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Curing of concrete in shelters
Turenne, R. G.
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Curing of Concrete in Shelters
by R.G. Turenne
A N A L Y Z E D
Reprinted from
Third International RlLEM Symposium
on Winter Concreting, Nov.
25
-
27,
1985
Technical Research Centre of Finland, Espoo
VTT, Symposium
61,
p.
53-63
(IRC Paper No.
1391)
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Reprinted from: Third International RILEM
CURING OF CONCRETE IN SHELTERS Symposium on Winter Concreting, NOV. 25-27,
1985. Technical Research Centre of Finland. Espoo 1985.
VTT,
Symposium 61, pp. 53-63.R.G. Turenne
Head, Technical Information Group Division of Building Research National Research Council of Canada Ottawa. Canada
Abstract
Because of the long and cold winters common throughout Canada, contractors have had to develop their own methods for placing and curing concrete in cold weather. These include the building and heating of enclosures, the use of insulation and the monitoring of strength development. This paper is a summary of the current practice in Canada.
1 INTRODUCTION
In Canada, the practice of curing concrete in heated enclosures goes back more than 50 years, but it was not until the mid-1950s that the federal government began to actively promote winter construction not only as a means of reducing unemployment in winter but also of sustaining a stronger economic growth at that time of the year. The government initiated a Municipal Winter Works Incentive Program whereby it would pay up to one half of the payroll cost of approved municipal winter works projects. The initiative was so successful that by the end of the 1960s the government was able to discontinue its financial support. By that time the technology had developed to the point where winter construction added only between 1 and 14% to the total cost of a project. And because a three-to-four-month shutdown in winter added considerably to the cost of interim financing for a project and delayed occupancy, causing a loss of revenue for the owners, winter construction was no longer just a desirable practice; it became an economic necessity. This was especially true with double-digit inflation in the 1970s.
Owners and designers began to schedule large projects so as to derive maximum advantage from the changing seasons, realizing that certain
operations should be done in warm weather while others such as concreting could be carried out in cold weather.
Concrete placed in cold weather must not only be protected against freezing at an early age lest it suffer irreparable damage but often m a t be allowed to gain sufficient strength to support all live and dead loads imposed on the structure during the early stages of construction. This is particularly important for concrete placed at the beginning of the cold season since, once allowed to freeze, the structure may remain frozen for up to four months, gaining little strength if any in the interval.
In Canada, the regulations dealing with cold weather concreting are contained in a national standard entitled, "Concrete Materials and Methods of Concrete Construction" (1). The section on Cold Weather Protection states that "Protection shall be provided for newly placed concrete by
cans
of suitable enclosures, coverings and/or adequate insulation during the necessary curing period when the mean daily air temperature is less than 5°C. Provision shall be made to heat the enclosures if necessary....".
The duration of the protection period varies with the required concrete strength, as follows:a) for resistance against early frost damage
-
3 days or 7 MPa b) for structural safety--
as determined by the authorityc) for strength and durability
-
minimum 7 days at 10°C or higher.Most construction contracts in Canada are awarded on the basis of lump sum tenders which means that the successful bidder must include the cost of protecting and heating all concrete placed in cold weather in his bid price. To do so he must prepare, at the time of tendering, a detailed work schedule to determine which phases of the project are to be constructed in cold weather in order to estimate the additional costs involved. He m s t then price all materials and labour to build the enclosures, estimate heat losses and calculate the quantity of fuel needed based on weather data for the area. This is not an exact science, however, and estimators rely heavily on experience.
When designing protection for cold weather concreting, an estimator is faced with many choices in terms of materials, fuels, heaters, etc. We will review some of these here.
