Marshal Stability Test Exp#10

8/8/2019 Marshal Stability Test Exp#10

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Experiment NO: 10

MARSHALL STABILITY TEST

1 SIGNIFICANCE:

Important significances of Marshal Stability test are given below.

Optimum %age of bitumen is obtained for given type of aggregate and traffic

intensity.

Used in designing and evaluating the asphalt concrete mixes.

Density, voids and stability parameters are determined during the Marshall test.

2 Related Theory:

a. Stability

The stability of mix is the maximum load carried by a compacted specimen at a

standard temperature of 60 C.

b. Flow

The flow is measured as a deformation in units of 0.01” between no load and

maximum load carried by the specimen during stability test.

3 APPARATUS

Following apparatus is required for this test.

Specimen mold assembly

Specimen extractorCompaction hammer

Compaction pedestal

Specimen mold holder

Breaking head

Loading machine

Ovens

Hot plates

Mixing apparatus

Water bath


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4 PROCEDURE:

The Marshall Mix design method consists of 6 basic steps:

Aggregate selection

Asphalt binder selection

Sample preparation (including compaction)

Stability determination using the Marshall stability and flow test

Density and voids calculations

Optimum asphalt binder content selection

Detail procedure is given below.

Place all the aggregates in fixed proportion on hot plates at a temp of 250F

Bitumen of specific grade is heated to approximately 350F

The different size of aggregates are put together in desired proportion and

predetermined amount of asphalt is added

The mixing operation is carried manually

After mixing , the mix is placed in a preheated(200-300F) compaction mould

Place the compaction mold on compaction pedestal

With the help of hammer , 75 blows are given on each side

Extract the specimen from the mold with the help of extractor

The specimen is cooled in the air and sample is identified with suitable mark

Bulk density of specimen is determined by weighing the specimen first in air

and then in water.The specimen is immersed in water at a temperature of 60C for 30 to 40 min

The specimen is removed from the water bath and is placed with its horizontal

axle into the test heads

The complete assembly is quickly placed on the base plate of Marshall loading

machine

Place the flow dial gauge and proving gauge at required place and adjust to

zero reading

Start the machine in such a way that base plate moves at rate of 2” per min

Record the values of maximum flow dial gauge and load dial gauge andmachine is reversed

The elapsed time for the test after the removal of specimen from water bath to

maximum load determination should not more than 30 min

The stability values of sample are corrected when the height of sample is

greater than or less than 2.5” by using correction factor

when height > 2.5” then C.F. < 1

when height < 2.5” then C.F. > 1


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3.1 COMPACTION WITH HAMMER:

Each sample is heated to the anticipated temperature and compacted with a Marshall hammer.

Key parameters of the compactor are:

Sample size = 102 mm (4-inch) diameter cylinder 64 mm (2.5 inch) in height.Tamper foot = flat and circular with a diameter of 98.4 mm (3.875 inches)

corresponding to an area of 76 cm2 (11.8 in2).

Compaction pressure = specified as a 457.2mm free fall drop distance of a hammer

assembly with 4536 gm (10 lb) sliding weight.

Number of blows = typically 35, 50, 75 on each side depending upon anticipated

traffic loading.

Simulation method = the tamper foot strikes the sample on the top and covers almost

entire area. After a specified number of blows, the sample is turned over and the

procedure is repeated.

F igur e: Marshal Stabil ity Test Apparatus


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4 Calculations:

Three basic purpose of test are

1. Determination of bulk specific gravity

2.

Void analysis

3. Stability flow analysis

4.1 VOID ANALYSIS

Following equations are used for void analysis.

Vv = [(Gt – Gb)/ Gt]*100

Where

G b = Bulk specific gravity

Gt = Theoretical specific gravity of specimen

Gt = 100/[(W1/G1) + (W2/G2) + (W3/G3) + (W4/G4)]

Where

W1 = %age of coarse aggregate in mix

W2 = %age of fine aggregate in mix

W3 = %age of filler aggregate in mix

W4 = %age of asphalt in mix

VMA = Vv + Vb

Where

V b = volume of bitumen or asphalt

Vb = (W4/G4)*Gb

VFB = (Vb/VMA)*100


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F igur e: Volumetric Properties

4.2 GRAPH

Following graphs need to be drawn:

%age of asphalt binder by Weight ~ %age of air voids

%age of asphalt binder by Weight ~ Density or unit weight (psf)

%age of asphalt binder by Weight ~ Flow (0.01”)

%age of asphalt binder by Weight ~ Marshall stability (lbs)

%age of asphalt binder by Weight ~ %age of VFA%age of asphalt binder by Weight ~ %age of VMA

4.3 OPTIMUM ASPHALT CONTENT

Following method used for optimum asphalt content calculation.

Determine the asphalt binder content that corresponds to the specifications median

air void content (typically this is 4 %)Determine properties at this optimum asphalt binder content by referring to the

plots.

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.


