Concrete Mix Design: Controlling Temperature and Time

Intentionally inducing DEF in a laboratory – an experiment for QC managers

By Frank Bowen

Editor’s Note: This is the fourth article in a year-long series that explores the science of concrete to provide a better understanding of mix design. The series is collaboratively written by Paul Ramsburg, technical sales specialist at Sika Corp., and Frank Bowen, business development representative with Rosetta Hardscapes. Click here for the third article in the series.

Author’s Note: Before we dive into this, I invite you to read ACI 305, “Guide to Hot Weather Concreting,” and ACI 306, “Guide to Cold Weather Concreting,” as well as sections 4.4.6, “Hot Weather Precautions,” and 4.4.7, “Cold Weather Precautions,” in the NPCA Quality Control Manual for Precast Concrete Plants. In addition to these guidelines, I also suggest you read the article “Thermal Shock of Concrete” by Kayla Hanson, P.E., published in the July-August 2016 issue of Precast Inc. To avoid any redundancy and to keep you riveted, I would prefer to introduce new ideas and concepts rather than repeat what was already covered in a conclusive editorial. In her article, Hanson reviews the bulk of the casting and curing rules we should be following regarding temperature regulation while manufacturing precast concrete. My article, however, is intended for readers who want to break those rules and learn from their findings.

As we find ourselves in the peak of summer’s heat, it’s a good time to review the production and curing practices in our facilities. Temperature limits – even though I am about to ask you to cross them – are set in place for a very good reason. As precasters, we need to cast our forms constantly to remain profitable. Time is money, and labor isn’t cheap. Therefore, we want to produce as many castings as possible in the shortest amount of time allowed. This is where we can run into serious problems, especially if we are not careful of our constraints. Production teams who are more focused on the number of units produced than the quality of units produced can risk the dangers of delayed ettringite formation (DEF) if proper mitigation techniques are not employed.

Time can be on your side

“We need to pour those forms twice a day!” We have all heard this before. Accelerating admixtures, shorter curing cycles and rapid-paced manufacturing are tools that bring precasters the efficiency they crave. But what happens if we reach the limits of what we can produce and fall behind schedule? When construction projects go ahead of the planned schedule, praise is often given to suppliers and manufacturers. The “behind the scenes” production time we provide our customers will always be a factor in our bids to improve our share of work. Time, as it constantly reminds us, is without break and more valuable each day we press on. When deadlines approach and we find ourselves behind schedule, cheating time can become increasingly appealing. But if we stick to a few simple rules, we can plan for things to go exactly as we hope.

The problem

What is the problem I am addressing in this article, what is it caused by and how do we avoid it? Too much heat during curing of concrete can cause DEF but it can be avoided by following American Concrete Institute and National Precast Concrete Association guidelines. Let’s look at what is happening in the curing stages of a precast casting.

I define the stages of precast curing in four categories. The first is a well-known category called “initial set.” Initial set is defined as the time from when concrete is placed and finished to the point it takes 500 psi to penetrate the mortar 1 inch per ASTM C403, “Standard Test Method for Time of Setting of Concrete Mixtures by Penetration Resistance.” This stage is easily monitored by means of a penetrometer and can help us determine when we can introduce additional curing techniques. The second stage of curing is from the point at which concrete reaches handling strength. I call this stage “initial cure.” It’s during this stage that it becomes more reliant on the quality control manager’s tacit knowledge than his or her explicit knowledge to predict the time it takes to achieve this point. This is because many penetrometers are accurate only from about 100 psi up to about 700 psi. Most of the current hydraulic strength testing machines are not deemed accurate for testing results below 1,500 psi on 4-inch specimens. This leaves a gap between testing for strengths between 700 psi and 1,500 psi that is best understood by years of casting, recording and analyzing set time data.

PRO TIP: In general, we need the concrete to achieve the designated stripping strength before we can lift it out of the form. This, of course, is dependent on the thickness of the member, the mass of the member, the lifting devices used and numerous other variables. Each casting should be analyzed properly to determine stripping, lifting and handling strength limits. Some castings may need less, some may need more. Conclusive internal testing is the only way to prove a member is ready to be removed from the form.

