13.1-i 

Table of Contents

Section 13 ..................................................................................................................................................

Advanced Inspection 

Techniques 

 13.1 Timber.................................................................................................. 13.1.1 

 

 13.1.1 Introduction............................................................................. 13.1.1 

 

 13.1.2 Nondestructive Testing Methods ............................................ 13.1.1 

 Pol-Tek ............................................................................. 13.1.1 

 Spectral Analysis.............................................................. 13.1.2 

 Ultransonic Testing .......................................................... 13.1.3 

 Vibration........................................................................... 13.1.4 

 13.1.3 Other Testing Methods............................................................ 13.1.4 

 Boring or Drilling............................................................. 13.1.4 

 Moisture Content.............................................................. 13.1.6 

 Probing ............................................................................. 13.1.6 

 Shigometer........................................................................ 13.1.7 
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TOPIC 13.1: Timber 

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13.1.1

Section 13 

Advanced Inspection Techniques 

Topic 13.1 Timber 

13.1.1 

Introduction Advanced inspection techniques give inspectors the ability to further evaluate

suspected defects found during a visual inspection. They can also be used to 

perform inspections on members that are not accessible. Advanced inspection 

techniques usually require calibrated testing equipment, a professionally trained 

technician to perform the testing and a professional that has expertise in 

interpreting the advanced inspection results. However, bridge inspectors should 

have an understanding of the various advanced inspection techniques. 

There are two main classifications of advanced inspection techniques. The first is 

labeled nondestructive testing or evaluation (NDT or NDE). This classification 

pertains to all advanced inspection techniques that do not impair the usefulness of 

the member being tested. Other testing, the second main classification, covers all 

advanced inspection techniques that affect or destroy the structural integrity of the 

member being tested. 

New technology is making the use of these highly technical systems economically 

feasible for bridge inspection. From this fact, advanced inspection techniques are 

becoming more popular for the routine inspection of all bridge members. Current 

and future studies have been, and will be focusing directly on relating results from 

advanced inspection techniques into Bridge Management Systems ratings. 

This Topic describes the different types of nondestructive and other test methods 

for timber bridge members and the general procedures for each. 

13.1.2 

Nondestructive 

Testing Methods 

Pol-Tek Pol-Tek is a sonic testing device that is used to detect rot or other low density 

regions in timber poles. Starting about six inches below the ground line, probes are 

pressed on opposite sides of the timber member. A trigger trips a hammer that 

sends a sound wave down one probe, through the member, and up the other probe 

to a dial (see Figure 13.1.1). 
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TOPIC 13.1: Timber 

13.1.2

 

Figure 13.1.1 Pol-Tek Sonic Testing Apparatus 

This method eliminates the need for making holes in timber members. Members 

testing positive for rot are then drilled or cored to determine the nature of the 

defect. A dial reading that is low, compared with that of a good member of similar 

diameter, indicates rot or another low density region that delayed the sound wave 

within the member. However, several readings should be taken on the member 

since the readings are nearly instantaneous, and the Pol-Tek should be checked 

frequently for proper calibration. 

Used by trained personnel, Pol-Tek works well with Douglas fir and western red 

cedar. However, it does not work as well with southern pine members because of 

the high incidence of ring shakes. 

Spectral Analysis Spectral Analysis, sometimes called stress wave, uses sonic waves to produce 

stress waves in a timber member. The stress waves are then used to locate decay 

in timber members. The stress waves travel through the timber member and 

reflect off the timber surface, any flaws, or joints between adjacent members at the 

speed of sound. It is known that stress waves travel slower in decayed members 

than in sound members. If the member dimensions are known, the amount of time 

it takes for a stress wave to travel the known distance can indicate that defects are 

evident due to longer stress wave timings. 

Stress waves are also used to determine the in-situ strength of timber members. 

Sound timber members transmit waves at higher velocity than decayed wood. The 

velocity of the stress wave can be calculated by obtaining time of flight readings 

over a set length. The velocity can be converted into a dynamic modulus of 

elasticity, which in turn, allows professionals to estimate the strength properties of 

the wood. 
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TOPIC 13.1: Timber 

13.1.3

First, a stress wave is induced by striking the specimen with an impact device that 

is instrumented with an accelerometer that emits a start signal to a timer. A second 

accelerometer, which is held in contact with the other side of the specimen, serves 

to detect the leading edge of the propagating stress wave and sends a stop signal to 

the timer. The elapsed time for the stress wave to propagate between the 

accelerometers is displayed on the timer. 

The use of stress wave velocity to detect wood decay in timber bridges and other 

structures is limited only by access to the structural members under consideration. 

It is especially useful on thick timbers or glulam timbers where hammer sounding 

is not effective. Note that access to both sides of the member is required. 

The transmission time is affected by such properties as growth ring orientation, 

decay, moisture content, and preservative treatment. 

 

Figure 13.1.2 Stress Wave Timer 

Ultrasonic Testing Ultrasonic testing (UT) consists of high frequency sound waves introduced by a 

sending transducer. Discontinuities in the specimen interrupt the sound wave and 

deflect it toward a receiving transducer. The magnitude of the return signal allows 

a measurement of the flaw size. The distance to the flaw can be estimated from the 

known properties of the sound wave and of the material being tested. Ultrasonic 

testing can be used to detect cracks, internal flaws, discontinuities, and sub-surface 

damage (see Figure 13.1.2). 
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TOPIC 13.1: Timber 

13.1.4

 

Figure 13.1.3 Ultrasonic Testing Equipment 

In timber bridge members, ultrasonic testing can be used to determine the in-place 

strength of timber bridge members, both above and below water. The load 

carrying capacity of the member is correlated to the member's wave velocity 

normal to the grain and to its in-place unit weight. 

Vibration A newer type of nondestructive testing that can determine the condition of timber 

bridge members deals with the use of vibrations. This nondestructive testing 

method is based on the philosophy that all sound timber members vibrate at a 

certain frequency. While testing a timber member, if the member vibrates at a 

different frequency than the established theoretical frequency, the member may 

have defects present. Vibratory testing methods in timber members are basically 

used to determine the member's modulus of elasticity. From this, other properties 

of the timber member can be established. 

13.1.3 

Other Testing 

Methods 

Boring or Drilling Boring is the most dependable and widely used method for detecting internal 

decay in timber. Boring permits direct examination of an actual sample from a 

questionable member. An increment borer is used to extract wood cores for 

examination (see Figure 13.1.3). 
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TOPIC 13.1: Timber 

13.1.5

 

Figure 13.1.4 Increment Borer 

Drilling is performed using a rechargeable drill or a brace and bit. An abrupt 

decrease in drilling resistance indicates either decay or a void. However, wet 

wood and natural voids can falsely suggest decay. While samples are generally 

not attainable, observation of the wood particles removed during the drilling 

process can provide valuable information about the member. The depth of 

preservative penetration, if any, can be determined, and regions of discolored 

wood may indicate decay. 

A decay detection device is a newer drilling technique. It operates upon the 

principle that a drill moving through sound wood will encounter more resistance 

that a drill moving through decayed, and/or soft wood. It records the resistance, 

using a pen, paper and rotary drum arrangement, so that a permanent graphic 

record of the test is generated. Sound wood produces a series of near vertical 

markings on the record, however, when decayed wood is encountered, the 

resistance drops and the markings assume a more horizontal or diagonal pattern. 

