How non-destructive testing can enhance the art and engineering of bridge inspection
The collapse of Nanfang’ao Bridge in Taiwan, China (October 2019),1 and Ponte Morandi in Genoa, Italy (August 2018),2 have raised concerns about the safety and reliability of existing bridge structures around the globe. An extensive number of bridge structures in North America are in poor structural condition. According to the National Research Council Canada, one-third of Canada’s highway bridges have some structural or functional deficiencies and a short remaining service life.
According to the ASCE Infrastructure Report Card (2017),3 9.1 per cent of the nation’s bridges (approximately 56,000 bridges) were structurally deficient in 2016. Structural and durability performance of these structures remain a significant risk for the near 188 million trips that are made across them each year.3
Owners and maintenance managers of these assets are looking for cost-effective and reliable inspection and monitoring tools to evaluate the condition of bridge structures to effectively prioritize and plan any repair or replacement. This is also true about new bridges. The Canadian Highway Bridge Design Code (S6-14, 2014)4 now considers a service life of 75 years for newly constructed bridges (the New Champlain bridge in Montreal is designed for 125 years in service). The new design and detailing procedures such as the use of waterproofing materials, increased concrete cover thickness and the use of glass fibre reinforced polymer reinforcement have made this new service life achievable.5 However, many things can change during this long service life since the bridge will be exposed to harsh environments and rapidly evolving climate for a much longer period. This article will review some of the existing challenges in bridge maintenance and explore how non-destructive testing (NDT) can help engineers in effective inspection and testing.
Existing bridges: the challenges
The poor condition of concrete bridge decks is an important challenge in the U.S., and Canada (SHRP2, 20136 and Lounis 20087). Highway bridge deck slabs in cold areas are often exposed to de-icing salts, which might contain a significant amount of chloride, that can penetrate into concrete through cracks in the asphalt topping or existing cracks in the concrete deck. Once the concentration of chloride ions on the steel reinforcement reaches a critical level (referred to as threshold level), corrosion of steel rebar begins. The resulting corrosion products (referred to as rust or red rust) have a significantly larger volume (six to seven times) than steel. The increase in the volume induces significant stresses in the concrete. Once the maximum tensile stress exceeds the tensile strength of concrete, internal cracks initiate at the interface of steel and concrete and propagate to the concrete surface. These new cracks create new paths for the chloride ions to penetrate concrete, reach the steel reinforcement and accelerate corrosion.
The most common method in bridge inspection procedures is a routine (annual or biannual) close-up visual inspection of the bridge elements. In this approach, a qualified engineer inspects bridge elements and records all major defects. Such defects could be concrete delamination, spalls, corrosion or cracks in asphalt topping. The overall condition of the bridge is then evaluated and quantified using a standard system. This allows asset owners and maintenance managers to prioritize the repair or replacement policies. In Ontario, bridge inspection is performed according to the Ontario Structure Inspection Manual (OSIM, 2008).8 While this guideline provides a cost-effective and practical approach for management of bridge structures, it does not cover further testing and inspection.
If a defect is observed during the inspection, further testing might be needed to assess the location and extent of such defects. In this stage, NDT methods can help engineers identify the extent of defects and help assess the quality and structural integrity at select areas.
Non-destructive testing for bridges
NDT methods can be helpful in detecting the damage mechanism at early stages, when no apparent sign is observed. The early detection of damages minimizes the cost of maintenance work. The Second Strategic Highway Research Program6 has identified various NDT techniques for condition survey of bridge decks. The report ranks these methods based on their effectiveness in detection and characterization of four major deterioration types: delamination, concrete degradation, reinforcement corrosion and vertical cracking. The Strategic Highway Research Program 2 is a useful review of some of the NDT methods such as the impact-echo (IE), half-cell potential, ground penetrating radar (GPR) and Ultrasonic Tomography for bridge deck evaluation. The following will briefly describe these methods.
1. Impact-echo method
In an IE test, a stress pulse is generated at the surface of the element. The pulse spreads to the test object and is reflected by cracks, flaws or interfaces, and boundaries. The surface response caused by the arrival of reflected waves is monitored using a high precision receiving transducer (Malhotra and Carino, 2004).9
When stress waves travel within the concrete element, a part of emitted acoustic waves by the stress pulse on the surface is reflected over the boundary layers where the different material stiffness changes. The data received by the transducer is analyzed in the frequency domain to measure the wave speed and the thickness. The test is widely used to evaluate the location and extent of delamination in concrete decks. This procedure has been standardized as the ASTM C1383, “Standard Test Method for Measuring the P-Wave Speed and the Thickness of Concrete Plates Using the Impact-Echo Method.”10 The application of the IE method for detection of delamination in concrete decks with asphalt overlays is somewhat limited to low temperatures. Another major complication in the test occurs when engineers want to identify the boundaries of the delaminated area. In this case, a very dense test grid is required which would make the test more labour-intensive and time-consuming.
