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The destruction of thousands of buildings during World War II gave the people questions about what to do with the rubble. It wasn t until the 70 s that the potential of rubble to be recycled was explored.
Today, concrete is the most widely used building material in the world. We use it for our houses, roads, buildings, bridges, and most of our structures around the world. Concrete after it s used, is sent directly to be disposed of in our landfills.
With a lot of attention going into the wellness of our environment, we recycle items such as paper and plastics, to reduce our impact on the environment. We believe that by recycling concrete, not only will our impact on the environment be reduced, but its economical advantages are enough reason to recycle concrete.
The purpose of this paper is to educate ourselves and the reader about what we can do to reduce our impact on the environment by utilizing rubble from demolition of buildings.
Environmental Impact http://www.torontoenvironment.org/gravel/impacts
To extract virgin aggregates, we have to dig open-pit mines. The aggregates are then blasted out or dug out. The most common effects are dust, pollution, noise, sedimentation.
The physical change to the surrounding land causes the most impact. All the vegetation must be removed causing a loss of wildlife and biodiversity. Improper engineering leads to improper drainage which causes erosion. In some cases, water used to wash the aggregates seep into the ground and contaminate ground water.
After a quarry is used, it is rehabilitated to be restored to its former condition. A study was done in Ontario and less than half of the land disturbed for aggregate production between 1992 and 2001 has actually been rehabilitated.
By using recycled aggregates, we can reduce the need to mine aggregates. We can use the energy that would otherwise be used for mining, transporting, and processing, to recycle used concrete from demolished structures.
Recycling concrete is a great alternative to disposing concrete. The use of recycled concrete reduces the waste that would otherwise be dumped into landfills. The amount of solid wastes at a municipal landfill is comprised of 23 - 33% of waste from demolition of buildings. Landfills don t get smaller. They will increase in size and reduce land that could be used for residential and commercial uses.
Economics of Recycled Concrete
Concrete is the most widely used building material in the world. Because we are highly dependent on
Concrete is cheaper to recycle than sent to landfills but there are limitations on economic savings. Location is a big factor when looking at the costs of recycling concrete. Typically, remote areas have very little to no access to recycling sites raising transportation costs.
During our research, we found that there is a limit where the cost of hauling to a recycling plant exceeds the cost of dumping in a local landfill. Here in Lethbridge for example, it costs $22.25 per tonne to dispose concrete rubble in landfills. The nearest recycling plant is in Calgary and would cost hundreds of dollars to transport the rubble.
Mobile concrete crushers can be used to recycle but they should only be used on large projects. Portable crushers are not widely available in all parts of the country. In some cases, rental and labour costs would exceed dumping costs.
Average hauling cost is $0.13/ton/km.
Processing costs for the recycler costs between $2.76 and $6.61 per metric ton. The larger the plant, the more efficient and more output it produces.
RCA products sell for between $1 and $18 per cubic meter, with the higher end being in aggregate-poor areas. Natural aggregates normally sell for around $8.
A big advantage of recycling is that you can charge disposal fees to companies that want to dispose their concrete rubble. The plant can dictate the cost of this fee and compensate for the lower recycled aggregate prices.
On the other hand, the cost of recycled concrete depends on market prices of virgin aggregates. Natural aggregates dominate the markets thus they dictate the prices of RCA.
Starting a concrete recycling business will require an investment of $4.40 to $8.80 per metric ton of annual capacity. USGS (U.S. Geological Survey) has done a thorough analysis on the costs of different sizes of recycling plants:
Based on their research, a big factor that affects the feasibility of starting a recycling plant depends on the amount of rubble that can be recycled. When a plant is not operating near full capacity, the profitability of the plant reduces.
It was found that small recycling plants will have a tougher time making profits. Small plants are heavily affected by the changing market conditions. But they can increase tipping fees or increase product price to compensate for the small profits. But in most cases, product pricing is out of control of the producer.
On the other hand, medium and large recycling plants are more profitable. Larger operations have lower operating costs (table 4) and do not need a tipping fee to have a rate of return of 12%.
Nearly all buildings today are partly made of concrete. A big part of recycling is what the material can be used for. There would be no point of recycling if there was no use for the end product.
The most common way of using aggregate is using it as a layer under roads called a sub base. The crushed concrete provides additional strength to the foundation of the roadway.
RCA can also be used as pipe bedding for underground utilities such as sewage and storm pipes. The RCA provides a stable foundation to lay the pipes on.
