Watertightness Critical to Septic Tanks - MC Magazine Fall 2000 - Concrete Publications

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Watertightness Critical to Septic Tanks

As the demand for quality septic tanks rises, precast companies are required to prove their septic tanks are watertight to ensure consumer safety and compliance with environmental regulations.

Currently, precast concrete septic tanks are being used for on-site water disposal for more than 40 million households in the United States. As a result, watertightness is now a critical characteristic of precast septic tanks and has heightened environmental awareness to the point that regulators, specifiers, and departments of health nationwide are demanding watertight structures.

Precast concrete septic tanks are considered the most structurally sound, which makes them a common choice for primary treatment of on-site sewage because of their durability, strength, performance, and long service life, all of which are important to a tank’s most significant feature: watertightness. A precast septic tank can be made watertight through proper planning during the engineering and design, manufacturing, installation, and quality assurance processes.

Engineering and Design
When designing a watertight precast concrete septic tank, specifiers should account for a variety of factors, including regional weather elements, site conditions, appropriate tank geometry and type, and the stresses that will be placed on the tank before and during service.

Pre-service stresses include stripping, handling and storage, leakage test, and backfilling, while in-service stresses consist of surface surcharges (e.g., snowfall), periodic wheel loads from passing vehicles, buoyancy and hydrostatic forces from a rising groundwater table, and lateral soil pressures.

To attain watertightness in a septic tank, experts recommend that the product's design follow the American Society of Testing and Materials (ASTM) C1227, Standard Specification For Precast Concrete Septic Tanks, or NPCA's Septic Tank Best Practices Manual, which recommends a minimum wall thickness of three inches and a minimum compressive strength of 4,000 psi at 28 days. Inserts for lifting devices should be designed with a minimum safety factor of four.

The most common types of precast concrete septic tanks are one-piece with a separate lid, two-piece, and monolithic, which are one continuous unit, including the lid. There are benefits and drawbacks to all three types: Monolithic tanks are best for regions with severe site conditions but are heavy; two-piece tanks are comparatively light but are more difficult to handle; and one-piece tanks are lighter than monolithic tanks but heavier than two-piece tanks.

Batching and Mixing
The first step to producing a watertight precast concrete septic tank is to select quality raw materials, which include aggregates, cement, water, and sometimes mineral and chemical admixtures.

Aggregates used for watertight structures should conform to ASTM C33 and must be well-graded, sound, non-porous, and free of substances that might promote the deterioration of hardened concrete. Well-graded aggregate composition within the hardened concrete minimizes the voids between aggregate particles and reduces permeability.

Mineral admixtures such as blast furnace slag, fly ash, and silica fume can be added to reduce the permeability of hardened concrete. Chemical admixtures can enhance the properties of fresh and hardened concrete, air-entrainment (at less than 7 percent) increases resistance to cycles of freeze-thaw, and water-reducing agents help maintain a mix's appropriate water-cement ratio.

ASTM C150-compliant cement should be selected and used based on performance to suit regional conditions and manufacturing operations. The water used to batch concrete must be potable and free of visible suspended solids and organic materials.

To ensure a septic tank's watertightness, the mix design—the most critical concern for precasters—requires low water-cement ratios, high concrete strength, well-graded aggregates, and proper consistency. Imperative to a septic tank's watertightness is a minimum 28-day compressive design strength of at least 4,000 psi, aggregates that are well-graded to achieve higher densification, and a concrete consistency that is workable to achieve proper consolidation.

The most significant of these elements is the water-cement ratio, (water-to-cementitious ratio if using mineral admixtures), which should be no higher than 0.45 to produce a strong, hardened concrete that will not let water pass through easily. Water should have a pH greater than five and should be carefully monitored during batching to reduce the possibility of using excess cement.

Brookline Concrete Products, Brookline, Ontario, which uses a mix formula of over 5,000 psi for overnight stripping strength, has successfully used electronic moisture detecting systems in each of its four batch plants since 1990. “The bottom line is that you want the concrete to be consistent batch after batch, and that’s what it does,” says Chris G. Radjohn, company controller. “No matter what kind of aggregate comes in, you can get the same concrete going out.”

