Concrete Septic Tanks Are Watertight
The article, “Finishing Touches,” in the June Onsite Installer included information about backfilling tanks and watertightness and stated “Tanks with mid-wall seams have a high probability of not being watertight.”
We would like to address this statement by clarifying a few things. Watertightness of an underground tank depends on the answers to three fundamental questions:
• Is it strong enough to withstand all the anticipated earth loading and hydrostatic pressures, whether it is empty or full? If not, it will begin to fail and collapse, and watertightness becomes the least of our problems.
• Is the material itself able to stop fluids from entering and exiting the tank?
• Are the joints and connections configured and sealed so as to prevent the ingress or egress of fluids?
Precast concrete septic tanks should be designed and built according to ASTM C1227–Standard Specification for Precast Concrete Septic Tanks. That standard refers to 25 other ASTM, ACI and ANSI standards and contains requirements for material quality control, manufacturing, sealants and connectors, structural and physical design, and performance test methods, including vacuum testing.
ASTM C1227 does not contain any reference to the location of the joint. Why? Because a mid-seam joint or a top-seam joint will be equally effective in keeping the tank structurally sound and watertight. Mid-seam joints are configured in an interlocking configuration, such as a tongue-in-groove or lap joint, that will prevent lateral movement and maintain structural strength. The joints must fit tightly — within 3/8 inch. When a high-quality joint sealant is applied properly to the joints, the result is a watertight seal.
Regardless of the seam’s location, precast concrete septic tanks must be manufactured with high-quality raw materials in a controlled environment with an emphasis on quality control. Pre-pour inspections ensure that reinforcement is properly sized and placed. Post-pour inspections ensure that there are no cracks, voids or any other defects that may diminish the structural capacity of the tank.
Proof testing will simulate field conditions. ASTM C1227 stipulates two methods of verifying that the tank is watertight and strong enough to counter anticipated forces: hydrostatic testing and vacuum testing. They are performed according to detailed requirements so that they simulate actual anticipated loads.
Many NPCA members manufacture mid-seam septic tanks, and specifiers and homeowners know these tanks are reliable and strong enough to endure exposure to varying loads and conditions. That is why they are still widely specified and used today across the country. For more information on the manufacturing, quality control and installation of precast concrete septic tanks, please refer to our Onsite Wastewater Tanks Best Practices Manual at www.precast.org/onsite_bpm.
For further questions, please visit NPCA’s website at www.precast.org, or contact our technical department at 800/366-7731 or technical@precast.org.
Claude Goguen, P.E., LEED AP
Director of Technical Services
National Precast Concrete Association
