Prescribing a Bonded Post-tensioning System

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Publication: Structural Engineer

Pharmaceutical companies are charged with developing and distributing medicines that allow patients to live longer, healthier, and more productive lives. Therefore, it is essential that the buildings in which they operate are efficient and durable, and that any downtime associated with maintenance is minimized. That is why post-tensioning — a method of reinforcing and prestressing concrete, masonry, and other structural elements — was selected as the solution for a 23,000-square-foot monolithic slab-on-ground foundation project for a major drug manufacturer in the Midwest.

The industrial-strength slab-onground is located in a production area of the one-story facility and is subjected to forklift traffic with areas of heavy storage.

The owner sought a durable flooring solution that would withstand these stresses and, because of spill containment guidelines, would provide the greatest potential for resisting cracks.

Unique bonded solution

The overall structure consists of a 6-inch-thick, post-tensioned slab-onground with 36-inch-deep post tensioned edge beams, external and internal concrete masonry unit (CMU) walls, steel columns, and roof trusses.

Although a conventionally reinforced slab was considered, bonded post-tensioning was selected as a means to meet the strict requirements of the pharmaceutical setting. Additionally, all construction joints and crack control joints were eliminated because the slab was poured monolithically and the post-tensioning system provided satisfactory pre-compression on the slab. The structural engineer, Jacobs Engineering of Pasadena, Calif., relied on the post-tensioning company, VSL of Baltimore, Md., to provide the slab design and the post-tensioning specifications.

The project was located in the middle of the existing plant so access and organization were critical factors. It began with the essential planning necessary to ensure that the construction of the structure would not affect the rest of the plant's daily routines and production. To complicate the scenario even further, construction was slated to occur during the winter months with temperatures averaging below freezing during the day — the toughest time to pour concrete.

With bonded post-tensioning, the prestressing steel is installed at the job site after the formwork is erected and typically after concrete is poured. It is housed in a sheathing or duct to prevent it from bonding to the surrounding concrete. After the concrete has hardened, the prestressing steel is gripped at both ends and then pulled and anchored to prestress the concrete. This complete assembly of steel, duct, and anchors is known as a tendon. Although unbonded post-tensioning is a common solution for slab-on-ground applications, the use of bonded post-tensioning in a slab-onground application is unique.

The design team sought optimal durability and a means to prevent any spilled material from penetrating the slab through cracks and contaminating the subsoil. As such, a bonded, posttensioned solution was implemented.The terms bonded and unbonded post-tensioning refer to the steel and whether or not it is bonded to the surrounding concrete after tensioning. Despite the fact that there are few projects in the United States in which a bonded system is used in a slab-on-ground application, the design team chose to capitalize on the benefits of such a system, including improved corrosion protection, durability, and greater overall crack control. The design provides greater crack control because of the strain compatibility between the prestressing steel and surrounding concrete. This means that where there is a localized increase in tension in the concrete, there is a corresponding increase in resistance to this tension by the prestressing steel. As such, a bonded post-tensioned system provides ultimate crack control by compressing the slab through the bonded prestressing steel within the slab.

All in the details

To provide a true encapsulation system, VSL selected a 1/2-inch, twostrand, bonded tendon system with flat, 1-inch by 2-inch high-density polyethylene duct for the slab post-tensioning.

To protect the tendons from any corrosive materials used within the structure, the construction specifications would not allow the grout tubes for venting the tendons to project through the top or bottom of the slab. Therefore, the team located the grout entry and exit ports on the slab faces only. An added benefit of this feature is that it was not necessary to finish around grout tubes that penetrate the top of slab (which is necessary in grout-filled systems), resulting in a smoother, more uniform slab surface.

Another benefit of the system used on this project was the slim design of the slab tendon bearing plates. These bearing plates fit easily in the 6-inch slab edge areas where there were no thickened edge beams. Further, the 1-inch-thick duct allows for tendons to run in both directions within the 6-inch slab while maintaining the required 1.5-inch concrete coverage. The design of the edge beams and their slopes also was a crucial element in reducing the resistance of the soil on the slab during stressing.

