Example Using Absolute Volume Method
Determine the mix proportions for a retaining wall, using the following specifications
and conditions:
Required 28-day 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
Dry-rodded unit weight of coarse aggregate
Dry-rodded unit weight of fine aggregate
Slump
Cement
3,000 psi
3/4 in.
Moderate freeze-thaw 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 17-3, the air content should be 5 percent
(3/4-inch aggregate, air-entrained concrete, moderate exposure).
2. Estimate the mixing water content. From table 17-3, you should use 305 pounds
of mixing water per cubic yard of concrete (3-inch slump, 3/4-inch aggregate, air-entrained
concrete).
3. Determine the water-cement ratio. From table 17-2, a water-cement ratio of 0.59
will satisfy the strength requirement for 3,000 psi concrete. From table 17-1, you find that a
water-cement 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 water-cement ratio (Step 3), you can determine the cement content as
follows:
5. Calculate the coarse aggregate content. By using table 17-5 and interpolating
between fineness moduli of 2.6 and 2.8, you find that for 3/4-inch aggregate having a fineness
modulus of 2.7, the volume of dry-rodded 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 dry-rodded 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.
17-6
