Sprayed concrete

Gunning is the process in which concrete is sprayed at high speed, with pressurised air through a hose, onto a support. It is an indispensable process in the structural support and lining of tunnels and slopes, among other qualities due to its resistance. It has also become a technology that places Spain, due to its orography, as a world reference in terms of machinery, products and qualified personnel. Gunite is a procedure used in the most demanding works, and at Soeiro one of our commitments is the quality of each construction.

The materials of which the shotcrete prepared for spraying is composed are those listed below. It is advisable to follow a quality control, to have an adequate dosage. In addition, the processing and working place, the machinery and the workers must be suitable.

The aggregates must be homogeneous. It must be taken into account that fine sands can lose volume (once projected) and coarse sands are more likely to bounce out.

The cements used should as far as possible be standardised compositions, or at least tried and tested.

Water shall be clean, without substances that could damage the concrete. It will be present in different forms: in the mixing and in the humidity of the aggregates.

Additives: in dry mixes, they can be accelerators, setting stabilisers, silica fume, steel fibres, fly ash. In wet mixes it is advisable to use substances that reduce rebound and superplasticisers. At Soeiro we use colloidal silica, which allows for greater cohesion of the mixture, increased resistance, reduction of rebound and reduction of dust in the working area. It is, in fact, a material that is the result of innovation and technological progress in the construction sector.

The way to apply reinforced concrete is as follows: first the cement and aggregates are mixed in a homogeneous composition. The mixture is deposited in a feeder, from where it is distributed to the hose. It is conveyed with the help of pressurised air to a nozzle, and it is here that water spray is applied to the cement and aggregates. The mixture, wet, is sprayed. This is a valid, if somewhat generic, explanation: progress means that the incorporation of additives is becoming more and more sophisticated. It is desirable that the composition, no matter how modern the method, does not produce dust, having total control of the amount of water in cement.

The rejection when spraying, i.e. the rebound that can occur when directing the concrete against the substrate, is what determines how well the spray gunner does. We are talking about the components that do not adhere, and that cause the performance of the technical equipment to drop. Bounce can be the result of a multitude of reasons: the ratio of water and cement, the type of aggregate, and the speed is fundamental. These are factors that can be calculated with greater or lesser accuracy, however something that distinguishes cleanly executed projects from those that drag on, is the expertise of the sprayer. At Soeiro we are aware of the importance of using qualified workers to offer the best service.