🕑 Reading time: 1 minute
There is no exact procedure for selecting a suitable vibration system to compact freshly placed concrete. This is due to variations in several concrete parameters; for instance, modifications in the concrete mix make each construction case unique.
Concrete mix variations can originate from modifications in the slump, chemical additives, aggregate sizes and shapes, cement content, consistency of the mixture, weather conditions, and even the type of formwork used.
So, each construction case is different and needs to be evaluated individually. Nonetheless, certain general rules have been laid out that site engineers and contractors can refer to while selecting suitable vibration machines for the work under consideration.
How to Select the Right Vibrating System for Concrete Consolidation?
Selection of Internal Vibrators
Internal vibrators consolidate by a vibrating head which is inserted into fresh concrete. The number of vibrations per minute (frequency) and the deviation of the head from the neutral position as it oscillates (amplitude) describe the vibration motion.
Parameters that need to be considered when selecting internal vibrations are the availability of vibration equipment, head size, and flex-drive length.
Equipment availability could control the selection of vibration. This is because the contractor may not own the best and suitable vibration equipment for the work under consideration.
Head size and type of vibration are other criteria required to be accounted for. Commonly, the contractor prefers the largest head size since it has a greater area of influence and, consequently, finishes the work early. However, the effectiveness of vibration equipment is controlled by its amplitude and frequency.
The effective compacted area is 1.6 times the head area of vibrator equipment. Reinforcement spacing, formwork dimension, and concrete workability control the selection of the head size. For instance, a small head-size vibrator should be used for small reinforcement spacing, shallow formwork, and high slump concrete.
Regarding flex-drive length, a contractor would normally like to employ the shortest flex-drive that permits complete access to the concrete that is being consolidated. ACI 309R -5 (guide for compaction of concrete) provided a table to guide contractors to choose suitable internal vibration equipment. The data provided in the table are empirical, i.e., based on previous works. A summary of the table is presented below:
Table-1: Selection of Internal Vibrators Based on Equipment Head Size, Amplitude, Radius of Influence, and Concrete Placement Rate
|Diameter of the head, mm||Vibrator recommended frequency in concrete, Hz||Average amplitude in air, mm||Radius of influence or fully consolidated concrete, mm||Rate of concrete placement, m3/h||Consolidation Applications|
|20-40||150-250||0.4-0.8||75-150||1-4||Plastic and flowing concrete in shallow members and confined places, supplement larger vibrators, and compact laboratory samples.|
|30-65||140-210||0.5-1.0||125-250||2-8||Plastic concrete in thin walls, columns,beams, precast piles, thin slabs, and alongconstruction joints.|
|50-90||130-200||0.6-1.3||175-350||5-15||Concrete with a slump of less than 75 mmin general construction like walls, columns, beams, prestressed piles, and heavy slabs.|
|75-150||120-180||0.8-1.5||300 -500||11-31||Mass and structural concrete of 50 mm slump placed in quantities up to 3 m3 in relatively open forms of heavy construction such as powerhouses, heavy bridge piers, and foundations.|
|125-175||90-140||1.0-2.0||400-600||19-38||Mass concrete in gravity dams, large piers, and massive walls.|
Selection of External Vibrators
The workability of concrete and formwork rigidity should be considered while the external vibration system is selected. For instance, plastic concrete with a slump greater than 75 mm can be consolidated adequately with high-frequency vibration. In contrast, high-amplitude vibration is required for stiffer, fresh concrete with a slump less than 75 mm to initiate fluidization.
External vibration with a speed between 3000 and 12000 rpm is suitable for form vibration. However, the natural resonant frequency of Portland cement ranges from 9000 to 12000 rpm, and pneumatic-powered vibrators are the only available equipment that produces this required frequency. Frequently, it would be required to use more than one vibration equipment to generate the force needed to consolidate the concrete properly.
After determining the total weight of the fresh concrete and the formwork, Table-1 can be used to select the suitable vibration equipment. If the specific weight of concrete is not available, then use standardized weight 2400 kg/m3 as an approximation.
Table-2: Selection of External Vibrator Based on Concrete Consistency, Weight, and Force of Vibration Equipment
|Concrete consistency||Slump,||Vibrator selection|
|Very stiff concrete||< 12||The force of the vibrator shall be equal to 200-300% of the total weight of the concrete and form.|
|Stiff or stiff plastic concrete||12-50||The force of the vibrator shall be equal to 130-150% of the total weight of the concrete and form.|
|Plastic or flowing concrete||> 50||The force of the vibrator shall be equal to the total weight of the concrete and form.|
Consolidation is the process of removing trapped air in the concrete mix just after it has been poured.
Internal vibration is the vibrating and subsequently consolidating of concrete from within the formwork.
External vibration is the vibrating and hence consolidating concrete from outside the formwork.
The concrete vibration should be stopped when the emergence of air bubbles from concrete has stopped.
1. Equipment availability
2. Head sizes and shapes
3. Flex-drive length
4. Concrete workability
5. Dimension of the formwork