Nondestructive Testing to Identify Concrete Bridge Deck Deterioration (2012)

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12 The literature review provided a number of NDT technolo- gies that have the potential to detect and characterize deterio- ration in concrete bridge decks. The following 14 techniques were considered for grading and ranking on the basis of the literature search and possible inclusion in the validation testing program: • Impact echo; • Ultrasonic pulse echo; • Ultrasonic surface waves; • Impulse response; • Ground-penetrating radar; • Microwave moisture technique; • Eddy current; • Half-cell potential; • Galvanostatic pulse measurement; • Electrical resistivity; • Infrared thermography; • Visual inspection; • Chain dragging and hammer sounding; and • Chloride concentration measurement. The following sections provide brief descriptions of the principle of operation and applications of each technol- ogy with respect to concrete deck deterioration detection. Advantages and limitations of the technologies according to the literature search are also described. The advantages and limitations of these technologies were not explicitly stated in most materials reviewed. In those cases, they are defined on the basis of the review of the reported results and reported or perceived technology performance. Because none of the participants in the validation tests used micro- wave moisture, eddy current, and chloride concentration measurement devices, these three techniques are not described or included in the final evaluation. Visual inspection is not described either, since it was not included in the validation testing. Impact Echo Description The impact echo (IE) method is a seismic or stress wave–based method used in the detection of defects in concrete, primarily delaminations (Sansalone and Carino 1989). The objective of the IE survey is to detect and characterize wave reflectors or “resonators” in a concrete bridge deck, or other structural ele- ments. This is achieved by striking the surface of the tested object and measuring the response at a nearby location. Sim- ple or automated devices, such as those shown in Figure 3.1, can be used for this purpose. Physical Principle The operation of the IE method is illustrated in Figure 3.2. The surface of the deck is struck by various means, such as wire-mounted steel balls, automated projectile sources, or solenoid-type impactors. The response is measured by a nearby contact or air-coupled sensor. The position of the reflectors is obtained from the frequency spectrum of the deck’s response to an impact. In a more rigorous sense, the response is related to the first symmetrical Lamb wave mode in the deck structure. The frequency of the reflection, called the “return frequency,” can be identified in the response spectrum of the recorded signal. The depth of the reflector can be obtained from the return frequency, fT or ft, and the measured or estimated compression-wave velocity of con- crete, VP, using the simple relationship shown in Figure 3.2. Because strong reflectors will be generated at all interfaces where there is a contrast in acoustic impedances of materials, such as the one between concrete and air, delaminated areas are typically recognized as shallow reflectors. In the case of a sound deck, the dominant reflector will be from the bottom of the deck, another concrete–air interface. Other reflectors may include voids, tendons, supporting structural elements, and so forth, and responses from various defects (initial delamination or cracking). C h a p t E r 3 Candidate Methods for Deterioration in Concrete Bridge Decks

13 Different authors interpret the severity of the delamination in a given deck with the IE method in various ways. One of the ways used in this study is shown in Figure 3.2. A test point is described as intact if the dominant return frequency corre- sponds to the bottom of the deck. A delaminated point in the deck will theoretically demonstrate a shift in the return fre- quency toward higher values because the wave reflections occur at shallower depths. Depending on the extent and continuity of the delamination, the partitioning of the wave energy reflected from the bottom of the deck and the delamination may vary. The initial or incipient delamination, described as occasional separation within the depth of the slab, can be identified through the presence of return frequencies associated with the reflections from both the bottom of the deck and the delamina- tion. Progressed delamination is characterized by a single peak at a frequency corresponding to the depth of the delamination. Finally, in cases of wide or shallow delaminations, the dominant response of the deck to an impact is characterized by a low- frequency response of flexural-mode oscillations of the upper delaminated portion of the deck. This response is almost always Figure 3.1. Stepper (left) and bridge deck scanner (right). Figure 3.2. Grades for various degrees of deck delamination.

