Last week BS8500, the complementary British Standard to EN 206 (the European Standard on Concrete) came of age. Although I live reasonably near the BSI HQ in Chiswick, I was not disturbed by any wild parties, in fact I’m pretty sure the 18th birthday passed by unnoticed by all. In the UK, becoming 18 marks the point when you are legally entitled to, amongst other things:
- Serve on a jury
- Get a tattoo
- Buy an alcoholic drink in a pub
BS 8500 cannot partake of these new rights, but I bet that grappling with BS 8500 has driven more than a few engineers to drink over the years. You would have thought that after 18 years we would know how to use the Standard, but I keep coming across examples of it being incorrectly applied. Back in 2011 when BS 8500 was still young, I was motivated to write an article for the Structural Engineer magazine highlighting a common error that engineers made when using the Standard, and it’s still being made.
So, as my birthday present to this fundamental Standard of our industry, I’m going to go over the issue again and see if we can prevent a few more of you from making this error.
Technical jargon warning
The article gets a bit techy from here
Those of you familiar with the Standard will know that Tables A.4 and A.5 are the key tables in which for a given exposure environment, you determine your cover to reinforcement, concrete limiting values (i.e. strength, minimum cement content and maximum water cement ratio) and cementitious material type to achieve your required design life. Table A.4 gives the requirements for a 50 year design life and Table A.5 for a 100 years.
The two Tables give the specifier options. To achieve the design life, they can either specify a:
- higher cover with a lower quality concrete
- higher quality concrete with a lower cover
The quality of concrete can be improved by using a better performing cement and/or a lower water cement ratio (and associated higher minimum cement content and strength).
So, for each different exposure condition the specifier has a range of options to choose from. For example, consider a concrete element exposed to a marine splash zone (exposure class XS3) and a 50 year design life. Table A.4 gives the specifier 23 options with strengths varying from C20/25 to C40/50 (and all grades in between), 4 different groups of cements and minimum cover to reinforcement from 45mm to 80mm inclusive. In terms of the Standard, all these options are equally valid.
The common mistake I keep coming across is that the specifier who may have an element that requires a design characteristic strength of say C32/40, believes that they must look up that strength in the XS3 exposure class row in Table A.4 and then use one of the two combinations they find i.e.
C32/40 mcc 360 w/c 0.45 with IIB-V or IIIA cement and 60mm cover
C32/40 mcc 360 w/c 0.45 with IIB-V (min 25% fly ash) or IIIA (min 46% slag) cement and 60mm cover
Worse still, the specifier will often compound this error by restricting the cement type, e.g. there is no C32/40 option in Table A.4 for cements with a high supplementary cementitious materials content, so the specifier will exclude IIIB or IVB-V cement.
While this is a solution that meets the requirements of the Standard, it is unnecessarily restrictive and could be technically poor, e.g. if it is a large element that requires a low-heat cement to minimise the risk of thermal cracking.
Instead it should be noted that the limiting values in Tables A.4 and A.5 are a minimum. If the specifier wants to use IIIB cement at 50mm cover then Table A.4 says you need a minimum specification of C28/35 mcc 360 w/c 0.45 to meet durability requirements. If you need C32/40 for structural reasons then specify C32/40 IIIB cement mcc 360 w/c 0.45 with 50mm cover and your concrete will comply with both structural and durability requirements. Four of the 23 options have durability limiting values with a strength greater than C32/40. The specifier can still use these options but they will have to increase the limiting values, including strength, to match the requirements in Table A.4.
#concrete #BS8500 #EN206