## USBR Method of Concrete Mix Proportioning

Principle

In this method of mix design, the water content of air entrained concrete and the proportions of fine and coarse aggregates are determined for a fixed workability and grade of fine aggregates.

Procedure of mix proportion

Table 1 recommends maximum slump for various types of concrete construction works for which an appropriate selection may be made.

Water Cement Ratio

• The water cement ratio for the target mean 28 days compressive strength is determined from table 2 for either air entrained concrete with water reducing, set-controlling admixtures.
• The water cement ratio then be checked against the maximum permissible value for the type of exposure and construction work specified in table-3.
• lower of two values be adopted.

refer ppt

• Approximate air and water contents and the percentages of sand coarse ag­gregate per cubic meter concrete are determined from the table 4, for con­crete containing natural sand fineness of modulus 2.75 and having worka­bility of 75 to 100m.
• Adjustment of values in water content and percentage of sand or coarse ag­gregate are made (Table 5) for changes in the fineness modules of sand ,slump of concrete ,air content ,water-cement ratio and sand con­tent other than the reference values in Table 4.

refer ppt

• The cement content is calculated using the selected water-cement ratio and the final water content of the mix is arrived after adjustments.
• Proportions of aggregates are determined by estimating the quality of coarse aggregate from the table 4 or by computing the total solid volume of sand and coarse aggregate in the concrete mix and multiplying the final percentage after  adjustment.

Example Problem
Data Given:-
1.Type of structure: Other structure.
2.Type of exposure: Severe climatic (Class B). Refer Table 3.6.
3.Mean cylinder strength: 250kg/cm(sq) at 28 days.
4.Slump: 75mm.
5.Fineness modulus of sand: 2.75.
6. Coarse aggregate:
a) Maximum size of aggregate: 40mm.
b) Dry rodded unit weight: 1750kg/m(cube).
7.  Specific gravity:
a) Cement: 3.15
b) Coarse aggregate: 2.68
c) Fine aggregate: 2.63

Mix Design Procedure

Mix design procedure:
Step 1: Read water cement ratio for the mean compressive strength of 25N/mm( square) at 28 daays from table.
Water cement ratio : 0.55
Step 2: Maximum permissible water cement ratio from table 3.6 for severe condition is 0.5.
So adopt water cement ratio : 0.50
Step 3: Read value of air content water content percentage dry rodded unit weight for maximum size of aggregate equal to 40mm from table 3.7:
Water content : 145 Kg/m(cube).
Air content : 4.5 %
Sand: 34%.
Therefore weight of cement : 145/0.5= 290Kg.
Volume of sand: [(1-0.45)1000-290*1000/3.15-145*1000-244.1*1000]*0.34
=244.1×10^3 cm(cube)

Weight of sand: 244.1×2.63=641.98Kg.
Volume of coarse aggregate: [(1-0.45)*1000-290*1000/3.15-145*1000-244.1*1000] =473.84×10^3 cm(cube).
Weight of coarse aggregate: [(1-0.045)*1000-290*1000/3.15-145*1000-244.1*1000] =473.84X10^3 cm(cube).
Weight of coarse aggregate: 473.84*2.68= 1269.88 Kg.

Step 4: Alternately when the aggregate is proportioned by the dry -rodded unit weight of coarse aggregate, the amount of coarse aggregate is calculated as:
Dry-rodded unit weight = 73% from table.
Weight of coarse aggregate: 0.73*1750=1277.50 Kg.
The volume of sand and coarse aggregate  can be calculated as in Step 3.
REFER PPT for better understanding 