BSI Flex 350 v2.0:2024-09
BSI Flex 350 v2.0 was published September 2024. It is a document that describes a method by which concrete produced containing a lower carbon non-standardised cement (binder) can be assessed for use in structures.
What is a BSI Flex?
A BSI Flex is not a British standard but is considered a Code of Practice. In that respect, they can be considered to be similar to a PAS (Publicly Available Standard). Both a PAS and Flex are sponsored documents produced in response to a need, are written by a Technical Author, aided by an Advisory Group (AG). Where they differ from each other, is that the Flex is produced using an iterative process in which the document is published for public comment with those comments being reviewed by the AG and guidance given to the Technical Author. This process can go through a number of cycles with the aim that final iteration will be highly relevant to those using it. Another difference between a PAS and Flex is that, if required, the Scope of a Flex may be changed in response to public comment.
Why was BSI Flex 350 needed?
All standards, whether they are international (ISO), European (EN), British (BS), or industry best practice, are produced to meet a need for an agreed way of doing things.
The LCCG (Low Carbon Concrete Group) identified a need for a document that would provide guidance on how to assess the performance of binders (cements that may, or may not, contain Portland cement clinker) for “lower carbon” concretes that are not covered in the current cement standards. Without having this guidance, it was believed that the specification of novel materials would be too difficult, so specifiers would follow the path of least resistance and specify to EN 206/BS 8500.
The Flex is aimed at those manufacturing lower carbon binders and concrete producers, but is also of interest to Engineers and Architects, and anyone else involved in the specification of lower carbon concrete.
BSI Flex 350 has replaced PAS 8820:2016 - Construction materials – Alkali-activated cementitious material and concrete – Specification, which has now been withdrawn. It has been through two reviews with v2.0 being the latest. At the time of writing, there are no plans for a further revision but could be if there was need.
What is a lower carbon concrete?
There is no agreed definition of lower carbon concrete as, for example, a concrete with a higher embodied carbon could be more durable (so have a longer life) than a lower carbon concrete when used in the same application.
However, the Flex 350 Advisory Group believed that a definition was necessary in the context of a code that has the aim of reducing the embodied carbon of concrete. The definition (which only applies to Flex 350) is “concrete with an embodied CO2 equivalent (kg CO2 e/m3) 15% lower than the average for concretes in use across the market for the relevant strength class”.
In the UK, the LCCG Benchmark shows the market average embodied CO2 equivalent by strength class. It may appear that 15% lower than the mean is quite a high figure to be classed as lower carbon, however, the embodied carbon of concretes in the UK when categorized by strength class follows a normal distribution with most concrete supplied somewhere around the mean. 15% lower than the average would be approximately the lowest 25% of concretes supplied.
How does it work?
Flex 350 lists the test methods that should be used to assess the properties of concrete produced using a non-standardized binder. These are split into separate chapters, one for concrete properties (fresh and mechanical), and one for durability. The producer can select the range of testing required, so, for example, if the intention is that the concrete is to be unreinforced, or reinforced with a non-corroding reinforcement, there would be no requirement to assess whether embedded mild steel would remain in a passive state.
The concrete to be assessed is compared to one that has a known performance. This is similar to the EN 206 Equivalent Concrete Performance Concept (ECPC) which allows the use of standardized constituent materials for which there are no national provisions.
Annex B provides guidance on the selection of the reference concrete in which is normative for conforming to the requirements of Flex 350.
Flex 350 does not provide guidance as to how to interpret the test results however the equivalent performance concept should show whether the binder performs as well as, or better, than the reference concrete.