Concrete contractor Landavazo Bros. Inc., Hayward, California, recently fulfilled a contract to place and pump the 110,000 cubic yards of concrete needed for the expansion of San Francisco's Moscone Convention Center. The 100-million-dollar project involved construction of a 487,500-square-foot underground wing connected to the existing convention center by a series of tunnels. Crews poured a 168,000-square-foot, post-tensioned waffle slab comprising 22,500 cubic yards of concrete. The waffle slab forms part of the street-level roof topping the underground wing. Engineers chose a waffle-slab because it can distribute heavy superimposed loads in two directions and can accommodate the large spans needed for open exhibit space.

Unlike typical waffle slab construction, this project called for pouring joists separately, before the slab. The project, which has 205,000 square feet of joists, required 18 joist pours averaging 300 to 400 yards of concrete each. Despite the complexity and large scale of the waffle slab, the project took only 5 months to complete. Placement constraints, however, resulted in the need for massive concrete pours to complete the project on schedule. Because the city only allowed pumps on two lanes of traffic and required that pours be scheduled for times that were the least disruptive to convention traffic, Landavazo had to place the slab in just three pours.