Experiment – 7
MARSHALL METHOD OF MIX DESIGN
The Marshall Method of mix design is intended both for laboratory design and field control of bituminous hot-mix dense graded paving mixtures. Originally developed by Bruce Marshall of the Mississippi State Highway Department, the US Army Corps of Engineers refined and added certain features to Marshall’s approach and it was then subsequently formalized as ASTM D 1559 and AASHTO T245.
Outline of Method
It uses standard cylindrical test specimens (64 mm high and 102 mm in diameter). Two principal features of Marshall Method of mix design are Density Void Analysis and Stability Flow Test of compacted test specimen.
Stability of test specimens is the maximum load resistance developed by standard test specimen at 60°C in Newton.
Flow value is the total movement or displacement occurring in the specimen between no load and the point of maximum load during stability test in units of 0.25mm.
Selection and combination of Aggregates
Selection of the aggregates to be used in a given paving mixture is a very important phase of the design phase. In normal procedure, both coarse and fine aggregates available in the near vicinity of the proposed work are sampled and carefully examined for compliance with the individual specifications for these materials. In case no suitable single aggregate is available then aggregates from several different sources may have to be blended to get the required specified specimen. The proportions selected must be within the specification and far enough from its extremes to provide room for the job mix tolerance so that when it is added or subtracted the mixture will not be outside the original specification master range. Sieve Analysis of the aggregates can most economically be used in this case as determined by AASHTO methods of T27 and T37.
MINERAL AGGREGATE AND MIX COMPOSITION
|Passing Sieve Designation||Retained on Sieve Designation||Percent by weight|
|¾ in. (19.0 mm)||½ in. (12.5 mm)||0-6|
|½ in. (12.5 mm)||3/8 in. (9.5 mm)||9-40|
|3/8 in. (9.5 mm)||No.4 (4.75 mm)||9-45|
|No.4 (4.75 mm)||No. 10 (2.00 mm)||8-27|
|Total Coarse Aggregate||No. 10 (2.00 mm)||50-65|
|No. 10 (2.00 mm)||No. 40 (0.475 mm)||6-22|
|No. 40 (0.475 mm)||No. 80 (0.177 mm)||8-27|
|No. 80 (0.177 mm)||No. 200 (0.75 mm)||5-17|
|No. 200 (0.75 mm)||——||5-8|
|Total fine aggregate & filler||Passing No. 10||35-50|
|Total mineral aggregate||——||100|
|Total mineral aggregate||92-95|
It is obvious that the fine aggregate and coarse aggregate only, in any combination, cannot meet the requirements of the pecifications for total mineral aggregate. So, mineral fillers must be used in the mixture.
SIEVE ANALYSIS OF AGGREGATES (PERCENTAGE USED FOR EXPERIMENT)
|Passing Sieve Designation (Percent by weight)||Retained on Sieve Designation (Percent by weight)||AGGREGATE TYPE|
|COARSE AGGREGATES||FINE AGGREGATES||MINERAL FILLER|
|¾in.(19.0 mm)||½in.(12.5 mm)||5||—-||—-|
For first trial, the amount of mineral filler is arbitrarily set at 8 percent. The total coarse aggregate in the mix must be from 50 to 65 percent, and this figure is set as 52 percent. The remaining 40 percent must be fine aggregate.
Calculations made using the indicated proportions in determining the sieve analysis of the combined aggregates are as follows. A comparison of the figures in the last column of the table with the requirements of the specification will show that this combination of aggregates meets the stipulated requirements. This combination will therefore be judged satisfactory and no additional trials are made here.
CALCULATIONS FOR SIEVE ANALYSIS
|Passing Sieve Size||Retained on Sieve Size||Percent by weight||Agg. Wt. (gm)|
|COARSE AGGREGATE||FINE AGGREGATE||MINERAL FILLER||TOTAL AGG.|
|¾in.||½in.||0.52 x 5 =||—-||—-||2.6||31.2|
|(19.0 mm)||(12.5 mm)||2.6|
|½in.||3/8in.||0.52 x 32 =||—-||—-||16.6||199.2|
|3/8in.||No.4||0.52 x 37 =||—-||—-||19.2||230.4|
|No.4||No.10 (2.00mm)||0.52 x 22 =||0.40 x 7 =||—-||14.2||170.4|
|No.10 (2.00mm||No.40 (0.475m||0.52 x 4 =||0.40 x 28 =||—-||13.4||160.8|
|No.40||No.80||—-||0.40 x 39 =||0.08 x 5||16||192|
|No.80 (0.177mm||No.200 (0.75mm)||—-||0.40 x 24 =||0.08 x 30||12||144|
|No.200 (0.75mm||—–||—-||0.40 x 2 =||0.08 x 65||6||72|
DESIGN BITUMEN CONTENT
When determining the design bitumen content for a particular blend or gradation of aggregates by Marshall Method, a series of test specimens is prepared for a range of different bitumen contents so that the test data curves show well-defined relationships.
