Category: Articles – Precast Concrete

Article – Precast Inc. Magazine; January – February 2008
Cold-Weather Concreting

Several practices must be addressed in order to produce quality products that meet specifications.
By Evan Gurley

For most precast producers, it is that time of the year when the cold weather plays an immense role in the procedures, requirements, and practices that encompass precast concrete production. For manufacturers in the warmer climates, cold-weather concreting is not so much of a concern, but it is better to be prepared for extreme climatic conditions.

ACI 306 defines “cold-weather concreting” as the operations concerning the placing, finishing, curing and protection of concrete during cold weather. More specifically, it defines “cold weather” as a period of three or more successive days during which the average daily outdoor temperature drops below 40 degrees F (4 degrees C)and the air temperature is not greater than 50 F (10 C)for more than one-half of any 24-hour period.

Cold-weather concreting practices need to be addressed in order for precast manufacturers to produce quality products that meet specifications. Problems associated with cold-weather concreting are freezing of concrete at an early age; lack of required strength; improper curing procedures; rapid temperature changes; and improper protection of the structure consistent with its serviceability.

By observing a few principles, these problems can be avoided. Use discretion when deciding what is sufficient for dissimilar applications. What works for one application may not be the best for another, but generally, these principles will help you make quality precast products during cold weather. The main principles that should be defined are as follows:

  • Concrete that has attained a compressive strength of 500 psi or more and has been protected from freezing during this period will not be affected by a single freezing cycle.
  • Concrete described above will be able to establish its potential design strength even if exposed to further cold weather. This also means that there is no need for further protection of the concrete.
  • Design strengths which must be attained in a short time span (a few days or weeks) must be sheltered at temperatures above 50 F (10 C).
  • Little or no added external moisture is needed for curing during cold weather unless located in a heated enclosure.
  • Take special precautions when using calcium chloride as an accelerator (hardening and setting); especially when the concrete contains embedded metals. (See ACI 318 for acceptable limits of calcium chloride in reinforced and non-reinforced concrete members.)

When faced with cold-weather concreting situations, the manufacturer must decide whether it is profitable to operate during this period of time or whether it makes more sense to wait until warmer weather. Statistics have shown that the cost of adequate cold-weather concreting is not extreme when considering the products manufactured. If one decides to follow through and manufacture precast products during cold weather conditions, do not take any shortcuts. This will ensure that the products are of the highest quality.

General requirements

To manufacture quality precast products during cold weather conditions, a number of general requirements have been established to serve as guidelines. These guidelines for cold-weather concreting include planning, protection during “non-cold” weather seasons (spring and fall), concrete temperature, temperature insights, exposure and slump.

Planning. One of the first steps when preparing for cold-weather concreting is planning. When planning for cold- weather concreting the owner, concrete contractor and concrete supplier should meet to discuss how and which specific methods should be used. Planning should take place well before any freezing temperatures are expected.

Protection during non-cold weather seasons. According to the National Oceanic and Atmospheric Administration (NOAA), non-cold weather seasons (spring and fall) still have the potential for frost or freezing and should be considered when planning for protection of freshly exposed concrete surfaces. NOAA issues cold-weather advisories long before temperatures drop to dangerous levels. If in doubt, always take the safe route to avoid jeopardizing the quality and structural integrity. All concrete surfaces should be fully protected from possible freeze damage for a minimum of 24 hours after being placed.

Concrete temperature. During cold-weather concreting, the temperature of concrete at the time of placement (after mixing) should follow ACI 306, Table 3.1 – Recommended concrete temperatures (see Figure 1). In addition to the recommended placement temperatures in Table 3.1, these concrete placement temperatures should be controlled for a length of time (called the protection period) specified by ACI 306, Table 5.1 – Length of protection period required to prevent damage from the early-age freezing of air-entrained concrete (see Figure 2). The protection period depends on factors such as cement type (Type III reduces time) and amount (higher cement content reduces time), accelerators (reduce time) and service. When concrete is placed, the temperatures should be right around the minimum temperatures described in Table 3.1. These placement temperatures should never be higher than the minimum placement values by more than 20 F (11 C). Concrete placed at lower temperatures is sheltered from freezing and can develop a higher ultimate strength and durability from the longer curing time it experiences. Placement at higher temperatures speeds up the finishing process in colder weather but may weaken the long-term properties of the concrete.

