Progression of Performance Testing of Precast Concrete Onsite Wastewater Tanks

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1 Progression of Performance Testing of Precast Concrete Onsite Wastewater Tanks Aaron Ausen 1 and Claude Goguen 2 Background According to the EPA s (Environmental Protection Agency) Septic System Fact Sheet, the decentralized onsite wastewater industry continues to grow. In 2007, an estimated 20 percent (26.1 million) of total U.S. housing units were served by septic systems. This is an increase of 1.54 million septic systems since 1985., (EPA Web Site As the decentralized wastewater system grows, so does the responsibility of providing efficient quality systems while protecting our natural resources and public health. There have been many advances in the field of treatment; however, all this technology is redundant without a durable watertight vessel to contain it. Precast concrete is the most widely used material for manufacturing onsite wastewater structures including septic tanks. Not all concrete is the same. Concrete is a composite of multiple materials properly designed and proportioned to produce a quality output. What is the same however, is the expectation of a structure that will withstand severe stresses and environmental conditions on the inside and outside for many years while keeping wastewater in and groundwater out. Specifiers and end-users expect a structurally sound and watertight tank. The current testing standards address those requirements. Many states such as Florida, Minnesota and New Jersey as well as counties and have been or are beginning to adopt requirements to specify performance testing of precast concrete septic tanks. There exists some uncertainty about which tests or standards to adopt and how, when and why these tests should be performed. Nearly one in four households in the United States depends on an individual septic system (also referred to as an onsite system) or small community cluster system to treat their wastewater. (EPA Web Site EPA concluded in its 1997 Response to Congress that "adequately managed decentralized wastewater systems are a costeffective and long-term option for meeting public health and water quality goals, particularly in less densely populated areas. There are many factors that go into a properly functioning decentralized wastewater system. This paper will focus on the precast concrete structures that serve as septic tanks, pump tanks, aerobic treatment chambers, distribution boxes or any other containment system. Well-built onsite wastewater tanks are remarkable engineering accomplishments. These are tanks that must withstand various loading conditions from all sides, keep groundwater and surface water out, while keeping wastewater in, and do all this while many times harboring an aggressive exposure environment. 1 Aaron Ausen, Vice President, Dalmaray Concrete Products. 405 South Arch St. Janesville, WI aaron@dalmarayconcreteproducts.com 2 Claude Goguen, Director of Technical Services, National Precast Concrete Association City Center Dr. Suite 200, Carmel, IN, cgoguen@precast.org

2 There are many reputable precast concrete manufacturers capable of producing a quality structure that will accomplish previously mentioned feats. However, how does an inspector or an owner obtain assurance that the tank will perform as needed? Putting the structure in a simulated environment that exceeds anticipated conditions can provide that quality assurance. Testing of precast concrete structures has been performed for many years; however, the testing methods have evolved. It is important for the authority having jurisdiction and the owner to know what tests should be performed, and more importantly, how are they done properly. In testing a precast concrete wastewater structure, there are mainly two objectives to validate: its ability to withstand stresses from dead and live loads, and its ability to retain water. ASTM (American Society of Testing and Materials) C1227 Standard Specification for Precast Concrete Septic Tanks. has served as the standard for precast concrete onsite wastewater structures since The standard has always contained requirements on load testing and water tightness testing. Proof of Design In section 6.1, under Structural Design Requirements, ASTM C1227 currently states, Structural design of septic tanks shall be by calculation or by performance. ( Design by calculation must be done by Strength Design Method or by Alternate Design Method as outlined in ACI (American Concrete Institute) 318. Both types of proof of design contemplate all applicable site, soil, dead load, live load and surcharge loading conditions and should be performed by qualified engineering personnel. Performance testing must be done to show that the tank will not fail under loading that is at least 1.5 times the anticipated actual loads. Section 6 goes on to stipulate live and dead loads to be used. Many jurisdictions specify loads that are in excess of those in ASTM C1227. These jurisdictions may justify this additional loading due to particular soil type or increased burial depth to keep tank below frost line. However, safety factors are used to account for these circumstances and often making the additional loading requirements unnecessary. The designer or person performing the test must be aware of the applicable required loading conditions. Those in section 6 can be used when no loads are specified locally. 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 below) 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. A vacuum test is sometimes referred to as an air test, but it s actually a lack of air test. The process works by removing air within the tank introducing negative pressure in comparison to the atmospheric pressure on the outside of the tank.

3 The tank should rest on a yielding foundation (i.e. sand bed) to ensure that the tank dead load does not control the bottom slab design. The top and bottom slab may have a larger structural capacity than the side walls. Vacuum pressure is applied equally to all surfaces. In order to increase the allowable capacity of the top and bottom slabs over and above the capacity of the side walls, additional uniform loading can be added to the top slab. Figure 1 Proof Testing the Tank Prior to Installation Some plants will use sandbags or other means to simulate additional loading on the top slab during performance testing. It is important to note that these tests are intended to be performed prior to backfill. Introducing soil and perhaps water around the outside of the tank can significantly impact the amount of vacuum to pull. Trying to conduct the same test with the same vacuum pressure when the tank is backfilled could damage the gaskets, the tank itself, and pose a safety risk to personnel nearby. Watertightness Testing When building vessels for the treatment of wastewater, watertightness is of primary concern. Leaky tanks not only reduce the effectiveness of the system to do its job in mitigating the harmful constituents in the wastewater, it also poses an health and environmental risk. Concrete is inherently porous, but with good raw materials and production practices, can be made dense enough to resist water ingress far in excess of the tank s service life. Watertightness is not

