Gunite Technique

Learn about the Gunite technique...

Gunite is the ''dry spray method of spraying concrete, widely regarded as a leading method of construction for high quality, luxury swimming pools.

Guncast is a market leader in this field, see below details of the process and its benefits.
  • Excavation

     

    Generally the ground is excavated carefully and accurately to form a ”mould” for the concrete.

    Where this is not possible, e.g. poor ground / cave-ins or out-of-ground construction, a back-shutter is required. This would normally be made from a lightweight timber skeleton framework with commercial grade hessian stretched and fixed across it, or standard hardboard. In all cases a back-shutter is fixed at the upper periphery of the pool, to the datum level for the pool shell.

    A 150mm thick sub-base of 1½" reject stone is then laid to the base of the excavation providing a clean surface to work from and under pool drainage. In many contracts the excavation and sub-base is undertaken by others.

  • Reinforcement

     

    Steel reinforcement is fixed throughout, which is in accordance with current British Standards and designed by qualified structural engineers. For most residential pools, one layer of 10mm high tensile steel fixed at 200mm centres in both directions satisfies engineering standards. Design calculations can be provided for Local Authorities' or other interested parties' examinations.

  • Gunite Pool Shell Construction

     

    The excavation for a Gunite Pool Shell should be carried out as carefully as possible, as the walls of the excavation are generally used as the back-shutter for the concrete. However, pools can be built partially out of ground, or entirely above ground; but in both cases some formwork is needed, although such formwork need not be the heavy shuttering associated with normal concreting work. A lightweight skeleton framework of timber covered with hardboard or hessian is normally sufficient. A drainage bed of reject stone is laid to the base of the excavation and then the steel reinforcement is fixed in position. The Gunite is applied and built up to the required thickness and finally cut back to the design shape.

  • Gunite

     

    Gunite is a small aggregate and cement which is mechanically mixed together and fed, in a dry state, into the hopper of a "gun", then metred by a rotating barrel into the delivery hose, where the mixture is forced by air, supplied by a 600cfm compressor, to the nozzle, at which point water is added, and the wet mixture sprayed at high velocity on to the surface to be gunited.

  • Spraying

     

    The most satisfactory structural design incorporates a cove at the wall to floor junction. If necessary, a 90° detail can be incorporated.

    The walls are gunited sections to full height and thickness, when these are complete the central portion of the floor is gunned.

    The thickness of the applied Gunite is determined by wires stretched across the face of the work to the finished levels.

    Final finishing of the Gunite is achieved by cutting down the concrete surface with long handled, large steel trowels.

  • Personnel

     

    The Gunning Crew:

    Nozzleman - Most skilled operative, responsible for placing the concrete and controlling its hydration.

    Gunman - Regulates the supply of the mix and the air from the gun.

    Mixer Operator - Ensures the the sand and cement are correctly proportioned and of good quality.

    Labourer(s)

  • Advantages and Properties of Gunite Pool Shells

     

    1 - The method of construction complies in all respects with BS8110 and BS8007. Structural calculations can be provided to satisfy Local Authority requirements.

    2 - High speed of construction.

    3 - The shell is monolithic with no joints that can be subject to cracking through movements.

    4 - Normal construction technique requires no backfilling between excavation and pool shell obviating the risk of damage to pipework whilst backfilling, or through subsequent settlement.

    5 - No need for waterproof rendering prior to accepting marbline plaster.

    6 - For larger pools, where reinforced concrete would be used, a gunited pool would almost certainly be a cheaper alternative.

    7 - Difficult access sites require only a 50mm diameter hose to reach the working area, which can be up to 80 metres away from the plant & materials.

    Properties of Gunite



    28 Day Strength - usually specified as 35 N/mm² although strengths of 70 N/mm² are frequently reported.

    Drying shrinkage - 0.06 - 0.10%

    Water / cement ratio - 0.35 : 0.50 by weight