Specifications for mass concrete require thermal control plans to minimize cracking. Traditionally, mass concrete meant dams. But other structures may be large enough—or have enough portland cement—to treat as mass concrete.
ACI 207.1R defines mass concrete as, “any volume of concrete with dimensions large enough to require that measures be taken to cope with the generation of heat from hydration of the cement and attendant volume change to minimize cracking.”
Many state DOTs require contractors to provide thermal control plans for bridge elements. Thermal control plans ensure that the differential between the core temperature and the surface temperature is never high enough to cause cracking. The plan can either use a maximum allowable temperature differential of 35 °F or demonstrate that the imposed stresses never exceed the cracking strength of the concrete. Beton has completed many thermal control plans for bridges. We can help you develop a specification for a thermal control plan—or meet a specification someone else has written.
Why is heat generation a problem?
Cement hydration generates heat, causing the concrete to expand. Eventually, hydration slows, the heat begins to dissipate, and the surface starts to cool off. However, it can’t contract because the warm concrete core restrains it. That generates tensile stresses, and if those stresses are high enough, the concrete will crack. These cracks are not small—they can be wide enough to stick your finger into and extend through the thickness.
Other problems result when concrete cures at elevated temperatures. The first is that the hydration products form hard, dense shells around the cement grains, leaving a network of coarse, interconnected pores. Chloride ions, for example, penetrate much more readily into concrete hydrated at 120 F than at 68 F. The effect of that increase in hydration temperature is comparable to that of increasing the water-cement ratio from 0.40 to 0.50. There’s no specific limit below which temperature has no effect on pore structure.
The other problem is delayed ettringite formation. Ettringite is a normal product of portland cement hydration. However, at curing temperatures somewhere above 160 F, ettringite is not stable and monosulfoaluminate (monosulfate for short) forms instead. When the concrete cools, monosulfate transforms to ettringite. This expansive reaction is highly destructive. Delayed ettringite formation does not normally occur without some other process to cause cracking and requires the presence of liquid water for the reaction to take place.
To avoid these problems, thermal control plans limit both the temperature gradient and the maximum temperature.
Keeping it cool
Limiting the maximum temperature helps minimize the thermal gradient. In mass concrete, you want to minimize the amount of portland cement because that’s what generates the most heat. One strategy is to substitute as much supplementary cementitious material as you can. Fly ash and slag cement hydrate more slowly and generate less heat than portland cement. Small amounts of silica fume can help if you need high strength at an early age—for example, in posttensioned members.
Supplementary cementitious materials offer additional benefits in mass concrete. They can prevent delayed ettringite formation and help develop a discontinuous pore structure even at temperatures well above 160 F.
Another strategy is to minimize the amount of cement paste by optimizing the aggregate grading. The aggregate suspension method (ACI 211.6) uses as much aggregate (and as little cement paste) as possible while still producing a workable mixture. For long spans or members that are prestressed or posttensioned, this is the best way to minimize creep and loss of prestress.
You can also lower the placement temperature using the same methods you’d use in hot-weather concreting. The lower starting temperature not only slows the initial rate of hydration, but also reduces the maximum temperature. Thermal modeling can tell you the maximum placement temperature to use.
As a last resort, you can install pipes to carry cooling water into the concrete core. Because this is so expensive, we prefer to use some combination of the other strategies.
Thermal modeling for prediction and control
In addition to minimizing the core temperature, there are some other things you can do to limit the temperature gradient. Thermal modeling will help you decide how to manage them.
Thermal modeling predicts the core temperature of the concrete over time. The core temperature depends on the concrete mixture proportions—how much of each type of cementitious material is present—and the initial placement temperature.
Insulation alters the surface temperature to keep the temperature differential within acceptable limits. The thermal model can determine what type of insulation is necessary and when it can be removed. It’s especially helpful to use several layers instead of a single thick one. That way you can gradually cool the surface by removing the layers one at a time.
