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788 Concrete grade confusion in software

Report ID: 788

Published: Newsletter 54 - April 2019

Report Overview

A reporter noticed that the design calculation report generated by a proprietary software package called for "concrete characteristic strength = 45N/mm2", but did not confirm whether this refers to the cylinder or cube strength.

Report Content

A reporter reviewed a third-party design of a composite floor deck and noticed that the design calculation report had been generated by a proprietary software package which called for "concrete characteristic strength = 45N/mm2". There is no information in the calculation confirming whether this refers to the cylinder or cube strength.

The drawings correctly called up that C45/55 concrete was required, however, unfortunately grade C35/45 was poured on site. This appears to have been caused because whoever ordered the concrete was more familiar with the calculation report than with the drawings. Upon seeing "concrete characteristic strength = 45N/mm2", they thought that this meant C35/45.

Fortunately, cube test results taken when the floors were poured showed that the concrete mix eventually achieved the 45N/mm2 needed. However, there is a lesson to always be clear as to what concrete grade is being specified, and to make sure the drawings take precedence over the calculations to show what is to be built.

The reporter has contacted the software developer to suggest they amend their software.


For designed concrete, the 'concrete characteristic strength' is the lower number and is meant to be a true representation of the strength as opposed to the 'characteristic cube strength', which is a measure of the strength expected from control cubes. The double designation was meant to improve understanding, however, it does rely on a level of competency from designers and contractors.

The Concrete Centre publication How to specify concrete for civil engineering structures using BS 8500 and the National Structural Concrete Specification (NSCS) provide further information on specifying concrete, including inspection and testing requirements.

In 2014, the BBC published an article Great miscalculations: The French railway error and 10 others about failures due to mis-matches in units. One example was the 1999 $125m Mars Orbiter, designed as the first interplanetary weather satellite, that was lost because the NASA team used metric units while a contractor used imperial. An investigation said the "root cause" of the loss was the "failed translation of English units into metric units" in some software. A far cry perhaps from mixing up C35/45 and C45/55 concrete, but in the history of engineering failures there have been catastrophic examples consequent on mis-interpreting units.

A fundamental aim of any structural design is clarity of intent and clarity on the specifics of any product or material used. A recent CROSS report on steelwork pointed out that it was impossible to differentiate between grades simply by appearance and the same goes for most concrete. That said, a contractor should seek clarification from the designer in case of doubt about differences between calculations and drawings. Normally the drawing would be assumed to be correct.

A typical Swiss cheese model for avoiding failure provides a series of checks and one of these should be to make sure that designs and drawings are compatible.


The following additonal text was published in Newsletter 55 in July 2019:

The feedback received on report 788 Concrete grade confusion in software, published in Newsletter 54, highlighted some differences in understanding across different sectors of the industry.

Concrete specified to BS8500 has a dual designation, for example C45/55. From a testing point of view, the first number is the characteristic strength found from testing cylinders with a height to diameter ratio of 2. The second number is the characteristic strength of the same concrete found through testing cubes where the height to width ratio is 1. From a design point of view, the first number reflects a value close to the uniaxial strength of the concrete. The uniaxial strength is the true strength of the concrete, independent of any confinement such as that caused at the ends of specimen by friction with the surface of the testing machine.

Historically, UK design practice was to use the concrete compressive cube strength, appropriately reduced, for design and to specify just the cube strength to the supplier. Since the introduction of the Eurocodes, structural concrete design has been based on the characteristic cylinder strength as this better reflects the structural strength of the concrete. Cube samples still usually form the basis of UK conformity testing.

Therefore, there is a risk, as highlighted in the original report, that a designer using the characteristic concrete strength of 45MPa in the design, and then specifying a concrete strength of 45MPa, would end up with a weaker concrete than assumed in the design due to a 45MPa characteristic concrete cube strength being supplied.

This highlights the need for clear and consistent concrete specification, preferably in accordance with BS8500. Where the specification is not clear, the supplier should seek clarification. Where prebagged materials are used, similar caution should be exercised as product names may relate to either the expected cube or cylinder strength and reference to the technical data sheet is required to clarify.


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View other CROSS reports published in Newsletter 54


Explanation of the compressive strength class notation (The Concrete Centre, How to specify concrete for civil engineering structures using BS 8500, 2007)

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