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 items 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?
There have been numerous companies over the last half-century that have attempted to build prestressed concrete tanks. Those companies choosing to construct tanks in accordance with AWWA D110 standards have survived and have been very successful. Those using other methods have come and gone and have left many municipal and industrial customers with an inferior product. The Crom Corporation has become the preeminent prestressed concrete tank construction company by successfully building tanks using the AWWA D110 - Type II method. The Crom Corporation currently constructs over 100 tanks per year with over 3600 tanks currently in service.
1. Floor Design and Construction
2. Wall Design and Construction
- A membrane floor(a thin and 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 shrinkage cracking. The idea that thicker is better is not necessarily true when it comes to floor slabs of water storage tanks. We have demonstrated this in over 50 years of building high-quality watertight tanks with thin floor slabs.
- The tank core wall is constructed using shotcrete. Shotcrete is pneumatically applied (high velocity) concrete with a high cement content and low water cement ratio. It is an important component in the construction of the tank core wall in Type II tanks. Shotcrete is much less permeable than traditional ready mixed concrete. Shotcrete provides the long-life and strength associated with the advantages of concrete tanks.
- Steel Shell Diaphragm - Another important 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 are sealed watertight by epoxy injection (as opposed to relying on a coating to seal the joints in the tank wall). The steel shell diaphragm provides the watertightness generally recognized in steel tanks.
- Single wire prestressing is the most fail-safe process used for prestressing concrete ground storage tanks. Only with machine-wound high strength wire is the steel tensioned before the wire touches the wall. This assures that the prestressing steel is tensioned uniformly. Other methods jack the prestressing after the steel wire or cable is in contact with the wall - resulting in friction buildup and non-uniform tensioning.
- Each 8-gauge wire is individually bonded to the tank wall by completely encapsulating it in shotcrete, thus minimizing the potential for wire corrosion. This is important. If a small area of the tank experiences localized damage, the effect is limited to the specific area. In other types of systems, such as unbonded tendon prestressing, any localized damage can result in the loss of the prestressing for the full tank circumference.
5. Prestressed Free-Span Concrete Dome
- Over the years The Crom Corporation has found that 1" of shotcrete over the prestressing wire is the proper way to protect wires from corrosion. This is supported by over 3600 successful implementations.
- The high cement content and dense nature of the applied shotcrete provides a barrier to corrosion.
EXPERIENCE and RESPONSIBILITY
- The permanence of concrete in compression.
- No interior support columns that could require maintenance.
- No column point loads on foundation that could cause differential settlement.
- Specialization: Building water-holding structures is a specialized field requiring many years of tank construction experience on the part of designers and construction crews. Crom engineers have this experience in the design and management of many different tank projects. Crom superintendents have an average of over 20 years of field construction experience building tanks only. 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.
- Responsibility: The Crom Corporation provides a turnkey design and construction service. Only expert tank builders can accept the full responsibility for such specialized construction.
- Proven Methods: Various construction techniques have already been tried and tested by the experts. Crom composite tanks are known for 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. But, as history will testify, our basic design and construction techniques are very sound.
- Look for the prestressed concrete tank builder with the longest and best experience. Look for the long-established prestressed concrete tank builder whose staff consists of registered professional engineers and expert constructors, and who will bear the undivided and complete responsibility for tank design and construction.