Post-Tensioning Update - ShoreMaster News

Post-Tensioning Update
Advances in Corrosion Protection and
Design Flexibility Give Contractors New Options

By Scott Greenhaus, president of VSL, and John Crigler, PF, vice president and technical manager of VSL
June 2005
Construction Bulletin

Although post-tensioning has been a proven solution in the marketplace for almost five decades, older systems focused more on obtaining the desired prestress force and less on durability. Today, however, the industry has evolved to offer systems that deliver the desired prestress force while providing improved protection for the prestressing steel. These advancements in corrosion protection are especially important in areas that experience significant exposure and damage from freeze-thaw cycles, de-icing salts, seawater, salt spray, and other deterioration mechanisms. Many owners, designers, engineers, and contractors also are looking at post-tensioning as a solution that offers the high level of structural flexibility necessary to meet changing user requirements. With so many options and advances available today, successful implementation of a post-tensioning system depends on matching the right system to each project.

Defining Post-Tensioning
Post-tensioning is a method of reinforcing and prestressing concrete, masonry and other structural elements. Simply, concrete and masonry are very strong in compression but relatively weak in tension. In comparison, steel is very strong in tension. Combining steel with concrete or masonry therefore results in a product that can resist both compressive and tensile forces. Further, if concrete is prestressed or "squeezed together" with the help of the steel (known as prestressing steel) during the construction phase, its resistance to cracking increases significantly.

There are two methods of prestressing: pre-tensioning and post-tensioning. In pre-tensioning, the prestressing steel is stressed at a precast manufacturing facility before the concrete is cast. With unbonded post-tensioning, the prestressing steel is installed on the job site just before concrete is poured. The prestressing steel is greased and encased in an extruded plastic sheathing to prevent it from bonding to the concrete. After their concrete hardens, the prestressing steel is gripped at both ends, tensioned and anchored to prestress the concrete.

The completed assembly of steel, sheathing and anchors is known as a tendon. Unbonded tendons generally consist of a single strand. With bonded post tensioning, the prestressing steel is placed in a corrugated metal or plastic duct that has been cast into the concrete.

The columns of the four-level 419,000 square-foot parking garage at the Financial Center
complex in Owings Mills, Md., required 60-foot beam spans and 27-foot-long slab bays.

The prestressing steel is usually placed after the concrete has been placed. A bonded post-tensioned tendon typically contains more than one prestressing steel strand and can range from several strands to 55 or more strands in a single tendon, while the anchorage assembly consists of confinement reinforcing steel, bearing plate, anchor head, wedges, and grout cap. The strands can be stressed individually or simultaneously with a monostrand or multistrand hydraulic jack. After stressing, the duct is filled with a low-shrinkage, low-bleed flowable cementitious grout to achieve bond to the concrete member and to protect the prestressing steel from corrosion.

Today, post-tensioning is used for a wide range of applications including office buildings, condominiums, hotels, parking structures, slab-on-ground foundations, ground anchors, storage tanks, stadiums, silos, and bridges. Other application include post-tensioning in pavement, masonry, bridge decks, seismic walls, and single-family homes. It also can be effectively combined with other structural steel, reinforced concrete, masonry, and timber structures, as well as enhance and extend the capabilities of precast, pre-tensioned elements. Examples include spliced precast bridge girders, segmental bridges and hybrid precast moment resisting frame building. One of the most significant reasons for its growth is that post-tensioning allows designers to achieve longer spans with shallower concrete sections, providing owners with the economical advantages of lower floor-to-floor height. This allows architects and engineers to design and build lighter and shallower concrete structures without sacrificing strength. Other key benefits of post-tensioning include functional flexibility, improved deflection and vibration control, crack control and reduced maintenance.

With bonded post-tensioning, the prestressing steel is placed in a corrugated metal or plastic duct that has been cast into the concrete. The prestressing steel is usually placed after the concrete has been placed.