By studying the resulting record, an experienced operator can determine if decay 

exists and, because the record is marked in millimeters (mm) of penetration, can 

estimate the approximate location and size of the decayed area (see Figure 13.1.5).
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TOPIC 13.1: Timber 

13.1.6

 

Figure 13.1.5 Decay Detection Device 

The use of increment cores for assessing the presence and damage due to bacterial 

and fungal decay requires special care. Cleaning of the increment borer is 

necessary after each core extraction to eliminate transfer of organisms; 

trichloroethane has been found to work well. Core samples that do not show 

visible signs of decay can be cultured to detect the presence of potential decay 

hazards. Many laboratories can provide this service. Core samples are more 

commonly used to detect the presence of internal decay pockets and to measure the 

depth of preservative penetration and retention. 

All bore holes can provide an entrance for bacterial and fungal decay to gain 

access to the member. As such, the holes must be treated with a preservative and 

must be plugged after testing. 

Moisture Content Moisture meters can be used to determine moisture content in a timber member. 

Moisture contents exceeding 20% indicate the condition of the wood is conducive 

to decay. As a sliding hammer drives two electrodes into the wood, a ruler 

emerging from the top of the hammer measures the depth. These electrodes can 

measure moisture content to a depth of approximately 2 1/2 inches. Because the 

high moisture content of decaying wood causes steeper than normal moisture 

gradients, the meter is useful for determining the extent of decay. 

Probing Probing consists of inserting a pointed tool, such as an ice pick, into the wood and 

comparing its resistance with that of sound wood. Lack of resistance or excessive 

softness to probe penetration may reveal the presence of decay. 

Two forms of probing are a pick test and a shell-thickness indicator. A pick test 

consists of removing a small piece of wood with a pick or pocketknife. If the 

wood splinters, it is probably sound wood, and if it breaks abruptly, it is probably 
SECTION 13: Advanced Inspection Techniques 

TOPIC 13.1: Timber 

13.1.7

decayed wood. 

A shell-thickness indicator is a thin, metal, hooked rod used to determine the 

thickness of solid, but not necessarily sound, wood. The rod is inserted into a hole 

made by coring or drilling and is then pulled back with pressure against the side of 

the hole. The hook should attach to the edge of a decay pocket, making it possible 

to determine the depth of the decay and the solid wood. 

Shigometer The Shigometer measures electrical resistance to detect decay in timber members. 

It should be used in wood with a moisture content of at least 27%, a value 

indicative of decaying wood. A probe is used consisting of two twisted, insulated 

wires with the insulation removed near the tip. This probe is inserted to various 

depths into a hole 3/32 inch in diameter. If the electrical resistance changes as the 

probe goes deeper, this indicates rot or a defect. 

While this device effectively detects decay, it can also produce misleading 

readings on sound timber. Consequently, drilling or coring should be done on 

suspect members. Like the Pol-Tek, the Shigometer should be recalibrated 

frequently. 
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TOPIC 13.1: Timber 

13.1.8

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13.2-i 

Table of Contents

Section 13 ..................................................................................................................................................

Advanced Inspection 

Techniques 

 13.2 Concrete .......................................................................................... 13.2.1 

 

 13.2.1 Introduction............................................................................. 13.2.1 

 

 13.2.2 Nondestructive Testing Methods ............................................ 13.2.1 

 Acoustic Wave Sonic/Ultrasonic Velocity 

 Measurements................................................................... 13.2.1 

 Electrical Methods............................................................ 13.2.2 

 Delamination Detection Machinery.................................. 13.2.3 

 Ground-Penetrating Radar................................................ 13.2.3 

 Electromagnetic Methods................................................. 13.2.4 

 Pulse Velocity................................................................... 13.2.5 

 Flat Jack Testing............................................................... 13.2.5 

 Impact-Echo Testing ........................................................ 13.2.6 

 Infrared Thermography .................................................... 13.2.7 

 Laser Ultrasonic Testing................................................... 13.2.9 

 Magnetic Field Disturbance ............................................. 13.2.9 

 Neutron Probe for Detection of Chlorides........................ 13.2.9 

 Nuclear Methods ............................................................ 13.2.10 

 Pachometer ..................................................................... 13.2.10 

 Rebound and Penetration Methods................................. 13.2.10 

 Ultrasonic Testing .......................................................... 13.2.10 

 Smart Concrete............................................................... 13.2.10 

 

 13.2.3 Other Testing Methods.......................................................... 13.2.11 

 Carbonation .................................................................... 13.2.11 

 Concrete Permeability .................................................... 13.2.11 

 Concrete Strength ........................................................... 13.2.11 

 Endoscopes and Videoscopes......................................... 13.2.11 

 Moisture Content............................................................ 13.2.12 

 Petrographic Examination .............................................. 13.2.12 

 Reinforcing Steel Strength ............................................. 13.2.12 

 Chloride Test .................................................................. 13.2.12 
SECTION 13: Advanced Inspection Techniques 

TOPIC 13.2: Concrete 

13.2-ii

 Matrix Analysis .............................................................. 13.2.12 

 ASR Evaluation.............................................................. 13.2.12 
13.2.1

Topic 13.2 Concrete 

13.2.1 

Introduction Advanced inspection techniques give inspectors the ability to further evaluate 

suspected defects found during a visual inspection. They can also be used to 

perform inspections on members that are not accessible. Advanced inspection 

techniques usually require calibrated testing equipment, a professionally trained 

technician to perform the testing and a professional that has expertise in 

interpreting the advanced inspection results. However, bridge inspectors should 

have an understanding of the various advanced inspection techniques. 

There are two main classifications of advanced inspection techniques. The first is 

labeled nondestructive testing or evaluation (NDT or NDE). This classification 

pertains to all advanced inspection techniques that do not impair the usefulness of 

the member being tested. Other testing, the second main classification, covers all 

advanced inspection techniques that affect or destroy the structural integrity of the 

member being tested. 

New technology is making the use of these highly technical systems economically 

feasible for bridge inspection. From this fact, advanced inspection techniques are 

becoming more popular for the routine inspection of all bridge members. Current 

and future studies have been, and will be focusing directly on relating results from 

advanced inspection techniques into Bridge Management Systems ratings. 

This Topic describes the different types of nondestructive and other test methods 

for concrete bridge members and the general procedures for each. 

13.2.2 

Nondestructive 

Testing Methods 

Acoustic Wave 

Sonic/Ultrasonic Velocity 

Measurements 

A full evaluation of concrete decks can be accomplished with sonic ultrasonic 

acoustic wave velocity measurements. This method delineates areas of internal 

cracking (including delaminations) and deteriorated concrete, including the 

quantification of strength characteristics (elastic moduli values). A mobile 

automated data acquisition device with an impact energy source and multiple 

sensors is the principle part of a computer-based monitoring and recording system 

for detailed evaluation of bridge decks. Bridge abutments and concrete support 

members are tested using the same recording system with a portable, hand-held 

sensor array (see Figure 13.2.1 and 13.2.2). The system works directly on either 

bare concrete or through wearing surfaces such as asphalt. It can distinguish 

between debonded asphalt and delaminations, and it is effective for a detailed 

evaluation of large areas. 
 SECTION 13: Advanced Inspection Techniques 

 TOPIC 13.2: Concrete 

13.2.2 

Figure 13.2.1 Portable Hand Held Sonic/Ultrasonic Testing Sensor Array System 

 (http://www.ndtcorporation.com) 

 

Figure 13.2.2 Acoustic Emission Sensors 

Electrical Methods Half Cell Potentials are used to evaluate the corrosion activity of reinforcing steel 

embedded in concrete. Commonly known as CSE (Copper Sulfate Electrode) 

tests, reinforcing bar networks are physically accessed and wired for current 
 SECTION 13: Advanced Inspection Techniques 

 TOPIC 13.2: Concrete 

13.2.3 

detection. Half cell electrical potentials of reinforcing steel are measured by 

moving the CSE about the concrete surface. As the CSE contacts concrete over an 

actively corroding rebar, voltage is registered. Measured potential values reflect 

levels of corrosion activity in the rebar. Higher potential measurements indicate 

corrosion activity. This kind of survey can be used to determine core sample 

locations. 

