Asphalt Mix Design 71-06-101

MAIN ROADS Western Australia Document Number 71/06/101 Page 1 of 14 TRIM D11#103379 Date Issued 13/05/2011

Document No. 71/06/101 Issue Date May 2011

ASPHALT MIX DESIGN This document is owned and authorised by the Pavements and Surfacings Manager. The Project Manager Surfacings is the delegated custodian. All comments for revision should be submitted to the delegated custodian.

AUTHORISATION

As owner of this procedure I authorise it for issue and use.

…………………………………………………………………………….. PAVEMENTS AND SURFACINGS MANAGER

Date: __/__/__

All controlled copies shall be marked accordingly


1. INTRODUCTION ....................................................................................................... 4

2. TYPES OF ASPHALT .............................................................................................. 4

3. PURPOSE OF MIX DESIGN .................................................................................... 4

4. MIX DESIGN FOR CONTRACTS ............................................................................. 5

5. MIX DESIGN PROCEDURE ..................................................................................... 6

6. APPROVAL PROCESS .......................................................................................... 10

7. FURTHER READING ............................................................................................. 11


Revision Status Record

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1. INTRODUCTION

This guideline describes the purpose and objectives of asphalt mix design, the design procedure used by Main Roads Western Australia and the process for approving asphalt job mix designs that ultimately creates specifications for asphalt properties. The relevant TDP documents for asphalt are Specification 502 “Stone Mastic Asphalt”, Specification 504 “Wearing Course Asphalt”, Specification 510 “Full Depth Asphalt” and Specification 511 “Materials for Bituminous Treatments”.. The information in this guideline is specific to the types of hot mixed asphalt listed in Section 2 and does not cover cold mixed asphalt, various types of warm mix asphalt or thin open graded asphalts such as Novachip.

2. TYPES OF ASPHALT

Asphalt is sometimes referred to as asphaltic concrete or bituminous concrete and is a mixture of coarse and fine aggregates (crushed rock), filler (dust and hydrated lime) and bitumen, heated and mixed together whilst hot in a mixing plant. There are variants to typical asphalt where specialised materials are used in the production process to manufacture asphalt with specific functions. Other materials used at times include liquid adhesion agents, red oxide to colour the asphalt, polymer modified or multigrade binders, bitumen modifiers to alter the viscosity of the bitumen, fibres to increase the amount of bitumen in the asphalt and fine fibres such as Kevlar to improve the fatigue and deformation characteristics of asphalt. Types of hot mixed asphalt used on the Main Roads WA network are:

dense graded asphalt (DGA) open graded asphalt (OGA) stone mastic asphalt (SMA)

DGA, the most common type of asphalt, provides optimal structural strength and generally good resistance to deformation. OGA is designed to drain water through the asphalt layer to remove excess water from the tyre/road surface. OGA is used in high speed applications such as Mitchell and Kwinana Freeways, Roe Hwy and locations where it is important to remove water from the road surface such as hills like Greenmount on Great Eastern Hwy. SMA is similar to OGA but has a high proportion of dust and high binder contents to achieve an improved fatigue life. SMA has a textured surface but does not drain water through its layer as does OGA. Further information on the selection and use of asphalt can be found in the following documents:

Main Roads guideline “Selection of Bituminous Surfacings Treatments”, document no. 6706/04/153, available on the MEB website

Austroads “Guide to Pavement Technology Part 4B: Asphalt” available at austroads.com.au.

3. PURPOSE OF MIX DESIGN

The aim of designing an asphalt mix is to determine the optimal proportions of bitumen and crushed aggregates to produce asphalt that meets specified properties and is workable and durable. To achieve an optimal mix design work needs to be undertaken in a materials laboratory to determine the best proportions of the available aggregates and bitumen to give a product (asphalt) that is durable, workable, has resistance to deformation and premature fatigue and provides an adequate surface texture for its intended purpose. There has to be sufficient bitumen in the asphalt to completely coat the aggregate particles and bond them together. If the design of an asphalt mix is not optimised it is likely to have performance issues such as early fatigue, rutting or shoving, ravelling of the aggregate particles, have a low surface texture creating


a skid resistance problem or stripping of bitumen from the aggregate particles in service, any of which may require premature replacement of the asphalt. Further information on the production of asphalt can be found in Austroads Pavement Technology Series Part 4B “Asphalt”.

