Thermal Control Plans for Mass and Thermally Controlled Concrete
As schedules on construction projects continually accelerate and service life requirements become more demanding, structures are being built larger and to last longer. With this trend, the performance of concrete has also improved. Because the cement in the concrete generates heat as it hydrates, there has also been an increase in the number of “mass concrete” pours. Thankfully, the awareness of mass concrete and the thermal behavior of concrete has also increased over the years.
Historically, mass concrete is widely used in dam structures, bridge piers, matt foundations, and other large structures that require, substantial volumes of concrete. Additionally, thermally controlled concrete, which may not have the same volume and dimensional characteristics as traditional mass concrete, also require thermal considerations because of the higher cementitious content that leads to high internal temperatures. For the purpose of this article, thermally controlled concrete will also be referred to as mass concrete.
There is no universal definition that defines a pour as mass concrete. In many cases, the oversimplified definition is that if the element’s smallest dimension is greater than 4 feet, then it should be categorized as mass concrete. According to the American Concrete Institute, mass concrete is any volume of structural concrete in which a combination of dimensions of the member being cast, the boundary conditions, the characteristics of the concrete mixture, and the ambient conditions can lead to undesirable thermal stresses, cracking, deleterious chemical reactions, or reduction in the long-term strength as a result of elevated concrete temperature due to heat from hydration.” If precautions are not implemented on a mass concrete pour, these undesirable results can have substantial financial consequences.
In document ACI PRC-207.1-21, the American Concrete Institute developed a figure to provide guidance on when a pour should be treated as mass concrete. The figure accounts for the primary influencers: the concrete mix and the minimum dimension of the element. If the pour falls in the red area, the pour should be treated as mass concrete. It should be understood that the figure is a guide, and if there is any doubt, consultation with a thermal control concrete engineer is recommended. The engineer will review all the characteristics surrounding the pour, and if it is determined to be mass concrete, the next step is the development of a thermal control plan.
Figure 3.1.1 from ACI 2017.1 R-21(Gajda et al. 2018)
Understanding thermal control plans
A thermal control plan (or TCP) is a document that demonstrates a contractor’s methods to comply with mass concrete specifications. A TCP provides a foundation to ensure that high thermal stresses and cracking of the mass concrete will be avoided and that the long-term durability of the concrete will not be negatively affected. This is attained through the control of both the maximum temperature in the concrete (at the core) and the temperature differential between the core and outside faces of the concrete elements.
Educated owners and designers will require contractors to provide thermal control plans if there is a potential for the pour to fall under mass concrete. However, even if the specifications do not state a thermal control plan is required, project staff should be educated on the topic and learn to recognize when a pour may be considered mass concrete, as the contractor will often bear the financial burden if thermal cracking develops.
A thermal control plan should be prepared by a registered professional engineer and subject matter expert on concrete thermal control. It will often contain the following items:
- Project Review
This includes an understanding of the size of the pour(s), mix design, general schedule of concrete placement, pour sequence, equipment that the contractor intends to use, and other variables that may affect the temperature of the concrete.
- Thermal Analysis
Based on the review of the aforementioned items, a thermal analysis will be conducted to determine if and what action items will be necessary to mitigate the potential undesirable temperatures and stresses in the concrete. Typically, a thermal control plan will require 1) the maximum core temperature to remain under 160° F, and 2) the maximum differential between the core and surface to remain below 35° F. In most cases, maintaining the temperature differential between surface and core (vs. limiting the maximum temperature) is the primary challenge.
- Mix Design Optimization
Generally, the most cost-effective method to reduce internal concrete temperatures is by optimizing the mix design. This is often achieved by replacing some of the portland cement with a supplementary cementitious material (SCMs) such as fly ash or slag cement, which have a lower heat of hydration.
- Concrete Cooling Options
The plan may also provide guidance on the alternative methods for reducing concrete temperatures. The options include pre-cooling options such as using chilled water, ice, and in more extreme cases, the use of liquid nitrogen. The pre-cooling options would usually be performed by the concrete supplier. A post-cooling option through the use of pumping chilled water through cooling pipes may also be recommended.
- Surface Temperature Control
As mentioned earlier, the primary challenge is limiting the temperature differential. The use of insulation or blankets on the forms reduces the rate of heat loss from the surface of the element, helping to minimize the temperature differential between the core and the surface. The TCP will often provide insulation requirements based on the ambient temperatures and concrete temperature at the point of placement.
- Guidelines on Temperature Monitoring
This section of the TCP will include the # of temperature sensors, location of sensor placement, reporting format, frequency of the temperature data recording, and recommendations on sensor brands and vendors. There have been major advancements in concrete sensors in the last few years. Newer technology has led to sensors that can log temperatures, transmit the data wirelessly and in real-time, and even provide alerts and notifications when certain thresholds are exceeded. Concrete sensors such as Brickeye’s Lumicon concrete sensors contain all of these features (and more!) and can provide the contractor and owner’s representative with extremely accurate data in real-time. This benefit allows the contractor to anticipate issues before they become costly mistakes.
This list is a summary of the items that may be included in a thermal control plan. Like many aspects of construction, pre-planning and open communication between the owner’s representative and the contractor can help identify issues before they become costly mistakes. If there is any concern that a pour may classify as mass concrete, it is highly recommended to talk with a thermal control engineer. Brickeye works closely with thermal control engineers throughout the United States and Canada. If you need recommendations, please don’t hesitate to reach out to us.
About the author:
Based in Colorado, Eric Van Dixhorn holds a bachelor’s degree in civil engineering and a background in construction, with a specialty in heavy-highway and infrastructure construction. Van Dixhorn brings a unique perspective to his role as Director of Technical Sales, having previously served as the Vice President of Engineering at NITROcrete.