Delamination Detection 

Machinery 

Delamination detection machinery is based on sonic responses and can be used to 

inspect concrete decks (see Figure 13.2.3). The portable electronic instrument 

consists of three components: a tapping device, a sonic receiver, and a signal 

interpreter. The instrument is moved across the deck as acoustic signals are passed 

through the deck. These signals are then received and electronically interpreted, 

and the output is used to generate a plan of the deck showing delaminated areas. 

This method can be used on concrete decks with asphalt covered surfaces, 

although accuracy decreases. 

 

Figure 13.2.3 Delamination Detection Machinery 

Ground-Penetrating 

Radar 

Ground-Penetrating Radar (GPR) is a geophysical method that uses high 

frequency pulsed electromagnetic waves to acquire subsurface information. An 

important benefit of this method is the ability to measure the thickness of asphalt 

covering. It can also be used to examine the condition of the top flange of box 
 SECTION 13: Advanced Inspection Techniques 

 TOPIC 13.2: Concrete 

13.2.4 

beams that may otherwise be inaccessible. 

An electromagnetic wave is radiated from a transmitting antenna, and travels 

through the material at a velocity which is determined primarily by the electrical 

properties of the material. As the wave spreads out and travels downward, if it hits 

a buried object or boundary with different electrical properties, then part of the 

wave energy is reflected or scattered back to the surface, while part of its energy 

continues to travel downward. The wave that is reflected back to the surface is 

captured by a receiving antenna, and recorded on a digital storage device for later 

interpretation (see Figure 13.2.5). The most common display of GPR data is one 

showing signal versus amplitude, and is referred to as a trace. A single GPR trace 

consists of the transmitted energy pulse followed by pulses that are received from 

reflecting objects or layers. 

GPR is used to map geologic conditions that include depth to bedrock, depth to the 

water table, depth and thickness of soil and sediment strata on land and under fresh 

water bodies, and the location of subsurface cavities and fractures in bedrock. 

Other applications include the detection of delaminations/flaws in a reinforced 

concrete bridge, location of objects such as pipes, drums, tanks, cables, and 

boulders, mapping landfill and trench boundaries, mapping contaminants, and 

conducting archeological investigations. It has been used for concrete bridge deck 

and tunnel lining inspection. 

Ground Penetrating Radar for Bridge Decks

Ground penetrating radar (GPR) technology is nearing full acceptance as a method 

to assess the condition of bridge decks, in particular, delaminations between 

concrete and rebar. 

Importantly, GPR is an NDE technology, as opposed to cutting core samples from 

concrete decks. It can provide information on asset condition that can be used to 

plan and execute effective and efficient repair programs. The principal issue with 

GPR technology is slow rate of data capture when the depth of evaluation is more 

than approximately 75 mm (3 inches). 

Electromagnetic Methods Advancements in ground penetrating radar have lead to the development of the 

High Speed Electromagnetic Roadway Measurement and Evaluation System 

(HERMES) Bridge Inspector. This system was built by the Lawrence Livermore 

National Library to detect delaminations in concrete decks caused by 

reinforcement corrosion. The HERMES Bridge Inspector sends high frequency 

electromagnetic pulses from 64 radar antennas into a bridge deck while travelling 

over the structure. The device is set up in a trailer mounted towing vehicle and is 

made up of a computer workstation, storage device, survey wheel, control 

electronics, and the 64 antenna modules or transceivers (see Figure 13.2.4). The 

system can inspect up to a 1.9 m (6’-3”) width at a time with maximum speeds of 

up to 96 kilometer/hour (60 mph). At speeds of around 32 kilometer/hour (20 

mph), the system can sample the concrete deck every 1.5 cm (9/16”) in the 

direction of travel. Output information can be reconstructed to show cross-sections 

of the deck being inspected. The depth of penetration depends on time and the 

material type. A 30 cm (11-13/16”) penetration in concrete can be accomplished 

in about 6 nanoseconds. In the near future, a new system, called HERMES II, will 

update the original HERMES Bridge Inspector based on experience gained from 

the original. 
 SECTION 13: Advanced Inspection Techniques 

 TOPIC 13.2: Concrete 

13.2.5 

Figure 13.2.4 The HERMES Bridge Inspector 

Pulse Velocity Pulse Velocity techniques are used to evaluate relative quality of concrete and 

estimate compressive strength. The pulses pass through the concrete and the 

transit time is then measured. The pulse velocity is then interpreted to evaluate the 

quality of the concrete and to estimate in-place concrete compressive strength. 

This equipment analyzes concrete in decks by measuring velocity of sound waves. 

Some equipment generates sound waves by shooting BB's onto the deck. The time 

for the waves to return depends on the integrity of the concrete. 

Flat Jack Testing The flat jack method was originally developed to test the in situ stress and 
 SECTION 13: Advanced Inspection Techniques 

 TOPIC 13.2: Concrete 

13.2.6 

deformation of rock and is now being applied to masonry structures. A portion of 

the horizontal mortar joint is removed, and the flat jack (an envelope made of 

metal) is inserted and pressurized to determine the state of stress. For deformation 

testing, two flat jacks are inserted, one directly above the other and separated by 

five or six courses. 

Figure 13.2.5 Schematic of Ground-Penetrating Radar 

Impact-Echo Testing Sound wave reflection is a method for nondestructive evaluation of concrete and 

masonry, based on the use of impact-generated stress (sound) waves that propagate 

through the structure and are reflected by internal flaws and external surfaces. 

This method can be used to determine the location and extent of flaws such as 

cracks, delaminations, voids, honeycombing and debonding in plain, reinforced 

and post-tensioned concrete structures. It can locate voids in the subgrade directly 

beneath slabs and pavements. It can be used to determine member thickness or 

locate cracks, voids and other defects in masonry structures where the brick or 

block units are bonded together with mortar. This technique is not adversely 

affected by the presence of steel reinforcing bars. 

A short-duration mechanical impact, produced by tapping a small steel sphere 

against a concrete or masonry surface, produces low-frequency stress waves that 

propagate into the structure and are reflected by flaws and/or external surfaces (see 

Figure 13.2.6). The wavelengths of these stress waves propagate through concrete 

almost as though it were a homogeneous elastic medium. Multiple reflections of 
 SECTION 13: Advanced Inspection Techniques 

 TOPIC 13.2: Concrete 

13.2.7 

these waves within the structure excite local modes of vibration, and the resulting 

surface displacements are recorded by a transducer located adjacent to the impact. 

The piezoelectric crystal in the transducer produces a voltage proportional to 

displacement, and the resulting voltage-time signal (called a waveform) is digitized 

and transferred to a computer, where it is transformed mathematically into a 

spectrum of amplitude vs. frequency. Both the waveform and spectrum are plotted 

on the computer screen. The dominant frequencies, which appear as peaks in the 

spectrum, are associated with multiple reflections of stress waves within the 

structure, or with flexural vibrations in thin or delaminated layers. 

Figure 13.2.6 Impact-Echo Testing Equipment 

Infrared Thermography NDT inspection using thermography is based on imaging surface temperatures of a 

specimen in order to infer subsurface delaminations or defects. The basic theory is 

that heat conduction through a material will be altered if a delamination is present. 