Norwalk Concrete Industries, Norwalk, Ohio, which has been producing 500- to 2,000-gallon standard septic tanks since it was founded in 1906, also uses 5,000 psi concrete because it's richer in cement content and higher in design strength, preventing it from being damaged during shipping or in yard storage. This is a critical stage for septic tanks that can result in stress cracks if the mix design is not of premium quality.

Since Front Range Precast Concrete, Inc., Boulder, Colorado, stopped producing its own mix in 1994, the company uses a ready-mixed formula from a company two miles away, an approach Doug Jatcko, president, finds to be efficient and cost-effective by saving one hour of labor every day.

Using the right mixing formula, however, is no guarantee that every now and then problems won’t occur—even with the most well-made septic tanks. Consolidation is also an important factor in achieving watertightness and should immediately follow the placement of concrete using internal stinger vibrators or external vibrators that attach to the formwork.

Proper vibration allows all entrapped air to rise to the surface without promoting segregation, making a smooth, dense concrete surface. Poor consolidation from insufficient vibration produces entrapped air voids and areas of honeycombing that allow water to pass through the concrete structure. Excessive vibration segregates the constituents, resulting in a non-watertight structure. “If you pour good concrete but don’t have proper consolidation during vibration, water might migrate through the cement, creating voids,” explains John Lendrum, president of Norwalk.

Reinforcement and Fiber
Septic tanks are continually exposed to various load conditions, causing the need for adequate compression and should be determined by design. This is the main reason precast concrete septic tanks are more effective and stronger than cast-in-place tanks, according to Terry Collins, concrete construction engineer for Skokie, Illinois-based Portland Cement Association.

"With poured-in-place, it's more difficult in the field to control the location of reinforcement, which results in higher costs," he explains. "Precast gives you better control in the manufacturing process because it's repetitive and in a controlled environment." To prevent corrosion, reinforcing steel should be completely embedded in concrete, free from form oil.

While using reinforcing fiber helps increase a tank's durability by reducing shrinkage cracks and surface chipping, it should not replace structural reinforcement cages. Lendrum, whose company has been adding polypropylene fiber to its concrete mix for the last seven years to improve durability and workability, agrees. “Fiber is good for preventing chipping and minor cracks," he says, "but when you’re talking about loading, I believe you should use structural reinforcement.”

The Curing Process
The curing process begins once concrete has been cast into formwork and involves two important factors: Maintaining concrete moisture and constant concrete temperature. The most common practice for maintaining moisture content is by either applying plastic coverings or a curing compound. Concrete moisture can also be maintained by wetting, fogging, or spraying. Both methods are effective and allow for reduced man-hours and next-day stripping.

A precaster's choice of curing methods should be contingent on availability, production facilities, aesthetics, and economics. If plastic coverings are used, they should overlap about 18 inches to prevent water loss and must comply with ASTM C171, which specifies a minimum thickness of four millimeters. Curing compounds can be applied after stripping the forms.

In hot weather, when the concrete temperature is between 90 (F) and 150 (F), shading the aggregate pile and cooling the mixing water are viable solutions, as is heating the water and aggregates to increase the concrete temperatures in cold weather. Once the concrete has reached a minimum 500 psi, freezing has limited impact. For next-day stripping, when the concrete temperature is >50 (F), the concrete should have reached the minimum compressive strength.

Sealants, Fittings, and Joints
The most likely area for leakage in a septic tank occurs at the joints and fittings, making it extremely important that all joints are intact and clean, properly sized sealants are used, and that all fittings are flexible and watertight.

Tongue-and-groove joints and lap joints are the most common and are effective for watertightness because they leave an annular space to be occupied by a watertight sealant. Watertight bitumen or butyl rubber sealants must adhere to the performance requirements in section 10 of ASTM C990, Standard Specification for Joints for Concrete Pipe, Manholes, and Precast Box Sections Using Preformed Flexible Joint Sealants.

Sealants also must work well within a wide range os temperatures, adhere to clean, dry surfaces, have a proven performance over time, resist shrinkage, and be pliable enough to compress a minimum of 50 percent within the annular space of interlocking joints. "If you use too much sealant in a joint," Lendrum says, "you will negatively impact the sealant capability."

To be watertight, sealants must be applied as a continuous ring around the tank, spliced parallel or side-by-side by working the ends together by hand. Mechanical fasteners or secondary pour of lids on bases might be necessary to promote watertightness in areas with severe site conditions. Because fittings and connections are a possible significant source of leakage in the presence of differential settlement, Lendrum advises using flexible joint sealants that contain less than 3 percent volatiles as defined in ASTM D6 A.

