What mix? CEMI or CEMII or CEMIII?
We all know concrete contains cement! Lowering the cement content will reduce the speed of the set (and reduce its strength). The minimum recommended cement content is 340kgs per m³ for the type of work we are involved in.
But do we know what we get if we order a P340 which is CEMI, CEMII or CEMIII mix, or if we order a P340 which is a ‘blended’ mix?
Put simply:
- A CEMI mix contains 100% cement. (it’s not actually 100% but for simplicity, we’ll say it is)
- A CEMII mix is up to 35% cement replacement, (therefore, contains 65% cement).
- A CEMIII is up to 55% cement replacement, (therefore, contains 45% cement).
Please note: These percentages refer to cement replacement referred to as ggbs / slag / PBF. The percentage replacements are different for PFA, but PFA is not recommended for pattern imprinted concrete work.
Cement replacement will reduce the speed of the set, which can of course be useful in the summer. Please be aware though, that a CEMIII mix is not recommended because unless it is cured correctly, (which is not always possible to do in the pattern imprinted concrete world, or indeed the general construction industry) the concrete can fail DRAMATICALLY. Certainly, for a range of different conditions, a superior installation will be constructed using CEMI. It is OK to use CEMII, but we would strongly recommend against using a CEMIII.
In fact, to make it clear, NEVER order a CEMIII.
If you order or are offered a ‘blended’ mix, the concrete supplier will often supply a CEMIII rather than a CEMII. It is worth checking if you are offered or require a blended mix / a mix with a cement replacement, that it is a CEMII mix and NOT a CEMIII mix.
Many of you will use cement replacements, but a number of installers also stick with a CEMI – regardless of the temperature.
You can read more detailed information about cement types and mix designs in the PICS – Pattern Imprinted Concrete Mix Specification booklet sections 5 & 6.
Dr Concrete
