PLC Concrete: Suitable for Vertical Slip-Forming
In October 2010, owners of a cement plant in Leeds, Alabama commissioned a major upgrade to replace the original structure, built in 1906. The replacement, a state-of-the-art, environmentally friendly cement storage silo and loadout facility, is engineered to last 100 years while improving efficiency: it reduces load times, increases loading capacity to 300 tons of cement per hour, and virtually eliminates dust.
Along with dust-free operation, the construction is environmentally friendly because low-carbon concrete mixes were maximized for service life and used for all applications of the project, including the silo, surrounding foundations, pavement, creek relocation, and soil stabilization. Portland-limestone cement (PLC) mix designs for the silo and its foundation contained 60% high-early strength PLC (with 12% limestone content) and 40% slag cement. Some of the pavement mixes followed the AL DOT mix design with 20% Class C fly ash. All agreed that the PLC concrete mixes worked well with fly ash, slag cement, and typical commercial admixtures.
The large silo, 20,000 metric ton capacity measuring 240 ft tall by 80 ft in diameter, was built using slip-forming, which requires tight control over concrete workability and setting. Anyone familiar with the slip-form technique understands the necessity of predictable concrete performance, underscoring the consistent performance of the PLC concrete. Despite the complexity and 18-month timeline for the silo pour, with two layers of reinforcement, slip-forming proceeded at 12 vertical in. per hour, with no significant changes in the placement process. Similarly, the contractor found no issue placing reinforced concrete pavements surrounding the area. Using a PLC designated as high-early strength was not only beneficial for the slip-forming, it also accelerated the construction timeline to reopen the pavement sections to traffic, thereby reducing disruption to local commuters.
Following the success of the cement plant installations, nearby states Alabama and Georgia were impressed enough with the results to specify variations of this same concrete mixture for their own streets and local road construction.