tensile strength than the compressive strength.
- Hardness: Hardness defines the confrontation of a particular material to the deformation of a localized plastic. The recycled concrete has the hardness that is characterized by the compressive strength of RCA and it is measured in PSI (“Pounds per square”).
- Lower density: The density defines the overall quantity of aggregate concrete per unit area, unit strength, or unit volume. RCA concrete has a lower density as compared to other concrete. The lower density provides thermal insulation, better compatibility, and internal curing. This means the RCA concrete has a lower quantity of mass per unit length as this concrete consists of fewer particles. The lower density of aggregate is attributed to the higher porosity than the natural aggregate.
- Specific Gravity: Specific gravity defines the ratio between the density and a specified substance. As per the view of Tang et al. (2019), the specific gravity of RCA is in the range of 2 to 2.5 without the treatment of the surface and it becomes 7.5-8% with the absorption of water. Several aggregates of concrete have a specific gravity in the range between 2.5 and 2.6. This specific gravity is lower than the natural aggregates.
- Sand Bulking: The bulking of aggregate defines the phenomenon and occurrence of an increment in the volume of sand as the moisture content increases. The sand bulking is calculated by measuring and determining the gap between the saturated volume and real volume. The recycled concrete has a bulk density that is lower than the natural aggregate.
- Absorption and porosity of water: Water absorption occurs through two types of mechanisms: passive absorption and active absorption. This defines the water amount that is absorbed by the RCA concrete. The water absorption of RCA concrete is in the range of 5 and 6%. Porosity defines the amount of space where the water is held under the surface and ground. As per the view of Tang et al. (2021), the aggregate concrete has a porosity of about 3-4% and in the case of RCA concrete, it is 15-16%. In the case of concrete, the average value of porosity is in the range of 8-10%. The water absorption of recycled concrete is higher and it is utilized to maintain the conditions of SSD (saturated surface dry).
The overall performance of RCA concrete is estimated from these types of mechanical properties. In addition, the other mechanical properties are gradation, the shape of concrete, bulk density, surface texture, abrasion resistance, organic impurities, and fineness modulus. The characteristics of porosity, water absorption, and density are the main focus of measuring the concrete. In addition, the other mechanical properties are gradation, the shape of concrete, bulk density, surface texture, abrasion resistance, organic impurities, and fineness modulus. The characteristics of porosity, water absorption, and density are the main focus of measuring the concrete.
Study on Microstructure of RCA
As per the discussion of Mataret al. (2019), the angular conventional shape of concrete has a clean surface aggregate, it is less permeable, as well as it is free from losing the particles like dust. The microstructure of RCA concrete is different from conventional concrete. The angular shape as well as the rough surface of the concrete seems to be more complicated sometimes. The surface is permeable and is covered with crumbly loose particles. The specimens of RCA are manufactured for incorporating RCA to display the distinct past decorations. The reaction of Alkali-Silica, shortly termed ASR is the most detrimental mechanism that leads to the early deterioration of the infrastructure of the concrete. The waste of demolition can be transformed into RCA.
ASR mechanism is such a distress technique; its recurrence potential in RCA can lead to adverse effects. The crack propagation investigation of RCA concrete exposes the distress properties that may vary in a wide range. As a result, it can indicate distinct mechanisms in the RCA concrete. RCA produces concrete of high performance that can be limited to the insufficient correlation between the outcoming microstructure with the influence of it in case of mechanical performance while reproducing. The concrete microstructure can be analyzed systematically and also can be characterized with the help of scanning the microscopic electron that may result in the correlation of compressive strength, and the ability to absorb water.
As per Mikhailenkoet al. (2022), replacing the natural aggregate with the influential RCA along with distinct sources of strength level of concrete of silica fume. As a result, the developed microstructure is dependent on the composition of concrete. The analysis of the microstructure of RCA can be done by the backscattered electrons. The water-consuming ratio of “carbonated Recycled Concrete Aggregate” or CRCA may be lower than the RCA which is untreated. The density of CRCA should be modified significantly. The results of the concrete strength indicate the mixture of CRCA adopts two stages of the mixing approach that give a view of compressive strength as compared with the other mixes. The analysis of the microstructure can demonstrate the mix is much denser than the untreated RCA.
Applications of RCA
Recycled concrete is one of the favorite selections as a material of construction among engineers. It is recommended for longer life, low cost of maintenance, and better performance. The applications and use of RCA concrete are affected by some factors like material specifications, and phasing which means the timing of availability, and volume of obtained RCA. There are several applications of this RCA concrete:- Constructing pavements: The recycled aggregate concrete can be utilized for constructing pavements and gutters. The gutters are a channel that is designed to transfer and collect rainwater to a drain.
- Building revetments: There are several pieces of recycled aggregate concrete. The comparatively large pieces of this RCA concrete can be utilized for constructing revetments that are very applicable and useful in handling and controlling erosion of soil. These revetments are constructed as structures of sloping as well as permeable using concrete blocks and natural stones. Consequently, the absorption of wave energy is enhanced and reflection is reduced.