A problem with using RCA for structural grade concrete is that the density of the aggregate depends on the density of the original concrete. In some cases, the density of mortar reduces the strength of the aggregate. Typically, a mix of natural aggregates and recycled aggregates are used when RCA is used in structural grade concrete. It was found that up to 20% of RCA can be used before the compressive strength of the concrete is reduced. Before it is used in structural grade concrete, it should be tested and it should meet certain specifications set by ASTM. Concrete made from RCA is typically used for low-rise buildings but its use in large buildings is very limited.
Recycled concrete is being used for curbs, meridians, sidewalks, and gutters. The reduced specifications allow the RCA-concrete to be used in structures that don t require resisting heavy loads.
Recycled concrete doesn t necessarily have to be used as an aggregate. Rubble can and has been reused for other purposes in its natural state.
Recycled concrete is relatively cheap and abundant which makes it perfect for landscaping. Slabs of concrete can be shaped and used as pavers, while larger pieces can be stacked and laid with mortar to be used as a retaining wall.
Large pieces can also be used as riprap. The riprap acts as a barrier and absorbs impact from waves to reduce erosion along shorelines. It can protect structures from damage where water erosion is a problem.
Noise barriers can be built in residential areas along highways. The noise barrier can be built with RCA or it can be built similar to retaining walls to make it aesthetic.
Reasons to Recycle
Natural aggregates are inexhaustible. So why should we recycle?
Countries like here in Canada have enormous amounts of natural resources. We will never use all our resources but the distribution of resources around the world is different. Countries like Japan have very limited amounts of land. They rely heavily on importing raw materials.
Location is a big factor. Quarries can not be built anywhere. They have to be carefully planned and approved by the government before they can be built. Remote areas often have very little to no access to raw materials so it s more feasible to use what s already available.
With increasing attention towards global warming governments are setting regulations to reduce CO2 emissions. Companies are under a lot of pressure to reduce those emissions. Recycling concrete allows us to avoid transportation of materials which leads to less consumption of fossil fuels. Recycled concrete has a carbon footprint 65% less than equivalent products from quarried stone. (RMIT Life Cycle Analysis). If your company aims to reduce its environmental impact, recycling concrete is a great alternative.
Companies are afraid that using recycled aggregates will reduce the qualities of their products. Recycling concrete is a fairly new technology so companies are skeptical of its future. When the word recycled is mentioned, it is assumed that the quality is reduced but that is not necessarily true. Several research organizations have concluded that with proper mixtures, RCA can outperform natural aggregates in some ways.
Another reason to recycle is because of market conditions. Landfill space is becoming scarce and municipal governments are increasing landfill dumping costs. Recycling allows us to eliminate the landfill cost. In cases where portable plants are used, hauling costs are also eliminated. So not only are you potentially saving money, you are also reducing the concrete that goes to landfills.
As mentioned before, recycling concrete is new to the industry. Most companies have not been exposed to the new technology and are still reluctant to give it a chance.
It is a known fact that RCA has a lower quality than virgin aggregates. The quality varies from site to site. You will need a lot of quality control testing to ensure that the RCA will not reduce the quality of the concrete. This variance in quality does pose concerns about the quality of structures being made.
Most companies simply lack the experience with recycling and recycled products. When new construction methods are introduced, sufficient knowledge is required to achieve a high quality end product.
Future of recycling concrete
We believe that recycling concrete is the future of the industry. As time goes, more people will be exposed to this technology. The benefits far outweigh the disadvantages.
It will be widely accepted in the future whether its 5 years from now or 20 years from now.
When buildings are demolished, much of the left over materials are comprised of concrete, wood, and rebar.
The process begins after the building has been demolished. The recyclable materials are trucked to the recycling plant or a portable crusher is brought into the site. The main equipment used is a crusher. The rubble is fed into the crusher and the fragments of concrete are crushed into small pieces.
The crushed pieces are put on a conveyor belt where it leads to a magnet. The magnet picks up the reinforcing from the crushed materials and separates it into its own pile. Other materials such as wood are generally removed by hand.
The product from the magnet is then screened and separated into separate sizes. The crushed rubble goes through a screen mesh where desired sizes can fall through while larger pieces can be isolated. Depending on the specifications of aggregate required, the larger pieces can be sent to a secondary crusher where it can be crushed further.
The crushed pieces are separated into their own piles according to size. They are stockpiled until they are sold.
