Just the Cure for You

A discussion on accelerated curing for precast concrete.
By Eric Carleton, P.E.

There are several ways to cure concrete, but accelerated curing has become very popular among precast plants. This type of curing is advantageous when early strength gain is important or when additional heat is required to accomplish hydration. Accelerated curing reduces costs and saves time to better meet precast concrete production demands and finished product quality. Here’s a review of the basic phases of accelerated curing and how to determine the best results.
For efficient precast concrete plant production, it is imperative to cycle product forms as quickly as possible. For wet-cast operations, product is typically poured one day and stripped the next so the form is ready to accept another pour. For dry-cast operations, the speed is increased. Forms are stripped immediately for reuse after casting and consolidation, and achieve handling strength the next day. To accomplish either, precasters must employ techniques to accelerate curing and consequential early strength gain. Accelerated curing is accomplished in one of two ways. The first is through the use of chemical admixtures, primarily calcium chloride. Calcium chloride has an excellent 100-year history in effectively accomplishing accelerated strength gain in concrete mixes, yet it has some problematic characteristics as identified in the Federal Highway Administration Materials Group document, “Accelerating.” ¹
The best way to implement accelerated curing is by adding heat to the precast component while reducing process water evaporation through the use of steam, mist or product tarps. Research has shown this heat-and moisture-induced accelerated curing may lead to a slight compressive strength loss over the final 28-day period. However, the benefits of achieving high early strength far outweigh any minor mix design modifications to product design strength requirements. If a production method includes heat-induced accelerated curing methods, it’s good practice for the manufacturer to include a certain frequency of 28-day cylinder breaks to determine if the specific process exhibits this minor strength loss for record keeping, if not required by specification.
The National Precast Concrete Association has previously published two articles describing the basic processes of precast concrete curing: “To Cure or Not to Cure” in the May-June, 2011, issue of Precast Inc. and the “Curing Wet-Cast Precast Concrete” Tech Note. These documents use Figure 1, which shows the atmospheric steam-curing cycle.  This important graphic interpretation of the accelerated curing process needs to be clearly understood by precast quality control and quality assurance personnel and the production team.

The process is divided into four primary sections based on temperature and time.  The graph’s vertical axis shows the temperature of the atmosphere surrounding the precast unit. However, it is important to also note what temperature the concrete product is at the beginning, during the curing process and at the end prior to storage. This provides a better understanding of mix characteristics and offers another data point if any unexplained quality issues occur.

Pre-steaming

The first phase is a very significant time during the early life of new concrete. This period goes by a number of names such as initial delay, preset, pre-steaming, holding or setting. This is when the cement paste begins to become rigid due to hydration. During this period, the concrete mixture is in a vulnerable state. If it’s too cold it can freeze, and too much heat can prematurely accelerate hydration products and cause internal stresses. The setting time begins when water is added to the concrete mix, and ends when the temperature of the concrete noticeably increases and when the volume of the concrete slightly increases. ASTM C403 has become the standard to determine the mix setting and consequently the setting time. It is developed for mixtures with a slump greater than zero and is based on using a standard Proctor soil probe pushed into the mix to a depth of 1 inch. When the mix stiffens to develop a 500-psi penetration resistance, it has set. The actual length of time from when water was first introduced into the mix to this set point is the pre-steaming time or initial delay time.

Most mix designs used for precast concrete products set between 45 minutes and two hours. The American Concrete Institute’s 517.2R, “Accelerated Curing at Atmospheric Pressure” standard states, “… The drier earth-moist mixes used in machine pipe and dry-cast process are able to withstand relatively short preset periods without damage because of their lower water content.  At a total moisture content of 5% some mixes have shown no significant decrease in compressive strength with a preset period as short as 1 hour.” ³

It is generally agreed that during this period the only heat needed is to ensure the ambient temperature and corresponding concrete is above freezing with an optimum temperature above 50F. The setting time will decrease with moderate increases in temperature, but a temperature above 85F will cause a detrimental effect.

Controlled heating

The next important phase in accelerated curing is the controlled heating section.  This is where the precaster can begin to add heat via hot air and mist water, hot steam for products outside and electric heating blankets or heated forms for products not yet stripped from forms. However, the term controlled heating is there for a purpose, as the heat increase needs to be applied at a uniform rate of roughly 40F to 20F per hour.

It was previously believed concrete could safely be brought up to temperatures approaching 180F. However, research has shown these high curing temperatures can potentially lead to delayed ettringite formation or visible displacement and cracking.

During this phase and into the next constant temperature or soaking time phase, when exposed concrete surfaces of dry-cast or wet-cast products which are still in their forms, they must be kept moist for proper curing. Additionally, if the formwork consists of wood without an impervious barrier coating, it may work like a sponge to draw out water from the precast product. If the wood form must stay in place, be sure to keep it wet and saturated when possible. If a steel form is used, ensure it hasn’t moved or expanded enough to pull away from the precast product. Also, it is important for precasters to carefully check the blankets or steam enclosure curtains being used to ensure there are no major tears or leaks.

Steam loss from opening in curing tarps (photo Mel Marshall)

After the concrete temperature has caught up with the ambient temperature through hydration, external heat sources may be turned off or reduced. The time in this phase will vary depending on the products soundness of the enclosure and temperature used for curing.

The final phase is controlled cooling. Similar to controlled heating, the expectation is to conduct a gradual, uniform reduction of curing temperatures. The suggested reduction in temperature is between 40F to 20F per hour.

Document your findings

It is important for the plant QA/QC personnel to note and document specific temperatures and curing times within the curing cycle that works for their plant, products, process and climate. Quoting from ACI, “There is no one curing cycle that is best for all plants.  Each plant is unique and the curing cycle that is optimum for one plant may not be effective in another.  Many factors act and interact in the curing cycle, and they influence the strength and other properties of the product …”⁴

A staff and work crew knowledgeable about your plant’s accelerated curing process will provide the best assurance to improve the quality of the finished product and to remedy any curing issues which may be noticed.  A well-documented accelerated curing procedure and corresponding positive testing results for your plant’s unique conditions may save you time and trouble if an inspector visits your plant. It will also help you revise your production protocol based on an understanding of precast concrete manufacturing standards.

Eric Carleton, P.E., is NPCA’s vice president of Technical Services.

Resources:

1. Federal Highway Administration Guidance Document, Infrastructure Materials Group “Accelerating”
2. Portland Cement Association, Design and Control of Concrete Mixes, 15^th Edition, Figure 15-9
3. ACI 517.2R Accelerated Curing at Atmospheric Pressure – This document has been discontinued by ACI and is available for informational purposes only.
4. ACI 517.2R-7 Effect of Variations in the Curing Cycle on Compressive Strength and Other Properties