By Adam D. Neuwald
Adam Neuwald was formerly a Technical Services Engineer with NPCA and a member of the NPCA TechTeam.
Consolidating and compacting freshly placed concrete with an internal vibrator is a fairly simple and straightforward process. However, precast concrete manufacturers are continually spending unnecessary amounts of time and money repairing defects often associated with the improper use of internal vibrators. A majority of honeycombing, rock pockets, bugholes, sand streaks, pour lines and voids can be avoided by taking the time to properly train and educate employees on the proper use of internal vibrators and the future problems that may arise from incorrect vibration practices.
Freshly placed concrete can contain 5 percent to 20 percent entrapped air, ultimately reducing the concrete’s density. Concrete with a high percentage of entrapped air will likely have a reduced strength and increased permeability, which will greatly reduce the durability of the concrete product once in service.
In general the two most common mistakes made when using an internal vibrator are removing the vibrator head too quickly and dragging the vibrator head through the concrete. Both of these practices often seem like logical shortcuts to the untrained individual, while the trained individual will understand that simply leveling the concrete will not remove entrapped air voids, bugholes and honeycombing. The trained individual will also realize that dragging a vibrator through the concrete will form a mortar channel in the concrete, creating a structurally weak area in the finished product.
When training individuals on the proper use of internal vibrators, it is important to explain the basic principals of how an internal vibrator works. Simply put, vibratory impulses liquefy the mortar, drastically reducing the internal friction between aggregate particles. The mixture becomes unstable, allowing entrapped air to rise to the surface while the heavier aggregates settle under the force of gravity into a dense matrix.
The amount of time that the vibrator must remain in the concrete will depend on the vibrator’s frequency and amplitude and the concrete’s workability. The frequency is the number of vibration cycles per minute and is often presented as revolutions or vibrations per minute (rpm or vpm). Frequency will have an effect on lighter masses, moving sand and slurry around aggregate and ultimately causing the mortar to liquefy. The amplitude is the maximum distance a point on the vibrating head moves from its position of rest. Amplitude will have an effect on heavier masses, moving coarse aggregate and ultimately determining the radius of action or influence. The radius of action is the area of concrete influenced by the vibrator.
Smaller-diameter vibrators will have higher frequencies and lower amplitudes, while larger-diameter vibrators will have lower frequencies and higher amplitudes. As a general rule, the diameter of the vibrator should be a quarter of the wall thickness of the product being cast. Smaller-diameter internal vibrators with higher frequencies are typically used for consolidating higher-slump concretes used in manufactured concrete products.
When using internal vibrators it is important to lower the vibrator vertically into the concrete, allowing the head to descend under its own weight. Internal vibrators should not be forced down into the concrete. The vibrator head should penetrate previously placed lifts of concrete by 6 inches (150 mm). If there is a considerable amount of time lapse between the placement of subsequent lifts, it may be necessary to revibrate the previous lift prior to placing additional concrete to minimize the potential for pour lines and cold joints.
As noted earlier, vibration times will vary depending on the size of vibrator used, the concrete’s workability and depth of the concrete member. According to the Portland Cement Association, an insertion time of 5 to 15 seconds will usually provide adequate consolidation. Another general rule of thumb is to allow the vibrator to sink under its own weight and then remove the vibrator at a rate of about 3 seconds per vertical foot (300 mm). Concrete should move to fill the hole left by the vibrator; otherwise briefly reinserting the vibrator nearby should solve the problem. The vibrator should then be reinserted close enough to the last location so that the radius of action overlaps the last one.
Employees should familiarize themselves with each vibrator and mix design used at the plant. Changes in the surface appearance of the concrete and the sound or feel of the vibrator are often good indications that the concrete has been adequately consolidated. Large air bubbles should stop rising to the surface as large aggregate particles embed themselves into the concrete, and a thin film of mortar (sheen) should appear on the surface. According to the American Concrete Institute, the pitch or tone of an internal vibrator will typically decrease when the vibrator is initially inserted into the concrete, because the frequency will be reduced. The frequency will then increase slightly then level off when the concrete is free of entrapped air. Vibrator operators should become familiar with these simple indicators.