Temperature insights. When concrete is placed, the concrete’s surface temperature is primarily what determines the success of protection. Testing and monitoring the temperature is therefore significant. When recording and monitoring the temperature there are certain things to look and test for that could make or break the effectiveness of the protection. First, you must recognize that the edges and corners of the concrete are more prone to freezing. This is simply because it is harder to maintain the corner spaces. Another important issue is the monitoring of the concrete temperature, outdoor temperature, time of readings and weather conditions. Monitor the concrete’s temperature daily and choose these readings sporadically in order to gain a range of values. These readings are typically taken by thermometers under some sort of temporary cover.

Exposure. Take precautions if it is likely the concrete in its saturated condition will be exposed to freeze and thaw cycles. In order to help protect the concrete, it should be air entrained, have a water-to-cement ratio (w/c) not exceeding limits recommended by ACI 201.2R, and not be allowed to freeze and thaw (in saturated condition) before gaining a compressive strength of 3,500 psi. Depending on the intended use, this w/c value is usually between 0.30 to 0.50.

Slump. Cold-weather concreting calls for a lower slump (less than 4 inches). This is desirable because it will have less bleed water on the surface. If this bleed water remains too long, it may affect the finish. The lower slump is also desirable because if this excess bleed water is blended into the concrete during trowelling, surface strength will be lower resulting in possible dusting and freeze-thaw damage.

Temperature

Placement temperature. As mentioned earlier, the temperature of the concrete at placement has a great deal to do with the protection of the concrete during cold weather. The placement temperature should be determined by recommended limits in ASTM C 1064. For placement temperatures that are much higher than line 1 of Table 3.1 (Figure 1), this does not protect against freeze-thaw as well due to the higher rate of heat loss for greater differences in the outside air and concrete temperature. In addition to the rapid heat loss, higher-temperature concrete usually calls for more mix water; as stated earlier, the lower the w/c the better. When more water is used in the mix design, more problems can arise (quick setting, increased thermal contraction).

Mixing temperature. The recommended concrete temperature during mixing is shown on lines 2, 3 and 4 of Table 3.1. The mixing temperature should not exceed 15 F (8 C) above recommend values in Table 3.1. One of the ways to offset the heat loss at the time of placement is to increase the concrete mixing temperature. This is only in correlation when the ambient air temperature is decreasing.

Heating. Heating of aggregates and water is an excellent way to obtain a desired mix and placement temperature. It is a difficult task to uniformly heat aggregates to a desired temperature and less challenging to heat mix water, but both are reasonable methods.

Heating mix water. Heating of the mix water is an easier method to obtain a higher mix temperature. While it may be easier in terms of preparation, there are many important rules to follow in order to avoid problems such as flash setting or clumping. If the temperature of mix water used is above 140 F (60 C), the order of mixing your ingredients may need to be changed. Instead of adding cement and water consecutively, water and aggregates may need to be added and then the cement.

Heating aggregates. Before heating of aggregates can take place, ice and any type of frozen materials must be removed. There are several methods of obtaining the desired temperature, but heating devices such as steam heaters and gas burners are typically used. The general consensus is that when air temperatures are consistently below 25 F (-4 C), heating the aggregates is necessary.

Overheating. Having a concrete mix with higher than recommended temperatures can cause major problems in precast products. It is impossible to have water that is too high in temperature since you can use water up to its boiling point. However, concrete temperatures must be within the limits shown in Table 3.1 in order to avoid flash setting. Aggregates, on the other hand, can cause problems if overheated. The average temperature of the aggregates should not exceed 150 F (66 C) as they are placed into the mix. This excess temperature could make the overall mix temperature exceed the required limits.

