Mechanical vibration has been a standard practice in concrete construction for decades. It is used to consolidate fresh concrete, eliminating pockets of air so that the mixture can fill all the available space within the form and bond well to reinforcing steel. When a vibrator is inserted into fresh concrete, its motions exert shear stresses in the plastic concrete. The vibrator’s accelerations liquefy the concrete, using available water to lower the friction coefficient of particles in the concrete mix, and allowing entrapped air to move upward. The shear stresses increase with the speed of the vibrations, which also increases the amount of water separation. For a comprehensive explanation of concrete consolidation, see “Shaken, Not Stirred” a 2007 article.
Although the basic facts about vibrating concrete have not changed much over the years, design and construction practices have evolved. For example, many of today’s concrete structures are more heavily reinforced, which means that the space within formwork is more limited. Using pumps to place concrete also has become more common. Both of these factors have led to more flowable concrete mixes with higher slumps, finer aggregates, and more supplementary cementitious materials. Epoxy-coated rebar has grown in popularity, especially in northern climates for bridge decks, highway pavements, parking garages, and other applications that are subject to infiltration of water mixed with deicing salts. Formwork has taken on new shapes and incorporated new materials over the years. In response to these changes, manufacturers have developed new vibration equipment that’s better suited to 21st-century jobsites.
Development of vibrator standards
Most concrete vibrators today are designed to a set of standards that was developed by the Army Corps of Engineers in the late 1960s. ACI Committee 309 on Vibration (now called Consolidation of Concrete) adopted the Army Corps standards to limit vibration speed to a maximum of 11,000 vibrations per minute (vpm). Vibration exceeding that speed was believed to cause a separation of the component materials in the concrete mixture.
The traditional electric vibrator consists of a motor, a flexible shaft, and a vibration head. The power of the motor, the length of the shaft, and the size and weight of the vibrator head used vary depending on the requirements of the particular project. Most motors that drive vibrator units are uncontrolled. They start at a high vibration speed, and then vibrate more slowly as greater loads are imposed through increased shaft length, head weight, and concrete viscosity.
Today’s higher-slump concrete mixtures offer less resistance, and therefore tend to reduce the vibration speed less than was common when the vibrator standards were developed. Vibration speeds that are too high can adversely affect the quality of the concrete.
Under high-speed vibration, water is driven away from the vibrator head and moves downward to find a leak path in the forms. Water that pools against the inside of the forms creates washouts and water voids. In other cases, excessive vibration speeds cause concrete to shed its bleed water to the bottom of the form. The resulting surface defects need at least to be patched; if severe enough to expose reinforcement, they require more drastic repairs.
Learning lessons from pavements
Vibration speed became a matter of concern partly due to experience with concrete pavements that contain air-entraining admixtures to increase freeze/thaw durability. In some cases, consolidation at excessive speed seemed to eliminate not only objectionable entrapped air voids, but also microscopic entrained air bubbles. Research at the Portland Cement Association showed higher vibration frequencies could adversely affect air-void spacing and freeze/thaw performance.
Based on this research and field experience, the Federal Highway Administration, Federal Aviation Administration, and individual state transportation departments have recognized the importance of proper vibration. Some agencies are setting new standards and monitoring performance to make sure contractors produce concrete pavements with adequate air-void systems.
Controlling vibration speed
Drawing on its experience producing vibration equipment for concrete pavements, Racine, Wis.-based Wyco Tool Co. set out to produce a new kind of vibration system for formed concrete structures. The system’s key component is a vibrator motor that runs at a constant speed regardless of any internal or external forces. Wyco’s new Sure Speed motor is designed to maintain a consistent vibration speed, with any concrete mixture and any combination of shaft and vibrator head.
Wyco national sales director Ted Grant says, “We performed rigorous testing of vibrators set to different speeds. Some tests were done in-house, others were done in front of witnesses, and still others were done in collaboration with contractors that were experiencing serious surface defect problems. Over and over again, the story was the same: Reducing vibrator accelerations markedly reduced the occurrence of concrete form face surface defects.”
Grant expects further testing and field experience with the defined speed vibrator to yield a better understanding of how vibration speed affects concrete quality. Table 1 summarizes common applications and factors affecting vibration recommendations. The vibration speeds shown for specific types of work are selected to minimize damage by ensuring compatibility with the concrete.
Coping with tight spaces
The prevalence of more heavily reinforced concrete structures has made consolidation more difficult. Sometimes the space within forms is so congested that it’s difficult, if not impossible, to fit a standard vibrator in between lengths of rebar and between the rebar and form faces. Such tight spaces also can make it hard to avoid having the vibrator head impact and damage the interior of the forms. This damage not only necessitates repair or replacement of forms, but also produces a flawed concrete finish when the forms are removed.
Oztec Industries Inc. developes products specifically to alleviate these problems. The RubberHead vibrator head features a dimpled outer shell of urethane that won’t damage forms on impact. Oztec president Fred Oswald says the product’s unique dimpled design “holds the concrete tight to the vibrator, improving the vibrator’s ability to transmit vibration into the mix.” The RubberHead also is recommended for consolidating concrete around epoxy-coated rebar, because it won’t damage the epoxy coating and potentially compromise its corrosion protection.
Another solution to consolidating concrete in congested forms is to vibrate the rebar itself. Oztec’s Rebar Shaker, when placed on top of vertical rebar for 5 to 10 seconds, transmits vibrations in all directions (both toward the form wall and toward the center of the structure), along the entire length of the rebar. The Rebar Shaker also serves to rotate the rebar, helping to ensure that it bonds with the concrete on all sides.
When pouring additional lifts before the previously poured concrete sets up, the Rebar Shaker will revibrate the earlier lift, improving consolidation. If the previous lift has hardened, on the other hand, the rebar vibrator generates forces too small to separate the rebar from the hardened concrete. The vibration is simply absorbed by the concrete mass.
Depending on wall thickness or column diameter, you may still need to use an internal vibrator to consolidate the entire mass. When vibrated, each rebar works as a vibrator head of similar diameter, so its influence is comparable.
Outside the box
External form vibrators are another consolidation option, one commonly used in precasting operations, but less often for cast-in-place. Some forming systems accommodate external vibrators easily, but others require contractors to attach mounting brackets by bolting, welding, or other laborious means. Some specialized forming systems, such as ICFs and tubular cardboard or polyethylene column forms, can’t support mounting brackets.
Vibco Inc., Wyoming, R.I., developed a new product that enables contractors to use external vibration for these applications. The company introduced its Stick-It Vibrator Mounts at World of Concrete 2012. These brackets can be installed on any smooth surface, curved or flat, by means of a venturi air mount system that creates positive suction with a steady air supply. They require 80 psi and draw 8 cubic feet per minute.
The Stick-It Mounts are easy to attach, easy to remove, and easy to relocate as needed. They come in three sizes, to accommodate 40 different Vibco vibrator models. The vibrators are powered independently from the mounting system, so the line of compatible vibrators includes pneumatic, electric, hydraulic, and DC powered models.
Contractors who have been frustrated by surface defects related to overvibration, by the difficulty of consolidating concrete in rebar-congested forms, or by the inability to use external vibration on ICFs or Sonotubes, should investigate these innovative tools.
Kenneth A. Hooker is a freelance writer based in Oak Park, Ill.