Jan 04

Minimize Rebounding in Shotcrete

Shotcrete, a.k.a gunite, is a wet mix technique that produces a high velocity cone-shaped spray pattern via pneumatic mechanism onto a horizontal or vertical surface. Increase the air from the shotcrete nozzle to increase the velocity and the compaction of the particles and produce a denser concrete. Shotcrete uses either aggregates or mortar.

When to use shotcrete?
When you need to apply concrete vertically but do not want to deal with formwork and poured in concrete. Shotcrete works best for covering or repairing curved surfaces like swimming pools, bridges, tunnels, dams, slope reinforcement…

Strength of Shotcrete
Shotcrete uses a wet mix so the water/concrete ratio is very important and controlled by the nozzle man. It needs to be low (under 0.5) to obtain a denser and water-resistant mix.  This will also help reduce shrinkage and lower permeability in the finished product.

What is rebound?
One of the problems with shotcrete is the loss of material from concrete ricochets off the target surface due to the high velocity of the spray. This causes having to use more concrete, longer application times etc. The result can be vastly different when it comes to the properties of the in-placed concrete mix. If you use steel fibers as a distribution solution, the problem is even more problematic than with aggregates. According to Austin and Robins (1995), aggregates rebound is about 20-30% whereas with steel fibers, rebounding can reach 35-78% (Morgan et al., 1987; Banthia et al., 1992) with dry-mix shotcrete. With such high rebound, compaction is reduced and the concrete strength ends up greatly compromised.  The rebound of non-adhering material during a gunite operation (using a dry process) is greater than during a shotcrete job (wet process) because gunite requires a much larger air compressor. On a gunite job, the rebound material can be reused, whereas, on a shotcrete job, the non-adhering material is wasted. Removing it is time consuming, slows progress and can be costly. So choosing the right technique for the job is essential.

Thick or thin applications?
It turns out that the percentage of loss is affected by the thickness of the layer sprayed. In other words, the thinner the layer of shotcrete, the larger the percentage loss. The highest amount of rebound happens in the first spray on concrete. With subsequent coats, rebound diminishes. Another factor affecting the rate of rebound is aggregate density. A lower density will result in lower rebound rate.

Some solutions
Some polymer additives (Etonis) or modified starches can improve the adhesion and flow properties of concrete and reduce crack formation.

What pump is best for your job?
 If you are not sure what concrete pump is best for your next job, call Dick Hibbard at (503) 283-2105. With over 40 years experience in all things concrete, no question is too simple or too complex for him!

Some great pumps he recommends are:

Schwing WP 1000 concrete pump

A Schwing Wp-1000X






A Reed B-50 or C50Hp

Reed B50 and C50HP concrete pumps







Check all the great deals he has on Mayco, Schwing, Olin, Reed and Putzmeister used shotcrete pumps on United Equipment Sale’s website:
If you you need a gunite machine, check our site for our latest great deals.

Reference material
Parametric Studies on Fiber Rebound in Dry-Mix Shotcrete
The American Shotcrete Association
Polymer modification of shotcrete