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.
The advantages of consolidating concrete by vibration are:
- 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
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.
Choosing the right vibrator depends on several factors
- 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
Source: wackerneuson