**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/mm^{2}), 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/mm^{2}) to Grade IA (4500 psi or 31.3N/mm^{2}).

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/mm^{2}) 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/mm^{2} mix in CP 114, Mix B is equivalent to 25.5N/mm^{2} and Mix C is equivalent to 30N/mm^{2}. The Handbook also includes Designed Mixes D and E, with 28-day cube strengths of 40 and 50N/mm^{2} 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.