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Poly vs Pre-cast concrete OSD tanks

4 Myths about plastic tanks that could be costing you money.

For over ten years, Australian Tanks has been at the forefront of OSD tanks (on-site stormwater detention systems). 

When we first began, engineers would mostly consider in-situ block and slab installation; previous experience suggesting it was the leading and most practical way to achieve maximum efficiency. But block and slab came with its limitations and the industry soon embraced the concept of pre-cast detention systems as the preferred option. 

Australian Tanks continued to work with engineers to design and develop solutions that best meet their needs. Today there’s no doubt that pre-cast onsite detention systems can minimise risk, provide a cost-effective solution and offer quick and efficient installation to maintain critical path activities. 

With the move from in-situ to pre-cast, alternative players have emerged in the market, predominantly using HDPE (High Density Polyethylene) and other ‘plastic’ systems. This has led to some general misconceptions regarding installation, cost-efficiency and durability of pre-cast concrete solutions when compared to these systems. 

To dispel some of the myths on the topic of concrete OSD tanks vs plastic OSD tanks we’ve put together this list. The following information to assist our customers making decisions on what system best suits their client’s needs.

The Parthenon OSD tank
The Parthenon OSD tank system being installed.

Myth 1: Polyethylene OSD tanks are quicker to install

One of the key marketing messages of poly systems is the quicker installation time. And when compared to in-situ block and slab systems, this can be a valid argument. However, our experience has shown the following needs to be taken into consideration when comparing installation times between pre-cast concrete and poly:

  • Project footprint: Due to the shallow internal height of HDPE structures (max. 1.2m), poly installations need a greater overall footprint – on average, 2x the designed footprint size. With a greenfield installation, this may not be a cause for concern, however, when installing on a brownfield site, time and cost considerations need to include:
    • Relocation of above and/or below-ground services in the expanded footprint
    • Creating a safe environment to redirect pedestrian and vehicular traffic in highly populated areas
    • A redesign of the detention system by consulting engineers accommodating the reduced head height 
  • Structural stability: When construction is complete and the pavements are laid and finalised, the site often requires high-traffic loading capabilities. Poly systems rely heavily on the relation between structural granular backfill and the structural integrity of the polymer unit for stability and durability. Sufficient time must be included for sourcing, placing and consolidating these gravel layers and columns. 
  • Base slab requirements: Installing a system with a structural square base such as the Australian Tanks Parthenon, allows the units to butt up against each other, forming modular units that can be installed to most standard design configurations. With the structural base already in place, there’s no need for an in-situ base slab, so you save time. Also, you’re not installing directly onto the earth, thereby creating a more controlled and stable outcome.
  • Laydown areas: Pre-cast concrete tanks save time and space as they do not require a dedicated laydown area. The units are delivered to the site, craned into position and set in place as required. There are minimal workers in the excavation, which is then backfilled immediately, ensuring a rapid turnaround to maximise the use of the total development footprint. It is routinely found that an 800kL tank system can be delivered, installed and ready for backfill within 1-2 days. 
  • Installation duration: As mentioned in the previous point, it’s common to see large storage systems installed and completed within 1-2 days. Experienced contractors throughout Australia understand the value in minimising the time an excavation is exposed to inclement weather; be it 2-3 days with pre-cast concrete or 2-3 weeks with block and slab. 
  • Coating systems: Where appropriate (i.e., septic tanks), coating systems can be partially installed prior to arrival. This can speed up the installation and commissioning process onsite. 
Truck driving over storm water detention tank
Australian Tanks Parthenon system is immediately trafficable

Myth 2: Poly OSD systems are more cost-efficient

When it comes to evaluating the costs of installation, it is essential stakeholders consider the full life cycle of the system, not just the supply and delivery. When measured only on a cost-per-unit criteria, poly will seem to be the cheaper option when viewing the industry averages of cost per sqm. 

