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With fly ash to spare, Silvi Concrete mobilizes for record Philadelphia mat pour

Sources: Silvi Concrete, Fairless Hills, Pa.; CP staff

SilviSet to become Philadelphia’s largest reinforced concrete building, the W Hotel got off to a productive start earlier this month as general contractor Tudor Perini teamed with concrete contractor TP Carney and ready mixed producer Silvi Concrete to execute the largest continuous pour in city’s history.

The 20,000-sq.-ft. hotel foundation is roughly 9 ft. thick and bears 650 tons of rebar. Due to proximity of the heavily trafficked Philadelphia City Hall, construction proved challenging, but workable thanks to closure of four major downtown streets over a 36-hour weekend window. A high-performance concrete mix and temperature control requirements—necessitating the foundation core remain below 155°F—were among the most complex local practitioners have seen. To counter concrete’s heat of hydration effects, TP Carney crews placed six miles of cooling tubes in tandem with the rebar cages, and ran chilled water through them for one week following the pour.

Silvi Concrete was tasked to design a 10,000-psi mix containing 50 percent fly ash, or 356 lbs./yd., and deliver it below 90°F amid prospective peak summer temperatures. The contractors inquired about the pour with less than two weeks’ notice, confirming Silvi Concrete as the only ready mixed producer with sufficient fly ash supplies. In order to complete the monolithic placement per engineer’s specification, the producer committed to deliver nearly 5,900 yards in a continuous sequence, deploying 136 mixer trucks and four central mixed plants. A combination of narrow one-way streets and open sidewalks limited TP Carney to four pumps, capping hourly concrete placement to just over 200 yd. and stretching the pour to nearly 30 hours.

Since the mix contained almost as much fly ash as cement, Silvi Concrete stockpiled the former in portable blimps. Eyeing Monday morning delivery commitments—on the heels of the W Hotel pour—the producer amassed raw materials equivalent to 10,000 yards going into the weekend.