Bitumen content can be estimated from following sources
- Ø Experience
- Ø Using filler-to-bitumen ratio guideline ( ranges 06 to 1.2 )
- Ø Computational Formula
P = 0.035a + 0.045b + K c + F
P = Approximate mix bitumen content, percentage by weight of total mix
a = Percentage of mineral aggregate retained on sieve No.10 in whole no.
b = Percentage of mineral aggregate passing sieve No.10 and retained on sieve No.200
c = Percentage of mineral aggregate passing sieve No.200
K = 0.15, 11-15% passing sieve No.200.
= 0.18, 6-10% passing sieve No.200
=0.20, 5% passing sieve No.200
F = 0 to 2%, based on the absorption of light and heavy aggregates. F = 0.7 incase no data is available.
Preparation of test specimens
- Separate the aggregate in desired fractions by dry sieving.
- The aggregates are first dried to a constant weight at 105 to 110°C.
- Amount of each size fraction required to produce a batch that will give 63.5+1.27 mm high compacted specimen is weighed in a separate pan for each test specimen. It is about 1.2kg of dry aggregates.
- Prepare at least three specimens for each combination of aggregates and bitumen.
- Dry the aggregates to the required mixing temperature.
- Add heated aggregates in a mixing bowl and the required quantity of bitumen is added. Mixing is carried out until all aggregate particles are fully coated with bitumen.
- Optimal viscosity of bitumen for compaction is between 2 Pa.s and 20 Pa.s. If viscosity is too low, the mix will be excessively mobile resulting in pushing of the material in front of the roller, high viscosities will significantly reduce the workability of the mix and little compaction will be achieved.
- Depending upon design traffic category (light, medium and heavy), the compacted mix is expected to withstand 35, 50 and 75 blows respectively applied with compaction hammer to each end of the specimen.
- After compaction, specimens are allowed to cool in air at room temperature until no deformation results on removal from the mould.
- Sieves conforming to ASTM Standards
- Compaction Hammer
- Flow meter
- The Marshall Testing Machine, a compression testing machine
In Marshall Method, each compacted test specimen is subjected to following tests and analysis in the order listed.
- Bulk Specific Gravity Test
- Stability and Flow Test
- Density and Void Analysis
Bulk Specific Gravity Test is performed on freshly compacted specimens after they have cooled to room temperature.
Then immerse the specimen in a water bath at 60°C for 30 to 40 minutes and perform Stability and Flow Tests.
The Testing Machine will apply loads to test specimens through cylindrical segment testing heads at a constant rate of vertical strain of 51mm per minute. Loading is applied until the specimen failure occurs.
MARSHALL STABILITY VALUE
The force in Newton required producing failure of the test specimen. The applied testing load is determined from calibrated proving ring.
The magnitude of deformation of the specimen at the point of failure. The point of failure is defined by the maximum load reading obtained.
- v All fractions of aggregates are heated to a temperature of 250°F. Bitumen of specified grade is heated to a temperature of 350°F. Bitumen should not be heated for more than an hour. The required quantity of aggregates and bitumen is mixed manually or electrically at a temperature of 200 to 300°F. After mixing place it in a compaction mould and give 75 blows to the sample on each side. The specimen is then immersed in water bath at a testing temperature of 60°C for 30 to 40 minutes. Then remove the specimen from water bath and place it on a base plate of Marshall Loading Machine. The proving ring and flow gauge are adjusted to zero reading. The base plate of machine moves upward at a rate of 2 inches per minute. The value of maximum load and dial gauges are recorded and machine is reversed. The elapsed time for the test after the removal of specimen is noted.
Percentage Air Voids, Va
Va =[100 X ( Gmm – Gmb )] / Gmm
Va = air voids in compacted mixture as a percentage of total volume.
Gmm = maximum specific gravity of a paving mixture,
Gmb = Bulk specific gravity of a compacted mixture.
Gmm = 100/( W 1 / G 1 + W 2 / G 2 + W 3 / G 3 + W 4 / G 4 )
W 1 = %age weight of coarse aggregates
W 2 =%age weight of fine aggregates
W 3 = %age weight of mineral aggregates
W 4 = %age weight of bitumen
G 1 = Specific Gravity of coarse aggregates
G 2 = Specific Gravity of fine aggregates
G 3 = Specific Gravity of mineral aggregates
G 4 = Density of bitumen (g/cm3)
Percentage of Voids in mineral Aggregates, VMA
VMA = Vb + Va
Where, Va = %age of voids in aggregates , Vb = %age of voids in bitumen
Vb = Gmb X ( W 4 / G 4 )
Percentage of voids filled with bitumen, VFB
VFB = ( Vb / VMA ) X 100
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