The traditional track system, a ballasted track, is still widely regarded as one of the favored options for new railway construction projects due to low construction costs and easy maintenance. However, this type of track requires frequent repairs as a result of periodic train loads [1
]. Lee and Pyo [1
] developed a quickly converting track system that converts ballasted railway tracks into concrete tracks using quick-hardening materials.
During the service time of ballasted railway tracks, fine abrasion dust particles (FADPs) of aggregates are generated from the deterioration of aggregates under repeated train loads [2
]. Lee et al. [2
] experimentally evaluated the influence of FADPs at the interface on the strength of quick-converting track concrete (QTC). They concluded that surface cleaning of aggregates is necessary in order to achieve target strength. Lee et al. [4
] additionally assessed the effects of FADPs on interfacial fracture toughness (IFT) between quick-hardening mortar (QM) and ballast aggregates in order to develop a suitable QM with high IFT. They revealed that the use of coarser silica sands and silica fume could produce the required QTC strength with a minimum cleaning process of existing ballast aggregates.
However, the effects of FADPs on the IFT between the aggregate and the QM—and subsequently on the strength of QTC—are not yet fully understood. For instance, ballast aggregates are generally placed in an outside environment and can be easily exposed to water through rain or snow. It is well known that the amount of water in mixing concrete should be adjusted to correspond to the content of water saturation and surface condition of aggregates because the W/C ratio would change if water saturation of the aggregate is not constant [5
]. Thus, it can be expected that the water saturation content of the aggregate would substantially affect the compressive strength of QTC.
This experimental study aims to further understand the effects of the aggregate surface condition on the strength development of QTC. The detailed purposes are to experimentally evaluate the effects of FADPs that are adhered to the surface of aggregates and the effects of different water saturation content of the aggregate on the IFT between the aggregate and the QM and, subsequently, on the strength development of QTC.
2. Aggregate Conditions and the Properties of Concrete
Many studies have been carried out to characterize the influence of aggregate conditions, e.g., aggregate moisture content, type, roughness, and surface deformation, on the mechanical properties of concrete [6
]. Aggregate water saturation generally produces negative effects on the interfacial bonding between the matrix (cement paste or mortar) and inclusions (coarse aggregate) [6
]. Oliveira and Vazquez [6
] investigated the influence of the moisture of recycled aggregate on the strength and durability of concrete and reported that saturated aggregates showed lower flexural strength. Poon et al. [7
] revealed that air-dried aggregates produce a better compressive strength than that of water-saturated aggregates. On the other hand, Lee and Lee [8
] reported that saturated surface-dry aggregates exhibit higher concrete strength than air-dried and sun-dried aggregates because oil palm shell aggregates are more efficient for internal curing because of the higher absorption capacity of oil palm shell aggregate.
The aggregate type also significantly affects concrete strength [9
]. Ozturan and Cecen [9
] claimed that basalt and limestone generate higher concrete strength than gravel aggregates. The effects of aggregate type on concrete strength show significantly different results in high-strength concrete [10
], where crushed quartzite aggregates indicate higher concrete strength than marble aggregates. Beshr et al. [11
] also reported that the influences of the type of aggregate on concrete strength are considerable in high-strength concrete. Petros et al. [12
] investigated the interpretation of the adverse effects of the secondary products in two types of rocks during their performance as concrete aggregates. They reported that abnormal hydration reactions and considerable swelling of the smectite result in the appearance of defects in the concrete, hence contributing to its low performance. Petros et al. [13
] investigated the effects of the aggregate type on concrete strength. They reported that the mineralogy and microstructure of the coarse aggregates affected the strength of concrete.
Aggregate surface roughness [14
] and the aggregate shapes [16
] also have substantial effects on the mechanical properties of concrete. Rao et al. [14
] reported that an increase of the roughness of aggregates contributes to an increase in the interfacial bonding between the aggregate and mortar. In addition, Hong et al. [15
] revealed that concrete strength variation would correspond to the roughness of the aggregate. Rocco and Elices [16
] reported that the concrete that uses crushed aggregates shows better mechanical properties of concrete than the concrete that uses spherical aggregates.
In addition, surface-coating of the aggregate with pozzolanic materials produces noticeable effects on concrete properties [17
]. Kong et al. [17
] concluded that the surface-coating of the aggregate with pozzolanic particles consumes calcium hydroxide (CH) in the pores and interface between the aggregate and mortar, thus forming new hydration products. This phenomenon improves the microstructures around interfacial transition areas, further enhancing the strength and durability of concrete using the recycled aggregate. Choi et al. [19
] revealed that the aggregate coated with inorganic powder strengthens the interfacial transition zone, thereby preventing micro-cracking and improving the mechanical performance of the concrete. Petros et al. [20
] investigated the effects of three types of recycled materials (beer green glass, waste tile and asphalt) on concrete strength. In addition, the effects of beer green glass with quartz primer and waste tile with quartz primer on the concrete were studied. They reported that the material coated with quartz primer was suitable for obtaining optimal compressive strength results.
Pyo et al. [21
] investigated the mechanical properties of ultra high performance concrete (UHPC) incorporating coarser fine aggregates with maximum particle size of 5 mm. They reported that the UHPC mixtures with dolomite and steel fibers with more than one volume percent achieved more than 150 MPa of compressive strength at the age 56 days, and showed strain hardening behavior and limited decrease in tensile strength compared to typical UHPC without coarser fine aggregates.
As many research studies found in the literature point out, the condition of aggregates is a critical factor for the properties of cement-based materials. However, the effects of aggregate conditions on the QTC strength, especially using quick-hardening mortar, are not fully understood. Specifically, investigation is needed of how aggregate water saturation affects the strength development of QTC. It is important to clarify the influence of aggregate water saturation on QTC strength to obtain the target strength of railway tracks using the proposed quick-converting method because ballast aggregates are under various climate conditions.
The compressive strength of QTC and IFT between the aggregates and QM was investigated by changing the amount of FADPs on the surface of the aggregate and the content of aggregate water saturation. The key conclusions can be summarized as:
FADPs attached on the aggregate surface deteriorated the interfacial bonding between the aggregate and QM. Accordingly, the compressive strength and IFT decreased as FADPs increased.
For the specimen without any FADPs on the aggregate surface, the IFT decreased as the content of aggregate water saturation increased. The compressive strength of the QTC was also reduced because the porosity at the interface increased due to aggregate water saturation.
However, in the case of 0.15 wt% FADPs on the aggregate surface, both the compressive strength and IFT increased as the content of aggregate water saturation increased. The reduction in the W/C ratio at the interface resulted from the absorption of water by the particles on the surface of the aggregate, and the cleaning effects of the aggregate surface increased the compressive strength as well as IFT.
The results obtained in this study provide fundamental knowledge of the importance of the aggregate surface conditions for the strength development of QTC. Therefore, in order to efficiently apply the quick-converting method, further research is required to improve interfacial bond strength according to aggregate surface conditions. Therefore, research with additional variables is essential. The effects of precipitation on the strength of QTC during the curing period in the actual track system should be investigated in addition to the effects of aggregate water saturation.