The vacuum test detects leakage and determines watertightness.
The vacuum test detects leakage and determines watertightness.

What are the criteria that satisfy the requirements of a structural proof of design test and a watertightness test for a precast concrete onsite tank? Ask 10 different people in the onsite industry (regulators, specifiers, health officials, precasters, and sanitarians) and you will probably get 10 different answers.

The answers will be different for many reasons. How can you sift through all the information and end up with the right answer? A look at the standard answers the question. In sections 6 and 9 of the ASTM C 1227, “Standard Specification for Precast Concrete Septic Tanks,” the requirements are outlined for both of these tank tests.

In section 6.1, under “Structural Design Requirements,” ASTM C 1227 states, “Structural design of septic tanks shall be by calculation or by performance.” Both of these methods contemplate all applicable site, soil, dead load (DL), live load (LL), and surcharge loading conditions and should be performed by qualified engineering personnel. The calculation methodology compares the calculated loading against the structural characteristics of the given tank design (4 inches Hg) with the appropriate ACI 318 specifications.

The standard continues in the remaining parts of Section 6 to further stipulate minimum design, loading, and performance requirements that each tank design must satisfy, unless specific project specifications or requirements are more stringent.

Section 9 of the standard outlines the performance test methods for tanks. The first two parts of this section address proof testing to demonstrate strength of the tank to resist all anticipated external and internal loads.

The performance test method uses the calculated applied loading conditions derived in Section 6 (usually in pounds per square foot, or psf) and uses a 1.5 multiplication factor for the performance test loading. This performance test load can then be converted to an appropriate vacuum level (negative pressure) for the test. A qualified engineer should witness and sign off on the test.

For a leakage or watertightness test, Section 9 of the standard states that this can be performed by using either vacuum testing or water-pressure testing. To perform the vacuum test, seal the empty tank and apply a vacuum (negative pressure) level of 4 inches of mercury for two minutes. A tank passes if 90% of this vacuum level is held for two minutes.

In the water pressure test, the tank is sealed, filled with water, and left to stand for 24 hours. After this time, the tank is topped off, assuming there has been some water absorbed into the concrete. The tank is approved if the water level is held for one hour.

For example, suppose your engineer has derived a worst-case, installed, applied tank loading of 368 psf based upon the design criteria. Per ASTM C 1227, Section 6.1.2, the performance test load would be 1.5 times this amount, or 552 psf.

To convert this loading value to a partial vacuum into inches of mercury, divide the 552 psf by 71 psf per inch of mercury. The result yields a vacuum test value of about 7.8 inches of mercury.

Then, to satisfy the C 1227 requirements, plant personnel choosing proof of design by performance and watertightness by vacuum would most likely run the tests in reverse order. That is, they would perform the watertightness level test first and then continue the test up to the structural proof level.

Phillip Cutler is the National Precast Concrete Association's director of technical services and is a member of the NPCA TechTeam. For more information,