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Project: Cherry Street Parking Garage
Owner: State of Vermont
Location: Burlington, VT
Description: Non-destructive testing and vacuum impregnation used to restore structural integrity to parking garage
Completed: July 2004

The Cherry Street parking garage owned by the State of Vermont was originally built as an open structure. After one year of use it was closed for the construction of a three story steel framed office building above it. The harsh northeastern winters took their toll on the garage with spalled concrete damaging cars and potentially harming pedestrians.

Structure Characteristics
The parking garage has 388 bays over three levels covering 120000 square feet. The lower level is on grade and the upper two levels are raised having 78000 square feet.

The raised levels were constructed using a “filigree” deck system; a 2 1/4 inch thick pre-stressed precast concrete deck which functions a as a permanent stay-in-place form with a 3 3/4 inch reinforced cast-in-place concrete composite slab for a total thickness of 6 inches.  The precast deck/form is dished 6 inches deep at the columns thus creating a beam of 12-inch total thickness by 8 feet wide. Composite action is developed from the precast to the cast-in-place slab with trusses cast in the precast deck.

Problems that Prompted Repair
A condition assessment documented rust staining throughout the underside of the slab leaking of salt and water as well as cracking up to 1/8 inch.  In many locations the bottom surface of the filigree slab had fallen off /spalled damaging vehicles and imposing a serious threat to pedestrians.

Inspection / Evaluation Methods
In January 2002 the owner had 14 core samples taken at areas of sound construction and at damaged or suspect areas in the raised slabs. A specialize testing laboratory was retained to perform petrographic analysis of two core samples and to document the chloride-ion content at various depths on all 14 cores.

Local engineers McFarland-Johnson Inc. made a condition assessment. Balvac was retained by them to perform non-destructive testing using impact echo to determine the structural integrity of the composite construction specifically the possibility of debonding of the composite slab. 

Test Results
Findings from the January 2002 petrographic report indicated that the concrete mix used for the deck was relative good having the physical properties required for an aggressive environment. Carbonation was identified to depths of 3/8 inch.  Traces of oxidation were observed in all 14 cores.

Conditions such as debonding and poor bond of the cast-in place slab while not prevalent were found at random locations throughout the 10 areas tested.  With the exception of the entrance area defects such as honeycombing and delaminations were detected in approximately 30% of the test areas identified in the previous investigation. The most common flaw observed was at the reinforcing in the precast filigree slab.

Causes of Deterioration
The garage is used by several clients: from 6:00am to 5:00pm by employees of the State of Vermont Board of Health their associate organizations and employees of Vermont Department of Justice; and through the day until 10:00pm by Vermont Community College.  As the State of Vermont experiences long and severe winters there is very limited opportunities for preventative maintenance.

The slab is exposed to constant water and salts with all 388 parking spaces occupied for the majority of the year. The deck surface is also subjected to steel studded tires oils and chemical deposits from vehicles.

The constant intrusion of water and chlorides through the cracks and spalled areas accelerated if not exacerbated the deterioration process. Caulking of the joints had also failed and had contributed to further demise of deck integrity.

Repair System Selection
Restoration required a combined effort of conventional and specialty techniques.  It was essential to the overall repair program to first repair delaminated concrete and spalled areas and fill selected surface open cracks by use of vacuum injection.  Conventional removal and replacement of slab joints that had failed throughout the garage followed. All calked joints were removed and re-caulked.

The Balvac vacuum injection and impregnation process was selected to repair conditions such as through cracks debonded and poor bond of the composite construction and delaminations at reinforcing.

Upon completion of the vacuum impregnation an epoxy protective coating was applied to top of the slab. Grit was applied to provide a non-skid surface.

Repair Process Execution
The extent of damage and disrepair to the composite slab was identified with non-destructive testing using impact echo and by visual observations. This was found to be localized and limited; however testing over the entire bays in the areas of spalled concrete indicated internal conditions such as debonding and poor bond at the interface of the composite section. Further testing identified flaws at the truss reinforcing both at the top of the truss in the cast-in-place slab and in the bottom of the truss in the precast filigree slab. No areas of the garage slab tested under this program damaged or otherwise gave concern for a loss of structural integrity.

The deck top surface required conventional shall depth patching with a cementicious material compatible with the impregnation resin prior to the vacuum impregnation.  As a composite design the Balvac vacuum injection and impregnation process was the first stage of repair employed to fill any area of de-bonding to mitigate further loss of bond at the slab interface and reinforcing. 

Upon completion of the deck vacuum injection/impregnation the above noted conditions of spalling surface delaminations and deck cracking were repaired. The underside spalling and leaking cracks not only presented a liability in regards to vehicle damage they were also a safety concern. A structurally compromised slab would inevitably result from a worsening of these conditions.

The second stage of repair was the application of deck surface coating. Water penetration would inevitably accelerate concrete deterioration and oxidation of the reinforcing. Furthermore the high levels of chlorides would continue to compromise the slab unless moisture and oxygen was shut out. The only effective coating was a resin impregnated with an anti-slip finish.

Installation of Vacuum Shroud and Vacuum Ducts