The quality of concrete can be significantly improved by vibration. The savings achieved and the improved quality of concrete are now so well established that vibration is an accepted job site practice.
Internal vibrators are the most common for concrete consolidation of thick slabs, walls and columns. Because of their design, size, weight, maneuverability and price, they are a very practical means of vibration.
Typically, internal vibrators are not considered “big ticket” equipment items, but they are important to providing a quality product. Timing is the name of game and when the pour is ready, so must the contractor with the right equipment for the job.
- Greater density of the concrete
- Greater strength
- Improved bond with reinforcement steel
- Greater bond on construction joints
- Greater durability
- Improved product finish
- Reduction in the amount of required cement through the use of drier mixes
The basic principal of concrete vibration is to consolidate the mix by removing entrapped air. Vibration is a series of rapid compressive impulses which greatly reduces the surface friction between the various size particles of the mix. The result is the rearrangement of these particles into a denser mass. During the vibration process, unwanted trapped air escapes to the surface.
When the vibrator is immersed in the concrete, the amplitude (circular motion) of the vibrator converts stiff concrete into a fluid, flowable mass. There is a noticeable movement of the concrete under the influence of the vibration, however it must be remembered that movement of concrete is not the primary objective, to remove air is. The concrete should be placed as near as possible to the final position and not moved along the forms with the vibrator. This will avoid segregation of the concrete mix.
Once the concrete is in place and a fairly level surface has been attained, a systematic pattern of vibrator insertion and removal needs to be done to rid the mix of the remaining entrapped air. The entire area of placement must be consolidated. Under-vibration (withdrawing too fast) coupled with hit and miss vibrator placement is very common and does not get the job done properly. It is very important that the operator sees to it that all the concrete is vibrated, that form corners are reached and overlapping is done if necessary. The distance between insertions should be about 1 ½ times the head’s effective consolidation radius or radius of influence. The vibrator’s area of influence is determined by observing the air bubbles breaking the surface surrounding the head.
Proper vibrator handling entails quickly submerging the vibrator into the fresh lift, then withdrawing it at that rate of one inch per second (never more than two inches per second). This is because air bubbles naturally rise at the same rate in normal concrete. The hole must close completely behind the vibrator after total withdrawal. If not, the mix is too stiff or the vibrator frequency is incorrect. The vibrator has done its job when a thin film of glistening “cream” appears on the surface of the concrete, cement paste rises where the concrete meets the forms, and large air bubbles are no longer surfacing. When pouring walls and columns, the concrete is typically placed in the form in 20 inch (recommended) to 48 inch layers (lifts). The vibrator head should be inserted quickly all of the way to the bottom and left there for 5 -15 seconds. It should then be slowly withdrawn bringing the entrapped air to the surface. Each lift should be vibrated; and when each following lift is vibrated the head should penetrate about six inches into the preceding layer and jigged for 5-15 seconds to bond or knit the two lifts together.
- Type of pour: wall, floor, column, curb and gutter, etc.
- Height of the wall or structure
- Width of the wall or structure
- Electrical availability on the jobsite
- Electrical source distance
- Amount of reinforcing steel
- Job specifications
- Concrete slump
- Concrete mix design
There are many different types of consolidation vibration equipment available for internal consolidation. They may include electric flex-shaft vibrators, electric high-cycle vibrators, gas powered flex-shaft vibrators, and gas powered backpack modular vibrators.
Electric flex-shaft models are most commonly found on residential and commercial jobsites for all types of applications. Most have interchangeable heads and shafts to accommodate varying job demands. High-cycle vibrators are prevalent on larger jobs. They are unique in that they convert single phase (1~) energy into three phase (3~) for more consistent power while immersed in the concrete. These two types of vibrators require an electrical power source and therefore the cords must be managed by the operator to prevent damage or interference with other jobsite processes.
Gas powered internal vibrators offer versatility on the jobsites by eliminating the power cord but require gas engine maintenance. Piece work for curb and gutter, vibration at construction joints, and slab consolidation are a few good applications for a backpack vibrator. Some limitations may occur with the length of the shaft and head size with these units however. There may also be jobsite limitations regarding hazardous materials.
By asking some basic questions such as the job size and type of project will help determine which vibrator is best for the job at hand. Following these simple handling techniques will provide a quality concrete product.
Written by Wacker Neuson's Product Application Specialists Luke Sevcik (ACI Certified Flatwork Finisher) and Vince Hunt (ACI 309 Committee Member).