Planning Mass Concrete Placements for Radiation Control

Proton therapy is a cancer treatment that enables doctors to direct proton beams at cancer cells with sub-millimeter precision. The generation and delivery of the proton beams is a complicated process housed within a maze of mass concrete sections to contain radiation generated as part of this process. Radiation shielding provided by the mass concrete elements is an integral part of the treatment facility’s design.

In late 2011, Baker Concrete Construction commenced the challenging work on a Knoxville, Tenn.-based proton therapy facility. The facility has a relatively small footprint but contains massive structural and shielding elements including:

  • 18-inch-thick mat foundation
  • 3- to 12-feet-thick walls (6 feet on average)
  • 6-foot-deep elevated slabs

The construction team for this project consisted of engineers, physicists, contractors and the concrete supplier who cooperated throughout construction to overcome numerous challenges.

 

Mass concrete challenges

Projects involving mass concrete present unique challenges that must be addressed by the construction team. When concrete is placed, the hydration of cementitious materials in the concrete mixture generates heat. In large sections, or “mass” placements, heat is generated faster than it can be dissipated, and the temperature rise within the concrete can be excessive. Uncontrolled temperatures can lead to cracking, internal damage to the concrete and long-term durability issues. The most common considerations when dealing with mass concrete involve:

  • Concrete mixture optimization to reduce heat generation
  • Maintaining temperatures below a maximum specified limit
  • Controlling internal temperature differentials

Pre-construction planning is essential for identifying concrete mixture options, placement practices and insulation/cooling techniques that will be beneficial to the project. The planning needs to consider the potential for thermal cracking, ambient weather conditions and scheduling impacts. The radiation shielding component unique to this project added to the complexity of the mass concrete design. The construction team worked together to overcome challenges including tight mass concrete specifications and weather considerations such as hot summer placement conditions to develop and place a mass concrete mixture that minimized heat gain while meeting strength, density and radiation shielding requirements.

 

Tight mass concrete temperature requirements

Compared with standard industry requirements, the mass concrete specifications for this project were tight. Section 8 of ACI 301-10 “Specifications for Structural Concrete” requires that a thermal plan be developed to monitor and control temperatures on mass concrete projects. The mass concrete requirements of ACI 301 and the Knoxville project specifications are presented in Table 1 for comparison.

While the project team worked within the tight Knoxville temperature requirements, they did relax the 3 inch maximum aggregate size requirement. After discussing placement concerns the project team decided it would be reasonable to utilize a 1-½ inch maximum nominal aggregate size gradation to enable the concrete to be placed by pumping.

 

Weather challenges

An unusually wet winter delayed concreting operations that were to begin in early 2012. The combination of clay soil and excessive rain at the jobsite hindered early construction activities. As a result, concrete placements originally scheduled during the cooler months of the year were pushed into the summer. This shifted the bulk of placements from ambient temperatures expected to be between 40 and 60° F to that in the 70 to 90° F range.

Planning for ambient temperature conditions is critical to the success of projects involving mass concrete. The Knoxville project imposed limitations on placement temperature, maximum temperature rise due to the heat of hydration, and temperature differentials within the concrete section. In cooler weather, placement temperatures benefit from the natural cooling of raw materials making it easier and more economical to meet both maximum placement and temperature rise requirements. As a result of the early weather delays on this project, the expected increase in ambient temperature required the concrete mixture to be redesigned to meet the challenges of summer placements and the strict mass concrete requirements.

 

Concrete mixture optimization

Since the mass concrete placements were pushed to the warmer summer months, the construction team decided to redesign the original concrete mixture to optimize it for placement under warmer ambient conditions. The supplementary cementitious material (SCM) content of the original mixture was doubled to reduce its adiabatic temperature rise potential by nearly 20° F. As a result of the mixture change, the placement temperature could be increased by approximately 20° F and still maintain the same maximum temperature rise level as the original mixture that would have been placed at cooler temperatures. A comparison between the adiabatic temperature rise of the original and optimized mixture is shown in Figure 1. A secondary benefit of increasing the SCM content resulted in an extended setting time which was helpful during consolidation of deeper sections.

 

Concrete density challenges

Since the original mass concrete mixture was optimized to contain additional supplementary cementitious material, the density of the new mixture had to be re-evaluated to determine if it would still meet the minimum density requirements of 145.2 pcf. The concern was that the substantial increase in SCM content of the new mixture may cause a significant reduction in density since a portion of the heaviest mixture component (cement) was being replaced by a lighter material (SCM).

According to the shielding consultant’s requirements, the density of the concrete mixture had to be conservatively analyzed based on its dry density tested in accordance with ASTM C567. One concern discussed by the construction team was that evaluating the 24-hour dry density as required by ASTM C 567 would not provide an accurate representation of the new mixture’s potential density. The hydration process of mixtures with a higher SCM content is slowed due to the fact that the SCM continues to form secondary hydration products and bind water into the concrete matrix over an extended time period. In other words, the density of the high SCM mixture was expected to increase over time. Since the actual structure would be allowed to hydrate for several months before being placed into service, the shielding consultant agreed that the oven-dry density of the mixture should be evaluated over an extended time period to determine if the mixture’s density would increase. The measured increase in oven-dry density of the proposed mixture over a 56-day period (Figure 2) successfully determined that the density of the proposed mixture was adequate.

 

Compressive strength must still be met

As a result of the increased SCM content of the optimized mixture, the construction team evaluated the compressive strength of the new mixture over an extended time period. A plot of the strength gain from 7 to 120 days is presented in Figure 3. The project specifications required a minimum 28 day compressive strength of 3,000 psi. Based on the size and loading of the structural elements, the project team agreed on extending the specified compressive strength from 28 to 56 days.

The project team maintained excellent communication and effectively dealt with the numerous challenges as they arose during the project. This project is an excellent example of how preconstruction planning and cooperation among the various disciplines is key to a successful project.

 

About the authors

Ron Kozikowski, PE, a construction and materials engineer with North Starr Concrete Consulting, P.C. Erich Breitenstein is superintendent with Baker Concrete Construction, Inc. Dennis Knose is regional operations manager with Baker Concrete Construction, Inc. Matt Glasshagel is operations manager with Lithko Restoration Technologies, LLC.

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