Smithfield-Liberty Helix Ramp Rehabilitation
This article contains content from our submission for the ICRI Project of the Year in 2018. Our team was awarded as a 2018 Project of the Year Finalist within the Longevity category for exemplary solutions and an efficient project approach.
Longevity Features
The rehabilitation of the Smithfield-Liberty Parking Garage was undertaken in 1997 and, over 20 years later, the structure exhibits no additional deterioration. Exposed to several hundred cars each day, this critical structure was rehabilitated using materials and methods designed and installed to provide a long-term service life. The success of the project is best evidenced by the fact that no additional structural repairs or rehabilitation have been required since the work was completed. The design and construction processes used to complete this project in the late 1990s is a testament to the longevity that the project has enjoyed. Based on its current state of performance, it is expected that the ramp will likely experience another 20 year extension of its service life without the need for any significant attention.
History
Originally constructed in 1964, the helix ramp of the Smithfield-Liberty parking garage in downtown Pittsburgh, Pennsylvania, is six levels of post-tensioned concrete slab with perimeter knee wall cantilevered from a conventionally reinforced concrete cylinder. The post-tensioning system for the structure is composed of solid bar reinforcing encased in a grouted conduit. The solid bars have plates at each end to provide compressive force transfer into the slab. The primary visible distress of the structure was exposed, corroding post-tensioned bar anchor plates at the perimeter of the ramp. This circular ramp provides the only means of exit from the top six levels of the attached seven-story parking structure, so it was decided that conventional rehabilitation project delivery methods would not work for this rehabilitation.
Evaluation
In late 1996, an evaluation of the helix ramp was undertaken to establish the cause of the observed distress in the structure. The evaluation included a condition assessment of the structure, concrete material and corrosion testing, a review of existing structural drawings, and a thorough structural analysis.
The condition assessment included a visual survey to record visible surface defects, including cracks, spalling, and exposed corroding steel elements. Material and corrosion testing included the following:
Petrographic analysis
Concrete compressive strength testing
Acid-soluble chloride ion testing
Carbonation testing
Electrical continuity testing
Half-cell potential testing
Corrosion rate testing
Reinforcing steel location and cover measurements
The results obtained from the condition assessment, structural analysis, and various testing methods were necessary to fully evaluate the distress mechanisms occurring within the structure.
Diagnosis
After evaluating all of the results, the cause of the distress in the structure, consisting of delamination and spalling with exposed reinforcing steel, was determined. Based on the evaluation, three primary causes for the observed distress were identified:
The cause of the distress recorded on the top of the ramp slabs was determined to be high chloride ion levels in the lower five levels of the six-level ramp. On these levels, chloride contents exceeding the threshold amount necessary to induce corrosion of the reinforcing steel were found in the top 2.5 inches of the slab.
The cause of the distress observed on the underside of the ramp slabs was cracking at old patches in the top slab.
For the deterioration occurring on and below the concrete knee walls, insufficient concrete cover on the steel reinforcing and plates was identified as the cause of the distress.
Once the causes of the distress were identified, solutions were developed to address them in consideration of alternative service life expectations.
Solution Analysis
Chloride Ion Content
To address the chloride ion content problem with the ramp slabs, removal and replacement of the top 2.5 to 3 inches of the concrete floor slab was ultimately selected to offer a long-term service life expectation. This solution represented a potential structural problem, however, given that the slab was post-tensioned. Prior to being able to recommend that the top portion of the slab be removed, a structural analysis had to be performed. The analysis was necessary to determine the post-tensioned reinforcing forces on the original slab section, the reduced slab section (once the top portion of the slab was removed), and the final slab section (with the original slab and new topping slab). Upon completion of the analysis, it was determined that the top slab section could be removed if the perimeter of the ramp was shored and supplemental post-tensioned cables were added to the final cross section.
Supplemental Post-Tensioned Cables
The requirement for the supplemental post-tensioned cables influenced the decision to recommend a high quality conventional concrete material with a compressive strength of 6,000 psi to closely match the existing concrete strength. In addition, the use of a shrinkage-compensating admixture was recommended to minimize cracking in the new topping slab.
Underside Slab Condition & Water Infiltration Problems
To address the underside slab condition and water infiltration problems, conventional partial-area patch repairs were recommended in conjunction with the application of a hybrid polyurethane fluid-applied membrane with epoxy wear course with specialized aggregate on the top ramp surface. The patch material recommended for the underside slab repairs was a polymer-modified cementitious repair material to facilitate use of the form-and-pump repair technique. The repair material was selected to have compressive stiffness characteristics that closely matched that of the existing concrete. This was necessary to provide uniform compressive stress distribution throughout the concrete slab when stressing supplemental post-tensioned reinforcement.
