Accelerated curing methods offer many benefits beyond early-age strength development. Understanding how they work and when to use them is key to implementing them successfully.
By Kayla Hanson, P.E.
We live in an age of instant gratification. We can have groceries delivered to our doorsteps within hours, communicate with people across the globe within seconds and watch most any movie instantly with the click of a button. So why do we wait days for concrete to cure?
While the raw materials, mix designs, and controlled manufacturing and curing conditions used in precast concrete manufacturing enable products to reach design strength in a few days or less, traditional curing methods aren’t always fast enough.
What is accelerated curing?
Accelerated curing uses heat, or a combination of heat and moisture, in the early stages of the curing process to increase the rate of cement hydration. This expedites concrete’s early-age strength development. Accelerated curing is used for many reasons, such as counteracting slow strength gain during cold-weather concreting, providing an optimum curing environment for dry-cast concrete or helping ensure precast products reach a desired stripping strength quickly.
Keystones of curing
Cement hydration and pozzolanic reactions
When portland cement comes into contact with water, cement hydration reactions begin almost instantly. The reactions, along with those of supplementary cementitious materials, initiate the process by which the paste gains strength, stiffens and ultimately creates a resilient matrix, giving way to concrete as we know it.
Three keys to curing: Time, temperature and moisture
The conditions fresh concrete is exposed to throughout the curing process directly impact its hardened properties. Ensuring concrete is allowed sufficient time to cure in an appropriately warm and humid environment will help optimize the concrete’s properties and ensure the product is as strong and durable as possible, given the mix design and structural design.
The impact of maintaining a moist curing environment on concrete compressive strength development is depicted in Figure 1 (at left). The figure reveals that curing concrete in a moist environment compared with curing concrete in air without moisture could almost double compressive strength in one year if all other factors are constant.
Heat causes cement hydration reactions to occur at an expedited rate, which causes concrete to develop strength at a faster rate. Conversely, reactions occur at a slower rate when the fresh concrete or the surrounding environment is cooler. The impact of casting and curing temperatures is shown in Figure 2 (below and left). Note that concrete cast and cured at warmer temperatures shows higher early-age compressive strengths, but long-term strengths may be slightly compromised.
How accelerated curing works
Accelerated curing methods alter one or two of the key curing influencers – temperature and/or moisture – to shorten curing time. Curing concrete at high temperatures in low humidity can cause cracks and may reduce long-term strength gain, therefore accelerated curing tends to employ both increased temperature and moisture throughout the process, typically using steam.
Because of heat’s impact on cement hydration reaction rates and concrete strength gain, most accelerated curing methods are heavily dependent upon it.
“It’s advantageous,” said Mark Kraft, regional director – North America of Kraft Curing Systems, Inc. “You’re creating a 100% saturated environment at an elevated temperature. Steam is also the most efficient heat-transfer method available.”
The specific approach for each situation depends on factors such as the shape and size of the concrete product, allowable accelerated curing time, finish requirements and cost.
Other methods to achieve higher early-age strengths
Other methods to expedite early-age concrete compressive strength development include using heating coils under formwork, heating forms prior to casting, incorporating accelerating admixtures, using warm mix water or incorporating Type III cement in the concrete mix design.
Accelerated curing processes and methods to increase the rate of early-age strength gain are not universal. Each mix design, product, manufacturing facility and curing environment is different, so the methods should be carefully considered before selecting the best option.
“You have to look at the cost of your mix design versus steam curing,” Kraft said. “That will play a role. The chemicals, the cement, and the accelerated curing have an expense.”
Accelerated curing using steam
Steam curing is a popular accelerated curing method for precast products and can be conducted using live steam at atmospheric pressure or a high-pressure autoclave. The autoclave, which is similar to a pressurized oven, is best suited for curing small products or masonry units. Due to their size, most precast products are steam cured at atmospheric pressure within an enclosure. Steam curing precast products requires careful attention to time and temperature, as well as to the curing enclosure.
Steam-curing enclosures function like kilns and are typically made of tarps, polyethylene sheeting or similar materials capable of retaining both heat and moisture. Some precasters use a separate room for steam curing instead of creating a temporary enclosure.
“You really want to make sure your kiln is well-sealed,” said Marcus Barnett, Hamilton Kent territory manager and 37-year-veteran of the precast industry. “You can lose a lot of heat through fluing.”
No matter their design, ensure curing chambers are properly sealed and free of gaps or openings so the enclosure retains steam, prevents drafts and maintains the anticipated curing conditions.
Steam curing cycles
Plants must establish a curing cycle that outlines durations and curing temperatures specific to each application.
“The mix design is going to determine the curing specifications,” Kraft said. “Steam curing can affect the aesthetics of architectural and colored products. You could get staining, change of color or efflorescence if it’s not carefully controlled.”
Product dimensions are also a factor in tailoring each accelerated curing cycle.
“Thin-walled sections are going to go faster than a thick product,” Kraft said. “Thin sections will reach their curing temperature quicker and need less accelerated curing time than a large product.”
A general steam-curing cycle is shown in Figure 3 (at left). The curing cycle, durations and temperatures will vary depending on each scenario and must be monitored and documented throughout the accelerated curing process.
Sensor technology advancements have streamlined steam curing for precasters. Many sensors on the market today allow precasters to upload the data directly to a computer for easier monitoring and correlation between sensor data and other recorded concrete data.
Barnett advised precasters to pay close attention to the sensor location within the curing enclosure. Placing a sensor on the ground or near the top of the enclosure may not provide an accurate representation of conditions throughout the chamber. Placing sensors in the same position each day is also important to maintain consistency in data collection.