2. Corrosion inspection and monitoring
Corrosion inspection in concrete bridge decks is mainly done through Half-Cell Corrosion Potential Survey (ASTM C 876).11 Half-cell tests provide valuable information about the likelihood of corrosion activity in concrete bridge decks. The test is relatively rapid and enables scanning a large area in a relatively short period. A major challenge in performing a half-cell potential test is preparing the test points in the decks with asphalt overlay or decks with waterproofing. Also, the test results might not be conclusive in the case of epoxy coated rebar.
3. Ground penetrating radar
GPR is a very useful technique for non-destructive evaluation of concrete. GPR uses pulsed electromagnetic radiation to scan concrete. It can be used to locate rebar, voids and delamination in the depth of concrete decks. When it comes to testing bridge decks, GPR has a great advantage as it can detect defects from the asphalt overlay. Sneed et al. reported that “GPR can be used to evaluate the condition of a concrete bridge deck with or without an asphalt or concrete overlay.”
GPR consists of a transmitter antenna, a receiver antenna and a signal processing unit. GPR emits electromagnetic pulses (radar pulses) with specific central frequency to scan the subsurface medium. The reflected waves from subsurface layers and objects are captured by the receiver antenna. The scanning apparatus can be mounted on a truck or a special vehicle and performs the scan at the traffic speed, which eliminates the
need for extended road closures. The main advantage of GPR method is the speed of the test. The test can provide a cost-effective and rapid tool for rapid screening of bridge decks and identification of repair needs.10
Another application of GPR is to locate steel reinforcement and prestressing tendons in concrete girders. This becomes critical when signs of corrosion or other deterioration mechanism is observed in the girder. GPR can also help engineers identify the best location for taking cores from concrete piers, girders and decks.
4. Ultrasonic Tomography
Ultrasonic Tomography methods use shear waves to determine the thickness of concrete and to identify the location of voids or defects. Tomography methods can help detect location and depth of delamination and evaluate the integrity of concrete.
The concept behind this method relies on the propagation of stress waves through materials. An array of transmitters introduces a stress pulse into concrete at an accessible surface. The pulse propagates into the test area and is reflected by flaws or interfaces. The emitted impulse and the reflected acoustic waves are monitored at the receiving transducers. The signals are analyzed to calculate the wave’s travel time. If the wave’s speed in the material is known, this travel time can be used to evaluate the thickness of the medium. Depending on the multi-layer system under investigation, the travel time of shear or compressive waves is used to evaluate the thickness of each layer. The application of these methods for bridge deck scanning can be time consuming since a proper scan requires very close spacing between the test locations.
Concluding remarks
Most of the tests that are currently included in most bridge deck condition surveys can be replaced or complemented with commercially available NDT solutions. Most NDT solutions can deliver results with equal or higher accuracy, while reducing the inspection timeline and minimizing intrusion in the structure, thus reducing further damage to critical elements.
Application of different NDT methods for detailed bridge condition surveys have been studied by many researchers and engineers. These techniques can be enhanced when combined with more advanced structural health monitoring solutions to bring real-time data collection and monitoring capabilities for bridge owners and maintenance managers.
References
https://en.wikipedia.org/wiki/Nanfang%27ao_Bridge – Last visited 11 May 2020.
https://en.wikipedia.org/wiki/Ponte_Morandi.
www.infrastructurereportcard.org/cat-item/bridges/ Last visited 11 May 2020.
The Canadian Highway Bridge Design Code (S6-14, 2014).
Mermigas, K.K. (2018) “Evolution of Bridge Practices in Ontario, Canada.” Structures Standing Committee.
Nenad Gucunski, National Research Council (U.S.). Transportation Research Board, Second Strategic Highway Research Program (U.S.) (2013) “Nondestructive Testing to Identify Concrete Bridge Deck Deterioration.”
Zoubir Lounis and Lyne Daigle (2008) “Reliability-Based Decision Support Tool for Life Cycle Design and Management of Highway Bridge Decks.”
Ministry of Transporation (2008) “Ontario Structure Inspection Manual (OSIM).
M. Malhotra, Nicholas J. Carino, (2003) “Handbook on Nondestructive Testing of Concrete.”
ASTM C1383-15, Standard Test Method for Measuring the P-Wave Speed and the Thickness of Concrete Plates Using the Impact-Echo Method, ASTM International, West Conshohocken, PA, 2015, www.astm.org.
ASTM C876-15, Standard Test Method for Corrosion Potentials of Uncoated Reinforcing Steel in Concrete, ASTM International, West Conshohocken, PA, 2015, www.astm.org. 