Concrete Cover Issues
To address the concrete cover problem on the post-tensioned reinforcing anchor plates on the perimeter of the ramp, a new concrete cap was recommended to provide suitable cover for the plates. In addition, a drip edge was recommended to prevent water from running down the underside of the ramps. Although this repair detail reduced the depth of the reveal at the slab perimeter, the architectural appearance of the ramp was not significantly changed. To address the concrete cover problem on the existing reinforcing steel in the knee walls, it was recommended that patch repairs be slightly over-built to obtain suitable concrete cover on the reinforcing steel. The visual effect of the patch overbuild was reduced by enlarging the patch area to the extent where existing reinforcing steel had sufficient concrete cover.
Rehabilitation
The repair solutions described above were incorporated into the contract documents and issued for bidding by experienced repair contractors. The successful bidder was awarded the contract in the summer of 1997 and the work was immediately scheduled to be completed in under 10 weeks during the summer to coincide with the garage’s off-peak season.
Garage Ergonomics
Prior to beginning repairs, the traffic in the garage required re-routing. This was necessary since the helix was the only means of exit for the upper levels of the garage and closing the garage was not an option. After considering alternate scenarios to solve this dilemma, a solution was developed to convert the one-way traffic flow into two-way traffic. To accommodate the two-way traffic on the upper six levels, turn-around areas were established on alternating levels to facilitate cars changing direction. Although the turnaround areas resulted in a reduction in parking spaces, the traffic flow was not significantly hampered and the disruption to patrons was minimized.
Fast-Tracked Construction
Once parking traffic was re-routed, the helix ramp was closed, and construction commenced. Given the aggressive construction schedule and limited work area, methods to expedite the repair process had to be implemented. The primary time saving measure utilized during the rehabilitation was hydro demolition, which is a process utilizing water under very high pressure (about 10,000psi) to demolish concrete. This method was used in lieu of conventional jackhammers to remove the top section of chloride-contaminated concrete on the ramp slabs and resulted in significant time savings.
Specialized Equipment
Prior to initiating repairs, the contractor requested the substitution of specified specialty repair materials for materials that they had success with on previous projects. This practice is common on concrete rehabilitation projects but is one that must be carefully considered. For this project, different areas, and even different levels, had different design criteria that had to be met. For example, the post-tensioned areas required a material with a compressive stiffness closely matching that of the existing concrete due to the need to evenly distribute new post-tensioning stresses induced in the slab. The cap placed around the helix ramp perimeter required a material easily placed using the form-and-pump method, but one that did not require similar stiffness characteristics to the existing concrete.
Completion
Following industry standard concrete repair practices and incorporating state-of-the-art materials, the rehabilitation of the ramp was completed on schedule. One of the most important reasons for the success of the project was that the correct process was followed. Involving the correct experienced parties and identifying and addressing the causes of the distress in the structure and developing and implementing a repair approach aimed at providing a long-term service life extension.
Past, Present, & Future
Prior to implementing the rehabilitation described above, another repair of the helix ramp had been undertaken. Partial-area patches completed during that project were found to be deteriorated at the time of the condition assessment performed in 1996. Past repairs are often found to be deteriorating due to the ongoing internal corrosion mechanisms within the structure. Short of implementing cathodic protection or complete replacement, the corrosion process will likely be ongoing, to varying degrees, in most parking structures throughout their service lives.
Given this situation, the concern becomes whether past repair methods and materials addressed the cause(s) of the distress in the structure. Unfortunately, past evaluation and testing techniques fall short of the standards in place today. Rate of corrosion testing, for example, was in its infancy as of 10 years ago. The lack of specialized knowledge about the corrosion process is one of the primary reasons that past repairs are more prone to premature failure than repairs completed today. Another common problem encountered with past repairs was the practice of engaging in an inexperienced contractor to “fix a problem.” In most situations, that contractor didn’t thoroughly evaluate or understand the problem, nor did he properly identify the cause of the problem.
Today, knowledge in the field of concrete structures and corrosion mechanisms is steadily growing through experience and research. Experienced contractors are more prevalent, as are experienced engineers and material specialists. Present day materials and products are being produced to provide a higher quality product. The topping material used on the helix ramp project, for example, incorporated a shrinkage-compensating admixture to control cracking of the material over the existing concrete substrate. Use of this product resulted in a crack-free topping placed in two separate pours on five levels of the ramp. It is expected that these trends toward more specialized experience and product quality will continue.
Although it is difficult to predict the future of concrete rehabilitation, the continued expansion of knowledge and experience in the field is virtually guaranteed. Continued research should expand the understanding of the corrosion process and corrosion control mechanisms. This, in turn, will lead to the development of materials and products that can better control the corrosion process and facilitate longer lasting concrete repairs.