To stress the importance of proper sealant application, Front Range holds demonstrations for contractors and counts on its drivers to remind contractors how to properly apply sealants. “Don’t cut the polyethylene gaskets with a pocket knife,” Jatcko instructs. “Take it out with a blunt instrument and rip it along the edge; don’t try to cut along it. If you skip any steps, you’ll defeat the system.”

Quality Control and Testing
All precasters should establish a thorough quality-control program for the production of septic tanks to inspect the work done after each manufacturing process to continually ensure tank watertightness.

A key factor to Norwalk’s success in producing watertight septic tanks is consistent quality-control checks. “Every day, strength cylinders are taken, so we know what the strength of our concrete is and what impact the weather is having on it,” Lendrum says.

Pre-pour operations should include checking to make sure forms, reinforcement, and other embedded items are proper and should verify that the forms are clean and properly set, the reinforcement cages are stable and properly spaced, blockouts are secure, and lifting apparatus are properly positioned.

Once precast septic tanks reach designed handling strengths, they must be removed from forms and inspected for conformance to fabrication drawings. Some things to look for include surface defects (honeycombing), cracks, damaged lifting devices, and exposed reinforcement.

Quality control testing for compressive strength of cylindrical concrete specimens must be performed and recorded on test cylinders for every 150 yards of concrete poured, according to ASTM C39. Leakage testing must be conducted by randomly selecting one out of every 20 septic tanks.

Testing for watertightness varies with local jurisdictions. Using the ASTM C1227 performance testing method, either vacuum testing or water-pressure testing procedures can be used. Vacuum testing verifies that a tank is watertight if it holds 90 percent of a two-inch vacuum of mercury for two minutes.

Water-pressure testing determines a tank's watertightness by maintaining a certain water level for one hour after a 24-hour absorption period. Both methods can be performed at the job-site and should include testing of inlet and outlet connections and pipe joints. Properly designed and manufactured precast concrete tanks will always pass watertightness tests, regardless of the number of joints.

Although regulations for sealants don’t exist, watertightness and loading testing are required, according to industry standards. At Front Range, they wholeheartedly agree with testing for watertightness and challenge all precast producers to do the same. “If you don’t test. How do you know?” Jatcko asks.

As with most precast companies, though, Norwalk does not test every tank for watertightness. “We’ve done enough vacuum testing of products that we’re confident we’re producing a watertight tank,” Lendrum contends.

Installing a Watertight Tank
The careful transportation of precast concrete septic tanks to job sites is also essential to ensure watertightness. Precast tanks should only be lifted with manufacturer-supplied lifting devices. When placed on flatbeds, concrete tanks should be secured so that the structural or watertight integrity of the tanks is not compromised.

Excavation sites need to be accessible to trucks weighing up to 80,000 pounds, and trucks must be able to get within three to eight feet of the site for unloading. Sites with silty soils, high water tables, or other problem characteristics must have specially designed bedding and bearing surfaces. Correct compaction of the underlying soil and bed is required to ensure that there is no differential settlement. Tank inlet penetrations should face the residence and the tank must be level.

Backfilling must be placed uniform with mechanically compacted layers less than 24 inches thick and must be free of debris. Each layer should be adequately compacted. To prevent flotation, fill the tank with water, place soil on top of the tank, or keep water pumped out until backfilling is completed. In very cold weather, it might be necessary to insulate the tank before backfilling.

The Future of Precast Septic Tanks
Well-designed and manufactured precast concrete tanks will be watertight (whether monolithic, one-piece, or multiple-pieces) and provide long-term, reliable performance as long as they're connected with flexible fittings, sealed with ASTM-compliant sealants, and handled, transported, and installed properly.

Precast concrete manufacturers can continue to produce high-quality, watertight, and structurally sound precast concrete septic tanks by consistently complying with standard specifications. “As precasters, we have to face the fact that our customers and regulators demand watertight, durable precast concrete structures," Lendrum says. "If we’re not willing to meet those rules and supply those types of tanks, plastic and fiberglass manufacturers are going to do it for us. The potential for future growth is tremendous.”

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