Concrete is used all around the world for its versatility. This mixture of cementitious material and aggregate, which forms a hardened mass on the setting, is the reason it is used in skyscrapers, underwater structures and suspension bridges. Concrete made from cement is especially ideal for its high compressive strength. The compressive strength is obtained as a result of strength gain by series of hydration chemical reactions.
The reactants gradually hydrate, set and become hardened. The surrounding temperature plays a vital role in this hydration process as water evaporation depends on it. If the water required for hydration of mineral compound becomes unavailable due to temperature anomalies, the end concrete product will become weak and unusable.
The best range of temperature for concreting is between 40℉ to 60℉ If the temperature drops below 40℉, hydration process becomes slower, setting time delays and concrete becomes weaker. Setting time of concrete at various temperatures is given below:
|Temperature (℉)||Approximate Setting Time (Hours)|
If water available for hydration freezes, it expands and originates cracking in concrete. Thus, it is necessary to have concreting performed in the suitable temperature range, but it is easier said than done. Weather conditions all around the globe vary along with the region. In some areas of the world, the region’s average annual temperature falls below the ideal range, hence, making conventional concreting practices obsolete.
Definition of cold weather according to American Concrete Institution is a period of more than three consecutive days, when average daily temperature is less than 40℉ and the air temperature is less than 50℉ for more than one-half for any 24-hour period. The strength of concrete depends on its setting time which in turn is dependent of ambient temperature. It is really important to protect concrete in its fresh state. Therefore, concreting in cold weather is performed accordingly to gain designed structural strength and keep weather adversaries in check.
Problems in Concreting in Cold weather
Concreting in cold weather involves the following problems:
Delayed setting time: As discussed above, the average temperature is the crucial factor controlling the setting time of the concrete. If the average temperature of the region is low, the hydration process will become slower. Hence, concrete will take longer to set and develop its required strength.
1)Freezing of concrete at an early stage
When the temperature of the surrounding environment of concrete falls below zero degrees Celsius, the water available for cement hydration in fresh concrete freezes, as the volume of water increase by 9 per cent upon freezes, the concrete swells up; it expands, and the hydration process stops. This expansion leads to a decline in the strength and propagation of cracks.
2)Unsuitable curing techniques
Concrete curing provides adequate water to hydrate cement, but the temperature is the second important aspect for curing. The heat of hydration is released when water reacts with the cement, so an inner portion of concrete is warm. Suppose the outer part of concrete becomes extremely cold due to curing in cold weather. In that case, thermal cracking is induced in concrete due to a colossal temperature gradient between core and surface.
3)Placing of concrete
The placement of concrete is another difficulty that is encountered in cold weather. Concrete cannot be placed on frozen ground for foundation or subgrade. Swift movement of heat from fresh concrete to the icy surface will retard the setting time of concrete. It will also result in differential settlement of foundations and cracking.
Solutions for Concreting in Cold Weather
Following practices and solutions can be opted to deal with concrete problems encountered owing to cold weather and ensure good strength gain according to the design requirements:
Accelerating admixtures are used to accelerate the setting time of concrete. If the cold weather delays the setting time, adding a chemical admixture can help combat this delay. Accelerating admixtures don’t necessarily prevent freezing the concrete, and they just offer protection to fresh concrete against freezing. Concrete in the early stage is most vulnerable, so adding an accelerating admixture doesn’t compromise its strength and helps in early and ultimate strength gain.
Insulating materials in colder climate regions are an excellent choice to prevent the freezing of concrete. If the concrete members are sealed, the heat of hydration produced remains entrapped inside the concrete. Also, insulation material protects the external freezing temperature, ensuring early strength gain and preventing thermal cracking.
Fig (1): Insulation of concrete members by polythene sheets
Using an external heating system is also an option in this case. Large scale external heaters can be utilized to provide an ambient temperature of concrete. But this option has its limitations as the availability and transport can become quite difficult and expensive, thus making it a viable option for only large-scale projects.
Concrete sealers are used the seal the surface of the concrete to prevent any imperfections. Sealers can offer the best resistance against freezing. Water-repellent sealers also provide an extra layer of protection.
5)Early strength gaining cement (Type-Ⅲ)
Type-Ⅲ cement is similar to ordinary Portland cement, but it attains its 28-day compressive strength in just three days. The use of Type-Ⅲ cement to achieve early strength gain in a colder climate is a great choice. Its rapid setting time allows good strength gain of concrete members. Moreover, its high heat of hydration makes it perfect for use in colder regions as it prevents cracking from freezing concrete.
6)Air entraining agents
Introducing air-entraining agents is another option for concreting in a cold climate. Air entraining agents produce air bubbles throughout the concrete structure. When water freezes, ice expands and swells up the concrete. These air bubbles accommodate the volume expansion of ice preventing swelling and cracking by providing pressure relief.
In cold weather, heated water and sometimes other constituents of concrete such as heated aggregates are employed to achieve a concrete mix having a temperature between 50℉ to 77℉. Warm formworks, tools and reinforcement are proven to be effective. However, heated cement is not used.
8)Heat curing boxes
For controlled curing of concrete between 60℉ to 80℉, heat curing boxes are used. Members are placed inside it along with a thermometer that records the temperature. Thus, the desired temperature can be easily achieved. Such types of heat curing boxes are actively employed in the curing of test cylinders.
Precaution in Concreting during Cold Weather
Following are some precautions to be taken for concreting in cold weather:
- Concrete should not be allowed to freeze after 24 hours of its placement.
- The concrete surface should remain covered to retain heat and prevent freezing.
- The outer surface should be cleaned of ice and snow frequently.
- Formwork should support the members for a more extended period as setting time is retarded due to cold temperatures.
- Concrete structures should not be subjected to sudden cooling to prevent thermal cracking.
- Proper temperature ranges should be sustained for a longer time to ensure concrete protection against weather adversaries.
- Concrete should be poured on the subgrade that has wholly thawed from the top surface to the bottom-most layers.
- Concrete should be inspected frequently using temperature monitors to help identify a problem early so that swift action can be taken to prevent excessive damage.
- If the temperature falls below zero, avoid wet curing techniques such as the sprinkling of water and ponding. Opting for steam curing is a better decision as it accelerates setting time and prevents freezing.
- Cementitious materials like fly ash and slag should be avoided as they can cause a delay in setting time.
Fig (2): Steam used for curing of concrete