Cold Weather Concreting
As per ACI 306-R10, Cold Weather Concreting is defined as a concreting process – mixing, pouring and curing, below atmospheric temperature of 4o Celsius (40 o F) over the recommended protection period as per the specifications. Protection period is defined as the time required for the concrete to develop early strength of 3.5 MPA or until a desired strength is attained depending on service condition.
For example, for Normal-set concrete without any admixtures, protection period is 2 days if concrete element is not subject to load or atmosphere. On the other hand, for partially loaded and exposed condition it can go up to 6 days. Thus, only if concrete is properly proportioned, placed, protected during cold weather, it can develop adequate and desired strength and durability. Early thawing and freezing cycles will reduce the final compressive strength of concrete.
To overcome these issues during Cold Weather Concreting, admixtures are added during concrete mixing to enhance the properties and performance of fresh concrete. Antifreeze admixtures are chemical compounds that are added to the mixing water of concrete for lowering the freezing point of the aqueous solution. It is necessary for concreting in cold weather. It can be used even at temperatures as low as -30o C.
Standard requirements of Cold Weather Concreting
ACI 306-R10, Table 5.1 defines minimum temperatures to be maintained while mixing, placing and curing concrete. This primarily depends on the stage of concreting, ambient temperature and the thickness of concrete element. Higher degree of control is required for thinner members since the temperature variation across depth will not be significant.
Code also defines Protection Period over which the recommended concrete temperature must be maintained. As mentioned above, protection period depends on the service condition during after concreting until the concrete reaches desired strength. Ensuring temperature control is critical for the removal of formwork since, the strength gain will different for Cold Weather Concreting compared to conventional concreting.
Problems in Cold Weather Concreting:
There are two main problems of cold weather concreting:
- It freezes before it gains sufficient strength.
- Setting of concrete occurs slowly. As heat is a catalyst to the hydration of concrete, the less heat, the slower the setting.
Normally concrete cannot develop strength at acceptable rates when the temperature is lower than 5°C. When fresh concrete is subjected to freezing or freeze thaw cycle, there arise several problems. The development of ice within the cement mortar paste disrupts the paste and ice lenses are formed. The strength of concrete is reduced by 20 to 40%.
The strength can be reduced up to 50% if freezing takes place before gaining 500 psi compressive strength of concrete. The bond between concrete and reinforcement is decreased by 70%. The durability factor is also reduced. Ice crystals can produce porosity of hardened cement. So to ensure that concrete does not undergo freezing, antifreeze admixtures are added in cold weather. It helps to avoid thermally induced cracking and ensures adequate strength for safe removal of formwork.
Methods applied in Cold Weather Concreting:
There are some processes to deal with cold weather concreting.
- By using heated water or heated aggregates to jump start the hydration reaction.
- By increasing cement content. As the heat of hydration is due to the exothermic reaction of concrete mixture, using more cement will generate more heat.
- By using high early strength cement. It contains high amount of C3S and C3A which leads to faster setting. Type III is a high early Portland cement.
- By providing adequate insulation so that heat can be retained in concrete. Blankets and boards can be used.
- By using heated formwork. As fresh concrete cannot bond with frozen concrete, heated formwork will avoid freezing.
Some advanced methods of cold weather concreting:
- By using antifreeze admixtures which will lower the freezing point of water mix.
- By using urea. It increases workability and prevents freezing. But it is not effective in lower that -5 degree Celsius temperature.
- By using accelerators which will accelerate setting and hardening. It can be used via tableting.
Here we will be discussing about antifreeze admixtures.
Chemical Composition of Antifreeze Admixture:
Antifreeze admixtures have two purposes- to depress the freezing point of the aqueous solution and to accelerate the setting and hardening of concrete at low temperature.
There are two groups of antifreeze admixtures:
This includes chemicals, weak electrolytes, sodium nitrite, sodium chloride and non-electrolytic organic compounds.
This includes potash and additives based on calcium chloride, sodium nitrite, calcium chloride with sodium nitrite, calcium nitrite -nitrate-urea and other chemicals
The first group is weak in accelerating setting and hardening properties. But the second group has effective accelerating properties.
But the admixtures are to be used in right proportion. For example, 2% of calcium chloride works as accelerators, but increasing the amount to 9% can act as retarders and increase setting time.
How Antifreeze Admixture Works:
Freezing and boiling points are colligative properties of liquid. It means they are depended on the concentration of the solutes. Freezing means the liquid becomes a rigid crystalline structure. When an antifreeze is added, it is difficult to slow down the molecules in its consecutive freezing point. The different kind of molecules block the attractive forces of solution. So the freezing point depresses.
How to use Admixtures for Cold Weather Concreting
Antifreeze admixtures can be powder or liquid. It is first added to the water or the premixed cement mortar. The dosages are dependent on the chemical type of the admixtures. Then it is homogeneously mixed for at least 60 seconds. Though the time depends on the mixing device. After placing, fresh concrete must be cured properly.
But it is must to test the dose of admixture with the proportioned concrete mortar in laboratory before using in the construction. Here is some example of dosage at different temperatures of 2 antifreeze admixture.
Major producers of Antifreeze and Cold Weather Concreting admixtures
Due to geographical situation, antifreeze admixture productions are based on North America, Latin America, Asia Pacific, Europe, Middle East & Africa. Asia Pacific dominates the antifreeze admixture market as their consumption rate is very high.
Some of the leading global antifreeze markets are BASF SE which is a German multinational chemical company, Fosroc constructive solution which is a UK based global manufacturer of chemicals for construction industries, Sika AG (a Swiss multinational chemical company), Ashland Global Specialty Chemicals Inc. (an American chemical company), Arkema Chemicals company.
SikaCem® winter is a liquid product of Sika AG. Every bag contains 500 ml of admixtures. Its shelf life is 24 months. It costs nearly £5.
Advantages of Antifreeze Over other methods
Antifreeze admixtures improve quality of concrete. As per Ratinov and Rosenburg at -10 degree Celsius temperature the compressive strength of a plain concrete at 28 days is 18.1MPa whereas for concrete mixed with admixtures it is 49.9MPa. It increases the rate of early strength development.
Antifreeze admixtures allows to place the concrete in cold and let the concrete be cold while developing acceptable strengths. So where a massive structure is cold, joint in precast concrete structure, repair of dams, tunnels are possible.
The selection of antifreeze admixtures depends on the type of structure, protecting methods employed in winter concreting, types of cement and aggregates used, proportion of cement and aggregates. This admixture improves plasticity of cement. It increases cohesiveness and minimizes sand streaking.
In building construction, floor slab and wall section can be placed without the need of temporary shelter as antifreeze admixtures accelerates setting and hardening. It can also save large construction cost than conventional Cold Weather Concreting methods.