For the edge beams, the design team used a 12-strand, 1/2-inch, multi-strand bonded system incorporating a steel/grout composite bearing plate with two, 3/8-inch round, high-density polyethylene ducts. Lighter, smaller, and easier to handle than a full steel bearing plate, the composite bearing plate enabled easy installation of the anchorage assembly. The grout vents exit through the slab edges with this anchorage.

Compared with galvanized steel duct, the polyethylene duct, water-tight duct couplers, and trumpet to duct couplers in both systems provide superior corrosion protection of the posttensioned strands. Both systems also are equipped with permanent polyethylene grout caps that protect the anchor heads, wedges, and strand tails. Each cap includes a grout vent on the face of the cap to ensure the cavity is filled completely with grout and that any air voids are eliminated.

Live loads, dead loads, forklift loads, and impact loads all were incorporated into the design solution. VSL performed a complete floor analysis to calculate the amount of prestress required within the slab and edge beams. This design resulted in the use of two-strand tendons spaced at 24 inches on-center in each direction and two, 12-strand tendons in each edge beam. This tendon layout provided an average effective compressive stress of 330 pounds per square inch (psi) in each direction within the 6-inch slab and 300 psi in the edge beams.

The next phase of the project involved the material fabrication, supply, and installation portion of the foundation.

A unique, two-strand ram was used on this project. This ram is slightly larger than a typical monostrand ram because of its ability to stress two strands simultaneously.

The benefit of using this ram is that it allows simultaneous stressing and produces less friction loss between the strands, and thus a higher final effective force within each tendon.

Stressed out

The stressing operation began by placing an initial stress of 26 kips per slab tendon within 36 hours of concrete placement to resist tension that develops within the slab because of shrinkage.

Applying this initial stress along with proper curing of the slab are key strategies for minimizing the potential development of shrinkage cracks. Concrete strength also had to reach a minimum of 1,400 psi within the 36 hours to with stand the 26 kips. This initial force was calculated to ensure that the friction between the underside of the slab and the gravel base was overcome. By overcoming the friction resistance between the two surfaces, sufficient residual force was provided to control any early shrinkage cracks. This first phase of stressing was performed as soon as possible to meet the owner's requirements for crack control.

Eliminating the occurrences of cracks within the first 36 hours creates greater potential for a crack-free slab. Concrete strength of 4,000 psi was reached in just four days, even with cold temperatures.

Then, the slab tendons were stressed fully to 66 kips in the final stressing sequence. Also at this time, the 12-strand tendons in the perimeter edge beams were stressed using a 200-ton ram.

To grout the two, 12-strand tendons, a specialized admixture was formulated for blending with portland cement (Type II) and water to produce a fluid, pumpable, non-bleeding, shrinkage-compensated, high-strength grout with extended working time that meets published Post- Tensioning Institute requirements. This admixture was combined with the portland cement at a 1:20 mix ratio by weight and water at a maximum water cement ratio of 0.35, to produce a dense, durable, and stable grout. Used in combination with a high-shear colloidal grout mixer and pump, the resulting grout provided excellent corrosion protection for the prestressing steel and promoted an effective bond between the prestressing steel and concrete elements.

Distinctive solution proved successful

The post-tensioning portion of the slab-on-ground was completed in six weeks without any disruption to the plant's operations. By choosing a posttensioned slab and eliminating the expenses associated with a conventionally reinforced slab, such as shutting down the facility, as well as moving equipment and materials to perform the necessary repair and maintenance, the owner saved money. Most important, however, is the knowledge that the unique bonded posttensioned solution not only will meet the necessary crack control requirement, but also will offer a long-lasting, highly durable slab because of the improved corrosion protection and durability.