-what concrete temperatures can be reached?
-hpc
-ambiant temperature in factory building down to 8°C
-max. Diameter of element 30cm
The reaction with cement already produces heat. Because of the characteristics of high performance concrete, I imagine the content of cement is high. The larger the cement quantity, the bigger the exothermic reaction. That, though, can be a problem for cracking when hardened, so be careful.
If temperature is over zero you won't probably have problems with freeze, but you should prevent it. If you can avoid concreting these chilliest days, it is better. Try to pour concrete at higher temperatures during the day, and you can use accelerators to get it hard earlier. If you introduce hot water in the mix, the reaction will be faster and it also will help to harden earlier. If you reduce this time, you can get your concrete elements finished during the day (warmer temperature).
You can use special moulds that retains heat, such as timber-made, or even insulate them so that heat produced with the cement reaction is kept inside as much as possible. Also, covering with plastics during the day can have like a greenhouse effect and keep warm inside.
A final comment from my experience is that cement reaction is decreased with cool, even it can be stopped. We concreted one wall with chilly temperatures (probably below zero at night) and it needed 3 days to harden. Evidently, the resistance of this wall was decreased substantially and we had to demolish it.
Concrete curing below 4 degrees C is problematic, as the reaction is exponential with temperature. Also, freezing is a risk with sufficiently low temperatures.
Proper insulation of the mould is critical to prevent damage from temperature differentials between the surface and the centre of the concrete.
I would suggest:
- warm the materials to between 12 degrees C and 20 degrees C;
- warm the mould if reasonably possible (apply warm water or hot air);
- use insulated moulds (if using steel moulds wrap with R3 insulation bats, if timber wrap with R1.5);
- keep in mould for at least 3 days
- after demoulding, cure with water (20 degrees).
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