Concrete Vibrator Why Is It Important?

The vibration process of concrete in construction is of vital importance, especially when talking about the durability of the structure. It consists of subjecting fresh concrete to high-frequency vibrations immediately after being poured, using vibrators that operate with compressed air pressure or electricity. With this procedure, the concrete mixture acquires a slightly more fluid and liquefied consistency, allowing the spaces to be covered homogeneously; making it adhere to steel more easily.

The main objective of the vibrating process is to make air bubbles rise inside the mass of fresh concrete and thus go outside, eliminating with the environment and homogenizing. Due to the process of mixing, transporting and placing the concrete, air is trapped in the form of voids or pores, which vary both in size and distribution, and which must be removed to give it the character of a monolithic solid. As is known, these voids decrease the density of the concrete, making it more permeable, less resistant and less durable.

To remove the air, procedures such as “sucking with a rod or rod” are also used (recommended in small spaces; it is not recommended in large-scale works since they do not provide sufficient efficiency and can lead to other problems in the mixture), and vibration (internal or external). These seek to decrease friction between different concrete components (between concrete and reinforcing steel, and between concrete and formwork). Some of the most common vibration compaction methods are:

Internal vibration: it  consists of applying the action of the vibration directly to the concrete, inserting a vibrating rod inside the mass. Internal application vibrators, also known as immersion or needle vibrators, are the most common for consolidating concrete into walls, columns, beams, and slabs.

External vibration of the formwork: it  consists of attaching eccentric masses to the formwork, which are moved by an electric or pneumatic motor, so that when they rotate they transmit to the concrete, through the formwork, the chased vibration. Of course, the formwork must be sufficiently rigid and hermetic to avoid deformation of the element and leakage of grout. Form vibrators , also known as wall vibrators, are very common in the  precast concrete industry, in some construction works in which the use of the needle vibrators is restricted by very narrow or highly congested sections of reinforcement, in elements where it is required to complement the internal vibration, and especially in the consolidation of very dry mixtures in the that internal vibrators cannot be used. Additionally, its use is restricted to the construction of thin elements whose thickness does not exceed 300 mm.

Surface External Vibration - Surface  external vibrators, also called vibratory rulers, are used to compact concrete from the surface of the slabs. In these cases, the mixtures are designed so that the settlement does not exceed 75 mm, in order to limit the content of superficial mortar, since the superficial vibration of mixtures with greater settlements will result in the excessive accumulation of mortar and fine material. on the surface, and with it, not only will the wear resistance be reduced, but also the probability of appearance of surface cracks, eventual peeling and, in extreme cases, plastic shrinkage cracks will increase.

Vibrating tables:  Another external vibration system is the vibrating tables, which are frequently used in the production of precast elements. Normally, these are equipment developed specifically for the industrialized production of a particular element or set of elements. The vibrating tables consist of a rigid board supported elastically on fixed supports. To the board is attached below a vibration generating equipment, which can be eccentric masses moved by belts, or, vibration resistant motors, equipped with eccentric rotary tables, adhered directly to the board. Logically, the vibrating table must be solid since the mold of the piece is placed on it to produce more of the concrete mass that will form it.

Vibration by rollers:  in this type of external vibration of the concrete, there are the static smooth rollers and the vibratory smooth rollers that are used in the  construction of pavements  and dams built with dry concrete. Inside the static smooth rollers are the pneumatic wheels and the metallic smooth rollers.

Revibration -  Concrete revibration is used in some cases to improve adhesion between concrete and reinforcing steel, to release trapped water below horizontal reinforcing bars, and to remove additional trapped air voids. This practice does not generate any damage, if the concrete is still workable. However, it is important not to touch the reinforcing steel with the vibrator needle. Revibration transmitted through the reinforcements to the semi-plastic concrete causes loss of adherence of the bar with the concrete and cracks on the reinforcements.

Conventional vibrating devices on the market have a shaft rotation frequency between 3,000 to 14,000 vpm, which between 3,000 to 12,000 vpm (low and medium) mobilize fine grains; on the other hand, the frequencies (13,000 to 14,000 vpm) are able to mobilize the fines around the coarse ones, resulting in a good distribution of the same and facilitating the expulsion of the air bubbles retained within the concrete mix.

The most important reasons for vibrating concrete mixes and preventing further damage to the material are:

• Increase the strength  and durability of concrete.
• Expel excess air and distribute particles more evenly within concrete.
• Reduce the possibility of segregation of fresh concrete and volume changes by subsequent retraction.
• Achieve that the concrete behaves "like a liquid" within the diameter of the vibrator, allowing a better filling of the formwork.
• Obtain mixtures with a compaction that allows the occluded air level to be kept within the expected limits.