In today's lexicon, the word "concrete" has come to symbolise strength, stability and the image of being set in stone. So, not surprisingly, concrete is the most common solution for storage tanks in the water and wastewater industry. Beyond the inherent durability of concrete, a variety of post-tensioning and corrosion protection techniques are providing even longer-lasting and economical solutions.
The water and wastewater industry is undergoing tremendous changes due to population growth, aging infrastructure, security concerns and water quality regulation. All of these factors have contributed to industry growth, which calls for the construction of new storage and process tanks that reduce long-term maintenance costs. A basic understanding of prestressed, post-tensioned concrete and recently enhanced multistrand and monostrand systems can help industry professionals select the optimal solution for their next water storage or process tank.
In its basic composition, concrete has been around for almost 200 years. According to the Portland Cement Association (PCA), Joseph Aspdin first patented portland cement in England in 1824. Portland cement comprises 11% of the composition of concrete. He named it "portland" because of its resemblance to stone found in a quarry on the Isle of Portland. Today, the PCA describes portland cement as a calcium silicate cement.
Concrete is formed with a mixture of paste and fine and coarse aggregates. The paste, a blend of cement and water, adheres to the surfaces of the coarse and fine aggregates - the sand and gravel. A process called "hydration"' then begins as the paste hardens and the whole mixture eventually assumes the final form of concrete. Hydration continues as long as water is available to the cement. Proper placement and curing are absolutely critical to the durability and strength of concrete and curing techniques vary depending on factors, such as ambient temperature and relative humidity. Also, a number of variables enhance the strength and durability of concrete. For example, a decreased water/cement ratio can result in a higher strength concrete that is highly impermeable and the addition of flyash or microsilica can enhance the quality of the concrete.
According to the American Concrete Institute (ACI), prestressed concrete has had internal stresses induced to balance out, to a desired degree, stresses due to self-weight and externally applied loads. The objective of prestressing is to create compressive stresses (prestress) at the same locations as tensile stresses within the member so that the tensile stresses will be reduced or eliminated. This results in members that have fewer cracks or are crack-free at service load levels. This is one of the advantages of prestressed concrete over conventionally reinforced concrete, particularly in corrosive atmospheres, such as a moist environment.
The normal method for applying prestress force to a concrete member is through the use of high strength steel, 7 wire strands. Two basic methods will achieve the final prestressed member: pre-tensioning and post-tensioning. Pre-tensioning may be defined as a method of prestressing concrete in which the high strength steel stands are tensioned before the concrete is placed, while post-tensioning is defined as a method of prestressing concrete in which the tendons are tensioned after the concrete has hardened. Essentially, this equates to the introduction of stresses to a structural member with tendons that counteract the stresses resulting from the self-weight and superimposed loadings.
Post-tensioning has proven to be a high quality and economical solution for tanks. Since there are no coatings, joints or liners required for water tightness, post-tensioned tanks are relatively maintenance-free and eliminate construction and maintenance costs associated with construction joints.
The use of post-tensioning in water storage and water treatment tanks has increased significantly over the past several years. Post-tensioned tanks have superior corrosion protection for prestressing steel. All cast-in-place concrete members are prestressed bi-axially to ensure water tightness without joints in the floor or roof slabs. Further, since the prestressing steel strands are in tension and the concrete is in compression, each material is in its ideal state.
Advanced post-tensioning technology
Post-tensioned concrete storage tanks have been a proven solution for almost 30 years, but recent advances have improved durability. Post-tensioned concrete offers the attributes of thinner members; therefore more economical construction, reduction in necessary reinforcement, reduced congestion and simpler consolidation of concrete, improved crack control and water tightness.
A variety of multistrand and monostrand systems are being integrated as solutions for the water storage tank industry. The multistrand system can be used with anchorage types and high-performance cementitious grout. Further, the system offers simultaneous stressing of all strands in a tendon, and simple and reliable equipment for installation, stressing and grouting.
The US company VSL designs, manufactures and installs bonded post-tensioning systems for tanks (walls, floors and roofs). The VSL Bonded Post-Tensioning System uses seven-wire strands bundled to form tendons that are enclosed in PT duct, stressed, and secured with end anchorages up to five strands contained in flat-shaped ducts and anchorages. Strands are individually stressed and gripped by wedge action. After stressing, the ducts are filled with high performance cementitious grout that fully bonds the strands to the surrounding concrete and provides corrosion protection to the strands. In comparison, the VSL Monostrand system is often specified for the floor and roof of concrete water tanks and uses 13-mm and 15-mm diameter strands. The strands are given a permanent corrosion-inhibiting coating and are encased in an extruded plastic sheath. The grease and plastic provide double corrosion protection, and prevention from any bonding between strands and surrounding concrete. The plastic sheath is made from polyethylene with a minimum 50- to 60-mil wall thickness. Special sleeves are used to join the sheaths to the anchorages and each anchorage is provided with a protective cap to ensure continuous corrosion protection in aggressive environments. The system features factory-applied corrosion protection, very low friction losses, and full use of structural depth. These light, flexible monostrands can be easily and rapidly installed, leading to economical solutions.
Technology in use
A good example of this technology is the post-tensioned water tank constructed for the East Cherry Creek Valley Water and Sanitation District in Aurora, Colorado, USA. The project features a 10-million gallon circular storage tank measuring 22-feet in diameter and 22-feet in height. The floor consists of a five-inch thick concrete membrane orthogonally-prestressed with fully-encapsulated unbonded post-tensioning tendons. Stage stressing of the tendons was used to provide pre-compression to the concrete as soon as possible after placement to minimise or eliminate early shrinkage cracking. The post-tensioned membrane floor was cast in a single pour to eliminate construction joints - a potential source for leakage into and out of the reservoir.
The 12-inch thick wall was cast-in-place in 12 full-height segments equal in length; four of the segments contain pilasters for stressing the horizontal tendons. Prestressing was accomplished with internal tendon-type bonded post-tensioning systems both circumferentially and vertically to ensure crack-free service. Multiple layers of corrosion protection on the prestressing strands include high performance grout injected into the polyethylene ducts encased in the densely compacted wall concrete. High levels of concrete pre-compression provided by the post-tensioning tendons under the most extreme loading conditions eliminate tensile stresses and ensure a durable structure for many decades. The roof is composed of a 7.5-inch column supported two-way prestressed concrete flat plate cast-in-place during a single pour. Similar to the floor slab, it is also orthogonally-prestressed with fully-encapsulated post-tensioning tendons and stage stressed.
"The long-term durability and lack of required maintenance associated with steel tanks, such as dewatering, sandblasting, refilling and repainting, allow the post-tensioned concrete tank to serve as an attractive option," said Robert Bates, PE, president and founder of Bates Engineering, who worked with the author on the project team. "Further, since the tanks can be completely buried in the ground, allowing for limited access and visibility, security and safety are increased - a concern for everyone in today's world."
Conclusion
Beyond the intangible safety benefits and elimination of corrosion, post-tensioned concrete tanks allow owners to realise immediate decreased life-cycle maintenance costs. The cost to maintain a post-tensioned tank is minimal; frequent painting of the tank is unnecessary and the tank is rarely out-of-service. Further, the tanks do not require coatings on the inside or outside. These attributes, however, must be accompanied by quality design and construction.