Inspection Techniques

The Inspection of concrete bridges has evolved in the recent years. Although visual inspection is still widely accepted as a common practice, many NDE techniques have been developed and widely adapted to objectify the process

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Bridges & structures that must be inspected

Minimum qualifications for program managers & team

leaders Maximum inspection intervals Inspection regulations &

references

State-Defined Aspects

Short interim inspections

of structures or components Testing methods, NDE Access policy for close up

& hands on inspections

Personnel requirements for complex structures/methods

and make it more fast and reliable. The following sections will discuss the most pertinent inspection techniques.

II.2.3.1 Visual Inspection

Visual inspection is considered the most basic, yet the most prevalent bridge inspection technique. The goal of field visual inspection is to report on the physical condition of bridges and to evaluate their status. This is mostly done on a routine basis by bridge engineers or inspectors, who carry out comprehensive visual inspections of structures on site to detect and evaluate the deterioration and spot damages on the different structural elements.

In general, visual inspection is required to follow well established procedures established by bridge inspection manuals and codes issued by transportation agencies. In addition, certain requirements are commonly set forth to regulate inspectors’ qualifications and data recording formats (FHWA 2002; MTQ 2012; MTO 2008). The amounts of funds, time and efforts involved in experimental investigations render visual inspection more practical and appealing as a condition assessment strategy. Visual inspection can provide valuable information on a bridge’s condition; especially that most bridge defects (such as cracks, spalls and leaching) can be visually detected. However, results obtained from visual inspection heavily depend on the expertise and judgments of bridge inspectors, yielding them to be primarily qualitative and subjective (Jain and Bhattacharjee 2011). Nevertheless, Data from Visual inspections are still regarded as the standard input to assist in maintenance decision making and evaluate needs for further investigations.

II.2.3.2 Non-Destructive Evaluation Methods

Due to the several drawbacks of visual inspection, many Non-destructive Evaluation (NDE) techniques have been introduced to augment the evaluation process. This section aims at providing the reader with a brief overview of some of the popular NDE testing methods that are implemented for onsite assessment of concrete structures. More focus was hereby given to NDE techniques that are most commonly used in practice, or predominantly cited in literature, as fitting the purpose of evaluating reinforced concrete bridges. Each of the below mentioned tests can be single-handedly applied to evaluate certain aspects of concrete bridges; but one test might as well be combined with a second or third test to cover a wider breadth of testing capabilities in a complimenting manner. Included in this section is the description of Half-Cell Potential test, Impact Echo test, and acoustic methods. Further NDE techniques are explained in detail in appendix A, including Concrete Resistivity, Infrared Thermography and Ground Penetrating Radar (GPR).

II.2.3.2.1 Half-Cell Potential Test (HCP)

This test is considered to be one of the most widely applied NDE tests for corrosion assessment and evaluation. It is much easier to conduct than many other methods including nuclear or radio-active tests, which are deemed to be more complicated. Relative ease of administration, fairly low cost, and simple data interpretation have promoted half-cell potential (HCP) to be a very popular test in measuring reinforcement corrosion in concrete structures. The test is conducted according to the configuration shown in Figure 6. As can be seen from

the figure, the apparatus consist of the reference electrode (half-cell), connecting wires, and a high impedance voltmeter.

The principle of the test, in its basic form, relies on measuring the potential difference between steel re-bars and the concrete surface. This is achieved by wire connecting an exposed steel reinforcing bar to one terminal of the voltmeter, while having the other terminal linked to a reference probe which rests on the concrete surface and forms the other half of the cell. The concrete cover must be moist enough in order for it to act as an electrolyte. This will allow excess electrons to flow from the corroded rebar to the reference probe through the damp concrete cover due to difference in potential. Therefore, a rebar with a higher corrosion probability will be identified by a greater potential difference pointed out by the voltmeter.

Figure 6: Half-Cell Potential Test Apparatus (ASTM C876-09)

In addition to the sufficient moisture required by this test, reinforcing steel bars should be free of any coating that might hinder their electrical connectivity (ASTM C876-09). Therefore, the test can’t be run on epoxy-coated bars. By manually placing the reference probe on predefined grid points, a map illustrating corrosion potentials can be generated. ASTM C876 provides general guidelines on performing Half-Cell potential test to evaluate the probability of corrosion activity in reinforced concrete structures. According to those guidelines, potential readings are interpreted as per Table 2 to indicate corrosion probably. Gucunski et al (2010) stated that the HCP technique has been extensively utilized by bridge engineers as a standard corrosion measurement; however, the measured potential values may be influenced by concrete resistivity and cover thickness. In most traditional cases of performing HCP test, holes through concrete cover have to be dug so as to connect the bars to the voltmeter. Therefore, many sources, including Lai et al. (2012), argue that traditional HCP test is partially destructive and may not be looked at as a pure NDE technique. The later study proposed a modified HCP method that alternatively reports deferential potential with the use of two probes, by having both of them placed and moved on the concrete surface.

