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What is the difference between wet and dry spraying?

Wet spraying is currently the most common method of shotcreting. It is mainly used in tunnelling. For wet spraying, the concrete is mixed in the normal way and then sprayed with heavy pumps. The advantages are higher outputs, less rebound, less dust and more consistent quality of the shotcrete. Disadvantages are a rather high investment in equipment and less flexibility. Dry spraying method is mainly used for repair, slope stabilization and small job-sites. Here the ingredients are mixed dry and water and accelerator are added at the nozzle exit.

Therefore a lot depends on the nozzleman’s skill in controlling water-cement ratio and thus producing a uniform shotcrete.

What is rebound and how can I reduce it?

When shooting or guniting concrete at high speed against a target, not all parts of the concrete adhere to that target, where some portion of the shotcrete falls down or rebounds. Generally the rebound of steel fibres is higher than the rebound of the other aggregates. Therefore the fibre dosage rate in the hardened concrete is generally lower then the batched dosage rate. As the customer has to pay for the totality of the fibres and materials he is of course interested in having the lowest amount of rebound possible. The percentage of rebound depends on several parameters. 

The most important factor is the skill of the operator or nozzleman (concrete pump parameters, distance of the nozzle from the target, spraying angle, etc.). For the dry method the rebound will be higher  than that for wet-spraying.  When shotcreting overhead rebound will also be higher than when shotcreting horizontally. Some fibres like the FE (flat end) lead to lower rebound then other fibre types and heavier fibres have less rebound then very fine and light fibres as more of them have energy to stick to the concrete on the wall instead of rebounding. In addition to the mentioned parameters, the addition of micro-silica to the concrete helps as to reduce the rebound of fibres and aggregates and to reduce dust.