Lime – 0.55
Slag – 1.3
Ash – 1.1
Lime – 2.1
Cement – 3.14
Slag – 2.8
COLD WEATHER CONCRETING
Concreting in cold weather requires some special considerations
1. Concrete finishing times are extended due to:
Slowing of the evaporation rate at the concrete surface.
Concrete temperature rise being reduced due to cold materials, i.e. aggregate, sand, cement and water.
Concrete being placed on cold and/or wet ground.
Bleed water on the surface.
Working the concrete too early.
2. Other effects of cold weather concreting:
Decrease in early strength development. This requires the delayed removal of formwork.
The risk of scaling or delamination due to the concrete surface being over worked.
Dusting concrete surfaces due to the bleed water being worked into the surface of the concrete
Do’s and Don’ts
Do use an accelerator in the mix. This could take the form of an accelerating admixture or extra cement.
Don’t place the mix too wet. The wetter the mix the longer the finishing times due to more bleed water.
Don’t place the concrete on frozen ground. This will reduce the concrete temperature and cause delayed bleeding.
Do remove water, ice and frost from trenches, formwork and reinforcement.
Do compact the concrete by vibration. This will release the bleed water from the concrete matrix.
Don’t over bull-float the surface of the concrete. This traps the water in the concrete and retards bleeding. Tip: A bull-float with a piece of flat, dressed timber bolted to the face will open the pores of the concrete and allow bleed water to escape more readily.
Do remove excess bleed water from the concrete surface by dragging a length of hose or rope across the surface.
Don’t play with the concrete surface. Tip: Take a break, have lunch, set up the next job.Do the final finishing operation when the bleed water has disappeared and you can only just mark the surface with a press of the finger. Tip: Cement during hydration grows interlocking crystals, therefore every time you work the concrete surface the crystals are broken and they have to regrow, slowing the hardening process.
Do start curing as soon as possible.
DON’T FORGET: Finishing time is the time from when the last concrete placed was bull-floated to the completion of final finishing.