Protecting Concrete in Challenging Cold Weather

Protecting concrete in cold weather is a continuous challenge for concrete contractors and site supervisors. Placing concrete in the cold weather condition requires special preparation and protection. All necessary precautions should be taken in order to alleviate the negative impacts of cold weather. Special curing and protection are required in most cases. 

In my previous article, we reviewed what is considered cold for concrete construction, and what should be done prior to placing concrete. 

In this article, I will review some of the widely used protection techniques and strategies and the challenges in protecting the concrete from extreme cold. But first, let’s see what cold means for concrete:

Why Cold Temperature is critical

The hydration of cement is a chemical reaction. Extremely low temperatures, as well as freezing, can significantly slow down the reactions, thus, affecting the strong growth. In fact, freezing temperatures within the first 24 hours (or when concrete is still in the plastic state), can reduce the strength by more than 50%.

Protecting Concrete in Cold Weather

The CSA A23.1 specifies that protection shall be provided by means of:

1) Heated enclosures2) Coverings3) Insulation

The protection should be continued until required structural properties such as strength are achieved. The minimum strength before exposing concrete to extreme cold is 500 psi (3.5 MPa). CSA A 23.1 specified a compressive strength of 7.0 MPa to be considered safe for exposure to freezing. Traditionally, cast-in-place punch out cylinders is used to estimate the strength at certain intervals. The maturity method is gaining popularity with the recent advancement in wireless sensor technology. Wet curing during this period should be avoided.

Covering – Curing Blankets w/o Insulation

Covering with curing blanks is widely used in construction sites during the cold season. The heat generated from the hydration of cement is normally sufficient for many cases, should blanket are used properly. Blankets should remain for a couple of days. The required insulating value depends on the thickness of concrete, the amount of cement, and anticipated cold temperature. Consult ACI 306 chapter 7 for details on the insulation.

Site managers and engineers are responsible to assess if the concrete has reached desired strength. Temperature monitoring using infrared thermography from concrete surface or maturity method can be used for better decision making.

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The heat generated from hydration process should suffice in most cases, if appropriate insulating blankets of polyethylene sheets are used. Additional source of heat might be required based on area and temperature.

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When covering with blankets, special attention should be given to the corners and edges of the slabs. This area often requires further insulating layers. If covering cannot keep the concrete temperature at desired levels, the external heat source such as electric heating blankets, or hydronic heating pipes should be used.

Heated Enclosures

If blankets do not provide enough protection, or if the weather is extremely cold (even prior to placing concrete), then heated enclosures should be used. This technique includes enclosing the construction site (for example the story under construction) and heating the space. Certain challenges should be addressed:

1- Carbon Dioxide-Carbonation

One common challenge with heated enclosure is the problem of carbonation. The carbon dioxide produced by some of the commercially available heaters increases the chance of carbonation of freshly placed concrete. This can lead to the formation of a weak concrete layer which is often unacceptable. It is recommended to use heating systems that exhaust to the outside of the enclosure.

2- Rapid Drying / Uneven Heating

The use of heaters can result in very rapid drying of concrete, which will increase the chance of plastic shrinkage, and might lead to poor quality concrete (if water required for hydration process evaporates). It is recommended to move the location and direction of the heat source for a more uniform heating pattern.

3- Fire

Special attention should be paid to the heaters that use propane gas.

CONCRETE FRAME STRUCTURE GUIDE WITH DETAIL

Concrete frame structures are the most common type of modern building used nowadays internationally.  As the name conveys, this type of building consists of a frame or skeleton of reinforced concrete.

The structure is actually a connected frame of members, each of which is firmly connected to each other. In engineering phrase, these connections are called moment connections, which means that the two members are firmly connected to each other. 

There are other types of connections, including hinged connections, which are used in steel structures, but concrete frame structures have moment connections in 99.9% of cases. This frame becomes very strong and must resist the various loads that act on a building during its life.