Vibrator operators should also be conscious of heavily reinforced areas and the location of large blockouts, which may require increased vibration times. Noting the quality of formed surfaces and defects during a post-pour inspection are important steps in determining if the proper vibration equipment and procedures have been followed. According to the NPCA Quality Control Manual for Precast Concrete Plants, formed surfaces should be relatively free of bugholes and honeycombed areas. The presence of sand streaks, caused by heavy bleeding and mortar loss, and crazing, caused by drying shrinkage cracking of the mortar, are often indications of excessive vibration, moving concrete with a vibrator and the use of an improperly sized vibrator.
It should be noted that the concrete mix design, placement methods and condition of form work may also contribute to a number of the previously mentioned defects. However, providing production personnel with the proper vibration equipment and training should greatly reduce the amount of time and money spent filling in bugholes and patching honeycombed areas.
Rick Sanders says
when using an internal vibrator, where should I position the head in a one foot thick wall, with #5 rebar 2″ inside both sides of the form walls? Should the vibrator be positioned in the middle of the form or between the rebar and the form wall? Thank you. Rick
Sara Geer says
Thanks Rick for the comment. Phillip Cutler, P.E., one of our technical engineers, has provided the following answer: “Typically, a stinger vibrator would have an effective field strength of sufficient magnitude to consolidate a section thickness of 12 inches. It should be placed in the center of the section wall, allowed to fall through the concrete under its own weight and then removed slightly slower than it fell through the mix.”
Asiddin K. Arabain says
Is it allowed to insert the head of vibrator in inclination position?
Is there any effect to the concrete for the incline insertion of vibrator?
Sara Geer says
Thank you for your comment Asiddin. One of our technical services engineers, Claude Goguen, P.E., LEED AP, provided the following response: “Internal vibrators are designed for maximum effectiveness when inserted vertically into the concrete. There are circumstances where casting thin elements will not allow this, so the vibrator head may be inserted at an angle or horizontally in order to keep it completely immersed in the concrete. It is important that the vibrator head not be moved around laterally while consolidating in this manner.
In terms of effects on concrete due to inserting the vibrator at an angle, the effectiveness of consolidation can be compromised. Vibrator heads contain an eccentric weight that rotates at high speed and exerts vibratory action perpendicular to the axis of the head. If the head is inclined, there is a risk that the radius of action of the vibrator will not consolidate some portions of the concrete.”
Is it allowed to use vibration during lime pointing?
Sara Geer says
Thank you for the comment Mark. I’m glad you enjoyed reading the article. For an accurate response to your question, I recommend you contact the Masonry Institute of America as they have more knowledge on this subject. Since lime pointing is not common in precast concrete production, it is not a familiar practice with our technical service engineers. However, if you do have other questions pertaining to precast production or can provide more information about your project, I’d be happy to help. Thank you.
Im going to pour a 14 inch by 17′ tall wall that calculates out to 220 plus yards. Its about 300 square feet around this boxed wall. Thinking of running 2 vibrators at once it hopes to keep up with the hose while getting maximum vibration. Any suggestions on how high I should pour my lifts…
Bob Telligman says
John, do you own or are you renting your vibrators? If renting I would get a 3″ head with a 20 ft. whip. This would eliminate the second vibrator ” IF” you have an experienced vibrator hand. Otherwise two 2″ vibrators will be required. Timing on your delivery/ age of concrete/mix design etc. will determine how your product can turn out. Do you have door bucks, window bucks or other block outs,conduit etc. in this wall? Obviously you will have a double curtain of re-bar in this shear wall so you will want to run a trim hose so you can insert it down into your wall so you are not dropping your mud the full 17 ft.length.That much force can cause pillowing and blowouts on your forms and rock pockets through separation of the aggregate . I would not pour higher than 4 ft per lift unless you have block outs ie. door/window bucks per lift unless say they are at 5ft. You will need to seal the bottoms on the first lift. Run as high of a slump as allowed to insure there are no voids .Good luck. P.S. Are these walls exposed? If so run you vibrator
down the side of your forms that are not exposed.The vibrator head pulls the air to its self and away from the exposed surface.
What influence does the vibrator frequency has to the finished concrete quality?
There are vibrators with 50Hz, 200Hz and then there are some vibrators on the market with adjustable frequency from 0-200Hz.
What is the influence of frequency on different types of concrete?
Are there any hints what frequency and when should be used?
