CONCRETE (COMPACTION BY) VIBRATION

WHY?Cast-in-place concrete and concrete for prefabricated elements is vibrated in order to compact it, ensure it flows in between the rebar and throughout the formwork, filling all voids and to reduce the quantity of entrapped air. Normal concrete does not flow under its own weight!

Well compacted concrete is stronger. It is also more durable, reacts less to outside corrosive factors. Well-compacted concrete is homogeneous and has about 1-2% entrapped air. Its surface after removing the formwork is smooth. It is also more impervious. Strength decreases with 10% for 2% volume of voids and with 30% for 5% volume of voids per total volume.Although good quality concrete is delivered (and we pay a lot for it), poor vibration can significantly decrease the quality of the cast element.

WHAT?Concrete can be compacted in many ways. Using rods, vibrators, vibrating tables, vacuum or centrifugal equipment.One of the most common and best methods for compacting cast-in-place concrete is using vibrators, which are mobile, require less manual labor and if best practices are followed give good quality results.Concrete vibration sets in motion the fresh concrete due to the vibrations induced by the tools, causing the concrete to flow and fill gaps, also eliminating entrapped air in the paste.

HOW?Concrete vibrators used depend on the size and shape of the concreted element and on the concrete properties (workability, max. aggregate size, etc.), also on the space between rebar and the distance between them. The usual diameter of a vibrator is of 5 7 cm.The general rules for vibration are: Uniform vibration, in conformity with the radius of action of the vibrator. Vibrators should be placed at 1 radius of action of the vibrator between them.

Fast insertion, slow removal (1 second for every 10 cm removed) vibration between 5 30 seconds. Insert straight Vibrator has to reach at least 10-15cm into the previously cast concrete layer Do not touch rebar, it causes the bars to displace Do not push concrete laterally with vibrator, it should flow because of the vibration Vibrate until very little air escapes the concrete, until surface is level and shiny, but no cement water is at the surface (indicator of the segregation of concrete) Do not over-vibrate can cause segregationCannot use very fluid (workable) concrete, because normal concrete losses strength with fluidity due to more added water. Admixtures increase the price of concrete, so thats not very handy.PROBLEMS due to under-vibration: Excessive entrapped air Honeycombing Sand streaks Cracking Cold jointsPROBLEMS due to over-vibration: Segregation Sand streaks Form deflection