2 ENCLOSURES
Enclosures consist of light frames, usually wood or light steel members, either self-supporting or attached to the formwork or the permanent structure, which support a light envelope made up of plywood, wood fibre panels, polyethylene, or insulated tarpaulins. Although Canada is a cold country and fuels are expensive,
polyethylene remains a most popular choice for covering material. It is economical to purchase and install, it allows daylight into the work area and it is discarded on completion of the project. Its main disadvantages are that it deteriorates rapidly when exposed to the elements and offers little resistance to heat flow. As a result, insulated tarpaulins are gaining in popularity in the colder areas of the country because of their ruggedness, thermal properties and greater wind resistance.
Insulated forms are sometimes used in conjunction with heated
enclosures to achieve the desired protection. The most common example is in the construction of concrete walls where only the interior surfaces are exposed to a heated space. The faces of the exterior framework are then insulated to reduce heat flow and thus ensure a higher and more uniform concrete temperature.
As mentioned previously, temporary enclosures are most vulnerable to wind and materials such as polyethylene can be damaged by high winds. Air leakage thus constitutes not only a major source of heat loss but is also responsible for uneven temperature distribution within an enclosure, where concrete close to the outside wall on the windward side may be exposed to cold air blowing into the enclosure. It is thus important for the builder to construct reasonably airtight envelopes and to repair rips and holes in the fabric quickly. When estimhting the heax logs for the purpose of determining the quantity
of fuel required, an equivalent heat loss must be included for air leakage. This figure can be calculated in terms of number of air changes per hour. Again one must rely on experience, but four air changes is probably a reasonable number to use, while 10 or 12 air changes per hour may have to be assumed for a loosely built enclosure on a windy site.
3 HEAT SOURCES
The three most common fuels used in Canada are natural gas, liquid propane, and oil. Natural gas is preferred if it is available at the site, that is, if a gas line has been installed in the immediate vicinity. Natural gas is not only the most economical fuel but where it is also to be used for heating the building many of the costs involved in servicing the job site are included in the contract and thus borne by the owner and not the builder.
If the contractor wishes to use liquid propane he will have to install
tanks to store it since it is normally delivered in bulk. He will
also have to install a vaporizer. Both the storage tanks and the vaporizer can be leased from the supplier. The building contractor is obviously responsible for installing the lines to feed his heaters either from the riser in the case of natural gas or from the bulk storage in the case of liquid propane. Most heaters can burn natural gas or liquid propane which simplifies inventory control and reduces the contractor's investment. The size of these heaters varies, with outputs ranging from 117 to 586 kW and rates of air flow between 566 and 3446 L/s.
Fuel oil is used mainly in areas where the other fuels are not readily available. Oil heaters require more attention and maintenance than do gas or propane heaters. They are usually refilled manually, they are heavier than gas heaters and more difficult to move on a construction site.
One of the by-products of the burning of gas or oil is carbon dioxide. If this gas is released wiihin an enclosure and settles on fresh concrete, it will combine with the calcium hydroxide in the concrete to form a weak layer of calcium carbonate on the surface. Since this layer turns to dust underfoot, it does not constitute an acceptable finish and usually has to be completely removed by grinding. Since this reaction only takes place very early after placing of the concrete it is preferable to place the heaters outside the enclosure for the first 24 hours.
Electric heaters are used in projects where other forms of heating are impractical such as on slip-firm operations; these heaters usually have relatively small outputs and are used in small, well-built enclosures. Powerful heaters having the capacity required on large projects usually require high voltages which are not always available on a construction site in the early stages and relocation on a site can also be costly because of the wiring involved.
4 HEATING TECHNIQUES
Two techniques are used to heat the enclosed space: recirculation and pressurization. Recirculation involves placing the heater, usually natural gas or propane, within the space and circulating the air through the heater. The air within the enclosure is maintained at the same pressure as the air outside and the wind will force cold air into the space on the windward side and remove warm air on the leeward side; this results in uneven heat distribution.