Steam curing process
Phase 1 – Initial Delay
For wet-cast concrete, Phase 1 could last up to 6 hours from the time of casting. This duration is shorter for dry-cast concrete products. During this period, the products cure in the enclosure without using heat or moisture.
If heat is applied too soon, the concrete could experience permanent damage, delayed ettringite formation (DEF) or compromised long-term strength development. This initial delay also ensures any moisture on the concrete surface is allowed to naturally evaporate instead of drying prematurely as a result of heat being applied too soon.
Therefore, the initial delay is critical to allow the cement paste to reach initial set – at least 500 psi – prior to applying accelerating curing practices. Initial set is typically tested in accordance with ASTM C403, “Standard Test Method for Time of Setting of Concrete Mixtures by Penetration Resistance,” and can occur 2-to-6 hours after batching.
“When the concrete gets to initial set, that’s when you’re going to get the most from accelerated curing,” Barnett said.
Conversely, it’s also important not to wait too long to begin accelerated curing practices. Steam curing is less effective when applied after concrete reaches final set. When producing numerous pieces over the course of an entire shift, consider creating an adjacent curing enclosure so the steam curing process can run in waves.
“Sometimes, it could be 8-or-10 hours before you can start the steam,” Barnett noted. “If you can, it’s best to start the steam as you go.”
Figure 4 (at left) shows the relationship between concrete compressive strength at 18 hours and the initial delay period prior to employing accelerated curing practices. Regardless of the steam temperature in each scenario, the 18-hour compressive strength was optimized when a 5-hour initial delay was used.
Figure 5 (below and left) shows the rate at which concrete mixes at different temperatures can reach initial set. Similar to Figure 2, Figure 5 depicts how increased temperatures significantly impact the rate of initial set.
Phase 2 – Ramping Up
Phase 2 generally lasts 2-to-3 hours. During this “ramping up” period, the enclosure is filled with steam as the temperature steadily increases. The temperature increase within the enclosure must be carefully controlled and limited to a 20-to-40-degree increase per hour – until the enclosure temperature reaches about 140-to-150 degrees. This prevents thermal shock and avoids damaging concrete volume changes.
Steam curing concrete at temperatures greater than 140 degrees is not shown to significantly improve compressive strength development.
Phase 3 – Constant-Temperature Steam Curing
Phase 3 consists of holding the enclosure temperature constant – around 140 degrees – for 6-to-12 hours or until the concrete has reached the desired compressive strength. The duration primarily depends on the mix design and the curing temperature. Mixes made with Type III cement, which provides enhanced early-age strength development, may require less time to reach the desired compressive strength, while products made with Type I or other types of cement may require longer curing times. Likewise, steam curing at temperatures toward the high end of the acceptable range – around 140 degrees – will expedite curing and strength gain.
Monitoring both the temperature within the enclosure and the concrete’s internal temperature during this period is paramount. Cement hydration reactions are exothermic, meaning they generate heat. Therefore, while the curing environment within the enclosure may be an appropriate temperature, the concrete temperature during steam curing could be significantly higher. In no case shall the concrete temperature exceed 150 degrees, unless the precaster employs measures proven to avoid DEF.
“The cement hydration reactions are going to add a lot of heat, too, so you have to be careful not to overshoot it,” Kraft advised. “On a larger product, for instance, when you see the internal temperature get to 140 degrees, you can cut off the heat source and you’ll see the internal temperature keep rising due to the heat of hydration.”
Phase 4 – Ramping Down
Phase 4 typically lasts 2 hours or longer. The steam curing process continues during this period, but the temperature within the enclosure is gradually reduced at a rate of about 20-to-40-degrees per hour until the enclosure temperature is within 20 degrees of the ambient environment. Like the “ramping up” phase, this “ramping down” phase must be carefully controlled. Excessive temperature drops could shock the concrete or cause abrupt and damaging volume changes.
Once the enclosure temperature is within 20 degrees of the surrounding environment, the enclosure can be opened and the cured products removed and transported to their storage location.
Myriad benefits await
Accelerated curing methods can offer benefits beyond expediting early-age strength development. In comparison to curing concrete at 73 degrees, which has been shown to be optimal in terms of long-term strength development potential (Figure 2), curing concrete around 140 degrees can reduce drying shrinkage and creep.1
Precasters should consult their suppliers to determine how accelerated curing practices will affect each mix design and specific product, and to determine the ideal accelerated curing cycle for different applications. Depending on the specific scenario, other methods to promote early-age strength gain – such as using accelerating admixtures, Type III cement or warm mix water – may be viable solutions.
Sidebar: Delayed Ettringite Formation
Ettringite, or calcium sulfoaluminate, is a naturally occurring mineral in portland cement concrete. Gypsum and other sulfate compounds react with calcium aluminate to form ettringite in the early stages of the hydration process. When the concrete is exposed to elevated temperatures at early ages, some of the existing ettringite can deteriorate. Then, in the presence of moisture, it can reform much later once the concrete has cured. The reformed ettringite, or delayed ettringite formation (DEF), can cause internal expansive pressures that can lead to hardened concrete cracking. Therefore, the recommended maximum concrete curing temperature should be limited to 158 degrees.1
Kayla Hanson, P.E., is NPCA’s director of technical services.
1 Kosmatka, Steven H. and Wilson, Michelle L., Design and Control of Concrete Mixtures, EB001, 16th edition, Portland Cement Association, Skokie, Illinois, USA, 2016, 632 pages.