What is the future of bridge deck scanning? What is beyond traditional scanning methods such as chain dragging? This article briefly describes application of NDT methods for scanning concrete bridge decks. Regular monitoring and maintenance of concrete bridge decks is a challenging task. The Inspection and monitoring of concrete bridge decks using traditional chain dragging can be labor-intensive and time-consuming. In addition, the results might be accurate enough. Bridge owners and operators are looking for alternative methods that are rapid, and at the same time, provide more information about the condition of concrete deck.
The Second Strategic Highway Research Program (SHRP 2) has identified various NDE techniques for condition assessment 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. SHRP 2 recommends the use of ground penetrating radar (GPR), impact echo (IE), ultrasonic surface waves (USW), half-cell potential (HCP), electrical resistivity (ER), and chain drag/hammer sounding for bridge deck evaluation.
Chain dragging is widely used to detect delamination in concrete bridge decks. The concept behind this very simple method is the unique hollow sound that is made by dragging chain across the delaminated surface. The test is used to identify potentially delaminated areas on the deck slab.
The main advantage of the method is that it is very simple, and no special equipment is required. However, chain dragging in the field needs some training. The practice is difficult when vehicles are moving over the deck. In this sense, distinguishing the hollow sound can be very subjective.
The main disadvantage of the chain dragging method is that they can only provide results if the defect and deterioration are in well advanced stages. Early diagnosis of damage mechanism is not possible using this technique. Application of the method is somewhat difficult during heavy traffic.
Beyond Chain Dragging
NDT methods for Scanning Concrete Bridge Decks
NDT methods can effectively be used for scanning concrete bridge decks. These methods can improve the speed of bridge deck scanning, and increase the accuracy of inspections. These methods can be automated to reduce the number of human labor in the field.
In Impact-Echo test, a stress pulse is generated at the surface of the element. The pulse spreads into 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). 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 different the material stiffness changes.
The data recieved by the transducer is normally analyzed in the frequency domain to measure the wave speed and the thickness. 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”.
The use of Impact-Echo method for detection of delamination in concrete decks with asphalt overlays is somewhat limited to low temperatures. The detection of the boundaries of delaminated area requires using a very dense test grid.
2. Ultrasonic Pulse Echo (UPE)
The Ultrasonic Pulse-Echo (UPE) method is used for thickness measurements, flaw detection, detecting delamination, and evaluation the integrity of concrete. The concept behind this method relies on the propagation of stress waves through materials. A transmitter introduces a stress pulse into the object at an accessible surface. The pulse propagates into the test object and is reflected by flaws or interfaces. The emitted impulse and the reflected acoustic waves are monitored at the receiving transducer. The signals are analyzed in the time domain, to calculate the wave travel time. If the wave speed in the material is known, this travel time can be used to evaluate the thickness of medium. Depending on the multi-layer system under investigation, the travel time of shear or compressive waves are used to evaluate the thickness of each layer.
The application of UPE method for bridge deck scanning can be time consuming, since a proper scan requires very close spacing between the test locations. The application of the method is somewhat difficult over rough surfaces.
3. GPR in Bridge Deck Investigation
Ground penetrating radar (GPR) is a very useful technique for nondestructive 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 deck. When it comes to testing the 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 is currently the only non-destructive method that can be used to evaluate a concrete bridge deck with an asphalt overlay. The practice has been standardized by ASTM D6087, 2008.
GPR consists of a transmitter antenna and 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 perform the scan at the traffic speed. This will eliminate the need for extended road closures.
The main advantage of GPR method is the speed of test. Large areas can be scanned in a limited period. The scanning antenna can be installed on a moving vehicle, and can scan the bridge deck at highway speed. Application of GPR in concrete bridge deck evaluation can be as simple as locating the reinforcement, or thickness of concrete cover. It can also be used to identify potentially delaminated areas.
GPR can not directly detect delaminated areas on the concrete deck. Delaminated areas can only be detected if there is enough moisture in them. GPR can not provide useful information about the mechanical properties of concrete, nor corrosion of steel reinforcement.
4. Electrical Resistivity Measurement
The use of electrical resistivity method for quality control and evaluation of concrete durability is becoming very popular among scholars and engineers. The measurement of electrical resistivity of concrete provides useful information about the microstructure of concrete material. The test procedure has since been standardized by AASHTO TP 95.
This method is adapted by several Department of Transportation (DOTs) to replace the labour-intensive and time-consuming methods such as the ASTM C 1202, “Electrical Indication of Concrete’s Ability to Resist Chloride Ion Penetration”, generally known as the Rapid Chloride Permeability Test (RCPT). The application of electrical resistivity measurement for scanning concrete deck was studies in studied as part of a research program (read more).
Electrical resistivity measurement is easy and fast to perform over concrete bridge deck; It can provide useful information about the resistance of concrete material to penetration of chloride ion.
Electrical resistivity measurements are effected by moisture content, salt content of concrete, and presence of steel rebar. This makes the task of data interpretation difficult.
5. Half-Cell Corrosion Potential Mapping
Half-cell potential mapping is a well-known test method for determining the probability of corrosion from the surface of concrete. A reference electrode (usually copper/copper sulfate) is used along with a voltmeter to measure potential different of grid points with regards to a reference point. A single connection to rebar mesh is required for performing the test. This is normally done by removing concrete cover over a small area (drill size hole over rebar would do the job), and connecting to the rebar network.
Half-cell potential mapping can identify locations where there is higher chance of corrosion. Half-Cell potential mapping can be used as an effective method for scanning bridge decks. Half-cell potential measurements can be used to identify locations where there might be more severe corrosion activity. Half-cell can be a very rapid test, if the access to the concrete surface is provided.
Half-cell potential measurement is easy to perform over large areas by using novel techniques and procedures. The data management, and obtaining test results is straight forward.
Half-cell test results are very sensitive to humidity, cover thickness, and quality of concrete materials. Test result can also be affected if the concrete is carbonated. However, the main disadvantage of the test is that the electrode should be placed directly over the surface of concrete. What it means is that the asphalt overlay should be removed at the location of testing.
6. Automated Acoustic Scanning
The very concept that is used during chain dragging was automated by a group of researchers at the University of Nebraska – Lincoln. In this method, small metal balls are used to impact the concrete surface. The sound generated through this impact is collected using microphones, and are directed to a signal processing unit. The method shows promising result in automating one of the most traditional methods available for bridge deck scanning.