AGGREGATE GRADATION FOR JOB MIX FORMULA

OBJECT: DETERMINATION OF AGGREGATE GRADATION FOR JOB MIX FORMULA.

PERFORMANCE OBJECTIVE:

Upon successful completion of this lab experiment, students will be able to,

1. Perform sieve analysis tests on various aggregate stockpiles.
   
2. Select optimum proportions of different aggregate stockpiles that meet the design specification.
   

APPARATUS AND MATERIALS

1. Aggregates
   
2. Set of Sieves.
   
3. Oven
   
4. Mold assembly
   
5. Loading machine
   
6. Flow meter
   
7. Balance
   

THEORY:

Bruce Marshall, formerly a bituminous engineer with Mississippi State Highway Department, USA formulated the Marshall Method for designing bituminous mixes. Marshall’s test procedure was later modified and improved by the U.S. Corps of Engineers through their extensive research and correlation studies. ASTM vide designation D1559-62 T has standardized the test procedure. Generally, this stability test is applicable to hot mix design using bitumen and aggregate with a maximum size of 25 mm. The particle size distribution, or gradation, of an aggregate, is one of the most influential aggregate characteristics in determining how it will perform as a pavement material. In HMA, gradation helps in determine almost every important property including stiffness, stability, durability, permeability, workability, fatigue resistance, frictional resistance, and moisture susceptibility. The coarse aggregates, fine aggregates, and filler material should be proportioned and mixed in such a way that the final mix after blending has a gradation within the specified range. These specified gradations of mineral aggregates are given in Table 9.2.

PROCEDURE

1. Select grading to be used.
   
2. Select aggregates to be employed in the mix.
   
3. Find out the percentage passing of each size of the aggregate.
   
4. Determine the proportion of each aggregate required to achieve the design gradation.
   
5. Determine the specific gravity of the aggregate combination (Effective) and the Bitumen.
   
6. Makeup trial specimens with varying asphalt contents.
   
7. Determine the specific gravity of each compacted specimen.
   
8. Make a stability test on each specimen.
   

PREPARATION OF TEST SPECIMENS

The coarse aggregates, fine aggregates, and filler material should be proportioned and mixed in such a way that the final mix after blending has a gradation within the specified range. These specified gradation of mineral aggregates are given in Table 9.2. The aggregates and filler are mixed together in the desired proportion as per the design requirements and fulfill the specified gradation. The required quantity of the mix is to be taken so as to produce a compacted bituminous mix specimen of thickness of 63.5mm approximately.

Approximately 1200 g of aggregates and filler are taken and heated to a temperature of 175^oC to 109^oC. The compaction mold assembly and rammer are cleaned and kept pre-heated to a temperature of 100^oC to 145^oC. The bitumen is heated to temperature of 121 to 138^oC and the required quantity of the first trial percentage of bitumen (say 3.5% by weight of total mix) is added to the heated aggregate and thoroughly mixed using a mechanical mixer or by hand mixing with a trowel. The mixing temperature for 80/110 grade of bitumen may be around 154oC and that for 60/70 grade about 160oC. The mix is placed in a mold and compacted by a rammer, with 75 blows on either side. The compacting temperatures may be about 138^oC for 80/110-grade bitumen and 149^oC for 60/70-grade bitumen. The compacted specimen should have a thickness of 63.5 mm. The weight of aggregate taken may be suitability altered to obtain a thickness of 635 mm. At least three specimens should be prepared at each trial bitumen content which may be varied at 0.5 percent increments up to 5.0 or 5.5 percent.

Table 9.2: Specified Gradation of mineral aggregates