BLENDING OF AGGREGATES

BLENDING OF AGGREGATES

The mixing of two or more aggregates so as to obtain different aggregate properties.

A good asphalt concrete pavement requires more than asphalt, aggregates and equipment.

It also requires knowledge, skill, and workmanship. Part of this knowledge and skill is the

ability to blend aggregates to meet a specified target,known as the job-mix formula.

Asphalt concrete requires the combining of two or more aggregates, having different

gradations, to produce an aggregate blend that meets gradation specifications for a

particular asphalt mix.

Definition of a Job-Mix Formula

In its simplest form, a job-mix formula consists of two parts:

1. The Combined Gradation of the aggregates to be used in the production of the

asphalt concrete mixture.

2. The Asphalt Content necessary to produce a satisfactory mix meeting all the

specification requirements.

Method for Combining Aggregates

Mathematical procedures are available to determine an optimum combination of

aggregates, but the “Trial and Error Method” guided by a certain amount of reasoning is

the most practical procedure to determine a satisfactory combination and the one we will

demonstrate.

Trial and Error Method

Step 1 - Obtain the required data.

a. The gradation of each material must be determined.

b. The design limits for the type of mix must be obtained.

Step 2 - Select a target value for trial blend.

The target value for the combined gradation must be within the design limits

of the specifications. This value now becomes the target for the combined

gradation.

Step 3 - Estimate the proportions.

Estimate the correct percentage of each aggregate needed to get a combined

gradation near the target value. For example, if aggregates are combined, a

possible combination may be 30% of Aggregate 1 and 70% of Aggregate 2.

Step 4 - Calculate the combined gradation.

This calculation will show the results of the estimate from Step 3. The

method of calculating the combined gradation will be shown in the example

problem.

Step 5 - Compare the result with the target value.

If the calculated gradation is close to the target value, no further adjustments

need to be made; if not, an adjustment in the proportions must be made

and the calculations repeated. The second trial should be closer due to

the “education” received from the first. The trials are continued until the

proportions of each aggregate are found that will come close to the target

value. If the aggregates will not combine within the design range, it may be

necessary to use or add different materials.

General Math Conversion:

Convert a percent (%)to a decimal, divide by 100 or move decimal place two places to

the left.

                         Example: 75%:                      75/100 = .75

BLENDING WORKSHEET

Use this worksheet to mathematically blend aggregates by hand.

Example Problem No. 1

Trial and Error Combination of Two Aggregates

Aggregate 1 and Aggregate 2. The mix type is an SM-12.5 A.

Step 1 - Enter the data (aggregate gradations and design limits) in appropriate columns.

Step 2 - Determine the target value

Target Value must be within design range.

The Target Value is provided by Mix Design Technician.

Step 3 - Estimate the proportions.

The first estimate might be 50% of Aggregate 1 and 50% of Aggregate 2.

Enter these figures on the line marked “% Used”.

Step 4 - Calculate the individual proportions on each sieve for each of the two

aggregates and enter in the column “% blend”. Add the two columns for each

sieve and enter in the column “Total Blend”.

Blend = % Pass x Percent Aggregate Proportion

Remember, change Percent Aggregate Proportion to decimal.

Total Blend = % Blend Aggregate 1 + % Blend Aggregate 2

Calculations:

% Blend:

Sieve                           Aggregate 1                                Aggregate 2

3/4                           100 x .50 = 50                              100 x .50 = 50

1/2                            100 x .50 = 50                                  94 x .50 = 47

3/8                             94 x .50 = 47                                  65 x .50 = 32.5

No. 8                           52 x .50 = 26                                 27 x .50 = 13.5

No. 200                         7.1 x .50 = 3.6                               1.2 x .50 = 0.6    

Total Blend:

Sieve        Aggregate 1     +       Aggregate 2     =         Total Blend

3/4                   50           +              50              =                    100

1/2                   50           +              47             =                      97

           3/8                   47            +           32.5             =          79.5 round to 80

         No. 8                  26            +           13.5             =           39.5 round to 40

No. 200                3.6             +           0.6              =                    4.2      

Step 5 - Compare this combined gradation

Compare the Total Blend with the Target Value.

Observations:

Sieves No. 3/8 and No. 8 are not close to target value, therefore an adjustment needs to

be made.

Make adjustment to the Aggregate Percentage being used.

Step 5 - Continued

For Adjustment:

Use one sieve to make an adjustment before recalculating all sieves.

This example will use the 3/8 sieve.

                       Mat’l                                       Aggregate 1               Aggregate 2

                  % Used              Trial 1                       50                                50

                                            Trial 2                       55                                 45

                                            Trial 3                       60                                 40

                                            Trial 4                       70                                  30

                                            Trial 5                       75                                  25

                                            Trial 6                       45                                  55

                                            Trial 7                       40                                  60

Conclusion from this table; Choose Trial No. 5 and recalculate rest of sieves.

Step 5 - Continued (Recalculated Blend)

Publised By: Rakesh Kumar Chhalotre
                   : राकेश  कुमार  छलोत्रे 

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