Technical Questions

ACI Committees, Membership, and Staff have answered common questions on a variety of concrete related topics.



How should the concrete be consolidated to obtain a complete filling of the forms and around the reinforcement?

Q. How should the concrete be consolidated to obtain a complete filling of the forms and around the reinforcement?

 

A. High-frequency internal vibrators have demonstrated a great advantage over other methods in a wide range of shapes and sizes of structural members (ACI 309 and ACI 309.1R). When the consistency of the concrete and the capacity of the vibrator are properly selected for the type of mixture and size of member, internal vibration will permit more thorough consolidation of lower water content and slump concrete than is possible using hand methods. The largest and most powerful vibrators that can be operated in the given work will usually produce the best results with the least labor. Vibrators attached to the forms or to the reinforcement system can be used with benefit where it is impossible to use immersion vibrators. No change in mixture or slump is required.

Under high-frequency vibration the concrete becomes fluid, and gravity moves the mortar and the concrete to fill any space remaining as the concrete is placed. Thus, with careful placement to avoid segregation and excessive depths of concrete before vibration, thorough vibration will solidly fill the concrete around steel bars, between them and the form, and into corners and irregularities. It is better to have a little extra vibration than not enough. When the rising entrapped air bubbles stop breaking the surface of the concrete, vibration can be discontinued. If this amount seems to be bringing an excess of fine material to the surface, it is better to reduce the slump rather than the amount of vibration.

Revibration as late as the operating vibrator will sink through upper layers by itself is beneficial to formed concrete.

In addition to increasing the concrete strength and improving bond under horizontal bars, this procedure largely prevents seepage under form ties left in outside walls, corrects plastic settlement cracking over openings and over upper horizontal bars and bolts, and noticeably reduces the number of surface air voids (“bugholes”) in the upper parts of formed areas.

 

References: SP-1(02); ACI 309R-05

Topics in Concrete: Concrete Fundamentals; Consolidation; Placing

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