Damage to road surfaces and their non-destructive detection

Marc Alsina, winter semester 2020/2021

Due to the current dimensions of the transportation infrastructure, continuously upkeep of road surfaces is a matter of maximum importance in order to reduce expenses in terms of maintenance and rehabilitation. Advanced non-destructive evaluation (NDE) plays an essential role by enabling an early detection of deterioration. ^[3][4]

Mechanisms of deterioration

Deterioration can be provoked by a wide variety of causes of chemical, physical or even biological character. Above all, corrosion stands out and in most cases ends up inducing cracking and concrete delamination.

When the concrete’s reinforcing steel bars corrode in an oxygen rich atmosphere, the iron generated on the bar’s diameter causes notable stresses in the concrete, which can lead to either vertical or horizontal cracking. With extended corrosion, the reinforcement layer horizontal cracks use to merge into a large crack plane parallel to the surface, which can finally cause the mentioned delamination. Although this defect can achieve notable length, the crack thickness stays rather small. Furthermore, it is also important to remark that reiterated overloading and fatigue are other reasons for delamination.

However, other types of deterioration such as alkali-silica reaction (ASR), delayed ettringite formation (DEF), plastic shrinkage or freeze–thaw cycles have to be taken into consideration. These affectations are related to concrete deterioration: variations of the elastic modulus, strength and electrical or chemical characteristics. ^[1][3][4]

First, the ASR is characterized by the formation of a silica gel that gains volume in presence of water, inducing internal and external cracking. Then, the DEF is the consequence of an incorrect heat curing of concrete and provokes non-desired expansions. Additionally, plastic shrinkage (volume diminution) can as well produce cracks. Finally, freeze-thaw can raise hydraulic pressure, so the road surface will collapse once the pressure goes beyond the tensile strength.

Apart from that, it is important to emphasize that once imperfections appear, moisture and chlorides will fill that zones. This scenario will further induce deterioration and affect the structural unity of the road surface. Moreover, these imperfections will also difficult the application of non-destructive evaluation (NDE) techniques. ^[1]

Detection strategies

The origin of the deterioration’s mechanisms resides underneath the ground, which is the reason why their effects cannot easily be seen by visual inspection or simple non-destructive evaluation (NDE) such as chain drag and hammer sounding. These techniques have limitations in terms of early detection with respect to their stage of progression.

Therefore, to be able to address anticipated deterioration palliation, the use of advanced non-destructive evaluation (NDE) methods is basic to determine which type of maintenance or rehabilitation is needed. The most common procedures are: electrical resistivity (ER), impact echo (IE), ultrasonic surface waves method (USW), infrared thermography and ground penetrating radar (GPR). ^[3][4]

Chain drag and hammer sounding

These simple non-destructive evaluation (NDE) methods are used to detect delamination in advanced stage and can be categorized as manual vibrational tests. The purpose is to detect the areas where the sound from dragging the chain or hitting the hammer variates from a clear sound to hollow sound (delaminated).

As a starting point, chain drag is a fast method to conclude the approximated position of a delaminated zone. Then, hammer sounding is a much slower practice, indicated for smaller regions and accurate positioning of defects. Therefore, best option is to use both methods to optimally define the position and dimensions of a delamination.

Fundamentals

The impact of either chain dragging or hammer sounding generates an oscillation of the road surface, which frequency usually is found in the 1 to 3 KHz range. Taking into account that this is audible range for the human ear, the presence of any kind of defect changes the frequency of oscillation.

Limitations

These techniques depend on the ability and experience of the labourer, what implies a huge subjectivity. Additionally, it is important to take into consideration that initial delaminations produce frequency oscillations outside of the human’s audible range, making therefore their detection not possible. ^[1]

Electrical resistivity (ER)

This method is used in zones where deterioration is initiated by a corrosive environment. As the environment becomes tougher, the generated corrosion will induce micro and macro cracking, which finally lead to delamination.

Electrical resistivity provides a description of the corrosive environment. Dry concrete will represent a high resistance to the current’s conduction. On the other hand, the existence of water and greater porosity due to wreck and damage will reduce resistivity. ^[4]

Fundamentals

This technique measures the voltage and current at the road surface. Resistivity is usually determined using the Wenner setup, which makes use of four equally spaced probes. The outer probes generate a current into the road surface, while the inner probes determine the resulting electrical field. The resistance is calculated according to the formula:

$//<![CDATA[ \begin{array}{l}ρ=2πaV/I\end{array} //]]>$

where:

               ρ: resistivity [Ω.m]

               a: electrode separation [m]

               V: voltage [V]

               I: current [A]

The interaction between ER and the corrosion degree is given in the following table:

Limitations

High electrical resistivity in the road surface concrete can induce unsteady quantifications, a problem which has to be solved by prewetting the area to measure. Then, in order evade variations due to this wetting, the first measurement has to be taken not earlier than a few minutes after. ^[1]

Impact echo (IE)

This method consists of a mechanical impactor and a receiver. When an impact is effectuated, road surface resonances are induced. These resonances provide characteristics of the delaminations. The road surface can be impacted with different tools such as wire-mounted steel balls, automated projectile sources, or solenoid-type impactors. On the other hand, the reaction can be quantified using a nearby contact or air-coupled sensor.

