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Reinforcement design strengths (historic)

The reinforcement stresses (in tension) to be used in design, as specified in the Code current at the time, have changed over the years as indicated below. Note that design stresses in compression were lower than in tension until 1972. For information on the Standards in force at the time and the specified yield stresses see Reinforcement Standards (historic)

CP 114: 1948 Steel with guaranteed f_y
0.5f_y but not more than 27,000 psi
Mild steel
Not more than 18,000 psi

CP 114: 1957 Steel with guaranteed f_y
0.5f_y but not more than 30,000 psi
Mild steel
Not more than 20,000 psi for bar sizes up to 1½ inches, not more than 18,000 psi for bar sizes above 1½ inches.

CP 114: 1957 amended 1965
High bond bars and fabric
0.55 f_y but not more than 33,000 psi for bar sizes up to 7/8 inch, not more than 30,000 psi for bar sizes above 7/8 inch.
Mild steel and plain bars
Not more than 20,000 psi for bar sizes up to 1½ inches, not more than 18,000 psi for bar sizes above 1½ inches.

CP 110: 1972 and BS 8110: 1985
f_y values of the relevant material standards.

Further information on how reinforcement has developed over the years is given in Concrete Society Technical Report 70, Historical approaches to the design of concrete buildings and structures. 

Acknowledgement: The Concrete Society

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Historic reinforcing bars and steel fabric

TR70 Historical approaches to the design of concrete buildings and structures