7 - hma mix design

8/12/2019 7 - Hma Mix Design

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MIX DESIGN OF HMA

oHot-mix asphalt (HMA) is produced in a hot asphalt mixing plant (or hot-mix plant)by mixing a properly controlled amount of aggregate with a properly controlled

amount of asphalt at an elevated temperature.

After compacting and cooling to air temperature, HMA is a very strong pavingmaterial with the ability to sustain heavy traffic loads while remaining flexible

enough to withstand ambient environmental conditions and stresses.

Probably over 98 percent of the hard-surfaced roads in the Nigeria are paved usingHMA.

RATIONAL DESIGN PROCEDURE

Steps in the procedure for a rational design are:

1. Select type of asphaltic concrete mix to design for (base course /wearing course)2. Select aggregates to be employed in the mix.3. Determine the specific gravity of the aggregate combination and of the asphalt

cement

4. Determine the proportion of each aggregate required to produce the designgrading.

5. Make up trial specimens with varying asphalt contents.6. Determine the specific gravity of each compacted specimen.7. Carry out stability tests on the specimens.8. Calculate the percentage of voids in each specimen. Calculate the VMA and the

percent voids filled with asphalt.

9. Select the optimum asphalt content from the data obtained.

1. Grading Analysis of Materials

The grading of aggregates denotes the distribution of sizes from coarse to fillers.The grading is determined by running the material through a series of sieves with

progressively smaller openings and weighing the material retained on each sieve.


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The total percentage passing method is very convenient for the graphicalrepresentation of a grading and is most widely used in graded aggregate

specifications (see Figure 1).

The particle-size distribution of combined coarse and fine aggregates and fillersshall lie within the following limits shown in Table 1.

TABLE 1: GRADING ENVELOPES FOR BINDER AND WEARING COURSE

BS SIEVE SIZE % BY WEIGHT PASSING

ASTM BS

BASE-COURSE

(1 in. Normal Size

Aggregate)

WEARING COURSE

(1/

2in. Normal Size

Aggregate)

1 4in. 31.8mm 100 -

1 in. 25.0 mm 90100 -

4in. 19.0 mm 7090 100

2in. 12.5 mm 5580 85100

8in. 9.5 mm 4770 7592

4in. 6.4 mm 4060 6582

No. 7 2.36 mm 2745 5065

No. 14 1.25 mm 2034 3651

No. 25 600m 1427 2640

No. 52 300m 820 1830

No. 100 150m 515 1324

No. 200 75m 27 714

Bitumen Content % By Weight Of

Aggregate

4.56.5 58.0

Source: The Ministry of works (General specification for roads and bridges (1994)

2. Select Aggregates to Employ In the Mix

The aggregates commonly used in asphalt pavements are crushed limestone, basalt,

gravel, slag, and sand.

In the manufacture of crushed limestone, solid ledges are reduced in the quarry by

blasting and then further reduced through a series of crushers. The limestone is

then screened to produce any desired size or sizes of aggregates.

Basalt is an igneous rock, green to black in colour, and is processed in the same manner

as crushed limestone.

Blast-furnace slag is a by-product in the manufacture of iron.

Sand is nearly always present in gravel deposits and is separated as part of the

manufacturing process


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Figure 1Typical Grading Envelope

3. Determine Specific Gravities

The specific gravity of asphalt cement is rarely determined in an asphalt-paving-mix laboratory. It is usually provided by the producer. The specific gravity of the

aggregate combination is nearly always determined in the paving laboratory.

Three types of specific gravity of the aggregate are employed: (1) Bulk specificgravity, (2) Apparent specific gravity, and (3) Effective specific gravity.

The bulk specific gravity involves the overall volume of the aggregate particleincluding its capillaries.

The apparent specific gravity involves only the impermeable portion of aggregateexclusive of the volume of capillaries which become filled with water upon 24-hr

soaking.

The effective specific gravity purportedly involves the volume of theimpermeable aggregates and the volume of capillaries unfilled with asphalt as it

exists in the pavement.


