Vibration of concrete: how and why it is done

What is the vibration of concrete?

Vibrating concrete is an important step of concreting that expels the entrapped air and excess water from freshly placed concrete and increases the density of the concrete. It is a must-done procedure of concreting immediately after pouring except for special types of concrete such as self-compacting concrete that does not require vibration.

It is implemented using vibration tools which are rotating masses with eccentricity that rotate with a certain frequency and amplitude or magnitude. The vibrators can be internally applied or externally mounted. External vibrators are fixed on formworks and mostly are pneumatic since they are more effective for external vibration.

The evolution of concrete vibration and the advent of CFV (controlled frequency vibration)

Concrete placement has evolved a lot from the time Wacker Neuson invented the first internal vibrator In the 1930s. As the use of admixtures was introduced the vibration specification had to be changed to maintain the high quality of concrete. Hence, as water-reducing agents become prevalent, CFV(controlled frequency vibration) was required to successfully monitor vibration frequencies in order to prevent defects. And, presently, the first battery-powered backpack internal vibrator, offering cordless and emission-free operation is realized by the Wacker Neuson company. 

Concrete vibration is essential for successful concrete pours, but it must be controlled due to various factors affecting concrete quality. Thomas Reading, a U.S. Army Corps of Engineers engineer in the late 1960s, established vibration placement recommendations based on vibration tests, limiting vibrator frequency to 10,000 vibrations per minute (vpm).

That was helpful in limiting excessive vibration frequency, which was leading to concrete material separation. 

In the 1970s, water-reducing agents (WRAs)were introduced to make concrete more workable for pumping, causing a disconnect in vibrator design and surface defects. Early surface voids were often mistaken for entrapped air but were later linked to vibration-frequency-forced bleed water.

In the mid-1990s, it was recognized that over-vibration was the root cause of these issues, prompting the development of Controlled Frequency Vibration (CFV) systems. Today’s standards recommend specific vibrator settings to control vibration energy effectively.

Changes in concrete mix designs, such as the use of water-reducing agents, necessitated adjustments in vibrator settings. The reduction in vibrator frequency from 13,000-17,000 vpm to 6,000-8,000 vpm reduced surface defects caused by excess water movement. Understanding vibration wave energy behavior is crucial, with CFVs providing data to bridge the gap between predictive vibration energy and batching, transporting, and pumping inconsistencies.

Research into CFVs and real-time on-site quality control tools has expanded, emphasizing the importance of controlling vibration energy and its impact on concrete workability. Contractors now use pre-construction trials and digital slump readings to ensure consistent results. Vibrator knowledge has grown, and CFV research will continue to improve concrete pouring outcomes.

Why vibration is needed?

Concrete is composed of coarse aggregates, fine aggregates, cement, and water. Coarse aggregates are irregular and coarse in size with cavities between the particles. Fine aggregates and Cement paste fill the cavities between the coarse aggregates. However, the concrete mix will always have voids that can occur because of void spaces created between the particles, that can contain entrapped air or water in liquid form.

Vibration of concrete is applied to remove the air content and unnecessary excess water to make the concrete stronger and more durable. Specifications also limit the air content according to the intended use of the concrete.

A freshly mixed concrete has 5-20 % of entrapped air content. Non-entertained concrete should have 1-2% of entrapped air content. Specification can require 4-7% of entertained air for concrete that is used in cold areas.

Air content is not needed because it decreases the strength and durability of the concrete. The rule of thumb is that an increase of 1% of air content over the design value means the concrete strength decreases by 3-5%.

Vibration should be performed properly since over-vibration can cause segregation bleeding. The lack of vibration can result in more cracks and honeycombing of concrete. 

Vibration is helpful to reduce the water-cement ratio since can improve the concrete’s workability. Hence, vibration will ensure proper mixing that does not risk the strength reduction as well as the workability of the concrete.

How vibration tools cause consolidation of concrete

Inducing Vibrations for Concrete Consolidation

Consolidation of particles in concrete mass

Concrete consolidation is a critical step in the construction process, ensuring that freshly poured concrete achieves the desired strength, durability, and finish. To achieve this, construction professionals rely on the art and science of vibration. Vibration concrete tools play a pivotal role in this process, imparting the right combination of force and frequency to transform a semi-liquid mixture into a solid structure.

The Mechanics of Vibration

At its core, concrete consolidation by vibration involves introducing mechanical forces that act in tandem with gravity. These forces, combined with the right frequency, lead to a series of intricate changes within the concrete mixture.

1. Force and Frequency

The first key element is the combination of force and frequency. When applied correctly, this duo initiates a transformation that is both fascinating and essential. The force component of vibration initially creates a chaotic dance among the concrete’s particles within the area influenced by the vibrator. This movement temporarily liquefies the mortar, reducing internal friction by a staggering 95%.

2. Achieving Fluidization

As internal friction decreases, the concrete becomes unstable, and gravity takes over. The concrete seeks its lowest level, densifying in the process. However, the magic truly begins with the right frequency—typically ranging between 9,000 to 12,000 vibrations per minute. This frequency resonates with the cement particles, exciting them.

