**2. Experimental methods**

*Advances in Structural Health Monitoring*

methodology is widely regarded as antiquated.

Even if the ballast around sleepers is scraped out, damage such as cracks cannot always be detected due to the clogging of cracks with soil dust. Moreover, scraping out ballast to inspect sleeper states is not realistic due to the enormous number of concrete sleepers that require inspection. Vibration-based structural damage detection, however, is a potential method that may be employed for effectively remedying this challenge. Vibration-based structural damage detection is a well-known concept and widely invoked within the domain of structural health monitoring [10, 11]. For civil engineered structures, many researchers and engineers have performed such related assessments [12, 13]. Thus far, varying degrees of success for state evaluations of actual structures focusing on local and higher vibration modes and vibration characteristic changes before and after earthquakes have been reported [14–16]. In contrast, however, substantial numbers of structural damage occurrences have been detected impractically, thereby resulting in general detection methodologies that do not specifically focus on actual structural systems or accrued associated damage thereof. Assuming a damage detection method for practical applications, however, characteristics of target structure types and their typically incurred damage modes should be investigated. From this point of view, an advanced methodology is not always required but rather typically depends on characteristics of the target structure. Concrete sleepers typically display a characteristic in which a singularly damaged sleeper does not usually prompt a major impact on train-running safety or riding-comfort provided they can transmit trainloads and retain a gage [3]. Only in instances of multiple and consecutive concrete sleepers maintaining serious damage levels are the potential impacts on safety or comfort. This means that a state evaluation method based on continuous monitoring or advanced detection techniques is not necessary for concrete sleepers. If one can ascertain whether or not a measured concrete sleeper needs replacement, it could sufficiently contribute to the achievement of an effective maintenance protocol for them even if the utilized detection

Several related references have previously pointed out the possibility of detecting concrete sleeper damage based on measured modal characteristics. Lam et al. [17, 18] and Kaewunruen and Remennikov [19, 20] developed ballast damage detection methods using the natural frequency and mode shape of in-situ concrete sleepers. In those studies, frequency-based damage detection for concrete sleepers was described as "an important future task". In addition, Kaewunruen and Remennikov [21] investigated the effects of rail pad stiffness on the modal parameters of sleepers, and Matsuoka et al. also investigated the modal properties of damaged sleepers [22]. Despite these contributions, the overall feasibility of the damaged-sleeper detection process remains uncertain because of the absence of the following three critical factors: impact of

*Illustration of concrete sleeper installations and typical bending moment distribution during train passage.*

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**Figure 1.**

In this research, in order to verify the proposed damage detection system, several experiment series were set up, and necessary concrete sleepers were collected for each experiment. First series are new concrete sleepers. These are used to investigate the relationship between damage and mode characteristics through the stepwise bending test. These are also used to evaluate the influence of other track members (i.e., pad stiffness and ballast-supporting stiffness) on modal characteristics of sleepers on the actual environmental tests. Second series are concrete sleepers with actual damaged. Comparative studies of these concrete sleepers can provide an empirical validation of the feasibility of vibration-based damaged-sleeper detection. In addition, measurements using both accelerometers and sound-level meters were ultimately performed; comparison of such results can provide substantive evidence of applicability for the effectiveness of the sound-based detection method.

#### **2.1 Test concrete sleepers**

**Figure 2** and **Table 1** provide design drawings and nominal specifications of test sleepers in this research endeavor. Specifically, this study focuses on 3PR and 3PO sleepers which are the most widely used types of sleepers on meter-gauged railway lines in Japan. 3PR/3PO are pre/posttension and mass/individual-production types, respectively. Posttensioning types utilize an unbonded system in which the steelconcrete bond is removed by an asphalt-based resin material.

