Thursday, 15 January 2009

bituminous material

Natural bitumen occurs in Trinidad and Venezuela (not in common use in other places)
Derivative bitumen is produced by fractional distillation of petroleum crude oil
Also known as asphaltic bitumen
Consists essentially of hydrocarbons and their derivatives
Solid or very viscous liquid
Black or dark brown in color
Other properties:
Soluble in carbon sulphide
Becomes soft at high temperature
Adhesive (to dry solid), hence used as binder
Resilient (free from cracks, making expansion joints unnecessary for road pavement)

Obtained by destructive distillation of coal, wood, shale etc. (if source not stated, imply coal tar)
Similar chemical compositions bitumen
Viscous liquid
Black in color
Other physical properties similar to bitumen
Also used as binder

Material used for the purpose of holding solid particles together
In this context, it means either bitumen or tar, usually fluxed, may be in other forms such as cutbacks or emulsions

The residual material from the fractional distillation process is often too hard to be used, the material may be softened by fluxing with non-volatile oils
Fluxing of bitumen
Bitumen can be fluxed to a desirable consistency
Fluxing of pitch
Pitch, the residual material obtained from the fractional distillation of coal, is too hard to use, the pitch is fluxed to yield tar
Fluxed bitumen or pitch (tar) with cementing qualities suitable for the manufacture of asphalt pavements is called asphalt cement

Cutbacks are mixtures of binders with light volatile oils (such as kerosene), the resultant mixture having a much lower viscosity than the original binder
After application in thin layers, the light oil quickly evaporates leaving a coat of the original binder
Allows the various handling operations to be carried out at a much lower temperature than would otherwise be required

An emulsion of binder in water
The binder exist in the form of many small globules dispersed in water and kept in suspension by means of an emulsifier
Emulsions are quite stable when stored in bulk, but break down or “crack” when applied to surfaces in thin layers, the water evaporating to leave a coating of the original binder on the surface

Properties of binders
The binders are used as binders because they are adhesive
The surfaces to be adhered to must not be dusty or wet or else the binders won’t stick to the surfaces
The binders may be stripped off the surfaces by water
Tar resist stripping better than bitumen
Harder (high viscosity) binders adhere better in the presence of water than soft (low viscosity) binders

Although the harder grades of binders exhibit some of the characteristics of solid, all of them possess the liquid property of viscosity
Viscosity at 20℃
Cutback bitumen 103 to 104 (x10-1Nsm-2)
300 pen bitumen 105 (x20-1Nsm-2)
100 pen bitumen 106 (x20-1Nsm-2)
15 pen bitumen 108 (x20-1Nsm-2)
All bituminous binders become much softer or of lower viscosity as their temperature increases, and harder or of higher viscosity as their temperature falls

Penetration test
Standard test for hardness (viscosity) of bitumen
Empirical, doer not yield actual viscosity value
In the test, a 1mm diameter sharp pointed needle, just touching the surface at the start, is allowed to fall under the wt. of 100g for 5 sec, the test temperature being 25℃
The penetration in tenths of a millimeter is used to define the grade of the bitumen
A 70 pen bitumen means bitumen which will be penetrated by 70x10-1mm in the test
The smaller the penetration value, the harder the bitumen is

Bituminous mixtures:
Binders, no matter the type, are seldom used alone. They are invariably mixed with aggregates.
All practical mixtures of binders and aggregates lie between two extremes.
Mastic asphalts consisting of binders with a small proportion of aggregate “floating” in the binder.
Open-textured macadam mixes consisting largely of single sized aggregate with a coating of binder as a cementing agent

Coarse aggregate
Crushed rock, gravel of slag
Fine aggregate
Crushed rock or sand
Fine material passing 75μ sieve
May be crushed rock fine, Portland cement or hydrated lime
Bitumen or tar

Mastic asphalt
Binder 13 to 20%
No coarse aggregate
In buildings used for damp proof course, tanking of basement, roof covering
In roadworks used for wearing course where thin thickness and high durability are required

Hot-rolled asphalt
Mainly for heavy duty dense road surfacing
Hot-rolled at about 180℃
Approximate proportions are
65% coarse aggregate
29% fine aggregate
6% binder
Wearing course
20% coarse aggregate
58% fine aggregate
12% filler
10% binder

Bitumen macadam
Mixture of cutback bitumen, graded aggregate and filler
For road construction by warm process at about 120℃
Approx. proportions
90% coarse aggregate
6% fine aggregate and filler
4% cutback bitumen

Uses of bituminous materials
Road surfacing
The soil on which the road is supported
Layer of granular material
Spread loading
Main structural element
Support upper layers and spread loading
Usually of bitumen macadam
Base-course & wearing course
Provide smooth running surface
Protect underlying layers
Usually of hot-rolled asphalt

Building works
Water-proofing, roofing, tanking, sealing of joints etc

Hydraulic works
Water-proof lining for canals, reservoirs
Water-tight upstream carpets for rock-fill dams

Required properties of bituminous mixtures
The mixture must be easy to handle and capable of being compacted to the desirable density. Workability of the mixture is dependent on the temperature of the mixture. Consequently, the temperature of the mixture must be well controlled.

Strength & flexibility
The mixture must be strong enough to resist deformation under considerable loads, both static and dynamic. At the same time, it must be sufficiently flexible to accommodate both long term movements of the material or structure that supporting them, the repeated short term loading of the type likely to cause fatigue failure.
Strength of a mixture can be obtained by using high viscosity binders in the mixture and/or by using crushed stone with rough angular surfaces as aggregate flexibility is most easily obtained by using a mixture with high binder content or a binder of fairly low viscosity.

Durability of the mixture is a function of the binder used. Exposure of the binder to air, particularly when hot, results in evaporation (of some of the lighter oils), oxidation and polymerization, all of which may occur together and all of which produce hardening of the binder.
Durability is also influenced by the adhesion between the binder and the aggregate. Poor adhesion is usually the result of water and/or dust films preventing intimate contact between binder and aggregate. The binder may also be stripped off the aggregate surface by water. Diesel oil dropped from vehicles may dissolve the binder.

Marshall test
A cylindrical specimen, 100mm in diameter and 63.5mm long is made by compacting the mixture in a steel mould with a standard compaction hammer. The specimen at 60℃ is placed in the coller-like testing device which offers no confinement at the ends and is loaded at a speed of 50mm per min. the maximum load and deformation of the specimen before failure are recorded.
Maximum load that the specimen can carry before failure
Deformation of the specimen at maximum load
The density of the specimen is also measure for determination of the void content.
The Marshall test can be used to evaluate the quality of a bituminous mixture and for the design of road pavement.