Lightweight External Vibrators Provide: Outside-In Consolidation

While conventional wisdom dictates the use of internal vibrators to consolidate concrete from the inside out for a wall pour, customary practice was not

While conventional wisdom dictates the use of internal vibrators to consolidate concrete from the inside out for a wall pour, customary practice was not equal to the massive amount of steel reinforcement within the core and shear walls of the 38-story Borgata hotel tower expansion in Atlantic City, N.J.

The new $400 million Water Club at Borgata will house 800 hotel rooms and suites as well as several pools, a full-service spa, meeting rooms and boutique retail shops. In addition to 700,000 square feet of Filigree cast-in-place wide slabs for floors and ceilings, the tower’s supporting walls and columns required 28,000 yards of varying 9,000-, 7,000- and 5,000-psi mixes poured at a 7-in. slump. Project specifications called for a total of 3,000 tons of rebar to be used for concrete reinforcement.

From the project’s May 2006 start, concrete contractor Weatherby Construction and Renovation Corp. of Ventnor, N.J., faced key decisions in the erection process. We could have detailed the rebar with couplers to reduce splicing and steel congestion, but that would have required a lot of crane time, says Vincent Fabi, the contractor’s general foreman and superintendent.

The subcontractor chose to splice the bars. Assembled first in mats at ground level, steel reinforcement was then lifted and secured within the forms. While saving crane time, this method essentially doubled the amount of steel in the already congested core and shear walls Û a challenge for consolidating concrete inside the formwork.

At most job sites, internal vibrators have been the tool of choice. External vibration, the oldest method of concrete consolidation, historically has been relegated to precast yards due to the unwieldy heavy motors and weld-on mounts the process required. Yet, a new generation of external vibrators offers a lighter weight design and removable clamping system for quickly attaching and removing the motors. High-frequency, low-voltage designs also help improve on-site operating safety.

When Weatherby initially attempted to use internal vibrators on the 16-ft.-tall, first-floor walls, crews had difficulty maneuvering vibrators through the jungle of intertwined steel. Moreover, company management was not completely satisfied with the wall finish quality. That it would be nearly impossible to use internal vibrators to consolidate the 300 linear feet of 26-ft.-tall, second-floor walls was evident.

Accordingly, two approaches remained: self consolidating concrete or external vibration. The building specs did not call for self consolidating concrete, explains Weatherby’s Richard Bischoff, vice president and general manager, so it was not an option for us.

Wacker Corp. and Edison, N.J.-based equipment dealer Gamka Sales Co., Inc., approached Bischoff with the external-vibration alternative. Demonstrations proved the vibrators effective at consolidating concrete from the outside in, reaching areas inaccessible to the internals due to rebar congestion. Bischoff affirms, We noticed a dramatic difference in quality Û both in consolidation and appearance Û with the externals.

Weatherby rented 80 AR36/6/042 external vibrators from Gamka Sales for consolidating the lower floors of the tower. Eight GP5600 generators supplied power to eight inverters to run the externals. The inverters convert the power from 240 to 42 volts, improving safety, says Jim Gilbert, Wacker Corp.’s regional concrete product specialist. The arrangement allows the motors to operate at a variable frequency up to 200 Hz.

On the Water Club project, external vibrators were fitted with a clamping system that matched the Doka forms in use. Each vibrator produced a 7- _ 4-ft. elliptical area of compaction, uniformly transmitting vibration through the formwork. External vibration also helps the concrete flow through heavily congested areas of reinforcing where it would have a chance to get caught, adds Gilbert.

Weatherby’s crews positioned the vibrators in a pattern that created overlapping vibration waves. As concrete filled the forms, the crews leapfrogged the rows of vibrators, following the concrete to the top of the form. Due to wall thickness ranging from 24 to 26 inches, vibrators were mounted to both sides of the forms to ensure proper consolidation throughout the concrete.

Project principals note that external vibration provided a satisfactory solution to achieve proper concrete compaction and keep the project on schedule. The externals left a smooth surface that required very little finishing, says Fabi. Bischoff adds, The vibrators gave us a nickel solution to a dollar problem.