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Bituminous surfacing takes two common forms:
Both surfacing types involve the use of:
The bituminous binder predominantly used in road surfacing work is of petroleum origin and in Australia is known simply as bitumen. It is composed mainly of hydrocarbons and their derivatives.
Natural bitumen is probably the oldest petroleum product to be used by man. The ancient Egyptians used it for embalming mummies and in jewellery. Through the ages it has been used in Middle Eastern countries for water-proofing and constructional jobs. The bitumen was obtained from natural seepages out of the ground in various parts of the Middle East. Although naturally occurring bitumens are still available they account for only a very small percentage of the bitumen used today.
Bitumens are produced from suitable crude petroleum oils, essentially by a process of distillation.
Australian crude oils are unsuitable for bitumen production as they are too light and too waxy in nature. Therefore bitumen production in Australia is almost exclusively from Middle East crudes.
The characteristics of bitumen which make it a good material for use in road surfacing are:
The classification of bitumen is based on its viscosity at 60 degrees C. For example Class 170 bitumen has a viscosity in the range 140 to 200 Pa.s. (170 being the mid-point of the range) at 60 degrees C.
The classes of bitumen generally used for road making purposes in Australia are 170 and 320.
The range of tests used for determining the characteristics of bitumen is as follows:
Flux and cutter are both petroleum which are added to bitumen to change the viscosity.
In fluxing the aim is to achieve a relatively long term change to the viscosity. A typical flux in use is diesel fuel oil. Fluxing the bitumen means that the binder remains soft for a period of weeks or months. Fluxed bitumen is used to manufacture pothole patching mixes which have to be stored for a period of time before use. Flux may also be added to the bitumen when sealing in colder climates during the cooler period of the year. In this case flux helps to keep the bitumen fluid during very cold spells, and helps prevent loss of aggregate from new seals due to bitumen brittleness at low temperatures.
Cutting is the addition of a fairly volatile oil which produces a temporary reduction in the viscosity of the binder. Cutter is intended to be mainly lost by evaporation within a period of minutes or hours. The use of cutter in sealing work is mainly to give enough time to incorporate the aggregate particles firmly in the sprayed bitumen before it becomes too hard, and subsequently to allow the development of a good bitumen-aggregate bond. The cutter commonly used is power kerosene.
Fluxing and cutting may both be carried out at the job site before sealing work is performed. However cuback bitumen can also be purchased in bulk from bitumen refineries.
Fluxes and cutters have fairly low flashpoints (eg power kerosene 38 degrees C) and therefore strict safety procedures must be adopted when using these materials.
Cutback bitumen is classified into three categories:
Curing refers to the time required for the altered bitumen to return to its original properties. Medium curing cutbacks are generally used for roadmaking purposes. Typical uses are:
| Grade of Cutback Bitumen | Percent of Kerosene in Cutter |
Use |
|
AMC00, AMC0, AMC1 |
56, 44, 34 |
Precoating, Priming |
|
AMC2, AMC3, AMC4 |
27, 21, 16 |
Primersealing, Light sealing, Cold Mix |
|
AMC5, AMC6, AMC7 |
11, 7, 3 |
Sealing |
Bitumen emulsion is a mixture of bitumen and water, with the bitumen suspended in water as very fine droplets. In order to achieve this condition an emulsifying agent is used in the mixture. The stability of an emulsion (ie its ability to retain the bitumen in droplet form) is controlled by the amount and type of emulsifying agent used.
Emulsions are very fluid and have the advantage that they allow the bitumen to be applied to the road surface without the heating of the bitumen. However because of the very fluid nature of the material it can only be applied in a relatively thin layer (thicker applications attempt to run off the road surface). They are used for tack coats before the spreading of plant mix, for maintenance patching, and to a limited extent for spray sealing work. When emulsion is applied to a surface the emulsion "breaks", with the bitumen droplets coalescing and the water evaporating off to leave behind a thin bitumen layer.
Two types of emulsion are in common use and are described by the type of emulsifying agent used in their manufacture. The two type are 'anionic' and 'cationic'.
An adhesion agent may be used in bituminous surfacing work to assist with the adhesion between bitumen and aggregate particles. They may be applied to the aggregate or by incorporating in the bitumen. The performance of particular adhesion agents varies with different types of aggregateand it is necessary for them to be tested for effectiveness with the particular aggregate to be used. They are generally used in the proportion of about 0.5 to 1% of the volume of the bitumen.
Precoating agents are used to improve the bond between the bitumen binder and the aggregate particles. The precoating material is applied to the aggregate particles before the sealing work takes place. Materials used for precoating include bitumen based materials (such as AMC00 grade cutback bitumen) and oil based materials. The application rate for precoating material varies according to the nature, size and surface area of the aggregate, but is normally about 4 to 12 litres per cubic metre of aggregate.
