Flexibility is maybe not the first word that comes to mind when thinking about concrete blocks, but when planning your next ballast solar project, it should be considered.
Sure, the first concern when choosing your concrete foundations will likely be cost. How much will it cost to ship precast blocks to the site? Will it cost less to hire the necessary labor and bring a concrete truck on site for a cast-in-place (CIP) solution?
“Depending on where you are getting the concrete, the shipping numbers could be huge. It’s usually more cost effective for the truck to show up on site and then pour it that way,” says Nathan Knapp, design engineer at Solar FlexRack. “But regionally, you can have higher labor rates — New Jersey for instance — so yeah, if you’re paying a high union labor rate at the site versus paying a lower rate at the precast facility, it can pan out that precast is more effective even with the shipping.”
But the more intriguing consideration for us is that of flexibility, which mostly favors CIP — pouring foundations right there on site versus trucking in precast blocks from the manufacturer. Not having to move concrete blocks provides obvious convenience during the install, but the adjustability benefits go beyond that. Depending on the CIP system, you should be able to completely adjust the racking and line everything up prior to the modules arriving and prior to the concrete being poured.
Also, consider the site itself — there is always something wonky about the field that was missed or unaccounted for in the original plans, such as subtle local grade changes. A precast foundation form, with its flat bottom, was not set with those in mind; a CIP solution can sit on grade and level the system.
Contractors have noted load rating concerns in landfills and other saturated sites that might not be able to handle a truck with precast blocks, a small crane needed to move said blocks or even a big cement truck needed for CIP foundations. As a work around in the latter case, according to Knapp, some contractors use a concrete buggy, which is a four-wheel machine with a dump bed that runs back and forth between the cement truck and the jobsite until the load of the truck is acceptable.
Every site and job have their own challenges, which is why a company like Solar FlexRack will provide both precast and CIP solutions. According to Greg Huzyak, PV Structures/Product Development Manager for Solar FlexRack, the company’s CIP solution adds more flexibility as the CIP forms are roll-formed channel sections that are manufactured to create custom ballasts of virtually any size and nest for optimized shipping. After arriving on site, the forms are quickly folded, assembled and ready for concrete.
Filling the CIP blocks is streamlined too with internal bracing that doubles as reinforcement during the pour, eliminating the need for additional rebar or external supports. Huzyak notes that they are always filled to the top to avoid water filling the excess space on top.
When specing that next landfill PV job, just be sure to consider all of your options in terms of price, installation speed and flexibility.
— Solar Builder magazine