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Test Properties Curves/Marshall Design Curves

0

2

4

6

8

10

12

14

16

0 1 2 3 4 5 6 7 8

F l o w

% AC

% AC Vs Flow

Flow Linear (Flow)

2450

2500

25502600

2650

2700

2750

2800

2850

0 1 2 3 4 5 6 7 8

S

t a b i l i t y

% AC

% AC Vs Stability

Stability Poly. (Stability)


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0

10

20

30

40

50

60

70

80

90

0 1 2 3 4 5 6 7 8

V F A

% AC

% AC Va VFA

VFA Linear (VFA)

140

140.5

141

141.5

142

142.5

143

143.5

0 1 2 3 4 5 6 7 8

G m b

% AC

% AC Vs Gmb

Unit Bulk Density Poly. (Unit Bulk Density)


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0

1

2

3

4

5

6

7

8

9

0 1 2 3 4 5 6 7 8

% A i r V o i d s

% AC

% AC Vs Air Voids

% Air Voids Linear (% Air Voids)

18

18.2

18.4

18.6

18.8

19

19.2

19.4

19.6

0 1 2 3 4 5 6 7 8

V M A

% AC

% AC Vs VMA

VMA Poly. (VMA)


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Comments:

The Marshall Stability value increases with increasing asphalt binder content up to a

maximum after which the stability value decreases. Design curve shows that stability

value decreases with increasing asphalt content and do not have a peak value.

The Marshall Flow value increases with increasing asphalt binder content. In other

words, the higher the asphalt binder content, the greater the specimen deforms under

load.

The mixture air voids, Pa, decreases with increasing asphalt content, until it reaches a

minimum air void content.

Asphalt binder acts as lubricant while the mixture is being compacted, thus leading

that mixtures with a higher asphalt binder content have a lower VMA percentage.

What is usually observed is that the VMA generally decreases to a minimum value,

then increases with increasing asphalt binder content.

The percent voids filled with asphalt, VFA, increases with asphalt binder content.

This should be expected since the VMA is being filled with the asphalt binder.

Gmb is also increasing with increase in asphalt binder content. At an extent it increases

but after that it starts decreasing.The reason is that as we add more binder in aggregate

it fills the voids and that’S why increasing the overall Gmb but when voids are filled

with binder and binder starts making film then Gmb starts decreasing.


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Determine the Optimum Asphalt Binder Content

Method 1

According to specifications design asphalt is that which contains 4% air voids. So against the

4% air voids we select the asphalt content in asphalt content and air voids graph. At this

asphalt binder content we select the remaining Marshall Test properties values and this will

be the optimum asphalt content.

Method 2

Another method to select the optimum asphalt binder content is to select each individual

asphalt binder content from the Marshall Design curves based on the following criteria:

The asphalt binder content at the midpoint of the specification range for the mixture

air voids.

The asphalt binder content at the maximum stability value.

The asphalt binder content at the peak of the specification range for unit weight curve

(Gmb).

Then the three possible asphalt binder content values are then averaged to give the optimum

asphalt binder content.


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Method 1

0

1

2

3

4

5

6

7

8

9

0 1 2 3 4 5 6 7 8

% A i r V o i d s

% AC

% AC Vs Air Voids


0

2

4

6

8

10

12

1416

0 1 2 3 4 5 6 7 8

F l o w

% AC

% AC Vs Flow

Flow Linear (Flow)


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2450

2500

2550

2600

2650

2700

2750

2800

2850

0 1 2 3 4 5 6 7 8

S t a b i l i t y

% AC

% AC Vs Stability


0

10

20

30

40

50

60

70

80

90

0 1 2 3 4 5 6 7 8

V F A

% AC

% AC Va VFA

VFA Linear (VFA)


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Parameters Optimum Asphal t binder

content (%)

Value

Mixture Air Voids, Pa 6.9 4%

VMA 6.9 18.59%

VFA 6.9 78%

Marshall Flow 6.9 14(0.1in)

Marshall Stability 6.9 2750

Mixture Unit Weight, Gmb 6.9 142.85 pcf

140

140.5

141

141.5

142

142.5

143

143.5

0 1 2 3 4 5 6 7 8

G m b

% AC

% AC Vs Gmb


18

18.2

18.4

18.6

18.8

19

19.2

19.4

19.6

0 1 2 3 4 5 6 7 8

V M A

% AC

% AC Vs VMA

VMA Poly. (VMA)


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Method 2

0

1

2

3

4

5

6

7

8

9

0 1 2 3 4 5 6 7 8

% A i r V o i d s

% AC

% AC Vs Air Voids


2450

2500

2550

2600

2650

2700

2750

28002850

0 1 2 3 4 5 6 7 8

S t a b i l i t y

% AC

% AC Vs Stability



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Average Asphalt Content = 6.9+6.5+6.9/3 = 6.77 %

140

140.5

141

141.5

142

142.5

143

143.5

0 1 2 3 4 5 6 7 8

G m b

% AC

% AC Vs Gmb


Marshal Stability Test Exp#10

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