Asphalt: Acceptability of Marshall Flow Test Results

[Adesh_S]

I have to determine whether Marshall Flow values from three pucks prepared from the same sample of hot mix asphalt are acceptable according to the provisions of ASTM D6927 and ASTM C670. The three pucks were prepared by the same operator. The Marshall Flow values are 15, 21 and 26 (0.25mm).

The average of the flows is 20.7 (0.25mm)
The standard deviation is 5.51
The co-efficient of variance of individual measurements is 26.6%

From ASTM D6927 and C670:
Number of samples = 3
Multiplier for Table 2 = 5.7
Co-efficient of variance for acceptance, 1s% = 9%
Maximum acceptable range as a percentage of mean = 51.3%

My question pertains to the calculation of the range as a percentage of the mean.

Method 1

Is the range calculated as the difference between the highest and the lowest values, that is, 11 (=26-15), and then the range as a percentage of the mean is 11/20.7*100 = 53.1%? Therefore, in this case, the range as a percentage of the mean (53.1%) is outside of the maximum acceptable range (51.3%), and the results can be considered suspect.

Or is the range calculated as follows.

Method 2

Flow (0.25mm)

Range as a percentage of mean​

15

27.5% (=(20.7-15)/20.7*100))​

21

1.4% (=(21-20.7)/20.7*100))​

26

25.6% (=(26-20.7)/20.7*100))​

In this case, the range as a percentage of mean (1.4% to 27.5%) is within the maximum acceptable range (51.3%) and the results can be considered acceptable.

Any guidance on the above query will be sincerely appreciated.

Thank you.