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What is bitumen?

Bitumen is viscous, nonvolatile liquid or solid. Bitumen is a complex colloid system the chemical properties of which are dependent on the properties of crude oil from which it is produced. Pure bitumen is a colloid dispersion of microscopic asphalt particles (dispersion phase). The chemical composition of bitumen is a mixture of various hydrocarbons with molecules of oxygen, sulphur, and nitrogen. Hydrocarbons present in bitumen are mostly condensed naphthene and aromatic rings with small number of side paraffin chains. Mass fraction of hydrocarbons is 75-85%, hydrogen 9-10%, oxygen 2-8%, sulphur 5-7%, and nitrogen 0.1-0.5%. Bitumen is partially or completely soluble in various organic solvents. Dissolved fractions of bitumen in solvent are called maltenes or petrolenes, and undissolved fractions are called aspaltenes. The maltenes are a mixture of resins and oil, and they are a disperse agent. For the most part, physical properties of bitumen depend on dispersion degree of asphaltenes in maltenes.

Properties of Bitumen
  • 1- Adhesion: Bitumen has the ability to adhere to a solid surface in a fluid state depending on the nature of the surface. The presence of water on the surface will prevent adhesion.
  • 2- Resistance to Water: Bitumen is water resistant. Under some conditions water may be absorbed by minute quantities of inorganic salts in the bitumen or filler in it.
  • 3. Hardness: To measure the hardness of bitumen, the penetration test is conducted, which measures the depth of penetration in tenths of mm. of a weighted needle in bitumen after a given time, at a known temperature. Commonly a weight of 100 gm is applied for 5 sec at a temperature of 77 °F. The penetration is a measure of hardness. Typical results are 10 for hard coating asphalt, 15 to 40 for roofing asphalt and up to 100 or more for water proofing bitumen.
  • 4. Viscosity and Flow: The viscous or flow properties of bitumen are of importance both at high temperature during processing and application and at low temperature to which bitumen is subjected during service. The flow properties of bitumen vary considerably with temperature and stress conditions. Deterioration, or loss of the desirable properties of bitumen, takes the form of hardening. Resultantly, decrease in adhesive and flow properties and an increase in the softening point temperature and coefficient of thermal expansion.
  • 5. Softening point: Softening point is the temperature at which a steel ball falls a known distance through the bitumen when the test assembly is heated at a known rate. Usually the test consist of a (3/8)in dia steel ball, weight 3.5 gm, which is allowed to sink through a (5/8) in dia, (1/4) in thick disk of bitumen in a brass ring. The whole assembly is heated at a rate of 9 °F per min. Typical values would be 240 °F for coating grade asphalts, 140 °F to 220 °F for roofing asphalt and down to 115 °F for bituminous water proofing material.
  • 6. Ductility: Ductility test is conducted to determine the amount bitumen will stretch at temperature below its softening point. A briquette having a cross sectional area of 1 in2 is placed in a tester at 77 °F. Ductility values ranges from 0 to over 150 depending on the type of bitumen.

Penetration grade bitumen

The penetration grading system was developed in the early 1900s to characterize the consistency of semi-solid asphalts. This system classified bitumen according to hardness. The hardness of a sample is determined by inserting a specific type of needle into it, and measuring the depth to witch the needle is able to penetrate the sample within a giving time and temperature. This test is called the standard penetration test, and the bitumen that is tested in this way is called penetration grade or road grade bitumen (Paving grades). Penetration grading’s basic assumption is that the less viscous the asphalt, the deeper the needle will penetrate. This penetration depth is empirically (albeit only roughly) correlated with asphalt binder performance. Therefore, asphalt binders with high penetration numbers (called “soft”) are used for cold climates while asphalt binders with low penetration numbers (called “hard”) are used for warm climates.

