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Introduction to bituminous materials & applications of bitumen
Introduction to Bituminous materials
The term bituminous materials is generally used to denote substances in which bitumen is present or from which it can be derived. Bitumen is defined as an amorphous, black or dark-colored, (solid, semi-solid, or viscous) cementitious substance, composed principally of high molecular weight hydrocarbons, and soluble in carbon disulfide. For civil engineering applications, bituminous mate-rials include primarily aphalts and tars. Asphalts may occur in nature (natural asphalts) or may be obtained from petroleum processing (petroleum asphalts). Tars do not occur in nature and are obtained as conden-sates in the processing of coal, petroleum, oil-shale, wood or other organic materials. Pitch is formed when a tar is partially distilled so that the volatile constituents have evaporated off from it. Bituminous mixtures are generally used to denote the combinations of bituminous materials (as binders), aggregates and additives.
This article presents the basic principles and practices of the usage of bituminous materials and mixtures in pavement construction. In recent years, the use of tars in highway construction has been very limited due to the concern with the possible emission of hazardous flumes when tars are heated.
Applications of Bitumen
One of the most important uses for geotextiles is as a filter in drainage and erosion control applications. Drainage examples include trench and French drains, interceptor drains, blanket drains, pavement edge drains, and structural drains, to name just a few. Permanent erosion control applications include coastal and lakeshore revetments, stream and canal banks, cut and fill slope protection, and scour protection. In all these applications, geotextiles are used to replace graded granular filters used in conjunction with the drainage aggregate, perforated pipe, rip rap, and so on. When properly designed, geotextiles can provide comparable performance at less cost, provide consistent filtration characteristics, and they are easier and therefore cheaper to install. Although erosion control technically does not improve the soil, prevention of both external and internal erosion in residual and structured soils is an important design consideration.
Geotextiles can also be used to temporarily control and minimize erosion or transport of sediment from unprotected construction sites. In some cases, geotextiles provide temporary protection after seeding and mulching but before vegetative ground cover can be established. Geotextiles may also be used as armor materials in diversion ditches and at the ends of culverts to prevent erosion. Probably the most common application is for silt fences, which are a substitute for hay bales or brush piles, to remove suspended particles from sediment-laden runoff water.
Filtration Design Concepts: For a geotextile to satisfactorily replace a graded granular filter, it must perform the same functions as a graded granular filter:
- Prevent soil particles from going into suspension
- Allow soil particles already in suspension to pass the filter (to prevent clogging or blinding); and
- Have a sufficiently high permeability and flow rate so that no back pressure develops in the soil being protected.
The factors that control the design and performance of a geotextile filter are
- Physical properties of the geotextile
- Soil characteristics
- Hydraulic conditions, and
- External stress conditions
The level of design required depends on the critical nature of the project and the severity of the hydraulic and soil conditions. Especially for critical projects, consideration of the risks involved and the consequences of possible failure of the geotextile filter require great care in selecting the appropriate geotextile. For such projects and for severe hydraulic conditions, very conservative designs are recommended. As the cost of the geotextile is usually a minor part of the total project or system cost, geotextile selection should not be based on the lowest material cost. Also, expenses should not be reduced by eliminating laboratory soil–geotextile performance testing when such testing is recommended by the design procedure.
The three design criteria which must be satisfied are
- Soil retention (piping resistance)
- Permeability, and
- Clogging criteria
For both permeability and clogging, different approaches are recommended for critical/severe applications. Furthermore, laboratory filtration tests must be performed to determine clogging resistance. It is not sufficient to simply rely on retention and permeability to control clogging potential. Finally, mechanical and index property requirements for durability and constructibility are given. Constructibility is sometimes called survivability, and it depends on the installation conditions. The best geotextile filter design in the world is useless if the geotextile does not survive the construction operations.
Applications of bituminous materials:
Prefabricated Drains In the last few years, prefabricated geocomposite drainage materials have become available as a substitute for conventional drains with and without geotextiles. Geocomposites are probably most practical for lateral drainage situations geocomposites is the use of prefabricated vertical (“wick”) drains to accelerate the consolidation of soft compressible cohesive soil layers. Because they are much less expensive to install, geocomposite drains have made conventional sand drains obsolete.
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