DuraStor™ Prestressed Concrete Tanks

Prestressing is the introduction of stresses to a structural member, usually with high-strength steel 7-wire strands, that counteract the internal stresses resulting from the member’s self-weight and superimposed loadings.

Pre-tensioning is the method of prestressing associated with production of structural components at a pre-cast concrete facility. Post-tensioning is the method of prestressing associated with concrete members cast-in-place at the project site.

Concrete is a material with high compressive strength but relatively low tensile strength. Through the principles of structural design, the level of post-tensioning applied to a structural component can reduce or eliminate the tensile stresses in the concrete depending on code requirements and the desires of the structural design engineer.

Post-tensioning allows for large concrete placements without construction, expansion, or control joints. The floor and roofs slabs are each cast in single placements eliminating the maintenance and leakage associated with construction joints. Wall segments are cast full-height in lengths of approximately 80 ft. with no horizontal construction joints.

The ultimate stress of the prestressing strands is 270 ksi compared to 60 ksi yield stress for mild reinforcement allowing for greatly reduced reinforcement ratios. Reinforcement congestion is eliminated simplifying concrete consolidation.

Post-tensioning allows for thinner members resulting in reduced materials, placement labor, construction schedule, and costs compared with conventionally reinforced tanks.

Post-tensioned concrete is an inherently water tight material. Expensive coatings and liners, that require ongoing maintenance, are not required.

The post-tensioning tendons, which are the primary structural reinforcement, have multiple layers of corrosion protection including the highly pre-compressed concrete cover, thick polyethylene ducts filled with high-performance grout or corrosion-inhibiting coating, encapsulated anchorages, and watertight duct connections.

The construction sequencing of a post-tensioned concrete tank allows for multiple tasks to be accomplished simultaneously. For example, wall construction proceeds in parallel with column and roof shoring placement. The resulting construction schedule is highly efficient in comparison to other methods of tank construction.

Post-tensioning technology has been accepted widely in the United States since the 1950’s. The range of structural applications includes office and residential buildings, parking structures, bridges, foundations, ground anchors, containment structures, and structural strengthening – thousands of structures are constructed annually with post-tensioning as the primary reinforcement. It is a favored method of construction for its construction speed, economy, and durability.

Hundreds of post-tensioned concrete tanks have been built in the United States with the earliest being in service for over forty years; hundreds more have been constructed world-wide.

The design and construction of post-tensioned tendon-prestressed concrete water tanks is governed by the following building codes and standards:

• American Concrete Institute ACI 350-06 Code Requirements for Environmental Engineering Concrete Structures and Commentary
• American Water Works Association ANSI / AWWA D115-06 Tendon – Prestressed Concrete Water Tanks
• American Concrete Institute ACI 373R-97 (Report Recommendations) Design and Construction of Circular Prestressed Concrete Structures with Circumferential Tendons

The Post-Tensioning Institute provides the following technical reports and specifications:

• Design of Post-Tensioned Slabs Using Unbonded Tendons
• Acceptance Standards for Post-Tensioning Systems
• Design and Construction of Post-Tensioned Slabs-On-Ground
• Field Procedures Manual for Unbonded Single Strand Tendons
• Specifications for Unbonded Single Strand Tendons
• Recommendations for Prestressed Rock and Soil Anchors
• Specification for Grouting of Post-Tensioned Structures
• Anchorage Zone Design

Construction can proceed in freezing temperature conditions as the concrete members are protected from the elements with insulating blankets and heat is applied to the curing concrete as required

STRUCTURAL TECHNOLOGIES can provide many shapes to suit the project needs including circular, rectangular, rectangular with rounded corners, egg-shaped digesters, irregular-shaped polygons, oval, – there are no practical limitations with this method of construction. Economical capacities have been constructed ranging from 250,000 gallons to above 30,000,000 gallons.

Site conditions may require a fully-buried, partially-backfilled, above-grade, sloped backfill or conditions where ground water is above the tank floor – a DuraStor™ Prestressed Concrete Tank is adaptable to any of these conditions.

The design methodology and structural detailing of prestressed concrete tanks is well-developed. Concrete structures, and particularly prestressed concrete tanks, have the proven ability to withstand severe earthquakes.