**Table 2** provides a list of test concrete sleepers. Six pretension and seven posttension type sleepers, which had been previously used in actual railway lines in Japan, were collected. Sleeper Nos. 1 and 9 shown as intact in **Table 2** were given artificial damage by stepwise bending tests, with vibration measurements. Sleeper Nos. 2–5 and 11–14 had different degrees of cracking or steel rod fractures generated through actual operational history. **Table 2** also presents these damage levels as "cracked sketches." Sleeper Nos. 6 and 13 are destroyed sleepers resulting from bending tests, in order to evaluate excessively damaged sleeper states. Other vibration measurements in a full-scale test line (described later) were performed on six intact sleepers (Nos. 17–22). In contrast, sleeper Nos. 7, 8, and 16 were used for verification of a simple and efficient detection method per the use of a sound-level meter.

#### **2.2 Bending test method**

**Figure 3** presents the scheme of a bending test focusing on the cross-sections at the rail seats of Sleeper Nos. 1 and 9. This scheme complies with Japanese Industrial

#### **Figure 2.**

*Specifications of prestressed concrete sleeper on meter-gauged line prescribed in Japanese Industrial Standard: (a) the pretension type named 3PR and (b) the posttension type named 3PO.*


#### **Table 1.**

*Nominal specifications of test concrete sleeper.*

Standards E 1201 and 1202. In order to validate the impact of damage levels on modal characteristics, loading/unloading and vibration measurements were performed stepwise. Procedures for the bending test and the vibration measurement test are described as follows. First, a vibration measurement was performed in the intact state; second, vibration levels were measured after loading and unloading of the

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**Figure 3.**

**Table 2.**

*List of collected concrete sleepers and associated damage levels.*

*condition and (b) picture of installed concrete sleeper.*

*Application of a Frequency-Based Detection Method for Evaluating Damaged Concrete Sleepers*

design-proof load; and third, after loading and unloading of the "cracking load," vibrational levels were again measured. The cracking load was determined by visual inspection which was conducted during the loading of this latter step. Subsequently,

*Bending test scheme following the Japan Industrial Standards for concrete sleepers: (a) loading and supporting* 

*DOI: http://dx.doi.org/10.5772/intechopen.82711*

*Application of a Frequency-Based Detection Method for Evaluating Damaged Concrete Sleepers DOI: http://dx.doi.org/10.5772/intechopen.82711*


**Table 2.**

*Advances in Structural Health Monitoring*

**78**

**Table 1.**

*Nominal specifications of test concrete sleeper.*

**Figure 2.**

Standards E 1201 and 1202. In order to validate the impact of damage levels on modal characteristics, loading/unloading and vibration measurements were performed stepwise. Procedures for the bending test and the vibration measurement test are described as follows. First, a vibration measurement was performed in the intact state; second, vibration levels were measured after loading and unloading of the

*Specifications of prestressed concrete sleeper on meter-gauged line prescribed in Japanese Industrial Standard:* 

*(a) the pretension type named 3PR and (b) the posttension type named 3PO.*

*List of collected concrete sleepers and associated damage levels.*

*Bending test scheme following the Japan Industrial Standards for concrete sleepers: (a) loading and supporting condition and (b) picture of installed concrete sleeper.*

design-proof load; and third, after loading and unloading of the "cracking load," vibrational levels were again measured. The cracking load was determined by visual inspection which was conducted during the loading of this latter step. Subsequently, after loading and unloading 1.2 times the cracking load observed in the last step, the vibration level was again measured. Finally, after loading and unloading of the "ultimate load," the resulting vibration levels were once again determined. The "ultimate load" is defined as the maximum load. Bending tests with these procedures were conducted for both cross sections of rail seats for concrete Sleeper Nos. 1 and 9.

