MC Magazine

Making It Right

Let’s consider a general outline for quality – it is really rather simple. The first thing to consider is a proper structural design that begins with load calculations and careful consideration of any specific or special requirements imposed by the local authority having jurisdiction. Items such as burial depth, water table location, physical size, connections, risers and auxiliary equipment may be referenced in local codes for on-site systems and must be accommodated in the design.

We then move on to consider the reinforcement methods, materials and section dimensions. These factors must be properly determined in order for the structure to resist the anticipated loads. One important consideration is the minimum wall thickness. For reference, the minimum wall thickness recommendation as published in the “NPCA Septic Tank Manufacturing Best Practices Manual” is 3 inches. However, it should be noted that this is a minimum recommendation, and that engineering design calculations and performance testing may indicate additional wall thicknesses are necessary for a given tank configuration and loading scenario.

Once a properly reinforced structural design has been established, it is critical that a good concrete mix design be developed in support of it. While somewhat variable, the basic elements of proper proportioning consist of using clean, sound and properly graded aggregate, low water-to-cement ratios (typically less than 0.45), a minimum 28-day compressive strength of 4,000 psi, and have a consistency or workability suitable for proper placement and consolidation in the forms by conventional means.

The next step covers proper manufacturing methods and practices. It’s a fact that we can develop an optimal design and purchase high-grade raw materials, but if they are put together incorrectly the result will be a questionable structure that may not hold water. Each and every batch of fresh concrete must be mixed, discharged, placed, consolidated and cured using quality operations. This means all prepour and post-pour processes are appropriate, controlled, repeatable, reproducible, verified, validated and documented. Forms must be clean and have tight joints. Ensure that those product areas that ultimately become the sealing joints in the finished product are free from surface defects and are within tolerance.

We also cannot neglect cast-in components such as lifting devices and inlet and outlet boots/seals, as they certainly have an affect on the final performance of the tank. Methods used in stripping the forms, handling and storage, transportation and final delivery of product all impact the ultimate performance of the delivered tank.

As a final step in the process, verification of product performance should be performed. Both compressive strength testing of hardened concrete samples and watertightness testing of finished tanks should be performed. Regular compressive strength tests on cylindrical specimens as part of an established quality control plan are critical. The minimum number and frequency of specimens to be tested should be clearly spelled out in the plant’s quality control plan. Compressive strength test is verification that the concrete mix design meets the minimum strength requirement for use in the product being cast.

Watertightness testing is necessary to ensure that the final product will perform as required. Septic tanks and other wastewater treatment tanks must be watertight. If you do not test your tanks, how can you be sure that they will perform as they should?

Remember: It’s not just a septic tank that is buried and forgotten. It is a quality, structurally sound and watertight tank manufactured in your plant with pride using quality materials in a quality process. You must make it right.

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