Non-destructive bridge inspection methods

Technique Method Advantages Drawbacks

Acoustic techniques

Chain or hammer dragged and human ear discerns changesDetects delamination

Cheap and easy to do with limited training Does not work on bridges with asphalt overlaysDoes not provide 100% repeatable resultsCannot be used for planning repairs to deteriorated areas that are not yet delaminated

Half-cell potential

Assesses rebar corrosion by measuring the voltage between the rebar in the concrete and a reference electrode on the surface

Can detect corrosion before it has become delamination

Requires bare concrete – does not work with asphaltBridge has to be closedVery time-consuming

Infrared Changes in infrared radiation from the surface concerete indicates delaminations

Can be done quickly and with a moving vehicleMinimises bridge downtimeMaximises human safety

Must be done when there is a large thermal gradient between bridge/ambient temperatureCannot be done on bridges with asphalt overlays

Visual inspection

Inspectors visually mark cracks, spalling and potholes on a map

Simple low tech method for fi nding areas in immediate need of fi xing

Addresses problems only after they have resulted in most damage

Ground-penetrating radar

Uses radar pulses to image the surface Quantitative – not subjectiveWell suited for prioritising for budgetingCan be used for concrete and asphalt

Long learning curve

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n ASSET MANAGEMENT

Detection of deterioration on bridge decks can be carried out using a new system based on ground-penetrating radar. Michael Arvanitis explains

GOING UNDERGROUND

Many bridges in Europe are structurally defi cient or functionally obsolete. Since national budgets are insuffi cient to meet maintenance demands, it is important to use diagnostic tools that can provide an early warning of construction faults and detect bridge deterioration before it leads to a tragedy like a bridge collapse.

Inspection methods are also needed to make sure the investment in new bridges is sound and construction can stand up to years of service. Ground-penetrating radar stands out as the best method for assessing the quality and uniformity of an asphalt or concrete surface, and detecting deterioration on bridge decks. New GPR technology is available to improve bridge deck condition assessment and also help avoid premature road failures that result in costly and time-consuming repairs.

The problem of bridge obsolescence in Europe is diffi cult to quantify because bridge inspection requirements and methods, condition assessment, and maintenance procedures differ so much from country to country. Nation-specifi c construction processes and historical and political differences greatly affect bridge maintenance. One thing is certain — the number in structural danger is on the rise and the risk of catastrophic bridge failures continues to grow.

Aside from the obvious public safety, concerns about bridge inspection, economic concerns are also critical. Experts warn that newer EU states have primarily focused on constructing new roads and are not maintaining existing infrastructure. In some central European countries, roads and bridges have deteriorated so severely that they cannot handle their current load, let alone the increases expected as commerce grows.

Inspection methods are usually divided between non-destructive testing methods — those that evaluate the properties of a material, component, or system without causing damage — and destructive methods, like coring and chipping.

Ground-penetrating radar technology has a number of advantages over other available non-destructive inspection methods for assessing the quality and uniformity of

an asphalt or concrete surface, and detecting deterioration on bridge decks. The method is well suited to prioritising for budgeting purposes; whereas acoustic methods are subjective, GPR data is quantitative and noise does not affect readings.

GPR uses radar pulses to image the subsurface. Electromagnetic radiation in the microwave band (UHF/VHF frequencies) of the radio spectrum detects the refl ected signals from subsurface structures. The two most common types of GPR for bridge surface measurement are ground-coupled and air-launched. Ground-coupled systems rely upon an antenna that is placed very close to the roadway/surface while air-launched systems use directional antennae aimed at the surface from a height of 300-500mm. Ground-coupled antennae have a reputation of being less prone to radio frequency interference from mobile phone towers and TV broadcasting, but typically operate at speeds that are below normal highway limits. Air-launched antennae can collect data even when travelling at more than 100km/h, and are located at a safe distance from the surface, typically some 400-500mm above the ground.

Typically, a cart-based system is used for bridges; data is collected at a walking pace or from a vehicle travelling about 8km/h. This will include the antenna and a controller. The radar technology looks for weakness in the returning radar signal — ie weaker amplitudes — from the reinforcing steel. The weaker the signal, the more deteriorated the concrete. The technology can show the location and depth of rebar, tie bars, and dowel bars.

Condition assessment can be performed using both air-launched horn antennae and ground-coupled antennae. The latter provide better horizontal resolution, which is suffi cient to enable imaging of individual rebars in the top mat, typically not possible with horn antennae. Ground-coupled antennae collect densely spaced measurements along lines that are oriented so they cross over the top rebar in the upper mat at right angles (or close to a right angle if the rebars are skewed). The amplitude of the radar wave refl ection from each rebar is recorded versus its location on the bridge. Relative

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AsseT MANAGeMeNT n

Scanning vibrometerS achieve higher SenSitivity

changes in the rebar reflection amplitudes are typically indicative of the condition of the rebar and/or concrete above it.

