A water tightness test is a procedure that is used to determine if a water-holding vessel is free of leaks. Water tightness testing includes three main steps: filling the vessel(s) with water, monitoring and measuring the water level of the vessel(s) over a prolonged period of time, and analyzing the measurements and observations recorded during the test. Depending on the results and how they are interpreted, the contractor may be required to complete repairs on the vessel(s) and conduct the test a second time. The American Concrete Institute (ACI) provides guidance on how concrete structures should be water tested as well as the expected results. All language pertaining to water tightness testing is found in Section 350.1-10 of the ACI code.
The ACI is adamant that any water tightness testing must be conducted prior to the vessel(s) receiving any type of finish, sealant, or waterproofing. This may seem like a backwards methodology; however, concrete, by nature, should be watertight. Therefore, it must be tested prior to receiving any type of coating or finishing layer. The pool finish should be thought of as a secondary means of water containment, with the first being the natural characteristics of concrete. In theory, a plaster finish, waterproofing membrane, or any other form of finish, may temporarily hide or cover imperfections in the concrete vessel(s). Hidden imperfections may become exposed over time and begin to leak.
The water tightness test is a procedure that occurs over a minimum of six days. It is important that the concrete vessel has cured for at least 28 days prior to the test taking place. Once the 
concrete has cured and gained sufficient strength, it is ready to be tested. It is essential that all penetrations through the concrete vessel(s), such has drains, lights and inlets, be thoroughly sealed off before the test is conducted. Once all penetrations have been sealed and the shell has cured, the contractor can begin the test. The vessel(s) is expected to lose water through concrete absorption during the first few days of the test. Concrete is porous by nature, and will soak up water. It is unknown how long this will take or how much water will be absorbed, so the contractor is advised to monitor and refill the vessel(s) over the first three days. After three days or whenever the water absorption has stopped, whichever comes first, the contractor may begin taking down measured recordings. The contractor must take measurements of the vessel(s) water level(s) every 12 hours over a span of three days. It is essential that the recording process remain consistent throughout the test to generate accurate results. After three days of recording, the results shall be compiled and analyzed to determine if the water loss meets or exceeds allowable levels.
Evaporation and precipitation can greatly affect the results of a water tightness test, and must be considered in the test calculations. The contractor shall fill a restrained, calibrated, open container with water and allow the container to float within the concrete vessel(s) during the testing period. The open container will be used to measure evaporation and precipitation throughout the test. Every time the contractor measures the water levels of the concrete vessel(s), they must also measure the water levels of the open container(s). The water loss or gain seen in the open container(s) is a direct result of evaporation or precipitation and should be used to analyze the concrete vessel(s) recordings. For example, if the open container is measured at ¼” below the starting measurement after 12 hours, and the concrete vessel is observed at ½” below its starting measurement, we can attribute ¼” of water loss in the concrete vessel to evaporation. Furthermore, it can be concluded that the remaining ¼” of water loss seen in the concrete vessel in those 12 hours is the actual water that was lost due to issues with the concrete vessel.
Water tightness testing is one of the few, if not the only, surefire ways of determining whether a concrete vessel has been built watertight. It is important to understand the time, money, and resources needed to conduct a proper water tightness test. Each test typically takes at least one week, requires thousands, if not millions, of gallons of water, and adds to the overall project cost. While the test requirements seem immense, they guarantee the concrete vessel will be watertight once complete. If the vessel is not tested, and experiences leaks over years of use, the cost and time needed to fix any damage related to the leaks could be much greater than the initial cost of testing the vessel. Counsil-Hunsaker believes that every concrete vessel should be tested for water tightness to verify the quality of the build and to certify its longevity.