Example Using Absolute Volume Method
Determine the mix proportions for a retaining wall, using the following specifications
and conditions:
Required 28day compressive strength (f´_{c})
Maximum size aggregate
Exposure condition
Fineness modulus of fine aggregate
Specific gravity of portland cement
Specific gravity of fine aggregate
Specific gravity of coarse aggregate
Dryrodded unit weight of coarse aggregate
Dryrodded unit weight of fine aggregate
Slump
Cement
3,000 psi
3/4 in.
Moderate freezethaw exposure—
exposure to air
2.70
3.15
2.65
2.60
102 lb/cu ft
100 lb/cu ft
3 in.
Type IA
To determine the mix proportions, proceed as follows:
1. Estimate the air content. From table 173, the air content should be 5 percent
(3/4inch aggregate, airentrained concrete, moderate exposure).
2. Estimate the mixing water content. From table 173, you should use 305 pounds
of mixing water per cubic yard of concrete (3inch slump, 3/4inch aggregate, airentrained
concrete).
3. Determine the watercement ratio. From table 172, a watercement ratio of 0.59
will satisfy the strength requirement for 3,000 psi concrete. From table 171, you find that a
watercement ratio of 0.50 will satisfy the exposure conditions. Since 0.50 is the smaller of
the ratios, that is what you should use.
4. Calculate the cement content. By using the weight of the mixing water content
(Step 2) and the watercement ratio (Step 3), you can determine the cement content as
follows:
5. Calculate the coarse aggregate content. By using table 175 and interpolating
between fineness moduli of 2.6 and 2.8, you find that for 3/4inch aggregate having a fineness
modulus of 2.7, the volume of dryrodded aggregate per unit volume of concrete is 0.63.
Therefore, the volume of coarse aggregate needed for 1 cubic yard of concrete is
0.63 x 27 = 17.01 cubic feet. Since the dryrodded weight of the coarse aggregate is 102
pounds per cubic foot, then the weight of the coarse aggregate for a cubic yard of the concrete
is 17.01 x 102= 1,735 pounds.
176

