Binders play a vital role in the construction of roads and their performance over time, as they bind the aggregates together to withstand traffic load and harsh weather conditions. Despite significant inroads by Portland Cement as a binding medium, it is the bituminous binders of different types and grades that are predominantly used for road and highway constructions.
What are Bituminous Binders?
Bituminous binders are made of bitumen, which is a dark-coloured substance found as a byproduct of crude oil that becomes solid when cold and viscous when heated or processed with some diluting agents.
Roadway construction extensively utilises bituminous binders due to their excellent binding characteristics, waterproofing properties, and relatively low cost. They act like glue to hold together aggregates (such as crushed stone, sand, and gravel) in asphalt mixtures, creating a durable, flexible surface.
Chemical Composition of Bitumen
- Bitumen is a condensed hydrocarbon mixture made up of 300 to 2000 chemical components.
- It is composed of 87% carbon and 8% hydrogen. It also contains 5% sulphur, 1% nitrogen, and 1% oxygen, as well as other metals.
- Asphaltenes, Polar and Nonpolar aromatic hydrocarbons, and Saturates are among the chemical components of bitumen.
What are the different forms of bitumen binders for road construction and maintenance?
Depending on the proper binders or modifiers, bituminous binders used in road construction can be of many types and forms. Modifiers or binders are compounds that lessen the susceptibility of bitumen to temperature changes over a wide range of temperatures. Bituminous Binders are classified as follows:
- Cutback Bitumen
- Bitumen Emulsion
- Bituminous primer
- Modified Bitumen
Asphalt is made up of fine and coarse aggregates and bitumen as the binder. Asphalt has a bitumen percentage ranging from 4% to 7%.
Asphalt mix is commonly used for making the surface layer in road construction. The quality and characteristics of the asphalt mix aggregate is highly dependent on the type, size, and amount of aggregate used in the mixture. These characteristics can be altered to produce the road construction ideal for expected traffic load, climatic, and other conditions. Asphalts are classified into three types:
- Hot asphalt:
Hot asphalt is produced by heating bitumen and aggregates in a factory. About 95% of the particles in hot mix asphalt are made up of stone, sand, or gravel that has been bitumen-bonded.
- Cold asphalt:
Cold mix asphalt is made by combining unheated aggregates with emulsified bitumen, cutback bitumen, or foamed bitumen. Because this technique does not require aggregate heating, cold asphalt is both cost-effective and less-polluting.
- Protective asphalts:
Protective asphalt is typically employed as a temporary coating in road construction and is not one of the essential components of the road body. This layer is usually applied to keep the main asphalt layer impermeable and protected from eroding. It also strengthens the surface layer of the road.
Cutback bitumen is created by mixing a solvent with bitumen to make it less viscous and more liquid. Its main advantage is that it does not need to be heated before use. Petroleum extracts such as kerosene, naphtha, diesel oil, and furnace oil are added to bitumen to act as a solvent. To reduce the requirement for pre-application heating. However, once the bitumen is sprayed, the solvent gets evaporated.
This procedure is used to temporarily reduce bitumen viscosity, making cutback bitumen especially suitable for cold areas. The binder cures when the solvent is evaporated, cementing the aggregate particles together. The type of solvent employed impacts the rate at which the bitumen cures when exposed to air. Curing refers to the rate at which the solvent evaporates and the binder hardens.
Depending on the use of different types of solvents, cutback bitumen can be classified into three types:
- Rapid Curing (RC); the solvents used are Naphtha or petrol
- Medium Curing (MC); the solvent used is kerosene
- Slow Curing (SC); the solvent used is light oil
Each type of cutback bitumen is further classified into various grades.
RC grades are: RC 70, RC 250, RC 800, and RC 3000
MC grades are: MC 30, MC 70, MC 250, MC 800, and MC 3000
SC grades are: SC 70, SC 250, SC 800, and SC 3000.
Bitumen emulsion is a liquid product in which bitumen is mixed with water. But since bitumen is an oily product and cannot mix with water, an emulsifier is added to water before mixing. The emulsifier works as a surface active agent that helps to break bitumen into minuscule droplets suspended and dispersed.
In a bitumen emulsion, the bitumen percentage is kept around 60% and the rest is water. When sprayed on the road, the water particles in the emulsion are released and evaporated, causing the bitumen to settle and harden. The time taken for setting of bitumen varies with the different grades of bitumen.
Bitumen emulsion can be classified in two different ways:
- Based on surface charge
- Based on setting time
Based on Surface Charge
The emulsifiers used in bitumen emulsifiers are ionic types that create surface charge. So, depending upon the type of surface charge bitumen emulsifiers are divided into the following two types:
- Anionic Bitumen Emulsion
- Cationic Bitumen Emulsion
Anionic bitumen emulsions contain electro-negatively charged bitumen particles, whereas cat-ionic bitumen emulsions contain electron-positively charged bitumen particles. The cationic bitumen emulsion is most commonly used today.
The mineral composition of the aggregates used for construction determines the type of bitumen emulsion to be employed (anionic or cationic). For example, if silica-rich aggregates are used, the surface will be electro-negatively charged, and the bitumen emulsion to be used should be cationic type. This will aid in bitumen spreading and binding with aggregates.
Based on Setting Time
When bitumen emulsions are sprayed on aggregates, water begins to evaporate, resulting in bitumen separation from water. The surface of the aggregates then becomes covered with bitumen, acting as a binding material gradually getting hardened.
So, when based on the pace at which water evaporates and bitumen particles separate from water, bitumen emulsion is categorised into three categories
- Rapid Setting Emulsion (RS)
- Medium Setting Emulsion (MS)
- Slow Setting Emulsion (SS)
Bitumen primer is low-viscosity asphalt applied as an adhesive to create bonding between the loosely held aggregate particles. This is done to prepare the base course on top of which the surface course of asphalt mixes is laid. The application of bitumen primer helps in the following ways:.
- Coat and adhere loose particles on the base of the surface layer.
- Harden or toughen the base course to create a stable platform for construction equipment.
- Plug capillary gaps in the base course surface to avoid moisture migration.
- Ensure adhesion between the base course and the upper layer asphalt course.
Bituminous primers are typically made on road construction sites by combining penetrating bitumen with petroleum distillate.
Modified bitumen is bitumen that has been treated with various modifiers. Bitumen modifiers are additives or blends of additives that can improve the qualities of bitumen and bituminous mixes. Polymer modified bitumen (PMB)/ crumb rubber modified bitumen (CRMB) should be utilised only in wearing courses where extreme climate fluctuations are required. The following are the benefits of utilising modified bitumen:
- Increased protection of road surfaces from temperature fluctuations
- Higher deformation resistance at high pavement temperatures
- Improved ageing resistance characteristics
- Longer fatigue life of the asphalt mix
- Improved aggregate and binder adhesion
- Crack prevention and reflective cracking