14 in the audible frequency range, unlike responses from the deck with incipient delamination that may exist only in the higher- frequency ranges (Gucunski et al. 2006; Cheng and Sansalone 1995; Lin and Sansalone 1996). Applications The applications of IE can be divided into four general cate- gories as follows: • Condition assessment of reinforced concrete elements with respect to delamination; • Characterization of surface-opening cracks (vertical cracks in bridge decks); • Detection of ducts, voids in ducts, and rebars; and • Material characterization. The IE technique is primarily used to detect and characterize delaminations with respect to its horizontal and vertical posi- tion and its stage of development. The method has also been used with some success for the characterization of the depth and primary direction of surface-opening cracks and in the detection of rebars and ducts in cases in which the diameter- to-concrete cover ratio is above a certain threshold (roughly one-third). The detection of rebars and ducts requires a scan- ning approach and usually a higher level of expertise. It should be noted that, although the horizontal position of a rebar or duct can be defined, it is difficult to accurately define the con- crete cover thickness. The IE method has also been used to evaluate concrete modulus and estimate concrete compressive strength. Other applications of the method include the charac- terization of grouting in tendon ducts and overlay debonding detection on decks with overlays. Limitations The IE method can detect delaminations on decks with PCC overlays. However, on decks with asphalt concrete overlays, detection is possible only when the asphalt concrete tem- perature is sufficiently low, so that the material is not highly viscous, or when the overlay is intimately bonded to the deck. It is necessary to conduct data collection on a very dense test grid to accurately define the boundaries of the delaminated areas. In the rare cases of impact echo test- ing on bridge decks near the deck edges, boundary effects should be taken into consideration. Such boundary condi- tions will produce reflections that will interfere with the sought signal. The boundary interference problem is more common during IE testing on other structural elements of limited dimensions (such as girders, piers, and pier caps) than on bridge decks. Ultrasonic pulse Echo Description Ultrasonic pulse echo (UPE) is a method that uses ultrasonic (acoustic) stress waves to detect objects, interfaces, and anomalies. The waves are generated by exciting a piezoelec- tric material with a short-burst, high-amplitude pulse that has high voltage and current. Civil engineering applications, for reinforced concrete structures in particular, were realized only in the recent past because of two reasons. First, tradi- tional ultrasonic testing would lead to high scattering and attenuation of the transmitted pulses, mainly because of the very heterogeneous nature of concrete. Second, the trans- ducers (ultrasonic probes) had to be coupled to the surface of the tested element using grease or wax. To overcome scattering problems, low-frequency transducers have been introduced, of a center frequency between 50 and 200 kHz that can be dry-coupled. A dry-point contact, ultrasonic transducer unit consisting of 24 probes is shown in Figure 3.3 (left). Twelve probes in the Figure 3.3. Shear-wave probe array A1220 (left), and automated A1220 measurements using stepper (right).

15 Possible delaminaon Backwall Backwall Backwall Backwall Figure 3.4. Bridge deck survey using MIRA ultrasonic system. B-scans (top) and equipment and data collection (bottom). array act as pulsers, while the other 12 act as receivers. Depend- ing on the transducer unit, these probes can emit both compressional and shear waves. The UPE test can also be per- formed by mounting the probe on an automatic device, like the one shown in Figure 3.3 (right). Physical Principle A UPE test concentrates on measuring the transit time of ultrasonic waves traveling through a material and being reflected to the surface of the tested medium. Based on the transit time or velocity, this technique can also be used to indirectly detect the presence of internal flaws, such as cracking, voids, delamination or horizontal cracking, or other damages. An ultrasonic wave is generated by a piezo- electric element. As the wave interfaces with a defect, a small part of the emitted energy is reflected back to the sur- face. Defects, in this case, are identified as any anomaly of acoustical impedance different from the concrete element tested. The wave is then detected by a second piezoelectric element. In regions where there is significant deterioration or microcracking, concrete will have a noticeably lower velocity compared with concrete in intact regions. A UPE B-scan (vertical cross section) of a deck along two survey lines is illustrated in Figure 3.4, using an ultrasonic system A1040 MIRA with almost real-time synthetic aperture focusing technique, or SAFT (Kozlov et al. 2006; Bishko et al. 2008; Gebhardt et al. 2006). Applications Ultrasonic pulse echo surveys have been used for thickness measurements on objects with only one-sided access. The UPE is capable of assessing defects in concrete ele- ments, debonding of reinforcement bars, shallow cracking, and delamination. The UPE was also successfully used in the detection of material interfaces, based on phase evalu- ations of the response. Examples include the interfaces between concrete and steel (e.g., reinforcement) or con- crete and air (e.g., grouting defects) (Taffe and Wiggenhauser 2006; Afshari et al. 1996; Krause et al. 2008; Hevin et al. 1998).

