Sung-Ho Joh

Accelerated Stiffness Profiling of Aggregate Bases and Subgrades for Quality Assessment of Field Compaction

As a compaction control parameter, stiffness has been investigated as an alternative to dry unit weight, because the design of pavement is based on the elastic modulus of pavement layers. A reliable and practical quality assurance system was proposed to accelerate the quality assessment of field compaction. The proposed system, which has evolved from the spectral analysis of surface waves (SASW) method, profiles the shear wave velocity of the unbound aggregate base and subgrade. The system consists of dedicated hardware for surface wave measurements and an analysis algorithm to automate the interpretation and analysis procedures. The measurement hardware consists of two sensor units and one source unit and can be operated by one person. The automation in interpretation and analysis was implemented with the peak-trough method and the frequency wave number technique. Consequently, the surface wave measurement for stiffness profiling is reduced to a 10- to 15-min task. The validity of the proposed hardware a...

Pavement Integrity Scanner to Characterize Modulus Contrast Between Near-Surface and Deeper Material in Concrete Pavements

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.

Deformation characteristics of discontinuous section of quick-hardening concrete tracks under full-scale test

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.

Experimental Verification of Displacement Estimation Algorithm using Velocity Time History

In this study, displacement estimation algorithm, which is not requiring an absolute reference point unlike the conventional displacement measurement method, is developed using the geophone. To estimate displacement of the bridge, measured velocity time signal is integrated in the frequency domain. And, the estimated displacement is compared with the measured result using a conventional method. Based on the dynamic field test results, it was found that the estimated displacement by the present algorithm is similar to that of a conventional method. The displacement estimation algorithm proposed in this paper can be effectively applied to measure the displacement of a structure, which is difficult to install a displacement transducer at the fixed point.

Nondestructive Identification of Freezing-Induced Cracks in Concrete Sleepers of High-Speed Railways in South Korea

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 material stiffness or depth to multiple reflections was chosen as a key parameter for assessment of the structural integrity of a sleeper. The results allowed the design of a two-stage testing procedure: the flexural rigidity assessment of concrete tracks by antisymmetric Lamb (FRACTAL) waves technique for preliminary scanning and spectral analysis of surface waves (SASW) tests for in-depth investigation. A total of 18,177 sleepers that required testing were examined using FRACTAL tests, and 387 of these sleepers were double-checked using SASW tests. Before the overall examination of sleepers, a series of statistical analyses were performed to establish the criteria for defining the quality of a sleeper. The results of the FRACTAL test agreed with those of the SASW test in identifying sleepers with hairline cracks or possible latent cracks.

ESTIMATING SUBGRADE STIFFNESS AND BEDROCK DEPTH: COMBINED FALLING WEIGHT DEFLECTOMETER AND SIMPLIFIED SPECTRAL ANALYSIS OF SURFACE WAVES MEASUREMENTS

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.

Estimation of the Compressive Strength of Concrete Using Shear Wave Velocity

Compressive strength of concrete is a very important parameter in the quality control of both new and existing concrete pavement. It has been widely used to evaluate the concrete strength both in the laboratory and in the construction field regardless of whether numerous mix designs or identical mixtures are used. Errors usually happen in the compressive strength test even if the mixture is evenly mixed and compacted into test specimens. This is caused by either by the hand-operated testing machine or by the eccentricity between the specimen and the testing machine. If concrete specimens with identical mix design need to be tested at various curing ages every time concrete is used in the construction field, the result is significant waste of time, money and efforts are required. Consequently, there is a real need to replace the repeated compressive strength tests with the introduction of nondestructive tests to estimate compressive strength of concrete. In this study, a nondestructive test method using elastic waves was used because shear wave velocity is not affected by restraint conditions. The compressive strength of concrete was estimated according to various curing ages as well. In the results, shear wave velocity was very closely related to the compressive strength. The results further showed that the estimation of compressive strength of concrete using shear wave velocity is very effective and reliable.

Dynamic Deflection of a Railroad Sleeper from the Coupled Measurements of Acceleration and Strain

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.

Cross-Sectional Imaging of Elastic Modulus for Railway Trackbed under Ballast for Identification of Potential Settlement

Recently a limited section of trackbed with ballast at KTX railway were reported to have settled down and led to problems such as reduced speed and passenger discomfort. Therefore, an urgent remedy for the settled trackbed is required to recover normal operation of KTX trains. In this paper, a new technique is proposed to visualize the elastic modulus of cross sections at railway trackbeds under ballast for practical identification of potential settlement. The proposed technique is based on ICSW technique, enabling use of impact source and overcoming inherent limitations of CSW method. To verify validity and feasibility of the proposed method, the method was employed to construct cross-sectional images of elastic modulus of two railway trackbeds and compared with other tests such as SASW, PBT, DCP and portable FWD tests.