GPR can also be used for quality assurance and quality control of deck thickness and concrete cover assessment on a new bridge, to confirm whether the depth of the rebar and the thickness of concrete meet the required specifications. These are often done during quality assurance of the bridge deck after it has been poured to ensure the top rebar mat is at the depth range specified in the bridge plans.

One example of GPR technology for bridge deck condition assessment is the Bridgescan system, a structured approach to collecting, processing and interpreting GPR data developed by GSSI. The procedure provides a map of rebar reflection amplitudes.

The areas with the lowest rebar reflection amplitudes (yellows and reds) correspond to portion of the bridge deck containing the most distress in terms of concrete deterioration and/or rebar corrosion.

Another example of new technology that can be used for bridge construction is the Pavescan RDM asphalt density assessment tool, which provides real-time data for on-site results by correlating the dielectric measurements with the density of the new pavement. This technology enables users to obtain critical density data for quality assurance and quality control of new pavements. Unlike nuclear density gauges or radioactive alternatives, Pavescan does not result in any site hazards nor does its use require closing off any work areas.

The tool offers an easy and affordable way to non-destructively determine asphalt density during application. The idea is to uncover problems that occur during the paving process, including poor uniformity and significant variations in density. By avoiding these problems, Pavescan RDM helps avoid such premature failures as road raveling, cracking, and deterioration along joints.

Despite its advantages, using GPR tools for bridge inspection is not yet widespread in Europe. However, interest is building, as governments look for diagnostic methods that can help them make the best use of limited maintenance funds.

A study by authors Marelli, Oppioni and Lommori, which was presented at the International Conference on Bridge Maintenance, Safety & Management in 2012 discusses how GPR was used along with a laser scanner to evaluate the condition of a 40-year old highway bridge in Tuscany, Italy and assess what repairs were needed. The technique yielded an extremely accurate 3D geometrical model for the whole structure. According to the authors, “The good correspondence between GPR final output and physical, mechanical and chemical in situ and laboratory tests was encouraging, particularly

for significant and sensitive regions on the road track.” In this case the results showed that, even though GPR demonstrated the presence of cracks, there were no important structural faults and the general state of the bridge is acceptable. The evidence meant repairs could be conducted as routine rather than an emergency.

Another study conducted by researchers at the University of Zilina in Slovakia set out to test the usefulness of GPR, recognising the importance of diagnostic methods that are non-invasive, simple, safe, time efficient and non-intrusive from a traffic flow standpoint. The researchers were concerned that the lack of financial resources for road infrastructure was causing poorly-executed repairs, which were wasteful of material and time. They concluded that GPR measurements allowed them to determine the thickness of paving layers as well as the cause of damage to the surface. Based on the GPR analysis, they believed they could better determine the optimal repair or reconstruction of the deformed bridge n

Michael Arvanitis is sales manager for GSSI

Laser-scanning vibrometry has proven its worth as a non-contact, non-invasive, fast and full-field way

of measuring vibrations. Polytec is now offering the tried-and-tested PSV-500 and PSV-500-3D series vibrometers with the ‘Xtra’ option, which significantly increases optical sensitivity. Optical sensitivity defines how powerful a scanning vibrometer is; it not only determines the signal strength and therefore on which surfaces measurements can be carried out, but it is also responsible for the signal-to-noise ratio of the measurement, the maximum achievable stand-off distance and therefore the size of the scannable area too.

The Xtra concept is based on an infrared laser that produces more power without sacrificing the eye safety. In addition, more of the infrared light is reflected from the surface. This additional light provides a signal that is at least eight times stronger than that produced using the basic model, which is far more resistant to noise as a result.

The signal-to-noise ratio is improved and resonance frequencies in the Fast Fourier Transform spectrum become even more clearly visible. This results in more precise data analysis and finite-element model validation — with shorter measuring times too, since the number of averages that

are needed drops significantly. Also, there is no longer any need to treat surfaces or to reposition the instrument from one scan area to another due to the greater measuring distance, saving time and effort. Users can therefore carry out many more measurements per day, while complex

product developments can be turned into reality more efficiently. The new PSV-500 Xtra is available both as a complete system and as an upgrade for the basic models.

Polytec www.polytec.com

Top: Calculated dielectrics with the Pavescan system. Bottom: GPR software denot-ing areas of concrete deterioration

Weaker reflections Stronger reflections

DielectricDistance: 0+39.00 ftChannel: CenterValue: 4.04