2431 N. Clark St. – The Full Story
The 91,000 SQFT "building" at 2341 N. Clark Street in Chicago, located a few blocks north of where the St. Valentine's Day Massacre was perpetrated, is actually the assembly of two buildings that date back to the 1920s when Al Capone ran Chicago's North Side. About 40 years ago, these buildings were joined to create a 66,000 SQFT parking garage that was constructed along with 25,000 SQFT of commercial space and seven storefront shops and restaurants to serve residents of a high-end apartment building that overlooks Lincoln Park and Lake Michigan. The parking garage takes up the entire second floor of both buildings, as well as the eastern half of the first floor.
Completely Renovated Garage
The Problem
The owner of the property noticed several cracks and structural delaminations throughout the garage. This caused water to leak into high-end shops at the west end of the property, leading to unsightly water damage on ceiling tiles and complaints from shopkeepers. Over time, the cracks, water leakage and delaminations raised the possibility that concrete could fall near cars and pedestrians at the east end of the property, leading to citations from city inspectors. At the east end of the garage on the first floor, temporary shoring was used to prop up delaminated areas of the ceiling and tarps were put in place to prevent mineral leeching and paint tarnishing on resident vehicles – all of which looked quite bad to anxious residents and had them wondering why they were paying up to $30,000 per year for a parking spot and if they should fight for parking spots outside on the street instead.
Additionally, cracks were found at the west foundation wall of the north building, the south garage's wooden truss ceiling needed reinforcement, and a lack of drainage was present throughout the east end of the second floor of the parking garage. It was later discovered that the floor drains were there, but that over the years they had been by covered with asphalt and concrete patching and overlays.
The Cause
Preliminary core samples taken two years prior found high readings of chloride in the concrete. After making a few investigative openings, the restoration contractor discovered that the elements and salt infiltration from cars bringing in snow and salt during sub-zero winters, combined with expansion from 100 degree-plus heat in summer, had caused the structure's rebar to corrode and the concrete to crack, leading to an overall failure of the 4" thick structural slab, ceiling beams and joists.
The entire west end of the parking garage featured a waterproofing membrane buried under a layer of asphalt, but this membrane was well beyond the warranty period and consisted of cold tar pitch – a material that has since been replaced by far more effective waterproofing technology.
Example Full-Depth Repairs
Mitigating Factors
Because several necessary full-depth repairs were directly above the commercial storefronts, the stores underneath were required to close down for up to 48 hours so that the work could be completed without creating a safety issue for tenants and customers. This required the restoration contractor’s project manager to coordinate all store closures alongside the property manager with each individual shop owner to minimize business disruption.
The Solution
Zera Construction began work by removing 18,000 SQFT of the asphalt overlay with Bobcats equipped with "teethed" buckets and jackhammers. Rotary scarifying machines were then used to remove the existing buried membrane system down to the structural slab. With the top layer of asphalt removed, the restoration contractor then evaluated the condition of the structural slab with the nondestructive chain drag method, in accordance with ASTM D 4580. Many delaminations were found, leading to partial and full-depth repairs to the structural slab, including those that went through to the stores below.
While most shop owners were accommodating, some expressed varying degrees of resistance. By working closely with them, the restoration contractor was able to develop an overall schedule that not only fit in all the work but also minimized closures and performed work on slow days for each business. In addition, the overall project schedule was kept in line with the property owner's expectations. This was done by working in three shifts to rig up shoring, completely encasing the area in plastic for dust protection, breaking out the concrete, framing the area with plywood, injecting expandable foam (Great Stuff) to prevent debris from falling into commercial storefronts, and then pouring in the new concrete. In order to cut 24 hours off of the schedule, the wooden formwork was secured and left in place once the shores were removed. This formwork was abandoned and left in place to aid in the curing process, and was invisible to the stores below once the finished drop ceilings were reinstalled.
Anodes Installed & Tied to Rebar
Lightweight air hammers were used to remove the unsound top surface delaminations, and concrete was removed below the existing reinforcing steel as well as so that the new concrete patch would properly engage. The reinforcing steel was then sandblasted and cleaned, and supplemental reinforcing steel was added to areas that experienced too much section loss. The existing reinforcement was then coated with a layer of protective zinc-rich protective coating. Galvanic anodes were also placed around the perimeter of the patches for corrosion prevention and control. 2,000 SQFT of repairs were also made to the underside of the structure, including beams, joists, walls and columns.
Concrete Beam Repair and Rebar Reinforcement
Complete beam replacement was then planned for three concrete ceiling beams but further investigation revealed three additional beams that also required restoration. In order to make all repairs as cost-effectively as possible, in accordance with the owner's and structural engineer's wishes, partial-depth repair was used in conjunction with carbon fiber reinforcement from BASF. The combination of partial depth repairs and the use of carbon fiber enabled the restoration contractor to prevent having to perform more expensive full beam removal and replacement. This saved the owner thousands of dollars.
Once all the structural concrete repairs were completed and cured, the restoration contractor then applied 35,000 SQFT of American Hydrotech 6125 Monolithic Membrane over the areas that would receive the asphalt topping. A 48-hour water test was then performed to verify that the repaired areas were leak free. Installation of the asphalt topping presented some challenges since the portions of the garage to receive the paved overlay were located on the second floor, within tight quarters in a congested urban environment, leading the restoration contractor to bring in all asphalt with Bobcats and then place and roll it with small pavers and rollers to meet the height and weight restrictions of this garage.
Installation of BASF Carbon Fiber Reinforcement
Installation of American Hydrotech 6125 Monolithic Membrane
Portions of the garage that did not receive the asphalt overlay were protected with BASF Conipur II Deck Coating System.
Additional work included:
- • Injection of urethane into cracks at the west foundation wall of the north garage
• Tuck-pointing of deteriorated brick areas within the garage walls
• Repairs to the wooden truss of the south garage to strengthen structural integrity
• Installation of all new MEPs, including 40 new drains and lighting
In fact, the restoration contractor was able to reuse nine drains that had been covered by asphalt. The parking garage was then completely repainted and the parking spaces re-striped and numbered.
Completion
Work began on July 28, 2008 and was completed on December 11, 2008. All project dates were met on the bid sheet, even though many unforeseen repairs were needed. This could only have been accomplished because all restoration contractor associates worked together closely with the structural engineer, property owner, site engineer, and tenants.