Penetration grading quantifies the following asphalt concrete characteristics:

  • Penetration depth
  • Flash point temperature
  • Ductility
  • Solubility in Trichloroethylene
  • Thin film oven Test

The Penetration numbers obtained by means of the penetration test indicate the hardness of the bitumen. 15”pen” bitumen is the hardest and 450pen bitumen the softest. In the construction industry, the term “penetration grade bitumen” is often abbreviated to “pen bitumen or simply “pen bit”.

The common penetration grades are listed in the table, these grades are used in paving regularly.

Penetration Grade (ASTM Method) Comments
Specific Gravity @ 25/25 °C 1.01-1.06
40-50 Hard grad; Used for warm climates
60-70 Typical grades used
85-100 Typical grades used
120-150 -
200-300 Softest grade; Used for cold climates
Cutback bitumen

Cutback bitumen is a range of binder that are produced by blending (mixing) penetration grade bitumen and a hydrocarbon solvent such as paraffin or mineral turpentine. Cutbacks are used because they reduce asphalt viscosity for lower temperature uses (tack coats, fog seals, slurry seals, stabilization material). Similar to emulsified asphalts, after a cutback asphalt is applied the petroleum solvent evaporates leaving behind asphalt cement residue on the surface to which it was applied. A cutback asphalt is said to “cure” as the petroleum solvent evaporates away. The use of cutback asphalts is decreasing because of

Cutback asphalts contain volatile chemicals that evaporate into the atmosphere. Emulsified asphalts evaporate water into the atmosphere.

The petroleum solvents used require higher amounts of energy to manufacture and are expensive compared to the water and emulsifying agents used in emulsified asphalts.

  • Environmental regulations.
  • Loss of high energy products

In many places, cutback bitumen use is restricted to patching materials for use in cold weather. Cutback bitumen gets its name from the solvent that is involved in the process, because the solvent “Cutback” or evaporates, leaving behind the binder to “get on with the job”. The solvent used in cutback bitumen is called the “cutter” or “flux”. Three types of solvents are used for the blending process: slow-curing, medium-curing solvents. The letter two are the most common in South Africa. The choice of solvent determines the rate at which the bitumen will cure when exposed to air. A rapid-curing (RC) solvent will evaporate faster than a medium-curing (MC) solvent. Curing relates to the evaporation rate of the solvent winch influenced the setting time of the bitumen. The viscosity of the cutback bitumen is determined by the proportion of solvent added: the higher the proportion of solvent, the lower the viscosity of the cutback. Cutbacks differ from penetration grade bitumen in that they are more workable in other words, they can be more easily reshaped. Less heat is required to liquefy Cutback bitumen than penetration bitumen, making it easier to use at lower temperatures. Typical cutback bitumen’s are MC30 and RC250. The letters in the name refer to the curing action of the solvent, and the number to the viscosity of the binder.

Application of Cutback Bitumen

Cutback bitumen’s suitable for primer sealing can also be used in the manufacture of pre-mix asphalt, which is used in patch repairs. Cutback bitumen’s are used extensively in sprayed sealing applications, particularly in cooler weather where they provide improved initial stone retention due to their lower viscosity. Typically, a single application of the appropriate cutback bitumen is sprayed onto the primed pavement onto which aggregate is laid.

    Other Application
  • Surface Dressing
  • Prime Coating
  • Tack Coating
  • Semi –Grouting /Penetration Macadam
  • Slurry Sealing
  • Fog Sealing /Mist Spraying
  • Dust Binding
Emulsion Bitumen

An emulsion is a stable mixture of extremely small particles of one liquid dispersed (spread) in another liquid in which it does not dissolve. A bitumen emulsion, for example, consists of fine droplets of bitumen dispersed in a liquid such as water, in which it cannot dissolved. An emulsion can be created by using an emulsifier or emulsifier agent which soap-like substance is used to coat each individual particle of another substance and so prevent these particles from sticking together. Emulsions are used because they effectively reduce bitumen viscosity for lower temperature uses (tack coats, fog seals, slurry seals, bituminous surface treatments (BST), stabilization material).

