COLD WEATHER CONCRETE

OUTLINE

ACI definition of cold weather

Setting time of concrete at various temp.

Effect of cold weather on concrete

Precautionary measure for conc. in cold weather

Conclusion

ACI DEFINITION

Cold Weather - A period when, for more than 3 consecutive days,

the following conditions exist:

The Average daily air temperature is less than 40 ºF (5 ºC), and

The air temperature is not greater than 50 ºF (10 ºC) for more

than one-half of any 24 hour period.

SETTING TIME OF CONCRETE AT VARIOUS TEMPERATURE

Temperature, °F Approx. Setting Time , hrs.

70 6

60 8

50 11

40 14

32 Concrete Freeze

PROBLEM OF COLD WEATHER ON CONCRETE

Delay setting time

Freezing of concrete at early age

Placing concrete on ground

Plastic concrete problem

1. DELAY SETTING TIME

The rate of hydration of cement depends upon the ambient temperature.

If the ambient temp. is low , hydration process will go slow.

Than concrete takes a longer time to set and to develop strength.

2. FREEZING OF CONCRETE AT EARLY AGE

When the temp. of concrete falls below

freezing point , the free water held in the

plastic conc. Freeze.

Due to freezing of water , conc. Expand and

hydration of cement will be stopped.

3. PLACING CONCRETE ON GROUND

Concrete should never be placed on a frozen subgrade

On a frozen subgrade, heat will migrate rapidly away

from the bottom of the concrete retarding setting time

Thaw the subgrade not just the surface

4. PLASTIC CONCRETE PROBLEM

Concrete bleeding

Bleed water can freeze on surface

Bleed water capillary channels can freeze within the concrete

Cold subgrades

Rapid migration of heat from concrete will affect setting time

Uneven settlement may occur, causing cracking

PRECAUTIONARY MEASURE FOR CONCRETE IN COLD WEATHER

A. Use of materials

B. Precautions in field

A. MATERIAL USED IN FIELD

Use of type III cement

Air entraining agent

Admixture

1. HOT WATER

During cold weather, it may be necessary to heat

one or more of the concrete materials (normally we

use hot water).

An increase in water temperature of 50oC will

result in a rise in concrete temperature of

approximately 10-15oC.

The temperature of the water added should be no

more than 60-80oC.

2. TYPE III CEMENT

Type III cement is a form of Portland cement.

Type III cement achieves about 70% of its 28 day

strength after 3 days. That is indeed a little

faster thanType I.

3. AIR ENTRAINING AGENT.

Air entrainment is the intentional creation of tiny

air bubbles in concrete.

Air-Entrained concrete has higher durability than

the ordinary concrete under freezing conditions.

Modify the pore structure of the concrete and

improve the resistance to frost attack.

4. ACCELERATING ADMIXTURES

Earlier finishing of slabs

Increased early and ultimate strength

Reduced protection time in cold weather

Earlier stripping and reuse of forms

B. PROTECTION IN FIELD

Delayed Removal of formwork

For slower rate of gain strength formwork is required to be kept in place for

longer time

Covering the concrete surface

During cold weather, all conc. Surface shall be covered as soon as the conc.

Has been placed in order to prevent the

i. Loss of heat

ii. Prevent freezing