Recycled Aggregate Concrete
Recycled Aggregate Concrete (RAC) is basically concrete mix that is prepared from crushed aggregates and concrete which has already been used in construction works. In the sector of construction engineering the key role of an engineer is to provide adequate quality products with the available materials. The products should be economical and ecological which will sustain the possible adverse situations.
Use of good quality materials all the time is not a sign of good engineering as it can be costly and energy and natural resources consuming. From this point of view, the idea of recycling of concrete to produce recycled aggregate (RA) is developed. The use of recycled aggregate concrete in this time of modern civilization opens a whole new range of possibilities.
The requirement for coarse recycled concrete aggregate (RCA) and coarse RA is specified in BS 8500-2: 2002 which is given the image below.
Importance of Recycling Aggregates for Concrete
Recycled aggregates are collected from CDW (Construction and demolition waste) as the amount of CDW has increased significantly over past few years. From a study it is estimated that nearly 70 million tones CDW is produced each year. A construction has to be demolished after it has passed its service life. So recycle and reuse of CDW have become necessary for environment.
Due to rapid industrialization and civilization there is a need of huge amount of primary aggregates. Use of recycled aggregates are required for sustainable developments as it conserves natural aggregates and reduces the space required to dispose demolition wastes. It also reduces consumption of energy.
Recycling of concrete is economical as it reduces the cost by 34%-41%. It also reduces Carbon Dioxide emission by 23%-28% as the cement can also be retrieved from the recycling process. Therefore the emission of CO2 in the production of new cement is less than before. As a result recycled aggregate is considered to be green construction material.
Here we are reusing crushed hardened concrete in the form of aggregate. From experiments it has found that the compressive, tensile and flexural strengths of recycled aggregate concrete is nearly of 85% to 95% of natural aggregate concretes. It has adequate durability and can be used versatile. From experiment it has found that 30% of natural course aggregates can be replaced by recycled aggregates without affecting the significant characteristics.
Sources of recycled aggregate
A huge amount of aggregates for recycling is available from different sources. Here are some examples
- Demolished waste generated due to natural disaster
- CDW generated for development of new buildings replacing old ones
- Tested specimen from laboratory
- Crushing of Portland concrete pavement
- Concrete debris generated from war destruction
When a structure is demolished, the particles are of larger sizes. So to make the processing easier the particles need to be crushed down to 300mm.
Steps included in recycling aggregates
75% of the concrete is made of aggregates. Firstly, the contaminants like reinforced steel, foundation material, plastic, wood, bitumen and soils are removed from concrete. . Methods of separation are as follows:
- Magnetic screening
- Using electromagnets
- Air separation
Secondly, the concrete is crushed mechanically or manually in the desired grain size which is usually from 20mm to 50mm. Different types of crusher are used that use impact force to crush larger stones. Namely,
Concrete is compressed between a stationary and a moveable plate by jaws. Thus concrete is reduced in size while travelling along the length of the wedge between two plates.
Here concrete is compressed between two cone shaped plates. This machine typically produce aggregate of 1.5” minus.
The mechanism involves a spinning rotor with bars that fling the concrete to one or several solid plates or rods.
Some other examples are: gyratory crusher, cone crusher, mineral sizer, horizontal and vertical shaft impactors etc. These crushers can be portable, mobile or stationary.
Finally, they need to be washed. Fine and course materials are separated as per size requirement. Here the recovered aggregate contains coarse aggregate and coarse sand. Fine sands are settled down from suspension and wash water is recycled for reuse.
Contaminants in Recycled Aggregate Concrete
Depending on the source of CDW for example concrete, bricks, tiles, rubber, reinforced concrete, the types of contaminants are different. This contaminants reduce the quality of RAC but cannot be removed fully for economic reasons. In turn, an allowance is made ensuring a certain quality control and so recycled concrete aggregate is a marginal material, not a good quality material. Followings are some examples of contaminants and their effects
- Recycled aggregate derived from rigid pavement with layer of bitumen has a reduction in compressive strength by 30% due to the presence of asphalt by 30% volume.
- Again existing mortar causes degradation in concrete properties but the removal process is costly.
- Organic matter includes wood, paper, fabrics, joint seal and these materials are unstable when aggregate is subjected to freezing and thawing.
- Chlorides and sulfates are harmful for reinforced concrete as they can cause corrosion of steel. Again sulfate is responsible for excessive expansion of concrete in damp condition.
Uses of Recycled Aggregate
Recycled aggregates can be of two types – processed and unprocessed. Different types can be used for different types of construction works.
- Unprocessed recycled aggregates are used for:
- Bulk fill
- Base fill for drainage structure
- Aggregate for road construction
- Riverbank protection
Processed recycled aggregates are used for:
- Aggregate in lean concrete
- Aggregate in bituminous concrete
- Structural works in bridges
- Median barriers
Microstructure of Recycled Aggregate Concrete
The interfacial transition zone (ITZ) is the most important part of RAC as it has more ITZs than NAC. NAC has one ITZ between the original aggregate and the hardened cement paste. But in case of RAC there are two ITZs. One is old ITZ between original aggregate and residual material. Another is between the residual mortar and the hardened cement paste.