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Specific Gravity Measurements

An aggregate has three components in its volume, viz:

(i) Volume of solids = Vs(ii) Volume of capillary voids which will be filled with water after 24

hours soaking = Vc, and

(iii) Volume of impermeable voids, Vi

Let Ws be the weight of solids.

(i) Apparent specific gravity=

voidspermeableexcludingaggregatesofVol

aggregatesofWeight

.

=)( iS

s

VV

W

(ii) Bulk specific gravity,=

voidspermeableincludingaggregatesofVol

aggregatesofWeight

.

=)(

CS

s

VViV

W

(iii) When a number of aggregate fractions are blended, the average specific gravity,

or the specific gravity of the blended aggregate mix, is calculated from:

Gavg =

3

3

2

2

1

1

100

G

w

G

w

G

w

whereGavg = Average specific gravity of combined aggregate

w1, w2, w3, etc = respective percents by weight of aggregate 1, aggregate 2, etc.

G2, G3Gn = respective specific gravities of aggregate 1, aggregate 2, etc.


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Example 1: Coarse aggregate, fine aggregate and filler having respective specific

gravities of 2.58, 2.72 and 2.70 are mixed in the proportion 55.3, 36.8 and 7.9 percent

respectively. What is the average specific gravity of the mix?

Solution:

Gavg =

70.2

8.7

72.2

8.36

58.2

3.55

100

= 2.64.

4. Proportioning of Aggregate to Produce Grading

The aggregate proportions selected to produce a given grading may be derived

mathematically or by actually blending them in the laboratory on a trial-and-error basis

until a satisfactory grading is reached.

5. Make up trial specimens with varying asphalt content

A number of asphalt mixture specimens are prepared in the laboratory and testedfor stability and flow (see Step 7 for definition of Marshall Stability and flow).

The exact method of preparing and compacting the specimen is a function of the

test method employed.

The four most commonly used methods are: (1) The Marshall Method, (2) TheHveem method, (3) The Hubbard-Field method, and (4) Superpave method. The

Marshall method is currently in use in Nigeria

The test specimens are prepared with varying asphaltic cement content with %increments such that at least two values are above and two are below the

optimum.

Usually, six (6) values of asphalt cement content are selected and for each, threespecimens are needed.

To prepare the specimens, samples weighing 1.2kg each are used. The specimens are prepared by heating the aggregates and binder (within the

range of 135oC (275

oF)) and mixing them.


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The asphalt mixture is compacted in a 101.6-mm (4-inch) diameter cylindricalmould by a Marshall compaction hammer, which is 6.5 kg (10 pounds) in weight

and dropped from a height of 457 mm (18 inches). The specimens are compacted

by giving 50 blows on both top and bottom by the hammer.

The compacted specimen is 101.6 mm (4 inches) in diameter and approximately63.5 mm (2.5 inches) in height

The procedure is adequate for highway pavements designed for a tyre pressure of0.7 MN/m

2. For airfield pavements and heavily trafficked highway pavements

designed for a tyre pressure of 1.4MN/m2, 75 blows are given on each face.

6. Determine the specific gravity of each compacted specimen

The theoretical (because in practical scenario, a mixture cannot be totally void-less)

maximum specific gravity Gmmfor void-less bituminous paving mixtures is determined

by:

Gmm = 1

..........

100

3

3

2

2

1

1

n

n

G

w

G

w

G

w

G

w

where

w1 = the percentage by weight of bitumen i.e. binderG1 = the specific gravity of the bitumen (AASHTO Designation T228)

w2, w3 wn = the percentages by weight of different aggregate fractions

G2, G3Gn = the specific gravities of the respective aggregate fractions

The procedure for determining d, the bulk specific gravity of a compacted specimen, is

given in AASHTO T166 as

Gmb =CB

A

where A = weight of the dry specimen in air, in grams

B = weight of saturated surfacedry specimen in air, in grams

C = weight of saturated specimen in water, in grams


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If the specimen has an open and porous surface, it must be covered with a paraffin

coating before being placed in the water. The formula for determining Gmb, the bulk

specific gravity of a compacted asphaltic mixture when the specimen is coated with

paraffin, is as follows:

Gmb = F

ADED

A

where

A = weight of the dry specimen in air, in grams

D = weight of the specimen plus paraffin coating in air, in grams

E = weight of the specimen plus paraffin coating in water, in grams

F = bulk specific gravity of paraffin specimens are allowed to cool

overnight before testing

7. Stability Tests on Compacted Specimens using the Marshall Method

1. The principal features of the method are a density-void analysis and stability-flowtest of the compacted specimen.

2. The Marshall testing machine is an electrically powered testing device designedto apply loads at a constant rate of strain of 5cm per minute and equipped with a

calibrated proving ring to measure the load.

