מבחן וֶבֶּה (Vebe)

The Vebe test measures the workability of concrete, particularly for dry mixes. It uses a cylindrical chamber, slump cone, and transparent disc-shaped rider on a vibrating table. The test quantifies how much energy is needed to fully compact concrete, expressed in Vebe seconds. This measurement helps engineers assess concrete's ease of handling and placement.

The Vebe test apparatus consists of a cylindrical chamber measuring nine and a half inches in diameter and eight inches in height, a standard slump cone, a 6-pound transparent disc-shaped rider, and a vibrating table. The transparent rider sits on top of the concrete and becomes obscured when compaction is complete, allowing technicians to measure the time required for full consolidation.

Concrete is placed into a slump cone within the cylindrical chamber and compacted in a standard manner. The cone is then removed, leaving concrete in a truncated cone shape. A transparent disc-shaped rider is placed on top, and the vibrating table operates at 50 cycles per second. The test ends when the rider becomes fully obscured, indicating complete compaction with no visible voids.

Vebe seconds measure the time for concrete to transition from a slump cone shape to a fully compacted cylinder under vibration. Higher Vebe seconds indicate lower workability, meaning the concrete is stiffer and harder to compact. Lower Vebe seconds suggest higher workability, with concrete being more fluid and easier to handle and place.

The vibrating table operates at 50 cycles per second, inducing vertical movements that facilitate concrete compaction within the cylinder. These vibrations help consolidate the concrete and eliminate air voids. The duration of vibration needed until the transparent rider becomes fully obscured represents the energy required for complete compaction, directly reflecting the concrete's workability.

When the transparent rider becomes fully obscured by concrete, it indicates that compaction is complete and no visible voids remain on the concrete surface. This moment marks the end of the test, and the elapsed time from the start of vibration is recorded as Vebe seconds. This measurement quantifies the concrete's workability and its resistance to compaction.

The Vebe test is designed to measure workability for dry concrete mixes, which are stiffer and require more energy to compact than wet mixes. The vibrating table's mechanical action effectively consolidates these drier mixtures, making the test ideal for assessing their handling characteristics. The Vebe seconds measurement provides a reliable indicator of how much effort is needed to achieve full compaction in stiff concrete.