#### **2.3 Vibration and sound pressure test method**

**Table 3** provides the specifications of measurement equipment associated with this study. In order to identify vibration characteristics up to 1 kHz, all acceleration and sound pressure responses were recorded on a Laptop PC with a 5 kHz sampling rate via a preamplifier and an A/D converter. Accelerometers and sound-level meters were employed (models PV85 and NL62, Rion Co. LTD.) that have frequency ranges up to 7 and 20 kHz, respectively. Sound pressures were measured by a sound-level meter with a flat input window. An impulse hammer was adopted per PCB PREZORTONICS INC. model 086C03 with a 2.25 mV/N sensitivity load cell to excite up to a level of 1 kHz. The weight of the unit was 136 g. **Figure 4(a)** shows the observed input forces of the impulse hammer and indicates an overall relative flat-frequency response. **Figure 4(b)** represents the distribution of all input forces, with the average of the forces being 0.079 N.

**Figure 5** describes the four vibration test methods and shows TEST I using 11 accelerometers in order to validate the impact of artificial or actual damage on mode characteristics of sleepers. Each concrete sleeper was supported by a soft urethane mattress of 600 mm thickness to simulate a free-free boundary condition [23]. TEST I was conducted on Sleeper Nos. 1–7 and 9–13. **Figure 5(b)** presents TEST II using nine accelerometers for the purpose of estimating the influences of other track members. TEST II was performed in a test line on the premises of the Railway Technical Research Institute, in which sleepers with two rails and rail pads were laid on ballast similar to an actual railway environment. Concrete Sleeper Nos. 17–22, all which were intact, were evaluated in TEST II.

**Figure 5(c)** and **(d)** presents TEST III and IV using two and one accelerometers and a single sound-level meter to validate the feasibility of an effective detection method. These tests were conducted on Sleeper Nos. 7, 8 and 16. TEST III and TEST IV adopted the supporting method via the use of a soft urethane mattress and ballast, respectively. TEST III also investigated the impact of other vibration modes, excitation points, and sound-level meter distances to investigate the optimal measurement method and to clarify the potential limitations of usage in practical applications. In all tests, free-vibration responses excited by an impulse hammer were measured. Impact forces were applied at the midspan (and at the rail seat for TEST III) of sleepers, with responses being recorded three separate times for the respective sleepers.


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**2.4 Identification methods**

*free boundary condition; and (d) TEST IV with ballast support.*

*Application of a Frequency-Based Detection Method for Evaluating Damaged Concrete Sleepers*

*Input force by impulse hammer: (a) time and frequency series and (b) distribution of input force.*

Two well-known methods, ERA (Eigensystem Realization Algorithm) [23] and peak-picking were employed to identify the modal characteristics, natural frequency, modal damping ratio (ERA only), and modal shape (ERA only). Peak-picking was embedded into portable equipment for practical applications. Both identification methods, however, were neither capable of considering nonlinearity nor nonstationarity that may be caused by damage, such as breathing cracks. Several publications [24] have previously indicated that unsteady modal characteristics can express smaller structural damage scenarios. Concrete sleeper management, however, does not require the detection of such smaller-type damage events; being able to exclusively detect damaged sleepers that are in need of exchange is sufficient. Thus, this study adopted well-known time-invariant identification methods. As for ERA, more information about identification via the ERA method can be found in Refs. [25, 26]. Peak picking, which is the most simple and straightforward identification method for natural frequencies, was employed to develop an efficient detection system in the practical field. A 0.5-s acceleration just after impact excitation at the sleeper midspans was used

*Sensor arrangement and impact position of vibration and sound pressure test methods: (a) TEST I assuming free-free boundary condition; (b) TEST II assuming actual boundary condition; (c) TEST III assuming free-*

for identification; thus, the resulting frequency increments were 2 Hz.

*DOI: http://dx.doi.org/10.5772/intechopen.82711*

**Figure 4.**

**Figure 5.**

**Table 3.** *Specifications of measurement equipment.*

*Application of a Frequency-Based Detection Method for Evaluating Damaged Concrete Sleepers DOI: http://dx.doi.org/10.5772/intechopen.82711*

**Figure 4.**

*Advances in Structural Health Monitoring*

**2.3 Vibration and sound pressure test method**

with the average of the forces being 0.079 N.

which were intact, were evaluated in TEST II.

after loading and unloading 1.2 times the cracking load observed in the last step, the vibration level was again measured. Finally, after loading and unloading of the "ultimate load," the resulting vibration levels were once again determined. The "ultimate load" is defined as the maximum load. Bending tests with these procedures were conducted for both cross sections of rail seats for concrete Sleeper Nos. 1 and 9.

