You never know what’s happening while you’re asleep. As most of us were snug in our beds last Friday night, 70 concrete trucks were steadily rolling down Interstate-10 into downtown Tucson, Ariz., dumping load after load of concrete at one of Sundt‘s jobsites. From midnight until shortly after eight o’clock the next morning, the trucks took turns unloading more than 3,500 cubic yards of material for the foundation of the new Pima County/City of Tucson Joint Courts Complex, being built at the corner of Toole and Stone Avenues. A crew of twenty-two laborers and foreman from Sundt (and an equal number of testing agents and flagging personnel at street level), spent the wee hours of Saturday morning finishing the 330 truckloads of concrete as the sun rose over nearby high-rises. See the three-minute time lapse video here: http://tiny.cc/ehcrfw
That was just “Act 1.” The second half of the 24,000-square-foot foundation will be placed in a similar overnight operation on June 29.
Why so much concrete for a seven-story building? The site’s soil is exceptionally sandy, which makes it difficult (and costly) to construct a traditional foundation formed around caissons drilled deep into the ground. Instead, the team has opted to create a “compensating mat slab” foundation 25 feet below grade to support the weight of the building. Spanning just over an acre, the five-foot-thick slab will require a total of 7,300 cubic yards of concrete – necessitating two of the largest concrete placements in Tucson’s recent history.
It took two concrete batch plants – one on Tucson’s far northwest side and one in the nearby community of Vail – to make and supply enough concrete for the placement last weekend. By performing the operation at night, the team was able to manage and/or avoid a number of headaches, such as:
- Traffic delays. Concrete should be placed within 90 minutes of being mixed. After that, it begins to harden and can’t be placed correctly. That means getting the trucks from the batch plants to the site had to be quick and efficient. Sundt developed the best delivery routes weeks in advance and worked with City of Tucson traffic engineers to adjust the traffic signals on several streets entering downtown from I-10.
- Heat. The ideal temperature for placing concrete (again, so that it doesn’t set prematurely) is 70 degrees Fahrenheit – that’s hard to achieve even at night during the summer in Tucson. To help lower the concrete’s temperature, it was mixed with chilled water at the batch plant and cooled with water spray bars on the concrete trucks while in transit.