However, along with supply and delivery, the following project costs need to be factored into the final decision-making process:

  • Total installation costs: Poly solutions necessitate geotextile and plastic wrapping, impermeable liners and crushed stone for backfill. As these are essential for the installed units to be structurally stable a thorough assessment requires that these (and their associated costs such as labour and open excavation time) are included as part of the price per unit cost.
  • Whole-of-life costs: One of the biggest concerns clients have when discussing detention systems is accessible after installation. Australian Tanks include 900mm x 900mm openings as standard to allow for maintenance and repair works if needed. The internal height of the concrete tank systems better aids in allowing maintenance to be performed internally, often without needing confined space training. The flexibility in manhole and opening sizes allows larger maintenance equipment access to the tanks.
  • As a rule, poly tanks require the use of small gravel for their layering system to provide structural integrity and this is known to cause clogging. The inability to access tanks after installation to clean out any gravel, or only insert a small pipe to extract sediment, means there is difficulty efficiently maintaining the system or repairing it if damaged. 
  • Additional treatment systems: When future-proofing designs, detention solutions often need the flexibility to include additional treatment systems to be fit for purpose. Therefore, Australian Tanks design solutions with an internal height of up to 3.2m. This allows for the additional treatment systems to be located within the main detention tanks, rather than as an external attachment. This can mean fewer manhole access points and reduced interconnecting pipework leading to a more integrated and overall, aesthetic system.  
  • Additional footprint requirements: As mentioned previously, for poly tanks to meet stormwater requirements, they often require a footprint twice as large as pre-cast concrete installations. To accommodate this, concept designs may require significant changes that increase the overall project cost, requiring redesign and changes to approved plans.
  • Compatibility: Ensuring the proposed system can utilise standard filtration systems.

Myth 3: Poly OSD systems are more durable

While both poly and concrete tanks offer significant structural and design warranties, the resilience and strength of reinforced pre-cast concrete cannot be overlooked. This proves particularly advantageous when the area requires high-volume or high-load vehicles.

The increase in poly installations has also seen an increase in incidents where the tanks have been unable to support specified vehicle loads. While this may not be the result of the structural integrity of the tanks themselves (but from the quality of gravel and substrate needed for ground stability), it does call into question the durability of the overall system.

Other points to consider when assessing durability:

  • Concrete pre-cast tanks are typically designed to withstand high traffic loads. This can be achieved over a very large footprint that suits installation beneath industrial laydown areas, carparks and beneath or adjacent roads. 
  • Every OSD project is different. Requirements for incoming and outgoing services may require penetrations in different or multiple locations. Various arrangements of sumps, pits, manholes and weirs can be accommodated without compromising the strength and integrity of the system. 
  • Finally, there is flexibility in how concrete is reinforced, depending on the type of tank and its use. Australian Tanks products use steel reinforcing bar and mesh, as well as plastic fibres, which can offer improved durability in environments where a more onerous exposure classification is required. 
  • Pre-cast concrete has a greater ability to handle flotation generated by high water tables. Plastic systems often rely on the introduction of large volumes of concrete to counteract these effects, while the inherent heavy nature and other methods of preventing flotation, make concrete OSD systems a wiser choice. 

Myth 4: Poly systems are better for the environment

While poly tanks are constructed from recycled plastic and should be fully recyclable, if they are damaged or removed from the site, they can only be recycled into non-structural products due to the inconsistencies in their mechanical properties. This, coupled with a lack of facilities to recycle the poly, means in most instance, the tanks are transported to land fill. 

In contrast, turning concrete into recycled aggregates is now common practice, with regulatory bodies such as Austroads providing specifications for the use of recycled material in construction. Fly ash (a by-product of energy production) is a significant additive to structural concrete thus supporting the industry wide effort to reduce waste and recycle materials where possible.

Conclusion

As experts in the supply and installation of on-site stormwater detention systems for commercial projects, Australian Tanks understand the needs of design engineers. While there is certainly a market for poly tank solutions, our experience has found that they are not suitable for all situations. The cost savings that appear upfront during the tendering process do not often account for the full scope of related construction costs and delays, nor the whole of life costs for the project.

Our mission at Australian Tanks is to work collaboratively with engineers to develop solutions that solve design problems for better project outcomes. We have a well-earned reputation for integrity and quality products, and we aim to provide the industry leading detention tank service through trust and honesty in our service. 

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