The critical questions for the vibrator operator
are how far apart to insert the vibrator and
how deep to penetrate into the preceding lift.
The first decision to make in consolidating concrete is whether to use internal vibration or external vibrators mounted on the outside of the forms.
For this article, we will cover only internal vibration, because that is the most common technique used in the field. External or form vibration is nearly always used in precast plants and sometimes in field applications where the forms arc getting multiple uses and being moved as an assembly (gang forms) from place to place.
Internal vibration equipment is available from a variety of manufacturers although it all works in similar ways:
•Concrete vibrators develop vibration by spinning an eccentric weight within a housing (the vibrator head) typically at a very high speed. This action creates vibrations in the range of 10,000 to 17,000 vibrations per minute.
• Flexible shaft vibrators are the most common type used, where a flexible drive shaft in a housing spins the eccentric weight inside the vibrator head. The core of the flexible shaft is made from high-strength braided steel wire and the casing is steel reinforced rubber. Shafts come in various standard lengths up to about 20 feet long, although shafts can be coupled up to 65 feet.
• The shaft may be driven by an electric motor or by a gasoline or diesel engine. Some power units turn at speeds high enough to develop the required and others must be geared
For this article, we will cover only internal vibration, because that is the most common technique used in the field. External or form vibration is nearly always used in precast plants and sometimes in field applications where the forms arc getting multiple uses and being moved as an assembly (gang forms) from place to place.
Internal vibration equipment is available from a variety of manufacturers although it all works in similar ways:
•Concrete vibrators develop vibration by spinning an eccentric weight within a housing (the vibrator head) typically at a very high speed. This action creates vibrations in the range of 10,000 to 17,000 vibrations per minute.
• Flexible shaft vibrators are the most common type used, where a flexible drive shaft in a housing spins the eccentric weight inside the vibrator head. The core of the flexible shaft is made from high-strength braided steel wire and the casing is steel reinforced rubber. Shafts come in various standard lengths up to about 20 feet long, although shafts can be coupled up to 65 feet.
• The shaft may be driven by an electric motor or by a gasoline or diesel engine. Some power units turn at speeds high enough to develop the required and others must be geared
to achieve the needed speed. There are also pneumatic motors available to drive flexible shaft vibrators.
• Some manufacturers have developed quick connect systems between power units and shafts and between shafts and heads.
• Gas power units often come rigged to he carried like a backpack, freeing a worker from having to hold the power unit in one hand.
• Electric motor-in-head vibrators are also available; in these vibrators a small three-phase induction motor is powered by a heavy electrical cable that also serves as the vibrator handle.
• Pneumatic vibrators are used when compressed air is readily available and when vibrators must run for extended continuous periods, such as in mass concrete. The pneumatic motor is in the vibrator head and tends not to heat up like other vibrator s.
• Radius of influence (or radius of action) is the critical parameter in vibrator selection—this is the distance from the center of the vibrator to the farthest distance where complete consolidation will occur in the concrete. Vibrator heads are rated for radius of influence, although this distance varies with the concrete slump— with high slump mixes it can he double the listed value.
• Vibrator heads come in various shapes, diameters, and lengths.
There is no definitive evidence that one shape works better than another, although some dimpled geometries seem to be more efficient. The head size should be matched to the desired radius of influence and the power unit should he matched to the head size.
• With large headed vibrators and larger power units (as big as about 5.5 hp), the radius of action can he as high as 18 inches. Vibrator heads come with regular steel heads or with urethane (rubber) heads. Rubber heads are required with epoxy- coated rebar, where a standard steel head can chip the epoxy and allow corrosion of the steel. Most DOTs require non-metal vibrator heads with epoxy-coated bars.
• Rubber tips are available for steel heads to protect form panels from damage by vibrator tips.
• Undersized extension cords for electric drive motors can lower a vibrator’s performance and even burn out the motor. For example, a 15 0-ft extension cord powering a 2.25 hp motor should have 8 gage wires.
• Some manufacturers have developed quick connect systems between power units and shafts and between shafts and heads.
• Gas power units often come rigged to he carried like a backpack, freeing a worker from having to hold the power unit in one hand.
• Electric motor-in-head vibrators are also available; in these vibrators a small three-phase induction motor is powered by a heavy electrical cable that also serves as the vibrator handle.
• Pneumatic vibrators are used when compressed air is readily available and when vibrators must run for extended continuous periods, such as in mass concrete. The pneumatic motor is in the vibrator head and tends not to heat up like other vibrator s.
• Radius of influence (or radius of action) is the critical parameter in vibrator selection—this is the distance from the center of the vibrator to the farthest distance where complete consolidation will occur in the concrete. Vibrator heads are rated for radius of influence, although this distance varies with the concrete slump— with high slump mixes it can he double the listed value.
• Vibrator heads come in various shapes, diameters, and lengths.
There is no definitive evidence that one shape works better than another, although some dimpled geometries seem to be more efficient. The head size should be matched to the desired radius of influence and the power unit should he matched to the head size.
• With large headed vibrators and larger power units (as big as about 5.5 hp), the radius of action can he as high as 18 inches. Vibrator heads come with regular steel heads or with urethane (rubber) heads. Rubber heads are required with epoxy- coated rebar, where a standard steel head can chip the epoxy and allow corrosion of the steel. Most DOTs require non-metal vibrator heads with epoxy-coated bars.
• Rubber tips are available for steel heads to protect form panels from damage by vibrator tips.
• Undersized extension cords for electric drive motors can lower a vibrator’s performance and even burn out the motor. For example, a 15 0-ft extension cord powering a 2.25 hp motor should have 8 gage wires.
• Manufacturers have developed rebar vibrators that have proven to effectively consolidate concrete and also grout in reinforced masonry. A study at the University of Tennessee on reinforced masonry showed that rebar vibrators were “an acceptable alternative to conventional pencil vibrators.”
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