Bitumen stabilized materials (BSMs) are produced from soft road bitumen and their popularity has gained traction owing to the development of the mix design and construction technologies with economic and environmental benefits. The two commonly used BSMs are:

  • emulsified bitumen 
  • foamed bitumen

Both can be adopted to stabilize a wide range of pavement materials. In the stabilization process, the bitumen is dispersed in a continuous matter, which is the unique feature of the BSM. If the bitumen, like cut-back bitumen, disperses in a material in a continuous manner, it cannot be considered as a stabilization process.

In this article, Global Road Technology evaluates BSMs looking at their advantages, their different types, and their properties. 

What are the advantages of BSMs?

There are several advantages of BSMs and in our discussion, they are given in comparison to cementitious stabilization agents. These include:

  • relatively flexible 
  • less shrinkage cracking
  • better cohesion
  • open to traffic immediately after construction
  • prevent the problem of raveling 
  • enhance material strength
  • resistance to moisture damage

Bitumen emulsions – manufacture, properties, and chemistry on application.

In the emulsification process, bitumen and water are put through a high shear mill with an emulsifier chemical, and eventually, the bitumen is dispersed in the water, and an oil-in-water condition forms. The emulsifier dominates the charge of the bitumen emulsion. Bitumen emulsion is commonly produced in plants and can be kept in barrels for several months if stored appropriately. Since the surface of the bitumen droplets is charged, the interface between the bitumen emulsion and aggregate is enhanced when the aggregate particles are oppositely charged. When emulsified bitumen is mixed with aggregate, the bitumen tends to separate from the water and adhere surround the surface of the aggregate particles. This process is known as breaking. The mineral source and moisture of aggregate have a significant influence on the emulsion breaking. In this process, the bitumen emulsion places the role of lubrication agent, and the breaking is required to occur before compaction. From the appearance observation of the treated materials, a speckled surface is observed owing to the non-uniform distribution of bitumen coated on fine and coarse particles. The material stabilized with bitumen emulsion is known as BSM emulsion. 

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What research has been done on emulsified bitumen? 

Emulsified bitumen has been extensively researched and studied over the past 15 years. The effects of curing time and moisture content on the properties of cold in-place recycled (CIR) mixtures have been investigated. The results revealed that compared to CIR-emulsion, CIR-foam specimens had less moisture content and more tensile strength at the same process, which may have a negative effect on the coating properties and further on the mixture performance. Polymer-modified bitumen emulsions offer better stability and durability than recycled mixtures. Cationic slow-setting emulsions are often mixed with aggregate containing high content of fine particles, for instance, the dense-graded mixtures, to endow the mixture with improved stability. No solvent is required in the process and the workable duration is sufficient. 

To achieve good mixture quality, the cationic slow-setting emulsion is required to mix with wet recycled asphalt pavements since the water in the emulsion tends to separate from bitumen through evaporation. The construction duration is prolonged when the slow set emulsion is used, and it is very sensitive to rain. The cationic medium set emulsion has also been used as a faster set emulsion. Cationic vs anionic emulsions have been studied with cement to evaluate the mechanical properties of mortars. The results showed that the cationic emulsion of the mortars had a higher elastic modulus and strengths when compared to the anionic emulsion. Designations of the different types of emulsions can change for different parts of the world, so it is important to determine which type of emulsion will perform best for which application. 

Foamed bitumen – manufacture, properties, and chemistry on application.

The foaming of bitumen is a physical process in which a predetermined amount of cold water is injected into hot bitumen in a series of individual expansion chambers in a base recycling machine. The process produces thousands of tiny bitumen bubbles that live contemporary meta-stable lives. As the colloidal mass cools at ambient temperature, the steam in the bubbles condenses causing bubbles to collapse and the foam to decay. This foaming process usually occurs in relatively small thick-walled steel tubes and the water along with the air is ejected out at high pressure in less than one minute. The expanded bitumen has a resulting high surface area available for bonding with the aggregate. Usually, the expanded volume of the asphalt can approach 10-20 percent. The bitumen bubbles will burst and produce lots of tiny bitumen splinters during mixing. At the same time, these bitumen splinters will disperse in the aggregate matrix and form a mastic by adhering with finer particles. The dispersing effect of the bitumen has a close relationship with the mixed moisture. If the mixture is compacted, the bitumen particles will bond with larger aggregate particles just like spot welding. The material stabilized with foamed bitumen is called BSM-foam. 

What research has been done on foam bitumen? 

Foamed bitumen used in cold recycling projects is attractive to agencies due to the advantages of high strength, the flexibility of BSM-foam, less construction period, less work of maintenance, and quick traffic opening. Results have shown that foaming temperature had a close relationship with mechanistic properties of cold recycling mixture treated with foamed bitumen. Comparison between BSM-emulsion and BSM-foam on trial sections showed similar performance between the two although foamed bitumen is a more promising BSM. At 1.3% water content studies have shown that optimum foaming characteristics can be produced. After several tests, the optimum foamed bitumen content was adjusted to 2.5%, with wet Indirect Tensile Strength (ITS) recommended as an effective index to determine the optimum foamed bitumen content. Findings have also shown that flow numbers varied significantly as the foamed bitumen content changed. However, when the content increased from 1 to 3%, the flow number did not change. 


Many factors determined the selection of suitable stabilization agents including price, availability, material characteristics, durability as well as the local government policy. The primary concern is always the unit cost. In some countries, the authorities often choose agents based on empirical experience rather than objective evaluation. There is no doubt that new proprietary products are on the way and no single stabilization agent can be employed for all applications. Great efforts have been made to study the application effectiveness of different stabilization agents. 


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