Experimental Details for Investigation of the Concrete Railroad Ties In High-Speed Railway

A total of 441 PSC ties were collected from the ballasted track of the Gyeongbu HSR in Korea, which is located at 183.195−183.460 km from Seoul Station as shown in Figure 50 (a). Figure 50 (b)- (d) show the photographs taken at the field investigation to collect the sample ties in 2019.

All collected PSC ties were manufactured by Taemyung industrial Co., Ltd., South Korea; a high strength concrete of 60 MPa (28 days) for the ties was made of Type III portland cement without adding any supplementary cementitious materials (e.g., fly ash) and prestressing steel wires of 1,705 MPa (yield) was used. The PSC ties were installed in the Gyeongbu HSR in 2002 and were used for field testing for two years during the trial run of HSR from 2002 to 2004; after then, the ties had been used in service for 15 years from 2004 to 2018.

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Figure 50. Information on the collection site and collected ties: (a) the route of Gyeongbu HSR, (b) to (d) photographs of collection sites and 441 collected PSC ties.

After visual inspection, the collected ties were classified into three groups: (1) the ties without visible cracks (denoted Non-cracked), (2) the ties with large horizontal cracks on the sides (denoted Longitudinal-cracked), and (3) the ties with reticular cracks (or map cracks) on the surfaces (denoted Map-cracked). As control samples for comparison, three brand new PSC ties (denoted Brand-new) were obtained; they were manufactured in 2018, but never used. Photographs and details for these PSC ties are summarized in Figure 51 and Table 10. The Brand-new PSC ties showed very smooth and clean surfaces without any crack. In the Non-cracked PSC ties, a small number of minor cracks and rough textures were observed on the surfaces likely due to the long-term outdoor exposure and abrasion by ballast gravels. However, the Longitudinal-cracked PSC ties noticeably exhibited visibly wide longitudinal cracks (average crack widths = ~5-6 mm). Lastly, in the Map-cracked PSC ties, reticular cracks, spalling of concrete, and exposure of coarse aggregates were observed.

Among the collected 441 PSC ties, 211 ties (47.8%) were classified as Non-cracked and 228 ties (51.7%) were categorized as Longitudinal-cracked. Only two ties were identified as Map-cracked; thus, although the use of three samples, at least, is normally necessary to conduct static flexural loading test, only two samples were tested for Map-cracked.

SEOUL

BUSAN SOUTH

KOREA

YELLOW SEA

EASTSEA Site: 183.195 km−183.460 km from Seoul station (Yeongdong section)

Year of tie production: 2002

Period of field test: 2002−2004

Service period of ties: 2004−2018

Year of dismantlement: 2018

Year of sample collection: 2019 Site location where the used

PSC ties were collected

(a) (b)

(c)

(d)

Locations of station Collection site

Route of High-Speed Railroad

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(a) (b)

(c) (d)

Figure 51. Photographs of PSC ties: (a) Brand-new, (b) Non-cracked, (c) Longitudinal-cracked, and (d) Map-cracked.

Table 10. Summary of the collected 441 PSC ties and the brand new 3 PSC ties for the static flexural loading test and microstructural analysis.

Sample label

Year of production (service life)

Number of

samples Results of visual inspection

Brand-new 2018

(unused) 3 - No crack on surfaces

- Clear and smooth surfaces

441 PSC ties

Non-cracked

2002 (15 years)

211 - Slight surface abrasion - Generally good condition Longitudinal-

cracked 228

- Long and severe transverse cracks on sides

- Some surface peelings

Map-cracked 2 - Reticular cracks (or map cracks)

- Exposure of coarse aggregates Brand-new

PSC tie

Smooth and clean surface

Non-cracked PSC tie

Surface wear

Longitudinal-cracked PSC tie

Visible longitudinal crack

Map-cracked PSC tie

Reticular cracks and exposed aggregates due to surface peeling

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Static flexural loading tests were conducted on rail seat sections, as shown in Figure 52, in accordance with the European Standards (EN) [168] for all collected PSC ties and Brand-new ties. The distance between loading point and each end support was 300 mm, and the distance from the edge of each PSC tie to the point of loading was 545 mm. After the loading tests, the fractured pieces of PSC ties were collected for performing subsequent microstructural analyses.

Figure 52. Schematic view of test setup and instrumentation (modified after EN 13230-2 [168]).