Preparation before concreting

To make a quality precast concrete product during cold-weather conditions, preparation is key.

Temperatures of surfaces in contact with fresh concrete. Preparation for cold-weather concreting primarily consists of ensuring that all surfaces in contact with early aged concrete are above temperatures that will cause early freezing or draw out the setting time. As long as the surfaces that come in contact with the early aged concrete are a few degrees above freezing and within 10 F (-12 C) of the minimum required placement temperatures, you should be OK.

Removal of ice and snow. Remove all ice from the aggregates. Failure to do so can ultimately disrupt the water content of the mix design. If the concrete’s temperature is too high (in cold weather, this is hard to accomplish), then crushed ice may be added to the mix. The volume of the ice should not replace more than 75 percent of the batch water. The maximum temperature reduction from the use of ice is limited to about 20 F (-7 C). If you add ice to the mixing water, then the ice must be completely melted by the time mixing is complete.

Protection against freezing

Prevention of early-age freezing. Early stages of concrete production are the most crucial when protecting against cold-weather concreting setbacks. Early-age freezing prevention must be accomplished directly after the concrete is poured. The temperature and moisture recommendations need to be achieved in order for full protection to take place. Concrete that gains the initial compressive strength of 500 psi without freezing can withstand a cycle of freezing and thawing. If the precast concrete structure is larger in size or has a lower cement content, expect a longer protection period.

Additional protection. Minimum protection requirements that prevent air-entrained concrete from one cycle of freezing and thawing is defined by ACI 211.1, Table 5.1 (Figure 2). This ultimately means that if early-aged concrete experiences one cycle of freezing and thawing, the concrete’s durability is not affected. This is true under the conditions that proper curing has taken place, concrete did not or will not freeze when critically saturated, and the concrete is air-entrained.

Stripping of forms. Stripping of the forms is an important issue when dealing with cold-weather concreting. If the precast plant is a heated enclosure, forms also can serve the purpose of evenly distributing heat. For best results, leave the forms on for no less than the minimum protection time. This is not always possible, so forms can be removed at the earliest design age that will not jeopardize the integrity of the piece, according to ACI 347. After the forms are removed, the product needs to be covered or kept in a sheltered environment for the set time recommended in the chart “Concrete Set Time.”

Temperature drop after removal of protection. After the concrete has been in a protected environment for the recommended time, it is essential to let the concrete cool gradually. Concrete needs to experience gradual cooling down to the ambient temperature in order to reduce shrinkage cracking. The heat source needs to be slowly reduced and the insulation layers removed, observing the maximum temperature drops in ACI 306, Table 5.5 – Maximum allowable temperature drop during first 24 hr. after the end of protection period (see Figure 4).

Allowable temperature differential. Typically the concrete is cooled down to the ambient temperature, but a temperature difference is permitted. Using ACI 306 Figure 5.6, the maximum allowable difference between the ambient temperature and concrete temperature can be determined.

Protection for structural concrete

Structural concrete requires a higher level of design strength than non-reinforced concrete. Cold-weather concreting requires more protection beyond the minimum requirements shown in Table 5.1. For structural members, the requirements change in the duration of the removal of forms and shores. The difference is that removal of forms from structural concrete is now solely based on the in-place strength and not the duration for which they are secured.

Testing of field-cured specimens. To ensure that the concrete is holding up to standards, testing should be administered to concrete cured in the field. Test for in-place strength before the forms are removed and curing takes place. Testing should conform to ASTM C 31 standards.

In-place testing. Non-destructive strength testing can be performed on concrete that is cured in place and in the field using hand-held, portable instruments. Methods such as the pullout test (ASTM C 900) and the probe penetration test (ASTM C 803) are the most common.