Most of the important properties of an emulsion depend on the type and amount of emulsifier used. Emulsifiers are divided into two categories: anionic and cationic emulsifier. Anionic Type: In which the bitumen particles are negatively charged and the emulsifier is fatty acid metallic soap. These suitable for use with calcareous aggregates like limestone. Cationic type: In which the bitumen particles are positively charged and the emulsifier used is a long chain amine. These are suitable for use with siliceous aggregates like quartzite, sandstone, granite etc.

Classification according to per setting characteristics

Bitumen emulsions are classified based on how quickly they set, along with their electrical charge. Based on the precise mix of ingredients, an emulsion bitumen may be designed to be rapid-setting (RS), medium-setting (MS), or slow-setting (SS). Cationic emulsions are designated with a letter C in front of the emulsion grade, while anionic emulsions are not given an extra letter. The complete list of emulsion grades is shown in the table below.

Anionic Emulsions Cationic Emulsions
RS-1 CRS-1
RS-2 CRS-2
MS-2 CMS-2
MS-2h CMS-2h
HFMS-1 -
HFMS-2 -
HFMS-2h -
HFMS-2s -
SS-1 CSS-1
SS-1h CSS-1h

A suffix of “h” or “s” at the end of the emulsion grade indicates that a hard or soft base asphalt was used. Some anionic emulsions are designated “HF” for “high float,” which indicates that chemicals were used to give the asphalt a thicker film. When applied to an aggregate surface, this helps more of the asphalt film stay in place on the aggregate particles without draining into the pavement structure.

Application of Bitumen Emulsion

AsBitumen emulsions behave more like water than bitumen; they can be used at much lower temperatures than bitumen, quite often at ambient temperature, and can be readily mixed with water. Bitumen emulsions are much less sensitive to problems caused by damp or dusty aggregate and cool conditions due to their water base. Anionic emulsions are best used with positively charged aggregate surfaces such as basalt, dolomite and limestone, whereas cationic emulsions are preferred for use with negatively charged silicious aggregates such as quartz, granite, sandstone and river gravel. In general, cationic emulsions can be used with a wider range of aggregates, will tolerate greater quantities of moisture, and will break at a lower ambient temperature. The main application areas of Emulsion bitumen are surface dressing, tack coats, slurry seal, and cold mixing.

1. Surface dressing
Surface Dressing is a method of resurfacing an existing road by binding a new layer of stone chippings to the road surface with bitumen. The process is not designed to reprofile the road, or to provide it with additional strength. Surface Dressing is a highly efficient and cost effective way of restoring a road's grip characteristics while sealing cracks to prevent water penetrating and damaging the lower layers and foundation. The main type of Surface Dressing binder is bitumen emulsion.

2. Tack coats
A tack coat is a thin bituminous such as, emulsion or cutback layer which applied between Hot mix Asphalt (HMA) pavement lifts to promote bonding. Adequate bonding between constructions lifts and especially between the existing road surface and an overlay is critical in order for the completed pavement structure to behave as a single unit and provide adequate strength. If adjacent layers do not bond to one another they essentially behave as multiple independent thin layers – none of which are designed to accommodate the anticipated traffic-imposed bending stresses. Inadequate bonding between layers can result in delamination (debonding) followed by longitudinal wheel path cracking, fatigue cracking, potholes, and other distresses such as rutting that greatly reduce pavement life.

3. Cold mixes
Cold mix asphalt is produced when a special type of emulsified bitumen is mixed with aggregates. The emulsion used reduces the viscosity of the asphalt, making it pliable even at cold temperatures. Cold mix asphalt can be premixed and stored for months at a time, making it ideal for road repairs and other low-volume uses. Cold mix asphalt also has a number of environmental benefits. Low-temperature mixing reduces fuel consumption and resulting fumes. Production of cold mix asphalt doesn’t require high temperatures, therefore, emissions from the production facility are also reduced.