4. MIX DESIGN FOR CONTRACTS

Main Roads WA Specifications 504 and 510 provide two options for the contractor to use with regard to asphalt mix design when tendering for the works, being: Conforming Mix Design – where Main Roads specifies the binder content and particle size

distribution (PSD) of various standard mixes which it is known will satisfy the Marshall Properties. The standard mixes have been designed using granite aggregates from the Perth region and are only applicable to asphalt produced using these aggregates. When a contractor chooses to use a conforming mix design manufactured from Perth granite aggregate, a job mix design is not required. Where other than granite aggregate from the Perth region is used for the mix design the contractor shall produce a Job Mix Design.

Job Mix Design – this is where a contractor produces an asphalt mix design to satisfy the Marshall Properties and other requirements specified in Clause 504.26.03 or 510.26.04 of the two specifications. A Job Mix design must be authorised before a contractor can proceed with laying asphalt manufactured using a job mix design. .

4.1 Conforming Mixes Examples of conforming mixes include: 10mm Dense graded granite 20mm Dense graded granite (Intermediate mix) 14mm Dense graded granite (Intersection mix) 10mm Open graded granite 7mm and 10mm Stone mastic asphalt Open graded (OGA) and stone mastic asphalt (SMA) can ONLY be produced as a conforming mix and job mixes are not allowed in the relevant specifications. Therefore this guideline will focus on the process for the design of dense graded mixes only. Conforming mix designs in Specifications 504 and 510 shall not be used for asphalt produced in regional areas unless the asphalt supplier has transported Perth granite aggregates to the region. Aggregates from different quarries will have varying specific gravities (density) and will vary in size and shape of the aggregate particles. Differences in the properties of the crushed aggregate impacts upon the properties of the produced asphalt and for this reason mix designs for one region should not be used in another region where the properties of the aggregate differ. For example some changes as a result of differing aggregates include:

Bunbury basalts – have a higher density than Perth granite and as a result the percentage by mass of bitumen in the mix design is lower than Perth mixes. This also applies to any dolerite type aggregates used in regional areas.

Broome sandstone – the density of this aggregate is much lighter than Perth granite and as a result of this and absorption of bitumen in the aggregate the 14mm Intersection Mix for Broome has a bitumen content 1.1% higher than the conforming Perth mix design.


4.2 Job Mixes Examples of job mixes include: Mixes produced by regional asphalt plants Job mixes for mobile plants Mixes produced by Perth plants with small variations to the Conforming Mixes. This has

been done to suit the combination of the producer’s plant and materials they use. Where Main Roads does not specify a conforming mix, such as for mixes outside the Perth region, it is important that the asphalt supplier demonstrate that its job mix is capable of satisfying the Marshall properties. There have been occasions where Main Roads has instructed a contractor to produce a mix that may not conform fully to the Marshall properties or the PSD and binder specification. An example could be where a sprayed seal is fatty with active binder. Main Roads may instruct a contractor to produce a mix with higher design air voids to cater for the potential of binder migration from the seal into the voids in the asphalt. In this scenario an asphalt supplier may alter its mix proportions to change its PSD and possibly reduce the binder content slightly.

5. MIX DESIGN PROCEDURE

5.1 Design Method The design method specified by Main Roads is the Marshall method of mix design. The aim of the method is to determine via a series of laboratory produced trial mixes an optimal combination of aggregates and bitumen to satisfy specified design criteria. The first stage is to determine the type of mix required and what Marshall Properties have been specified. The Marshall Properties for asphalt mixes produced for Main Roads WA are shown in Attachment A. The first table shows properties for wearing course asphalt as shown in Specification 504 whilst the second table shows properties of the mix for intermediate and basecourse asphalt as shown in Specification 510. The properties in the tables include:

Marshall stability – is the strength of a cylindrical test specimen subject to loading at a test temperature of 60°C.

Marshall flow – is the deformation of the test specimen in the stability test at its maximum strength.

Voids in mineral aggregate (VMA) – is the total volume of voids within the mass of compacted aggregate. The VMA has to be large enough to accommodate the volume of effective binder in the mix (i.e. the binder other than that which may be absorbed into the aggregate) and the air voids in the compacted mix.