Figure 13.2.7 graphically shows this concept. In this example the temperature of 

the deck is greater than the surrounding air. With no internal defect, heat flow 

through the deck will be relatively uniform. An image of the surface temperature 

of the deck will then produce an image that is relatively uniform. If a delamination 

is now present inside the specimen, the heat flow will be altered. In this example 

the surface of the deck above the delamination will appear to be lower in 

temperature than the remainder of the deck. The resulting image of the deck 

surface will show a corresponding area with a lower temperature. 
 SECTION 13: Advanced Inspection Techniques 

 TOPIC 13.2: Concrete 

13.2.8 

Figure 13.2.7 Schematic of Thermal Imaging 

Thermographic measurements are complicated by a number of issues. Probably the 

most significant is that a thermal camera does not directly measure the temperature 

of a specimen. The camera measures radiant flux that needs to be converted to 

temperature. The measured radiant flux is not only a function of the surface 

temperature, but is a function of the emissivity of the specimen. Emissivity is a 

material property that describes how well an object emits or absorbs energy. Two 

objects at the same temperature but with different emissivities would appear as 

different intensities in an infrared image. Shadows or other uneven heating of a 

specimen are also a concern. Other environmental factors, such as water, snow, or 

ice on a specimen, will alter results as well. Also, the technique is sensitive to 

material property differences on the specimen surface. Surface defects, such as oil 

stains, water, skid marks, will show up in the infrared data. 
 SECTION 13: Advanced Inspection Techniques 

 TOPIC 13.2: Concrete 

13.2.9 

 

Figure 13.2.8 Infrared Thermography Testing Equipment 

Laser Ultrasonic Testing Laser ultrasonic testing provides information about flaws in concrete and about the 

position of steel reinforcement bars, which cannot be obtained with the non-laser 

ultrasonic testing described in this Subtopic. Laser-generated acoustic wave 

measurements with high stress amplitudes provide information about the quality of 

the concrete at various depths from the surface. Reinforcing steel does not cause 

misleading results in laser ultrasonic testing as it does in non-laser ultrasonic 

testing. 

Magnetic Field 

Disturbance 

Advanced inspection techniques have been developed that can evaluate fatigue 

damage to steel reinforcement in concrete members. The device is known as the 

magnetic field disturbance (MFD) system and can be used on reinforced and 

prestressed concrete. The system maps the magnetic field across the bottom and 

sides of the beam. A discontinuity in magnetized steel, such as a fracture in a rebar 

or a broken wire in a steel strand, produces a unique magnetic signal. While the 

research has been encouraging for detecting fatigue-related damage due to the 

significantly different magnetic signals for corroded reinforcing, MFD has not yet 

been demonstrated for detecting in-service corrosion damage. 

Neutron Probe for 

Detection of Chlorides 

A neutron probe can be used to detect chlorides in construction materials. The 

materials are bombarded with neutrons from a small portable source. Measuring 

the gamma rays bouncing back provides a spectrum showing different elements, 
 SECTION 13: Advanced Inspection Techniques 

 TOPIC 13.2: Concrete 

13.2.10 

one of which is chloride. A major potential application that remains to be tested is 

measuring chlorides in reinforced concrete to determine corrosion hazard. Another 

potential application includes inspecting suspension bridge cables. 

Nuclear Methods The primary use of nuclear methods is to measure the moisture content in concrete 

by neutron absorption and scattering techniques. These moisture measurements 

are then used to determine if corrosion of reinforcement is likely to occur. A more 

direct measurement of the rate of corrosion would be more useful to the bridge 

inspector, and this method is therefore more research oriented than operational in 

the field. 

Pachometer A pachometer is a magnetic device used in determining the position of 

reinforcement. Magnetic methods do not detect concrete defects or deterioration 

directly. However, they can detect regions of inadequate cover, which is often 

associated with corrosion-induced deterioration. Magnetic methods can be used to 

measure cover in the range of 0 to 75 mm (0 to 3 inches) to an accuracy of about 6 

mm (1/4 inch). 

Rebound and Penetration 

Methods 

Rebound and penetration methods measure the hardness of concrete and can be 

used to predict the strength of concrete. The Schmidt hammer (also known as the 

Swiss hammer) is probably the most commonly used device to measure the 

penetration resistance of hardened concrete. A spring-loaded device strikes the 

surface of the concrete, and based on the response, the compressive strength of the 

concrete can be determined. This inspection technique can be used to compare the 

quality of the concrete in different parts of concrete bridge components. However, 

only the surface of the concrete is being tested, and the strength value is relative. 

Another common penetration device is called the Windsor probe. A pistol-like 

driving device fires a probe into the surface of the concrete. The probe is 

specifically designed to crack aggregate particles and to compress the concrete 

being tested. 

Both of these tests are considered practical primarily with concrete that is less than 

one year old. However, when used in conjunction with core sampling, these tests 

can also be used to determine significant differences in concrete strength of older 

bridges. 

Ultrasonic Testing Ultrasonic testing can provide valuable information regarding the condition of 

concrete bridge members. However, the method can be difficult to use with 

reinforced concrete members, and some skill is required to obtain usable results. 

Large cracks and voids can be detected, since the path of the pulse will travel 

around any cavity in the concrete and time of transmission is therefore lengthened. 

The presence of steel parallel to the line of transmission provides a path along 

which the pulse can travel more rapidly, causing misleading results. Therefore, it 

is generally desirable to choose paths that avoid the influence of reinforcing steel. 

Smart Concrete Carbon fiber-reinforced cement can be used as a strain-sensing coating on 

conventional concrete. This coating allows the sensing of strain similar to strain 

gauges. The resistance can be measured by having electrical contacts attached to 

the member. 
 SECTION 13: Advanced Inspection Techniques 

 TOPIC 13.2: Concrete 

13.2.11 

Strain gauges are expensive compared to the structural material, and they are often 

become detached during use. This method could be much more reliable in sensing 

strain in structures. 

Smart concrete is in early stages of development. 

13.2.3 

Other Testing 

Methods 

Core sampling is a destructive form of concrete advanced inspection techniques, 

and it can weaken a member. Cores can be used for many of the following 

destructive tests. Usable cores can normally be obtained only if the concrete is 

relatively sound. If possible, cores should have a diameter three times the 

maximum aggregate size. All core holes should be filled with non-shrink concrete 

grout. 

Carbonation Carbonation of concrete is the result of the reaction of carbon dioxide and other 

acidic gases in the air, and it can cause a loss of protection of the reinforcing steel 

against corrosion. The depth of carbonation in a concrete bridge member can be 

measured by exposing concrete samples to a solution. Uncarbonated concrete 

areas change color, while carbonated concrete areas remain colorless. 

Concrete Permeability Air and water permeability can be measured by drilling a small hole into the 

concrete, sealing the top with liquid rubber, and inserting a hypodermic needle. 

Air permeability can then be determined by filling the hole with water and 

measuring the flow into the concrete at a pressure similar to that of rainfall. 

However, this method is seldom used in bridge inspections. 