Once at least three 4-inch cylinders are tested – I recommend casting one additional cylinder for the stripping-strength test, so you can break one early to get an estimate of the load increase — and recorded within the range limits of the designated stripping strength, the casting can then be moved from the form, post finished, labeled and moved to its third stage of curing. I refer to this stage as the “production plant cure.” This is the stage where the casting is moved to an area where further curing can take place. The final stage of the cure is after the casting is installed in what I refer to as the “on-site/installed cure.”

For this article, I want to address the two most important stages of curing to a precaster regarding the economy of form production cycle times, initial set and initial cure. But please allow me to digress for just a moment to make sure that you have addressed a couple of cost-saving measures prior to diving into an all-out thermal assault on your concrete castings.

The process

There are numerous ways to increase throughput in a precast plant. Some options, such as purchasing more forms or adding a new batch plant/building are not always viable. When this is not an option, it forces us to consider improving our output by decreasing existing production cycle times. This was the driving factor for the creation of dry-cast concrete manufacturing. Regarding wet-cast operations, I suggest continued lean review.

First and foremost, find your waste – most of it will be found in lost time – exploit it, correct it and teach others how to sustain the change. When applying the lean methodology to production of precast, observe the crew’s habits and process flow to see how long it takes to complete a given task. Do this to reevaluate the plant layout and, specifically, the steps and distances used in the process. There is free money on every plant floor if you know where to look. The most effective way to maximize efficiency at a precast plant is by adding a properly functioning second, or even, third shift. Adding an evening production crew eliminates the underutilized downtime in a manufacturing plant. This isn’t always an easy process to implement and usually requires a great deal of management attention to coordinate the two shifts. If your customer demand increases and you want to use more of what you already own, by all means, keep trying to achieve this goal, regardless of the constraints you encounter.

Now, let’s get back to understanding concrete temperature in a rapid-cycle production facility.

In a very brief synopsis, here are six key notes from NPCA’s QC Manual regarding temperature limits found in sections 4.3.5 and 4.3.6.

In cold weather conditions:

1. Avoid adding freezing aggregates to the mix.
2. Water at temperatures exceeding 180 degrees Fahrenheit should never be introduced to the mixer.
3. Fresh concrete temperature at the time of placing should be between 45 and 90 F. If heat or steam is used to assist in curing, initial set should be achieved prior to the introduction of heat and/or steam.
4. For this, initial set should be recorded at a minimum of once per month, per mix design to verify when the heat and/or steam can be introduced.
5. When accelerated curing is implemented, the rate of temperature rise should be monitored closely and never be allowed to exceed 40 F per hour. On a micro level, while monitoring a digital temperature gauge that shows accuracy to a tenth of a degree, this would average about 9 seconds between each tenth of a degree increase.
6. The maximum internal temperature of the concrete should never exceed 150 degrees during the cure unless DEF mitigation procedures are employed. See the commentary in the NPCA QC Manual on pages 59 and 60 for a few options suggested to mitigate DEF in higher cure temperatures.

PRO TIP: Keep in mind that when steam curing in your plant, auditors for the NPCA Plant Certification program will consider the intent of the added heat being introduced into your plant when judging if internal thermal recording is necessary. For example, turning up the heat on a central air thermostat that operates the plant’s overall ambient temperature may not be considered intentionally increasing the curing temperature. On the other hand, pointing the hot end of a torpedo heater in the direction of a precast form should show the intent of the added heat as a pre-planned process to accelerate the cure, thereby requiring proper internal monitoring.

During the initial set, the chemical reaction of hydration begins, and as the earliest formations of crystalline silica develop, it is critical the casting is protected from handling, moving or vibrating. The newly developed matrix is as fragile in this stage as it ever will be. After an initial release of increased heat, crystallization proceeds slowly with a period of low heat evolution. Depending on the casting shape and function of the form, demolding an outer form wall may be possible in only a couple hours and before even 500 psi is achieved, but it is inherently risky.