3. The testing method is by loading the specimen in compression to failure in theMarshall testing machine. The machine then measure both stability and flow that

develop at failure.

4. The Marshall stability is the maximum load the specimen can withstand beforefailure when tested in the Marshall Stability test.

5. The Marshall flow is the total vertical deformation of the specimen, in units of0.01 inch, when it is loaded to the maximum load in the Marshall Stability test.

8. Calculation of Percent of Maximum Density, Percent Voids, and Voids in the

Mineral Aggregate

Volumetri c Propert ies of Asphalt M ixtures

A compacted asphalt mixture consists primarily of aggregate, asphalt and air.


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Volumetric properties of asphalt mixtures are properties that are directly related tothe proportioning of the volumes of these three components.

Although there are only three components in a compacted asphalt mixture,numerous different volumes can be computed when different combinations of the

three components are combined.

Note that some asphalt can be absorbed into the aggregate and occupy part of thebulk volume of the aggregate.

The representation of the different volumes in a compacted mixture is shown in Figure 2.

Figure 2: Representation of volumes in a compacted asphalt mixturewhere:

Vma = Volume of voids in mineral aggregate

Vmb = Bulk Volume of compacted mix

Vmm = Void-less volume of paving mix

Va = Volume of air voids

Vb = Volume of asphalt

Vba = Volume of absorbed asphalt

Vbe = Volume of effective asphalt

Vsb = Volume of mineral aggregate (by bulk specific gravity)

Vse = Volume mineral aggregate (by effective specific gravity)

Percent Air Voids

The percent air voids (Pa) of a compacted mixture is the ratio of the volume of air voids

to the total volume of the mixture. It can be expressed by the following equation:


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Pa= %100xV

V

mb

m (1)

Percent Voids in Mineral Aggregate (VMA)

Percent voids in mineral aggregate (VMA) is the ratio of the volume of voids in mineral

aggregate to the total volume of the mixture. It can be expressed by the following

equation:

VMA = %100xV

V

mb

ma = %100xV

VV

mb

bea (2)

Percent Voids Filled with Asphalt (VFA)

Voids filled with asphalt (VFA), is the ratio of the volume of effective asphalt to the

volume of the voids in mineral aggregate. It can be expressed by the following equation:

VFA = %100xV

V

ma

be = %100)(x

VV

V

abe

be

(3)

Practical Computation of Marshall Test Data

The maximum specific gravity (Gmm

) of the asphalt mixture (already calculated in step 6)

is needed in order to calculate the percent air voids. The maximum specific gravity is the

specific gravity when there are no air voids in the mixture.

The percent air voids (Pa) can be computed from the maximum specific gravity (G

mm)

and the bulk specific gravity of the mixture (Gmb

) as follows:

Pa

= %100xG

GG

mm

mbmm

The percent voids in mineral aggregate (VMA) can be computed as follows:

VMA = 100 -avg

smb

GPxG

Where:Ps= aggregate content percent by total weight of the mixture

Gmb= bulk specific gravity of aggregate

Gavg= specific gravity of aggregate mix (step 4)


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The percent voids filled with asphalt (VFA) can be computed as follows:

VFA = %100xVMA

PVMA a

9. Selection of Optimum Asphalt Content

a) Marshall Mix design criteriaThe Marshall mix design method as recommended

by the Asphalt Institute uses five mix design criteria. They are:

(1) a minimum Marshall stability,

(2) a range of acceptable Marshall flow,

(3) a range of acceptable air voids,

(4) percent voids filled with asphalt (VFA), and

(5) a minimum amount of VMA.