Attainment of design strength. Ultimate design strength is attained when certain factors come together to produce a structurally sound member. One of the main reasons that concrete does not meet its specified design strength during cold weather is due to improper or lack of curing. Concrete must be cured for a specified amount of time in order to meet design strength. Tests have shown that when concrete specimens have been removed from the curing process before the required duration, full design strength is jeopardized. In cold-weather concreting, precast
structural members need to gain a sufficient high early strength so that they will be protected from exposure to freezing weather.

Holding early strength. The most important factors when determining when to remove forms and shores are those that affect strength development. These are concrete initial placement temperature, type of cement, temperature at which the concrete is maintained after placing, type of admixtures and accelerator admixtures (if any), and curing and protection. There are many instances when accelerated manufacturing of precast members is essential to running an efficient plant, and in this case, the duration of protection may be reduced.

Cooling of concrete. Gradual cooling of the concrete member after the setting period is also essential for structural precast applications. This is explained in Table 5.5, which defines maximum cooling rates. Concrete needs to experience gradual cooling down to the ambient temperature to reduce shrinkage cracking.

Estimating strength development. In cases where no testing was performed to determine the strength of the concrete, you can estimate it. However, this is true only if the concrete was properly cured and protected. ACI 306 Table 6.8 provides the duration of recommended protection for a standard-cured, 28-day-strength specimen.

Removal of forms and supports. Removal of forms and shoring must be in accordance with ACI 347.

Methods of protection and materials

Protection is a crucial aspect of cold-weather concreting. First, the placement temperatures should represent Table 3.1 as well as the recommended protection period discussed in Table 5.1. Protection of early age concrete can range from one extreme to another. Applications may need only insulating blankets covering the concrete for a short amount of time, or in other applications, enclosures and heating devices may be needed to protect the concrete
and maintain the desired temperature for longer duration of time. The key is that if you manufacture concrete products in colder weather, do not jeopardize the integrity of the member because of cost or any other restraints.

Insulating materials. One of the nice things about concrete is the fact that it generates heat when going through its chemical reaction (hydration), hardening the concrete. So in most applications, if the heat generated from hydration is contained (insulating blankets, etc.), outside heating sources may not be needed to prevent freezing. This is generally true for the first three days, and then if the protection period calls for a longer cover time, then additional insulating blankets are generally adequate. Some typical concrete insulators include polystyrene foam sheets, urethane foam, mineral wool/cellulose fibers, foamed vinyl blankets, batt insulation or even straw. Different
applications call for different insulating materials (ACI 306).

Enclosures. If you are looking for the best type of protection for an application, then an enclosure is what you want. Enclosures offer the best type of protection, but with the best of anything comes a higher price tag and therefore they may not be cost-effective for certain applications. The need for an enclosure depends on certain variables such as the nature of the structure and the weather conditions. In cases where the ambient air temperature is less than -5 F (-21 C), enclosures are generally required for placing operations. Generally, if you are operating in those types of conditions, then you may experience larger problems (equipment failure, etc.) in the manufacturing of your precast products.

Internal electric heating. Another method used to keep the concrete temperature at a steady median above minimum requirements is by internal electric heating. This is accomplished by using embedded coiled and insulated electrical resistors. A low-voltage current is passed through the coils, which in turn raises the temperature of the concrete internally.

Covering. As soon as the concrete has been poured and placed in cold-weather conditions, the concrete needs to be protected. An easy way to do this is by covering the concrete with some type of insulating material. Layering is an effective way to insulate the concrete, as this helps retain any heat that is generated.

Insulated forms. Insulated forms are used alongside enclosures. When insulated forms are used, the concrete’s temperature needs to be closely monitored so that the concrete does not become heated beyond the recommended temperature.

Curing requirements and methods

Curing refers to the natural phenomenon that happens within the concrete. When water is added to a concrete mix, a chemical reaction takes place. The reaction forms a calcium silicate hydrate gel that “glues” everything together. As this glue hardens, the cement hydrates and the concrete cures. Curing directly affects concrete strength.

