Because a water storage tank is a long-term investment and is such a critical part of the water supply infrastructure, it is important for consulting engineers and their customers to evaluate the type of tank that they are purchasing and the tank builder that they are choosing.
There are several substantive factors that make AWWA D110 – Type II tanks superior to other water storage tanks and, more specifically, other types of prestressed concrete tanks. In our opinion, the most important items to consider in specifying and purchasing a new tank are:
- What is the history of the construction system proposed?
- What are the specifics of the design proposed?
- What experience does the tank builder have and who is responsible?
We believe that addressing each of these questions will clearly demonstrate the supremacy of AWWA D110 – Type II tanks.
There have been numerous companies over the last half-century that have attempted to build prestressed concrete tanks, and the companies that have chosen to construct tanks in accordance with AWWA D110 standards have survived and been very prosperous. Those using other methods have come and gone and have left many municipal and industrial customers with an inferior product. In contrast, CROM has become the preeminent prestressed concrete tank construction company by successfully building tanks using the AWWA D110 – Type II method. The success of the AWWA D110 – Type II method is evident in the fact that CROM currently constructs over 1,000 tanks per year with over 3,600 tanks currently in service.
The design of AWWA D110 – Type II tanks is another exemplary quality that elevates this type of water storage tank above all others. Each of the major components of AWWA D110 – Type II tanks possesses the most sensible and effective design that is guaranteed to maximize the function and the lifespan of the tank. The major components bolstered with such an advantageous design include:
Membrane Floor: A membrane floor consisting of a thin, flexible, highly reinforced concrete slab is used in the construction of the vast majority of the tanks we build. The thin, flexible slab allows for angular rotation when a water tank is loaded and the underlying soil consolidates. The high percentage of reinforcing in the slab resists cracking from shrinkage. The idea that thicker is better is not necessarily true when it comes to floor slabs of water storage tanks. CROM has demonstrated this truth by building high-quality watertight tanks with membrane floor slabs since 1953.
Floor/Wall Joint: Another unique feature of CROM tanks is the interface at the floor/wall junction. CROM has invested heavily and successfully in developing a base joint that precludes leakage and does not require a coating for sealing the floor/wall joint.
The Composite Wall
Shotcrete Core Wall: The tank core wall is constructed using shotcrete, a pneumatically applied (high velocity) concrete with a high cement content and low water to cement ratio. It is a crucial component in the construction of the tank core wall in Type II tanks. Shotcrete is much less permeable than traditional, ready-mixed concrete, and it adds long life and strength to the list of advantages of concrete tanks.
Steel Shell Diaphragm: Another essential component of the tank wall is the steel shell diaphragm. The steel shell, embedded in shotcrete, creates a composite wall. Vertical joints in the steel shell diaphragm become watertight when sealed with a well-proven epoxy injection, which is the more effectual alternative to relying on a coating to seal the joints in the tank wall. The steel shell diaphragm produces the watertightness generally recognized in steel tanks by providing an impermeable positive water barrier.
The Prestressing Materials
Prestressing: Single wire prestressing is the most fail-safe method used for prestressing concrete ground storage tanks. Only with machine-wound high strength wire is the steel sufficiently tensioned before the wire touches the wall, and this process confirms that the prestressing steel is tensioned uniformly. Other methods tension the prestressing steel after the wire or cable is in contact with the wall, which results in friction buildup and non-uniform tensioning.
Shotcrete Cover: Each 8-gauge wire is individually bonded to the tank wall by being completely encapsulated in shotcrete, and this step minimizes the potential for wire corrosion. The step is also significant because if a small area of the tank experiences localized damage, the effect is limited to the specific area and is therefore much more manageable. In other types of systems, such as unbonded tendon prestressing, any localized damage can result in the loss of the prestressing for the entirety of the tank circumference.
Over the years, CROM has found that applying one inch of shotcrete over the prestressing wire is the proper way to protect the wires from corrosion. Over 3,600 successful implementations of this strategy support our discovery. The high cement content and dense nature of the applied shotcrete provides a unbeatable barrier to corrosion.
The Prestressed Free-Span Concrete Dome
Some of the benefits of the AWWA D110 – Type II tank with a free-span dome are:
- The permanence and strength of concrete in compression.
- The absence of any interior support columns that could require maintenance.
- The lack of any column point loads on the foundation that could cause differential settlement.
Experience and Responsibility
Knowing the level of experience and responsibility that the builder of your water storage tank has achieved is a subject of equal weight to the history and design of the tanks. CROM maintains our merit in these areas with our specialization, our means of delivery, our accountability, and our proven methods.
- Specialization: The professional ability to build water-holding structures lies in a specialized field that requires each member of the design and construction teams to possess many years of experience. CROM engineers have accumulated this experience by designing and managing many different tank projects, and our superintendents have on average over twenty years of field construction experience building only tanks.
- Means of Delivery: As an owner or consultant, you will deal directly with an engineer that specializes in the design and construction of water storage tanks, not a manufacturer’s representative. The same individual will manage your project from the initial inception to the tank completion and the final payment. At CROM, there is never a hand-off from the sales division to the construction division.
- Responsibility: CROM provides comprehensive design and construction services that supply fully functional, ready-to-use water storage tanks directly to our customers. Only expert tank builders such as those at CROM can accept the full responsibility for such specialized construction.
- Proven Methods: Various construction techniques have already been tried and tested by the experts, and CROM concrete tanks have earned the industry’s respect with their superb longevity and low-maintenance costs. Through our Research and Development Department, we will continue to improve the design and construction of AWWA D110 – Type 2 tanks. However, as history has testified, our basic techniques are very sound, because CROM tanks are designed and constructed with permanent materials that ensure enduring watertightness.
Type II Construction with CROM
CROM is committed to designing and building only the best water storage tanks available for our customers, and this deep-set commitment has driven us to specialize in AWWA D110 – Type II tanks. CROM is a long-established prestressed concrete tank builder whose staff consists of registered professional engineers and expert constructors, and we will bear the undivided and complete responsibility for our tank design and construction. With its reputable history and perfected design, we believe that the AWWA D110 – Type II tank is ready to meet your water storage needs, and with our talented team and excellent experience, CROM is ready for you to believe in us.