The argument over shotcrete and gunite  strength hinges on what kind of mix should be used. Should engineers specify 4,000 psi, thus requiring a higher cement content, or is 2,500 psi adequate for areas without severe weather or soil issues?

Then again, concrete strength and durability also depend on the techniques used in its application. Here are some basics.

  • Don’t add water. A concrete mix is designed to contain specific amounts of aggregate, cement and water, and adding too much of the last ingredient will weaken the finished product. When additional workability is needed, try to use admixtures instead.
  • Attain the right velocity. Part of what makes shotcrete strong is the velocity at which it’s applied. This compresses and densifies the material, helps prevent sagging, and ensures proper and thorough coverage of the rebar.
  • Experts say the material should move at a rate of approximately 400 feet per second. To accomplish this, they recommend compressors that generate air at a minimum of 250 cubic feet per minute. Ratings of 325- to 350 cfm are even more desirable. Professionals are advised to go even higher in severe-weather areas, where the compressors must work harder.

  • Practice proper nozzling. The shotcrete needs to hit the substrate with maximum impact. To accomplish this, nozzlers should stand no more than 4 feet back from the intended surface and always hold the nozzle at a 90-degree angle. Shooting from farther away or at a different angle will diminish the impact thus lessening the compaction.
  • Dispose of the rebound. As the concrete hits the surface, some pieces of aggregate will bounce off the wall and fall to the ground. That’s because these bits aren’t coated with enough cement to help them bond with the concrete.

Some applicators will bury the rebound inside stairwells or in the pool or spa floor. This practice compromises the integrity of the shell since rebound doesn’t have the right cement content and hasn’t been applied at a high velocity.
Rebound should be shoveled into a wheelbarrow and taken off the property. Not only is this good practice, but it’s specified in the International Building Code.