People have built backyard pools as far back as the 1950s. While the pools may have undergone aesthetic remodeling, including new tile and coping, the shell remains sound and watertight, thanks to proper concrete requirements. Shotcrete technology has come a long way since then and the concrete standards of today are even better than in decades past.

If you don’t start with the right materials, it’ll be difficult to build a solid concrete shell using the shotcrete method. This entails designing the proper mix-schedule for the installation conditions and installing the material within the time frames prescribed.

The material mix will be treated at the batch plant where the concrete mixing trucks are loaded if you’re using the wet form of shotcrete application.

Let’s pretend you’re shooting a shotcrete pool shell for the sake of argument. All of the plumbing has been designed correctly and pressure checked, and the soil has been determined to be structurally sound, which means it can be used as a substrate in a one-sided type application. Once we’ve got all of those things in place, we’ll move on to the mix design.

We ensure that any piece of aggregate in the mix is fully encapsulated by the cement paste, make sure the aggregate-to-cement ratio is within recommended proportions.

The rule is simple: four parts aggregate (sand) to one part cement (binder), or three to one if you want to cover the aggregate with more cement paste. Since there will always be some loss in the distribution lines and/or overspray, we like to use a 3:1 ratio to ensure that we have enough cement to cover all of the aggregate surfaces. Industry guidelines state that ratios greater than 4:1 are not appropriate.

The size of the aggregate is also significant, as the smaller the aggregate, the more surface area that needs to be encapsulated by the cement paste. ACI recommends (gradation 1) aggregates no larger than 1/4 inch for dry-mix, and (gradation 2) aggregates no larger than 3/8ths of an inch for wet-mix.

When it comes to ensuring proper cement hydration, the water-to-cement ratio is especially important. If you use too much water, you’ll end up with porous concrete that lacks the required compression power, as we’ll see later. It would be vulnerable to water penetration into the matrix, which can result in corroding rebar, efflorescence, and a variety of other issues later on. However, if you don’t have enough water, you won’t be able to hydrate any of the cement particles, compromising structural integrity.

The water-to-cement ratio is determined by dividing the amount of water by the amount of cementitious content. For most applications, the optimal range would be between.35 and.45. Under normal conditions, the American Society of Anesthesiologists recommends no more than.50.

With a good mix design in place, we must understand the time and distance the material must travel to reach the nozzle and, eventually, the substrate. Without the use of retardant admixtures, the wet-mix process takes about 90 minutes from the time the material is blended to the time it is applied. We’ve discovered that using retardants will extend the time period (up to 6 hours +/-), but this raises the material cost and necessitates a thorough understanding of concrete chemistry. Based on the moisture content of the material, a 45-minute window is usually the rule once the dry gun is primed.

Without admixtures, you want to keep the pumping distance as low as possible from the mixing truck (wet) or hopper (dry). We’ve discovered that if you go any farther than 100 feet without taking such steps in mixed transportation, you’ll start to get into trouble (i.e. lubrication, paste content, etc.).

As a general rule, the higher the cement or binder content required, the further the material is pumped. Plasticizers, pumping aids, retardants, and accelerators are among the admixture chemicals that can be used to preserve viscosity for pumping, regulate the rate of hydration, and promote workability.