Optimizing Mass Concrete Construction with PBTDL
The Performance-Based Temperature Differential Limit Method
During the construction of mass concrete projects, contractors face a multitude of challenges when concrete fails to meet performance requirements, often leading to delays and complications. With new construction developments breaking ground every day in major cities across North America, contractors, project managers, and building owners are working to build modern-day infrastructures more efficiently – meaning built faster and designed to last longer. As a result, the overall performance of concrete has improved, especially on projects of this scale.
Traditionally used in large structures requiring substantial volumes of concrete, mass concrete requires significant thermal considerations due to the higher cementitious content that leads to high internal temperatures. The American Concrete Institute (ACI) currently defines mass concrete as, “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.”
ACI 301, Specifications for Structural Concrete, states that after placement, the maximum temperature should not exceed 160°F, and the maximum temperature difference between the core and the surface should not exceed 35°F. While these guidelines are safe and cautious specifications, a more thorough analysis, such as a thermal control plan (TCP), allows engineers to steer away from such requirements to enhance the concrete construction process.
Figure 1. Guide to identifying mass concrete [Ref. ACI 207.]
How mass concrete is, or isn’t, recognized by DOTs
Depending on location, different Departments of Transportation (DOTs) set forth unique requirements for mass concrete, from maximum temperature and temperature differentials to thermal monitoring and maximum thickness. The DOT in each U.S. state holds specific specifications, reflected below in Figure 2, to show how mass concrete is – or isn’t – included in standard specifications, and whether or not it includes provisions for a TCP or other requirements for mix design, size constraints, and temperature limits.
Figure 2. Mass Concrete Specification Groups
Moving beyond TCPs
A TCP provides a foundation to ensure that high thermal stresses and cracking of mass concrete will be avoided, thus ensuring overall long-term durability. TCPs allow contractors and engineers to mitigate a number of risks during the placement of mass concrete through the analysis of maximum temperature and temperature differentials within the concrete placement and allow specifiers to deviate from some requirements set forth by the ACI, such as ACI 301.
While this safe and conservative approach is simple to implement and is standard across mass concrete projects, it doesn’t fully consider that as the concrete cures, the allowed thresholds should increase as it gains strength. This shortcoming has led to the development of new methods that, when implemented, focus on a temperature differential approach based on time.
Adopting the PBTDL Method
PBTDLs provide a temperature difference limit that is directly tied to the concrete’s compressive strength, completely replacing the 35°F temperature differential set limit. Through real-time data and analytics on mass concrete projects, the PBTDL approach enables an increased allowed temperature differential, while still maintaining a safe threshold, through monitoring of the maximum temperature and temperature differentials.
With initial testing completed on the mix to determine material properties, such as elastic modulus, tensile strength, and coefficient of thermal expansion, these properties are then used to develop a chart that provides the temperature differences that the concrete can resist without thermal cracking.
Thanks to ruggedized IoT-enabled sensors and devices, project stakeholders can then remotely and continuously access data in real-time, transmitted directly from the job site to the cloud, without the need to be onsite to assess temperature or strength data. Because it provides key insights into data on concrete temperatures, temperature differentials, and strength, the PBTDL approach enables contractors to be more proactive from both a quality control and operational standpoint.
The set temperature differential method is overall easy to understand – and even easier to implement. While the approach may require added pre-planning, the operational, environmental, and time savings outweigh any additional engineering costs and will benefit mass concrete projects for years to come. As awareness of the limitations of the 35°F set temperature differential increases, coupled with advanced concrete sensing technology, the use of a PBTDL approach is rapidly gaining popularity across the globe.
Figure 3. Different approaches to mass concrete temperature differential limits