16 Limitations Ultrasonic pulse echo surveys require very close spacing between test points to develop images of the tested medium, making it time-consuming. The data quality depends strongly on the coupling of the sensor unit, which may be difficult on rough surfaces. Very shallow flaws may remain undetected because the surface waves mask the needed compressional or shear-wave signals. Also, as UPE works with lower frequencies, some of the defects might remain undetected. Ultrasonic Surface Waves Description The ultrasonic surface waves (USW) technique is an offshoot of the spectral analysis of surface waves (SASW) method used to evaluate material properties (elastic moduli) in the near- surface zone. The SASW uses the phenomenon of surface wave dispersion (i.e., velocity of propagation as a function of frequency and wavelength, in layered systems to obtain the information about layer thickness and elastic moduli). The USW test is identical to the SASW test, except that the fre- quency range of interest is limited to a narrow high-frequency range in which the surface wave penetration depth does not exceed the thickness of the tested object. In cases of relatively homogeneous materials, the velocity of the surface wave (phase velocity) does not vary significantly with frequency. The sur- face wave velocity can be precisely related to the material mod- ulus, or concrete modulus in the case of bridge decks, using either the measured or assumed mass density, or Poisson ratio of the material. A USW test consists of recording the response of the deck, at two receiver locations, to an impact on the sur- face of the deck, as illustrated later in Figure 3.7. One of the devices that can be used for that purpose is shown in Figure 3.5. Physical Principle Surface waves are elastic waves that travel along the free surface of a medium. They carry a predominant part of the energy on the surface, in comparison to body (compressive and shear) Figure 3.5. USW testing using a portable seismic property analyzer (PSPA).

17 waves. This is illustrated in Figure 3.6, where the arrival of the surface (Rayleigh) wave follows the arrival of the two body- wave components because it is the slowest one (Nazarian et al. 1993; Stokoe et al. 1994; Yuan et al. 1999). The surface waves propagate radially from the impact source, forming a cylindrical front with a velocity dependent on the elastic properties of the medium. The waves propagating in a heterogeneous medium are dispersive; that is, waves of different wavelengths or frequencies travel with different velocities. Thus, information about the subsurface can be obtained through the measurement of the phase velocity versus frequency relation- ship, termed dispersion curve, and backcalculation of the dis- persion curve to obtain the profile of the tested system. Unlike many seismic methods that base evaluation on the detection and measurement of first wave arrivals, the USW velocity evalu- ation is based on the spectral analysis of the recorded signal. The body of a surface wave extends to the depth of approxi- mately one wavelength. Therefore, if the measurement is lim- ited to wavelengths not exceeding the thickness of the deck, the velocity of the surface waves will depend only on the concrete modulus. As sketched in Figure 3.7 for a two-layer half-space, at wavelengths less than or equal to the thickness of the layer, the velocity of the surface wave is more or less independent of wave- length. For the same reason, in the case of a sound and homo- geneous deck, the velocity of the surface waves will show little variability. An average velocity is used to correlate it to the con- crete modulus. Significant variation in the phase velocity will be Figure 3.6. Typical time record used in surface wave method. Phase velocity Wavelength VR1 VR2 VR1 VR2 Dispersion CurveTwo-Layer Half-Space IMPACT SOURCE RECEIVERS SS 1 2 Coherence Phase Phase velocity Young's modulus W av el en gt h De pt h DISPERSION CURVE YOUNG'S MODULUS PROFILE Wavelength considered less than layer thickness Figure 3.7. Schematic of surface wave velocity versus wavelength (top) and evaluation of a layer modulus by SASW (USW) method (bottom).