Concrete Strength Actual concrete strength and quality can be determined only by removing a 

concrete core and performing such laboratory tests as: 

3⁄4 Compressive strength 

3⁄4 Cement content 

3⁄4 Air voids 

3⁄4 Static modulus of elasticity 

3⁄4 Dynamic modulus of elasticity 

3⁄4 Splitting tensile strength 

Endoscopes and 

Videoscopes 

Endoscopes and videoscopes are viewing tubes that can be inserted into holes 

drilled into a concrete bridge member (see Figure 13.2.9). Light can be provided 

by glass fibers from an external source. Some applications of this method include 

the inspection of the inside of a box girder and the inspection of hollow 

posttensioning ducts. Although this is a viewing method, it is considered to be a 

destructive method because some destruction is necessary for its proper use in 

concrete. 
 SECTION 13: Advanced Inspection Techniques 

 TOPIC 13.2: Concrete 

13.2.12 

 

Figure 13.2.9 Remote Video Inspection Device 

 

Moisture Content Moisture content in concrete serves as an indicator of corrosion activity. Moisture 

content can be determined using nuclear methods (refer to Topic 13.2.2) or from 

concrete samples taken from the bridge and oven dried in a laboratory 

Petrographic 

Examination 

Petrographic examination is a laboratory technique for determining various 

characteristics of hardened concrete, which are useful in determining the existing 

condition and predicting future performance. This advanced inspection technique 

will detect Alkali-Silica Reaction (ASR) products. 

Reinforcing Steel 

Strength 

The actual properties of reinforcing steel can only be determined by removing test 

samples. Such removal of reinforcing steel can be detrimental to the capacity of 

the bridge and should be done only when such data is essential. See Topic 13.3.3. 

for the tensile strength test of steel. 

Chloride Test One of the current standard test methods used to assess the resistance of concrete 

to penetration of chloride ions is the rapid chloride permeability test. This test, 

officially known as AASHTO T 277-93, “Electrical Indication of Concrete’s 

Ability to Resist Chloride,” measures the charge passed through a concrete 

specimen subjected to 60 volts DC for six hours. Variable results have been 

reported with the rapid chloride permeability test when certain mineral admixtures 

such as silica fume were included in the concrete mixture and when calcium nitrite 

(included in some corrosion inhibitors) or reinforcing steel have been present. The 

test specimens are 50-mm (2") long and 100 mm (4") in diameter in the rapid 

chloride test. The rapid chloride test uses sodium hydroxide ponded on the top of 

the specimen, and a solution of sodium chloride at the bottom of the specimen. 

The specimen is initially subjected to 30 volts DC, and the resulting current 

determines the voltage to be applied for the duration of the test. The voltage is 

applied for three different time periods varying anywhere from 2 to 96 hours. 

Following the test, the specimen is split in half and a silver nitrate spray is applied 

to identify the depth of chloride penetration into the specimen. 

Matrix Analysis Additional Test 

ASR Evaluation The test for ASR evaluation, often referred to as the accelerated mortar bar test, 

has been accepted by ASTM and AASHTO. The test involves casting mortar bars 

that contain the subject aggregate (either coarse or fine), which is processed to a 

standard gradation. The mortar bars are then removed from their molds after 24 

hours and placed in water at room temperature. The temperature of the water is 
 SECTION 13: Advanced Inspection Techniques 

 TOPIC 13.2: Concrete 

13.2.13 

then raised to 80 °C (176° F) in an oven, and the mortar bars are stored in this 

condition for the next 24 hours. After the bars are removed from the water, they 

are measured for initial length and then submersed in a 1 normal (N) NaOH 

solution at 80 °C (176° F), where they are then stored for 14 days. Length change 

measurements are made periodically during this storage period. The total 

expansion at the end of the 14-day soaking period typically is used in 

specifications, although the expansion limits specified by different agencies vary. 
 SECTION 13: Advanced Inspection Techniques 

 TOPIC 13.2: Concrete 

13.2.14 

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13.3-i 

Table of Contents

Chapter 13 ..................................................................................................................................................

Advanced Inspection 

Techniques 

 13.3 Steel .......................................................................................... 13.3.1 

 

 13.3.1 Introduction............................................................................. 13.3.1 

 

 13.3.2 Nondestructive Testing Methods ............................................ 13.3.1 

 Acoustic Emissions Testing ............................................. 13.3.1 

 Computer Programs.......................................................... 13.3.2 

 Corrosion Sensors............................................................. 13.3.2 

 Smart Paint 1 .................................................................... 13.3.3 

 Smart Paint 2 .................................................................... 13.3.3 

 Dye Penetrant ................................................................... 13.3.3 

 Magnetic Particle.............................................................. 13.3.4 

 Radiographic Testing........................................................ 13.3.5 

 Computer Tomography .................................................... 13.3.5 

 Robotic Inspection............................................................ 13.3.6 

 Ultrasonic Testing ............................................................ 13.3.7 

 Eddy Current .................................................................... 13.3.8 

 

 13.3.3 Other Testing Methods............................................................ 13.3.9 

 Brinell Hardness Test ....................................................... 13.3.9 

 Charpy Impact Test .......................................................... 13.3.9 

 Chemical Analysis.......................................................... 13.3.10 

 Tensile Strength Test...................................................... 13.3.10 
SECTION 13: Advanced Inspection Techniques 

TOPIC 13.3: Steel 

13.3-ii

This page intentionally left blank 
13.3.1

Topic 13.3 Steel 

13.3.1 

Introduction Advanced inspection techniques give inspectors the ability to further evaluate 

suspected defects found during a visual inspection. They can also be used to 

perform inspections on members that are not accessible. Advanced inspection 

techniques usually require calibrated testing equipment, a professionally trained 

technician to perform the testing and a professional that has expertise in 

interpreting the advanced inspection results. However, bridge inspectors should 

have an understanding of the various advanced inspection techniques. 

There are two main classifications of advanced inspection techniques. The first is 

labeled nondestructive testing or evaluation (NDT or NDE). This classification 

pertains to all advanced inspection techniques that do not impair the usefulness of 

the member being tested. Other testing, the second main classification, covers all 

advanced inspection techniques that affect or destroy the structural integrity of the 

member being tested. 

New technology is making the use of these highly technical systems economically 

feasible for bridge inspection. From this fact, advanced inspection techniques are 

becoming more popular for the routine inspection of all bridge members. Current 

and future studies have been, and will be focusing directly on relating results from 

advanced inspection techniques into Bridge Management Systems ratings. 

This Topic describes the different types of nondestructive and other test methods 

for steel bridge members and the general procedures for each. 

13.3.2 

Nondestructive 

Testing Methods 

Acoustic Emissions 

Testing (American 

Society of Nondestructive 

Testing Designation 

(ASNT) - AE) 

Acoustic emission (American Society of Nondestructive Testing Designation 

(ASNT) - AE) detection has been around for many years, but is now becoming 

more of a standardized, available procedure. 

This inspection technique detects elastic waves generated within a test object by 

such mechanisms as plastic deformation, fatigue and fracture. Acoustic emission 

listens for sounds from active defects and is very sensitive to defect activity when 

a structure is loaded beyond its service load in a proof test. This process can detect 

flaws and imperfections such as the initiation and growth of fatigue cracks in steel 

structural members: the failure of bonds, fibers and filaments in composite 

materials and the appearance of potentially hazardous flaws in metal or synthetic 

pressure vessels. 

Bridges contain a large number of joints, welds and connections that are potential 

initiation points for fatigue cracks. Acoustic emission monitoring is used for early 

detection of fatigue cracks in fracture critical bridge members and to monitor the 

relative activity of existing fatigue cracks. Advanced signal processing and 

correlations to parametric measurements are used to separate noises generated by 

dynamic loading, loose connections, rivets and the crack growth. 
 SECTION 13: Advanced Inspection Techniques 

 TOPIC 13.3: Steel 

13.3.2 

Commercial systems are available, based on wave propagation properties. When 

energy is released (for example: high-tensile wire failures or concrete cracks), 

waves propagate in the material. Acoustic sensors distributed along the structure 

can detect and record the signal. Computer processing of the signal will then 

provide valuable information about the event: location, origin, energy, frequency, 

etc. 