The IE technique is mainly used to determine the horizontal and vertical position of a delamination, as well as its degree of advancement. However, it is also utilized to rate concrete modulus and compressive strength. ^[1]

Fundamentals

This technique is based on the transmission of high-frequency mechanical (sound) wave into the pavement and measurement of the P-wave echo between the top and bottom of the road surface. The amplitude of the measured vibration (echo) is transferred to the frequency domain as amplitude versus frequency. In absence of defects, there is a resonant or dominant frequency, which at the same time is proportional to the thickness of the road surface. This mentioned resonant frequency it also known as “thickness resonance”. ^[2] The frequency data is converted into thickness according to the formula:

$//<![CDATA[ \begin{array}{l}T=βCp/2f\end{array} //]]>$

where:

               T: thickness [m]

               Cp: assumed P-wave velocity [m/s]

               f: frequency [Hz]

               β: correction factor [-]

Note: Despite the fact that no theoretical principle has been provided for the ß factor, several investigations have enabled the determination of this shape factor. Due to the observed inconsistencies between the theory and the practical results, the correction factor value for the study case is ß=0.96. ^[6]

In case of a uniform pavement without defects, the determined thickness resonance is fairly unfluctuating. Nevertheless, in existence of a delamination, lower frequency modes will appear. According to the previous formula, the lower frequency modes will provoke an unusual high thickness calculated value. ^[2]

The first stage of the delamination process, also known as incipient delamination, can be recognized because of the existence of returning frequencies due to the reflections from the bottom of the road surface as well as the delamination. Next stage of delamination, known as progressed delamination, is distinguished by a single peak at the frequency that represents the depth of the defect. Eventually, in case of wide or superficial delaminations, the generated response to an impact belongs to the lower frequency range. In most cases this last type of delamination is situated in the audible range, differing from the incipient delamination, which is characterized by high frequencies. ^[1]

Limitations

The IE technique can spot delaminations on surfaces with PCC overlays. Nevertheless, when asphalt concrete overlays are used, spotting is only feasible when the temperature is enough low, so the viscosity is also relatively low. In resume, a very dense test grid is needed in order to be able to precisely characterize the delaminated area. ^[1]

For additional information, consult the already existing Wiki article named Impact-Echo.

Ultrasonic surface waves (USW)

The USW is based on the recording of the response to an impact of the road surface. This technique is used in quality control of road surfaces and grading of material damage. As in the impact echo test, the elastic waves can be generated by steel balls, automated projectile sources, or solenoid-type impactors. The lecture of the response is done by a pair or an array of receivers and registered by a transient recorder.

This method is a derivate of the spectral analysis of surface waves (SASW), which enables the determination of material properties such as the elastic modulus, in zones close to the surface. The SASW technique benefits from the surface wave dispersion to acquire data about layer thickness and elastic modulus. Here it is important to remark that the USW test is exactly like the SASW test, excluding the fact that the frequency range is constrained to a narrow high-frequency range in which the depth of the surface waves is littler than the thickness of the examined road surface. Given a measured or an assumed mass density or Poisson ratio, the surface wave velocity permits the calculation of the material modulus.

Fundamentals

Surface waves are distinguished because of a higher energy density on the surface, in contrast to the body. After an impact, they spread radially generating a cylindrical propagation, with a velocity directly related to the elastic characteristics of the environment. The subsurface data is acquired by the determination of the correspondence between phase velocity and speed, known as dispersion curve, and backcalculation of the dispersion curve to identify the profile of the examined road surface.

In the case of a homogenous deck, the velocity of the elastic waves will be pretty constant. It is important to mention that average velocity is utilized to relate it to the concrete modulus. A huge fluctuation in the phase velocity would imply the existence of a delamination or any kind of defect.

Limitations

This method can only determine accurate modulus values in zones in pristine condition, with any kind of defect. In addition, to be able to provide information about the deterioration degree, complex understanding and experience with the method are needed. Finally, it is important to add that the USW (SASW) modulus characterization becomes complex when working with layered systems, such as asphalt concrete overlays. In these cases, the modulus between layers can be relatively distinct. ^[1]

Infrared thermography

Infrared (IR) thermography is utilized to spot imperfections such as cracks, delaminations or even concrete disintegration in road surfaces. Moreover, this method can also be used to detect voids in shallow ducts and asphalt concrete segregation in quality supervision processes.