Optical microscope (Axio Zoom Microscope, Zeiss, Germany) was used to observe interface between aggregate and mortar matrix and to identify reaction rims around aggregates. To this end, after strength testing, the remains were sealed in plastic bags, and then sliced with a thickness of 5 mm using a precision saw to obtain cross sections showing microstructures between cement paste and aggregate.

Using a steel brush, coarse aggregates were separated from the collected PSC samples, and then finely ground. After brushing, powders of cement paste were also collected separately using sieves; to prevent further hydration and carbonation, the powders of cement paste were immersed in isopropanol and vacuum-dried for 2 days.

Patterns of powder X-ray diffraction (XRD) (D/MAX2500V/PC, Rigaku, Japan) were taken for the ground aggregates and the powders of cement paste, respectively. The 2θ scanning range was 5−60°

with an incident beam of Cu-Kα radiation (λ = 1.5418 Å). The measured XRD patterns were analyzed using the X’pert High-Score program [119] with the Inorganic Crystal Structure Database (ICSD) [120]

and the International Center for Diffraction Data (ICDD) PDF-2 database [143].

Thermogravimetric (TG) analysis (Q500, TA Instruments, USA) was conducted for the powders of cement paste in a nitrogen gas with an alumina pan and a heating rate of 30 °C/min from room temperature to 1,000 °C [65].

Force

300 mm 300 mm

545 mm

Rail seat section

Support Support

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Inductively coupled plasma-optical emission spectroscopy (ICP-OES) was performed to examine the potential alkali-silica reactivity of the used aggregates in the PSC ties according to the American Society for Testing and Materials (ASTM) C 289 [169] by using a spectrometer (700-ES, Varian, USA) with a 40 MHz free-running radio-frequency generator and axially viewed plasma. 25 g of 0.15−0.30 mm sized aggregates were prepared and immersed in 25 mL of 1N NaOH, 1N KOH, and saturated Ca(OH)² solutions at 80 °C for 1, 2, and 7 days; at each day (i.e., at 1, 2, and 7 days), after filtering the samples, the liquid portions were tested to determine the concentrations of dissolved silicon (Si), aluminum (Al), sodium (Na), and potassium (K) by using ICP-OES, and to measure pH by using a pH meter (HI 3320, Hanna instruments, USA). Although the ASTM C 289 test method has been widely used for determining alkali-reactive aggregates [170-173], a few studies [174, 175] recently criticized the reliability of the method because reactive aggregates were often mistaken as non-reactive aggregates, and thus, the method might underestimate the reactivity of aggregates. Hence, although the ASTM C 289 specifies a sample immersion of 24 hours, the immersion was prolonged for a period of 1, 2, and 7 days. Although there are three reactive aggregate discrimination test methods as shown in Figure 53, we used ASTM C 289 among them. Because these two methods require large amount of aggregates in its intact form to make mortar or concrete samples. However, it was realistically difficult to collect all the aggregates in the concrete ties; thus, ASTM C 289 was more suitable method than the other methods because only a small sample of aggregate is required in this method.

Figure 53. Comparison of reactive aggregate test method

In addition, the petrographic examination was conducted to verify the testing results of ASTM C 289 by referring to a previous study [173] using a polarized microscope (Leica DM4 P, Leica Microsystems, Germany); for the test, thin-sectioned (< 30 μm) aggregate was prepared. The test identified constituent minerals of the coarse aggregates in the PSC ties and compared sizes of quartz grains among the samples.

The scanning electron microcopy (SEM) (Quanta200, FEI, USA) was employed to obtain secondary electron (SE) and backscattered electron (BSE) images of the samples. For the SE images, the fractured samples were used without cutting or polishing. For the BSE images, the PSC tie samples

ASTM C 289 in this study Standard Test Method for

Potential Alkali Reactivity of Aggregates

(Mortar-Bar Method)

Standard Test Method for Determination of Length Change of Concrete Due to Alkali-Silica Reaction

Standard Test Method for Potential Alkali-Silica Reactivity of Aggregates (Chemical Method)

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were sliced into 2 mm thickness with a precision saw and mounted using epoxy resin. The mounted samples were polished using an EcoMet 250 Grinder-Polisher (Buehler, USA).

6.3. Static Flexural Loading Test of the Concrete Railroad Ties In High-Speed Railway Used For