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
- 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
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
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.