There are several innovative uses of recycled aggregate, ranging from non-structural uses (like fill material and landscaping) to constructing structural elements like beams, columns, and more. Aggregates are indispensable materials in the construction of rigid structures as they make concrete mixes more compact and contribute to the mechanical strength of concrete. The construction industry needs large amounts of aggregates each year. But meeting this need by using natural aggregates is costly and has severe environmental implications. This is where Recycled Aggregates in Concrete (RAC) come in!
Recycled aggregates in concrete come from crushed concrete from demolished buildings and roads.
Recycled aggregates are becoming more appealing economically and environmentally. They cost less to produce.
They protect natural environments by avoiding mining in their production and reducing construction and demolition waste disposal through their use.
However, big questions surround using recycled aggregate concrete, such as “Where can you use recycled aggregate concrete, are they durable, etc?”
This article will answer these questions. We’ll discuss recycled aggregates in detail.
Types and Sources of Recycled Aggregates in Concrete
Construction waste and demolition debris are the sources of recycled aggregates.
After demolishing an old building or road, people often consider the concrete and other materials worthless and dispose of them as demolition waste.
However, that is changing with the advent of recycled aggregate concrete.
Nowadays, workers can remove and crush the concrete used in past construction projects to create recycled aggregate. Incorporating recycled aggregates into concrete serves as a crucial aspect of sustainable construction.
Sources of Recycled Aggregates in Concrete
Specifically, sources of RCA include:
Demolition Wastes
Demolishing concrete structures (such as buildings, roads, and bridges) generates a large volume of concrete debris. This debris can be crushed and processed into recycled aggregates.
Demolitions happen for various reasons (such as problems with a structural element, a change in building code, the need to replace an old building, etc.).
However, all demolitions share one common trait – they generate a significant amount of demolished concrete waste that can be recycled into aggregates. Recycling this concrete waste lessens the environmental impact of construction projects.
Construction Waste
Construction waste usually includes concrete debris, which can be Recycled Aggregates in Concrete instead of discarded. Recycling concrete debris from construction waste improves resource efficiency in the construction industry.
Concrete debris from construction waste includes excess or unused concrete during construction or reinforced concrete removed for modification.
These concrete elements include aggregates and, therefore, can be recycled into RCA (Recycled Aggregates in Concrete) instead of being discarded.
CDW from Old Concrete Pavements and Sidewalks
Concrete pavements, sidewalks, or curbs are mostly concrete. So when they are old or damaged beyond repairs, they are good sources of recycled concrete aggregates.
Rejected Concrete
Manufacturers usually reject concrete that fails to meet specific quality standards. However, they can process this rejected concrete into recycled aggregates for other applications instead of discarding it, since it consists of aggregates.
The quality control manager will always run tests on concrete before using it. The concrete mix is rejected if such tests (air entrainment, slump, etc.) come back outside specified ranges.
However, every concrete job is different, so concrete that is rejected for one application may be suitable for another.
Even when the concrete cannot be used, it remains a rich aggregate source. So, instead of being discarded as waste, it can be processed into recycled aggregates.
Concrete Test Specimens
Concrete test specimens are concrete samples molded and cured for concrete tests.
When carrying out strength tests (like compressive and tensile strength), concrete is molded and cured before testing to determine specific characteristics.
These lab tests can quickly leave behind an army of concrete cubes and cylinders. Instead of discarding them as concrete waste, they can be gathered and processed into recycled aggregates.
Natural Disaster Waste
Natural disasters like hurricanes, earthquakes, and floods generate concrete debris, which can be crushed and processed into Recycled Aggregates in Concrete post-disaster.
The sheer force of hurricane winds can pull down concrete structures to generate concrete debris. The shock waves of earthquakes displace the ground, rocking and bringing down buildings and bridges over it.
The concrete debris generated due to natural disasters can be crushed and recycled into aggregates for post-disaster construction projects.
War Waste
Explosives during war destroy buildings and other structures, leaving behind concrete debris that can be recycled into RCA post-war. Hostilities leave behind a distasteful sight, which usually includes ruins of cities and towns.
These include lots of concrete debris that can be processed into aggregate for post-war constructions.
Types of Recycled Aggregates in Concrete (RCA)
There are different classifications of recycled aggregates. However, they are most commonly classified using the production process or particle size.