Polymer modified binders (PMBs) have been in use in Australia since the 1970s. Early materials used natural or synthetic rubber at about 4 to 6 % by mass of binder.
PMBs can now be manufactured to produce specific binder properties. The binder properties desired often include rutting resistance, fatigue resistance, and crack control for asphalt binders; and crack control and aggregate retention for sprayed seal binders.
The Australian Road Research Board in Victoria has done a lot of work in recent years on the use of polymer modified binders. The work is described on their Web Site at Overview of Polymer Modified Binders.
Road surfacing aggregates are typically derived from:
An aggregate is characterised by its particle size distribution.
Road surfacing aggregates need to be:
The following tests are used for determining the properties of road surfacing aggregates:
The purpose of a bituminous surface on a road is to:
As discussed in section 8.1, bituminous surfacings can be classified into two broad groups:
Plant mix surfaces are used where higher traffic volumes prevail, and the change from the use of sprayed surfacing to plant mix surfacing occurs when traffic volumes exceed 5000 to 7000 vehicles per day.
Some informative sites are available on the Web regarding asphalt and related matters.
The National Asphalt Paving Association site (NAPA) contains a section 'Some Frequently Asked Questions About Asphalt'.
The National Center for Asphalt Technology (NCAT) contains several full text reports available on line in their NCAT Publications section. Both the NAPA and NCAT sites are based in the USA.
The European Asphalt Pavement Association (EAPA) site (click on Welcome) based in The Netherlands contains a page titled 'Everything You Always Wanted to Know', refering of course to asphalt.
The Australian Asphalt Pavement Association (AAPA) has a fairly new site. This site does not as yet contain very much technical information.
Selection of the appropriate surfacing for a particular situation depends on a number of factors:
Surfacing type will vary from a single coat, small aggregate sprayed seal for low traffic conditions,
to an asphalt surfacing for heavy traffic conditions.
The main functions of a primer are:
The type of primer and application rate should be such that the primer penetrates the surface up to a depth of 10mm, and leaves a continuous film of binder at the surface. Selection of primer type is usually influenced by availability, cost, and previous experience as to suitability.
The condition of the surface to be primed is assessed as to its 'tightness' (resistance to penetration by the primer) and this assessment is largely based on experience.
Typical primes would be cutback bitumens (AMC00 to AMC1) with application rates from 0.5 to 1.3 litres per square metre, depending on conditions.
A wide range of cover aggregates and bituminous materials can be successfully used. In the selection of materials consideration should be given to the type of pavement material, the condition of the pavement, traffic and life expectancy.
The aggregate size will generally be 7mm or less. Primerbinders are usually cutback bitumen, bitumen emulsion or tar.
The following design process is normally used:
Step 1. Select Type of Seal (e.g. on application of binder and one application of aggregate).
Step 2. Select Aggregate Size.
The nominal aggregate size/s used should be related to the conditions of the work such as:
Where two applications of aggregate are used, the nominal size of the aggregate in the second application should be approximately one half of that used in the first application.
Step 3. Select Aggregate Type.
Step 4. Select Aggregate Application Rate.
After rolling the aggreagte particles will lie on their flattest side and the average thickness of the seal is the average of their least dimensions (average least dimension = ALD).
Australian trials show that application rates should be about 700/ALD for larger aggregates, and 600/ALD for smaller aggregates.
Example: A nominal 14 mm aggregate is found to have an ALD of 8.0mm. The spread rate is then 700/8.0, or 88 square metres per cubic metre of aggregate.
Where multiple applications of aggregate are to be placed it is generally satisfactory to calculate the first application rate for aggregate using the method for single application work. The second application is designed so that particles lodge within the voids of the first aggregate. The rate for the second application is usually based on past experience. Typical application rates would vary from 110 square metres per cubic metre for 10mm aggregate, to 250 square metres per cubic metre for sand.
Step 5. Select Binder Type.
The function of the binder is to retain the cover aggregate and to provide a waterproof seal.
The types of binder commonly available are:
Step 6. Select Binder Application Rate.
The design aim is for the binder level to be between one half and two thirds of the ALD of the aggregate. this provides a good compromise between the levels required to hold the aggregate in place, waterproof the pavement, and provide adequate surface texture and durability.
