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The National Asphalt Paving Association site ( NAPA ) is a site for the national asphalt body in the USA. The site is somewhat complex and a high quality browser is required for access.
The European Asphalt Pavement Association ( EAPA ) site (click on Welcome) is based in The Netherlands.
The Australian Asphalt Pavement Association ( AAPA ) has a useful site. The site contains some technical information, for example in the Pavement Work Tips series, and has information on current activities in the Australian surfacing industry.
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.
The aggregate size will generally be 7mm or less. Primerbinders are usually cutback bitumen or bitumen emulsion.
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:
Step 3. Select Aggregate Type.
Step 4. Select Aggregate Application Rate .
After rolling the aggregate 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 900/ALD for larger aggregates (10mm and larger). For a 7mm nominal size aggregate the spread rate would be 200 to 250 square metres per cubic metre.
Example: A nominal 14 mm aggregate is found to have an ALD of 8.0mm. The spread rate is then 900/8.0, or about 110 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.
The function of the binder is to retain the cover aggregate and to provide a waterproof seal.
The types of binder commonly available are:
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:
The Design Voids Factor (VF) is determined by selecting a basic voids factor (Vf), and adjusting for aggregate characteristics (Va) and for traffic effects (Vt), such that
Design Voids Factor (VF) = Vf + Va + Vt
Values for Vf, Va and Vt are found from graphs and tables which are contained in the Austroads 2002 Practitioners guide to design of sprayed seals - revision 2000 method and which are reproduced in the written study notes for this course.The basic binder application rate may be adjusted to account for:
In this case the aggregates will be such that the second aggregate is approximately half the size of the first aggregate. 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.
Where both applications are on the same day, with little or no trafficking between applications, the first application is designed as for a single/single sprayed seal except the Design Voids Factor (VF) is reduced by between 10% (traffic > 2000 vehicle/lane/day) and 25% (traffic < 500 vehicle per lane per day), and the aggregate application rate is reduced by 10%. The second application is designed as a single/single reseal but with no allowances for surface texture or embedment.
In situations where the second application is delayed and trafficking will occur during this period, the first application is designed as a normal single/single seal. the second application is designed as a single/single seal with aggregate application reduced by up to 30%.
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.
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:
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.
The Fibredec system sprays an emulsion tack coat over the surface and chopped glass fibres are then spread over the emulsion. A second coat of emulsion is then sprayed, and the aggregate spread in the same way as a sprayed seal.
Page last modified 5 July 2006.