General Order 95
Section IV
Strength Requirements for All Classes of Lines
48.3 Concrete
A. Reinforced Concrete
Values used for ultimate strengths of reinforced concrete,
in conjunction with safety factors given in
Rule 44
, shall not exceed the following:
Reinforcing steel, tensile or compressive strength, pounds per square inch:55,000
Concrete, 1:2:4 
Age 
Compressive Strength 
7 days 
900 lbs per sq. in. 

30 days 
2,400 lbs per sq. in. 

90 days 
3,100 lbs per sq. in. 

6 months 
4,400 lbs per sq. in. 
If reinforced concrete is designed for higher
strength values which are proven by test, such values may be used in lieu
of the figures given.
B. Prestressed Concrete
The minimum strength of the materials used
in prestressed concrete structures used in conjunction with the safety factors
given in Table 4
shall be as follows:
Reinforcing Steel  yield strength 40,000 lbs per sq. in.
Prestressing Steel – yield strength 188,000 lbs per sq. in.
Concrete  compressive strength at 28 days 4,000 lbs per sq. in.
Other strength values may be used provided the strength values used for design are proven by tests.