Utilization of recycled concrete aggregate in bituminous mixtures

India is constructing roads at a record pace: however, the non-availability of quality aggregates is a problem. At the same time, consumption of large quantity of aggregates causes depletion of the natural resources. The  production of natural aggregates  is environmentally harmful  too due to  quarrying and blasting.  This is a major concern and challenge that the construction industry is facing nowadays.  Dr. Bhupendra Singh, Assistant Professor, Department of Civil and Infrastructure Engineering at  IIT Jodhpur writes on the advantages of replacing natural or virgin aggregates  with  C&D waste.

The majority of roads in India are flexible pavements, with most having a bituminous surface layer. In bituminous mixtures, or bituminous concrete, bitumen accounts for less than 10% of the total composition, while aggregates make up over 90%.  Another significant issue is the generation of construction and demolition waste. It consists of various materials, including concrete, bricks, wood, glass, plastic, broken asphalt, and metals – essentially, anything found in a building. Among these components, concrete is the predominant constituent of construction and demolition waste.

When these concrete blocks are recycled to extract aggregates, the result is known as recycled concrete aggregate. RCA comprises natural aggregates with an adhered layer of mortar, as these aggregates were once part of concrete structures, retaining some of the original mortar. Over the years, researchers have utilized RCA in various applications, including unbound pavement layers, cement concrete, and bituminous concrete.

Using C&D waste to replace natural or virgin aggregates offers several advantages. Firstly, it helps conserve natural resources by reducing the demand for virgin aggregates. Secondly, the production of recycled concrete aggregate consumes significantly less energy compared to the production of virgin aggregates. This lower energy consumption translates into additional benefits, such as reduced production costs and lower greenhouse gas emissions.

The production of RCA

 The process involves several steps, beginning with the generation of construction and demolition waste. There are two primary sources of C&D waste: artificial and natural. The artificial source arises from routine construction and demolition activities, while the natural source results from disasters such as earthquakes, floods, and cyclones.

The C&D waste obtained in the form of large concrete chunks undergo primary treatment, where they are broken down into more manageable pieces, resulting in crushed waste. The next stage focuses either on removing the adhered mortar layer or reinforcing it in place.

For removing the adhered mortar, several techniques are used, including mechanical, thermal, ultrasonic, and acid soaking treatments. Mechanical treatment is the most common approach, typically involving rotating equipment such as concrete mixers, Los Angeles abrasion machines, or specialized devices combined with steel balls. This process uses abrasion and attrition to remove the mortar layer adhered to the aggregate surface.

In thermal treatment, the adhered mortar is removed by heating the primary treated C&D waste to a high temperature. Due to the differing coefficients of thermal expansion between the aggregate and mortar, this heating generates thermal stress, causing the mortar to detach from the aggregate surface. Similarly, in the ultrasonic treatment method, the C&D waste is submerged in water, and ultrasonic waves are applied, which helps remove the adhered mortar. In the acid soaking method, the C&D waste is soaked in an acid solution, where the acid reacts with the hydration products in the mortar, effectively removing it from the aggregate surface.

The second category of treatment processes focuses on reinforcing the adhered mortar in place. One such method is carbonation treatment, where CO2 reacts with the adhered mortar to form calcium carbonate (CaCO2), which densifies and strengthens the mortar, securing it in place. Another approach involves treating RCA with cementitious or pozzolanic materials, which help to stabilize the adhered mortar. When using RCA for asphalt mixtures, the aggregates can be coated with bitumen emulsion.

This emulsion seals the pores of the RCA, enhancing the bond between the adhered mortar and the aggregates, thereby addressing the issue of mortar detachment in the mix. Additionally, the sealing of pores reduces water absorption, further improving the RCA’s performance. Researchers have also explored the use of liquid silicon resin for similar purposes, which helps to reinforce the bond between the adhered mortar and the aggregate. More recently, Microbial Calcite Precipitation(MCP) has been investigated, where bacteria are applied to the aggregate surface. These bacteria produce calcium carbonate crystals, which enhance the bond between the adhered mortar and the concrete.

Physical Properties

When comparing the physical properties of RCA with natural virgin aggregates, RCA typically exhibits higher water absorption, higher aggregate impact value, higher abrasion value, and lower specific gravity. To enhance the properties of RCA, researchers and practitioners have employed various secondary modification techniques, such as mechanical treatment, thermal treatment, acid soaking, pre-coating with slag cement, calcium carbonate precipitation and MCP, both individually and in combination. These methods have generally resulted in significant improvements, with many studies reporting increased specific gravity, reduced water absorption, and lower LA abrasion values.

The use of RCA in asphalt mixtures has produced mixed results, largely due to the variable nature of RCA itself. The properties of RCA are closely tied to the history of the original cement concrete. Some studies have reported positive effects of incorporating RCA into asphalt concrete. Factors such as the rearrangement of aggregate gradation due to crushed adhered mortar, residual cementing activity from unhydrated cement particles, and the rough surface texture of RCA contribute to these beneficial outcomes.

Researchers have further explored various methods to enhance the properties and performance of RCA in bituminous mixtures. These methods include calcination, applying pre-coating pastes to RCA surfaces, using organic silicon resin to activate RCA, MCP, pre-coating RCA with bitumen emulsion, and incorporating lime, cement, and other filler materials. Most of these treatment approaches have demonstrated significant positive effects, indicating that RCA can effectively replace a portion of virgin aggregates without compromising the quality or performance of asphalt mixtures.