Table 2: HCP Readings and Interpretations (ASTM C876) Half-Cell Potential Reading Interpretation

More positive than -200 mV 90% probability of no active corrosion Between -200 mV and -350 mV Corrosion probability is uncertain More negative than -350 mV 90% probability of active corrosion

II.2.3.2.2 Impact Echo Test

The Impact Echo (IE) test is one of the major techniques that belong to the ultrasonic tests family. It is generally based on seismic analysis and transmission-reception of low frequency impact-generated waves through the tested material. The IE test has found various applications in the depth measurement and internal flaws detection of concrete bridge decks. This includes the detection, location, and range approximation of subsurface concrete defects; such as: internal voids, honeycombing, and delaminations.

Standard guidelines for carrying out the IE test are fully described in ASTM C 1383. The test principle is based on an instant, mechanically induced, stress wave that propagates through the tested structural material, and gets reflected by internal defects or intruding substances. When the hemispherical fronts of the stress waves reach an internal interfaces or discontinuities, such as boundaries or voids, energy reflections (echoes) are mirrored in multiple directions within the structure; thus exciting local modes of vibrations that can be received and recorded by a transducer positioned near the small steel sphere that originates the impact. The transducer generates a voltage that is proportional to the received displacements or vibrations, and transfers a “voltage-time” signal to a processor where it is mathematically analyzed into a spectrum of amplitude vs. frequency. Frequency peaks in the spectrum are commonly associated with multiple reflections against thin or delaminated layers (Carino 2004).

Carino (2004) describes the test’s main principle as an analysis of the frequency of displacement waveforms. When the stress pulse is generated by mechanically 24

impact the surface of the tested material, it propagates back and forth between the internal defect and the surface. The reflection of this pulse creates a characteristic downward displacement every time it arrives to the top surface (Figure 7). Thus, a periodic wave is formed with a known wave length (period) calculated by dividing the travel path (2L) by the wave speed. As wave frequency is equivalent to the inverse of the period, f of the characteristic displacement pattern is equal to:

𝑓 = 𝐶𝑝𝑝

2𝐿 [3] Where 𝐶𝑝𝑝 is the plate P-wave speed; determined from performing the IE test on a part of the structure (or plate) with known thickness. Therefore, as the dominant frequency of the waveform is calculated, the depth or distance to the reflecting internal flaw can be determined as follows

𝐿 = 𝐶𝑝𝑝

2𝑓 [4] A relatively straight forward application of the IE test is to determine the actual depth of plate-like concrete structures such as slabs. This application has been standardized ASTM C1383, particularly for plate like structures in which any lateral dimension is at least six times the thickness. Defect detection capabilities of IE range from cases of delamination or internal voids to rather complex cases of micro-cracking (Carino 2004).

II.2.3.2.3 Acoustic Methods

Acoustic methods are based on the sound effect produced by a hammer or metal chain against the surface of concrete bridge elements. Metal chains are commonly used for approximating near surface delamination of concrete bridge decks, while hammer tapping is usually utilized to test vertical bridge elements. Chain dragging is perhaps the simplest and the most widely used test to detect areas of delamination over the top reinforcement bars right below the surface of exposed concrete decks. The test procedure and apparatus are described in the guidelines of ASTM D4580 with the latest version in 2012 (ASTM D4580-2012). It is mainly focused on detecting the subsurface delamination by dragging a steel chain over the top of concrete decks. While sound concrete areas will produce a clear ringing sound, areas of delamination can be recognized by the operator

D isp lac em en Time Impact 𝐿 Defect Receiver R-wave P-wave

Figure 7: Impact-Echo Method

when encountering a dull or hollow sound (ASTM D4580-2012). This method is mostly not intended to be applied on bridge decks that are overlaid with asphalt, since the overlay might act as an insulator that hinders the transmission of sonic signals. Chain drag test is best utilized for uncovered concrete decks. It is still, however, applicable on concrete decks that have been overlaid with portland cement concrete mixtures (Scheff and Chen 2000).