Watertight Precast Concrete Septic Tanks - MC Magazine July/August 2005 - Concrete Publications

	MC Magazine

Watertight Precast Concrete Septic Tanks

Separating the fact from fiction

By Phillip Cutler, P.E., and Dean Frank, P.E.
Phillip Cutler, a Technical Services Engineer, and Dean Frank, Technical Services Director, are members of the NPCA TechTeam.

Increasingly there is news about the continued pressures of global competition, cement shortages, high fuel prices and record consumption of scrap steel by Far East countries. Certainly many if not all of these factors influence how precast concrete producers conduct business today. To further compound these circumstances, precast concrete septic tank producers must also cope with negative information being pushed by competitors and others with respect to the performance, quality and testing issues related to septic tanks.

Those involved in the on-site wastewater industry have inevitably heard negative information firsthand, such as: “All precast concrete tanks leak, no matter what!” or “A water test to the top of the riser is the only real test for watertightness” or “A vacuum test is not a real-world test.”

Are these and other similar statements based on fact or fiction? Given that there are no absolutes in life, these statements likely have some elements of both embedded in them. The fact that such statements are made indicates that we all need to do a better job of educating the public regarding the benefits of precast concrete septic tanks, determine what can we can do to elevate product quality industry-wide and eliminate the doubts about the superiority of precast concrete septic tanks. To be sure, whenever you hear these types of statements, you should ask: “What facts are they based on, and where is the test data or standard to validate such a statement?”

Leaky tanks
Do the statements regarding leaky tanks stem from the fact that precast concrete tanks have joints in them? Certainly they have been criticized because they do have joints where leakage into or out of the tanks at the joints could occur. Fortunately, specially made sealants are readily available that provide a reliable, long-term method of preventing water from moving through the joint. So just because precast concrete tanks have joints does not mean they all leak.

Do the statements regarding leaky tanks stem from the fact that concrete is a porous material? Just because concrete is a porous material does not mean that water can readily flow through it. There is a difference between porosity and permeability. Porosity is a measure of the proportion of the total volume of concrete occupied by pores, whereas permeability refers to the flow of something (in this case, water) through the concrete under a pressure differential. The fact is that there is no flow of water through good-quality concrete.

Permeability is primarily affected by the connectivity of the pores and capillaries of the concrete and not so much by the total porosity. Since cement inside the concrete continues to hydrate with time, the connectivity of the pores is constantly being reduced and therefore the resistance to moisture permeability improves. In the case of a septic tank, moisture continuously in contact with the concrete is actually improving its strength and durability and reducing its permeability and potential for leakage over time.

Another reason such statements are made may be because if a tank cracks, it may be possible that leaks will occur at that location. As such, it is critically important that the tank is properly designed for the anticipated loading conditions and that it is bedded and installed correctly. It is a fact that watertight below-grade precast concrete structures are quite common. For example, multi-piece utility vaults are routinely utilized to protect costly electronic equipment in below-grade environments. It is obvious that utility and communications companies have a high degree of confidence in precast concrete and its ability to be used in watertight structures.

Today, like manufacturers of utility vaults, producers of quality, structurally sound and watertight precast concrete septic tanks know that properly designed, manufactured and installed tanks do not leak. Unfortunately, not all producers of septic tanks today make a quality product, resulting in situations where someone makes a statement such as “All precast concrete tanks leak, no matter what.” Consequently, all producers are encouraged to consider how their businesses and the industry can improve by embracing a model for producing quality precast products, especially watertight septic tanks (see the sidebar “Making it Right”).

Hydrostatic testing
It has been said that “A water test to the top of the riser is the only valid test for watertightness.” You should ask: “In what standard does this language appear?” Indeed the hydrostatic test is one alternative that can be used to evaluate the watertightness of a septic tank. However, it is certainly not the only valid test method for watertightness, nor is the need to fill to the top if the riser.