18 an indication of the presence of a delamination or other anom- aly. A schematic of the USW (SASW) test is shown in Figure 3.7. Elastic waves are generated by means of impacts (e.g., steel balls, automated projectile sources, solenoid-type impactors) detected by a pair or an array of receivers and recorded by a transient recorder. Applications The USW is used in condition assessment for the purpose of evaluating probable material damage from various causes: ASR, DEF, freeze–thaw, and other deterioration processes. It is also used in material quality control and quality assurance of concrete and hot-mix asphalt, primarily to evaluate material modulus and strength, the second one using correlations with modulus. One of the USW’s applications is the measurement of the depth of vertical (surface) cracks in bridge decks or other elements. Finally, some results point to the USW’s ability to indirectly detect delaminations in bridge decks. Limitations On deteriorated sections of a concrete deck, such as debonded or delaminated sections, the USW method cannot provide reliable modulus values. It can play only a supplemental role in deterioration detection, and experience is required for understanding and interpreting test results. The USW (SASW) modulus evaluation becomes significantly more complicated for layered systems, such as decks with asphalt concrete overlays, where the moduli of two or more layers differ significantly. Impulse response Description The impulse response method, also known as the transient dynamic or mechanical impedance method, is a nondestruc- tive testing method that has been mostly used in quality con- trol and condition assessment of pavements and deep foundations (Figure 3.8). The method was first developed in France in the late 1970s as an extension of a vibration test, used in the quality control of drilled shafts. Since then, the impulse response method has been used to determine the subgrade modulus and presence of voids or loss of support below rigid pavements, concrete tunnel linings and slabs, and in reinforced concrete bridges. The method was recently introduced as a screening tool for bridge decks, slabs, and tunnel linings for the detection of potentially damaged areas. The objective of the test is to measure the dynamic response of the element tested, often described in terms of the mobility or flexibility spectrum, in order to detect areas where the spectrum takes shapes and amplitudes significantly different from those at sound locations (Nazarian et al. 1993, 1994; Gucunski and Jackson, 2001; Jackson and Gucunski, 2002; Davis et al. 2001). Physical Principle The impulse response method is a dynamic response method that evaluates the dynamic characteristics of a structural ele- ment to a given impulse. The typical frequency range of inter- est in impulse response testing is 0 to 1 kHz. The basic operation of an impulse response test is to apply an impact with an instrumented hammer on the surface of the tested element and to measure the dynamic response at a nearby location using a geophone or accelerometer. The basic principle of impulse response is illustrated in Figure 3.9. The signal from the impact hammer sensor, the forcing function, and the response at the nearby displace- ment transducer, geophone (velocity transducer), or accel- erometer are transformed into the frequency domain to obtain the corresponding spectra. The ratio of the displace- ment and impact spectra represents a flexibility spectrum, in the case of the measured displacement. The inverse ratio is termed mechanical impedance (dynamic stiffness spec- trum). In some analyses, the flexibility spectrum is matched by a flexibility spectrum (response spectrum) for an assumed single-degree-of-freedom (SDOF) system. Once the two spectra are matched, the modal properties of the SDOF sys- tem provide information about the stiffness and damping properties of the system. The underlying assumption of this process is that a structure’s response can be approximated by the response of an SDOF system. If the measured response is velocity, the ratio of the velocity and impact spectra is termed mobility spectrum. Figure 3.8. Impulse response testing.

19 Even though the concept behind impulse response and impact echo is similar, there are significant differences in bridge deck testing. Impact echo is based on the excitation of particular wave propagation modes above the probable anomalies within the deck or between the top and bottom of the deck, which is typi- cally in a frequency range of about 3 to 40 kHz. On the other hand, impulse response relies more on the structural response in the vicinity of the impact and, therefore, the frequency range of interest is much lower—that is, 0 to 1 kHz for plate structures. Applications The impulse response method has been used in a number of pavement and bridge applications. These include the following: • Detection of low-density concrete (honeycombing) and cracking in concrete elements; • Detection of voids under joints of rigid pavements or under slabs; • Concrete delamination in slabs, decks, walls, and other re inforced concrete structures, such as dams, chimneystacks, and silos; • Load transfer at joints of concrete pavements; and • Debonding of asphalt and concrete overlays on concrete deck and pavements. Limitations The impulse response tests can be used to detect gross defects in structures while smaller defects might go undetected. In addition, reliable data interpretation is highly dependent on the selection of test points. Finally, automated equipment is not available even though the automated analysis tools are available. Ground-penetrating radar Description Ground-penetrating radar (GPR) is a rapid NDT method that uses electromagnetic waves to locate objects buried inside the structure and to produce contour maps of subsurface features (steel reinforcements, wire meshes, or other interfaces inside the structures). GPR can be used for a range of applications— namely, condition assessment of bridge decks and tunnel lin- ings, pavement profiling, mine detection, archaeological investigations, geophysical investigations, borehole inspec- tion, building inspection, and so forth. Antennas of different frequencies are used to facilitate different levels of needed detail and depth of penetration. In addition to ground-coupled antennas, like the one in Figure 3.10, air-coupled systems are used for faster bridge deck screening (Romero and Roberts 2002; Maser and Rawson 1992; Barnes and Trottier 2000). Physical Principle Ground-penetrating radar provides an electromagnetic (EM) wave-reflection survey. A GPR antenna transmits high-frequency EM waves into the deck or the structure. A portion of the energy is reflected back to the surface from any reflector, such as rebar (or any other anomaly), and received by the antenna. The remainder of the GPR energy continues to penetrate beneath this interface, and additional energy is Flexibility spectrum or impedance t t Test Impact AF A f Stiffness and damping SDOFResponse f F f AFFT FFT Figure 3.9. Principle of impulse response testing.