The main advantage of these systems is the recording and real-time analysis of the 

waves themselves, allowing automatic filtering by the acquisition unit according to 

preset criteria. The events of interest are stored in the acquisition unit and 

automatically sent to be analyzed by an engineer. 

 

A device known as Local Area Monitoring (LAM) can be used to monitor areas 

that already are cracked or cracked areas that have been retrofit. The device is a 

portable, modular eight-channel system that can be mounted close to the area 

being monitored (see Figure 13.3.1). The system can be directly connected to a 

computer or it can be accessed through wired or wireless modems for data 

collection. 

 

Figure 13.3.1 LAM System Showing Eight Sensors and Holding Magnet 

Computer Programs Computer programs have been developed to maximize the value of bridge 

inspections. In the pre-inspection routine, the inspector enters data on the bridge 

design and previously detected flaws. The computer responds with a customized 

checklist for the inspector, flagging critical areas of the structure. In the postinspection routine, the inspector enters data about the flaws encountered in the 

field, and the computer responds with information about if the crack is likely to 

propagate and how to repair the crack. This procedure allows the inspector to 

detect flaws early and to judge which ones need immediate repair. 

Corrosion Sensors Corrosion sensors are being developed that use environmental variables such as 

dirt and duration of wetness to indicate the degree of corrosion of a steel structure. 
 SECTION 13: Advanced Inspection Techniques 

 TOPIC 13.3: Steel 

13.3.3 

Smart Paint 1 The National Science Foundation’s ATLSS (Advanced Technology for Large 

Structural Systems) Engineering Research Center has developed “Smart Paint” –

paint with microencapsulated dyes that outline a fatigue crack in a bridge or other 

highway structure as the crack forms and propagates. 

Smart Paint 2 Japanese scientists have developed a new paint that sends out electrical signals 

which are picked up by electrodes placed on either side of the paint’s resin layer if 

the structure or material begins to vibrate. The greater the vibration, the greater the 

electrical signal. 

This paint could enable engineers to monitor vibrations throughout the lifetime of 

a structure, allowing them to calculate much more accurately when fatigue is 

becoming a problem. 

The new paint is a much easier way of measuring vibrations than conventional 

strain gauges. 

Dye Penetrant A dye penetrant test can be used to define the extent and size of surface flaws in 

steel members (see Figure 13.3.2). The test area is cleaned to bare metal, a dye is 

applied and allowed to penetrate the defect, and excess penetrant is removed. A 

developer is then applied, which draws the dye out of the irregularities and defines 

the extent and size of surface flaws. Bridge inspectors commonly use this method 

since it does not require extensive training or expensive equipment. A limitation 

of this method, however, is that it reveals neither the depth of cracks nor any 

subsurface flaws. Another important factor when performing dye penetrant testing 

is the penetrant dwell time. This is the amount of time that the penetrant is 

allowed to remain on the surface before the excess is wiped off. Factors that effect 

the dwell time include: 

3⁄4 Temperature of the member being tested and the penetrant type 

3⁄4 Ambient air temperature (higher temperatures require shorter dwell times) 

3⁄4 Humidity (low humidity causes penetrant to dry out rapidly) 

3⁄4 Size and shape of the discontinuity (hairline cracks need more time than 

large ones) 

3⁄4 Material type 

3⁄4 Penetrant removal type and manufacturer’s recommendations
 SECTION 13: Advanced Inspection Techniques 

 TOPIC 13.3: Steel 

13.3.4 

 

Figure 13.3.2 Detection of a Crack Using Dye Penetrant 

Magnetic Particle Magnetic particle or magnetic flux leakage testing is useful in detecting surface 

gouges, cracks, and holes in ferromagnetic materials. It can also detect subsurface 

defects, such as voids, inclusions, and cracks, which lie near the surface. A 

magnetic field is induced into the member, and cracks or other irregularities in the 

surface of the member cause irregularities in the magnetic field (see Figure 

13.3.3). This technique is also referred to as magnetic field disturbance. 

N

N

S

S

N S

S N

 

Figure 13.3.3 Schematic of Magnetic Field Disturbance 
 SECTION 13: Advanced Inspection Techniques 

 TOPIC 13.3: Steel 

13.3.5 

Radiographic Testing Radiographic testing is used to detect and locate subsurface defects such as cracks, 

voids, and inclusions. X-rays or gamma rays are passed through the member and 

are absorbed differently by the various flaws. When a piece of film is exposed to 

the rays, the defects appear as shadows on the film. This type of advanced 

inspection is typically used for full penetration groove welds during fabrication 

and construction. 

 

Figure 13.3.4 Radiographic Testing 

Computer Tomography Computer tomography uses X-ray and gamma radiation to visualize the interior 

defects of a steel member. The image is captured by a detector array, it is 

processed by a computer, and it is then reconstructed. This method is similar in 

many ways to medical CAT scans, and it has great potential for locating 

discontinuities of all types in steel members (as well as concrete members). 
 SECTION 13: Advanced Inspection Techniques 

 TOPIC 13.3: Steel 

13.3.6 

Robotic Inspection Several companies are currently developing and marketing a system which uses 

high-resolution video cameras on robotic arms attached to permanent falsework 

underneath the bridge. By remote telescanning, details can be visually monitored, 

with magnification if needed, without the inspector having to climb to gain access 

to a detail each time an inspection is desired. While the primary material 

application for robotic inspection is steel, it can also be used on timber and 

concrete bridges. 

In recent years, the California Department of Transportation (Caltrans) has been 

working on an aerial robotic inspection system (see Figure 13.3.5). This system, 

in the testing and development stage, can allow bridge inspectors to view elevated 

bridge members from the ground. It is controlled by a remote control that is 

connected to the system through a 100 ft (30 m) electrical cord. A fiber optics 

cable transfers information and images from the aerial device to the ground station. 

This type of inspection may reduce traffic delays and increase the level of safety 

for motorists and bridge inspectors. 

 

Figure 13.3.5 Robotic Inspection 
 SECTION 13: Advanced Inspection Techniques 

 TOPIC 13.3: Steel 

13.3.7 

Ultrasonic Testing Ultrasonic testing is frequently used in steel applications and can be used to detect 

cracks in flat, relatively smooth members, as well as pins (see Figure 13.3.6). It 

can also be used to measure the thickness of steel members, providing detailed 

information concerning loss of cross section. Ultrasonic testing also has many 

applications in the inspection of welds, detecting porosity, voids, inclusions, 

corrosion, cracks, and other discontinuities. Refer to Topic 13.1 for further details 

about the principles of ultrasonic testing. 

 

Figure 13.3.6 Ultrasonic Testing of a Pin in a Moveable Bridge 
 SECTION 13: Advanced Inspection Techniques 

 TOPIC 13.3: Steel 

13.3.8 

Eddy Current This type of electromagnetic technique uses AC currents. Eddy current testing can 

only be performed on conductive materials and is capable of detecting cracks and 

flaws as well as member dimensions and variations. The system works by 

monitoring the voltage across a coil that has an AC current flowing through it. 

When the coil is placed next to the conductive member, the member produces 

eddy currents that flow opposite to the direction of flow from the coils. Defects in 

the member disturb the eddy currents, which, in turn, effect the induced current. 

The effected induced current is monitored through the voltage across the coil. 

Eddy current testing devices can be hand held devices (see Figure 13.3.7). 