Horizontal members of this frame are called beams, and vertical members are called columns.  Humans walk on flat planes of concrete called slabs. In frame structure building the column is the most important, as they are the primary load-carrying element of the building. If we disturb* a beam or slab in a building, this will affect only one floor, but the disturbance to a column could bring down the entire building. |*| Damage/Break

When we say concrete in the construction industry, we actually mean reinforced concrete.  Its full name is reinforced cement concrete or RCC.  RCC is concrete contains steel bars these are called reinforcement bars, or rebars.  That unification works very well, as concrete is very strong in compression, easy to produce and inexpensive, and steel is very strong in tension.  To make reinforced concrete, one first makes a mold, called formwork, that will contain the liquid concrete PCC (Plain Cement Concrete) & give it the form & shape we need.  Then according to the structural engineer's drawings, the steel reinforcement bars are placed & tied them in place using steel wire.  The tied steel is called a reinforcement cage because it is shaped like one.

Once the steel is in place, we can prepare the concrete, by mixing cement, sand, stone chips with the nominal ratio in a range of sizes of aggregate, and water in a cement mixer, and pouring the liquid concrete into the formwork till exactly the right consistency is reached.  The concrete will become hard in a matter of hours but takes a month to reach its full strength. 

(Check out: The Initial & Final Timing Of Mixed Concrete ). 

Therefore it is usually propped up until that period.  During this time the concrete must be cured or supplied with water on its surface, which it needs for the chemical reactions within to proceed properly.

(Check out : Best Method for Curing Of Concrete )

Working out the exact 'Ratio', or proportions of each ingredient is a technique in itself. It is called concrete mix design. A good mix designer will start with the properties that are desired in the mix, then take many factors into account, and work out a detailed mix design. A site engineer will often order a different type of mix for a different 

purpose.

For example, if he is casting a thin concrete wall in a hard-to-reach area, he will ask for a mix that has more workability than stiff. This will allow the liquid concrete to flow by gravity into every corner of the formwork. For most construction applications, however, a standard mix 1:2:4 is used.

Common examples of standard mixes are M20, M30, M40 concrete, where the number refers to the strength of the concrete in n/mm2 or newton per square millimeter. Therefore M30 concrete will have a compressive strength of 30 n/mm2.

A standard mix may also specify the maximum aggregate size. Aggregates are the stone chips used in concrete.

If an engineer specifies M30 / 20 concrete, he wants M30 concrete with a maximum aggregate size of 20mm. He does NOT want concrete with a strength of between 20-30 n/mm2, which is a common misinterpretation in some parts of the world.

The concrete frame rests on foundations, which transfer the forces from the building and on the building to the ground.

Some other important components of concrete frame structures are:

Shear Wall:

Shear walls are important structural elements in aerial buildings. Shear walls are essentially very large columns - they could easily measure between 400mm thick by 3m long making them expose like walls rather than columns. Their behavior in a building is to help take care of horizontal forces on buildings like wind and earthquake loads. Normally, buildings are subject to vertical loads. Shear walls also carry vertical loads. It is important to understand that they only work for horizontal loads in one direction. These are usually not required in low-rise structures.

  

Elevator Shafts:

Elevator shafts are vertical boxes in which the elevators move up and down normally each elevator is enclosed in its own concrete box. These shafts are also very good structural elements, helping to resist horizontal loads, and also carrying vertical loads.

WALLS IN CONCRETE FRAME BUILDINGS

Concrete frame structures are strong and economical. Hence almost any walling materials can be used with them. The heavier options include masonry walls of brick, concrete block, or stone. The lighter options include drywall partitions made of light steel or wood studs covered with sheeting boards. The former is used when strong, secure, and sound-proof enclosures are required, and the latter when quick, flexible lightweight partitions are needed.

When brick blocks or concrete blocks are used, it is common to plaster the entire surface brick and concrete with a cement plaster to form a hard, longlasting finish.

CLADDING OF CONCRETE FRAME STRUCTURES

Concrete frame buildings can be clad with any kind of cladding material. Common cladding materials are glass, aluminum panels, stone sheets, and ceramic facades. Since these structures can be designed for heavy loading, one could even clad them in solid masonry walls of brick or stone.

Concrete Frame structure system is extremely economic, it requires no painting, finishing or fire protection. It is a suitable structural form for industrial buildings, warehousing, sports halls, community halls etc.