MARSHALL METHOD OF ASPHALT-CONCRETE
MIX DESIGN
Bituminous
mixes (sometimes called asphalt mixes) are used in the surface layer of road
and airfield pavements. The mix is composed usually of aggregate and asphalt
cements. Some types of bituminous mixes are also used in base course. The design of asphalt paving mix, as with the
design of other engineering materials is largely a matter of selecting and
proportioning constituent materials to obtain the desired properties in the
finished pavement structure.
The
desirable properties of Asphalt mixes are:
1.
Resistance to permanent deformation: The mix should not distort or be displaced
when subjected to traffic loads. The
resistance to permanent deformation is more important at high temperatures.
2. Fatigue
resistance: the mix should not crack
when subjected to repeated loads over a period of time.
3.
Resistance to low temperature cracking.
This mix property is important in cold regions.
4.
Durability: the mix should contain
sufficient asphalt cement to ensure an adequate film thickness around the
aggregate particles. The compacted mix
should not have very high air voids, which accelerates the aging process.
5.
Resistance to moisture-induced damage.
6. Skid
resistance.
7.
Workability: the mix must be capable of
being placed and compacted with reasonable effort.
8. Low noise
and good drainage properties: If the mix is to be used for the surface
(wearing) layer of the pavement structure.
Marshall
Stability and Hveem stabilometer tests are largely used for the routine
testing. Criteria for the suitable mix
design have been specified by the Asphalt Institute.
OBJECTIVE:- To
design the Asphalt concrete mix using Marshall method.
MARSHALL
METHOD OF MIX DESIGN:-
In this
method, the resistance to plastic deformation of a compacted cylindrical
specimen of bituminous mixture is measured when the specimen is loaded
diametrically at a deformation rate
Of 50 mm per
minute. There are two major features of
the Marshall method of mix design. (i)
density-voids analysis and (ii) stability-flow tests. The Marshall stability of the mix is defined
as the maximum load carried by the specimen at a standard test temperature of
60°C. The flow value is the deformation that the test
specimen undergoes during loading upto
the maximum load. Flow is measured in
0.25 mm units. In this test, an
attempt is made to obtain optimum binder content for the type of aggregate mix
used and the expected traffic intensity.
STEPS OF
DESIGN:-
1. Select
aggregate grading to be used (Table 11.2)
2. Determine
the proportion of each aggregate size required to produce the design grading.
3. Determine
the specific gravity of the aggregate combination and asphalt cement.
4. Prepare
the trial specimens with varying asphalt contents.
5. Determine
the specific gravity of each compacted specimen.
6. Perform
stability tests on the specimens.
7. Calculate
the percentage of voids, and percent voids filled with Bitumen in each
specimen.
8. Select
the optimum binder content from the data obtained.
9. Evaluate
the design with the design requirements.
APPARATUS:-
1. Mold
Assembly: cylindrical moulds of 10 cm
diameter and 7.5 cm height consisting of a base plate and collar extension
2. Sample
Extractor: for extruding the compacted
specimen from the mould
3.
Compaction pedestal and hammer.
4. Breaking
head.
5. Loading
machine
6. Flow
meter, water bath, thermometers
Marshall test
PROCEDURE:-
In the Marshall test method of mix design three compacted samples are
prepared for each binder content. At
least four binder contents are to be tested to get the optimum binder
content. All the compacted specimens are
subject to the following tests:
• Bulk density determination.
• Stability and flow test.
• Density and voids analysis.
Preparation of
test specimens:-
The coarse aggregate, fine aggregate, and the filler material should
be proportioned so as to fulfill the requirements of the
relevant standards. The required
quantity of the mix is taken so as to produce compacted bituminous mix
specimens of thickness 63.5 mm approximately.
1200 gm of aggregates and filler
are required to produce the desired thickness. The aggregates are heated to a temperature of
175° to 190°C the compaction mould assembly and rammer are cleaned and kept
pre-heated to a temperature of 100°C to 145°C.
The bitumen is heated to a temperature of 121°C to 138°C and the required
amount of first trial of bitumen is added to the heated aggregate and
thoroughly mixed. The mix is placed in a
mould and compacted with number of blows
specified. The sample is taken out of
the mould after few minutes using sample extractor.
Bulk
density of the compacted specimen:-
The bulk density of the sample is usually determined by
weighting the sample in air and in water.
It may be necessary to coat samples with paraffin before determining
density. The specific gravity Gbcm of
the specimen is given by
Stability test:-
In conducting the stability test, the specimen is immersed in a bath of
water at a temperature of 60° ± 1°C for a period of 30 minutes.
It is then placed in the Marshall stability testing machine (Fig. 7.1)
and loaded at a constant rate of deformation of 5 mm per minute until
failure. The total maximum in kN (that
causes failure of the specimen) is taken as Marshall Stability. The stability value so obtained is corrected
for volume (Table 11.1). The total
amount of deformation is units of 0.25 mm that occurs at maximum load is recorded as Flow Value. The total time between removing the specimen
from the bath and completion of the test should not exceed 30 seconds.
RESULTS AND
CALCULATIONS:-
Following results and analysis is performed on the data obtained from
the experiments
Bulk specific
gravity of aggregate (Gbam):-
Since the aggregate mixture consists of different fractions of coarse
aggregate, fine aggregate, and mineral filler with different specific
gravities, the bulk specific gravity of the total aggregate in the paving
mixture is given as
Maximu
specific gravity of aggregate mixture (Gbam):-
The maximum specific gravity of aggregate mixture should be obtained as
per ASTM D2041, however because of the difficulty in conducting this experiment
an alternative procedure could be utilized to obtain the maximum specific
gravity using the following equation:
Percent voids
in compacted mineral aggregate (VMA):-
The percent voids in mineral
aggregate (VMA) is the percentage of void spaces between the granular particles
in the compacted paving mixture, including the air voids and the volume
occupied by the effective asphalt content.
Percent air
voids in compacted mixture (Pav):-
Percent air voids is the ratio
(expressed as a percentage) between the volume of the air voids between the
coated particles and the total volume of
the mixture.
DETERMINATION
OF OPTIMUM BINDER CONTENT:-
Five separate smooth curves are
drawn with percent of asphalt on x-axis and the following on
y-axis
• unit weight
• Marshall stability
• Flow
• VMA
• Voids in total mix (Pav)
Optimum binder content is selected as the average binder content for
maximum density, maximum stability and specified percent air voids in the total
mix. Thus
EVALUATION AND
ADJUSTMENT OF MIX DESIGN:-
The overall objective of the mix design is to determine an optimum
blend of different components that will satisfy the requirements of the given
specifications ,This mixture should have:
1. Adequate amount of asphalt to ensure a durable pavement.
2. Adequate mix stability to
prevent unacceptable distortion and displacement when traffic load is applied.
3. Adequate voids in the total compacted mixture to permit a small
amount of compaction when traffic load is applied without bleeding and loss of
stability.
4. Adequate workability to facilitate placement of the mix without
segregation.
If the mix design for the optimum binder content does not satisfy all
the requirements of specifications, It is necessary to adjust the original
blend of aggregates. The trial mixes can
be adjusted by using the following guidelines.
1. If low voids : The voids can be increased by adding more
coarse aggregates.
2. If high voids : Increase the amount of mineral filler in the
mix.
3. If low stability: This condition suggests low quality of
aggregates. The quality of aggregates
should be improved. (use different aggregate or use cement coated aggregate).
Thank you!
