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.