Sara Geer says
Thank you for the comment Janis. I forward your question to our Technical Services engineers. The following response is from Claude Goguen, P.E., LEED AP, director of sustainability and technical education.
Vibrators used to consolidate concrete are characterized by two factors: amplitude and frequency. The amplitude is the maximum distance a point on the vibrating head moves from its position of rest. Amplitude will have an effect on heavier masses, moving coarse aggregate and ultimately determining the radius of action or influence. The radius of action is the area of concrete influenced by the vibrator. The frequency is the number of vibration cycles per minute and is often presented as revolutions or vibrations per minute (rpm or vpm) or in hertz (Hz). Frequency will have an effect on lighter masses, moving sand and slurry around aggregate and ultimately causing the mortar to liquefy. Generally, as the diameter of the vibrator head increases, the frequency decreases and the amplitude increases. If you have a thinner concrete member, generally you would use a higher frequency and a lower amplitude as opposed to larger pieces, which will require less frequency and more amplitude.
Using improper frequency in consolidating concrete can result in durability and finish issues. Using too high a frequency can end up losing entrained air, which can impact durability. Also, using a high frequency can result in higher cement paste and water concentrations at the surface, which can result in drying shrinkage and crazing. If frequency is too low, the entrapped air can remain, which can affect the durability and finish quality of the structure. Guidelines for internal vibrator frequency and amplitudes are given in ACI 309R-05 (Table 5.1).
For example, for concrete in thin walls, columns, beams and piles, ACI recommends 8,500 to 12,500 VPM (140 to 210 Hz). This is the frequency while the vibrator is immersed in concrete.
Further guidance can be obtained by contacting internal vibrator suppliers directly. Suppliers can be found using our “Find Precast Products & Supplies” website feature.
Hello, I haven’t used a concrete vibrator before, so I’ve been experimenting with some test cubes to try it out, but I am finding that the poker is leaving a hole that just loosely fills with cement paste and sand.
The cubes I made were about 7 inches square and 5 inches deep, I’m using a1:1.5:2.5 mix with 10mm quarry gravel. I have a 1 inch poker driven by a 3000rpm motor, I’m removing the poker straight and slowly but the surrounding concrete is well compacted and does not move into the hole. I’ve tried some other mixtures as well but it still leaves a hole.
I’m planning on doing a reinforced floor slab and also planning to do some building using hollow blocks filled with concrete and rebar and wanted to vibrate the concrete, I fill the blocks with, but now concerned about the poker leaving holes and weakening the concrete. Please can you give me any advice?
Sara Geer says
Thank you for your comment John. I forward your question to our Technical Services engineers. The following response is from Eric Carleton, P.E., director of codes and standards.
From your description, it would appear your trial concrete mix is very stiff. For such mixes with a low W/C ratio, it may be impractical to utilize an internal or “poker” vibrator style to consolidate the mix. Your description of the actual end use for your concrete would lead me to believe the concrete is a “wet mix,” cast-in-place condition.
Consequently, consideration might be given to utilize a water reducer admixture or if the mix design permits added water to provide fresh concrete properties (slump) which would permit this internal method of consolidation. Certainly you will need to increase the slump for workability to finish the floor slab.
If your mix does require a stiff mix for quick form removal (i.e. dry-cast methods, etc.) then consolidating the concrete mix is done by external vibration. In these cases, vibrators are connected to the formwork framing, or the entire form is rigidly attached to a vibrating table that applies appropriate energy to the concrete. Increasingly stiff mixes as described are being used in flatwork and mass concrete pours, but the consolidation is done by large vibrating rollers more commonly associated with asphalt paving processes. This consolidation process may not meet the interior floor tolerances required.
Can we used viberotar in Roof slab?
Whose thickness is 6″.and length is 100ft and width is 17ft,
Please Quick reply/
Sara Geer says
Thank you for your comment Najib. If you can share a bit more about the specific problem you are having using a vibrator, I can pass that along to our Technical Services engineers for a response. In the meantime, I recommend visiting our “Find Precast Products & Supplies” online feature to find a vibrator supplier to contact regarding which type of vibrator best serves your project’s purpose. You can search by typing “vibrator” in the product search box to narrow your results.
