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Gunite Pool

See also: Categories: Pool, In Ground Pool

Definition

Gunite Pool reffers to a small-aggregate concrete or mortar wet mix conveyed through a hose and pneumatically projected at high velocity onto a pre-formed surface reinforced by conventional steel rods.

Originally gunite method has been developed as a solution for tunnels, underground structures, slope stabilization and structural repairs.

Reason for concrete type of pool

The durability and the infinite variety of shapes and sizes into which concrete can be molded are two reasons for this option.

Terminilogy explained

Gunite was, at one time, a trademarked name that specifically refers to the dry-mix shotcrete process. In the dry-mix process, the dry sand and cement mixture is blown through a hose using compressed air, with water being injected at the nozzle to hydrate the mixture, immediately before it is discharged onto the receiving surface. Gunite was the original term coined by Akeley, trademarked in 1909 and patented in North Carolina. The concrete mixture is by pneumatic pressure from a gun, hence "gun"-ite.

The term "Gunite" became the registered trademark of Allentown Equipment, the oldest manufacturer of gunite equipment. Other manufacturers were thus compelled to use other terminology to describe the process such as shotcrete, pneumatic concrete, guncrete, etc.

Shotcrete has emerged as the all-inclusive industry term to correctly describe "pneumatically applied concrete," either by the wet or dry process. The term "Gunite" is a noun (product name) and should not be used as a verb (as in, to "gunite" something). According to the American Shotcrete Association (ASA), the correct terminology is "shotcrete—wet mix" or "shotcrete—dry mix."

Shotcrete types

Shotcrete could be either Dry Shotcrete (dry applied) or Wet Shotcrete (wet applied).

The dry shotcrete method

Ingredients are placed into a hopper and then pushed out pneumatically through a hose to the nozzle. The nozzle operator controls the addition of water at the nozzle that would eventually be combined once the material hits the surface.

Benefit of using dry shotcrete is that adjusting the water content being put into the mix instantaneously allowing for a better placement process without the needs of adding accelerators.

Recommend method when the process involves frequent stops during the application process.

Wet-mix shotcrete

For applications that normally uses pre-mixed concrete or ready-mixed concrete. Compressed air is applied at the nozzle to propel the wet mixture over the receiving surface.

Wet-mix is the most commonly used process as it produces less rebound and dust compared to a dry-mix process.

The greatest advantage of the wet-mix process is that larger volumes can be placed in less time.

Advantages over traditional concrete

  1. Material is placed and consolidated at the same time.
  2. The maximum aggregate size is 3/8”, which helps to improve quality and improves manageability.
  3. Material adhered to surfaces better than regular concrete
  4. Lower water-cementitious material ratio than commonly found in residential wall mixtures
  5. Can be used with steel fiber as a replacement of welded wire mesh
  6. With steel fibers better flexural strength, ductility, and toughness are obtained
  7. Offers lower costs when compared to traditional concrete
  8. Offers reduced shrinkage and lower permeability

Pneumatic application

Pneumatic application method can be used in the following applications: slope stabilization, dome construction, tunneling, retention walls, water tanks and pools, artificial ponds, ditches and channels, as structural reinforcement, mining applications, dikes and dams.

General applications tips

  • The shotcrete application site should be free of standing water.
  • Layer of proper size stone on the bottom of the pool will ensure the control of subsurface water movement.
  • Steel should be raised 2 to 3 inches above the floor and away from the sides. The American Concrete Institute specifies a minimum 2-inch coverage around any concrete encapsulated reinforcement.
  • Freestanding walls require support mechanisms so there is no vibration. These walls should be secured and fastened properly.
  • All plumbing lines should be mounted firmly in place if they are to be part of vertical wall. Otherwise, plumbing must be run in the floor stone to avoid vibration during the shoot
  • The plumbing must be pressure-tested before shotcrete application. The cost of correcting a plumbing problem afterwards can be catastrophic.
  • Success application largely depends on the skill of the nozzle operator, therefore preferably the operator should be ACI certified for the application.
  • The substrates receiving the concrete must be rigid and non-vibrating. It means that forms must not move during application and the soil has to be compacted enough to provide a solid support. Your steel and form installations should be built to structural specifications based on the soil conditions and set up to withstand the impact of the concrete as it’s shot into place.
  • Concrete joints should be designed as per regular concrete application
  • Any material that might lead to surface deterioration must be removed.
  • The same steel reinforcement requirements could be used on cast-in-place concrete, precast concrete or shotcrete.
  • Shotcrete needs to be protected from rain until it obtains its final set, usually 4 or 5 hours. Following final set, it should be wet cured for at least 4 days, preferably 7 days if possible.
  • There are different textures that can be applied to the finish surface of the shotcrete
  • Typical shrinkage varies in the range of 0.06 to 0.10 percent after 28 days drying. It is typically slightly higher than similar strength concrete
  • Good surface sealer must be applied.
  • The fire-rating of a concrete wall constructed by pneumatic application or pouring will be the same.
  • Shotcrete could be used to repair concrete areas without the need to have a bonding agent applied over the surface being repaired. However, the surface must be before applying.
  • Shotcrete required the cure time. Moist curing is the preferred method of curing or it also can be done by using membrane curing compounds.

Reduction in form-work

Pneumatic application requires forms only for the bond beam. The reduction in formwork represents a considerable saving in labor and materials, but this advantage is at least partially offset by the special equipment and the skilled craftsman required to do the job. Pneumatic application should be limited to soils that can be shaped the contour desired and which will retain the shape until the work is completed.

Quick Statistics

  • In 1957 pneumatically applied concrete (shotcrete) accounted for 72 percent of all the pools built
  • In 1959 pneumatically applied concrete (shotcrete) method of application accounted for only 54 percent
  • In 1959 cast-in-place concrete (pour concrete) accounted for 23 percent of all the pools built (up from 11 percent in 1957)
  • In the same year vinyl or nylon liner accounted for 15 percent
  • In the same year fiberglass, steel, aluminum and other materials for 8 percent

Quick Facts

  • If there is a movement of a pipe or a piece of steel, it’ll leave a shadow or gap in the concrete which increases porosity and permeability. This can also lead to cracking. In that scenario, even if the shotcrete applicator has done everything correctly, delivering concrete that measures 4,000 psi on a compression test, the flawed preparation results in an unsound structure, which in turn leads to surface failures or water loss.
  • Proper substrate, plumbing setup and steel preparation are essential to create a reliable concrete shell, no exceptions!
  • Higher material’s projection velocity will produces reliable compaction.
  • Velocity = compaction, compaction = density, density = strength, strength = water tightness.
  • Success application largely depends on the skill of the nozzle operator, therefore preferably the operator should be ACI certified for the application.
  • Pneumatic application methods represent about 60 percent of the pools constructed with concrete.





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