Air voids – is the volume of air voids within a compacted laboratory test specimen. A critical aspect of the mix design is to ensure that the volumetric properties are fit for purpose. This ensures that there is sufficient volume within the mix to accommodate the aggregates, especially the fine aggregates, and sufficient air voids after mixing and compaction to allow for expansion of binder when the pavement is subjected to its hottest condition. The latter comment is not applicable to open graded asphalt which is designed to have in the order of 20% air voids. 5.2 Selection of Materials The first stage is to identify sources of aggregates that could be used in a mix design. Typically an asphalt supplier will use local aggregates unless those materials fail to meet specified requirements for aggregates or are not suited to achieving an optimal gradation in the produced asphalt or may be too variable in properties. The aggregate shall meet the requirements specified for asphalt aggregates in Main Roads Specification 511 Materials for Bituminous Treatments. Typically an asphalt supplier will use


between three and six different sizes of aggregate from local quarries. The designer does a desk top design by combining the gradings (PSD) of the relevant aggregates on a proportional basis to achieve a PSD that when the materials are combined and mixed with bitumen are likely to conform to the Marshall Properties. A designer would typically use a gradation envelope used for previous works as a target PSD as a guide for a new mix. This provides better assurance that the new mix design is likely to achieve the specified Marshall properties. An example of the combination of different size aggregates to produce a 14mm dense graded asphalt is shown in Table 1.

Aggregate Type and Size Proportion in Mix Design by Mass

14mm Granite 22.0

10mm Granite 12.0

7mm Granite 26.0

Washed Quarry sand (5mm and less in size)

10.0

Quarry Sand (5mm and less in size)

28.5

Hydrated Lime 1.5

Table 1 Combination of Aggregates Quarry sand is crushed granite typically screened to about 5mm and less in size. This product provides most of the dust portion of the mix design and also the majority of the fine aggregates, i.e. less than 2.36mm in size. The quantity of dust in the 7mm to 14mm aggregates is negligible so it is important that the amount of dust going into a mix is controlled. This is achieved by using washed quarry sand which has less dust (removed by washing in the quarry) and allows the asphalt supplier to maintain control of the amount of dust in the mix. The type of bitumen to be used in designing a mix is shown in Specifications 504 and 510 and is either Class 170 or 320 residual bitumen. Although Main Roads uses Polymer modified binders (PMB) in asphalt these binders are never used in developing a mix design. When Main Roads uses a PMB it is substituted at the same quantity as the bitumen that has been specified for a conforming or job mix design. Mix designs should not be developed using products that are added to bitumen such as wax additives (Sasobit etc) and materials used to mask bitumen odours. Where fibres are used in an asphalt mix to increase the binder content they shall be added when a mix design is being produced. Mixes designed to include red oxide shall have the appropriate content of red oxide added during mix development. 5.3 Initial Trial Laboratory Mixes Once the designer is happy with the preliminary design a trial mix will be manufactured at the midpoint of the target PSD and bitumen content. Trial mixes are produced in the laboratory by proportioning the various aggregates together, heating in an oven, adding bitumen and mixing the asphalt in a bench mixer. The laboratory prepared mix is then tested for all properties specified for compliance with the design requirements. The designer would review the results from the preliminary design and adjust the proportions or ingredients used for the design.


The designer may also produce a series of trial mixes at the target PSD and various bitumen contents to determine the optimum bitumen content to achieve the midpoint air voids and VMA, as shown in Figure 1. If necessary another trial mix at midpoint of the target PSD and bitumen content would be produced to verify any design amendments.

Figure 1 Determining Optimum Binder Content When the midpoint of the mix design has been established the designer will produce further trial mixes with the same PSD but bracketing the bitumen content by plus and minus 0.3%. From the results graphical plots are prepared of the relationship between the bitumen content and: mix stability and flow maximum density of the mix percent of the voids in the aggregate mix filled with bitumen percent air voids in the mix. An example of such plots is shown below. An optimal bitumen content is selected that satisfies the Marshall Properties (usually the midpoint of a range) specified for the type of mix. This bitumen content becomes the design binder content.


At this stage the designer may be unable to satisfy all or some of the properties and may need to review the proportions of or materials used for the mix. When the design mix (midpoint) has been established tolerances are placed on the PSD and design binder content to accommodate variations in PSD and binder content that occur during the production process from a plant. This establishes a grading envelope for the asphalt. Table 2 shows tolerances for the PSD of a mix design as shown in Specifications 504 and 510 for designing dense graded asphalts. The tolerance for bitumen content is specified as ± 0.3% either side of the design binder content.

