THE APPLICATION OF GUNITE
All surfaces should be thoroughly cleaned and wet down before Gunite application begins. The surface can then be sprayed with the Gunite solution. If the surface to be gunited is heavily coated with material or has ingrained dirt it might need to be sand blasted or wet sand blasted using the guniting machine with a specially designed nozzle for this purpose. To continue with guniting from a previous day, surrounding areas and steel or mesh must be wet down before spraying commences.

When guniting in a confined space care must be taken not to contaminate the surrounding areas with overspray traveling ahead at the point of application. To prevent this from happening the nozzle operator should immediately apply a light coating of Gunite over the surface which will then act as a bonding agent absorbing any small amount of overspray. More than one pass with the nozzle, applying a think layer of the solution each time prevents accumulating sand on the wet surface. The distance of the nozzle depends on the type of application and will vary with air pressure, the amount of the solution being applied, and the skill of the nozzle operator. On a typical Gunite job, nozzle distance is from 1 to 15 metres from the working surface. However, when guniting thicker sections, to cover steel or mesh, the nozzle distance at times may be as close as 0.75 metres from the work surface.

Securing a sufficient bond is primarily a mechanical function, but there is some chemical reaction between the concrete and Gunite as well. On a clean surface the Gunite, on impact, forms a dense, cohesive mass having zero slump and penetrating every irregularity and remains held in place by the reinforcements. Reinforcing will, at all times, be in accordance with the site engineer’s instructions. The combined efforts of cohesion, capillary action and force between the existing surface and the hardening cement in the freshly applied Gunite surface forms an effect and lasting bond.

After wetting down the immediate area to be gunited with a blast of compressed air and water, the nozzle operator will reduce the flow of water at the nozzle, while compressed air is maintained and the Gunite solution is introduced. Insufficient water results in a sandy, dry appearance and increased ‘rebound’. Too much water and the solution will slough and slide off. Rebound is when partially coated sand or aggregate particles do not adhere to the surface. The amount of rebound depends on the distance of the nozzle from the surface and the velocity and the angle of the nozzle. Usually the nozzle stream is directed at right angles to the work surface. A correct combination of water, air pressure and Gunite solution along with a correct nozzle distance will result in a glistening appearance when applied to the surface.

It may, on occasion, be necessary to use welded mesh for additional strength and to hold the Gunite in place when applying it in thicker sections. The mesh is held in place by tying to existing steel or by means of studs embedded in the concrete or surface to gunite. Where mesh is used, no two wires should be parallel and should have an overlap of at least 1 to 1.5 squares established. Reinforcement bars are used to increase strength in heavier applications.

When continuing with reinforcement bars, they are not tied or wired together, but have at least 50mm of spacing between them. Then guniting in reinforcement bars, the nozzle is held close to the bars and the solution hydrated sufficiently to keep the front of the bars clean. The solution should be dry enough so that it is fairly firm behind the bars and does not run. Any accumulation of knobs, overspray or rebound should be removed from surrounding bars before they are encased. Vertical bars are sprayed from side to side and horizontal bars from top to bottom with the nozzle held about 45 degrees to the surface. This procedure carefully followed will prevent sand pockets and voids from forming behind the reinforcement.

THE FINISH
The initial layer or layers of Gunite should be finished or screeded in any way until the initial ‘set’ has taken place. The material may be lightly screeded with the side of a trowel or a screed after the initial set. It is not recommended to float or trowel the Gunite solution while in the process of setting up. With the limited amount of water contained in the material, any trowelling on the surface will bring moisture to the surface, thereby weakening the entire coating. If a smooth finish is desired, a final flash coat of 4mm thickness is applied over the previously set up and dampened surface and this final coating is trowelled or floated to the desired smoothness.

CURING THE GUNITE
As with concrete, Gunite shrinks as it dries. However, because of its low initial moisture content, properly applied Gunite should never have major shrink cracks. In thin sections particularly, as the moisture evaporates, slight surface cracks can occur unless provision has been made for early curing. The most universal method of curing is the application of water to the freshly finished surface either by fine spray, fogging, application of wet burlap or a spray coat of membrane curing compound. This treatment is important in inhibiting rapid loss of the moisture and improves the ultimate strength and prevents surface cracking because of an increased shrink factor. Proper curing is very important to the finished product regardless of the thickness.