4 dependent on porosity, but rather on permeability. 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. Hydrostatic testing The hydrostatic test is one of two alternatives that can be used to evaluate the watertightness of a septic tank. A review of existing industry standards reveals the fact that there are several accepted watertightness test methods. ASTM C1227 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, which references ASTM C1227, stating Concrete septic tanks shall comply with ASTM C1227, except when the requirements deviate from this standard in which case this standard will govern. (IAPMO PS ). 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, (IAPMO PS ) not to the top of the riser. Finally, the Canadian Standards Association publication CSA (Canadian Standards Association) 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, (CSA B Design, material, and manufacturing requirements for prefabricated septic tanks and sewage holding tanks) There are some authorities that require the water level be at the top of the riser section. This is not indicative of the actual anticipated loading during service unless the system experiences a serious failure. Filling to the lowest invert is recommended. Generally, the test involves filling the tank with water up to the specified level, leave it for 24 hours for any initial absorption to take place, and then refill it to the original level. This level must remain for 1 hour for the test to be considered successful. There are many variations to this test, but the majority includes these parameters. Vacuum testing 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.

5 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. 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 including any inlet and outlet pipes and applying a vacuum (negative pressure) level of 4 inches of mercury for five minutes (see Figure 2). A tank passes if 90 percent of this level is held for two minutes. If the pressure should drop below 4 inches during the test, the pressure is returned to 4 inches and the 5 minute test starts over. This has changed over recent years, the previous requirement was to hold 4 inches for two minutes with an allowable 10% drop. Figure 2 Vacuum Testing for Watertightness Example of both tests performed The engineer has derived a worst-case installed applied tank loading of 368 psf based upon a soil cover of 2 feet. 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.

6 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. Figure 3 Conversion Table for Proof of Design Load to Inches of Mercury New Standard ASTM International s new ASTM C1719, Standard Test Method for Installed Precast Concrete Tanks and Accessories by the Negative Air Pressure (Vacuum) Test Prior to Backfill, was developed under ASTM s technical sub-committee C27.30, which falls under the main ASTM Committee for precast products, C27. Prior to the release of the new standard, precast manufacturers, engineers, specifiers and regulators could rely only on a manufacturing reference for testing watertightness of precast concrete septic tanks ASTM C1227, Standard Specification for Precast Concrete Septic Tanks. The new C1719 standard contains elements of C1227 for septic tanks and is also similar to the previously developed ASTM C1244, Standard Test Method for Concrete Sewer Manholes by the Negative Air Pressure (Vacuum) Test Prior to Backfill. ASTM C1719 is different from C1244 in that it provides a vacuum test method for installed precast tanks and accessories as an installed system. The scope of the new C1719 standard covers tanks for onsite wastewater treatment and storage, grease interceptors, grit/oil separators, water storage and other applications requiring watertight construction and installation.

7 Test equipment and method The equipment required to perform a negative air pressure (vacuum) test is not sophisticated. As stated in section 6 of the C1719 standard, the testing equipment consists of pumps, vehicle vacuum devices and even a high-performance shop-style vacuum cleaner. Most devices are easily operated and the test can be performed in a few minutes using the vacuum method, unlike the hours or even days needed to perform the same test with water. Like ASTM C1227, C1719 tests apply a vacuum to the sealed system to a level of 4 in. of mercury (Hg) for a duration of five minutes. If the gauge holds without pressure loss, the system passes. If the vacuum level drops, it is reapplied and the test is restarted. A tank or system failing to hold vacuum may be repaired per the manufacturer s recommendations and/or evaluated for not holding negative pressure and retested following such repair. On Site Testing vs Plant Testing Most producers of precast concrete septic tanks conduct watertightness and proof of design testing at their plant prior to shipping. When testing at the plant is done correctly and verified by an engineer, a qualified quality control technician or through a third party certification program, this should provide assurance to the buyer and/or jurisdiction having authority as the integrity of the structure. Conducting these types of test on site, will result in higher costs and delays. Also, the tests referenced in this paper are all designed to be executed prior to the tank being backfilled. Performing a vacuum test on a tank after backfill can damage the structure and components and pose a safety risk for personnel if proper adjustments are not made to the vacuum level. These adjustments must take in account soil and groundwater pressure. Vacuum testing these structures after backfill is not recommended as these adjustment values can be difficult to determine and errors can be costly. Conclusion Vacuum testing, hydrostatic testing, and proof of design by calculation provides assurance to the owner or regulator that the precast concrete tank was designed and manufactured to resist inservice loads plus a safety factor to include unusual circumstances. This is but one of many steps in building a wastewater treatment system; however, it is crucial to the operation of this system. That is why the proper testing procedures and equipment must be used. In the absence of local specifications, ASTM C1227 and C1719 should be consulted as they contain proven and trusted methods of tank testing. It is important to keep in mind that testing is an instantaneous measure and does not imply long term durability of the structure. A tank can be designed and manufactured to the highest standards. It can pass tests with flying colors. However, if the tank is installed improperly, without proper bedding or proper backfilling practices, the prior efforts are wasted, as this will impact the performance and longevity of the tank, and the system. Therefore, testing should not be the sole means of quality assurance, but rather a component of an effective and thorough quality assurance program.

PRECAST CONCRETE ON-SITE WASTEWATER TANKS

PRECAST CONCRETE ON-SITE WASTEWATER TANKS OUTLINE Purpose Precast Advantage Design Goals Applicable Standards Structural Design Materials Production Watertightness Installation NPCA Plant Certification