Winter concreting

Question 1

In connection with winter-concreting there is a risks of problems and/or damages! Name five such problems/damages!

Answer 1

• Early freezing of concrete - total collapse in worst case
• collapse of wall in connection with form-stripping
• Excessive deflection of slab or collapse of slab
• Damages on concrete in connection with form-stripping

Question 2

In connection with winter-concreting damages on walls can occur in connection with form-stripping.
 Describe these damages!
 What does this type of damages indicate?
 If the wall stands alone, there can be a risk that the entire wall can fall. How can you evaluate this risk?

Answer 2

When you take the form of the wall will collapse and it indicates that the concrete strength is very low. I should measure the strength in the lower part of the wall and it should be at least 4-5 MPa.

Question 3

Delayed final treatment of concrete is one problem that can occur in connection with winter-concreting.
 Describe shortly what the problem can cause on a construction site
 How can you avoid this problem

Answer 3

To avoid this problem you should make sure the chemical reaction between the cement and water starts as soon as possible after pouring. Protect the surface as soon as possible against chilling

Question 4

What happens if concrete freezes after the pouring but before the setting of the cement?

Answer 4

It is a big risk that ice-crystals will be formed. This will give increased porosity, decreased bond between concrete and reinforcement and cracks in the concrete.

Question 5

What happens if concrete freezes after the setting of the cement but before the concrete has gained the strength 5 MPa?

Answer 5

It will be cracks in the concrete. The extent of cracking is dependent on which the concrete has obtained at the freezing.

Question 6

In connection with winter-concreting you note imprints from ice-lenses on a part of a concrete structure. How do interpret this sign?

Answer 6

Imprints from ice-crystals in the hardened concrete is thus a sign showing that the concrete has been subject for early freezing. The loss of strenght is normally considerable.

Question 7

You are working with the following concrete: K35, StdP, T. Which compressive strength must this concrete obtain before it is subject to early freezing if the concrete shall not be damaged?

Answer 7

jag vet inte

Question 8

Why shall you avoid water-reducers and super-plastizisers in connection with winter-concreting?

Answer 8

It prolongs the setting time of the concrete and shall there for be avoided as they will considerably increase the risk od damages due to early freezing or dammages in connection with form-stripping.

Question 9

Why shall you avoid silica-fume and fly-ash in connection with winter-concreting?

Answer 9

Because they supposes a lower cement content in the concrete. This is negative because it will reduce the heat release from the reaction between cement and water which is positive and important in connection with winter concreating.

Question 10

Name five “concrete technology precautions” in connection with winter-concreting!

Answer 10

• Use SH-cement instead of stdP -cement
• Increased grade, e.g K30 ->K40
• Use warm concrete, 15-30 C
• Use hot concrete, > 30 C
• Use chemical admixture: accelerator

Question 11

Name three “production technology precautions” in connection with winter-concreting

Answer 11

• Covering of concrete
• Insulation of concrete form-work
• Heating on construction site:
• Radiation heaters
• electrical heating cables
• tents
• hot-air equipment