Consolidation of Concrete TM-101 4.8 - 4th Edition

Описание: This video overview focuses on the essential process of concrete consolidation, detailing various methods and best practices to achieve a dense, uniform, and durable concrete mass.
Concrete must be uniformly consolidated using methods such as internal or external vibration, surface vibrators, spading, or impact. The primary goal of consolidation is to fully coat the coarse aggregate and reinforcement with cement paste, preventing air pockets around the reinforcement that could lead to corrosion. Proper consolidation results in a dense, uniform product with surfaces free of imperfections. The optimal vibration time depends on the vibrator type, the concrete mixture characteristics, and the configurations of the mold and reinforcement. Undervibration is a more common issue than overvibration.
For lightweight aggregate concrete, care must be taken not to overvibrate, as this can cause the lighter coarse aggregate particles to rise, leading to finishing problems, non-uniform strength, and differential shrinkage. When consolidating face mixtures, especially if aggregates will be exposed, concrete layers must be placed as level as possible to prevent segregation caused by lateral movement of the concrete by the vibrator. Mounds should be leveled by hand or screed, not by inserting a vibrator into their center.
Internal vibrators should not touch forms or molds for exposed architectural concrete surfaces, nor should they be inserted closer than 2 to 3 inches (50 to 75 mm) from the mold face to prevent marring, "stinger marks," or "shadow lines" caused by vibrating reinforcement. Vibrators should sink under their own weight and be withdrawn slowly to prevent air pockets. The distance between insertions should allow for overlap of the visibly affected areas. For epoxy-coated reinforcing bars, internal vibrators must have rubber or nonmetallic heads to prevent coating damage. Even high-slump, flowing concrete requires a minimum nominal vibration to eliminate large air voids, despite its self-leveling appearance.
External form vibrators must be securely fixed to molds for maximum effectiveness and to prevent damage. Surface vibrators are ideal for consolidating thin layers, such as facing mixtures, and include vibrating screeds, hand jitterbugs, and bull floats with mounted vibrators. These must be moved at a rate sufficient to embed coarse aggregate and bring paste to the surface for finishing, ensuring the full depth of the concrete layer is compacted. Vibrating tables are effective for flat or low-profile units, requiring careful determination of frequency and amplitude to ensure uniform vibration and avoid both poor consolidation from inadequate amplitudes and rolling/tumbling from excessive local amplitudes.