With pressurization, the heaters are placed outside the enclosure and the heated air is directed into the enclosure by means of a fireproof duct. This has the effect of blowing outside air into the enclosed space under pressure, in fact, pressurizing it, providing that the envelope itself is tight enough. Warm air is thus forced out through cracks and small openings in the walls instead of cold air blowing in under wind pressure. This results in a more uniform temperature distribution. The concept, however, imposes another criterion on the
design of the system in that not only must the heat output satisfy the needs of the builder to maintain the enclosure at the desired
temperature but the amount of air delivered by the heaters must also satisfy the leakage characteristics of the envelope. Too low a rate and the enclosure is subject to cold air infiltration; too high a pressure and heat is wasted through excessive exfiltration.
A common practice in Canada now is to place the concrete structure for high-rise buildings in winter. This allows the walls and windows to be installed in warmer weather. An example will help illustrate how a contractor might estimate the cost of protecting and heating a typical floor. In this case, the concrete for the floor is generally placed in the open air and finished before it is covered with an insulating tarpaulin. The heaters are located below the slab. The insulated tarpaulins are placed the day before and the heaters are turned on in order to warm up both the formwork and the reinforcing steel before placing the concrete. The tarpaulins also keep snow off the deck. It is also customary to build a wind barrier about 1.5 m high around the floor to protect both the concrete and the concrete finishers from the wind.
EXAMPLE
Floor dimensions 32 m x 40 m Distance between floors 3.5 m Inside temperature 15'C
Outside temperature -20°C (i.e. average daily minimum)
The space to be heated is contained between the last floor cast and the one to be poured. The perimeter is to be closed off with polyethylene (allow a one-metre space all around).
-
59-
Heat Loss Calculations
- - -
Thermal Temperature Heat Area Conductance Difference Loss
Component A=DxL U t.-t Q in kW Concrete floor 34 x 42 3.6 35 180 Concrete roof including insulating tarpaulin (R0.7) 34 x 42 0.92 35 46 Polyethylene 2(34
+
42) 3.5 11.4 35 212 Air leakage (assume four air changes per hour)where 1.2 = the density of the air, kg/m3 1000 = the specific heat of air, J/kg
Total: 661 Heaters required: two rated at 293 kW = 586
one rated at 117 kW =
117
703 kWThe above example is valid where the recirculation technique is used. If the contractor wants to use the pressurization technique, the following calculations can be made:
Total heat loss less air leakage = 438 kW Use two heaters rated at 293 kW = 586 kW Air flow rate for each heater = 1888 L/s
Total air flow 3776 L/s or 2.84 air changes per hour Heat loss due to air flow
Q = 1.2 x 3776 x 35 = 159 kW Total heat loss = 438
+
159 = 597 kW Installed capacity = 586 kWThe next step involves selecting the required pressure difference across the envelope based on wind speed records for the area. A 100-pascal pressure difference is equivalent to the stagnation pressure of a 46.5 km/h wind, while an 80 km/h wind exerts a pressure
of 300 pascals. The pressure difference across the envelope can be measured with a manometer. The tightness of the enclosure can than be adjusted to maintain the desired air pressure within.
While this example may imply that pressurization is a more economical method of heating an enclosure, this is not necessarily the case. The number of air changes where the recirculation method is used depends to a large extent on wind speed. On a calm day the number of air changes may thus be less than that resulting from pressurization allowing the builder to shut d o m one of the heaters.
5 CONCRETE ADMIXTURES
Air-entraining and water-reducing admixtures are generally specified especially in concrete requiring good freeze-thaw resistance.
Accelerating admixtures such as calcium chloride can be used except in specific applications such as concrete exposed to sulfate attack, in prestressed concrete, in nuclear-shielding concrete or where corrosion of the embedded metals might result. The addition rate of calcium chloride should not exceed 2% in flake form by weight of cement. Antifreeze admixtures to lower the freezing point of concrete are not used in Canada.
