The usage of construction and demolition wastes as an aggregate for the manufacture of concrete is considered in new research studies. The usage of construction and demolition waste aggregates helps in reducing the depletion of natural aggregates and problems related to mining the aggregates.
It is found that the quality of natural aggregates is better compared to the construction and demolition waste aggregates. Thus, demolition waste aggregates have limited usage.
But the reduced cost of manufacture of concrete with the help of construction and demolition waste aggregates must be gained appreciation. The rule over the reduction of disposal of demolition waste has prompted the usage of CDW as construction aggregates.
The life cycle of construction materials can be made more performing with the help of recycled construction demolition waste. There is government laws and regulation that are put forward to increase the use of recycled construction demolition waste material. This has made the recycling amount of construction and demolition waste material by an amount of 90%.
Standardization of Construction Demolition Waste Aggregate
The incorporation of demolition waste material as an aggregate in the concrete mix requires standardization similar to that compared to the natural aggregates. This is because the concrete mix almost composes 75% of the aggregates.
The construction and demolition waste aggregates are classified by BS 8500 (2002). The code classifies this into two types as:
- Recycled Concrete Aggregates (RCA)
- Recycled Aggregates (RA)
The concrete aggregate that contains 95% of crushed concrete is called as recycled concrete aggregates. And if the aggregate used in the concrete is 100% crushed, it is called as recycled aggregates.
The construction and demolition waste in its original state consists of wood, gypsum, plastics and many contaminating materials, that have to be removed because of its usage in concrete manufacture.
Preparation of Construction and Demolition Waste Aggregates
The CDW aggregates have been recycled and produced in many countries. The aggregate preparation is done by breaking the CDW into small pieces that of natural aggregates mechanically. The pieces are later crushed by means crushers into further smaller pieces.
Next, similar to the grading of conventional aggregates, these are screened by means of sieve analysis and hence used as standard aggregates.
As mentioned before the waste from demolition is contaminated by many waste materials that are used in construction. So, the removal of these materials will affect the performance of the aggregates.
The rubbles used in the construction, after demolition, are covered by a mortar that too must removed for the usage as aggregates. Two typical plants that represent the recycling of recycled aggregates are shown in figure-1 and figure-2.
Fig.1: A Construction and Demolition Waste Plant, As per Gonzalez – Fonteboa (2008)
Fig.2: Construction and Demolition Waste Plant as per Eguchi et.al (2007)
The production of CDW aggregates are factors that would determine the quality and the material composition. The good processing of concrete helps in a better quality of aggregates (Nagataki et.al 2004).
Composition of Construction and Demolition Aggregates
It was reported that the CDW contains some amount of some natural aggregates that have adhered mortar. These have the origin from the precast concrete and test specimens. The CDW composition depends upon the type of demolition waste as well as the type of aggregate used for that construction.
A typical construction and demolition waste aggregate is said to have 65-70% of major coarse as well as fine aggregates, with 30-35% of cement paste.
Table-1: Composition of different materials in construction wastes as per Coelho and de Brito 2011
Different Classification of Recycled Aggregates from the construction and demolition waste
The aggregate recycled from Construction demolition waste must satisfy certain requirement of particle size as well as the minimum presence of contaminants. These parameters stand along the requirement of durability and stability of these materials.
No kind of unexpected or depletion- causing reactions must be taken place by the aggregates, with the cement or the armour. It should possess perfect shape plus particle size that would help in acceptable workability for the concrete.
The various classification of recycled aggregates from the construction demolition waste by different institution in different countries. Some of them are:
Classification based on Water absorption and Loss in Weight
Table-2: Classification of Recycled Aggregates from Construction demolition waste as per Ministry of Construction Japan (As per Kawana, 2000)
Classification based on Origin
This type of classification was put forward by Rilem (1994) for aggregates that are recycled from the major material in construction as,
- Type I – Masonry Waste originated Aggregates
- Type II – Concrete Waste originated Aggregates
- Type III- Natural aggregates + recycled aggregates; This type will be having a range of 80% of natural aggregate and 10 % type I aggregates.
Fig.3. Recycled Aggregates from Masonry Construction Demolition
Classification Based on Constituents
This classification is put forward by the European standard, EN 12620. Here a detailed sorting of recycled aggregates based on its individual constituents is made as shown in table-3.
One example to show what exactly the code wishes to convey is, the names RCU90, RB10, RA5 means that an aggregate containing concrete 90% by mass, 10 % lesser masonry, bituminous material by 5 % and so on.
Fig.4: Recycled Waste Aggregates from Concrete Waste
Table-3: Different Categories of Recycled Aggregates based on Constituent Present
Table-4: The Constituents present in Construction Waste Demolition, as mentioned to table 3
Key Characteristics of Recycled Aggregates from Construction Demolition Waste
- Elongated aggregates
- Rough Texture
- High Porosity
- Fissured Surface
- Higher Water Requirement