Prestressed Concrete Tanks What to Look For...

Transcription

1 Prestressed Concrete Tanks What to Look For... THE CRO M CO RPO RA TIO N

2 Page 1 CROM Prestressed Concrete Tanks AWWA D110 - Type II The Crom Corporation has become the pre-eminent prestressed concrete tank construction company by successfully building tanks using the American Water Works Association (AWWA) Standard D110 -Type II method. The Company currently constructs over 100 tanks per year with over 3800 tanks currently in service. Because a water storage tank is a long-term investment and is such a critical part of the water supply infrastructure that will serve for many generations, it is important for consulting engineers and utility owners to closely evaluate the type of tank they are purchasing and the tank designer/builder that they are choosing. There have been numerous companies over the last halfcentury 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. There are several substantive items that make AWWA D110 -Type II tank structures superior to other water storage tanks and more specifically other types of prestressed concrete tanks. The purpose of this brochure is to call attention to the unique qualities and advantages of AWWA D110 -Type II tanks compared to other methods of design and construction. Specifically, the main items for consideration are the following: Foundation Concrete Core Wall with Steel Shell Diaphragm Prestressing Free-span Dome The Tank Builder s Experience Who Will Be Responsible.

3 Page 2 Foundation Membrane Floor- A membrane floor is a thin and flexible, highly reinforced concrete slab that is used in the construction of the vast majority of Crom tanks. The membrane slab allows for angular rotation when a water tank is loaded and the underlying soil consolidates. The high percentage of reinforcing steel 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. Crom has demonstrated this by building highquality watertight tanks with membrane floor slabs since Floor/Wall Joint- Another unique feature of a Crom tank is the interface at the floor/wall junction. Crom has invested heavily in developing a base joint that precludes leakage and does not require a coating for sealing the floor/wall joint. Composite Wall Steel Shell Diaphragm- A critical 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 a well proven epoxy injection technique. This steel shell diaphragm insures an impermeable positive water barrier. Shotcrete Core Wall- Another important component of the tank core wall is shotcrete. Shotcrete is pneumatically applied (high velocity) concrete with a high cement content and low water/cement ratio. Shotcrete is much less permeable than traditional ready mixed concrete. It provides the long-life and strength associated with the advantages of concrete tanks. Prestressing Prestressing- 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 tension the prestressing steel after the wire or cable is in contact with the wall resulting in friction buildup and nonuniform tensioning. Shotcrete Cover- Each 8-gauge prestressing 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 force for the full tank circumference.

4 Page 3 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. No interior support columns that would require inspection and maintenance. No column point loads on the foundation that could cause differential settlement. Three ID Free-span Concrete Domes Experience and Responsibility Method of Delivery- As a consultant (or owner), 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, from project inception to tank completion and final payment, will manage the project. There is no hand-off from the sales division to the construction division. 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 management, design, and construction of many different tank projects. Crom superintendents have an average of over 20 years of field construction experience building only tanks. Responsibility- The Crom Corporation provides a turnkey design and construction service. Only expert tank builders can accept the full responsibility for such specialized construction. Building a Crom Tank Proven Methods- Various construction techniques have already been tried and tested by the experts. Crom concrete tanks are known and respected for their superb longevity and low-maintenance costs. Crom tanks are designed and constructed with permanent materials to ensure enduring watertightness. What to Look For... 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.

5 Page 4 Anatomy of an AWWA D110 Type II Wall The tank s core wall is actually a composite design utilizing a Steel Shell Diaphragm. The inclusion of a Steel Shell Diaphragm provides an impermeable positive water barrier. The Steel Shell Diaphragm is encased on both sides in dense shotcrete. Mild reinforcing steel is added in accordance with the tank s design calculations. Steel shell Diaphragm Machine-wound high strength prestressing wire is tensioned uniformly. There are no losses due to friction build-up or non-uniform tensioning. Steel Shell Shotcrete Encased Bonded Prestressing Wires If the prestressing steel is not permanently bonded to the tank wall, corrosion at any single point along the tendon can cause the failure of the entire length of that strand. The proven way to avoid this serious and undetectable problem is to design the tank with prestressing wires encased in shotcrete - wherein each and every wire is effectively bonded to the tank s wall. Damage at any single point along the wire will be a localized problem only. The remaining length of the wire continues to effectively prestress the wall. Completed Composite Wall

6 Page 5 The 10 keys to outstanding prestressed concrete tank construction: 1. Steel Shell Diaphragm: 26-gauge minimum, meeting ASTM Specification A-1008; encased and keyed to dense shotcrete on both sides to make up a composite, watertight wall. 2. Prestressing Steel Wire: 8-gauge minimum, meeting ASTM Specification A-821type B; encased in high strength Portland cement shotcrete and bonded to wall structure. 3. Waterstops: Watertight elastomeric waterstops used in floor/wall joints of larger tanks. 4. Dome roof: Free-span dome roof, supported only at the circular tank wall. No interior columns. 5. Maintenance-Free Accessories: Interior and exterior ladders, hatch cover, roof ventilator, wall manhole, liquid level indicator, manufactured of fiber glass, stainless steel, or heavy aluminum. 6. Beautiful exterior painting: Two coats of a modified waterborne acrylate paint or one coat cement base primer plus two coats high quality masonry paint. 7. Professional Design: Tank should conform to applicable portions of ACI "Design & Construction of Circular Wire- and Strand- Wrapped Prestressed Concrete Structures" and AWWA D110 Wire and Strand- Wound, Circular, Prestressed Concrete Water Tanks All design calculations and drawings to carry the seal of a professional engineer registered in the state wherein the construction shall be carried out. 8. No Division of Responsibility: The entire tank structure to be designed and built by a single company that specializes in Prestressed Concrete Tank construction. 9. Expert Contractor: Tank to be constructed by expert tank building contractor with no less than 5 years actual field experience in complete tank construction, including footings, floor, wire-prestressed composite wall, and free-span dome roof. 10. Worth-While Guarantee: Tank builder to guarantee the tank for five years against leakage and defects in workmanship.

7 THE CROM CORPORATION 250 SW 36TH TERRACE GAINESVILLE, FL Phone: (352) FAX: (352) copyright c 2011 by The Crom Corporation. All Rights reserved