Table 2 shows the requirements for stability, flow, air voids and VFA, while Table 3

shows the requirements for VMA. A mix design to be adopted must satisfy all these five

criteria.

TABLE 2 MARSHALL MIX DESIGN CRITERIA FOR ASPHALT CONCRETE

MIXES

PROPERTY BINDER COURSE WEARING COURSE

Optimum Bitumen Content 4.5% - 6.5% 5.0% - 8.0%

Stability, not less than 3.5KN 3.5KN

Flow 2mm6mm 2mm4mm

Voids in Mineral Aggregate (VMA) 3.0% - 8.0% 3.0% - 5.0%

Voids Filled with Asphalt (VFA) 3.0% - 8.0% 3.0% - 5.0%

TABLE 3REQUIREMENTS FOR ASPHALT CONCRETE MIXES

NOMINAL MAX.

PARTICLE SIZEVOIDS (mm)

DESIGN AIR VOIDS (%)

3.0 4.0 5.0

1.18 21.5 22.5 23.5

3.36 19.0 20.0 21.04.75 16.0 17.0 18.0

9.5 14.0 15.0 16.0

12.5 13.0 14.0 15.0

19.0 12.0 13.0 14.0

25.0 11.0 12.0 13.0

37.5 10.0 11.0 12.0


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Determination of design asphalt content To facilitate the selection of optimum

asphalt content, the following six plots are made:

(1) Average density versus asphalt content

(2) Average percent air voids versus asphalt content

(3) Average Marshall stability versus asphalt content

(4) Average Marshall flow versus asphalt content

(5) Average VMA versus asphalt content

(6) Average VFA versus asphalt content

Practical Selection of Optimum Asphalt Binder Content

The optimum asphalt binder content is finally selected based on the combined results of

Marshall stability and flow, density analysis and void analysis (see Figure 2). Optimumasphalt binder content can be arrived at in the following procedure (Roberts et al., 1996):

1. Plot the following graphs:o Asphalt binder content vs. density. Density will generally increase with

increasing asphalt content, reach a maximum, then decrease. Peak density

usually occurs at a higher asphalt binder content than peak stability.

o Asphalt binder content vs. Marshall stability. This should follow one oftwo trends:

Stability increases with increasing asphalt binder content, reaches apeak, then decreases.

Stability decreases with increasing asphalt binder content and doesnot show a peak. This curve is common for some recycled HMA

mixtures.

o Asphalt binder content vs. flow.o Asphalt binder content vs. air voids. Percent air voids should decrease

with increasing asphalt binder content.

o Asphalt binder content vs. VMA. Percent VMA should decrease withincreasing asphalt binder content, reach a minimum, then increase.

o Asphalt binder content vs. VFA. Percent VFA increases with increasingasphalt binder content.


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2. Determine the asphalt binder content that corresponds to the specificationsmedian air void content (typically this is 4 percent). This is the optimum asphalt

binder content.

3. Determine properties at this optimum asphalt binder content by referring to theplots. Compare each of these values against specification values and if all are

within specification, then the preceding optimum asphalt binder content is

satisfactory. Otherwise, if any of these properties is outside the specification

range the mixture should be redesigned.

5.0 6.0 7.0

% Asphalt Binder by Weight

%AirVoids

2

4

6

8

5.0 6.0 7.0


MarshallStability

(lbs)

2400

2600

2800

3000

5.0 6.0 7.0


Density(pcf)

5.0 6.0 7.0


%VFA

50

60

70

80

141

142

143

144

5.0 6.0 7.0

% Asphalt Binder by We

Flow(0.01inc

h)

5.0 6.0 7.0

% Asphalt Binder by We

%VMA

18.0

18.4

18.8

19.2

11

12

13

14

1. Plot asphalt binder content versus measured values.2. Select the asphalt binder content corresponding to 4% air voids.3. Determine values of the other properties at this % asphalt binder and

ensure they are within specification. 2003 Steve Muench

Figure 2: Selection of Optimum Asphalt Binder Content Example

(from Roberts et al., 1996)

7 - hma mix design

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