X-treme Concreting (Part 1) Hot Weather Concreting Practices - MC Magazine Summer 2003 - Concrete Publications

In 1922, the temperature in Al Aziziyah, Libya, reached 136 F (57.7 C). It is the highest temperature ever recorded. It beat the U.S. record set nine years earlier, in 1913, in Death Valley, Calif.: 134 F (56.6 C). At those extreme temperatures, human activity slows to a crawl, or ceases altogether. As a result, that kind of scorching heat can also affect how things are manufactured, including precast concrete.

Unfortunately, the majority of research performed on concrete has been done under controlled laboratory conditions where temperatures remain relatively constant, so the exact adjustments you need to make during hot weather may not be apparent. But even if you’re not experiencing world record-setting temperatures, you may still need to make adjustments as the weather warms because your everyday mix can begin to perform differently as temperatures rise above a mere 75 F (23.8 C)!

Hot weather
In 75-degree F (23-degree C) temperatures you might not even break a sweat, and they are unlikely to cause a work stoppage. So the question is: What is “hot weather” as far as precast concrete is concerned? According to the National Oceanic and Atmospheric Administration's formulas that measure Heat Index, a combination of high temperature and high humidity is cause for alarm – hence, the infamous “but it’s a dry heat” debate. However, according to ACI 305, “Hot Weather Concreting,” hot weather is any combination of the following weather conditions:

Hot weather problems
It is important to understand how hot weather conditions affect concrete, because once concrete has been damaged by hot weather, it can never be fully restored. One of the biggest problems during hot weather is its effects on curing.

The ability of a mix to reach its design strength is determined by the efficacy of the chemical reaction that takes place between water and cement. That reaction is responsible for solidifying the entire concrete mass. As concrete hardens, cement is said to be hydrating and the concrete is said to be curing. Understandably, the terms “hydration” and “curing” are often used interchangeably, but the former is a more technical term that refers specifically to the chemical reaction between water and cement. Curing is often a less technical term referring to the concrete’s gain in strength. Although intimately related, the two terms are used very differently. So it is more technically accurate to say that it is the rate of cement hydration that can be adversely affected during hot weather.

Concrete Temperature. The hydration process inherently produces heat (like many chemical reactions), but climatic factors such as high ambient air temperature and/or direct solar radiation can contribute to a detrimental increase in concrete temperature. Deceivingly, an increase in concrete temperature can produce higher early strengths, but it also contributes to lower 28-day strengths and your final product may ultimately not reach its design strength.

Water Demand, Initial Set and Slump. In hot weather, a mix will tend to set sooner than expected (an approximate 30 percent decrease in set time for each 10-degree F (5.5-degree C) increase in concrete temperature, as shown in Figure 2), which can make handling, consolidating and/or finishing the concrete very difficult. An indication of a pending decrease in initial set time is a decrease in slump, referred to as slump loss. As a general rule, there is an approximate 0.8 inch (20 mm) change in slump for every 20-degree F (11-degree C) increase in concrete temperature. An increase in concrete temperature can therefore increase the water requirement of the mix – the water will offset decreases in initial set time and slump. However, the addition of water without the addition of cementitious material (which is most commonly the case) will cause an increase in the water/cement ratio (w/c) of the mix and will result in a decrease in watertightness, strength and durability of the final product.

Cracking. When adding water because of hot weather, it is very important to add cementitious material since high water contents can also result in a greater likelihood of drying shrinkage. However, even when w/c ratios are maintained during hot weather, cracking can still occur. Thermal cracking, for example, can occur when fluctuations in ambient temperatures (such as a hot day followed by a cool night) cause a rapid drop in concrete temperature during initial strength gain. In arid climates, or anywhere with low relative humidity, there is also an increased tendency for plastic shrinkage cracking.

While plastic shrinkage cracking is typically associated with concrete produced in arid climates, it is not exclusively a result of decreased relative humidity. High concrete temperatures, low relative humidity and high wind speed can all cause evaporation of surface water, alone or in any combination, because each can cause evaporation of surface moisture. Whenever the rate of surface moisture evaporation exceeds the rate at which water rises to the surface from within the concrete mix (bleeding), the potential for plastic shrinkage cracking increases.

Moisture Loss. Although plastic shrinkage cracking is rarely a problem in hot and humid climates, the moisture loss due to evaporation during hot weather can still cause trouble in those areas. Any time a dry or windy environment exists, the rate of moisture loss of newly placed concrete can increase because water can more readily evaporate from the concrete into the atmosphere under those conditions. This leaves less water in the concrete mix than was called for by design. Without proper precautions, the water that remains in the mix cannot completely hydrate the cement, resulting in decreased economy and, in the final product, decreased strength and durability.

Air Content. This one may come as a surprise. As concrete temperature increases, entrained air decreases, typically as a result of slump loss. An increase in concrete temperature will require an increased dosage of air entraining admixtures in order to preserve the air content of the mix.

The detrimental effect of hot weather on concrete cannot be refuted. Obviously, you must take precautions to preserve cement hydration and maintain the integrity of the concrete. Fortunately for the precast concrete industry, once the product is poured and consolidated, it is much easier to manage curing in a quality-controlled environment than it is out in the field. However, preventive efforts to combat hot-weather problems should start at the mix design stage.

Precautions during hot weather
It is important to know when hot weather conditions will strike because, while you may know how your concrete will behave in hot weather, you cannot adequately prevent potentially bad behavior unless you adjust your mix beforehand. It is unlikely that you will need to take all of the following precautions – you should analyze your particular hot weather situation and plan accordingly.