In addition to growth in end-use applications, the industry continues to invest in research, testing and quality control in order to create a better product. With consistent testing and development, engineers are breaking new ground for concrete building systems. Systems such as post-tensioned ductile frames and flatplate column slab joint systems allow building designers to meet demanding building codes with simultaneously providing safe structures.

New Technologies Promote Durability and Combat Corrosion in New Construction
Since post-tensioning was first used domestically in 1949, the industry has seen many technological advances, including seven-wire strand, low relaxation strand, improved analysis techniques and design software, the use of banded tendons, extruded sheathing, encapsulated anchors, and plastic duct systems, as well as the development of pre-packaged, non-bleed grouts. However, one of the greatest advancements is the progress made during the last decade in plastic duct corrosion protection. When some of the earliest unbonded post-tensioned buildings were about 15 years old, corrosion problems started to surface and it was apparent that some tendon sheathings and coatings could not adequately resist corrosion in the most aggressive environments, such as where de-icing salts are applied to slab surfaces or in coastal areas that have a high salt content in the air. Starting in the mid-1970s, the Post-Tensioning Institute (PTI) developed tendon material specifications designed to address the corrosion problems. PTI specified improvements in sheathing, coatings and, in the most aggressive environments, complete encapsulation of the tendons. More recent advancements have opened new doors for post-tensioning as a long-term, durable design solution.

One means to protect post-tensionig systems is by encapsulation - which, for unbonded tendons, evolved into a seamless extruded plastic sheathing around individual steel strands. While this technique protected the tendons along their length, it did not address protection of the anchorages. In the late 1980s, VSL's first generation of encapsulated anchorage components, CP+ (TM), was introduced. Although this technology allowed for great advancement in durability, it did not completely address encapsulation at construction joints. As such, VSL, recently launched Enhanced Protection Plus (TM) (EP+) - a new generation of systems offering end-to-end protection of the tendon. The EP+(TM) System features a unique tendon coupler that carries the encapsulation across construction joints, improving constructability, durability and quality. In the early 1990s, VSL pioneered the use of fully integrated plastic duct systems for bonded tendons with the introduction of the P-T Plus (TM) duct system. It features a robust plastic duct combined with couplers, anchorage connections, grout caps and vents to completely encapsulate bonded tendons.

For bonded tendons, grouting is a key element of the overall corrosion protection strategy. Experience gained over many decades with grouted post-tensioning tendons has proven that cementitious grout provides excellent protection for the prestressing steel. The principle objectives of grouting are to protect the prestressing steel from corrosion by encasing it in an alkaline environment and filling the tendon to minimize voids in the completed structure. Grouting also bonds the tendon to the structural member for bonded applications. However, it is important to note that the quality of the grouting is of prime importance for the durability of post-tensioning tendons for any kind of application (bonded internal tendons, external tendons, stressbars, and ground anchors).

Advanced Post-Tensioning Systems in Action
While the majority of post-tensioned concrete building applications use monostrand post-tensioning systems, bonded systems are becoming more popular with the owners of airports, hospitals, government agencies, and universities. Approximately 3.5 million square feet of bonded post-tensioning has been installed in the United States in building slabs since 1995, and this growth is attributed to an increased interest in life-cycle economics.

With unbonded post-tensioning, the prestressing steel is installed on the job site just before concrete is poured. The prestressing steel is greased and encased in an extruded plastic sheathing to prevent it from bonding to the concrete. After the concrete hardens, the prestressing steel is gripped at both ends, tensioned and anchored to prestress the concrete.

Bonded systems offer a significant design advantage which leads to lifecycle savings. The key design feature of bonded systems is the hardened grout that locks the movement of the post-tensioning strands to that of the surrounding concrete. Hence, the force in a bonded strand is a function of the movement of the surrounding concrete. This concept of strain compatibility allows for a more efficient use of the prestressing steel and a reduction in the amount of supplemental mild steel.

Another design advantage of bonded post-tensioning is the inherent capacity to provide resistance to progressive collapse. This may be especially important in the event of localized blast loading. Like mild steel reinforcement, a bonded post-tensioning tendon has a relatively short development length. In the event that an anchorage fails or a strand is severed, the loss of tendon force would retain its force at the development length, away from the failure point, and would remain functional. This functionality can be used in the design phase when planning for alternative load paths.