A review of existing industry standards reveals the fact that there are several accepted watertightness test methods. ASTM International standard C1227, “Standard Specification for Precast Concrete Septic Tanks,” states that “Testing for leakage is performed using either vacuum testing or water-pressure (hydrostatic) testing.” Further stated in ASTM C1227 are descriptions of these two test methods, the performance criteria for each and the requirements for approval of tanks.

Another industry standard is the International Association of Plumbing and Mechanical Officials (IAPMO) “Material and Property Standard for Prefabricated Septic Tanks” (PS-1), which references ASTM C1227, stating “Concrete septic tanks shall comply with ASTM C 1227, except when the requirements deviate from this standard in which case this standard will govern.” The IAPMO standard contains additional specific language regarding tank sampling and water testing and states, “Sample tanks shall be assembled per manufacturer’s instructions, set level, and water raised to the flow line of the outlet fitting …,” not to the top of the riser.

Finally, the Canadian Standards Association publication CSA B66, “Design, Material, and Manufacturing Requirements for Prefabricated Septic Tanks and Sewage Holding Tanks,” states that “Following the appropriate tests … remove the load or vacuum and fill properly bedded tank with water to its outlet or overflow level …,” not to the top of the riser.

It is important to note that all of these recognized industry standards include provisions allowing either a hydrostatic or vacuum test for watertightness. Given this fact, is the statement “A water test to the top of the riser is the only valid test for watertightness” fact or fiction?

Vacuum testing
We have also heard from time to time that “Vacuum testing is not a real world test.” Is this simply because tanks do not operate in a vacuum or that a vacuum test exerts a uniform pressure on all six sides of a tank, when in reality soil and ground water loads vary with depth? The fact is that a vacuum test can be designed such that it approximates the maximum loads the tank will experience in the ground. Although this is a conservative approach, it does not diminish its validity.

One of the benefits of vacuum testing is that it is a multipurpose performance evaluation – both watertightness and structural integrity can be evaluated during the same test cycle. Vacuum testing is commonly used to check other precast concrete structures, including installed manholes, and is gaining in popularity for checking septic tanks and wastewater-related products. This increase in popularity is primarily because the equipment is easily portable, and the test setup and procedures are relatively simple. For example, most tank delivery vehicles have a ready supply of air that can be used to run the vacuum testing equipment. The test equipment (gauges, hoses, lid, etc.) requires little space to transport to the installation site. Tank manufacturers can easily perform routine testing in the yard and also conduct any required performance testing on site.

Both ASTM C1227 and IAPMO PS-1 standards allow the performance of a vacuum test for watertightness evaluation as well as proof of structural design. For instance, the CSA B66 standard offers a vacuum test as an option for strength evaluation – both physical loading with sand bags and vacuum testing are allowed. Both tests are performed for approximately one hour and then the tank is checked for deformation and leakage. The strength testing is then followed by a watertightness test.

Again, we see that all three industry standards recognize the vacuum test as a perfectly viable performance evaluation method. It is certainly considered to be a “real world” test by these standards bodies.

Future offerings
Misconceptions and negative information regarding precast concrete septic tanks are not limited to watertightness issues. In the future, we will be covering such statements as: “New septic tanks should be filled with water prior to being put into service.” Again, where is the documented evidence that filling a septic tank improves startup? If a hydrostatic test is already taking place at the site, is this just an excuse to leave the water in the tank? Also, is it just a coincidence that some tanks require some amount of water to be in the tank during backfill operations? After a septic tank is put into service, it receives everything necessary to start and operate efficiently and effectively without being filled with water ahead of time. The water does not hurt anything, but certainly it is not required for the tank to function properly.

So whenever you hear statements such as those discussed in this article, ask questions. “What physical principle, scientific data or standard is available to substantiate or validate them?” Also, and maybe more importantly, be sure that you produce watertight tanks and your operations are not perpetuating these statements. Then you can be the judge of whether they are fact or fiction.

If you would like to contribute additional “facts or fiction” information, please send them to technical@precast.org. Your feedback is welcome.

For more information on watertight precast concrete septic tanks, please contact NPCA at (800) 366-7731.