Particle Size Distribution AS Sieve Size (mm)

Tolerances on Percentage by Mass Passing

4.75 and larger 7

2.36 and 1.18 5

0.6 and 0.3 4

0.150 2.5

0.075 1.5

Table 2 Tolerances for PSD Table 3 shows an example of the application of the tolerances based on an actual job mix design for a 14mm Dense Graded Intersection Mix for a Perth based asphalt supplier. For the 0.075mm sieve a restriction on the tolerance has been placed by Main Roads on the fine or upper limit of the PSD to reduce the maximum allowable mass of fines in the mix to 5.5%.

Particle Size Distribution

AS Sieve Size (mm)

% Passing Coarse Limit

(lower) Midpoint PSD Fine Limit

(upper)

19.0 100 100 100

13.2 91 98 100

9.5 76 83 90

6.7 61 68 75

4.75 48 55 62

2.36 32 37 42

1.18 21 26 31

0.60 15 19 23

0.30 8 12 16

0.15 5 7.5 10

0.075 3 4.5 5.5 (note)

Table 3 Application of PSD Tolerances


5.4 Further Trial Laboratory Mixes The designer then needs to then prepare a further six trial mixes at the coarse and fine limits of the PSD. For each PSD condition three trial mixes are produced at the design binder content plus and minus 0.3%. Again the mixes will be prepared in a laboratory, however proportioning of the component materials as was done for the midpoint PSD will not produce either the coarse or fine limits of the PSD. To achieve these limits the materials must be sieved or screened into sieve fractions and then recombined to produce the correct gradation representing the coarse or fine limit of the PSD. It may be necessary to wash excess dust from the aggregate particles to achieve the coarse grading limits. Specifications 504 and 510 include tolerances stating how close to the target fine or coarse limit and the bitumen content the laboratory should get for the results of its laboratory prepared mix. The tolerance allows for variation in the PSD or bitumen content of the laboratory prepared mix that could be due to material breakdown during mixing or variation in obtaining a test portion. The results of these trials are reviewed against the specified Marshall Properties to ensure that at the extremes of the production specification that the mix conforms to the specified properties. The final selected mix is termed the “Job Mix” and will nominate the type, source and proportions of components, target PSD and binder content along with the Marshall properties. It is important that the design procedure results in an asphalt mix that will perform satisfactorily during placement and compaction or under long term trafficking. Any change in the type and source of components or significant variation in proportioning generally requires redesign of the mix and determination of a new job mix. An example is with aggregates where a change in density of the stone will alter the binder content, which is expressed as a percentage by mass. Therefore a heavier stone such as diorite or basalt may occupy the same volume as granite aggregate but the density of diorite or basalt is greater, resulting in a lower binder content by mass. Changes in the shape of the aggregate will affect how the aggregate particles pack together, thus impacting upon the volumetrics in the mix design. Implementation of a standard mix design specified for use with Perth aggregates in different locations such as Kalgoorlie or Port Hedland would not be practical. It is very likely that the aggregates in Kalgoorlie or Port Hedland, and other locations, differ markedly from the properties of aggregates produced in the scarp quarries of Perth. In addition to the design process described to ensure that a mix design has an appropriate PSD to ensure that it meets the Marshall design criteria it is also important that the mix be resistant to deformation during its service. A further laboratory produced mix must be made at the midpoint PSD and binder content and compacted in a gyratory compactor for 350 cycles. This test determines the refusal density and Specifications 504 and 510 require that the air voids in the compacted mix shall not be less than 2.5%.

6. APPROVAL PROCESS

Specifications 504 and 510 require that a Job Mix must be authorised before it can be laid on the Works. For authorisation the asphalt supplier must submit the following information: Detailed description of the materials to be used for the mix Midpoint PSD and binder content of the mix Test results demonstrating that the Job Mix satisfies the requirements of the specification

over the range of PSD and binder content limits. This represents the midpoint, coarse and fine limits of the PSD each at the midpoint, upper and lower limits of the design binder content.

All tests to be done in a NATA accredited laboratory and presented on an endorsed test report.

