Non destructive testing of bridge decks

| Transport

Car park structure inspection

Jonathan Newell looks at how ageing road infrastructure can be quickly and easily inspected to assess reinforcement corrosion and bridge deck delamination.

Highway maintenance authorities in both Europe and America are faced with the increasingly complex dilemma of balancing maintenance prioritisation with tumbling budgets. As much of the infrastructure is ageing considerably, the maintenance challenges are becoming more critical, with many potentially disastrous defects being hidden well below the road surface.

Elevated sections and bridge decks in particular are prone to problems of delamination and corrosion of steel reinforcement bars. Inspecting for these defects has traditionally been achieved using coring and sampling the road, a technique which suffers from four significant problems:

* Disruption: Whilst samples are taken and repairs made, the section of the road needs to be closed.
* Speed: The process is slow, requiring samples to be collected and laboratory analysis to be performed.
* Damage: The process of coring damages the road and this has to be repaired before the section can be used again.
* Sample density: Due to the speed, disruption and damage caused by sampling, highway engineers need to choose coring positions very carefully and sparingly. Even with such care, the samples taken might not be representative of the overall condition of the section of road.

Now, modern technology is being used widely on US highways to assess the condition of bridge decking and elevated sections using a combination of Ground Penetrating Radar (GPR) and InfraRed (IR) Thermography.

NDT at normal road speeds

GPR can be used for the detection of reinforcement depth, its level of corrosion and the deterioration of concrete; IR Thermography is used to map delamination within the bridge deck. Both tests are performed at normal road speeds up to 50mph (80kph) and data are collected using a suitably equipped vehicle making a series of passes along the road in each lane to collate data across the width of the deck.

The sensor systems can be attached to highway maintenance vehicles for road analysis or to hand-held apparatus for measuring car parks and other reinforced concrete structures.

According to American GPR specialist, Infrasense, the analysis of the radar signals for assessing deck condition is easy, since strong, uniform reflections indicate good condition, whilst weak or inconsistent reflected GPR signals indicate deterioration of the bridge deck at the reinforcement level.

The IR Thermography survey generates images every 12 inches (300mm) that the survey vehicle travels. Deep delamination of the bridge structure can be detected even at the reinforcement level due to the uneven heating caused by the voids left by the delamination process. Such voids appear as brighter images on the survey.

Knowledge to action

A key aspect of surveying road and bridge condition is the presentation of the survey results to the highway maintenance department as a recommendation of work, or a job card. The Infrasense system produces survey results that are stitched together from multiple passes to present highway engineers with a condition map of the roads that have been studied.

Such maps which include visual representations of moisture content, sub-surface voids and IR hotspots provide a good indication of what the expected remaining life of the road would be as well as helping to focus on which areas of the road require the most attention. However, it doesn’t show maintenance crews where to start digging.

But using Infrasense’s software, highway engineers can use the delamination maps to create a plan of repairs and export files in a format readable by Computer Aided Design that can be used for showing which specific parts of the road require attention and generating job cards for maintenance crews.

Jonathan Newell
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