- Prevention of drainage structure: The RCA concrete is used to crush the extracted reclaimed mortar. It is utilized to blend materials and RCA and consider a sub-base. It considers cleaning RCA to decrease the deposits of insoluble residue. It uses the largest particle sizes of RCA.
- Ground development materials: The manufacturing of RCA concrete also assists to generate several by-products. The by-products of this RCA concrete have several applications like the addition of concrete, a filter made of asphalt, and material for ground improvement.
- Reduction of new landfills: This is one of the major benefits of RCA concrete. It preserves landfill space. Consequently, the construction of new landfills is required and hence more costs are saved.
- Reduction in costs and aggregates: The other benefit of this recycled aggregate is the recursion of the number of aggregates and consequently the evacuation of resources is reduced. In addition, it is cost-economical. It reduces the construction costs and it is one of the significant features of this recycled concrete.
- Unbound sub-base: These RCA concretes have been successfully and widely utilized in fill applications and “unbound sub-base”. Sub-base describes the main layer of load-bearing of the gutters and pavements.
- Preservation of natural resources: The recycled aggregate rubble is utilized as the aggregate of recycled course in concrete and it preserves the natural resources.
- Absorption of CO2: During the process of crushing a huge amount of CO2 is reduced. While the recycled concrete is crushed into fewer particles, a huge amount of CO2 is absorbed. Consequently, it reduces the overall amount of Carbon dioxide in the atmosphere.
There are several drawbacks to recycled concrete:- The overall quality of RCA concrete is downgraded.
- As per the view of Wang et al. (2020), the capacity of water absorption is increased from 4% to 10%.
- As per the view of Wang et al. (2018), the compressive strength of concrete is reduced from 10-25%.
- The workability of RCA concrete is reduced.
- Lack of guidelines and specifications.
- Durability is reduced. Durability of RCA concrete can be improved by combining it with materials such as fly ash.
Conclusion
This part is based on the mechanical and physical properties of RCA concrete and the overall impacts of RCA that are utilized on the properties of concrete material. The modulus for RCA concrete is lower as compared to the conventional concrete and the elastic modulus is lower. The demolition waste and recycling construction in the manufacturing of recycled concrete is one of the approaches and goals in the economy and environment. However, recently the natural and structural applications of recycled concrete are limited. This part also discusses the causes behind the inferior durability and mechanical performance of aggregate concrete. An analysis and a review of the durability and mechanical behavior of concrete are discussed in this part. As per the view of Wang et al. (2022), in facing several challenges, recycled aggregate is utilized to create and maintain sustainable construction which is the proper solution to reduce the consequences of building waste. The requirements of this RCA concrete provide a huge impact on non-structural concrete.
Reuse and recycling of wastes have been identified as a proper resolution to the dumping problems accompanied by natural aggregates. The several applications of this recycled concrete are valuable materials for building in economical, technical, and environmental respects. In this part, the brief knowledge about this RCA concrete is known. In addition, several mechanical properties like compressive strength, bond strength, tensile and flexural strength, elastic modulus, lower density, hardness, toughness, specific gravity, bulking, porosity, and absorption of water are identified. Moreover, several physical properties of RCA concrete-like frost resistance, dustiness, porosity, abrasion, absorption capacity, polishing, and mineralogical composition are discussed. Moreover, several applications of this RCA concrete-like building revetments, prevention of drainage structure, construction pavements, and ground improvement materials are discussed. In addition, several benefits like reduction of new landfills, unbound sub-base, reduction in aggregates and costs, prevention of resources, and absorption of CO2 are also identified.
The recycled aggregate controls comparatively fewer bulk density, impact and crushing values, and higher absorption of water in comparison with natural aggregate. The strength of recycled concrete is comparatively fewer, up to 10-14% than natural concrete. The variation turns on to obtained original concrete. In addition, there are several drawbacks of this concrete-like downgrading of quality, increment of water absorption, reduction of compressive strength, lack of guidelines, and reduction of durability are also discussed.
There are reliable applications to utilize recycled aggregate concrete in construction. However, more initiation and research for several applications of RCA concrete is required to modify the design codes, procedures, and specifications to utilize in recycled concrete. Moreover, the durability and microstructure of RCA concrete are also discussed. The durability tends to degenerate with the enhancement of recycling concrete. The recycled aggregate can be produced by using RCA concrete that is produced from construction and demolition waste. In addition, the properties and microstructure of this RCA concrete are also discussed.
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Authors
Neha, Research Scholar, Department of Civil Engineering, North Eastern Regional Institute of Science & Technology (Deemed To Be University, Nirjuli), & Lecturer in Government Polytechnic Kotana Baraut Bagpat;
Dr. Sudisht Mishra - Professor & Head, Department of Civil Engineering, North Eastern Regional Institute of Science & Technology (Deemed To Be University, Nirjuli), India,
Dr. Rajeev Goel, Senior Principal Scientist, Bridge Section, CSIR-Central Road Research Institute, New Delhi.