The 28-day strength is defined as the standard design strength or specified strength. So another important factor that comes into play when cold-weather concreting is proper curing. Proper curing techniques must be established in order for the newly placed concrete to be protected from drying. When the temperature is below 50 F (10 C), in most geographic areas excess drying will not be a major concern. Although excess drying may not be a major concern, freezing of the concrete when it is critically saturated becomes a problem. Protection is needed so that critically saturated concrete will not be exposed to freezing conditions.

Curing during the production period. Although excessive drying is not a major concern in most geographic regions, concrete that is being protected from the cold-weather conditions may be a different story. When warmer concrete (60 F/16 C) is in contact with warmer air (50 F/10 C or higher), then precautions must be taken in order to prevent drying. The most frequently used method in preventing drying, in this case, would be by steam heating. Other methods such as dry heating and warm-water heating are also options, although they are practiced less often.
If using the steam curing technique, it should be terminated 12 hours before the end of the temperature protection period, because it will need to follow the recommended gradual adjustment defined in Table 5.5.

NPCA curing requirements. NPCA’s Quality Control Manual lists its target curing temperatures in correlation with the concrete being produced (heavyweight, lightweight). For precast/prestressed concrete, the target maximum curing temperature is 150 F (66 C). The NPCA QC Manual states that when curing is done at too high a temperature, it kills the formation of ettringite while the hydration process is going on later. If the concrete is exposed to a moist environment, ettringite can form and then crack the product.

Curing following the protection period. After the protection insulation has been removed, additional protection from drying is not typically needed as long as the temperature remains below 50 F (10 C) and it is not windy. The drying of concrete members is typically defined by four main factors, which are the temperature of the concrete, ambient temperature, relative humidity and wind speed. If excessive drying is probable, then water curing is acceptable as long as the temperature remains above freezing.

Acceleration of setting and strength development

It is permissible to expect shortening of the required strength setting times by the addition of cement, type of cement or admixtures. This is often used in cold-weather concreting because it shortens the protection period, allows forms to be reused more quickly and requires less labor to finish the work. The accelerated setting can also raise the heat of hydration, which could come in handy when keeping the concrete’s temperature up to the required temperature. Additional information on accelerated setting can be found in ACI 212.

Admixtures. There are many accelerating admixtures available. A couple of the most common accelerating admixtures used today are calcium chloride and those that conform to Type E (ASTM C 494). Calcium chloride, which reduces the setting time and increases the rate of early-age strength development, is a very popular accelerating admixture. It must be limited in use since too much can be detrimental to the integrity of the concrete. ACI 318 defines the calcium chloride limits and regulations. Some Type E accelerating admixtures have also been found to increase strength gain and accelerate setting time.

Evan Gurley is a staff engineer with NPCA.

 

How to keep on pouring when the temperature’s soaring.
By Evan Gurley

Hot weather creates special challenges for precasters, and technically speaking, there are more obstacles to overcome when placing concrete in hot weather than in the cooler seasons. By understanding how heat, humidity, and wind affect the curing of concrete, you can adjust your mix and compensate in a variety of other ways to maintain high-quality standards.

While you will not likely need to take all of the recommended precautions stated below, each hot-weather scenario should be analyzed individually by qualified personnel, who should find the optimum mix of quality, practicability, and economy.

What’s hot?
To an inexperienced precaster, “hot-weather concreting” can be a misleading label. If the ambient temperature outside isn’t “hot” then why should we be concerned about hot-weather concreting problems? The fact is that adjustments may need to be made to your mix as the weather becomes just slightly warmer since your everyday mix can begin to perform differently as temperatures rise above 75 F (23.9 C).

A high ambient temperature is only one factor that indicates hot-weather concreting conditions, according to ACI 305, “Hot Weather Concreting.” Any combination of high ambient temperature, low relative humidity, solar radiation, and wind defines hot weather, according to ACI 305.

