No abstract

No abstract

A major distress problem in rigid concrete pavements is early deterioration of near-surface layers. The quality of near-surface material governs the long-term performance of concrete pavement. If the near-surface material of the concrete layer is cured sufficiently well at an appropriate moisture and temperature, the concrete pavement system can be expected to endure fatigue loading of traffic throughout its design life span. A field measurement device, referred to as a pavement integrity scanner (PiScanner), was developed for use in quality control and quality assurance programs for concrete layers of pavement systems. PiScanner is based on the enhanced resonance search (ERS) technique, which evaluates the vertical variation of elastic modulus or compressive strength of the concrete layer in pavement systems. This information can be used in the construction period of a concrete pavement to evaluate the curing process of near-surface materials. In the implementation of the PiScanner, two design schemes were incorporated: a dedicated hardware device to perform measurements and an automated analysis algorithm to minimize user expertise required for analysis. The resulting PiScanner allows ERS measurements to be performed in about 2 min with complete analysis of the measurements in about 3 min. Two field investigations were performed on a concrete test slab and at an express highway to prove the reliability and feasibility of PiScanner. Vertical variations of elastic modulus of concrete layer were successfully obtained by PiScanner.

Dynamic deflection of a railroad sleeper works as an indicator of ballast stiffness, reflecting the health conditions of a ballast track. However, difficulty exists in measuring dynamic deflection of a railroad sleeper by conventional deflection transducers such as a linear variable differential transformer (LVDT) or a potentiometer. This is because a fixed reference point is unattainable due to ground vibrations during train passage. In this paper, a patented signal processing technique for evaluation of pseudo-deflection is presented to recover dynamic deflection of a railroad sleeper using a coupled measurement of acceleration and strain at the concrete sleeper. The presented technique combines high-frequency deflections calculated from double integration of acceleration and low-frequency deflections determined from strains. Validity of the combined deflections was shown by the deflections measured with a camera target on a concrete sleeper, captured by a high-resolution DSLR camera with superb video capturing features and processed by computer vision techniques, such as Canny edge detection and Blob analysis.

The spectral analysis of surface waves (SASW) method is an in situ seismic technique that is used for evaluation of the stiffnesses of pavement systems at low strain levels. The stiffness of the surface layer can be determined by direct measurements in the field. Stiffnesses of the layers beneath the surface layer require forward modeling of SASW field data in order for the pavement profile to be obtained. The forward modeling process can be time consuming, especially if a three-dimensional model is used. A simplified procedure is proposed for determining an average value of the subgrade stiffness without performing forward modeling. Additionally, the simplified procedure can be used for determining the depth of shallow bedrock beneath pavement sites. The recommended procedure is based on SASW tests performed at 24 flexible pavement sections in the state of Texas. Parametric studies were also conducted with idealized rigid and flexible pavement profiles for the purpose of verifying the accuracy of the procedure and evaluating its limitations. An important point is that SASW measurements associated with the simplified procedure can be performed in conjunction with falling weight deflectometer (FWD) measurements using the drop weight as a source. The drop-weight source generates the frequency range required for SASW receiver spacings of 3, 6, and 9 m, which are recommended in the simplified procedure. The subgrade stiffness and depth to bedrock (if it exists) determined by the simplified procedure can be used as input parameters for enhancement of the backcalculation procedure associated with FWD measurements.

A quick-hardening concrete track has been developed to convert old ballast tracks into concrete tracks on operating lines. This method has been utilized to convert urban railways since 1997. With recent increases in train traffic and speed, maintaining track irregularities within design criteria has become essential to ensuring safety. On quick-hardening tracks, track irregularities are predominantly caused by irregular settlement around construction joints. These construction joints are inevitable in quick-hardening concrete; however, they create discontinuous sections that can affect the stable running of trains and structural durability. In this study, full-scale tests were performed with quasi-static and repeated loading on both continuous and discontinuous sections in which the earth pressure acting on the trackbed, accumulated settlement, and elastic displacement were measured. The results obtained indicate that construction joints are disadvantageous in terms of load transfer, settlement, and displacement. Additional field observations conducted on the Seoul Metro Line corroborated the results of the full-scale tests. The overall findings strongly suggest that construction joints on quick-hardening concrete tracks would need to be reinforced.

No abstract

a concrete slab track system with various thicknesses, ranging from 0.11 to 0.13 m. Recently, some newly constructed sleepers have exhibited both surface-opening and latent cracks, as shown in Figure 1. A total of 332 sleepers with surface-opening cracks were discovered among the 306,000 sleepers installed for the second-stage construction of the KTX railway from Taegu to Gyeongju, South Korea. A team of investigators organized by the South Korean government concluded that conical cracks had developed in the sleepers because of volume expansion of the freezing water entrapped in the embedded fastening assemblies during the winter. Among the 306,000 sleepers, 18,177 were identified to be suspicious or damaged by freezing. The problem was then to identify sleepers with internal damage for repair. Sleepers with surface-opening cracks are easily detected by visual inspection. However, sleepers with hairline cracks or latent cracks are hard to detect. To make matters worse, the KTX railway construction schedule required the inspections to be finished in a couple of months.The goal of the research described here was to design an optimal nondestructive testing (NDT) method for the identification of sleepers with hairline cracks or latent cracks. The impact-echo method (1, 2), the spectral analysis of surface waves (SASW) method (3), and the flexural rigidity assessment of concrete tracks by antisymmetric Lamb waves (FRACTAL) method (4) were used to investigate three typical sleepers: an undamaged sleeper, a sleeper with surface-opening cracks, and a sleeper with hairline cracks and possible latent cracks. The proposed optimal procedure involves two stages of testing: FRACTAL tests for preliminary scanning and SASW tests for in-depth investigation. The procedure was applied to all 18,177 of the suspicious sleepers, and some typical results are presented in this paper.In the impact-echo test, stress waves from an impulse source are applied to the top surface of a platelike structure and are then reflected back from the bottom of the surface or from internal anomalies within the structure. The multiple reflections between the two plate boundaries are analyzed to determine the thickness of the structure or the The construction of a high-speed railway in South Korea was recently impeded by the unexpected problem of cracks in sleepers. These were presumably induced by the volume expansion of freezing water entrapped in the fastening assemblies during the winter. A total of 306,000 sleepers had to be investigated on an urgent basis for hairline cracks and possible latent cracks. These findings suggested that an appropriate nondestructive testing (NDT) technique for the inspection of concrete sleepers should be designed to meet two requirements: speed and reliability in testing. Research was performed to identify an optimal NDT technique by using sleepers with three different crack conditions: without any cracks, with a surface-opening crack, and with a hairline crack and possible latent cracks. Flexural rigidity rather than ma...