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A watertightness test is conducted on water-containment structures to check whether undetected and incidental defects are leading to leakage of water. The types of linings used in a water-containment structure, their location (semiarid or arid area, covered or not), and ambient temperature are used as factors to determine the watertightness of a structure.
The test result is influenced by structural deflection, water absorption, temperature, precipitation, and evaporation. Therefore, one should consider the effect of these variables and correct the test result if needed.
The watertightness test is conducted after the structure has been filled with water for a minimum of three days to eliminate the effect of water absorption and structural deflection on the test results followed by analysis of how temperature, precipitation, and evaporation affect the test results.
The test should be continued for a period sufficient to produce a decline of 12.7 mm in water level due to the leakage occurring at the maximum permissible rate. The test procedure, which is presented in this article, is applicable for cast-in-place reinforced concrete water containment-structures like tanks, reservoirs, basins, and conduits.
Test Preparations and Precautions
- The test procedure should not cause a loss of water in the structure.
- The water-containment structure needs to be structurally complete and should be able to withstand the test water's hydrostatic pressure.
- It is better not to have backfill material around the structure to observe visible leakage and determine surface dampness.
- Groundwater should be below the floor of the structure.
- Expose the underdrain line close to the containment structure to monitor flow in the system during the test.
- All temporary bulkheads, cofferdams, pipe blind flanges, and closed valves should be checked to see that they form a complete seal at these outlets, and, if possible, these outlets should be observed during the test.
- Fill the structure with water after it has gained adequate strength to support water pressure.
- Seal all piping, channels, and conduits before the commencement of the test.
- During filling the structure, inspect joints for any visible leakage, monitor outlets for watertightness, and observe underdrain outlet for any increase in the flow.
- If visible leakage or any other problem is observed, they should be treated before the test begins.
- Retesting of the structure should be allowed provided that the result of the test becomes unreliable due to unusual precipitation or any other external factors.
Factors Affecting Watertightness Test Result
1. Water Absorption
Water-containment structures that are new or have not been drained for a long time would absorb water during and after filling. Therefore, to reduce the water absorption effect on the watertightness test, it is recommended to wait for three days after the structure is filled with water and then start the test.
This measure should be considered for conventional concrete water-containment structure. For stricter test criteria, the three days limit can be extended up to seven days or more.
2. Structural Deflection
As the water-containment structure is filled with water, it deflects under the pressure of water. The initial deflection and final deflection of the structure are different due to microcracking, creep, and relaxation that change the concrete's stresses and strains.
That is why it is recommended to have a three-day interval between the time the structure is filled with water and the beginning of the test to remove the effect of structural deflection on the test. Fill the water containment structure at a maximum rate of 1.2 m/hour, and permit air to escape freely.
The volume of water changes due to temperature; for instance, the reduction of average water temperature from 21.11 degrees to 20 degrees in a (6m x 6m x 6m) water-containment structure leads to a water volume reduction rate of 0.008 percent per 24 hours, and water surface reduction by nearly 0.8 mm.
Therefore, the water level in the water-containment structure should be measured and recorded at a 24-hour interval. The readings should be taken at the same time each day and nearly at the same temperature. If the variation of the water surface is considerable, then the test should not be conducted.
5. Evaporation and Precipitation
If a water-containment structure is uncovered, has proper venting, and is located in semiarid or arid regions, the evaporation and precipitation would have a considerable effect on the water level. So, the watertightness test results should be corrected for the water gained from rainfall or the water lost due to evaporation.
This difference can be measured by placing a transparent, floating, open container (calibrated and partially filled) in the structure. The container should be positioned away from the sides and any overhead obstruction that may shield or shade the container.
The container should have sufficient freeboard to accommodate the precipitation from normal rainfall and not be overtopped by the wind's waves.
Watertightness Test Procedure
- Measure the level of water in the water-containment structure at two points— 180 degrees apart; or four points— 90 degrees apart from each other.
- Record the temperature of water at around 45 cm below the water surface. If the specified leakage criteria are stringent, measure and record the temperature at a 1.5 m depth interval.
- Position a calibrated, partially filled open container in an uncovered water-containment structure to measure evaporation and precipitation. Record the water level in the container at a 24-hour interval.
- Examine the exterior of the structure for any visible leakage.
- The test should be continued for a period that is enough to produce a 12.7 mm drop in water surface based on the leakage occurring at the maximum permissible rate.
- At the end of the test, measure and record the water level at the same locations where the first measurements were taken.
- Measure and record the water level in evaporation and precipitation container.
- Calculate the leakage from the tank and, if necessary, correct leakage measurements for evaporation, precipitation, and temperature.
- The structure fails the test if one or a combination of the following occurs:
- The leakage rate of the water-containment structure surpasses the values of the leakage rate provided in Table-1.
- Flowing water is observed from positions of water-containment structure other than the underdrain system.
- Moisture, except from precipitation or condensation, can be transferred to a dry hand from the exterior surfaces.
Table-1: Watertightness Test Criteria
|Type of structure||Side water depth, m||Maximum leakage rate, percent of the water volume in 24 h|
|Unlined water-containment concrete structure||7.62 or less||0.1|
|Concrete water-containment structures with lined wall||9.14 or less||0.06|
|Concrete water-containment structures with lined floor||9.14 or less||0.04|
|Fully lined concrete water-containment structure||-||0.025|
The allowable leakage rate for concrete water-containment structures with greater side water depths than those in Table-1 should be selected using engineering judgment; special consideration should be given to the tank floor and the location and type of concrete joints.
Reinforced concrete structures are tested for watertightness to check whether the incidental defects and joints allow water leakage or not.
1. Water absorption of concrete structure
2. Deflection of concrete structure under the pressure of the test water.
4. Evaporation and precipitation
The reinforced concrete structure should be retested for watertightness if unusual precipitation or any other external factors make the test results unreliable.
A minimum of three days gap should be observed between the time the structure is filled with water and the test's beginning.
Concrete structure fail watertightness test when:
1. Test results exceed the maximum allowable leakage rate.
2. Water flows from locations of the structure other than the underdrain system.
3. Moisture, apart from condensation and precipitation, is transferred to a dry hand from exterior faces.
Types of Joints in Concrete Water Tank Structures and their Spacings