A blend of two aggregates is required to meet the specificat

A blend of two aggregates is required to meet the specifications of a DOT for a concrete mix design. The sieve analysis data is shown in the table below has been determined in the lab for the two aggregates. The design range for each size of the aggregates has been determined from the DOT specifications. A target value for each sized aggregate has been provided by the mix design technician and is also shown in the table below. Design a blend that will meet the specifications for the target values. For Trial #1, consider a 50%/50% blend. Material Aggregate 1 Aggregate 2 Target Value Design Range % Pass 100 98 Sieve 3/4\" 1/2\" 3/8 #8 #200 %Pass 100 100 100 100 12 100 95-100 90 max 34-50 2-10 100 98 49 5.5

Solution

For designing of Blend , there are various steps

Step 1 - Calculate total blend

Step 2 - Check whether total Blend and target values are closest or not, if not follow step 3

Step 3 - Apply trial method by changing the % of blend

Step 4 - By following step 3 , the closest value or one solution is obtained.

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

Trial 1 for 50%/50%

Calculations:

% Blend:

Sieve Aggregate 1 Aggregate 2

3/4 100 x .5 = 50 100 x .5 = 50

1/2 100 x .5 = 50 98 x .5 = 49

3/8 100 x .5 = 50 77 x .5 = 38.5

No.8 100 x .5 = 50 9 x .5 = 4.5

No.200 12 x .5 = 6 0 x .5 = 0

Total Blend

Sieve Aggregate 1 + Aggregate 2 = Total Blend

3/4 50 50 100

1/2 50 49 99

3/8 50 38.5 88.5

No.8 50 4.5 54.5

No.200 6 0 6

Observatory Table

Sieve % % % % Total Target Design

(in) Pass Blend Pass Blend Blend Value Range

3/4 100 50 100 50 100 100 100

1/2 100 50 98 50 99 98 95- 100

3/8 100 50 77 50 88.5 88 90 max

No. 8 100 50 9 50 54.5 49 34 – 50

No. 200 12 50 0 50 6 5.5 2 – 10

Now, Compare the Total Blend with the Target Value

we see that sieve no. 8 are not close to target value, therefore, an adjustment needs to be made.

Make adjustment to the Aggregate Percentage being used

Use sieve no. 8 to make an adjustment before recalculating all sieves

By hit and trial method

Trial 2 - consider 45% / 55 %

For sieve no. 8

Aggregate 1 Aggregate 2

100 x .45 = 45 9 x .55 = 4.95

Total = Aggregate 1+ Aggregate 2 = 45 + 4.95 =49.95

since, now the total blend is close to the total value, so now we recirculate rest of sieves.

Sieve Aggregate 1 Aggregate 2

3/4 100 x .45 = 45 100 x .55 = 55

1/2 100 x .45 = 45 98 x .55 = 53.9

3/8 100 x .45 = 45 77 x .55 = 42.35

No.8 100 x .45 = 45 9 x .55 = 4.95

No.200 12 x .45 = 5.4 0 x .55 = 0

After trial method, Total Blend

Sieve Aggregate 1 + Aggregate 2 = Total Blend

3/4 45 55 100

1/2 45 53.9 98.9

3/8 45 42.35 87.35

No.8 45 4.95 49.95

No.200 5.4 0 5.4

Now the recirculated table is

Sieve % % % % Total Target Design

(in) Pass Blend Pass Blend Blend Value Range

3/4 100 45 100 55 100 100 100

1/2 100 45 98 55 98.9 98 95- 100

3/8 100 45 77 55 87.3 88 90 max

No. 8 100 45 9 55 49.95 49 34 – 50

No. 200 12 45 0 55 5.4 5.5 2 – 10

This is one possible solution. The Total Blend can be adjusted to be closer to the target

value. By using such hit and trial method , the design of blend closest to target value can be accomplished.