Shotcrete for Concrete Repair

Dry or Wet: There are two ways to approach shotcrete: dry-mix or wet-mix. Dry-mix shotcrete, sometimes called gunite, uses air to convey the dry cement and aggregate mixture to the nozzle where water is added. Wet-mix uses a concrete pump to convey the already-mixed wet concrete to the nozzle where compressed air is supplied to increase the velocity. The material is then shot from the nozzle at high velocity. Shotcrete allows us to construct walls with one-sided forms. Dry-mix takes more skill to control the consistency of the shotcrete.

Dry or Wet: There are two ways to approach shotcrete: dry-mix or wet-mix. Dry-mix shotcrete, sometimes called gunite, uses air to convey the dry cement and aggregate mixture to the nozzle where water is added. Wet-mix uses a concrete pump to convey the already-mixed wet concrete to the nozzle where compressed air is supplied to increase the velocity. The material is then shot from the nozzle at high velocity. Shotcrete allows us to construct walls with one-sided forms. Dry-mix takes more skill to control the consistency of the shotcrete.

Surface Prep: As with any concrete repair, good surface preparation is essential to success (see How-To Zone, October 2014). All damaged concrete must be removed, edges cut and squared off, and reinforcing steel fully exposed with all corrosion removed.

Shooting Technique: Talk about March Madness, when first being placed, aggregate ricochets off the surface — what’s called rebound, like little basketballs. As cement paste builds up on the surface, the aggregate is propelled into the paste where it sticks forming good shotcrete.

Straight Shooter: To control a rebound in basketball, you box out your opponent, but to reduce rebound and get good dense shotcrete, the gun should be aimed straight at the surface, never at an angle other than 90 degrees (perpendicular). Fill the corners first and work towards the center or edge.

Encasing Reinforcing Steel: Encasing steel with shotcrete requires a skillful nozzleman to prevent a shadow from forming behind the bars and leaving a cavity that does not provide bond of steel to concrete. Consider using a shotcrete nozzleman certified by the American Concrete Institute.

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