Recycled Aggregates in Concrete (RCA Type) by the Production Process
Recycled Aggregates in Concrete are often classified as processed or unprocessed, depending on whether they were screened during concrete recycling.
- Processed recycled aggregates: These are recycled aggregates that are screened after the crushing process.
- Unprocessed recycled aggregates: These are recycled aggregates that have only been crushed and not screened.
When recycling concrete, the concrete debris is first separated from other debris and then crushed.
Screening often follows crushing to separate the crushed aggregate material into different sizes and remove contaminants.
Contaminants in recycled aggregates are varied, depending on the source of the RCA.
However, contaminants in recycled aggregates reduce the quality of RCA concrete. For example, when aggregates are recycled from pavements with a layer of bitumen, the compressive strength of the recycled aggregate concrete is less because of the presence of asphalt.
While removing all contaminants is costly and impractical, screening helps remove some contaminants, improving the recycled aggregate’s quality.
Thus, processed recycled aggregates have higher quality than unprocessed recycled aggregate.
RCA, Recycled Aggregates in Concrete, Type by Particle Size
Recycled Aggregates in Concrete are also classified by particle size, with the types ranging from coarse RCA to dust/ fines.
- Coarse RCA: These RCAs have larger particles, typically between 0.375 – 1.5 inches (9.5 – 37.5 mm) in diameter. They are often used as replacements for natural coarse aggregate.
- Fine RCA: These are RCAs with smaller particles, typically less than 0.375 inches (9.5 mm) in diameter. They are used in place of natural fine aggregates, often in non-structural concrete mixes.
- Crushed concrete fines: These are RCAs with very small particles. As such, they are also called “fines or “dust.” They’re commonly used as fine-fill materials for soil stabilization.
- Mixed RCA: These are recycled aggregates that feature a combination of coarse and fine RCAs. They have wide applications.
What are the Innovative Uses of Recycled Aggregates in Concrete?
There are different innovative uses of recycled aggregates, ranging from bulk fill material to bridge foundation structural grade concrete.
However, processed RCAs are generally more desirable than their unprocessed counterparts because of having fewer contaminants.
Innovative Uses of Unprocessed Recycled Aggregates in Concrete
Because unprocessed recycled aggregates have lower quality, they are primarily used in non-structural applications. Some specific uses of these recycled aggregates include:
Bulk Fill
Unprocessed RCA can be used as a fill material (to raise existing ground level) in areas where load-bearing is not critical (such as low-traffic roads and pavements).
Drainage Application
Unprocessed RCA can also be used as aggregate base fill material for drainage structures. They may also be used in sloping walls of drainage ditches.
Temporary Road Construction
Unprocessed RCA can be used as a base or subbase material for temporary roads where load-bearing is less critical. These include access roads, construction entrances, etc.
Embankment
Unprocessed RCA can be used in constructing embankments to prevent flooding.
Riverbank Protection
Unprocessed RCAs are good aggregate materials for constructing structures that provide bank fixation, preventing the lateral migration of rivers (erosion).
Noise Barriers
Unprocessed RCAs are good concrete aggregate materials for building noise barrier structures along highways or other noisy environments to mitigate the effects of the noise and protect inhabitants from noise pollution.
Landscaping and Beautification
Unprocessed RCAs has a wide range of landscaping and beautification application, including building decorative pathways, making garden beds, and more.
Innovative Uses of Processed Recycled Aggregates
Processed recycled aggregate can be used in many structural-grade construction works. These include:
Structural Concrete Applications
Processed RCA can be used instead of natural aggregate to produce structural concrete for components like columns, beams, slabs, foundations, etc.
Non-Structural Concrete Applications
Processed RCA can be used to produce concrete for non-structural applications like sidewalks, curbs, gutters, etc.
Base and Subbase Material
Processed RCA can be used as a base or subbase material when constructing pavements, parking lots, roads, etc.
Road Shoulders
Processed RCA can be used as an aggregate material for constructing road shoulders, which protect the pavement edges from traffic damage and help in erosion control.
Median Barriers
Processed RCA are excellent aggregate material for constructing median barriers that separate opposing traffic on a divided road.
Pipe Bedding Material
Processed RCA can be used as a bedding material for utility pipes to provide support and stability.
