Post tensioned tendons

Structural Details: In this building, the post-tensioned (PT) tendons act similar to beams running through the concrete slab. When the tension is removed from the cables of the PT tendons are locked-off, the result is similar to the floor losing its support beams. The typical solution to this situation is to place shoring under the affected areas to replace the support. However, in this case, shoring to support the adjacent bays would interfere with the tenants' business operations.

Our Approach: Because of the project constraints, it is possible only if at some place shoring directly under the slab opening, not under adjacent slabs, as that would result to less capacity similarly the cables were de-tensioned. This required the design team to create a phasing plan in which they determined which cables were allowed to be de-tensioned and in which order. This lock-off phasing plan allowed the crews to de-tension individual PT tendons, while maintaining the structural integrity of the entire floor system. The project was phased in small sections in order to minimize disruption to the existing businesses and most of the work was performed at night. Once the tendons were re-tensioned, the crews would begin the concrete demolition for the next lock-off phase. The total square footage of the project area was 3,600 square feet. 

Structural Conservation

Scope of work: The scope of work included supplying labor, equipment and construction materials to remove and replace all the deteriorated mortar joints (inside and out) and strengthening of the chimney to accommodate the new loads created by the cell antennas. After a century of natural deterioration, the chimney was in very poor condition, with existing joints badly deteriorated - in some cases, completely through the wall. Extensive re-pointing of interior and exterior joints was carried out to stabilize and restore the integrity of the chimney; with the bricks at the top 4 feet so much degraded that they required replacement. The challenge in doing this was not only rebuilding the unique architectural shape for historic purposes, but also finding a source for the same type and color of brick.

Our Approach: A structurally efficient, easy-to-install, and cost-effective strengthening option was achieved by using an externally bonded carbon-based FRP strengthening system that would increase the shear and flexural capacity of structural elements. The thin carbon FRP plates were bonded to the inside face of the chimney, serving as vertical tension reinforcement. The continuous FRP laminated plates were installed in single, 30.5-m (100-ft) long pieces extending to the chimney’s full height. Since carbon FRP is only a quarter the weight of steel, the plates were easily handled by workers inside a small diameter work area (only two workers could fit in the structure) and added only minimal weight to the structure.

Old deteriorated structure to New office space

Structure Characteristics: Structure was constructed of cast-in-place conventionally reinforced concrete beam and slab system. The structure was exposed to road salts and moisture from the environment for years, and the resulting corrosion-induced deterioration had compromised the structural integrity of the floor system. The deterioration had also resulted in concrete deterioration of the pre-cast façade panels.

 Our Approach: The renovation of the tenant space and old bus terminal into new office space prompted the repair project. Structural restoration work included: 45,000 square feet of strip patch full slab replacement; 3,000 square feet of beam repair; 500 square feet of column repair; expansion joint replacement; sealant work; structural strengthening; hot applied waterproofing membrane; and 190,000 square feet of deck coating.

Power plant's cooling tower

Problem: Deterioration, corrosion of embedded steel, and water leakage through the shells.

Structural Details: The towers that need repairs are about 370ft high above the normal ground level with the diameter of 248ft and repairs can only be carried out during scheduled outages to avoid disruptions in the area’s power supply.

Testing: Using visual inspection, hammer sounding throughout both towers, and laboratory testing of core samples, investigators identified the types and extent of concrete deterioration. In addition to the spalling, corrosion, and leakage already noted, cracks were found in the support columns and ring beams at the bases of the towers.

Challenges in repairing: Providing access to the interior and exterior of the towers to allow thorough inspection and repair was difficult. Careful structural analysis was required to maintain the towers structural integrity while repairs were under way. Repair materials had to be evaluated and selected for their ability to withstand the unusual conditions within the towers.

Our Approach: Repair crew removed the deteriorated concrete from the tower shells with pneumatic chipping hammers and filled the created voids with dry-mix shot-crete. They sealed the cooling tower’s interior surfaces with water and vapor proof epoxy coating. They cleaned out cracks in the supporting columns and ring beam and sealed them with epoxy. Meticulous quality control testing and inspection during repairs helped ensure the project’s success. 

