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Project: Watsontown Bridge
Owner: Commonwealth of Pennsylvania
Location: Watsontown, PA
Description: Vacuum injection of underside of nine spans/barrels
Completed: June 2004

The Watsontown Bridge was built circa 1927 as joint effort of the Board of Commissions of Northumberland and Union Counties PA.  The structure provides transportation over the west branch of the Susquehanna River between Watsontown and White Deer. 

Delamination and debonding especially around reinforcing of the bridge’s nine concrete barrel arches were identified using traditional and non-destructive impact echo methods.

Restoration of the concrete spans required a combined effort of conventional and specialty techniques.

Structure Characteristics
The structure is a nine span reinforced concrete arch bridge having a concrete abutment on either side of the river and eight intermediate piers to support the nine 120 foot fixed span arch barrels.  The arch barrels rise 19 feet above the spring line with a 21 foot 9 inch width.  The nine spans are constructed with eight open spandrels supported by spandrel column walls.  The center portion of each span is concrete filled.  The arches are nominally 30 inches deep at the center of the span.

The deck is a 10-inch reinforced concrete slab with a waterproofing and an asphalt-wearing surface.  The roadway is 20 feet wide with a concrete curb on each side and 4 foot sidewalk on the north (upstream) side.

Problems that Prompted Repair
Visual inspection identified extensive deterioration of the superstructure deck railing and edges of the arches.  Similar conditions of disrepair were observed on the piers above the spring line.  Much of the pier deterioration is attributed to the failure of the deck expansion joints allowing water and salts to deposit on the top and faces of the piers.  Portions of the structure were beyond repair and required complete removal and replacement of concrete.  The majority of identified flaws using non-destructive impact echo were found as delaminations and debonding/poor consolidation at reinforcing.
Total removal and replacement of the bridge was a last resort option.  Due to the various depths of delaminations poor consolidation at reinforcing generally poor quality of the concrete mix and the friable surface on the top of the arches the only reasonable alternative other than the Balvac vacuum injection process was to completely raze the bridge and replace it with a new structure.

Inspection / Evaluation Methods
Engineers Goodkind & O’Dea Inc. conducted a comprehensive rehabilitation study in November 1997.  This involved an in-depth inspection of the superstructure above the spring line and substructure. Sounding with hammers and visual inspection was performed to identify and classify the various conditions of damage and deterioration including cracks spalled areas scaling map pattern cracking efflorescence disintegrating and honeycombing.

In July 2003 Balvac was appointed to evaluate damage using a method of non-destructive testing called impact echo. The purpose of testing with impact echo was to identify conditions of disrepair not otherwise evident by visual examination or detectable by conventional hammer sounding / acoustic impact. Furthermore test results using this method were not dependent on the subjective interpretation of operators and therefore gave an objective assessment of damage.

A representative sampling of the arches on all nine spans using impact echo was initially based on a nominal three-foot grid pattern to establish the integrity of the concrete.  Spans three and five were tested over the entire length of the arch underside. In the other seven spans the 3 foot grid pattern was employed at the center of the arch span and a 6 foot grid was used over a portion of the remaining span. The abutments and wing walls were also tested on three foot grid pattern.

Test Results
The 1997 study by Goodkind & O’Dea Inc. found conditions such as spalling and shallow delaminations on the underside of all nine spans much of which was visually observable or detected by hammer sounding.

The conditions of disrepair had noticeably increased since the 1997 study.  Delaminations and spalled surfaces had grown by 10% to 20% by some estimates since then.

Conditions of disrepair identified by impact echo in the 2003 Balvac analysis were isolated throughout the entire span in the areas tested.  However in no instance did impact echo identify any condition of such large magnitude that would appear to have structurally compromised the overall integrity of the arches in respect to their physical properties.

Due to the numerous shallow depth anomalies in the areas of reinforcing the inherent property of poor consolidation and delaminations at varying depths impact echo wave propagation was generally limited to less than full depth of the arch.  Therefore any condition of disrepair located at the top of the arch was not detectable or identifiable.

There were transverse cracks in two locations in each span.  These cracks were the full width of each the arch barrel and appeared as cold joints.  Without exception the cracks were open and had clean evidence of active water penetration.  This condition required filling and sealing using vacuum injection.  Consideration was given to using a low viscosity resin with elongative properties to account for future thermal changes.

Causes of Deterioration
The failure of the roadway joints and drains are the most significant contributors to the deterioration below the deck surface allowing continued exposure of salt and road debris on the spandrel walls and fascia beams as well as the top and face of the piers and abutments. The walkway on the up stream side and the deck and railing were subjected to constant road salt and water spray from vehicles.  The outer surface of the concrete was badly eroded from age and the elements.  Once surface cracks opened the high chloride exposure and water penetration caused further damage from frees-thaw cycles. 

Repair System Selection
The Balvac process of vacuum injection and flushing was recommended to repair conditions on all visible surface open cracks particularly the two full width transverse cracks/joints on each span and friable surfaces on the top of the arches.  This process dries out the surface and related internal network of cracks and voids by evacuating air before the introduction of bonding polymers under a partial vacuum.

Conventional removal and replacement of delaminated concrete replacement of spalled areas and filling of selected surface open cracks was essential to provide longevity and serviceability to the arches.  Due to the extreme surface erosion on the underside of all spans a sealer was recommended to mitigate moisture intrusion through porous concrete and micro-cracking.  Without a sealer all existing surface repairs would eventually result in shallow delaminations and spalling.

Repair Process Execution
Due to the numerous areas of damaged and deterioration observed on the surface Impact Echo testing was performed on the under of the arches and abutments above the spring line. Access was provided by use of snooper from the deck.  A nominal two foot grid was used on the entire intrados of the arch of spans 2 and 5.  The remaining spans were tested on a two foot grid at the arch center where it was in contact with the deck and a four foot grid over the remaining surface up to the spring line.

The extrados could not be tested prior to construction as access was impaired or prohibited. Once the deck and railing was removed the extrados was test in a similar pattern as the intrados.

Impact Echo finds were studied and results plotted to identify the type and extent of the various conditions of the arch.  The results of these finds set the program for repair of internal delamination de-bonding poor consolidation and similar conditions of otherwise sound concrete. 

The Balvac vacuum injection/impregnation process was selected as the method for repair.  Surface open cracks were vacuum impregnated. Internal conditions were vacuum injected through access holes drilled to the depth of the various conditions.  The friable condition of the extrados of the arch was solidified and sealed by Balvac prior to the placement of the new deck slab.


View of the nine concrete barrel arches