Decrease Concrete Temperature and Reduce Moisture Loss. The hotter the ambient temperature, the more difficult it is to maintain a constant concrete temperature – especially because hydration inherently produces heat internally within the concrete. Nevertheless, maintaining the temperature of fresh concrete at approximately 55 F (13 C) will prevent many hot-weather problems. Since moisture loss can result from increased concrete temperatures, these precautions can also help maintain concrete water content (see Moist Curing related article).

Misting forms and reinforcement immediately before placement can help cool them and prevent unwanted temperature increases. However, ensure that form release agents are not adversely affected, and always avoid pooling water anywhere within the forms.

Since the amount of heat generated by the hydration process is proportional to the amount of cement in the mix, it may seem practical to cool the cement before batching and/or limit the cement in the mix to the amount that is required to provide the specified strength and durability. Since this amount of cement usually only comprises 10 percent to 15 percent of the mix volume, the temperature of the mix can be decreased by 1 degree F for every 8-degree F decrease in cement temperature. However, it is not always practical to cool cement.

Controlling water temperature is typically the easiest way to lower concrete temperature. It also has the greatest effect (per unit weight) on concrete temperature than any other ingredient. Water can be cooled to as low as 33 degrees F, but substituting ice for water is also a good option. During batching, ice should be added with water – that is you should not substitute ice for all of the water. Specifications usually limit the amount of ice to 75 percent of the required mixing water. Further, it is important to use crushed, shaved or chipped ice to ensure that all of the ice melts before mixing is completed. Typically, the temperature of the mix can be decreased by 1 degree F for every 4-degree F reduction in water temperature – but specifications usually limit how much you can reduce concrete temperatures by water cooling. Consequently, you should consider using other methods in combination when the concrete temperatures must be cooled to greater than 20 degrees F (11.1 degrees C).

While water has the greatest impact per unit weight on concrete temperature, aggregates have the most overall significant impact because aggregate can represent up to 80 percent of concrete volume. That is, although its impact per unit weight is less than that of water, a 1-degree decrease in concrete temperature can be realized with a 2-degree aggregate temperature decrease. Consequently, you should make every effort to keep aggregates cool during hot weather. Keep aggregate piles in the shade, away from direct sunlight, or cover them with a light colored tarp. If shading is not practical, or in the presence of low relative humidity, sprinkling the aggregates can help keep them cool. Keep in mind, however, that the water content of aggregates affects the water-cementitious ratio of the mix.

After concrete placement, prevent moisture loss by immediately covering with any moisture-retaining material such as burlap or a curing compound. Retention of moisture will optimize the cement hydration process.

Increase Initial Set Time. Substituting your current cement type with ASTM C 150 Type II cement, or ASTM C 595 Type IP or Type IS blended cements can help increase initial set time and can help with concrete handling. Although slower-setting cements can decrease the risk of thermal cracking, it can increase the potential for plastic shrinkage cracking. Consequently, careful consideration should be given to this option. You can also affect initial set with the use of ASTM C 494 set-retarding admixtures, and you should work closely with the admixture supplier to determine the ideal dosing rate for your particular hot-weather condition, cement content and cement type.

Decrease Slump Loss. ASTM C 494 water-reducing admixtures can help curb slump loss without affecting the water demand of the mix. Since the efficacy of chemical admixtures is conditional upon cement type, you should work with your admixture supplier for proper admixture selection and dosage rate.

Prevent Cracking and Loss of Air Entrainment. Admixtures that increase the bleeding rate can help counteract surface drying, but may also require additional consolidation after the majority of bleeding has subsided. Work with your supplier to optimize the dosage of any admixture you choose, including air-entraining admixtures, which may need to be increased depending on the combination of hot-weather precautions you take. The use of fiber reinforcement can help prevent drying shrinkage cracks and can be added to the mix per the manufacturer’s recommendations.

After placement, protect the product from fluctuations in ambient air temperature by keeping them indoors as long as practical and/or covering. The product should also be protected from wind or even slight breezes – windbreakers can help if the product must be kept outdoors.

Hot weather conditions can challenge the way you think about concrete, but should not impact the quality of your final product. As with any mix design, your hot-weather mix will likely be a result of trial-and-error – be patient. Cool cement, water and/or aggregates as needed to keep the temperature of fresh concrete within the optimum range, usually 50 to 60 degrees F (approximately 10 to 16 degrees C), and maintain moist-curing efforts as long as possible. Work closely with the supplier of any admixture you introduce to your mix or any admixture whose dosage rate you intend to change. This can be invaluable as suppliers are usually as determined to find a satisfactory mix design as you are.

It is important to remain informed about the weather conditions that will impact your facility so that you will be able to implement your hot-weather plans in a timely manner. Although it is unlikely that you will experience world-record temperatures like the ones experienced in Libya or Death Valley, make a habit of knowing the long-range forecast for your area at all times, especially during season changes when the weather changes quickly and frequently.

References
ACI Committee 305, “Hot-Weather Concreting,” ACI305R-99, American Concrete Institute, Farmington Hills, Michigan, 1999

Burg, Ronald G., “The Influence of Casting and Curing Temperature on the Properties of Fresh and Hardened Concrete, Research and Development Bulletin RD113, Portland Cement Association, Skokie, Illinois, 1996

Klieger, Paul, “Effect of Mixing and Curing Temperature on Concrete Strength,” Research Department Bulletin RX103, Portland Cement Association, http://www.portcement.org/pdf_files/IS015.pdf, 1955