Is it Necessary to Vibrate Concrete?

Concrete is considered a building material that has good strength. To get concrete with good strength, the process of applying concrete in buildings must also be considered carefully. One way is to pay attention to the problem of concrete casting.

Usually a good concrete casting process requires the help of several tools, one of which is a concrete vibrator . The presence of this machine is considered central enough to produce the best quality concrete buildings. What is a vibrator for concrete and why it has a central role for casting will be discussed further below.

What Is A Concrete Vibrator?

So far, the term concrete vibrator is probably quite familiar to those who are used to working in the construction sector. However, for ordinary people this term is still rarely found. For this reason, this time we will first discuss what a vibrator for concrete is.

Read: How to Calculate Cubic Meter of Concrete

So a vibrator for concrete is a tool used when the casting process takes place. This tool has a function to make concrete that automatically results in solid building construction . In general, this vibrator has three central parts. Here are the parts.

  • Machine part used to generate vibrations.
  • The hose that will be used later delivers the vibrations that are present from the machine.
  • The vibrator head is inserted into the concrete. Usually the vibrator head is made of a steel cylinder. For the size itself resembles a baseball bat.

In general, vibrator machines come with three types of vibrations. For the first vibration is the swept sine profile vibration. Then the second vibration is random vibration while the third vibration is shock synthesized.

Why Is It Important To Use Vibrators In Concrete Casting?

Earlier we have known briefly about the vibrator, now is the time to understand why it is important to use vibrators in casting. There are many reasons for this. The following are some of them.

1. Make the Concrete More Dense

One of the reasons for using this vibrator machine is to ensure that the concrete becomes denser. Dense concrete is certainly stronger than less dense concrete. Concrete density is an important thing in the construction process.

Read: Advantages and Disadvantages of Self Compacting Concrete

Concrete density requirements usually vary depending on the type of building. This also affects the size of the vibrator used. Apart from the size of the vibrator, the number of tools also affects the ability of the vibrator to reach a certain density within a predetermined time. For example, to reach such a density within a few hours it takes 4 vibrators.

2. Removing Air Trapped in the Concrete Mixture

Often times in the concrete mixture before it is poured there are trapped air bubbles. This should be watched out for because it causes the concrete to become weak. For this reason, a vibrator is needed whose vibrations can help remove the air bubbles in the concrete mixture. The discharge of these bubbles can help make the concrete strength more evenly.

3. Preventing Concrete Becoming Porous

If air bubbles (as discussed in point 2) are left in the concrete, it is likely that porous concrete will form. This porous concrete means the presence of holes in the concrete which originally contained the air bubbles. This has a big effect on the strength level of the concrete. Even the strength of the concrete becomes uneven because there are parts that are porous and there are parts that are not porous.

Read: Types of Upper Floors: Concrete, Timber and More

There are many benefits that come from using a concrete vibrator. The three benefits above are just a few of the other benefits that are present. So of course there is no doubt when you use a vibrator in the concrete casting process, right?

The main objective of the vibrating process is to achieve that the air bubbles rise within the mass of the fresh concrete and in this way go out to the outside, being eliminated with the environment and homogenizing. Due to the process of mixing, transporting and placing 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, not very 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 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 the friction between the different components of the concrete (between the concrete and the reinforcing steel, and between the concrete and the form). Some of the most common vibration compaction methods are:

  • Internal vibration: consists of applying the action of vibration directly to the concrete, inserting a vibrating rod inside the mass. Internally applied vibrators, also known as immersion or needle vibrators, are the most common for consolidating concrete in walls, columns, beams, and slabs.
  • External vibration of the form: consists of attaching eccentric masses to the form, which are moved by an electric or pneumatic motor, so that when they rotate, they transmit the desired vibration to the concrete through the form. Of course, the form must be rigid and watertight enough to prevent 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 needle vibrators is restricted by very narrow or very congested reinforcement sections, in elements where it is required to complement the internal vibration, and especially in the consolidation of very dry mixtures in the 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 screeds, 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 surface mortar content, since the surface vibration of mixtures with larger settlements will result in the excessive accumulation of mortar and fine material on the surface, and with this, not only will the resistance to wear be reduced, but also the probability of the appearance of surface cracks, the eventual peeling and, in extreme cases, plastic shrinkage cracks will be increased.
  • Vibrating tables: Another external vibration system is made up of vibrating tables, which are frequently used in the production of prefabricated elements. Normally, it is 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. Underneath the deck is attached to a vibration-generating equipment, which can be eccentric masses moved by belts, or vibration-resistant motors, equipped with eccentric rotary tables, attached directly to the deck. Logically, the vibrating table must be solid since the mold of the piece is placed on it to produce more than the mass of concrete that will make it up.
  • Vibration by rollers: in this type of external vibration of concrete, there are static smooth rollers and vibratory smooth rollers that are used in the construction of pavements and dams built with dry concrete. Within the static smooth rollers, there are those with 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 water trapped under 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. The vibration transmitted through the reinforcement to the semi-plastic concrete produces loss of adhesion of the bar with the concrete and cracks on the reinforcement.

Conventional vibratory devices found 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 the fine grains; on the other hand, the frequencies (13,000 to 14,000 vpm) are capable of mobilizing the fines around the thicknesses, resulting in a good distribution of the same and facilitating the expulsion of air bubbles retained within the concrete mix.

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

  • Increase the strength and durability of concrete.
  • Expel the excess air and distribute the particles more homogeneously within the concrete.
  • Reduce the possibility of fresh concrete segregation and volume changes due to subsequent shrinkage.
  • Get the concrete to behave “like a liquid” within the vibrator’s action diameter, allowing a better filling of the forms.
  • Obtain mixtures with a compaction that allows to maintain the level of occluded air within the established limits.
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