The information is submitted to the Materials Engineering Branch for evaluation. If the Job Mix meets the specified requirements a number is assigned to the job mix and the asphalt supplier


is advised in writing, including an attachment detailing the materials to be used in the mix, the proportions of the materials, the binder and PSD specification, Marshall properties and limits on the use of the mix, e.g. maximum and minimum thickness to be laid. At present there is not an expiry date on the mix approval, however, the ongoing approval is subject to satisfactory production records demonstrating conformance with specified properties and satisfactory field performance. An example of the attachment sent to an asphalt supplier advising of details of a job mix design is shown in Attachment B. Details of the asphalt supplier have been removed from the example. A listing of job mix designs approved by Main Roads is available on the Materials Engineering page of the Main Roads website. The listing shows:

The asphalt supplier and location of plant (if fixed) Size of the mix and type of aggregate used When the design was approved and its current status.

The type of aggregate used for the mix design is important as discussed previously but also means that a mobile asphalt plant can use a mix away from the town where the quarry supplying the aggregate used in the mix design is located. For example an asphalt supplier may have an approved mix design using aggregate from the Holcim Quarry in Walkaway near Geraldton. If the asphalt supplier carts that aggregate to another location such as Carnarvon or Meekatharra that mix design can then be used to produce asphalt on site.

7. FURTHER READING

The following publications provide further information on the selection, use, design and application of asphalt. AUSTROADS Guide to Pavement Technology – Part 4B Asphalt

Main Roads WA – Guidelines for Surfacing Type Selection

Main Roads WA – Guidelines for the Timing of Bituminous Surfacing Treatments

Austroads/AAPA Work Tips

AAPA Advisory Notes

Austroads publications are available at www.austroads.com.au AAPA is the Australian Asphalt Pavement Association and their publications are available at www.aapa.asn.au


Attachment A

Specification 504 Marshall Properties for Dense Graded Asphalt

Nominal 5mm, 10 mm and 14 mm Mixes

Parameter Min Max

Marshall Stability 8.0 kN -

Marshall Flow 2.00 mm 4.00 mm

Air Voids (WA 733.1):

nominal 10 mm – Perth and

Southern areas of the state

4.0% 6.0%

nominal 10 mm – Northern and

Eastern areas of the state

4.0% 7.0%

nominal 5 mm 3.0% 5.0%

nominal 14 mm (Intersection Mix) 4.0% 7.0%

Voids in Mineral Aggregate:

nominal 10 mm Laterite 15.0% -

nominal 10 mm 15.0% -

nominal 5 mm 16.0% -

nominal 14 mm (Intersection Mix) 14.0% -

TABLE 504.B1 MARSHALL PROPERTIES - DENSE GRADED ASPHALT

(75 BLOW COMPACTION)


Attachment A

Specification 510 Marshall Properties for Dense Graded Asphalt Intermediate Course and Base Course Mixes

Parameter Min Max

Marshall Stability 8.0 kN -

Marshall Flow 2.00 mm 4.00 mm

Air Voids:

Nominal 14 mm (intermediate course)

4.0%

7.0%

nominal 20 mm (intermediate course)

nominal 20 mm (base course mix)

3.5%

1.5%

5.5%

3.5%

Voids in Mineral Aggregate:

nominal 14 mm and 20 mm (intermediate course)

nominal 20 mm (base course mix)

14.0%

14.0%

-

-

TABLE 510.2 MARSHALL PROPERTIES – DENSE GRADED ASPHALT (75 BLOW COMPACTION)


Attachment B

Supplier XYZ Job Mix 14mm Dense Grade Intersection Mix

XYZ – 14IM – Date

MIX SPECIFICATION Particle Size Distribution Sieve Size (mm) % Passing by mass 19.00 100 13.2 91 - 100 9.5 76 - 90 6.7 61 - 75 4.75 48 - 62 2.36 32 - 42 1.18 21 - 31 0.600 15 - 23 0.300 8 - 16 0.150 5 - 10 0.075 3 - 5.5 Binder Class 320 Bitumen 4.4 – 5.0% by mass Marshall Properties Marshall Air Voids 4.0 – 7.0% Voids in Mineral Aggregate Not Less than 14.0% Stability Not Less than 8.0 kN Flow 2.0 – 4.0 mm Aggregate Details Aggregate shall be sourced from the same location as nominated when the job mix design was submitted for approval, as shown below: Quarry, Location 14mm Granite 22.0% 10mm Granite 12.0% 7mm Granite 26.0% Washed Quarry Sand 10.0% Quarry Sand 28.5% Filler (Hydrated Lime) 1.5%

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Asphalt Mix Design 71-06-101