 

Figure 13.3.7 Hand Held Eddy Current Instrument 

Information concerning various nondestructive testing can be found on the 

American Society of Non-destructive Testing website: www.asnt.org. 
 SECTION 13: Advanced Inspection Techniques 

 TOPIC 13.3: Steel 

13.3.9 

13.3.3 

Other Testing 

Methods 

Strength tests are normally considered destructive tests since they usually involve 

tests conducted on pieces of steel removed from the bridge. Small steel pieces cut 

out of steel members are called test "coupons." The removal technique and 

coupon size must be suitable for the planned tests. If a coupon is required, consult 

the bridge engineer to determine the most suitable area of removal. For instance, 

an inspector should not remove a coupon from the web area over a bearing. An 

inspector also should not remove a coupon from the bottom flange at midspan. 

Destructive tests may be necessary to determine the strength or other properties of 

existing iron or steel on bridges for which the steel type is unknown. 

The following tests can be conducted only by the destructive technique of 

removing a sample and evaluating it in a laboratory. 

Brinell Hardness Test The Brinell hardness test measures the resistance to penetration of the steel. A 

hardened steel ball is pressed into the test coupon by a machine-applied load. The 

applied load and the surface area of the indentation are used to calculate the 

hardness of the steel. For a steel that has not been hardened by cold work, its 

hardness is directly related to its ultimate tensile strength. 

Charpy Impact Test An impact test determines the amount of energy required to fracture a specimen. 

A common impact test for steel coupons is the Charpy V-notch test (see Figure 

13.3.8). A notched test coupon is placed in a vise, and a hammer is then released 

from an elevated position, swinging down and hitting the coupon. Since the force 

of the hammer is concentrated in a notch in the coupon, the stress goes into 

fracturing the specimen and not into strain. The energy required for fracture is 

determined based on the mass of the hammer and the distance that it fell. This test 

can be performed at different temperatures to determine if the steel is susceptible 

to brittle failure. 

 

Figure 13.3.8 Charpy V-Notch Test 
 SECTION 13: Advanced Inspection Techniques 

 TOPIC 13.3: Steel 

13.3.10 

Chemical Analysis The chemical composition of the steel is an important indication of whether a weld 

will crack, either from cold cracking or hot cracking. Tests can be performed on 

coupons to determine the chemical composition of the steel. 

Cold, or delayed, cracking can be approximated using a carbon equivalent (C.E.) 

equation that is based on the chemical composition of the steel. One such 

equation, based on the relative proportions of various elements in the steel, is 

presented in the ASTM A706 rebar specification: 

When the C.E. is below 0.55, the steel is generally not susceptible to cold 

cracking, and no special precautions are required for welding. However, when the 

C.E. is above 0.55, the steel is susceptible to cold cracking, and special precautions 

are required for welding. 

Hot cracking occurs as the weld begins to solidify. Hot cracks have almost been 

eliminated today due to modern welding material formulation. 

Tensile Strength Test The tensile strength is the highest stress that can be applied to the coupon before it 

ruptures. Once the yield strength has been exceeded, the coupon begins to 

elongate or "neck down" and eventually breaks if the load is not removed. The 

tensile strength of the steel can be easily determined. See Section P.2, Bridge 

Mechanics. 

The ends of the test coupon are placed in vises on a testing machine. The machine 

then applies a tensile load to the ends of the coupon. The machine measures the 

load at which the coupon fails or ruptures. This load and the cross-sectional area 

of the coupon determine the tensile strength of the steel (See Figures 13.3.9 and 

13.3.10). 

 

Figure 13.3.9 Brittle Failure of Cast Iron Specimen 

CE C

Mn

6

Cu

40

Ni

20

Cr

10

Mo

50

V

10

.. %

% % % % %%

=+ + + + - -
 SECTION 13: Advanced Inspection Techniques 

 TOPIC 13.3: Steel 

13.3.11 

 

Figure 13.3.10 Ductile Failure of Cold Rolled Steel 
13.4-i 

Table of Contents

Chapter 13 ..................................................................................................................................................

Advanced Inspection 

Techniques 

 13.4 Advanced Asset Assessment................................................................ 13.4.1 

 

 13.4.1 Introduction............................................................................. 13.4.1 

 

 13.4.2 Overview of Major Asset Assessment Techniques................. 13.4.3 

 Strain or Displacement Sensors........................................ 13.4.3 

 Other Available Sensors................................................... 13.3.4 

 Dynamic Load Testing ..................................................... 13.3.5 

 System Identification........................................................ 13.3.5 

 Practical Considerations for Selection and Use of 

 Advanced Asset Assessment Technologies ............... 13.3.7 
13.4.1

Topic 13.4 Advanced Asset Assessment 

13.4.1 

Introduction Today’s sensing devices capture and report highly accurate and objective data, which 

can be used to make “fact-based” evaluations for asset condition assessment and 

provide decision support for maintenance, repair or replacement actions, optimizing

the owner’s overall asset management plan. 

Advanced asset assessment technologies allow the owner to more objectively capture 

and evaluate known or suspect defects found during a visual inspection. Also, they 

may be used to perform periodic or continuous inspections on members that are not

readily accessible. Advanced asset assessment technologies usually require 

customized hardware and software, an experienced technician to install sensing 

devices or perform the testing and an engineering professional that has expertise in 

interpreting the advanced asset assessment results (see Figure 13.4.1). However, 

bridge inspectors should have a basic understanding of the various available 

advanced asset assessment technologies to allow them to participate in selection and 

use of appropriate asset assessment technology to further enhance rating 

determinations. 

Figure 13.4.1 Installation of Sensors 

There are two main classifications of advanced asset assessment technologies. The 

first is labeled nondestructive testing or evaluation (NDT or NDE). This 

classification pertains to all advanced asset assessment technologies that do not 

impair the usefulness (short term or long term) of the member being tested. The other 

classification consists of asset assessment technologies that affect or destroy the 

structural integrity of the member being tested. Most practitioners and owners today 

prefer the nondestructive technologies for obvious reasons. 

Objective asset assessment solutions can support routine bridge inspection. As a 

result, advanced asset assessment solutions can be useful for the purpose of 
 SECTION 13: Advanced Inspection Techniques 

 TOPIC 13.4: Advanced Asset Assessment 

13.4.2 

optimizing an owner’s asset management program. Methods are being developed to 

transfer results from near real-time advanced asset assessment technologies directly 

into Bridge Management Systems ratings and bridge management protocols (e.g. 

permitting) (see Figure 13.4.2). 

Near real-time advanced asset assessment solutions are made possible by the 

combination of a variety of sensing devices, wireless communication and Internet 

technologies. The ability to capture data on member strains, relative movement 

between members, crack growth and propagation, and other relevant structural 

parameters are the result of digital technology being applied to structural asset 

assessments. 

 

Figure 13.4.2 Viewing Real Time Data 
 SECTION 13: Advanced Inspection Techniques 

 TOPIC 13.4: Advanced Asset Assessment 

13.4.3 

13.4.2 

Overview of 

Major Asset 

Assessment 

Technologies 

Strain or Displacement 

Sensors 

Strain or displacement sensors can be used to monitor the response of a member to a 

live load and/or thermal excursions (temperature changes). These type sensors are 

available and include foil type, vibrating wire, fiber optic, and even a sensor that 

measures current and peak strains (without power) in one device (see Figure 13.4.3). 

Foil type sensors must be used in the axial direction of flat members, and single wire 

filament “vibrating wire” sensors can be used on flat members or cables. Portable 

strain reading instruments can be used to monitor sensors from a central location on 

or near the bridge in a manual data collection mode or fully automatic monitoring 

can be installed, allowing readings to be taken at user defined intervals and sent 

wirelessly to a central location for viewing over the Internet. 