Influence on properties of fresh concrete from Recycled Aggregates
Durability of RAC is affected by various features like aggregate replacement ratio, concrete age, W/C ratio, moisture content. Concrete has lower resistance to carbonation, chloride penetration, reinforcement corrosion. The carbonation depth in RAC is higher because of the presence of an extra interfacial transition zone. So high porosity is formed in RAC which reduces durability.
From freeze and thaw test, it is seen in case of coarse aggregates RA losses can be 1%-12% more than NA and in case of fine aggregates RA losses can be 10%-33% more than NA. This resistance depends on the air entrainment of both the hardened cement paste and the residual material like mortar.
From soundness test of concrete, it is found the volume loss is greater for RAC. This loss results from chunks of mortars present in RA as they are fracture and de-bonded from the aggregate.
If the mortar is of high strength, the volume loss can be less. So the properties of RAC particularly depend on the quality of the mortar as the recycled aggregate is made of natural aggregate and mortar. Use of fly ash, slag cement, silica fume can increase the durability.
Shrinkage and creep of RAC is about 25% to 35% higher than NAC. The shrinkage of RAC is about 0.55-0.8 mm/m at 90 days whereas the value for NAC is 0.3 mm/m.
Workability of Recycled Aggregate Concrete
RAC has higher water absorption capacity because the contaminants become the primary absorber. This increased absorption capacity and porosity result in low specific gravity and low bulk density. So high water content is required to achieve the same workability as NAC.
From slump test it has been found that more than 50% recycled coarse aggregate have workability related problems. Following is the physical properties of conventional and recycled concrete:
The air content in RA is 4%-5.5% higher than natural aggregates.
Influence Of Recycling on properties of hardened concrete
Compressive strength of concrete is reduced from 5% to 25% by using of 50% to 100% recycled aggregate. By using 30% recycled aggregates with 70% natural aggregates, full compressive strength is gained. However, 7day strength of recycled aggregate concrete (RAC) is less than natural aggregate concrete (NAC). So we can conclude strength gain of RAC is less than NAC.
Flexural and Tensile Strength
In case of flexural strength and shear strength, there is 10% reduction in 28 days strength of concrete when recycled aggregate is used compared to virgin concrete. Because during demolition, weaker spots are formed on recycled aggregate. So the effects of RA on tensile and flexural strength cannot be found with much clarity. A test result is given below.
The reduction in bond strength is only 10% with 100% recycled coarse aggregate. Thus, this effect can be neglected.
Modulus of Elasticity
Modulus of elasticity of RAC is found to be significantly reduced than NAC. By using 30% of recycled aggregate, 85% of modulus of elasticity of NAC is gained. Again, the lower the difference between the modulus of elasticity of hardened cement paste and the residual mortar, the lower the reduction in the total modulus of elasticity of the RAC.
Acid treatment is a treatment method used to restore the properties of RAC. These treatment methods are briefly discussed
- 10% N HCL increases the compressive strength of 28days by 18% for crushed granite from 20 year old demolished structure causing 1.5% mortar loss.
- 10% N H2SO4 increases the compressive strength of 28days by 20% for crushed granite from 20 year old demolished structure causing 5.6% mortar loss.
- 0.5M HCL increases the compressive strength of 28days by 10% for crushed concrete cube from lab specimen structure causing 3% mortar loss.
Codes for Recycled Aggregate Concrete
There has not yet been developed any code by ACI. Some countries use their code for recycled aggregate.
Concrete Recycling Industries
Fish Creek Excavating Ltd situating in Canada is a factory which is recycling concrete and asphalt since 1995. Construction & Demolition Recycling Association (CDRA) in Chicago is another company to make recycled aggregate.
ELKON Concrete Recycling Plant is another company based in Turkey. SmartCrusher bv is a renowned concrete recycling factory in Netherlands. Apart from these many concrete recycling plants are available through out the world.
Disadvantages of RAC
Using RAC has mixed impacts. Such as
- High percentage of RA can lead to cracks and several faults in the structure.
- If recycling plant is not nearby, transportation cost of RA becomes costly than using fresh aggregates.
- If not refined properly, dangerous structure failure can happen.
- The grading of recycled aggregate and natural aggregate should match properly to avoid uneven grading.
- Supplementary materials need to be added to make the structure as strong and durable as possible.
- Serviceability of the structure can be hampered if proper mixing is not done.
- Using of RAC in earthquake zones can be harmful.
As the present world is focusing more on green technology, use of RAC has been essential. However, the replacement percentage of RA with NA need to tested in laboratory for each construction project to meet the requirement of the structure.
More researches are being conducted at this time as all the characteristics of RAC has not been discovered yet. Hence, we can hope recycled aggregate concrete (RAC) will take a large space in the construction technology in future.