Bonded systems also offer several practical benefits, such as a reduction in the amount of mild steel needed, particularly at the top of slabs. Reducing mild steel is especially important, as most maintenance costs are due to repairs associated with spalled concrete and corroded rebar. Another benefit is complete encapsulation; the strands are fully protected by cementitious grout, plastic duct and surrounding concrete. The bonded systems also offer more flexibility in terms of structural modification for stairwell openings, utility access and future expansion.

In addition to the advances in durability, many designers today also are selecting post-tensioning to provide the high level of structural flexibility necessary to meet changing user requirements. Since post-tensioning provides greater spans with reduced structural member depths, larger column-free areas are possible. Such was the case for a four-level, 419,000-square-foot parking garage at the Financial Center complex in Owings Mills, Md. The columns of the concrete structure required 60-foot beam spans and 27-foot-long slab bays. Further slabs were 7 inches thick on all levels. These dimensional constraints were easily overcome with the use of cast-in-place post-tensioned construction.

To increase the durability of the structure and mitigate deflections of the beams and slabs, VSL used a fully encapsulated bonded post-tensioned system known as VSLAB(TM). Proven to offer a long term durable solution that optimizes life cycle costs, VSLAB (TM) uses up to five strands contained in flat-shaped plastic ducts and anchorages and permanent end-caps for both beam and slap tendons that completely seal the anchorages. The use of approximately 177,000 linear feet of the bonded VSLAB (TM) postensioning system in the slab set the garage apart from typical parking structures because the bonded post-tensioning system resulted in the reduction - even elimination in some areas - of rebar in the slab. Because of this reduction, anticipated maintenance costs associated with corrosion control were reduced, and the durability of the structure significantly increased. Furthermore, reduced amounts of rebar allowed concrete mixed without microsilica or calcium nitrate corrosion inhibitor to be used. The garage now provides more than 1,500 convenient parking spaces for the tenants of the Financial Center.

Technology Continues to Advance
The development of new technologies is likely to continue to expand the acceptance and growth of the post-tensioning method. Electronic monitoring is one example of these technologies. The embedded electronic sensing devices, designed to monitor a structure over time, are becoming increasingly popular as they allow for the determination of when and where a tendon breakage occurs. When load cells are attached to the tendons, the stressing forces can be directly read and monitored. Although these electronic monitoring devices are not yet commonplace, their usefulness and payback in terms of maintenance are quickly proving their worth.

Yet another reason post-tensioning continues to grow as a solution is the dissolution of the myth that adapting or modifying a post-tensioned structure is difficult. In reality, a qualified contractor can safely and effectively modify a post-tensioned structure. Additionally, because post-tensioning offers a variety of design solutions and great freedom with longer spans, it is being selected as the solution of choice for the most challenging projects, such as current project at a Tennessee university in which bonded multistrand systems are being incorporated into a new research facility. The new structure is being erected over an existing building and will be supported by a series of immense post-tensioned cast-in-place trusses.

What else is in store for post-tensioned construction? One growth area may be tall buildings - 20 stories and higher - where most framing historically has been structural steel. Post-tensioned concrete offers significant performance benefits in tall buildings, particularly in the areas of fire resistance, sound transmission and floor stiffness.

Post-tensioning the floors and frames of tall concrete buildings minimizes their weight and, combined with the use of high-strength concrete, make this a realistic design solution. One only has to look at the recent selection of post-tensioning as the building method of choice for an office building in Manhattan, as well as a high-rise in Miami that stands nearly 700 feet, to recognize that the system continues to make inroads. Post-tensioning allows for design flexibility and great versatility.

Scott Greenhaus, president of VSL, has 23 years of experience in the structural repair, strengthening and protection of existing structures, as well as comprehensive repair analysis.

John Crigler, PE, vice president and technical manager of VSL, has served the company for more that 25 years. Positions held include division/chief engineer, national manager of bridges and marketing manager.