Concrete mix design

Question 1

What is the mix design process?

Answer 1

Stage 1: Selection of target water/cement ratio
Stage 2: Selection of free-water content
Stage 3: Determination of cement content
Stage 4: Determination of total aggregate content
Stage 5: Selection of fine and coarse aggregate contents

Question 2

How to do stage 1?

Answer 2

If previous information concerning the variability
of strength tests comprises less than 20 results,
the standard deviation (S) to be adopted should
be that obtained from line A in Figure 1.

If previous information is available consisting of
20 or more results, the standard deviation of such
results may be used provided that this value is not
less than the appropriate value obtained from line
B

Calculate:
Margin (M) = k * s
Target mean strength (fm) = fc + m (fc is specific characteristic strength)

Obtain compressive strength value from table 1 for strength of mix made with free-water/cement ratio of 0.5 according to AGE, CEMENT TYPE, and AGGREGATE TO BE USED.
Then, draw a line parallel to the others to plot that value on figure 2.
e.g. strength is 49, so go to (0.5, 49) on graph and draw parallel line until line reaches y = TARGET MEAN STRENGTH (fm), then use the x value.
That value is the free-w/c ratio.

Question 3

How to do stage 2?

Answer 3

Determine free water content (W) from table 2 according to type and maximum aggregate size.
If coarse aggregate and fine aggregate are of DIFFERENT TYPES, then use this formula:
2/3 * W(fine) + 1/3 * W(coarse)

Question 4

How to do stage 3?

Answer 4

Cement content (C) = Free-water content/w/c ratio

If content is lower than min. value, use min value and either
1. Keep the same water content and make new w/c ratio smaller than stage 1
2. Keep same w/c ratio and make new water content larger

If content is larger than max. value, mix should be redesigned.

Question 5

How to do stage 4?

Answer 5

Look at figure 3, if relative density is known, use that value and match with (x-value) free-water content, get the y-value.
If unknown, 2.6 for uncrushed aggr, 2.7 for crushed aggr.
This is the wet density of concrete mix (D).

Total aggr content = D - C - W

Question 6

How to do stage 5?

Answer 6

Look at figure 4, using relevant info, find the proportion of fine aggr needed.
Fine aggr content = Total aggr content * proportion
Coarse aggr content = Total - fine

For coarse aggr.
1:2 for 10 mm and 20 mm
1:1.5:3 for 10 mm, 20 mm and 40 mm