The concrete crew faced some unusual conditions on the first four floors of the 41-story tower addition to the Borgata Hotel and Casino, Atlantic City, N.J. Shear walls, cell walls, and columns ranged from 16 to 26 feet tall. One 2-foot-thick x 300-foot-long shear wall would require 40 truckloads of concrete. The No. 11 reinforcing bar was so congested in places that workers had to use hydraulic jacks to spread the bars enough to get the wall ties through. But the real challenge would be consolidating the concrete within those configurations.

“The construction documents called for a 9000-psi mix, which sets up quickly and doesn't give you much working time,” said Rich Beshoff of Weatherby Construction, Ventnor, N.J. “We couldn't get an internal vibrator down to the bottom of the wall and expect to get code-compliant results.” After completing some smaller elements, the concrete construction team realized that if they continued using a conventional internal vibrating system, they would run the risk of problems such as honeycombing and cold joints in the larger elements.

“We knew we'd have to be more creative in our approach to either placing the concrete or forming the wall,” said Beshoff. They started examining options such as slowing the process, installing extra bulkheads, and using smaller pours, which would have increased formwork and labor costs.

The first four floors of the 41-story addition to the Borgata Hotel and Casino feature shear walls, cell walls, and columns ranging from 16 to 26 feet tall.

A few weeks earlier, while on vacation in Miami Beach, Fla., Vince Forte, metropolitan jobsite specialist for Wacker Corp., contemplated the luxury condos going up next to the resort where he was staying. “I was looking at all the core walls, shear walls, columns, and No. 11 bars, and I realized, ‘this is where we've got to go with our new external vibrators',” said Forte. When he returned home to New Jersey, he gathered the demonstration team and they headed to the Borgata jobsite.

Congested reinforcing steel makes it a challenge to place and consolidate concrete at the bottom of the walls.

“They walked in the door one day and the timing was right,” Beshoff said. Forte, Wacker regional concrete products specialist Jim Gilbert, and representatives from distributor Gamka Sales Group, Edison, N.J., performed a one-pour demonstration of a three-box core connected to two wing walls, all monolithically joined. Wacker's Plug-and-play external vibration system—a lightweight, portable system designed for jobsite use—was used to consolidate the concrete.

Vince Forte demonstrates how to attach the clamp-mounted vibrator to the formwork. The vibrator/clamp assembly weighs as little as 20 pounds. The vibrators are also secured with safety cables in case the clamps loosen.
External vibrators are placed along the formwork in a pattern that will maximize coverage. The motors shake the forms, thus consolidating the concrete as it is placed. As the concrete travels up the wall, workers move the vibrators upward, maintaining the optimum spacing.

The vibrator motors are mounted to quick-disconnect clamps that are secured to the formwork. The vice clamps adjust to fit most steel and plywood forming systems. Each unit vibrates about 25 square feet of form surface and produces up to 6000 vibrations per minute. The vibrators transmit the pulse through the forms to consolidate the concrete. The force of the vibrators is adapted to the forming system by means of an adjustable frequency inverter, which handles up to four motors. Powered by a 240-volt, single-phase generator, the inverter transforms the output into 42-volt, three-phase power for operator safety.

The demonstration team showed the Borgata crew how to space the motors across the forms to achieve proper coverage and explained the importance of timing. Workers practiced activating the vibrators when the concrete reached the right level, clocking the vibration cycle, and leapfrogging the vibrators up the form-work as the concrete traveled up the wall.

To complete the first four stories of the tower, Weatherby rented 80 vibrators from Gamka. The structure of the upper floors, which house hotel rooms, was less concentrated, but Weatherby continued using external vibration, decreasing the rental order as needed.

Up to six vibrators can be connected to one low-voltage frequency inverter, which adjusts the vibration rate as appropriate for the forming system and conditions.
The columns were the first elements to be constructed using the external vibrators. The results were fully compliant with the specifications.

Beshoff observed that even in conditions where internal vibrators would work, using external vibrators could help speed construction. “With internal vibrators, the speed at which you can move the vibrators dictates how quickly you can place the concrete. With the external vibrators, that becomes a nonissue. Once you take vibration out of the time mix, vibration is no longer the critical task in the operation.”

“Every requirement was achieved with the external vibrators,” he added. “They may be more expensive than internal vibrators, but if internal vibration doesn't work, the cost comparison is theoretical. Ultimately it was more cost-effective—we would have had repairs on bigger walls if we hadn't come up with an alternative.”