20 continually reflected back to the receiver from other inter- faces until it is diminished. GPR measures specific signal responses caused by varia- tions in the electrical properties of the materials making up the deck. The signal responses are different for various inter- faces because of two changing electrical properties: electrical conductivity (inverse of resistivity) and relative dielectric permittivity (dielectric constant). Relative permittivity values (dielectric constant, er) for typical construction materials, including concrete, are shown in Table 3.1. These properties respectively govern (a) the ability of GPR energy to penetrate that particular medium and (b) the speed at which GPR waves propagate through the medium. In addition, the dielectric contrast between two adjacent materials will cause some of the penetrating GPR waveform to reflect back to the surface where it can be measured and recorded. The condition assessment of bridge decks is based on the evaluation of the attenuation of EM waves at the top rebar level. The key point is that EM waves cannot penetrate into metals, like rebars. Therefore, rebars are excellent reflectors of EM waves. However, most construction materials are fair-to- good host materials for GPR. Concrete that is moist and high in free chloride ions (or other conductive materials) can sig- nificantly affect a GPR signal’s penetration, or attenuation, in a measurable way. This is even more pronounced in cases where the deck is cracked or delaminated, and the cracks are filled with moisture, chlorides, and other conductive materi- als. An example is the GPR scan of a reinforced deck shown in Figure 3.11. The hyperbolae represent reflections from the top rebars. While most of the rebars on the right side can be described as providing a strong reflection, the far left rebar is somewhat fuzzy, which is an indication of strong attenuation at that location. To provide condition assessment, in most cases amplitude measurements from all the rebar locations are corrected for the rebar depth and plotted to create attenu- ation maps of a bridge deck. Zones of high attenuation are related to zones of likely deterioration, and vice versa. Applications GPR has been used in a range of applications, such as condi- tion assessment of bridge decks and tunnel linings, pavement profiling (pavement layer thickness evaluation) on both proj- ect and network levels, detection of voids and anomalies under pavements, mine detection, and archaeological inves- tigations. Typical GPR applications for bridge decks include evaluation of the deck thickness, measurement of the con- crete cover and rebar configuration, characterization of delamination potential, characterization of concrete deterio- ration, description of concrete as a corrosive environment, and estimation of concrete properties. Limitations Although GPR has many advantages, there are also certain limitations. One of them is the inability to directly image and detect the presence of delamination in bridge decks, unless they are epoxy-impregnated or filled with water. Also, GPR data can be negatively influenced by extremely cold conditions. Moisture in the deck that is completely frozen will influence the acquired signal because it will no longer be detected. The application of deicing salts during winter months can also negatively influence GPR by affecting the dielectric constant. Although GPR is capable of providing information about the layer structure, location, and layout of reinforcing steel, it cannot provide any information about the mechanical proper- ties of the concrete (e.g., strength, modulus). Also, GPR cannot provide definitive information about the presence of corro- sion, corrosion rates, or rebar section loss, even though it is sensitive to the presence of a corrosive environment and can be used to map the degree or severity of probable deterioration. GPR results generally require being correlated or validated by Figure 3.10. GPR testing. Table 3.1. Dielectric Constants of Different Materials Medium Dielectric Constant Medium Dielectric Constant Air 1 Sand 4–6 Water (fresh) 81 Gravel 4–7 Ice 4 Clay 25–40 Asphalt 4–8 Silt 16–30 Concrete 8–10 Silty sand 7–10 Crushed base 6–8 Insulation board 2–2.5

