Plastering In Cold Weather
Plastering when the temperature is at or near freezing causes special problems for the plastering industry, and the contractor must take special precautions to protect himself against the adverse conditions. Extra care should be observed if temperature is expected to drop below 50 degrees F. during the plastering operation, or below 32 degrees F. after the wall has been plastered.
One problem develops because water expands with great force as it freezes and becomes ice. The expansive power may fracture the plaster if the force occurs before the cement in the wall has gained strength. At 32 degrees F. and below, mortar will freeze on the wall and cause weakening of the plaster. Damage to the plaster may be so severe that mortar which has been frozen can be rubbed from the wall simply by brushing with the hand. Another problem is that efflorescense may form on the surface of the wall. Also, cold dry air at the surface of the new plaster absorbs warm moisture rapidly from the wall and can lead to plastic shrinkage cracks as well as impaired cure. Low temperature causes all portland cement plaster, concrete and brick mortar to take longer than normal time to set and gain strength, and delays finish troweling or floating, which raises labor costs needlessly. Another problem is that low temperature will increase the possibility of "dropouts" from soffits and "slip-offs" from walls. (Both may be caused by too much mix water, also.)
If a plastered wall has been frozen, excessive porosity may result. This porosity causes further deterioration during subsequent freezing and thawing if a driving rain should soak the wall and then freeze.
To counteract these understandable cold weather problems several precautionary steps must be taken. An interior job must be heated and an exterior job may have to be enclosed as well as heated. The temperature of the mix must be raised or an accelerator must be added to the plaster in the mixer. Often acombination of these protective methods must be used. If a contractor is unwilling to take these special precautions he must accept the fact that a longer time will be required before a crew can finish the job properly, or that a wall may be damaged.
When cold weather is near, the plastering contractor should plan in advance and have heaters, polyethylene film, calcium chloride in solution or other protective measures provided. Plastering can be done during freezing weather IF PRECAUTIONS ARE TAKEN TO PREVENT THE PLASTER FROM FREEZING.
Warming the sand and mix water is preferable to adding an accelerating agent such as calcium chloride to the plaster, because a better wall will be obtained.
Portland cement conforming to ASTM Type III (an early hardening cement) is preferred for use in adverse cold situations. This also applies to a cold storage room that is being plastered while in service. Use early hardening cement in the same manner that regular portland cement (Type I) would be used. That is, add enough hydrated lime or lime putty to make the mix fat. Some plastic cements entrain air as the plaster is mixed. Air entrainment is beneficial in mortar used on walls exposed to extreme temperatures. If plastic cement is used, no lime should be added.
For plastering inside a building in cold weather, space heaters can be used to heat the rooms and to allow the plaster to set in about a normal time period. Also, the mix water may be heated. If heaters are used inside, adequate ventilation must be provided for safety reasons and to reduce the possibility of carbonation of the plaster with a resultant tendency of the plaster to powder at a later time.
Excess carbon dioxide would be most apt to occur in the basement or other areas where thorough ventilation does not exist, but it must be considered anywhere in the structure.
For plastering the outside of a building, the mix water can be heated by directing the flame of a construction space heater against the side of the water barrel. If the water is heated to approximately 70 degrees F. but no higher, a quicker setting of the cement will be obtained. This will result in a rapid gain in plaster strength without ill effects. Avoid making the water too hot. However, because the setting time of the cement can be shortened too much and flash-set might occur.
Other precautionary measures are: heat the sand pile (particularly if ice is present in the sand), avoid over-sanding the mix, hold down on the amount of mix water to the practical minimum, install sufficient control joints to allow for contraction and expansion, water-cure the wall properly and retain heat in the plaster until the cement has developed good strength.
A practical way of retaining heat in the wall is to hang polyethylene or other sheet material from the top of the building. A source of heat must be provided between the polyethylene and the wall during and after the plastering because heat dissipates rapidly from wall surfaces. The heat from several light bulbs per wall is effective and safe. Hang a thermometer behind the sheets, check it periodically and be guided by it. The best curing range is between 50 degrees F. and 70 degrees F. Temperatures below 50 degrees F. describe cold weather plastering.
Tape the top of the sheets to the rakes and eaves to develop an effective seal along that line, and tape the edges of adjoining sheets along the wall as well as at the corners. Protect the sheets from being blown away from the wall by gusts of wind. The sheets should hang to the ground.
In lower temperatures use an electric heater or more light bulbs. Don't place the bulbs or heater too close to the plaster, to avoid drying patches of plaster and causing weak spots. To be safe, continue the heat day and night for at least three days. Temperatures generally drop during the nighttime, so provide additional bulbs or heaters at sundown. Hang the light bulbs at the bottom of the plastered wall. Heat will rise behind the curtain and protect the top of the wall.
Another method of causing portland cement plaster to set more quickly and to gain strength more rapidly is to add calcium chloride to the mix. (Calcium chloride sometimes may be desired in rainy weather also, since it allows earlier finishing of the brown coat.) It is obtainable in flake form, but must always be added to the mixer IN SOLUTION IN WATER. Generally speaking, from one half to one pound of calcium chloride per bag of cement is all that is required to accelerate the setting time of the plaster. NEVER exceed two pounds of calcium chloride per bag of cement. Don't add calcium chloride to a mix using early hardening (Type III) cement.
A standard solution of calcium chloride can be prepared ahead of the time that it will be needed and kept on hand for such an emergency. The solution can be stored for years with no deterioration in either steel or plastic container.) To make a fairly large amount of the standard solution fill a 55-gallon drum approximately 2/3 full of water. SLOWLY add two bags of calcium chloride while stirring the water. (Add two 100-pound bags of regular calcium chloride at 77 strength, or two 80-pound bags of concentrated calcium chloride at 94 strength. The bags may be used interchangeably because they have the same strength to weight relationship.) Stir the solution until all the flakes have dissolved completely. Then fill the drum to the 50-gallon level and stir again to be sure it is well mixed. After the solution has cooled it will be ready for use. Small batches may be used right away. One pint to one quart of this standard solution per bag of cement will furnish all the dissolved calcium chloride needed to hasten the set of portland cement plaster. A standard solution of calcium chloride ready for use is sold by some chemical firms.
To prepare five gallons of standard solution of calcium chloride dissolve 20 pounds of 77 strength calcium chloride in five gallons of water, or 16 pounds of 94 strength. This will be enough solution to accelerate 20 bags of cement when added at the rate of one quart of solution per bag of cement.
Calcium chloride is not an antifreeze and will not prevent freezing, but it will help the plaster to gain good strength more quickly and thus resist the breaking and softening action of freezing on uncured plaster.
The curing of plaster is extremely important in periods of low temperature. GOOD CURE MUST BE ACCOMPLISHED AND FREEZING MUST BE PREVENTED.
If proper precautions have not been taken and uncured plaster does freeze on a wall, the crew must do all in its power to help cure the plaster on the following day. In this manner at least part of the damage resulting from freezing of the plaster can be overcome. Curing can be helped by wetting the wall thoroughly, starting in mid-morning after thawing has taken place. If freezing temperature is anticipated on the following night, stop the wetting of the walls by mid-afternoon. Hang sheets of polyethylene film, canvas or other material from the top of the building to hold heat in the wall. Set a source of heat behind the sheet. If significant damage has been done and good strength cannot be developed in the wall by the measures described above, the plaster will have to be removed.
Check List for Plastering in Cold Weather
- Warm the sand and water.
- Consider the use of Type III Portland cement.
- Warm the air within the building.
- Avoid the use of too much mix water.
- Install control joints.
- Water cure the wall properly with fog-spray.
- Retain heat in the wall on exterior work.