By Phillip Cutler, P.E.
What are the criteria that satisfy the requirements of a structural proof of design test and a watertightness test for a precast concrete on-site tank? Ask 10 different people in the on-site industry (regulators, specifiers, health officials, precasters and sanitarians) and you will probably get 10 different answers. There may not be a huge variance in their responses, but the answers will be different for many reasons. How can you sift through all the information and end up with the right answer? Let’s have a look at the standard. In sections 6 and 9 of the ASTM C1227, “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 C1227 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 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 ASTM C1227 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 (see Figure 1 above). Proof testing is to be performed in such a way as to simulate the actual anticipated 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. The testing should be witnessed and signed by a qualified engineering professional.
For a leakage or watertightness test, Section 9 of the C1227 standard states that this test can be performed by using either vacuum testing or water-pressure testing. The vacuum test is performed by sealing the empty tank and applying a vacuum (negative pressure) level of 4 inches of mercury for two minutes (see Figure 2 at right). A tank passes if 90 percent of this 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 24 hours, 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.
Let’s take a look at an example. Suppose your engineer has derived a worst-case installed applied tank loading of 368 psf based upon the design criteria. Then by example, per ASTM C1227, Section 6.1.2, the performance test load would be 1.5 times this amount, or 552 psf. A conversion of this loading value to a partial vacuum into inches of mercury is accomplished by dividing the 552 psf by 71 psf per inch of mercury (Figure 3). The result yields a vacuum test value of approximately 7.8 inches of mercury. Then to satisfy the C1227 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.
Plants that apply for or participate in NPCA Plant Certification and On-site Accreditation must perform, pass, document and maintain records for structural proof of design and watertightness for on-site tanks to satisfy the requirements of these programs. For more information on these programs, visit NPCA’s Web site at www.precast.org.