Strength of historic concrete
The strength of the concrete in historic structures is likely to be very variable. The only sure way of determining it is to carry out appropriate tests (see Testing/Hardened concrete/Core testing for strength). However, if design information is available, guidance on the likely minimum concrete strength may be obtained from the Codes of Practice current at the time.
CP 114, Structural use of reinforced concrete in buildings, which was first published in 1948, specified three nominal concrete mixes on the basis of cement: fine aggregate: coarse aggregate ratios by volume, namely 1:1:2, 1:1½:3 and 1:2:4. Minimum 28-day cube strengths for the three mixes were given as 4500, 3750 and 3000lb/sq. inch respectively (i.e. approximately 31, 26 and 21N/mm2).
CP 115, The structural use of prestressed concrete in buildings, which was first published in 1959, specified minimum 28-day cube strengths of 6000lb/sq. inch (approximately 41N/mm2) for pre-tensioned concrete and 4500lb/sq. inch (approximately 31N/mm2) for post-tensioned concrete.
CP 116, The structural use of precast concrete, published in 1965, specified 5 grades of concrete ranging from Grade A (3000lb/sq. inch or approximately 21N/mm2) to Grade E (7500lb/sq. inch or approximately 52N/mm2).
CP110, The structural use of concrete, was published in 1972. Described as “The Unified Code” it brought together the separate codes relating to reinforced, prestressed and precast concrete. Design in CP 110 (and all subsequent Codes) was based on specified concrete grades. Minimum concrete grades were specified for different types of element: 20N/mm2 for reinforced concrete, 30N/mm2 for post-tensioned prestressed concrete and 40N/mm2 for pre-tensioned prestressed concrete.
Further information may be found in Concrete Society Technical Report 70, Historical approaches to the design of concrete buildings and structures, which also includes information on likely steel strengths, allowable materials etc.