Avoid sudden drops in temperature at the concrete surface, as they make it contract. The still-warm interior concrete acts as a restraint, and the resulting stresses may cause cracking. For bridge piers, that means there must be no water in cofferdams. Coverings must be well secured to protect the surface from wind and rain.
In working with mass concrete, you should also pay attention to shrinkage. Many times, I’ve seen owners who are upset over cracks in their mass concrete elements. The cracks are clearly shrinkage cracks, but they blame the contractor or the ready-mixed concrete supplier. The concrete doesn’t care what causes tensile stresses—it just knows that when they’re too high, it’s time to crack. Owners don’t care, either—they just don’t want to see cracks. So it’s up to you to make sure you’ve considered all tensile stresses to make sure they don’t add up to cracking.
Mass concrete elements are large placements. You would never place a typical concrete floor or driveway with 4000 psi concrete and not joint it. If you’re placing a foundation that is over 4 ft thick without joints, it will crack. The way to mitigate the cracking is to specify low-shrinkage concrete and to detail the reinforcing steel so that the cracks stay close together. The aggregate suspension method of mixture proportioning is a great way to minimize shrinkage. Wet curing will not eliminate shrinkage cracking—only postpone it.
Mass elements that are tall, such as columns and bridge piers, are less likely to crack from shrinkage or other restrained volume changes than elements like foundations and footings that have a lot of restraint at the base.
What should thermal control plans do?
To develop a thermal control plan, you need to do several things:
- Determine the heat output of the concrete mixture.
- Assess whether that concrete mixture will work for the element size, climate, ready-mix concrete producer, and prevailing conditions.
- Alert the contractor and ready-mix concrete supplier to treat the concrete as something out of the ordinary.
- Inform the ready-mix supplier of the maximum temperature of the concrete at placement.
- Tell the contractor how to keep the temperature differential between the core and the nearest surface below the specified limit.
- Inform the contractor/owner what to avoid.
Temperature limits in thermal control plans
Without getting into means and methods, which are the responsibility of the contractor, specifications should include:
- Smallest critical dimension at which a thermal control plan is necessary.
- Maximum limit on core temperature (160 F). ACI 301 says 158 F, the value as converted from Celsius to Fahrenheit. However, 160 F is a conservative limit for preventing delayed ettringite formation. Some researchers have determined that the interior concrete temperature can reach 180 F.
- Maximum limit on temperature differential. ACI 301 limits the difference in temperature between near-surface and core to a maximum of 35 F. This is very conservative, as the concrete gains strength with time. Even so, it is the most widely used limit across the United States. The Minnesota Department of Transportation specification allows the temperature differential to increase to 45 F after the first 48 hours and 60 F after the first 7 days.
- Limits on concrete shrinkage. The geometry of large foundations and footings is conducive to restraint of volume changes, whether due to temperature differential or shrinkage. Limit shrinkage to a maximum of 0.035%.
Specifying the instrumentation
Thermal control plans also include the placement and monitoring of sensors.
- How many sensors. Two at the centroid, two 2 in. inside the face closest to the centroid, and one for ambient temperatures are all you need. The external sensor can be in the shade or in the sun.
- How long to monitor. Specifications differ widely on this. Typically monitoring stops once the internal temperature reaches a maximum. However, the temperature differential is still of concern. Minnesota Department of Transportation specifications permit monitoring to cease after 72 hours (96 for bridge superstructures) or when the near-surface temperature is within 35 F of the average ambient temperature, whichever is later.
- Rate at which the concrete surface can cool off. This limit is not related to mass concrete, but to cold weather concreting. However, it often finds its way into mass concrete specifications. ACI 207 indicates that the concrete should not cool more than 20 F in 12 hours to prevent thermal shock.
- Frequency of monitoring. Temperatures should be monitored hourly, with the data transmitted to the thermal control engineer every 24 hours. More frequent data transmission may be appropriate for the first day or two.
- Temperature monitoring equipment. You may want to include something like this: “Use automatic sensing and recording instruments that record temperature at a maximum of 1 interval/hour. They should operate over a range of 0 F to 200 F with an accuracy of +/- 2 F”.