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). Most concrete vibrators today are designed to a set of standards that was developed by the Army Corps of Engineers in the late 1960s. In response to these changes, manufacturers have developed new vibration equipment that’s better suited to 21st-century jobsites. Formwork has taken on new shapes and incorporated new materials over the years. 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. Both of these factors have led to more flowable concrete mixes with higher slumps, finer aggregates, and more supplementary cementitious materials. Using pumps to place concrete also has become more common. For example, many of today’s concrete structures are more heavily reinforced, which means that the space within formwork is more limited. For a comprehensive explanation of concrete consolidation, see “ Shaken, Not Stirred” a 2007 article.Īlthough the basic facts about vibrating concrete have not changed much over the years, design and construction practices have evolved. The shear stresses increase with the speed of the vibrations, which also increases the amount of water separation. 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. When a vibrator is inserted into fresh concrete, its motions exert shear stresses in the plastic concrete. 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. Mechanical vibration has been a standard practice in concrete construction for decades.
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