This technique monitors the electromagnetic wave surface radiations associated with temperature fluctuations in the infrared wavelength. Defects are then interpreted as a variation of material characteristics such as density, thermal conductivity or specific heat capacity.  Infrared cameras capture the IR radiation released from the road surface, and transform it into an electrical signal. Finally these signals enable the confection of surface temperature plots.

Fundamentals

IR radiation belongs to the electromagnetic spectrum, with a wavelength situated in the 0.7 to 14 µm range. The principal three attributes that affect the heat flow through the road surface are thermal conductivity (λ), specific heat capacity (Cp) and the density (ρ). When radiation is emitted, the delaminations or voids have a distinct thermal conductivity and thermal capacity as they usually contain water or air. These delaminated zones have a lower thermal inertia and therefore, the heat up and cool down faster. For example, their temperature can be from 1 to 3 °C higher when environmental circumstances are advantageous.

Limitations

This method cannot determine the defect depth. Additionally, it is complex to detect the defect itself in large depth cases. Moreover, it is vital to emphasize that surface irregularities and boundary conditions have huge influence on the method. As an example, in the case when sunlight is utilized as a heating font, clouds and wind play an important role in terms of convective cooling. ^[1]

For additional information, consult the already existing Wiki article named Infrared Thermography.

Ground penetrating radar (GPR)

GPR is fast non-destructive test that takes advantage of electromagnetic waves to position items of the road surface and to confection their shape maps under the surface. Antennas of distinct frequencies provide diverse degrees of refinement and profundity of insertion.

This technique is applied in the evaluation of road surface thickness, detection of rebar layout, determination of delamination or deterioration stage, corrosive environment investigation and calculation of road surface properties. ^[1]

Fundamentals 

The GPR method is based on the sending of high-frequency radio waves into the road surface from a mobile antenna attached for example to a test vehicle. The principle of the technique is that waves reverberate back from imperfections, what generates noticeable reflections that are used to confection a damage areas map. ^[2]

GPR acquires data by fluctuations in two electrical characteristics: electrical conductivity (opposite of resistivity) and relative dielectric permittivity. Typical relative permittivity values (dielectric constant, ɛ_r) are show in the following table:

Note: The two electrical characteristics mentioned above are responsible for the capacity of a GPR wave to penetrate a certain environment and its travelling speed. Additionally, dielectric differences between two environments will provoke some of the GPR wave to reflect back, so it can be quantified and stored. In resume, the key factor of this technology is the capacity of measuring the reflection providing from conductive materials such as rebars and simply cracked or delaminated zones filled with moisture, chlorides and other conductive substances. ^[1]

Limitations

Despite the fact of the numerous advantages of GPR, this technology cannot spot delaminations unless they are epoxy-impregnated or water containing. Additionally, low temperatures can inadequately affect the GPR measurements. The reason is that frozen water will not generate reflections that can be detected due its low dielectric constant. Moreover, the presence of deicing salt will also modify the dielectric constant. Other disadvantages of the GPR method are inability of providing mechanical properties (strength modulus) or accurate data about corrosive environment. Finally, it is necessary to add that in most cases the GPR outcome needs to be correlated and confirmed by using other non-destructive evaluation (NDE) methods. ^[1]

For additional information, consult the already existing Wiki article named Radar.

Literature

1. Gucunski, Nenad; Imani, Arezoo; Romero, Francisco; Nazarian, Soheil; Yuan, Deren; Wiggenhauser, Herbert; Shokouhi, Parisa; Taffe, Alexander; Kutrubes, Doria: Nondestructive Testing to Identify Concrete Bridge Deck Deterioration. SHRP2 Renewal Research. Transportation Research Board, Washington D.C. (2013).
2. Heitzman, Michael; Maser, Kenneth; Tran, Nam H.; Brown, Ray; Bell, Haley: Nondestructive Testing to Identify Delaminations Between HMA Layers Volume 1-Summary (2013). Reports and White Papers. 17.
3. Popovics, John S (University of Illinois at Urbana-Champaign): Investigation of a Full-Lane Acoustic Scanning Method for Bridge Deck Nondestructive Evaluation. IDEA Program Final Report, Project NCHRP-134. Transportation Research Board of the National Academies (2010).
4. Gucunski, Nenad; Basily, Basily B.; Maher, Ali; Mahn La, Hung: Robotic Platform Rabit for Condition Assessment of Concrete Bridge Deck Using Multiple NDE Technologies. Center for Advanced Infrastructure and Transportation, Rutgers University, Piscataway, New Jersey, U.S.A (2013).
5. Mori, K.; Spagnoli, A.; Murakami, Y.; Kondo, G.; Torigoe, I.: A new non-contacting non-destructive testing method for defect detection in concrete. NDT&E International 35 (2002) 399-406.
6. Gibson, Alexander; Popovics, John S.: Lamb Wave Basis for Impact-Echo Method Analysis. Journal of Engineering Mechanics, Vol. 131, No. 4, © ASCE (2005).