Placing mass concrete—which is defined for practical purposes as concrete formed with a minimum dimension of 3 feet—presents challenges in any type of construction. Concrete contractors have to take the steps necessary to prevent thermal cracking and ensure that forms are braced adequately. When building an industrial project, they also may have to layout and place hundreds of anchor bolts that will be used to install and secure plant equipment. Locating anchor bolts accurately can be tricky, but it's key to successful placement of an industrial slab. Fortunately, there are innovative tools, foundation designs, and other resources that can make the job easier.
When concrete is placed in very thick sections, the heat generated during cement hydration can produce much higher temperatures at the center of a slab than at its surface. Unless measures are taken, these high temperature gradients produce differential volume changes that can lead to excessive cracking—which can compromise structural strength.
John Gajda, a principal engineer with Skokie, Ill.,-based CTLGroup who specializes in mass concrete issues, says structural thermal cracking can be serious. “If cracks develop through the thickness of a wind tower foundation, for example, you're left with a foundation that doesn't behave as intended, and the structure could potentially fail due to vibration, overturning, or overstressing.”
Thermal issues can be addressed by adapting the concrete mix design, reducing the delivered temperature of ready-mixed concrete, and using insulated blankets to cover mass concrete placements during curing.
Industrial projects can require unusual shapes that are difficult to produce using standard form system components. At the same time, though, mass placements make more demands on formwork than projects with thinner concrete sections do. Forms must be strong enough and braced well to withstand the hydrostatic pressure from massive amounts of fresh concrete.
Hydrostatic pressure on forms is a particular issue with mass concrete placements because the concrete mix typically is designed to set slowly. A slow-setting mix is less likely to develop the extreme temperature gradients that lead to thermal cracking. On the other hand, by remaining plastic longer, such concrete builds up more head pressure that can blow out forms if they are not adequately braced.
To ensure the formwork is up to these demands, contractors often rely on form manufacturers to engineer and supply appropriate forms. Most form manufacturers offer engineering services, and many also can provide technical representatives to oversee and train field crews in erecting and bracing forms correctly. Steve Karasik, lead engineer with PERI Formwork Systems Inc., Baltimore, says, “We can provide a range of services, depending on the experience and needs of the contractor. Sometimes a contractor will come in with drawings for a job he's about to bid, and we'll do a partial layout and preliminary materials list to help him develop an estimate.
“Once a contract is awarded, we'll do whatever engineering work the contractor needs in order to use our forms. The time frame varies from a couple of days for a simple project to a year or more for a large and complicated structure. One of the things we do is determine whether the formwork all has to be erected at once or if it can be done piecemeal. That decision obviously affects the quantity of formwork components that will be needed,” Karasik says.
Little Ferry, N.J.-based form system manufacturer Doka USA Ltd., prepared the engineering and field drawings for forms being used in the construction of liquid natural gas tanks in Freeport, Texas. Doka had all the main operating platforms and gangs delivered pre-assembled into 16x16-foot sections. The firm also sent a field service technician to the site to assist during the initial formwork installation and the completion of the first two lifts. Pre-assembly reduced labor costs for contractor Zachary Construction Corp., as well as saving time and avoiding the need for Zachary to train crews in formwork assembly.
Relying on formwork manufacturers to engineer and provide the formwork setup saves effort and allows the contractor to avoid design responsibility. CTLGroup's Gajda points out, however, that you may pay a premium for doing so. “If the form manufacturer is doing the engineering, it also can lead to significant overdesign if placement rates, anticipated concrete temperatures, and the concrete mix design are not fully communicated,” Gajda says. “It's also important to remember that forms for mass concrete often are left in place for the whole thermal control period, so the forms will be tied up longer than they would be in a typical placement.”
Joliet, Ill.-based Lindblad Construction specializes in industrial projects and has occasionally called on manufacturers to engineer and supply formwork for projects, such as a 4½-foot-thick mat slab for a power plant. Lindblad general superintendent Brian Long believes that the most important issue on mass concrete placements is the coordination needed to run smoothly.
“On large projects, the biggest challenge is making sure we have enough manpower and equipment available in the right place and at the right time,” Long says. “For the power plant slab, we had to place and finish almost 4000 cubic yards of concrete between midnight Saturday and 10 p.m. Sunday. We had to coordinate efforts among the ready-mix supplier, the power plant people, and town officials who were keeping the roads clear for us and the 500 ready-mix trucks delivering. We needed five laborers just to direct traffic. We had to provide space, time, and procedures to sample and test specimens from every 50 yards, and to wash out the trucks in accordance with environmental regulations. And we needed backups for all the equipment, because we couldn't afford any delays due to a breakdown.”
Anchor bolt challenges
Placing and embedding anchor bolts accurately is another key challenge in the industrial mass-concrete applications. Laying out the anchor bolts in the correct locations and keeping them in place through pouring and finishing are extremely important, but can be difficult to accomplish. In the old days, contractors used string lines and tape measures to do the initial layouts, and then called on surveyors to check all the points before moving ahead. Today's technology, such as electronic total stations, has streamlined the process considerably, not only saving time and effort, but also improving accuracy.
Gary Proffitt, a field superintendent for Lindblad Construction, has used a robotic total station to layout anchor bolts for a year. He says the unit has cut his bolt layout time in half. “We've always been pretty good about accuracy, but the robotic total station really makes accurate layouts faster and easier. We enter either x-y coordinates or measurements for each anchor bolt from the drawings into the data collector, and then set up the station at a known location. The technician takes the rod out onto the site and watches the screen to see when it's positioned right, then marks the bolt location.”
Concrete contractors who take the steps needed to prevent excessive thermal cracking, erect strong and efficient formwork, maintain jobsite coordination, and place anchor bolts accurately should be able to handle mass-concrete industrial projects successfully.
—Kenneth A. Hooker is a freelance construction writer based in Oak Park, Ill.