3. Releasing Entrapped Air

This excitement, or resonance, prompts the ultra-fine cement particles to release tiny air pockets that have clung to them. The mortar seizes this opportunity to fill the spaces around the aggregates. As a result, the concrete flows laterally, constrained by the formwork, and around any reinforcing steel embedments. This motion causes both large and small pockets of entrapped air to rise to the surface.

4. Completing the Vibration

The process continues until a sufficient amount of entrapped air has made its way to the surface, achieving the desired concrete consistency and intended strength. At this point, the vibration process is complete. It’s important to note that it’s neither feasible nor necessary to eliminate every single air bubble during consolidation.

The Influence of Magnitude and Frequency

The efficiency of concrete vibration is significantly influenced by the magnitude and frequency of the vibrations. High frequencies excite the fine particles, while the force moves the larger aggregates. This unique combination ensures the removal of air bubbles, even distribution of cement fines to coat the aggregates, and dispersion of the larger aggregates. The results are remarkable.

Benefits of Proper Vibration

Properly utilizing vibration not only reduces the need for excess water in the concrete mixture, resulting in stronger concrete, but it also eliminates air bubbles and ensures a consistent distribution of cement and aggregates. This, in turn, produces a smoother finish, enhancing both the structural integrity and aesthetics of the final concrete product.

In the world of construction, where precision and strength are paramount, the role of vibration in concrete consolidation cannot be overstated. It’s a dance of physics and engineering that transforms a formless mix into a solid foundation, ensuring that every structure stands the test of time.

Type of concrete vibration tools and methods

There are 4 main types of concrete vibrators that are classified according to the way of application in the concrete. The most common are internal vibrators that find application almost in every concrete pouring. Surface vibrators are applied in pavements and slab sections which are thin sections that can be compacted sufficiently by surface vibration. 

The other is mold vibrators which are used in the precast industry to vibrate the mold and augment internal vibrators in reaching areas that are difficult to use internal vibrators. Battery molds, compaction tables, tilt tables, and a wide range of forms and molds for girders, tees, columns, culverts, vaults, containers, septic tanks, concrete pipe, and other items are among the items that frequently have table vibrators. Roller compactors are machinery that are used to consolidate Roller Compacted concrete (RCC) that is used for pavements and highways.

Internal vibrators

Internal vibrators are also known as immersion, poker, or needle vibrators. They are the most common type of vibrator used for concrete compaction. Internal vibrators are inserted into the concrete mix and vibrate it from the inside out. This helps to remove entrapped air and achieve maximum density.

Internal vibrators are typically powered by electricity, gasoline, or air pressure in the case of pneumatic ones. They are available in a variety of sizes and frequencies, depending on the needs of the job. For example, smaller internal vibrators are often used for precast concrete, while larger internal vibrators are used for mass concrete pours.

How to use an internal vibrator

To use an internal vibrator, simply insert it into the concrete mix and move it around slowly. The vibrator should be inserted to a depth of at least 12 inches. It is important to keep the vibrator moving so that it does not overcompact the concrete in any one area.

Surface vibrators

Surface vibrators are also known as screed or pan vibrators. They are attached to the top of a concrete screed and vibrate the concrete as it is leveled. Surface vibrators are effective at removing entrapped air from the concrete near the surface of concrete slabs and pavements.

How to use a surface vibrator

To use a surface vibrator, simply clamp it to the top of a concrete screed. Start the vibrator and move the screed across the concrete surface in a back-and-forth motion. The vibrator should be kept moving to avoid overcompacting the concrete in any one area.

Mold or external vibrators

Mold vibrators are used to compact concrete in precast concrete molds. They are typically attached to the sides of the mold and vibrate the concrete as it is poured. Mold vibrators are effective at removing entrapped air from the concrete and ensuring a smooth, dense finish.

How to use a mold vibrator

To use a mold vibrator, simply attach it to the sides of the precast concrete mold. Start the vibrator and pour the concrete into the mold. The vibrator should be kept running until the concrete is fully consolidated.

Vibration Tables: Unlocking Precision in Concrete Casting

When it comes to crafting high-quality concrete products, precision is key. Whether you’re creating intricate molds for artistic sculptures or producing durable concrete countertops, achieving consistent results is essential. That’s where vibration tables come into play, revolutionizing the way we work with concrete.

What Are Vibration Tables?

Vibration tables are specialized equipment designed to improve the quality and efficiency of concrete casting. These tables are equipped with powerful vibrators that generate controlled vibrations. When concrete is poured into molds on these tables, the vibrations help to remove air bubbles, ensure proper compaction, and distribute the material evenly.

The Benefits of Vibration Tables

1. Air Bubble Elimination: One of the primary advantages of vibration tables is their ability to eliminate air bubbles from the concrete mix. These unwanted air pockets can weaken the structure and affect the surface finish. By vibrating the concrete, these tables ensure a bubble-free result.

2. Consistent Compaction: Achieving uniform compaction in concrete casting can be challenging, especially in intricate molds. Vibration tables make this process much more manageable. The controlled vibrations settle the concrete evenly, reducing the risk of voids and weak spots.

