What is 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.

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.

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.

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.

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.

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.

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.

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.

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.