Parking Garage needs major repairing

Task: To perform the repairing of concrete, replace expansion joints, and install a urethane deck coating on the condominium's parking garage.

Structural Details: The garage was designed with an elevated flat slab with banded post tension cables running over the column lines in two directions.

Problem: Deterioration of the mono-strand post tension cables because of the lack of proper coverage over the cables had allowed water infiltration.

Initial Survey: An initial Survey of the post-tension cable system was conducted. The initial phase of the survey examined the condition of the high and low points of the topside and underside of the slab to determine positive and negative reinforcement. Technicians also looked to see if water had infiltrated the bottom of the sheathing. While on the surface it was apparent that only 2 cables were broken, careful investigation located 9 others that were broken or close to breaking. Ultimately, it was determined that 7 main cables needed to be repaired, 8 anchors had to be replaced, 5 add-ons were required, and 108 anchor pockets needed cleaning and trimming.

Out Approach: The repair suggested is to be consisted of greasing and re-sheathing of the unbroken exposed strands, full length cable repairs, replacement of extensively corroded anchors with excessive slippage and loss of section, and cleaning and trimming of anchor pockets. Additionally, it has been recommended that sacrificial anodes should be placed every 20 to 24 inches per square foot area and the reinforcing bars treated with a corrosion inhibitor (except in areas where leads from the anodes tied onto the bar). The skill of the crew is a deciding factor for success of repairing by keeping concrete removal to a minimum level and the prompt delivery of materials.

Load Capacity of cast-in-place Slab

Task: the structural floor required an upgrade from a live load capacity of 100 PSF (pounds per square feet) to new load of 25 PSF of super-imposed dead load and 150 PSF of live load.

Structure Details: Floor of the structure consists of a one-way cast-in-place slab on precast pre-stressed joist system, simply supported on continuous cast-in-place concrete beams with precast pre-stressed soffits running between columns of the structure.

Our Approach: Rapid Load Testing has been used to evaluate the in-place strength of the loose floor components and to develop an optimized and economical strengthening solution. On the basis of load test, we concluded that slab was strengthened enough to bear up new loads but the joist should be rated to 125 PSF of live load and the beam for 135, all greater than the theoretical load.

An externally bonded carbon FRP strengthening system was recommended consisting of multiple plies of FRP strips attached to joists soffit to increase the positive bending moment capacity and additional FRP strips were wrapped around the joists at each end to anchor the FRP strips. The beams were strengthened to improve their positive and negative moment capacity by installing FRP strips the soffit of the beam for positive moment and on top of the slab for negative moment. On top of the slab, the required amount of FRP for beam upgrade was equally divided and installed of each side of the column in the direction of the beam.

Raker Beam (Basketball Court)

Problem: Cracks were observed on the beam during its construction phase only and on in-depth investigation it has been revealed that it requires additional tension reinforcement.

Structure Details: It was a 16,000 seat capacity new basketball arena and the raker beams that required strengthening were at the second level.

Our Approach: An optimum solution consisting of an external post-tensioning system and externally bonded FRP reinforcement was recommended. The external post-tensioning system was composed of high strength T-headed steel bars and special steel cross heads for anchorage. The post-tensioning system provided vertical and horizontal "clamping" forces that actively engage this reinforcing system and reduce beam cracking and deflection.

Installation: Core holes were drilled through existing transverse beams to install the vertical T-headed bars. To avoid damage to existing steel bars, small areas were chipped open on the topside of the beams prior to coring. Locations of the core holes were then adjusted to minimize damage to existing steel reinforcement. At less deficient locations, externally bonded FRP system was used to provide the additional tensile reinforcement. To address issues related to FRP development length and anchorage requirements, bonded concrete block was cast-in-place on the underside of each raker beam. At locations where pre-cast concrete tubs were already installed on the raker beams, a special jacking system was used to lift the tubs during strengthening installation. To achieve this, several hydraulic jacks were operated simultaneously to avoid overstressing the precast concrete elements.