The term “gunite pools” has been used for decades in the swimming pool industry and is commonly used to describe a pool using shotcrete placement, whether it is the wet-mix process or the dry-mix process. Pool builders and designers have often debated whether a pool should be “made from gunite or from shotcrete.” In this article, we clarify the terminology that leads to the debate and then provide arguments for both sides of the discussion.
Gunite and shotcrete are not materials, but rather are placement methods for pneumatically placing (shooting) concrete materials at high velocity. In the history of shotcrete, the term “Gunite” was a tradename used for the dry sand-cement mixture pneumatically shot from the Cement Gun Company’s equipment and hydrated at the nozzle. When the American Concrete Institute began writing standards in 1951, it adopted the term “shotcrete,” as proprietary trade names were frowned upon in technical standards. When reliable concrete pumping equipment allowed pneumatic placement of ready mixed concrete, the terminology was modified to include “wet-mix shotcrete,” while the original dry process became termed “dry-mix shotcrete.” Many companies still use the original term “gunite” to refer to dry-mix shotcrete. Thus, the term “shotcrete” can really be applied to either the dry-mix or wet-mix process.
Shotcrete has been considered the best way to place quality concrete for a swimming pool structure. Shotcrete requires less form-work, fewer touch-ups after form stripping, provides excellent strength and durability, and can be installed far more quickly than form-and-pour pool construction. Along with many other benefits of shotcrete, there is no question that shotcrete is how concrete pools should be constructed. However, with two shotcrete processes, which one is the best for swimming pool construction? The short answer is that both processes work exceptionally well when correctly placed with a well-trained crew using the proper materials, equipment, and placement techniques employed by experienced shotcrete companies.
Dry-mix is the process of conveying dry concrete materials, often just a coarse sand and cement mixture, through a delivery hose, injecting the majority of water at the nozzle, then shooting the newly hydrated concrete at high velocity onto the receiving surface. The wet-mix process pumps premixed concrete down the delivery line and adds air at the nozzle to accelerate the concrete to a high velocity.
Common points of debate are whether one method is stronger than the other and whether one method cracks more than the other. There are many variables that influence these points, including the mixture design and the placement of that mixture. If dry-mix shotcrete and wet-mix shotcrete both have a 5000 psi compressive strength, they are indeed the same strength. This seems elementary, but actual compressive strength is often ignored during debates of one method being stronger than the other.
Although a 2500 psi (17 MPa) 28-day compressive strength is commonly accepted by pool builders, the American Shotcrete Association’s position is that shotcrete must have a minimum 4000 psi (28 MPa) to allow for proper encasement of reinforcement, low permeability, and long-term durability. This is especially important in shotcrete pool shells that are expected to be watertight and provide decades of trouble-free service.
When it comes to cracking, the debate becomes more difficult. There are many reasons why pool shells crack. Common reasons include inadequate reinforcing bars, construction on poor soil, poor curing practices, seasonal temperature changes, concrete shrinkage, and even whether rebound is left in the pool rather than being removed during the installation. Most of the pool cracks that occur have nothing to do with the shotcrete placement. Shrinkage takes place in all concrete whether cast or shotcrete.
The wet-mix process typically uses a higher water-cementitious materials ratio (.40 – .45) than the dry-mix process (.35 – .40). The lower ratio reduces water in the concrete mixture and shrinkage cracking. Moreover, wet-mix concrete may have more water added if it has been aging and needs more fluidity to be pumped down the line. Unfortunately, adding additional water to the concrete mixture after the mixture has stiffened is a common practice in the pool industry, resulting in the concrete being placed into many pools having a ratio higher than 0.50. The higher water content reduces the strength of the shell, increases the permeability and creates the potential for more shrinkage. The negative effects of a too-high w/cm cannot be over emphasized in creating watertight pool shells.
A well-trained crew can mitigate this by using water reducing admixtures rather than adding water. Strength is not an argument for one shotcrete process over the other, as both, with the proper concrete mixture designs, can produce strong, functionally impermeable concrete shells. With proper attention to shrinkage, cracking is typically not an argument for one process over the other.
