Once all electrical work has been completed, you will need to get a Pre-Gunite City Inspection. Once this has been approved, you can start the concrete phase. Shotcrete (concrete applied pneumatically (via air pressure)) is the material that is used to form the ‘shell’ of the Koi pond. On average, Koi ponds are usually 6” thick, and 8” thick in the coves (the bent area between the wall and floor of the Koi pond). The top 6” of the Koi pond is called the bond beam. The bond beam is usually 12” thick to increase the structural integrity of the Koi pond.
Shotcrete is applied at 100 – 150 psi pressure. This works to compress the concrete, causing a stronger bond that can withstand more pressure, and has less of a chance of cracking. Shotcrete is the common method to pouring a Koi pond because no forms are needed. The concrete used is generally low-slump, meaning low in moisture. The low-slump mixture of concrete improves the overall strength. Once dry, the concrete can withstand 3000 psi and 4000 – 5000 psi after a month of curing.
The most common problem with concrete is cracking. The most common reason for this is shrinking concrete during the curing process. As water dries out of the concrete, it loses mass, causing it to shrink. It is important to keep the concrete moist during the curing process, so that it doesn’t dry out too fast, and crack. The first 24 hours is when most shrinkage cracks occur. Keeping the concrete moist during the process will increase its strength as well. Generally it is requested to hose down the concrete 3 to 4 times a day for the first week after being poured.
There are several pieces of equipment required in the shotcrete process. First you will need a line pump, to feed the concrete to the target destination. You will also need a compressor to create the pressure needed to deliver the concrete. You will also need a concrete hose, as well as an air pressure hose. The air pressure hose is fed into the concrete hose at the tip, to project the concrete at your destination with high pressure.
Before you begin to pour the concrete, you will want to make sure of several things. If you have any place in your Koi Pond that is going to be above the surface (waterfalls, fountains, etc.) you will want to add pegboard to the back of them, so you have something to shoot the concrete against as seen in the picture to the right. As a safety precaution, you can hammer in steel rods behind the Koi pond wall, and tie them to the steel frame of the Koi pond to prevent the wall from collapsing under the pressure of the concrete. Although this is not necessary, it is a great safety measure.
Be sure to put up canvas around the Koi pond, as shotcrete tends to spray everywhere, and you will want to contain as much as possible from splattering on your house, plants, or any other objects surrounding the Koi pond.
When you start applying shotcrete, you will want to make sure that your starting point is where you plan on exiting the Koi pond. The more time you give the concrete to dry, the better. You will notice that shotcrete is very fluffy, like a pillow. This makes it easy to shape and mold.
As you’re pouring the shotcrete, it is best to have someone else measuring all required measurements, and starting to smooth out all of the shotcrete. You want to try and smooth out the shotcrete as much as possible. It is best to use a significant amount of trowels, starting with a long narrow trowel to smooth out most of the concrete, and smaller trowels to detail cracks, crevices, etc. Shotcrete dries relatively quickly. After shaping the concrete, you can usually walk on it softly within an hour or so (I don’t suggest doing this, unless you have to smooth out another part of your Koi pond).
You will want to make sure that your Koi pond is level all that way around. To do this take a water leveler, and check every 2’ or 3’ to find where the zero bond beam is located. Once you make your marks, you can then take a trowel and continue the height around the entire Koi pond. If you have any holes that need to be formed for statues, etc. you will want to create them during this process. You could wait, but it is extremely difficult to drill through solid concrete.
After you have completed shotcreting the bond beam, and smoothing out the walls, start at one end and fill in the Koi pond floor. This is the last step of the concrete process. Next, you will want to install any rockwork that you may want.