21 some other NDE methods or limited destructive sampling (cores, chloride sampling and testing, or other ground truth). GPR surveys may be less cost-effective than other methods when applied to either smaller or individual structures. Finally, Federal Communications Commission regulations control- ling transmit power output and pulse repetition rate are limit- ing the ability to design and build newer systems, which would have a greater capability to cover larger areas in much less time using array platforms. half-Cell potential Description The half-cell potential (HCP) measurement is a well- established and widely used electrochemical technique to evaluate active corrosion in reinforced steel and prestressed concrete structures. The method can be used at any time dur- ing the life of a concrete structure and in any kind of climate, provided the temperature is higher than 2°C (Elsener 2003). Half-cell measurements should be taken on a free concrete surface, because the presence of isolating layers (asphalt, coating, and paint) may make measurements erroneous or impossible. Using empirical comparisons, the measurement results can be linked to the probability of active corrosion. Half-cell testing and equipment are depicted in Figure 3.12. Generally, the potential difference between the reinforcement and a standard portable half-cell, typically a Cu/CuSO4 standard reference electrode, is measured when placed on the surface of a reinforced concrete element. When the reference electrode is shifted along a line or grid on the surface of a member, the spatial distribution of corrosion potential can be mapped (Baumann 2008; Gu and Beaudoin 1998). Physical Principle When a metal is submerged into an electrolyte, positive metal ions will resolve (oxidation). Oxidation leads to a surplus of electrons in the metal lattice and a net negative charge at its surface. The positive metal ions will accumulate at the metal– liquid interface, which in consequence becomes positively charged, and a double layer is formed. Anions, from the elec- trolytic solution (in concrete –Cl- and –SO4 2-), are attracted to the positively charged side of this double layer and accu- mulate there, forming the so-called half-cell. A potential dif- ference between the metal and the net charge of the anions in the electrolyte builds up, which depends on the solubility of the metal and the anions present in the solution. If two different metals are submerged into an electrolyte (two half-cells) and are electrically connected by a wire, a gal- vanic element is created. The two different metals will cause different electrical potentials in their half-cells, which in turn will cause a current flow through the wire. The less noble of the two metals is dissolved (anode) and the more noble remains stable (cathode). In the surface layer of the less noble metal, a surplus of electrons is formed. The potential difference Figure 3.11. GPR principle.

22 between the two metals can be measured as a voltage with a high-impedance voltmeter (Figure 3.13). Applications The main application of the method is to identify the corro- sion activity of steel reinforcement in steel-reinforced concrete structures. Limitations Even though many bridge engineers have used the HCP method for years almost as a standard tool, the influence of concrete cover depth has not yet been thoroughly researched. As a con- sequence, correcting data for depth is not straightforward, just as it is not for moisture or salt content, which influence concrete resistivity. Galvanostatic pulse Measurement Description Galvanostatic pulse measurement (GPM) is an electrochemical NDT method used for rapid assessment of rebar corrosion, based on the polarization of rebars using a small current pulse (Figure 3.14). Estimating the corrosion rate of reinforcing bars using the HCP measurement is often unreliable when concrete is wet, dense, or polymer modified, and thus the access to oxygen is limited. The GPM overcomes those problems in the interpretation of corrosion risk because of a different physi- cal principle of operation. It provides more realistic measure- ments of the corrosion rate of steel rebars, which is often Figure 3.12. HCP testing. Figure 3.13. HCP principle.

23 underestimated because of concrete electrical resistance (Elsener et al. 1996; Böhni and Elsener 1991; Klinghoffer et al. 2000; Baessler et al. 2003; Newton and Sykes 1988). Physical Principle When a metal, such as steel reinforcement, is immersed in an electrolytically conducting liquid of adequate oxidizing power, it undergoes corrosion by two complementary mechanisms. First, metal ions pass into the liquid, leaving a surplus of electrons on the base metal that forms an anodic site. Second, the excess electrons flow to a cathodic site where they are con- sumed by oxidizing agents in the liquid. These processes induce a corrosion current between the anodic and cathodic sites. The corrosion current can be indirectly measured by gal- vanostatically (with constant current) imposing a short-time anodic current pulse between a counter electrode at the con- crete surface and the steel reinforcement. The applied cur- rent polarizes the reinforcement anodically (with respect to the free corrosion potential), resulting in a measurable electrochemical potential drop (Figure 3.15). Actively cor- roding reinforcement possesses an active current between its anodic and cathodic sites and thus has a low resistance to current flow. Because of the applied current, this results in a low electrochemical potential change relative to its steady- state free corrosion potential. Noncorroding reinforcement possesses no current and thus has a high resistance to current flow. Because of the applied current, this results in a high electrochemical potential change relative to its free corrosion potential. Applications The GPM is used primarily to identify the corrosion rate of steel reinforcement in reinforced concrete structures. Figure 3.14. GPM testing. Figure 3.15. GPM principle.