There are multiple types of crushers to consider:
* Jaw Crushers comprised of two jaws; one stationary and one moving. As the concrete moves down the plates, the concrete is crushed into smaller pieces. The spacing between the bottoms of the plates ensures that only small pieces make it through. The product ranges between 4 -8 in diameter. These types of crushers can be used as a primary or a secondary crusher.
* Impact Crushers Spinning rotors with bars or hammers fling the concrete into a solid plate causing the concrete to disintegrate into smaller pieces. The end product is a 2 aggregate. These types of crushers can be used as a primary or a secondary crusher.
* Cone Crushers - Concrete is crushed between two cone shaped plates. Cone crushers can handle pieces no larger than 8 which makes it suitable as a secondary crusher.
The location of the project affects the type of recycling plants to be considered.
* Stationary plants These are the same as the aggregate plants we see today. They are fixed to one spot. The material is trucked into the plant. Because they are stationary, they have no limitations on the size of the operations.
* Portable Recycling Plants These are generally put on a chassis and towed to the site. It remains stationary on site until it is moved to another location.
* Mobile Recycling Plant These are towed to the site like portable recycling plants but they have tracks which allow them to move around the site.
The type of plant used should be based on the amount of material to be recycled and it s location from the nearest plant. Stationary plants are more efficient at recycling large amounts of concrete due to larger size of operation. Portable and mobile recycling plants should only be considered where there is a large distance between the site and the nearest recycling plant.
Characteristics of Recycled Aggregate Concrete
For the mixture design the same process is followed as if you would use virgin aggregate. You would have to go through trial mixtures to get the right proportions and to check the concrete quality. Extra attention should be made on the higher absorption rate of the recycled aggregates, which influence would influence the batch mixture.
Mixing water and workability
Because of the high absorption of the recycled aggregates more mixing water and a higher starting slump may be needed. Recycled aggregates absorb water even after mixing, so to counteract this, the aggregate should be pre-wetted in their stock piles.
When starting it may be assumed that the compressive strength is the same for if you would use virgin aggregate as recycled concrete aggregate. The recycled concrete should contain both course aggregate and natural sand and the water-cement ratio should be adjusted accordingly if the compressive strength is lower than thought at first.
Compared to conventional concrete, in recycled aggregate concrete there should be higher cement content because of the higher fee water requirements for the recycled aggregate. Extra 5% cement would be required when coarse aggregate and virgin fines are used, and an extra 15% if both coarse and fine recycled aggregates are used.
Density and air content
New concrete will have a lower density ranging from 5% to 15%. The natural air content might be a little higher then conventional concrete and an air-entraining admixture is added if freeze-thaw durability is required.
The compressive resistance of recycled aggregate-concrete will be lower than that of concrete using natural aggregates. A study done by ECCO (Environmental Council of Concrete Organizations) concludes that recycled aggregate concrete will have a 5%-10% reduction of compressive resistance.
Characteristics of Recycled Concrete Aggregate
When the concrete rubble is crushed, the produced aggregates are screened according to size. They are then separated to desired gradation. Like natural aggregates, they are separated into fine aggregates or coarse aggregates.
Particle Shape and Size
Recycled coarse aggregate is similar in particle shape as crushed stone or natural aggregates. Fine and coarse aggregates are more angular which gives them a higher absorption which lowers the workability of concrete.
The amount of fine particles (<4.75mm in diameter) is estimated to be 5-20% of the total volume of the RCA.
Specific Gravity and Absorption
The specific gravity of RCA is much lower compared to natural aggregates. This is due to the cement and mortar that held the old concrete together before it was crushed. The SG of RCA ranges from 2.35 to 2.38.
The specific gravity increases as particle size as course particle size decreases. Te specific gravity greatly decreases as particle size decreases.
The porous nature of mortar and cement allows for higher absorption within the recycled aggregate. Coarse Aggregates generally absorb 2%-6% of water of its volume while fine aggregates generally absorb 4%-8% of their volume of water.
The bulk density of recycled concrete is generally lower than natural aggregates due to the lower density of the mortar.
The density of coarse RCA is around 2430 kg/m
The density of fine RCA is 2310 kg/m
Coarse virgin aggregates have a density of 2700 kg/m .
Coarse virgin aggregates have a density of 2590 kg/m .
The density of RCA will vary depending on the water/cement ratio and the type of concrete used in the original concrete mixture.
The ratio of density of the material to the density of water at a specified temperature is defined as the specific gravity of a material.
Compressive Strength of original concrete and recycled aggregate concrete for various water/cement and coarse/fine aggregate ratios