**Table 3** provides the specifications of measurement equipment associated with this study. In order to identify vibration characteristics up to 1 kHz, all acceleration and sound pressure responses were recorded on a Laptop PC with a 5 kHz sampling rate via a preamplifier and an A/D converter. Accelerometers and sound-level meters were employed (models PV85 and NL62, Rion Co. LTD.) that have frequency ranges up to 7 and 20 kHz, respectively. Sound pressures were measured by a sound-level meter with a flat input window. An impulse hammer was adopted per PCB PREZORTONICS INC. model 086C03 with a 2.25 mV/N sensitivity load cell to excite up to a level of 1 kHz. The weight of the unit was 136 g. **Figure 4(a)** shows the observed input forces of the impulse hammer and indicates an overall relative flat-frequency response. **Figure 4(b)** represents the distribution of all input forces,

**Figure 5** describes the four vibration test methods and shows TEST I using 11 accelerometers in order to validate the impact of artificial or actual damage on mode characteristics of sleepers. Each concrete sleeper was supported by a soft urethane mattress of 600 mm thickness to simulate a free-free boundary condition [23]. TEST I was conducted on Sleeper Nos. 1–7 and 9–13. **Figure 5(b)** presents TEST II using nine accelerometers for the purpose of estimating the influences of other track members. TEST II was performed in a test line on the premises of the Railway Technical Research Institute, in which sleepers with two rails and rail pads were laid on ballast similar to an actual railway environment. Concrete Sleeper Nos. 17–22, all

**Figure 5(c)** and **(d)** presents TEST III and IV using two and one accelerometers and a single sound-level meter to validate the feasibility of an effective detection method. These tests were conducted on Sleeper Nos. 7, 8 and 16. TEST III and TEST IV adopted the supporting method via the use of a soft urethane mattress and ballast, respectively. TEST III also investigated the impact of other vibration modes, excitation points, and sound-level meter distances to investigate the optimal measurement method and to clarify the potential limitations of usage in practical applications. In all tests, free-vibration responses excited by an impulse hammer were measured. Impact forces were applied at the midspan (and at the rail seat for TEST III) of sleepers, with responses being recorded three separate times for the respective sleepers.

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**Table 3.**

*Specifications of measurement equipment.*

*Input force by impulse hammer: (a) time and frequency series and (b) distribution of input force.*

**Figure 5.**

*Sensor arrangement and impact position of vibration and sound pressure test methods: (a) TEST I assuming free-free boundary condition; (b) TEST II assuming actual boundary condition; (c) TEST III assuming freefree boundary condition; and (d) TEST IV with ballast support.*

#### **2.4 Identification methods**

Two well-known methods, ERA (Eigensystem Realization Algorithm) [23] and peak-picking were employed to identify the modal characteristics, natural frequency, modal damping ratio (ERA only), and modal shape (ERA only). Peak-picking was embedded into portable equipment for practical applications. Both identification methods, however, were neither capable of considering nonlinearity nor nonstationarity that may be caused by damage, such as breathing cracks. Several publications [24] have previously indicated that unsteady modal characteristics can express smaller structural damage scenarios. Concrete sleeper management, however, does not require the detection of such smaller-type damage events; being able to exclusively detect damaged sleepers that are in need of exchange is sufficient. Thus, this study adopted well-known time-invariant identification methods. As for ERA, more information about identification via the ERA method can be found in Refs. [25, 26]. Peak picking, which is the most simple and straightforward identification method for natural frequencies, was employed to develop an efficient detection system in the practical field. A 0.5-s acceleration just after impact excitation at the sleeper midspans was used for identification; thus, the resulting frequency increments were 2 Hz.