24 Limitations High electrical resistivity in concrete cover leads to unstable measurements and, therefore, prewetting of the measure- ment area is essential. To avoid potential shifts due to wetting effects, the first reading should be recorded after a few min- utes. Because there is a difference between the passive and active reinforcement area affected by the electrical signal, direct measurements of apparent polarization resistance do not provide appropriate results for corrosion status in these two cases. Electrical resistivity Description The electrical resistivity (ER) method is often used for moisture detection, which can be linked to the presence of cracks. The presence and amount of water and chlorides in concrete are important parameters in assessing its corro- sion state or describing its corrosive environment. Dam- aged and cracked areas, resulting from increased porosity, are preferential paths for fluid and ion flow. The higher the ER of the concrete is, the lower the current passing between anodic and cathodic areas of the reinforcement will be. The relationship between ER and the normally observed corro- sion rate of reinforced concrete is given in Table 3.2 (after Gowers and Millard 1999). Physical Principle In practice, the voltage and current are measured at the sur- face of the object under investigation. The most common electrode layout in civil engineering applications is the Wenner setup (Figure 3.16). The Wenner setup uses four probes that are equally spaced. A current is applied between the outer electrodes, and the potential of the generated elec- trical field is measured between the two inner ones. The resis- tivity is then calculated according to the following: ρ pi= 2 aV I where r = resistivity (in W.m); a = electrode separation (in m); V = voltage (in V); and I = current (in A). The inverse of the ER is the electrical conductivity, s[S/m]. Building materials such as concrete, cement, or wood are ion conductors. This means that electrical conduction hap- pens through the interconnected pore space. The resistivity of fully saturated concrete is on the order of 100 to 1,000 W.m, depending on the conductivity of the saturating fluid. When oven dried, the ER of concrete is as high as 106 W.m and acts as an insulator. Because concrete is a composite material, its ER will always depend on its porosity, pore-size distribution, and factors such as the cement chemistry, water to cement ratio, types of admixtures, and so forth. To a large extent, resistivity values will also be a function of the ion type and content of the saturation fluid (Kruschwitz 2007; Hunkeler 1996; Bürchler et al. 1996). Phenomena such as carbonation, chloride attack (deicing salts), and secondary damage-like cracks also significantly influence the electrical properties of the concrete. Drying of the concrete surface, carbonation, and the presence of steel reinforcement in the vicinity of the electrodes significantly affect field resistivity measurements. Probably the most important parameter influencing the con- crete resistivity is the fluid salinity. The fluid conductivity within a sample will depend on the saturating fluid and on the solubility of the concrete. Applications Electrical resistivity is primarily used to characterize concrete’s susceptibility to corrosion by characterizing its corrosive Table 3.2. Correlation Between Resistivity Values and Corrosion Rates Resistivity (k  cm) Corrosion Rate <5 Very high 5–10 High 10–20 Moderate–low >20 Low Figure 3.16. Electrical resistivity testing using a Wenner probe.