Wind is not customarily associated with hot weather, for example, but it is an important factor because the wind accelerates the curing process in combination with temperature, humidity, and solar radiation. Efforts to preserve concrete quality on windy, sunny days are more critical than those required on calm, humid days, even if ambient air temperatures are identical (see Figure 1).

Effects of hot weather on concrete properties

Hot weather conditions can lead to problems in mixing, placing and curing hydraulic cement concrete that can adversely affect the properties and serviceability of the concrete. If precautions are not effectively implemented during hot weather, the concrete may be damaged through plastic-shrinkage cracking, thermal cracking and decreased 28-day strengths. Once damaged, the concrete can never be entirely restored.

Increased rate of cement hydration at elevated temperatures and the increased evaporation rate of moisture from the freshly mixed concrete are the causes of most of the problems associated with hot-weather concreting. The ability of a mix to reach its design strength is determined by the efficiency 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. In principle, curing refers to the concrete’s gain in strength, but technically speaking, the rate of cement hydration is what can be adversely affected during hot weather.
Potential problems associated with hot weather can be categorized into three different groups: problems for concrete in a freshly mixed state; problems for concrete in the hardened state; and problems related to other factors (See Table 1).

Temperature, water, and slump

While increased concrete temperatures produce higher earlier strengths, the concrete’s 28-day strengths are lower and the final product may never reach its optimal design strength, as seen in Figure 2. We know that when concrete cures, the hydration process creates added heat and raises the temperature of the concrete, but excessively high ambient temperatures and solar radiation also contribute to the heating effect.

Water is obviously a crucial component that must be carefully regulated in any precast mix design, but this is especially true for hot weather conditions. The higher the temperature of the concrete, the more water needed for the required slump (increases with time). If water is not added to the mix, placing and handling operations may be negatively affected.

An increase in water should be offset by a proportional increase in the quantity of cementitious material, which will increase production costs. If water is simply added to the mix without the addition of cementitious material, the water/cement ratio of the mix will be compromised, resulting in a decrease in water tightness, strength, and durability of the final product. The bottom line is that if extra water is needed for a given mix design, this water must be accounted for during mix proportioning.

In hot weather, a mix will tend to set sooner than expected. There will be about a 30 percent decrease in set time for each 10 F (5.5 C) increase in concrete temperature, as shown in Figure 2.

This decrease in set time can make handling, consolidating and finishing the concrete very difficult. When a decrease in initial set time is correlated with the decrease in slump, slump loss is taking place. As stated in ACI 305, there is about 1 inch (25 millimeters) of change in slump for every 20 F (11 C) increase in concrete temperature.

Cracking and shrinkage

Even if you plan ahead and maintain the water-cement (w/c) ratio at an acceptable level, cracking in hot weather conditions may still occur, which further emphasizes the need to maintain complete control over your mixing, placing and curing practices. Three types of cracking are most common:

Drying Cracking. Drying shrinkage typically occurs when the water content in a mix is increased without adjusting the amount of cementitious material, altering the w/c ratio.

Thermal Cracking. Thermal cracking may 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. Thermal cracking can also be caused by an increase in the concrete temperature in larger members. In larger precast concrete members, there is an increased rate of hydration and heat evolution that will increase the range of temperatures between the interior and exterior concrete, increasing the chances for thermal cracking.

Plastic Cracking. Plastic shrinkage cracking is typically considered an arid-climate problem, but humidity is not the only determining factor. Low relative humidity in combination with high wind speed and/or high concrete temperatures can cause problems in freshly mixed/placed concrete members. These factors cause accelerated evaporation of surface moisture and become a problem when the evaporation rate exceeds the bleeding rate (the rate at which water rises to the surface from within the concrete mix). The most commonly used bleeding rate value is 0.2 pounds per square foot (ACI 305). The potential for shrinkage increases when the evaporation rate exceeds the bleeding rate. Incorporating additional materials incorrectly into the concrete mix, along with hot weather conditions, increases the possibility for plastic shrinkage cracking and drying cracking. Fly ash, silica fume, and fine cement have a low bleeding rate. This makes the concrete mixture very sensitive even in moderately arid conditions, increasing the possibility of plastic shrinkage. Extra precautions should be taken when incorporating these materials into the mix, given that plastic-shrinkage cracks are hard to repair.