In the second part of the cure, which usually takes place overnight in a precast plant, a tremendous increase in heat of hydration takes place.

“There’s nothing like seeing the steam release and feeling the surface of a casting as the form is pulled opened the morning after placing concrete,” said Gary Knight of Lehigh Cement Co.

For the next couple of days, this concrete will have a rapid rate of increasing strength. At the early stage of hydration, the most soluble phases, tricalcium aluminate and tricalcium silicate, react first and contribute to initial set and early strength. At this stage, regulated temperatures at the upper end of the limits previously mentioned and maximum humidity provide the ideal environment (short of adding atmospheric pressure) to ensure this concrete reaches its strength and durability potential.

The experiment

It is now, during these two stages, I challenge you to break some rules (in the lab, of course). Specifically, I encourage you to break the maximum temperature limits, but test as many variables as you wish, so long as they are isolated for each experiment. To better understand the effects of curing in optimal, standard and substandard conditions, here is my recommended experiment in which you can intentionally induce DEF.

• Cast 22 test cylinders from a mix design used in your plant. Though it is not necessary for this experiment to work, choose a mix design, if available, that has no supplementary cementitious materials, uses Type 3 cement and is chemically accelerated. Try to have the fresh concrete temperature close to 90 F when placing it in the test cylinders. This would provide the most dramatic results.
• Cure six cylinders as you normally would for standard issue, in-house testing requirements, two for stripping strength breaks (let’s stay consistent here at 24 hours), two in the 73.5-degree F tank for 7-day breaks, and two in the 73.5-degree F tank for 28-day breaks. These are your control specimens. Compare all results to these breaks.
• Now, take 10 cylinders, directly after casting while the concrete is still in a fresh state, and immediately cure them in a controlled environment with 100% humidity at 200 F for 23 hours. After these sit for 23 hours, remove the cylinders from their molds, let them cool to 73.5 F in the 24th hour and keep them in a dry environment. Leave them exposed on the lab room counter if the lab can maintain a reasonable ambient temperature around 73.5 F.
• Break two at 24 hours. At the 4-day mark, place four cylinders back in the tank, break two at seven days and two at 28 days.
• Leave four cylinders on the counter, break two at seven days and two at 28 days.
• Take the remaining six cylinders through an optimal cure process. After casting, place two in a 100% humidity, 90 F environment until the predetermined initial set time of this specific mix under similar conditions. Once initial set has taken place, increase the ambient cure temperature at an even rate of 10 degrees every 15 minutes, for the next 90 minutes until the temperature is at 150 F and maintaining 100% humidity. In this environment, cure two of the cylinders for about 20 hours (just before the 24-hour break point allowing the cylinders to cool for handling at 73.5 F), two for 7-day breaks, and two for 28-day breaks.
• You will find, of course, that you can demold at record speed when overheating a casting during initial set. However, the problem in doing so is that moisture loss from the heat increase prevents the formation of calcium silicate hydrate, and this shuts down the chemical reaction of hydration before it was complete. When you demold your cylinders, there may be nothing obvious in appearance, but problems are now buried in the premature matrix. The partial crystallization that has already developed and matured to set is mixed among cement particles that have not yet begun to develop. What you have is a time bomb.
• The cylinders that were subjected to high heat early, left on the counter for four days, and then reintroduced to moisture should experience the expansive cracking that results from the delayed formation of the mineral ettringite. With high early heat in excess of the prescribed limit, ettringite, which is a normal product of early cement hydration, is choked off from development at the stage in its life when it is needed the most.

The lesson

Reading about this experiment may be helpful, but seeing it first hand and training a QC and production crew is even better. I hope you challenge your QC staff to test the limits and gain an understanding of accelerated curing techniques. There are great lessons learned by breaking the rules so long as these failures are recognized, and we learn from them. Failure is quite arguably a prerequisite to achieving success.

Frank Bowen, a 2013 Master Precaster graduate, received his M.B.A. from Middle Tennessee State University through the Concrete Industry Management graduate program in 2014 and is a business development representative with Rosetta Hardscapes in Charlevoix, Mich.