For many, the question commonly arises as to what type of concrete material makes the best water tight vessel. Cast-in-place concrete, shotcrete or gunite. While each have their own strengths, one must consider key items such as geographical/site location, soil conditions and more importantly, availability. There are many locations within the United States where shotcrete and gunite are not produced. Soil conditions can also play an integral role in the decision making. Questionable soils and/or remediated soils often call for thicker concrete placement due to the needed increase of structural steel (rebar). Let’s explore the primary difference between shotcrete and its close cousin, concrete. The placement method utilized for concrete is discharging it from a ready-mix truck down a shoot directly into form work or into a hopper equipped with a boom pump. The concrete is then pumped into forms and must be vibrated for compaction. By contrast, the shotcrete process, whether using wet or dry material feed, may only require single side forming or no forming at all by utilizing the earth as the back form in selected areas. This process utilizes a smaller pump with added air pressure via an accompanying compressor. The air is introduced at the end of the discharge hose nozzle, creating a high pressure discharge of the concrete mix. The pressure creates compaction, thereby enhancing design creativity and application flexibility, often resulting in a savings of time or money. The impact velocity of correctly placed shotcrete quickly compacts the material, yielding an “in-place” mix that’s richer in cement and higher in strength than the same mixture prior to placement. With shotcrete, what appears to be a waste of supplies known as “rebound” and also overspray, in reality ends up in thick, high -energy shotcrete as a portion of the mixture ricochets off the receiving area and away from the placement area. The loss through rebound will range relying on the dryness of the mix, the shooting distance from the surface area, wind conditions, and many others. The expected thickness is generally overshot, trimmed back to the design thickness reflected through “look-outs” and finished to the required surface consistency and appearance. This process commonly requires more labor for multiple finishers to cut and finish the concrete and labors to follow with clean-up and rebound removal. This compared to the additional materials and labor required to form both sides for the cast-in-place process can often times offset each other. However, cast-in-place does require additional labor for front and back form removal.
Characteristics of how concrete or shotcrete is delivered for pumping or placements are very similar. Typically, regardless of the concrete type, shotcrete or cast-in-place concrete has a 90-minute window from the time it is batched at the plant until it placed. Temperature of the material, air temperature and humidity levels can increase or decrease the set times of the concrete. Typically Ready-Mix companies hold back 10-15 gallons of water in the mix so that the contractor can adjust the slump of the concrete on site. A concrete truck typically hauls 8 to 10 yards of concrete and 1 cubic yard of concrete typically contains 38 to 40 gallons of water. Project specifications dictate the required mix design and testing agencies commonly verify the concrete slump of the first truck prior to pumping and periodically through the day. Adding 1 gallon of water over the design mix (amount of gallons of water per yard of concrete) can decrease the strength of the concrete by 200psi.
Often overlooked, the curing process is an integral step for optimum intended performance of concrete or shotcrete. The mix design can be perfect and the placement can be of the highest quality, but if not cured properly, adverse effects can impact the quality level dramatically. While there are many methods, curing concrete or shotcrete surfaces with water is by far the best method for maintaining adequate moisture and controlling shrinkage cracks during the hydration process. After some time the concrete will start the chemical reaction that will eventually harden the concrete. Curing vertical and horizontal surfaces with water can be easily accomplished with alternative curing methods considered due to project scheduling, the availability of fresh water and the ability to discharge the water being used. Curing compounds may also be applied to the concrete or shotcrete mix or to concrete surfaces after placement. However, these compounds can also have an adverse effect on the finish materials such as paint, plaster or tile mortar. It is suggested to consult with the finish contractor and the structural engineer prior to concrete placement to verify acceptance of curing compounds.
In closing, the question of which process is better? Well, as you can see, there are many variables to consider. While cast-in-place is better controlled and suited for close tolerance work, it can be a less economical option. Shotcrete, if applied properly, can also provide a superior water tight vessel. Make sure your concrete or shotcrete contractor is qualified to perform their work. Do your research. Ask for references and verify if their nozzleman is ASA (American Shotcrete Association) certified. Water test your pool “prior” to the placement of pool finishes. This process is detailed in ACI 350 (American Concrete Institute) and remember that pool finishes such as paint, plaster and/or tile are not designed to waterproof your pool. If the structure behind your finish is not watertight, your finishes will only serve as a temporary barrier.