3. Improved Surface Finish: Whether you’re crafting decorative concrete pieces or functional countertops, the surface finish matters. Vibration tables help in the even distribution of cement fines, resulting in a smoother and more aesthetically pleasing finish.

4. Reduced Labor: Traditionally, achieving proper compaction and bubble removal involved manual tapping, which is labor-intensive and less precise. Vibration tables automate this process, saving time and effort.

Applications of Vibration Tables

Vibration tables find applications in various industries:

– Concrete Artistry: Artists use vibration tables to create intricate concrete sculptures, ensuring that every detail is perfectly captured in the casting.

– Countertop Production: For countertops that require both strength and visual appeal, vibration tables help achieve flawless finishes and structural integrity.

– Precast Concrete: Manufacturers of precast concrete products, such as pavers and architectural elements, rely on vibration tables to maintain consistent quality.

– Laboratory Testing: Vibration tables are used in concrete laboratories to study material properties, helping researchers develop better concrete mixes.

Compactor concrete rollers

A compactor concrete roller, often referred to simply as a concrete roller, is a specialized construction machine used in the field of concrete construction and pavement work. This heavy-duty vehicle is designed to compact and smooth freshly poured concrete surfaces efficiently. 

Compactor concrete rollers find their common application in roller compacted concrete (RCC) that is applied in pavement concrete construction. Concrete rollers typically feature one or more large, heavy drums that exert significant pressure on the concrete as they roll over it.

The primary purpose of a compactor concrete roller is to remove air voids and achieve proper compaction within the concrete mix. This compaction process enhances the strength, durability, and overall quality of the concrete surface. It also helps create a smooth and level finish, making it suitable for a variety of applications, including road construction, building foundations, and concrete slabs for industrial or residential structures.

Concrete rollers come in various sizes and configurations, allowing them to handle different types of concrete projects. Some are equipped with vibratory capabilities, which further enhance the compaction process by settling the concrete mixture and releasing trapped air bubbles.

How to select concrete vibration proper tools

Manufacturers normally provide guides on how to select the proper tool for your projects. The Husqvarna construction guide is a good example of the process of selecting vibration equipment for various works. Another good guide is the global manufacturing pdf manual on how to select the vibrator tools that the company sells. 

The capability of a vibrator in concrete placement varies based on factors like slump, aggregate size, vibrator capability, and mixture agents.

Selection of vibration type with respect to type of structure

ACI 306 manual guides about the type of vibrator that shall be used for different types of structures. The vibrators used can be selected based on the type of structure, the thickness of the structure, reinforcement density, ease of access, and availability of power.

According to ACI 306, the following criteria should be considered in selecting an internal vibrator system and an external vibrator system:

Internal Vibrator System Selection:

1. Power Units Selection Criteria:

   – Determine the power source based on existing equipment, availability and type of power at the worksite, and worksite conditions/accessibility to the concrete.

2. Head Size/Type Selection:

   – Choose an appropriate head for the project, considering standard Steel Tip heads for most projects, rubber-tipped heads for lightweight or aluminum forms, and completely-encased heads for epoxy-coated reinforcement.

3. Head Diameter and Consolidation Efficiency:

   – Select the largest diameter head feasible for the project to achieve faster consolidation with higher amplitude vibration and fewer insertions.

4. Flex-Drive Length:

   – Use the shortest flex drive that allows complete access to the concrete being consolidated.

5. Selection Examples:

   – Provided examples based on project specifications, including head diameter, power unit, and flex-drive length selections.

 External Vibrator System Selection:

1. Vibrator Selection Criteria:

   – Consider the workability of the concrete and rigidity of the forms to choose external vibrators with appropriate speed and force output.

2. Orientation and Usage Tips:

   – Mount external vibrators with the internal rotating shaft in a horizontal position for effective vibration.

   – Ensure proper placement, spacing, and duration of vibration based on the concrete consistency and formwork to achieve successful consolidation.

3. Vibrator Spacing for Concrete Consolidation:

   – Follow the provided recommendations for vibrator spacing based on concrete consistency (very stiff to flowing) and slump.

4. Vibrator Selection Guide:

   – A guide for selecting the appropriate vibrator head diameter and power unit based on the application, slump, space limitations, and radius of action(radius of the vibrator’s reach) needed.

The above-mentioned vibrator selection parameters provide a set of criteria to select internal and external vibrator systems for effective concrete consolidation based on specific project requirements and conditions.

In summary, Concrete vibration is a critical step in construction that removes air and excess water from fresh concrete, improving its density and durability. It is essential for quality construction, improving compaction, and enhancing both structural integrity and aesthetics.

The evolution of vibration tools has been driven by the need to maintain concrete quality as construction practices evolved.

In the 1960s, Thomas Reading limited vibrator frequency to prevent concrete separation. The introduction of water-reducing agents in the 1970s caused surface defects, leading to Controlled Frequency Vibration (CFV) systems in the late 1980s to control vibration energy effectively.

Today, CFVs and vibration control tools are used for consistent results. Vibration helps remove air and excess water, enhancing strength and finish. Different vibration tools suit specific needs, guided by ACI recommendations for different structures and conditions.