Properties of Recycled Concrete Aggregates
Recycled concrete aggregate has certain properties that depend largely on the source of the RCA and the processing methods used to obtain them. Specifically, the properties of recycled aggregates include:
Particle Sizes
The size of recycled aggregates depends on the crushing and processing methods.
Crushing produces a mix of different particle sizes. Then further processing can separate them into different fractions, from fine to coarse aggregates.
Contaminants
Unlike natural/ virgin aggregate, recycled aggregate contains some contaminants. The specific contaminants depend on the source of the recycled materials and could include reinforcing steel, wood, and other material. Quality assurance measures are essential to minimize contaminants in recycled aggregates.
Processed RCA has fewer contaminants because of having undergone screening, but all recycled aggregates have residual adhered mortar particles.
These contaminants (especially residual mortar) degrade concrete properties by negatively affecting density, absorption, and other properties.
Bulk Density
The density of recycled aggregates is usually lower than that of natural aggregates because of the adhered mortar content.
The residual mortar is lighter in weight than the same volume of natural aggregate. So, recycled aggregates are always lighter than the same volume of primary aggregates, which decreases their density.
Porosity
Aggregates recycled from previously used concrete have higher porosity than natural ones.
While natural aggregate is all hard rock with few pores, recycled aggregate contains mortar with more plentiful and bigger pores. Thus, RCA has a higher porosity and greater water absorption capacity.
Specific Gravity
The specific gravity of recycled aggregates is also lower than that of natural aggregates.
Specific gravity is the ratio of aggregates’ weight to an equal volume of water.
Thus, the aggregate type with the higher weight will have a higher specific gravity.
Sadly, because of residual mortar, recycled aggregates are lighter than a comparable volume of natural aggregates. Hence, RCAs have lower specific gravity.
You can use this Gilson SG-20 Specific Gravity Bench to test the specific gravity.
Strength
Recycled concrete aggregate (RCA) is generally weaker than natural aggregate. This can be traced to the mortar content that makes RCA more porous.
Since recycled aggregate concrete has higher water absorption capacity, it performs lower than natural aggregate concrete in strength tests, including compressive and flexural strength tests. Check out our Concrete Compression Testing Machines.
Is Recycled Concrete Aggregate Durable?
You can use recycled aggregates to produce durable concrete for several construction applications. But only if you use high-quality aggregates, do proper testing, have good quality control and adhere to industry guidelines, have a good mix design, and use RCA for appropriate applications.
This is because the durability of recycled aggregate concrete depends on several factors, such as:
Quality of Aggregates
Residual mortar adhering to RCA degrades its chemical and physical properties, reducing the concrete’s strength and making it less durable.
Thus, using higher-quality recycled aggregates (with less residual mortar and other contaminants) helps make your concrete stronger and more durable. Understanding the material properties of recycled aggregates is crucial for producing durable concrete.
Quality Control
Suppose chlorides and sulfates are in the original concrete crushed to produce the RCA. In that case, these contaminants come into the fresh concrete and contribute to concrete deterioration mechanisms that reduce the durability of your concrete works.
For example, sulfates in concrete start the chemical breakdown mechanism called sulfate attack.
In the attack, the sulfate ions react with components of the hardened concrete, causing the concrete to expand. As the concrete pushes out, it can crack and disintegrate.
Also, chloride ions in concrete cause corrosion of steel reinforcement, undermining the structural integrity of concrete works.
Interestingly, proper testing and quality control can help you assess the level of these contaminants and take steps to avoid using recycled aggregates that compromise the durability of your concrete works.
Proper Testing and Evaluation
Recycled aggregates from certain sources contain high levels of siliceous minerals that react with the alkalinity of concrete.
The alkali-silica reaction (ASR) produces an expansive gel that causes expansion in the concrete after the cement sets, leading to cracking.
Proper testing and evaluation can help you identify siliceous minerals in recycled aggregates and avoid potential ASR issues that undermine the durability of your concrete works.
Check out our aggregate durability test equipment
Mix Design
Mix design increases the resistance of concrete against freeze-thaw – a process that can lead to the cracking of concrete in cold areas.
Freeze-thaw resistance is the ability of concrete to withstand the destructive forces of cyclic freezing and thawing.