25 environment (Figure 3.17). It can also help to identify regions of the deck or other structural elements susceptible to chlo- ride penetration. In addition, electrical resistivity surveys can be used to detect corrosion cells in tandem with another corrosion technique, such as half-cell potential, to map corrosion activity (Millard 1991; Gowers and Millard 1999). Limitations Even though the data processing is not complicated and easily reduces to plotting the raw data, the interpretation is more challenging. The reason is that the ER depends on a number of material properties (e.g., moisture, salt content, porosity), and the delineation of their specific contributions to the bulk result is difficult. Even though it is technically possible, auto- mated measurement systems for roads are not available on the market. Moreover, the electrodes need galvanic coupling to the concrete and, therefore, the surface of a test object has to be prewetted. Infrared thermography Description Infrared (IR) thermography has been used since the 1980s to detect concrete defects, such as cracks, delaminations, and concrete disintegration in roadways or bridge structures (Fig- ure 3.18 to Figure 3.20). To detect subsurface defects, IR thermography keeps track of electromagnetic wave surface radiations related to temperature variations in the infrared wavelength. Anomalies, such as voids and material changes, can be detected on the basis of variable material properties, such as density, thermal conductivity, and specific heat capac- ity. The resulting heating and cooling behavior is compared with the surrounding material. Infrared cameras measure the infrared radiation (wavelength ranging from 0.7 to 14 µm) that is emitted by a body, and this radiation is then converted into an electrical signal. These signals are further processed to create maps of surface temperature. A qualitative data analy- sis can be done from the thermograms (temperature coded images) (Maierhofer et al. 2002; Maierhofer et al. 2004; Maierhofer et al. 2006; Maser and Bernhardt 2000). Physical Principle Infrared radiation is part of the electromagnetic spectrum, with the wavelength ranging from 0.7 to 14 µm. Infrared cameras measure the thermal radiation emitted by a body, based on the thermal properties of various materials, and capture the regions with temperature differences. The three main properties that influence the heat flow and distribution within a material include the thermal conductivity (l), spe- cific heat capacity (Cp), and the density (r). When the solar radiation or a heater, in the case of active thermography, heats up the deck, all the objects in the deck emit some energy back (Figure 3.20). The delaminated and voided areas are typically filled with water or air, which have a different thermal conductivity and thermal capacity than the surrounding concrete. These delaminated areas heat up faster and cool down more quickly compared with concrete. They can develop surface temperatures from 1°C to 3°C higher than the surrounding areas when ambient conditions are favorable. Applications Infrared thermography is mostly used to detect voids and delaminations in concrete. However, it is also used to detect delaminations and debonding in pavements, voids in shallow tendon ducts (small concrete cover), cracks in concrete, and asphalt concrete segregation for quality control. Limitations The method does not provide information about the depth of the flaw. Deep flaws are also difficult to detect. Finally, the method is affected by surface anomalies and boundary condi- tions. For example, when sunlight is used as a heating source, clouds and wind can affect the deck heating by drawing away heat through convective cooling. Chain Dragging and hammer Sounding Description Chain dragging and hammer sounding are the most common inspection methods used by state DOTs and other bridge Figure 3.17. Electrical resistivity principle.

26 Figure 3.18. Infrared thermography testing. Infrared and Visual Cameras Source: Photographs courtesy of Dr. Ken Maser. Figure 3.19. Infrared thermography.

27 owners for the detection of delaminations in concrete bridge decks. The objective of dragging a chain along the deck or hitting it with a hammer is to detect regions where the sound changes from a clear ringing sound (sound deck) to a some- what muted and hollow sound (delaminated deck). Chain dragging is a relatively fast method for determining the approximate location of a delamination. The speed of chain dragging varies with the level of deterioration in the deck. Hammer sounding is much slower and is used to accurately define the boundaries of a delamination. It is also a more appropriate method for the evaluation of smaller areas. The application of the two methods is shown in Figure 3.21. Another technology that is close in principle to hammer sounding is called the rotary percussion system, which is also accepted as a standard procedure by ASTM D4580-86 (reapproved 1992) for measuring delaminations in concrete bridge decks. The rotary percussion system consists of multiple, gear-shaped wheels on the end of a pole. In some rotary percus- sion systems, a microphone and headphones are connected to the pole to amplify the sound and block out the ambient noise. The evaluation involves rolling the toothed wheel over a sur- face and listening for a change in the sound. Chain dragging and rotary percussion methods are relatively fast delamina- tion detection devices that are also very simple to use. Figure 3.20. Principle of passive infrared thermography. Figure 3.21. Chain dragging (left) and hammer sounding (right).

28 Physical Principle Chain dragging and hammer sounding are categorized as an elastic wave test. The operator drags chains on the deck, listening to the sound the chains make. A clear ringing sound represents a sound deck, while a muted or hollow sound represents a delaminated deck. The hollow sound is a result of flexural oscillations in the delaminated section of the deck, creating a drumlike effect. Flexural oscillation of a deck result- ing from an impact (the source of the impact can either be from chain dragging or hammer sounding) is typically found to be in a 1- to 3-kHz range. This is within the audible range of a human ear. The presence of any delamination changes the frequency of oscillation and, therefore, the audible response of the deck. Applications Chain dragging and hammer sounding are mainly used to detect the late stages of delaminations in concrete structures. Although chain dragging is limited to horizontal surfaces, hammer sounding can be used for a wider range of structures. Limitations Chain dragging and hammer sounding are dependent on the operator’s skill and hearing, which makes both methods sub- jective. Initial or incipient delamination often produces oscil- lations outside the audible range and thus cannot be detected by the human ear. As a result, they are not detected by chain dragging and hammer sounding. Both methods are generally ineffective for delamination detection on decks with overlays.