6 COOLING OF CONCRETE
To avoid cracking of the concrete due to sudden temperature change near the end of the curing period, the protection should not be completely removed until the concrete has cooled to the temperature differential given in the following table taken from the
Maximum Permissible Temperature Mfferential Between Concrete Surface and Ambient (Wind up to 24 km/h)
Maximum Permissible Temperature Differential, OC Thickness of Length to Height Ratio of Structure*
Concrete (m) 0** 3 5 7 20 or more - - 0.3 29 2 2 19 17 12 0.6 2 2 18 16 15 12 0.9 18 16 15 14 12 1.2 17 15 14 13 12 1.5 16 14 13 13 12
"According to the standard, length is considered the longer restrained dimension and the height the unrestrained dimension. **Very high narrow structures such as columns.
7 MONITORING STRENGTH DEVELOPMENT
Since ambient conditions in winter are generally not favourable for continuous strength development, it is important to ensure that the concrete has achieved sufficient strength before discontinuing heating. Various tests are used to determine the strength of the concrete in place. While no single test method is in itself all that reliable, a combination of tests can usually provide reliable information.
The most popular tests are field-cured cylinders, the pull-out test and the rebound method. The maturity concept is also used in special conditions.
Field-cured specimens consist of 150 by 300 mm cylinders prepared at the time of placing the concrete and cured in the enclosure under the same conditions as the concrete structure itself. The cylinders are then tested at specific intervals to monitor the strength development of the concrete.
The rebound hammer method requires no preparation and is simple to use. Although it cannot be applied to formed surfaces, it is
nevertheless useful for checking the uniformity of the concrete in a structure.
The pull-out test measures the force required to pull out a specially designed assembly cast into the concrete. The pull-out force must be rerated to the compressive strength of the concrete by means of test cylinders made of the same concrete. The pull-out test is superior to most other tests in that it measures quantitatively the strength of
concrete in place.
Concrete structures are generally not required to support heavy loads shortly after the protection has been removed. Builders rely heavily on reshoring of floors and beams to redistribute the load between three or more floors and thus minimize the possibility of damage to the structure.
8 CONCLUSION
The curing of concrete in heated enclosures is both practical and economical and has allowed contractors in Canada to work on large construction projects in cold weather and thus reduce construction times substantially.
In such surroundings the concrete can be finished without danger of freezing and can cure normally within predictable time limits. The workers for the most part are protected from the cold and snow is kept out of the immediate work area.
There are risks associated with this practice. These include fire. non-uniform strength development and carbonation. The benefits, however, outnumber the disadvantages a thousandfold.
REFERENCE
1. Canadian Standards Association, National Standard of Canada,
CAN3-A23.1-M77, Concrete Materials and Methods of Concrete
Construction, and CAN3-A23.2-M77, Methods of Test f o r Concrete,
Ottawa, 1977.
This paper i s a contribution from the Division of Building Research, National Research Council of Canada.
T h i s p a p e r i s b e i n g d i s t r i b u t e d i n r e p r i n t f o r m by t h e I n s t i t u t e f o r R e s e a r c h i n C o n s t r u c t i o n . A l i s t of b u i l d i n g p r a c t i c e a n d r e s e a r c h p u b l i c a t i o n s a v a i l a b l e f r o m t h e I n s t i t u t e may be o b t a i n e d by w r i t i n g t o t h e P u b l i c a t i o n s S e c t i o n , I n s t i t u t e f o r R e s e a r c h i n C o n s t r u c t i o n , N a t i o n a l R e s e a r c h C o u n c i l o f C a n a d a , O t t a w a , O n t a r i o , K1A OR6. Ce document e s t d i s t r i b u C s o u s f o r m e d e t i r 6 - 8 - p a r t p a r 1 ' I n s t i t u t de r e c h e r c h e e n c o n s t r u c t i o n . On p e u t o b t e n i r u n e l i s t e d e s p u b l i c a t i o n s d e 1 ' I n s t i t u t p o r t a n t s u r l e s t e c h n i q u e s ou l e s r e c h e r c h e s e n m a t i e r e d e b t t i m e n t e n 6 c r i v a n t