When water seeps into pores, it freezes and expands at low temperatures, producing a tremendous pressure that can cause cracking.
Interestingly, careful mix design after consideration of porosity and air entrainment will help you increase concrete’s ability to withstand freeze-thaw cycles.
For example, applying a lower water-cement ratio in the mix and adequate curing increase a concrete’s freeze-thaw resistance. Adding air entrainment agents/ admixtures to the mix to fight low porosity also increases free-thaw resistance.
Appropriate Applications
The choice of construction application for concrete with RCA can affect durability.
Recycled aggregates vary in quality. Some, like unprocessed RCA, are suitable for non-structural uses. However, processed RCA, screened for contaminants, can be used in structural applications.
Thus, to ensure the durability of recycled aggregate concrete, use the recycled material for appropriate construction applications.
Pros of Recycled Concrete Aggregate
Recycled concrete aggregate comes with many advantages. These include:
Reduced Costs
RCA is significantly less expensive than virgin aggregates. So using these recycled materials leads to significant cost savings in construction projects.
Lower Carbon Footprint
RCA has environmental benefits, as less energy is required to produce RCA compared to natural aggregate. Reduced energy consumption means reduced greenhouse emissions. So RCAs have a lower carbon footprint, making them more eco-friendly. Using recycled aggregates supports green building initiatives by reducing the carbon footprint of construction projects.
Conserves Natural Resources
RCA helps conserve our natural resources by not involving mining new aggregates. By minimizing the need for new aggregates, RCA contributes to making the construction industry sustainable.
Reduces Construction Waste
Construction wastes are typically sent to landfills. But with the construction technology now allowing crushing concrete elements and recycling them into aggregates, CDW that goes to landfills is reduced.
Demolished Portland cement concrete structures and other concrete elements are no longer considered waste but important materials for producing aggregates.
Versatile Use
RCA has many construction uses. It can be used in structural applications, such as constructing roads and bridge foundations.
They can also be used in several non-structural functions where using natural aggregate will be cost prohibitive (such as landscaping and beautification).
Enhanced Public Image
In this world of ethical considerations, using recycled construction materials like RCA can be a good weapon in your public relations arsenal.
With RCA having reduced environmental impact and contributing to resource conservation, using it in your construction projects will showcase your company as one committed to sustainable and responsible practices.
Cons of Recycled Concrete Aggregates
While RCA offers numerous advantages, it is not disadvantage-free. Some disadvantages of these recycled construction materials to consider before using them include the following:
Variable Quality
The quality of recycled aggregate varies based on different factors, including the source of the RCA, the processing methods, and specific contaminants.
When using different supply batches of these materials, you’ll face the problem of inconsistent quality, making it difficult to predict the performance of the concrete mix. Performance variability is a challenge when using recycled aggregates in construction.
Potential Contamination
Recycled aggregate comes with contaminants that degrade its physical and chemical properties, negatively impacting the performance of the concrete.
Reduced Strength
Adhered mortar on RCA makes it more porous. So, recycled aggregate concrete performs lower than conventional concrete (with natural aggregate) in strength tests.
Still Requires Natural Aggregate
Many applications require combining recycled aggregate with natural aggregate due to its reduced strength.
Also, when combining the two aggregate types in specific applications, determine the right ratio because a higher-than-normal percentage of RCA will significantly impact the strength of the construction work.
Uneven Grading
Combining RCA, Recycled Aggregates in Concrete, with natural aggregate introduces the problem of uneven grading. Properly matching the grading of the two concrete types increases the concrete’s workability and performance.
You may also like: Finding The Workability of Concrete With The Slump Test.
Takeaway: Use Recycled Aggregate for Your Construction Projects and Reap Economic and Environmental Benefits
Introducing waste materials into new products offers cost benefits. Advances in construction technology now enable the recycling and use of previously used concrete aggregates in new construction projects.
Recycled aggregate finds innovative uses in various applications, from serving as fill material and landscaping to constructing structural elements such as beams and columns.
By taking certain steps, including conducting proper testing and maintaining good quality control, you can produce concrete for various construction functions using recycled aggregate.
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Ready to take advantage of the cost savings and other benefits of RCA? Ensure you will get durable recycled aggregate concrete by equipping yourself with equipment for proper testing and quality control. See our wide range our aggregate durability test equipment!
