Weather And How To Minimize Its Effects On Concrete
By Matt McPharlin
Weather conditions at a jobsite—hot or cold, windy or calm, dry or humid—may be vastly different from the optimum conditions assumed at the time a concrete mix is specified, designed, or selected. Weather can adversely affect the properties and the serviceability of concrete unless certain practices are followed by the contractor and concrete producer. This technical note will address some of the common issues related to concreting in weather extremes.
COLD WEATHER CONCRETE
Cold weather is defined by ACI committee 306 as “a period when for more than 3 successive days, the following conditions exist: 1.) the average daily air temperature is less than 40°F, and 2.) the air temperature is not greater than 50°F for not more than one half of any 24 hour period. The average daily air temperature is the average of the highest and lowest temperatures occurring between the period from midnight to midnight. Normal concrete practices can resume once the ambient temperature is above 50°F for more than half the day.” Concrete can be placed safely throughout the winter months if certain precautions are met. During cold weather, preparations should be made to protect the concrete; enclosures, windbreaks, portable heaters, insulated forms and blankets should be ready to maintain the concrete temperature.
Concrete gains very little strength at low temperatures. Freshly mixed concrete needs to be protected against the effects of freezing until a compressive strength of at least 500 psi is attained. At normal temperatures this occurs in the first 24 hours. Up to 50% strength loss can occur if the concrete freezes within the first few hours after placement or before the 500 psi is attained. Temperature affects the rate at which hydration of the cement occurs. Low concrete temperatures will retard the initial set and strength gain.
In cold weather the temperature of newly placed concrete should be kept as close to 50°F as possible for at least 3 days. To shorten this time the concrete mix could be adjusted with the use of an accelerating admixture, the addition of an extra sack of cement, or the use of Type III cement. These practices will increase the amount of heat generated in the mix to offset the effects of cold weather. The following measures can be employed depending on the severity of the expected weather conditions.
Insulate the concrete to reduce heat loss. The heat generated from the internal chemical reactions will help to maintain favorable conditions for the development of early strength. During cold weather the protection offered by forms, except those made of steel, is often of great significance. Forms will insulate the concrete and allow for favorable curing temperatures.
Provide an additional heat source to maintain the concrete temperature. Combustion heating units must be equipped with heat exchangers that permit venting out the products of combustion (particularly carbon dioxide, CO 2 ). Exposure of fresh concrete to CO2 can result in poor durability of the wearing surface and deterioration of the concrete.
Modify the concrete mixture. Add accelerators, extra cement, or use a Type III cement. Calcium chloride should be avoided in any structural concrete containing steel reinforcement. Calcium chloride will cause corrosion of the steel and lead to deterioration of the structure.
Overcoming the problems of cold weather generally involves protecting the concrete. There are many ways of producing desired results but all of them have schedule and budget implications The most important element for success in cold weather is a plan of action by the contractor, which is coordinated with the architect/engineer, concrete supplier, and owner. Planning can help provide a structure of the required durability and quality, with minimal disruptions to the project schedule and budget.
HOT WEATHER CONCRETE
Concrete placed during the hot months of the year is subject to conditions that can adversely affect the properties and serviceability of the hardened concrete. Some of these conditions are:
- Increased water demand
- Increased rate of slump loss
- Increased rate of setting
- Increased tendency for plastic shrinkage cracking and drying shrinkage cracking
- Lower ultimate strengths
- Decreased durability
- Undesirable surface appearance
Hot weather as defined by ACI committee 305 1.2.1, is any combination of the following conditions that tend to impair the quality of freshly mixed or hardened concrete by accelerating the rate of moisture loss and rate of cement hydration, or otherwise resulting in detrimental results:
- High ambient temperature
- High concrete temperature
- Low relative humidity
- Wind velocity
The term “hot weather” can be misleading, as some of the undesirable effects on concrete can and do occur in the spring and fall seasons and in arid climates any time of the year. The ideal conditions for placing concrete occur when the temperature is 68°F to 72°F, the relative humidity is at 50% or higher, and wind speeds low. As the temperature rises the humidity begins to drop, the wind velocity increases, or any combination of these conditions occur, control procedures are needed to minimize the harmful effects.
CONTROLS FOR HOT WEATHER
- Using a concrete mixture that is properly proportioned for conditions expected in the field can help to minimize the rate of slump loss, and water demand.
- Consider the use of a set-retarding admixture to control set times. The use of fly ash can also reduce the effects of fast-setting concrete.
- Have adequate manpower and equipment available to handle the concrete at the desired rate of placement and to unload the concrete trucks promptly.
- Drying shrinkage cracking is greatly influenced by the amount of water in the mix. As the temperature rises so does the amount of water needed to maintain the specified slump. Add water only once—adjust the slump when the truck first arrives and follow through with prompt placement.
- Plastic shrinkage cracking is one of the most common types of cracks found in flat work. These cracks usually occur in the surface of fresh concrete within the first few hours after placement. These are usually short, random, discontinuous cracks. These type of cracks can be virtually eliminated by proper construction practices. When the rate of evaporation at the surface exceeds the rate at which bleed water rises to the surface, then these cracks are likely to occur. Low humidity and high winds greatly affect the surfaceevaporation rate. To minimize plastic shrinkage cracking, keep the surface wet (not saturated) by applying moisture with a fog sprayer until the final curing method is ready to be applied.
The key to successful placement of concrete in adverse weather conditions is planning. Knowing the locally available materials as well as what to expect out of the weather on the day you plan to place concrete is half the battle. Having an action plan to handle any of the conditions above can actually save you money and promote your business.