Preventing moisture loss
Any combination of high ambient temperature, high concrete temperature, low relative humidity, solar radiation, and wind can cause moisture loss. In windy, dry climates, moisture loss in freshly placed concrete can be accelerated and cause evaporation of water from the concrete member. This leaves less water in the concrete mix than was called for by design. Without proper precautions, water remaining in the mix cannot completely hydrate the cement, resulting in less than optimal economic efficiency and a decrease in strength and durability in the final product.
Here are a few precautions that help prevent moisture loss; most precasters will choose some combination of these precautions, based on local conditions and the upcoming weather forecast:

Water. While water seems to cause most of the problems in hot-weather concreting, controlling the water temperature is easier to execute and has the greatest effect per unit weight on the temperature of concrete. This is because water has a specific gravity that is four to five times that of aggregates or cement. In general, adding cool water to the mix will reduce the overall concrete temperature, but typically not more than 8 F (4.4 C). The ACI 305 document estimates that lowering the temperature of the batch water by 3.5 F to 4 F (1.9 C to 2.2 C) will reduce the concrete temperature approximately 1 F (0.5 C).

Ice. Adding ice chips to the concrete mix must be done properly to be effective. Ice must be crushed, chipped or shaved before it is added into the mixer. For maximum efficiency, the ice should not melt before it is placed into the mixer, but the ice should be fully melted before the mixing of the concrete is complete. As the ice melts, it absorbs the heat from the concrete at an estimated rate of 144 Btu per pound and lowers the overall concrete temperature. The ice should not comprise more than 75 percent of the batch water. If proper procedures are followed, ice can potentially lower the concrete temperature as much as 20 F (11 C). If a 20 F reduction in temperature is still not enough, injecting liquid nitrogen into the mixer is another option.

Cement. The more cement in the concrete mix, the higher the temperature increase from hydration. Therefore,      the amount of cement used in your mix design should be limited to that which meets strength and durability requirements. Also consider that if newly manufactured cement is delivered to your plant, its temperature may be elevated. According to ACI 305, concrete mixtures consist of approximately 10 percent to 15 percent cement. Using that estimate, each 8 F (4.4 C) increase in cement temperature will increase the concrete temperature by about 1 F (0.5 C).

Aggregates. If you consider that most mix designs include 60 percent to 80 percent aggregates, then the temperature and moisture content of aggregates should have the most significant impact on the concrete. A 1 F decrease in concrete temperature can be obtained by lowering the aggregate temperature 2 F, for example. Consequently, extra efforts should be taken to keep aggregates cool during hot weather.

Aggregate factors such as shape, size, and grading all affect the amount of water needed in a mix to produce the required slump. Crushed coarse aggregates provide better resistance to cracking than round aggregates, but they also require additional water. Blending two or more sized aggregates can reduce the mixing water demand and increase workability.

Cementitious Materials. Adding supplementary cementitious materials (fly ash, slag, etc.) should be considered when it is necessary to delay the setting time or lessen the temperature rise from hydration.

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

Post-Pour. After placing and finishing concrete, you can prevent moisture loss by immediately covering fresh concrete with any moisture-retaining material such as burlap or a curing compound described in ACI 306. Retention of moisture will optimize the cement hydration process and allow the concrete to develop its full strength potential. Failure to keep exposed surfaces from drying excessively fast may result in cracking and shrinking and jeopardizes the integrity of the product.

Air content and temperature
As concrete temperature increases, entrained air decreases. Reduction in entrained air content is typically a result of slump loss. An increase in concrete temperature will require additional air entraining admixtures in order to maintain the air content of the mix.