4. Slurry seal
Slurry shall consist of mixture of approved bitumen-emulsion, fine mineral aggregate, filler, water and specified additives, proportioned, mixed and uniformly spread over properly prepared surface. The completed slurry seal shall leave a homogeneous material, adhere firmly to the prepared surface, and have friction-resistant surface texture throughout its service life. Exact composition of slurry mixture shall be determined by the laboratory tests. The application of slurry seal will significantly extend the life of existing pavements by protecting the under-surface from effects of aging and the environment, filling smaller cracks, preventing of evolving of new cracks. It improves also the surface performance, friction resistance and aesthetic values of pavement.

Slurry seal technology can be applied on highways, streets and parking lots, squares etc.

    Advantages of Bitumen Emulsions
  • It can be used in damp environment on wet aggregates
  • Road repair work can be carried out in minimum time
  • Provides better tack coat with better workable conditions
  • Can be used in any season except in excessively cold environment or during rainfall
  • Long storage stability in clean containers/Tanks
  • Choice of viscosity for different applications.
Disadvantages of Bitumen Emulsion

Even though cationic bitumen emulsions have many advantages over cut-back binders, they also have certain disadvantages:

i) Climatic - A regular water supply is required and this can be a great problem in very hot countries, where water can be in very short supply.

Also aggregates in arid regions are often very dry and cationic emulsions work most effectively, especially mixing with moist aggregate.

ii) Stability Emulsions are inherently unstable with limited shortage life, (generally maximum 6 to 12 months).

iii) Plant Emulsion manufacture requires specialized plant and quality control procedures.

iv) Bitumen Special emulsifier grade binders are needed to make the best quality emulsions.

Tar and Natural bitumen

Petroleum bitumen is often confused with tar. Although bitumen and coal tar are similarly black and sticky, they are distinctly different substances in origin, chemical composition and in their properties. Coal tar is produced by heating coal to extremely high temperatures and is a by-product of gas and coke production. It was widely used as the binding agent in road asphalt in the early part of the last century, but has since been replaced by refined bitumen. Tar sands, also called oil sands, are deposits of a mixture of fine clay, sand, water, and variable amounts of bitumen which is a black, high-sulfur, tar-like, heavy oil. Tar sands are sedimentary rocks and the bitumen is an asphaltic substance. A typical tar sand is composed approximately of 83% sand, 13% bitumen, and 4% water (by weight percent). Tar sands containing more than 6% by weight of bitumen are considered to have commercial potential. The difference between oil shale and tar sand is that the bituminous matter in shale is a solid, whereas, the bituminous matter in tar sand is highly viscous liquid. Oil which is particularly thick and viscous is called heavy oil, or more colloquially, tar or asphalt.

Naturally-occurring bitumen, sometimes also called natural asphalt, rock asphalt, lake asphalt or oil sand, has been used as an adhesive, sealant and waterproofing agent for over 8,000 years. But it occurs only in small quantities and its properties are quite different from refined bitumen.

Tar Bitumen
Appears black when seen in large quantities Appears black wen seen in large quantities but brown in thin films
Has a strong distinctive smell The smell is not smell distinctive
Is more brittle at lower temperatures It is less brittle at law temperature
Respond easily to temperature changes Does not reach as quick to small temperature fluctuation
Spoils easily when overheated Is less likely to spoil except in extreme heating conditions
Can be removed from bulk containers much easier than bitumen Cannot be removed as easy from bulk storage containers
Spilling a petroleum solvent onto a tar surface will not affect it adversely A petroleum solvent spilt on a bitumen surface will soften it resulting in a loss of strength
A few of the thousands of chemical constituent of tars are known to be carcinogenic Not as carcinogenic as tar
Tar is toxic and must not be inhaled Is non toxic
Skin and eye irritation is worsened by exposure to the sun’s ultraviolet light In most cases contact with the skin or eyes can be treated with water