Figure 13.4.3 Strain Gage Used on Hoan Bridge 
 SECTION 13: Advanced Inspection Techniques 

 TOPIC 13.4: Advanced Asset Assessment 

13.4.4 

Locations for strain sensors should be selected based on the condition of individual 

members, accessibility, and the objectives of the monitoring program. 

Strain/displacement sensors can provide valuable information about: 

3⁄4 The actual transverse load distribution through the structural member 

3⁄4 The load sharing between elements of a multi-element member 

3⁄4 The effectiveness of the various members of the primary structural system 

3⁄4 The influence of deteriorated or defective members 

3⁄4 The growth and/or propagation of cracks in steel or concrete members 

3⁄4 The relative movement of members to fixed points due to loss of section 

(chemical) or load induced deterioration 

The principal use of strain sensors today is to ascertain the actual condition of a 

member or series of members and use that information to infer the safe load carrying 

capacity of the structure. In essence, sensors are used to provide data to allow 

additional decision making combined with a visual inspection. Strain sensor data 

can be used to ascertain the weight of vehicles crossing a bridge. This is known as a 

“weigh-in-motion” system. 

 

Sensing devices, coupled with electronic control equipment, can be used to update 

owners and bridge inspectors about ongoing deterioration of the structure. Such 

configured solutions, which can be integrated into one system, generally include 

strain or displacement sensors, a system controller on the structure, wireless data 

transmission, customized software, and other features, allowing secure data capture, 

data graphing, viewing over the Internet and even electronic alerts (e-mails) if 

strains or displacements exceed predetermined values. 

Other Available 

Sensors 

To complement strain or displacement sensors, newer sensors are being developed 

and deployed to enhance asset assessment. Typical sensing devices include 

tiltmeters (foundation movements), accelerometers (earthquake induced 

movements), temperature and humidity sensors, and even GPS systems to monitor 

movement of piers, towers, and decks on long bridges to 5 mm (3/16") accuracy. 

Other, more esoteric sensing devices include those to detect onset of fatigue 

cracking, actual stress in cables via electromagnetic fields, corrosion, and other 

member condition parameters. 

Generally speaking, price and functionality are directly related. That is, sensors 

meant to be used in outdoor environments for long periods of time (years) are more 

expensive than those meant for controlled environments (laboratories) or short 

duration use (weeks). Sensing devices can be utilized individually or as part of a 

system that is configured to provide a total solution. Specialized personal is 

required to integrate the variety of sensing devices with controller hardware and 

software for advanced asset assessment. 
 SECTION 13: Advanced Inspection Techniques 

 TOPIC 13.4: Advanced Asset Assessment 

13.4.5 

Dynamic Load Testing In recent years, an increasing number of short-span bridges have been assessed using 

measured response data from known loads. These assessments have provided useful 

information and, in some instances, have revealed bridges which need to be closed 

or restricted and those that can be safely upgraded (load restrictions removed). 

Use of this technique involves a novel combination of strain sensors, on-site data 

capture, and response modeling. A known load (weighed dump truck) is driven 

across a short-span bridge with no other traffic. GPS technology is used to precisely 

spot the truck’s position while strain sensors capture member displacements/strains. 

Data capture typically occurs in one day or less. The data is then used to “build” a 

rudimentary structural model for evaluation of actual load carrying capacity. The 

model is fitted to the actual structural response, allowing engineers to determine 

actual load carrying capacity (see Figure 13.4.4). 

This technology gains advantage over current load capacity protocols in that it can 

consider composite action of the members and contributions to load carrying 

capacity from other structural components (sidewalks and parapets) that are 

traditionally ignored with traditional analysis methods. Dynamic load testing has 

been used for over fifteen years and has proven its ability to provide accurate load 

carrying capacity determinations. 

Figure 13.4.4 Dynamic Load Testing Vehicle 

System Identification Using actual structural response data, the properties of the structure (e.g., areas and 

moments of inertia of structural members) can be calculated. The process of 

building a structural model from response data is called system identification. The 

primary use of system identification in structural engineering has been for 

earthquake engineering research. The historical accuracy achieved in this 

assessment methodology indicates that system identification can also provide a tool 

for detecting unseen structural flaws. 
 SECTION 13: Advanced Inspection Techniques 

 TOPIC 13.4: Advanced Asset Assessment 

13.4.6 

System identification can be performed using a variety of response data, such as 

modal and time history response. For modal response, the frequencies and mode 

shapes of the structure are obtained either from ambient vibration data or from the 

results of harmonic excitation. A time history response is the response (i.e., 

displacements or acceleration) of one or more points on the structure as a function of 

time due to a known loading function. For either type of response data, the results 

are used to determine structural parameters representing the structural integrity of 

the bridge. 

Initially, system identification is used to create a structural model, which accurately 

represents the in-service condition of the structure (see Figure 13.4.5). Subsequent 

analyses are then performed to determine which parameters are changing. Since the 

parameters represent structural properties (e.g., areas and moments of a inertia), the 

changes are indicative of structural deterioration. 

Since bridge inspections focus on individual members and system identification 

considers the entire structure, they are complementary processes. Therefore, system 

identification can be used to define the structural integrity of the entire bridge 

structure. 

Figure 13.4.5 Structural Model 
 SECTION 13: Advanced Inspection Techniques 

 TOPIC 13.4: Advanced Asset Assessment 

13.4.7 

Practical 

Considerations for 

Selection and Use of 

Advanced Asset 

Assessment 

Technologies 

Any advanced asset assessment technology should provide a reasonable return on 

investment. There is no reason to pay for technology unless that technology can 

provide a sufficient return for the asset owner. Over the past several years, 

significant research has been conducted to demonstrate new asset assessment 

technologies. Some worked well; others didn’t. But given the current asset 

assessment technologies, owners should expect adequate returns on projects. 

Returns can be calculated using a variety of financial metrics, but some of the more 

useful are provided below: 

3⁄4 Safe extension of asset life span, lowering life cycle cost of ownership 

3⁄4 Safe deferral of asset maintenance or repair programs 

3⁄4 Safe deferral of asset replacement programs 

3⁄4 Improved prioritization of limited funds 

3⁄4 Removal of unnecessary load restrictions to support commercial traffic and 

reduce detours, congestion and air pollution 

3⁄4 Identification of dangerous assets that must be replaced or repaired 

immediately, thereby lowering liability exposure and increasing safety 

Other considerations include the following issues that should be evaluated and 

decided upon before utilizing advanced asset assessment technologies: 

3⁄4 Is the asset assessment technology being used for a few bridges or across the 

entire system? 

3⁄4 Is the asset assessment technology capturing the “right” information to aid 

decision making and not a lot of extraneous information? 

3⁄4 Can the asset assessment solution be expanded easily and cost effectively if 

it is later decided to capture more data? 

3⁄4 Should a solution provider be used, capable of system configuration and 

installation, or integrate the hardware and software internally? 

3⁄4 Should the captured information be able to integrate with the existing 

information system? 

3⁄4 How long is the technology expected to be deployed – what is the reliability 

and durability of the hardware and software? 

3⁄4 Can the confidentiality of captured data, both on-site and for later viewing 

and downloading, be assured? 

3⁄4 Who has the responsibility for conversion of the structural data into useful 

information and subsequent analysis of that information? 

3⁄4 Can the solution (hardware) be used on other structures after project 

completion? 

In summary, the use of advanced asset assessment technologies can provide owners 

with information that promotes “fact-based” decisions. Care and judgment must be 

utilized when specifying and purchasing improved technologies, as well as use in the 

field. Always defer to those who have the experience and earned reputation to 

provide alternatives for consideration to drive best available returns on investment.