Specifications for mass concrete require thermal control plans to control cracking. Historically, mass concrete meant dams, but many other structures may be large enough —or have high enough portland-cement contents —to warrant treatment as mass concrete. Beton has completed many thermal control plans for bridges, for example.
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.”
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.
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. It may be necessary to provide layers of insulation so the contractor may remove them one by one, not all at once.
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. Supplementary cementitious materials further mitigate delayed ettringite formation.
- 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 (MnDOT) 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. Concrete shrinkage also causes cracking. Sometimes when shrinkage cracks appear on massive concrete elements, people attribute them to temperature differential. However, 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%.
Why does concrete temperature matter?
Thermal control plans focus on temperature differentials to minimize cracking. But curing temperature also affects durability.
The primary reason to limit the maximum hydration temperature is to prevent delayed ettringite formation. Ettringite is a mineral containing calcium-, sulfur-, and aluminum oxides. It’s a normal product of portland cement hydration. However, if the curing temperature is too high, ettringite is not stable and monosulfoaluminate (monosulfate for short) forms instead. When the concrete cools, monosulfate transforms to ettringite if there’s enough water and sulfate present. That’s a problem because it’s an expansive reaction, which is highly destructive. One way to prevent it is to keep the temperature low enough to favor ettringite formation in the first place.
Another reason to limit hydration temperatures is that at elevated temperatures the pores in the cement paste are coarser and more interconnected. This makes the concrete more permeable, so that it lets water and harmful chemical in easily. For example, chloride ions penetrate much more readily into concrete hydrated at 120 F than at 68 F. The effect of 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.
Fortunately, fly ash, slag, and silica fume mitigate both effects. They can prevent delayed ettringite formation even at temperatures above 160 F. They can also help develop a discontinuous pore structure at elevated temperatures. Even better, they reduce the heat of hydration so that concrete temperatures remain lower.
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”.
If you’re paying attention to 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-mix concrete supplier.
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. Wet curing will not eliminate shrinkage cracking—only prolong 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.
Heat of hydration
Estimating the heat of hydration of a concrete—to use in the thermal model to estimate how hot the center will get and what the differential could be—can be done in many ways. Each thermal control engineer will have his or her own way of estimating the heat of hydration. Whether that’s by calorimetry or calculation doesn’t really matter. What is important is that they use a method that they are comfortable with.
I liken it to a compression test. A standard compressive strength test (ASTM C39) doesn’t really measure the compressive strength of concrete. However, we’ve built a whole engineering specification, engineering design, and construction industry around the results of that test. We understand what the numbers mean even if they don’t exactly measure the concrete’s compressive strength. When you specify a thermal control plan, the heat of hydration will be calculated or measured somehow. There isn’t one best way to do it, so you don’t need to specify how it’s done.
Beton can help you develop and specify thermal control plans for your mass concrete projects.