QUESTIONS FROM THE FIELD IS A SELECTION OF QUESTIONS NPCA TECHNICAL SERVICES ENGINEERS RECEIVED FROM CALLS, EMAILS AND PLANT EVALUATIONS

PASCAL ASKS: When performing hydrostatic testing on our concrete structures, our state requires us to adhere to a maximum allowable leakage rate based on manhole diameter and water depth. How would I apply this formula to a rectangular pump station? Is there a reliable standard for allowable water tank exfiltration?

NPCA EXPERTS ANSWER: Until recently, only certain products, such as septic tanks and grease interceptors, have included procedures to test rectangular structures hydrostatically. For other types of structures, it has been necessary to convert the rectangular water surface area to an equivalent circular area and use the rate formula for circular structures by calculating an equivalent diameter. This often required some explanation and even confusion when trying to apply the formula to similar volumes with different water depths, also known as the static head.

Now there is a new standard for hydrostatic testing of rectangular structures, ASTM C1957 – Standard Practice for Onsite Exfiltration Acceptance Testing of Precast Concrete Water and Wastewater Structures. This new standard establishes a proven test method for precast concrete water and wastewater structures, including wet wells, pumping stations, stormwater detention structures, solar-heating reservoirs, cisterns, holding tanks, leaching tanks, extended aeration tanks and other similar structures. This new standard gives specifiers an established test standard to reference in their project specifications, and it gives manufacturers a clear set of performance expectations to meet when designing and manufacturing these types of structures. Test criteria are based on a combination of prismatic internal dimensions and the static water level, so that the user no longer needs to calculate internal volumes to compare to an allowable leakage rate based on gallons.

RIVER ASKS: I have a customer requesting a water leak test, which I have only seen used on septic tanks. However, on larger precast structures that are also very deep, I believe there is a greater risk of compromising the structural integrity by filling the structure with water prior to back-filling. Can you please provide some insight?

NPCA EXPERTS ANSWER: You are correct that performing a hydrostatic test on a precast concrete structure before any backfill has been placed can indeed be very detrimental to its structural integrity unless it has been specifically designed for that type and level of loading.
Precast concrete structures, particularly those that are designed for human entry, are often only designed to resist external loads such as lateral soil loads, hydrostatic loads and lateral surcharge loads. Filling a precast concrete structure with water without any soil support completely reverses that scenario.

Manhole sections designed to ASTM C478 specifications are generally designed to withstand internal pressure from a static water depth of not more than 10 feet, while rectangular structures not specifically designed for internal water pressure could exceed their capacity in as little as 3 feet of water. To allow for deeper water depths, additional reinforcing would need to be added in the walls and base slab to offset the increased internal water pressure.

If the precast structure has already been designed, approved and manufactured before realizing that testing was needed, then partial backfilling should be allowed as a compromise. Alternatively, vacuum testing in accordance with ASTM C1244 for circular structures or ASTM C1719 for rectangular structures offers safer alternatives.

Most importantly, testing requirements should be specified before the product is manufactured, and all parties should agree on who is responsible for conducting the test, providing the labor, water and measuring devices, and who is responsible for observing the test results and determining acceptance.

ASTM C1957 is in the process of being added and the rest of these standards can be found in your NPCA ASTM Compass subscription.