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Case Study

Highways Agency (Now Highways England) Mossback Viaduct

Highways England (formerly the Highways Agency) is a government-owned company with the responsibility of managing the core road network in England. It operates information services, liaises with other government agencies and provides staff to deal with incidents on the roads it manages. The company managed The Mossband Viaduct, which carried traffic over a roadway and railway until its demolition in 2008.

Project Details

Services
SoundPrint® Acoustic Monitoring – Bridges

Monitoring system commissioned in 2001

Operated continuously until bridge demolition in 2008

Bridge Type
Post-tensioned concrete
Monitored Length
836 ft (255 m)
Number of Spans
8
Number of Sensors
210

Project Highlights

System has performed in excess of 99% of efficiency over its life

SoundPrint identified and located 6 specific wire break events

Structure life extended over 7 years via structural health monitoring

Client estimated economic benefits $30 to $40 million

Challenge

The viaduct was comprised of twin decks – one deck was constructed of concrete girders butted up against each other, with a top cantilevered slab, while the second deck was a voided box-girder. Half of the spans were post-tensioned cast in-situ concrete table spans and four were suspended spans supported by the table spans on half-joints.

Conventional visual investigations were performed in 1990, 1995, and 1999.

The first investigation discovered water ingress at all the deck joints and next to the drainage pipes. This was believed to have been occurring over many years. General corrosion of the surface reinforcement in these areas resulted in surface spalling.

The second inspection revealed the presence of several corroded and broken tendons in the in-situ table spans. The damaged tendons were located in the deck over the pier supports, where the cable profiles approached the top surface of the deck. A deck construction joint within a meter of the pier support provided a direct water path to the tendon ducts. The cable profiles descended from this location into the mid-span of the table span and to the half-joint anchorage area.

As is often the case with selective visual investigations, one location often showed severe corrosion while an adjacent location appeared undamaged. In this case, the third inspection showed that some of the longitudinal tendons had all the strands completely corroded whereas only two meters away, they appeared in good condition. Clearly, the tendons at the half-joint locations were at risk at all 14 locations, but the extent of deterioration at every location was unknown.

Solution

The Highways agency implemented a comprehensive bridge management plan starting in 1999 to assess the rate of deterioration of the post-tensioning, and if necessary, to intervene and strengthen the structure. ­The plan consisted of:

  • Monthly visual inspections of critical sections
  • Installation of vibrating wire stain gauges to monitor cracks on the sides of the sections and soffit of construction joints
  • Load testing to compare stain changes
  • Installation of a SoundPrint acoustic monitoring system to monitor the rate of deterioration of the post-tensioning system

In late 2000, 210 acoustic sensors were installed along the 836 foot (255 m) length of the viaduct in three rows to fully monitor all post-tensioning tendons. ­The sensors were multiplexed at local junction boxes to minimize cabling and data acquisition requirements. Data was acquired via a single acquisition unit calibrated to reject the majority of non-wire events, with events of interest transferred to servers in Calgary, AB for analysis.

Results

The wire break rate found was lower than expected. Six wire breaks were detected during the extended life of the structure, giving engineers/owners confidence that the bridge management program was effective. Further, strain readings during the AIL annual load tests showed that the structure was not experiencing severe changes in deflections, and that the serviceability requirements were being met. ­ The crack growth was also monitored, and thought to be consistent with the rate of deterioration observed with the acoustic system.

 

In this case, the acoustic monitoring system was used to extend the life of the structure by seven years and 10 months, until a new bridge could be built as part of the A74 Cumberland Gap. Approximately $1.4 million was spent on the two monitoring systems and the load tests over 8 years, including the system-related inspection and reporting functions by engineers.

 

Client estimated that the economic benefit in delaying the permanent bridge replacement and not fast tracking a temporary structure was between $30-$40 million dollars.