In general, it is impractical to establish a maximum ambient temperature as your upper limit for production practices because of the other factors affecting the mix – concrete temperature, solar radiation, relative humidity, and wind. Plants in climate-controlled facilities, obviously, are not seriously affected by these factors. If your plant is not climate controlled, ACI 305 advises creating a set of measures that would include all the factors and testing your limits.

Chameleon Retractable Enclosures: The Ultimate Engineering Control

Many believe that Personal Protective Equipment (aka PPE) will be the solution to meeting the OSHA Silica Rule but it is not that easy or inexpensive.

Operations/hazards that have been identified as ones that will likely be above OSHA’s acceptable silica exposure limits:

  • Grinding and cutting of precast.
  • Media blasting.
  • Cleaning forms with compressed air.
  • Dust stirred-up around the plant by vehicles and material handling equipment.
  • And more… Silica dust is everywhere in a precast plant due to unenclosed dust creation processes.

Our initial impression is that PPE is the inexpensive and simple solution. If only using PPE for your employees, you will need to:

  • Monitor silica exposure levels throughout your property and provide PPE for all employees exposed to silica dust.
  • Maintain records of the exposure and training of all employees exposed.
  • Perform routine medical exams and maintain history on all employees exposed. Medical exams are costly and even more so for employers in rural locations.
  • Ensure that masks are fit-tested and all employees conform to facial hair requirements in order for masks to fit.
  • Ensure all employees are consistently and constantly trained on task, hazard and work practice engineering control.
  • Fines will be substantial for precasters whose employees fail to follow these requirements.

If PPE is not the ultimate solution, then what is?

Ultimately, the best solution is to ELIMINATE or SUBSTITUTE typical practices like grinding, cutting and media blasting but that is not typically an option in precast production.

Ultimate Solution: Engineering Controls – Isolate Employees from The Hazard

Silica dust is found throughout precast plants due to unenclosed dust creation processes. Chameleon Retractable Enclosures are a containment solution (engineering control) that will allow you to only expose a limited number of employees to silica dust since permanent enclosure solutions (ex: tents) do not efficiently work with cranes or concrete delivery systems.

The Ultimate Engineering Control: Chameleon Retractable Enclosures

  • Provide unrestricted 360-degree access to a work area, when retracted, allowing unhindered access to the precast product via crane, forklift etc.
  • Provide an environment where silica levels can be contained/controlled and easily monitored.
  • Protect production employees and forms/product from the weather – hot sun, wind, freezing temperatures, rain, etc. What is hard for your employees, is typically hard on precast production efficiency.
  • Engineered to your local wind and snow load requirements.
  • Chameleon supplies and retrofits various dust collector solutions.

The Bright Side for Precast Manufacturers:

The silica rule will only be harder for cast-in-place contractors. This should give precasters a competitive edge:

  • Precast concrete is manufactured in a factory and shipped to a construction site for assembly. That means all manufacturing processes creating dust occur at precast factories, not the job-site. You can apply engineering controls during manufacturing to capture dust in a factory where more costly engineering control solutions or time-penalizing job site segregation-of-operations will be required for those who cast-in-place on the job site.
  • Most openings are formed/molded into precast pieces (an engineering control) rather than cut in cast-in-place job sites. This means the additional costly dust control requirements, from cutting openings at the job-site, are avoided by precasting.
  • Precast concrete products are custom designed. You can adjust product layout and add reinforcing to minimize the amount of cutting required at the job-site.
  • Precast has a short construction duration. Coordination of subcontractors to prevent silica dust contamination between trades is likely to become a real concern at the job-site. The short construction duration, when precast is used, means job-site crews create dust fewer days, thus affecting other job-site trades people less.

All-in-all, the silica rule will be a challenge for everyone in construction but there are solutions and more so for those in the precast industry. Be prepared. The purpose of this OSHA rule is to decrease silica dust that does result in the terminal illnesses of thousands of tradespeople in our industry yearly.

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