The basic application rate is found using the formula:
Application rate (l/sq.m.) = ALD x Void Factor
The following table relates Void Factor and traffic volume (expressed in terms of the Annual Average Daily Traffic, or AADT)
| AADT per Lane | Void Factor |
| < 35 | 0.20 - 0.24 |
| 35 - 100 | 0.18 - 0.21 |
| 100-150 | 0.16 - 0.19 |
| 150 - 300 | 0.15 - 0.17 |
| 300 - 625 | 0.14 - 0.16 |
| 625 - 1250 | 0.13 - 0.15 |
| > 1250 | 0.12 - 0.14 |
The basic binder application rate may be adjusted to account for:
The application rates so designed are based on the residual binder at a temperature of 15 C and adjustment of the rate must be made to spray the hot binder (e.g. a rate of 1.2 l/sq m at 15 C may correspond to an application rate of 1.33 l/sq m at 175 C).
The rate of application of the binder should be determined as for a single application of aggregate, using the larger aggregate size as the basis for the design.
This type of treatment is applied by spraying the first binder application, applying the larger aggregate size, spraying the second binder application, then applying the second, smaller aggregate.
It is desirable that the first binder application be at a rate slightly less than that determined by the design method for a single application job, and this is achieved by reducing the void factor slightly. The determination of the second application rate for binder is usually based on experience. As a guide it could be based on the ALD of the second aggregate and assuming a smooth surface for the second coat.
(Refer to Study Notes).
Dense Graded Mixes
The mix should contain sufficient binder to coat the aggregate particles, to waterproof and bond them together when suitably compacted, and to provide flexibility, durability and stability of the compacted mass. A small volume of air voids (3 - 7%) will mean low permeability and improved durability. Dense graded mixes are often used for heavily trafficked roads and care must be taken that the voids in the mix are not overfilled with bitumen.
Open Graded Mixes
There must be sufficient binder to coat, waterproof and bond the aggregate.
Gap Graded Mixes
These mixes have some intermediate aggregate sizes omitted compared to a dense graded mix. Harder binders are generally used because gap graded mixes depend largely on the stiffness of the mixture for stability. Good durability may be obtained by the use of high binder contents and low air voids.
A number of design procedures are used in Australia and around the world. The most widely used procedure is the Marshall Testing Procedure and this will be the only method described in these notes.
Step 1. Selection of Mix Type.
Selection depends on the expected use of the mix, the type of road to be paved, existing pavement conditions, climatic conditions, and available materials.
Step 2. Combination of Aggregates.
The available aggregates, including the mineral filler, are combined in such proportions that their combined grading approximates that of a proven target grading. There are several methods available for aggregate grading determination, and these are the same techniques used in the blending of aggregates for Portland cement concrete.
Step 3. Binder Content Estimation.
The binder content is determined by the preparation and testing of trial mixes at a range of binder contents. Estimation of binder content for trial mixes is ususally based on previous experience, but will probably be in the range of 3 to 9% binder.
Step 4. Manufacture and Compaction of Trial Mixes.
Trial mixes are prepared by blending heated aggregate mixtures with various percentages of binder, and then compacting the hot mix into cylindrical moulds using a standard compaction process. The Marshall Test uses a cylindrical mould of 100 mm internal diameter and the compacted height of specimens aimed for is 64 mm. This test uses a compaction hammer with a dropping weight (a mass of 4540g falling through 475mm). The number of blows applied to each end of the cylindrical specimen is 35, 50 or 75 depending on expected traffic conditions for the mix in service (35 for light traffic, 50 for medium traffic, and 75 for heavy traffic). Once compacted and cooled the specimens are extruded for testing.
Step 5. Testing of Trial Mixes.
The test specimens are heated to 60C for 30 to 40 minutes in a water bath. When ready for testing the specimens are removed from the bath, seated in compression heads, and a force applied diametrically at a uniform rate of deformation of 51 mm/min. The complete test must take less than 30 sec to avoid excessive cooling of the specimen.
The maximum load resisted by the specimen (corrected for the actual height of the specimen) and the amount of vertical deformation undergone in reaching maximum load, are recorded as the Marshall Stability and Marshall Flow values respectively.
In addition to strength testing, the following properties are determined for each mix:
Step 6. Evaluation of Test Results.
Test results are best presented graphically to show the general level of their values and their trend with increasing bitumen content. The values of stability, flow, air voids, voids in the mineral aggregate, and bulk density are plotted against binder content on individual graphs.
The final binder content selected for the mix should give close to the maximum value for stability and bulk density, near the minimum value for VMA, and result in air voids and flow values within the specified limits.
Step 7. Adjustment of Mix Design.
The initial mix may need to be adjusted and further testing carried out to obtain a mix which meets specified criteria.
Step 8. Job Mix.
After the laboratory testing and selection of a design mix, it will be desirable to manufacture, spread and compact a quantity of the mix in a full scale field trial to ensure the mix meets expectations.
Page last modified 7 July 1999.