Dry-mix really excels in pools with less than 1000 ft2 (90 m2 ) of water surface area and pools with a high level of detail such as perimeter overflow pools or vanishing-edge pools. Pools in this size range can typically be finished in one day, barring any extravagant features that could cause delay. Pools this size are often found in backyards where there is very little area to work in and limited space for a concrete truck to clean out. Dry-mix shotcrete creates minimal mess and can be easily cleaned up, and that helps get crews in and out in a day.
The cleanup process after shooting dry-mix is also much easier than wet-mix. There are only dry concrete materials to scoop or vacuum up. There is no concrete wash water or leftover concrete to worry about. Wet mix adds hours of cleanup to the job, even with precautions in place.
Residential pools commonly have a lot of intricate detail. Many dry-mix shotcrete pool contractors use volumetric mixer trucks to produce their concrete mixtures. This allows the crew to stop and start as necessary to ensure a quality placement and finish, while eliminating the concern of set time of premixed concrete material not yet placed. The concrete materials in the volumetric mixer truck are dry and can sit as long as needed until the crew is ready to start shotcreting again. A great advantage to dry-mix shotcrete is that with the correct mixture and placement, the in-place material can handle foot traffic immediately, leaving little more than a footprint.
Unless a wet-mix concrete design, the batched concrete needs to be placed within 90 minutes of batching. By the time the concrete truck drives to the jobsite (hopefully not stuck in traffic), waits their turn to pump, and then is shotcreted, it is often difficult to meet the 90-minute time frame. Creating wet-mix concrete on site leaves the problem of walking on the freshly placed, relatively fluid concrete mixture and disrupting the surface.
Wet-mix shotcrete has significant benefits for pools larger than 1000 ft2 (90 m2) of water surface area and when more than 1 day is required to complete a pool structure. Wet-mix shotcrete has substantial production advantages over dry mix. At South Shore Gunite (SSG), a volumetric mixer truck is used to produce both dry-mix and wet-mix shotcrete. However, dry-mix production runs at about 8 to 12 rpm, whereas wet-mix concrete requires about 20 – 25 rpm. Both productions have similar output per rpm. Thus, when the job allows, using wet-mix can double the production rate of concrete as compared to dry-mix. The wet-mix process typically has less rebound (scrap concrete) than dry-mix, making areas with complicated layouts easier to manage with an experienced nozzleman and blow pipe operator.
Wet-mix shotcrete also allows the use of more complicated concrete mixtures than dry-mix. Wet-mix shotcrete is readily available from ready mixed plants with fibers, air entrainment, high-range water-reducing admixtures, accelerators, and other forms of concrete admixtures. Dry mix can include supplemental materials or admixtures, but it involves a more advanced setup, or use of prebagged, plant-produced materials.
SSG installs pools smaller than 1000 ft2 (90 m2 ) of surface area with dry-mix shotcrete (Fig. 5) and pools over 1000 ft2 (90 m2 ) with either method. SSG does not use wet-mix shotcrete for pools under 1000 ft2 (90 m2 ) because these pools can be done in 1 day, and thus require crews to walk on freshly placed concrete in the floor. With dry-mix shotcrete, the placed material is very stiff and walking on freshly shot material disturbs just the top surface (minor footprints) without disturbing the embedded reinforcing bars.
For strength and water tightness of the concrete pool shell, it is essential that all concrete is well placed with high velocity from the nozzle, providing thorough compaction of the material. This is equally important for both wetmix and dry-mix. Tight spaces with a lot of congestion in the formwork, embeds, pipe penetrations, or reinforcing steel— such as a spa—can create excessive rebound and in most cases, the floor should be shot or cast first.
SSG mostly uses its wet-mix equipment for larger jobs starting at around 200 yd3 (150 m3) and going up to 600 yd3 (460 m3 ) (Fig. 6). SSG typically works under tight schedules, so the ability to install a pool structure a week faster than with dry-mix is a huge advantage. However, before SSG started using the wet-mix process, it shot many large pools, including a 50 m (160 ft) pool for Brown University with dry-mix equipment. This pool had multiple layers of reinforcing bars and tricky areas. Having an experienced ACI-certified shotcrete nozzleman and an experienced blow pipe operator, SSG was able to build an exceptional structure that easily passed the water tightness test.