Watertightness Testing of Water-Containment Structures

Ensuring the integrity and durability of water-containment structures is crucial for their long-term performance. A watertightness test is a critical quality assurance step used to detect hidden or incidental defects that may lead to water leakage in structures such as tanks, reservoirs, basins, and conduits.

Several factors influence the accuracy of the test results, including the type of linings used, structural location (e.g., semiarid or arid regions), ambient temperature, and environmental conditions such as precipitation and evaporation. This article outlines the essential variables affecting test outcomes and provides a detailed procedure for conducting an effective watertightness test on cast-in-place reinforced concrete structures.

Factors Affecting Watertightness Test Results

1. Water Absorption

Newly constructed or long-unused water-containment structures tend to absorb water during and after filling. This absorption can mask actual leakage by temporarily reducing the observed water level drop.

Recommendation: Allow at least three days after filling before beginning the test to minimize this effect. For stricter testing criteria, extending this period up to seven days or more may be necessary.

2. Structural Deflection

Concrete structures deflect under hydrostatic pressure when filled with water. Initial and final deflections differ due to microcracking, creep, and stress relaxation within the concrete.

Recommendation: Wait at least three days between filling and starting the test to allow the structure to stabilize. Fill the structure at a controlled rate (not exceeding 1.2 m/hour) to prevent excessive stress and ensure proper air release.

3. Temperature Fluctuations

Changes in water temperature cause volume variations. Even minor temperature drops can affect water surface levels, potentially skewing leakage readings.

Example: In a 6m x 6m x 6m structure, a temperature drop from 21.11°C to 20°C leads to a 0.008% reduction in water volume per day and a 0.8 mm drop in water level.

Recommendation: Measure and record water temperature at consistent times daily, ideally around 45 cm below the surface. For stringent tests, take readings at 1.5 m intervals. If significant temperature variation occurs, delay testing until conditions stabilize.

4. Evaporation and Precipitation

Uncovered structures in semiarid or arid areas are particularly susceptible to water loss due to evaporation or gain due to rainfall. These environmental effects must be accounted for to avoid misinterpreting natural water level changes as leaks.

Recommendation: Use a calibrated, open floating container placed away from obstructions to measure net water change due to weather. Ensure it has sufficient freeboard to accommodate rainfall without overflow.

Test Preparations and Precautions

Before conducting the watertightness test, several preparatory steps must be followed to ensure accurate results:

• The structure must be structurally complete and capable of withstanding full hydrostatic pressure.
• Avoid backfilling around the structure to allow visual inspection for leaks.
• Ensure groundwater is below the floor level to prevent interference.
• Expose underdrain lines to monitor flow during the test.
• Inspect temporary bulkheads, cofferdams, pipe blind flanges, and valves for tight seals.
• Fill the structure only after the concrete has gained sufficient strength.
• Seal all piping, channels, and conduits prior to testing.
• Monitor joints and outlets during filling; address any visible leaks before starting the test.
• Retesting is permissible if unusual external conditions invalidate the initial test.

Watertightness Test Procedure

The following standardized procedure ensures reliable assessment of structural integrity:

Step 1: Measure Initial Water Levels

• Take readings at two points (180° apart) or four points (90° apart).
• Record water temperature at specified depths.

Step 2: Monitor Environmental Conditions

• Place a calibrated floating container in the structure to track evaporation and precipitation.
• Record its water level every 24 hours.

Step 3: Inspect for Visible Leakage

• Conduct an exterior examination of the structure for signs of moisture or flowing water.

Step 4: Conduct the Test

• Continue the test until a 12.7 mm drop in water level occurs due to leakage at the maximum permissible rate.

Step 5: Final Measurements

• Re-measure water levels at the same locations as the initial readings.
• Record the final level in the evaporation/precipitation container.

Step 6: Calculate Leakage Rate

• Adjust for temperature, evaporation, and precipitation effects.
• Compare the corrected leakage rate against allowable limits.

Step 7: Determine Pass/Fail Status

A structure fails the test if:

• The leakage rate exceeds the values listed in Table 1.
• Flowing water is observed outside the underdrain system.
• Moisture (other than condensation or rain) is transferred to a dry hand upon contact with the exterior surface.

Table: Watertightness Test Criteria

Note: For structures exceeding these water depths, use engineering judgment to determine acceptable leakage rates, with special attention to tank floors and joint details.

Conclusion

A properly conducted watertightness test is vital for verifying the integrity of water-containment structures. By understanding and correcting for influencing factors such as water absorption, structural deflection, temperature fluctuations, and environmental effects like evaporation and precipitation, engineers can ensure accurate and reliable test outcomes.