I’m doing slabs that are only 4″ thick and 24″ wide. They vary in length, from 8′ to 32′ long. I use a 3/8″ rebar grid in 8″ squares and want to know your thoughts on vibration. We pour a bag at a time and try to get the mix down around the rebar and then ultimately we burn the finish. Thanks, Don
Sara Geer says
Thank you for the comment Don. I forward this to our Technical Services engineers. The following response is from Evan Gurley.
If you are using conventional (slump concrete), you may want to use internal or external vibrators to consolidate the mix and to work it around reinforcing steel and any inserts or blockouts. Vibration or consolidation is an important component to address and is the most widely used method for consolidating concrete. When concrete is vibrated, internal friction between the aggregate particles is temporarily disrupted allowing the concrete to behave like a liquid. It settles in the form under the action of gravity and large entrapped air voids rise more easily to the surface. Internal friction is re-established in the mixture as soon as vibration stops.
Stinger vibrators are commonly used to vibrate concrete. They have high vibrations per minute – anywhere between 10,000-17,000 VPM depending on the amplitude and frequency used. However, they typically lose 20% of VPMs when inserted in the concrete.
Proper procedure for using internal vibrators include:
1. Drop vertically under own weight (about 1 second/foot)
2. Withdraw slightly slower than inserted (about 3 seconds/foot)
3. Place stick in each area only once
4. Overlap vibrating radius
5. When layering concrete, place stick about 6 feet into previous layer
6. Vibrate until surface is shiny and level, and bubbles no longer break
7. Avoid touching formwork with vibrator
You can also use external form vibrators to consolidate. Typically, these are attached to the metal formwork externally and powered.
How to use the vibrator when doing a 125mm X 225mm X 9′ columns?
Sara Geer says
Thank you for the comment Wilson. If you can provide more information about your project, or what exactly you’d like to know regarding internal or external vibration, I’d gladly direct your comment to our Technical Services engineers for a response. Thank you.
Kairi Gainsborough says
I didn’t realize that the air content of concrete makes such a drastic difference in its strength and durability. Finding the correct frequency and amplitude in the vibrating motor seems like an important part of removing those air bubbles. If I wanted to build a concrete addition to my house, I wonder if this is something I would need to consider. After all, I want my home to last for many generations, so strength and durability are necessary.
ADEDOKUN RAHMAN says
In Nigeria, on a civil engineering forum, we are arguing if the use of poker vibrator is suitable in slender suspended slabs that is 150mm thick. Some engineers are of the opinion that since the slab is slender, that vibration might cause cracks on the dry slabs. They say vibrators are more suitable in members with deep depth, e. g beams, columns, base slabs and others. What’s your take on this?
Sara Geer says
Thank you Adedokun for the comment. I forwarded your inquiry to our technical services engineers. The following response is from Eric Carleton, P.E., director of codes and standards.
The question of consolidation (vibration) depends more on the mix than on the geometry of the concrete structure. It is true that different element depths often dictate different consolidation methods or techniques, but traditional slump based concrete will require some type of consolidation, typically by vibration to densify the mix and remove entrapped air or honeycombing.
A 150-millimeter (6 inch) thick slab is thin and might be an excellent candidate for a self-consolidating concrete (SCC) mix, but if that is not the case then some type of vibration should be employed. Your question specifically is if a “poker” (stinger, spud, etc.) can be used to consolidate fresh concrete on a thin slab. The simple answer is yes; however, proper equipment and technique must be be used.
Below are some best practices to keep in mind with shallow forms:
– Shallow forms require a vibrator with a smaller length head. Many standard internal vibrator heads exceed 12 inches. Special 4 inch length vibrators are available.
– The vibrator needs to be inserted in multiple areas on flat sections in a grid pattern.
– Large flat sections must have all areas accessible for consolidation.
If a small head vibrator is not available, a larger head length vibrator may be used carefully and uniformly. Also, the normal holding time used in deeper wall vibration will need to be shortened. Regardless of the size, the vibrator should never be dragged within the flat slab formwork to move or consolidate concrete. Besides segregating the mix, this improper technique can also disturb steel placement. Proper vibration depends greatly on the skill of the operator and supervisor. Receiving Education and training for these unique conditions is important.
John Herscher Walter says
Okay,,this is always a debate. Are air holes on the side of a wall caused by overvibration, or undervibration.?????
