**1. Introduction**

A railway prestressed concrete sleeper is one of the important components supporting rails and distributing train axle loads into ballast [1]. Mono-block type concrete sleepers, one of the most common sleeper types, are conventionally used in countries such as Australia, Canada, China, Italy, the UK, the USA, and Japan [2]. In Japan, a large number of these sleepers have been continuously introduced into railway lines since the 1950s [3], with some exceeding 50 years of operational lifetime. Thus, sound methodologies and tools for achieving effective maintenance of such a vast number of concrete sleepers are ultimately required [4].

An impact axle load is a typical scenario that can promote cracks and influence the deterioration of concrete sleepers [5–9]. In particular, some impact loads due to wheel irregularities (e.g., flat wheels), rail irregularities, and rail joints would likely over time produce concrete cracks at the bottom of rail seats and at the top of midspans where the maximum or minimum bending moment arises as shown in **Figure 1** [3, 7]. A low-occurrence probability of such significant loads acting upon concrete sleepers actually exists; however, when they do occur, yielding of steel members, residual displacement, and/or open cracking of surrounding concrete will often result. Open cracking allows water to penetrate into the sleeper ultimately causing corrosion of reinforced steel, which then leads to a loss in bending strength [8].

Currently in Japan, the inspection of concrete sleeper deterioration/damage is typically carried out by visual inspection via foot patrol. However, concrete sleeper bodies, with the exclusion of their top surfaces, are usually covered by ballast (as shown in **Figure 1**). It is therefore difficult, in general, to detect damage by visual inspection.

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 methodology is widely regarded as antiquated.

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

**Figure 1.**

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

**77**

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

concrete sleeper damage on modal characteristics, influences of specification variations of other track members, and a simple and efficient measurement process for practical use. An important contribution of this study is the provision of valuable data relating to impacts of typical damage modes, such as cracks and steel rod fractures, on the modal characteristics of general concrete sleepers in Japanese railway. Another contribution of this study is to propose a simple and efficient detection method assuming a practical application. As for vibration measurement methods of concrete sleepers, they have not been investigated other than via the use of accelerometers [19]. It is, however, difficult to apply such practical uses in which more than several tens of thousands of sleepers would require inspection per a rail line. This study therefore focuses on a detection method based on a hammering sound with a well-known impulse hammer technique

to improve detection efficiency by omitting the installation of accelerometers.

of applicability for the effectiveness of the sound-based detection method.

concrete bond is removed by an asphalt-based resin material.

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

**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 steel-

**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.

**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

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

**2. Experimental methods**

**2.1 Test concrete sleepers**

**2.2 Bending test method**

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

concrete sleeper damage on modal characteristics, influences of specification variations of other track members, and a simple and efficient measurement process for practical use. An important contribution of this study is the provision of valuable data relating to impacts of typical damage modes, such as cracks and steel rod fractures, on the modal characteristics of general concrete sleepers in Japanese railway. Another contribution of this study is to propose a simple and efficient detection method assuming a practical application. As for vibration measurement methods of concrete sleepers, they have not been investigated other than via the use of accelerometers [19]. It is, however, difficult to apply such practical uses in which more than several tens of thousands of sleepers would require inspection per a rail line. This study therefore focuses on a detection method based on a hammering sound with a well-known impulse hammer technique to improve detection efficiency by omitting the installation of accelerometers.
