Specification of historic concrete
The First Edition of Reinforced Concrete Designers’ Handbook (1932) by Reynolds proposes a series of concrete mixes A to F, with proportions ranging from 1:3:6 up to 1:1:2. The corresponding working stresses for the hardened concrete range from 400 psi up to 875 psi (say 2.8–6.0N/mm2), though it is not clear what cube strengths these would equate to.
A later edition of the Reinforced Concrete Designers’ Handbook gives grades of concrete from the London by-laws, ranging from Grade V with a 28 day cube strength of 1100 psi (7.6N/mm2) to Grade IA (4500 psi or 31.3N/mm2).
The specification of concrete in accordance with CP 114, Structural use of reinforced concrete in buildings (first published in 1948), was on the basis of cement: fine aggregate: coarse aggregate ratios by volume, namely 1:1:2, 1:1˝:3 and 1:2:4. The code gave minimum values for the cube strengths associated with the three Nominal Mixes.
The Metric version of CP 114 (in 1969) retained the three Nominal Mixes but added a Table of Standard Mixes. There were three specified 28-day cube strengths (21, 25.5 and 30N/mm2) and mix proportions were given for different aggregate sizes and slumps.
The Ninth Edition of the Reinforced Concrete Designer’s Handbook by Reynolds and Steedman presents the same information (for 19mm aggregate only) but defines the mixes in terms of the letters A, B and C; Standard Mix A is equivalent to the 21N/mm2 mix in CP 114, Mix B is equivalent to 25.5N/mm2 and Mix C is equivalent to 30N/mm2. The Handbook also includes Designed Mixes D and E, with 28-day cube strengths of 40 and 50N/mm2 respectively.
CP 115, The structural use of prestressed concrete in buildings (1959) specified minimum cube strengths of 6000psi for pre-tensioned concrete and 4500psi for post-tensioned.
CP 116, The structural use of pecast concrete (1965) specified 5 Grades of concrete ranging from 3000psi (Grade A) to 7500psi (Grade E). These are broadly in line with the A to E mixes in the Ninth Edition of Reynolds.
With the introduction of CP 110 in 1972, concrete was specified in terms of the 28-day cube strength only.
Further information on specified concrete strengths may be found in Concrete Society Technical Report 70, Historical approaches to the design of concrete buildings and structures, which also contains information on allowable materials, reinforcement strengths, design methods etc.