Sara Geer says
Thank you for your comment John. I forwarded this to our technical services engineers. The following response is from Mitch Rector.
Performing consolidation is a very important part of eliminating voids in concrete. If concrete is not fully and properly consolidated, air voids will not be able to escape the concrete and can move to the sides of a wall. Conversely, overvibrating concrete can cause segregation which will cause the aggregates to sink to the bottom instead of being distributed throughout. To answer your question, typically, air voids are indicative of insufficient consolidation or undervibration.
A common mistake when vibrating wall panels is dragging the vibrator along the surface. This can risk pulling aggregates with the vibrator and cause segregation in the mix. Whenever possible, internal vibrators should be lowered and raised vertically into the concrete. ACI 309 contains information on proper vibrating procedures.
Okereke Ernest says
Please, at what area intervals should vibration be done in a 6″(150mm) thick slab using a poker??
Sara Geer says
Thank you for your comment Okereke Ernest. I forwarded this to our technical services engineers. The following response is from Mitch Rector.
Table 5.1 of ACI 309, “Guide for Consolidation of Concrete,” suggests using a diameter with a 1 1/4 to 2 1/2 inch head for thin slabs and a 2 to 3 1/2 inch slab for heavy slabs. Section 7.2, “Procedure for Internal Vibration,” recommends systematically inserting the vibrator vertically at a uniform spacing over the entire placement area. The distance between insertions should be approximately 1 1/2 times the radius of influence.
How to reduce sound or harsh noise made by vibrator during concrete setting?it affects people working in our industry..am also an engineer there..plz help me how to remove or reduce that sound almost to max level reduction..
Sara Geer says
Thank you for your comment Kaleem. I recommend contacting a vibrator supplier about your concern. You can visit our online search feature, search “supplier” and search “vibrator – internal” and/or “vibrator – external” for a list of NPCA associate members to contact. They would be more than happy to answer your questions. If you have any technical questions regarding consolidation best practices, I can forward those to our technical services engineers for a response.
Nick Goranov says
Could you please advise what characteristics should I look for in a vibration motor for vibrating 500x500x10-60mm concrete plates? I am planning to use a vibration table so I am looking for suitable motor. Also, how long do you think I should vibrate the concrete?
Sara Geer says
Thank you for the comment, Nick. I recommend visiting our “Find Precast Products and Supplies” feature on our site to contact one of our vibrating table precast concrete supplier members to answer your inquiry. If you have any other questions, please contact us at (317) 571-9500.
Could you please recommend what type of vibrator head to use when using the vibropak method of underpinning. Round? Square? Hexagonal? The concrete is overpoured by 6″ into the underpinning section and needs to fill all of the voids of a rubble footing.
Sara Geer says
Thank you for your comment, James. I recommend visiting our “Find Precast Products and Supplies” feature on our site to contact one of our vibrator associate members to answer your inquiry. If you have any other questions, please contact us at (317) 571-9500.
Wow, after going through your post, it seems very easy and simple to compact freshly placed concrete with an internal vibrator. Keep it up!
David Cole says
We have had trouble with a staff member not doing their part of the job properly – he was trying to keep his hands clean, and avoid the backache associated with pouring silage pit walling, which meant he was cutting corners, which has left us with pour level marking, subsidence cracks under tie-bars, as the tie-bars have supported the concrete above them, whilst the rest of the concrete continues to settle due to insufficient consolidation, and sandy areas as he has left the vibrator running in one place often for minutes at a time, as he either did other work, or simply stood and had a rest.
He has been told and told by myself of the importance of doing the job properly, and the consequences of not doing so, but has chosen to mockingly ignore all that he has been told – we now have two year old reinforced concrete walls, with large numbers of rust stains running from the subsidence cracks, as well as from poorly sealed tie-bar holes, as he has formed porous walls – hundreds of thousands of £s of structurally defective walling.
The proper vibration of poured concrete structures, is an important skill, and hard work, often dirty as well, can be wet and cold, or scorching hot, and a real risk of falling from height, but none the less, it requires a skilled and conscientious person to determine to do the job properly and cut no corners, paying attention to every detail – and there are several – if the finished product is to be structurally sound. It is a very skilled job if it is going to be don right.