### **2.5 Numerical calculation method**

In order to characterize the dynamics of concrete sleepers, many types of numerical models for concrete sleepers have been proposed. Grassie [5] proposed a simplified two-dimensional dynamic model in the free-free condition, with their analytical results having been verified by comparisons with 12 different types of sleepers. Dahlberg and Nielsen [27] developed a concrete sleeper model popularly denoted as the "Timoshenko Beam on an Elastic Foundation," for both free-free and in-situ conditions. Lam et al. [17, 18] also modeled the sleeper as a Timoshenko beam, the supporting ballast as discretized springs, and the rails as masses (with reference to previous studies [28, 29]). Furthermore, Kaewunruen and Remennikov [20] modeled the in-situ concrete sleeper as the sleeper, and the ballast and pads using the effective stiffness of the rails and pads.

In order to characterize affecting mechanisms of sleeper damage on modal characteristics, numerical calculations based on a finite-element model were performed in this study. **Figure 6** shows the subject numerical model for the concrete sleepers. Numerical analysis for the concrete sleepers was performed by LS-DYNA, version R8.0.0 [30]. A sleeper's concrete was modeled as hexahedral solid elements, and its steel wires and stirrups were modeled as beam elements. Supports and loading points for loading test analyses were modeled as rigid elements. A sleeper itself was modeled as a symmetrical model. Solid elements (concrete) and beam elements (steel wire and stirrups) share actual nodes to prevent slippage from occurring. **Table 4** shows the material properties of each element in the sleeper model. Young's modulus of concrete was set from the stress-strain curve. Uniaxial compression strength and uniaxial tensile strength of concrete was set from Young's modulus and Design Standards for Railway Structures and Commentary (Concrete Structures) [31]. For concrete, a material that can address cracking with tension softening and crushing was ultimately used [32]. Prestressing was reproduced by initial stress to the steel wires in the axial direction.

In order to reproduce the influence of bending damage upon modal characteristics, numerical simulations of loading and unloading under the same experimental bending conditions were first calculated, and then, the modal characteristics of damaged sleepers were investigated by eigenvalue analysis in each loading-unloading step.

**Figure 6.**

*Numerical model of a concrete sleeper: (a) finite-element of concrete and (b) reinforced steel.*


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**Figure 8.**

*Application of a Frequency-Based Detection Method for Evaluating Damaged Concrete Sleepers*

**Figure 7** shows the load-displacement relationship at the rail seat, and **Table 5** shows applied loads of each state. In all cases, the cracking loads and ultimate loads were overdesign-proof and design-ultimate oriented. It can be confirmed that the bending of sleepers tends to soften gradually after the load exceeds cracking levels (i.e., around 110 kN). Both the cracking and ultimate loadings of Sleeper No. 9 in the posttension system are larger than those of Sleeper No. 1 in the pretension system. Although bending tests were performed at the right and left rail seats, explicit

**Figure 7,** moreover, presents numerical calculation results. The results agree well with the experimental findings for pretension type Sleeper No. 1. A first crack occurred at the bottom of the rail seat as shown in **Figure 8**. The crack load is altogether depicted in **Table 5**, and the 112 kN cracking load agrees with the 111 kN

differences between the respective cases were not observed.

*DOI: http://dx.doi.org/10.5772/intechopen.82711*

**3. Results**

**Figure 7.**

**Table 5.**

*Bending test results.*

*Load-displacement envelope curve on bending test.*

*Contours of cracks at (a) 112 kN and (b) 184 kN applied loads.*

**3.1 Bending test results**

#### **Table 4.**

*Element types and specifications of the numerical model.*

*Application of a Frequency-Based Detection Method for Evaluating Damaged Concrete Sleepers DOI: http://dx.doi.org/10.5772/intechopen.82711*
