The demand for concrete fabrication is steadily increasing, especially in the area of concrete cutting employed in architectural renovation projects involving existing concrete structures, as well as in the demolition of nuclear power plants. Conventional material removal methods, such as waterjet cutting and diamond blade cutting, are still mostly used for concrete processing. Moon et al. [1
] reported that the performance to confirm whether the computer-aided design (CAD) based path planner could be fulfilled in the remote control of concrete surface grinding. Skarabis and Stöckert [2
] used diamond grinding to optimize the noise emission reduction of concrete pavement surfaces. Mostavi et al. [3
] investigated the applicability of using drill cuttings in concrete to produce cost-effective material. Furthermore, the effects of fly ash and silica fume on the compressive strength of concrete were evaluated. Foroutan et al. [4
] focused on testing the hypothesis that drill cuttings can be integrated into fine aggregates in the production of concrete. In addition, drill cutting is applicable for use in controlled low strength material (CLSM) as well as similar non-structural applications. Sitek et al. [5
] utilized the water jet technique in mending works on thermally affected concrete structures. Although the waterjet cutting method can be used to cut most cement-based materials, the method can be implemented only with limited power, leading to long operation times; in addition, the method generates unwanted contaminated wastewater. On the other hands, the diamond blade cutting method can handle only straight or circular geometries. In addition, diamond blade cutting causes micro-fracturing of the concrete because of vibration, thereby resulting in unnecessary removal of concrete.
Laser-aided manufacturing has been used in various fields, such as automobiles, aerospace, electronics, and semiconductors, because of its several advantages, e.g., ease of keeping the workpiece in the right position, low dust and noise generation, fast speed, and noncontact mode of operation. Especially in laser cutting, a small heat-affected zone can prevent the undesirable deformation of materials. Therefore, the laser cutting method has a high level of efficiency. Furthermore, lasers can provide a high energy density and focus on very small spots by optical manipulation. In addition, the laser cutting method has been actively applied to a wide range of materials such as wood, composites, rubber, and metal. Lee et al. [6
] proposed the feasibility of laser cutting of compressed cathodes and anodes for lithium-ion batteries. By analyzing the molten pool and temperature distribution after laser cutting on the anode, laser cutting can be used to predict and prevent defects of thermal stresses occurred during the laser cutting. On the other hand, the boundary between active electrode and current collector was clearly shown in the compressed cathode. However, the aluminum contaminated the active electrode when the compressed cathode was used. Wetzig et al. [7
] performed a high power fiber laser cutting of aluminum sheets, electrical sheets, and high strength sheets. Several cutting qualities, such as cutting edge, kerf width, and burr formation were analyzed. Pocorni et al. [8
] investigated the morphology of a cut front generated by fiber laser. In addition, high-speed imaging (HSI) was used to evaluate the fluid dynamics of the cut front during the laser cutting. Sun et al. [9
] proposed a morphology and physical mechanisms of laser-affected damage in laser ablation for thin glass sheets using picosecond lasers. They classified two types of damage morphology observed in a cross-section of a specimen. It was distinguished to be the damaged region caused by high-density free electrons and the heat-affected zone formed by the heat accumulation. Lee et al. [10
] fabricated the Spring Contact Probe (SCP) using laser cutting. Interaction relationships between laser and materials, such as a crater size, material removal zone, ablation depth, ablation threshold, and full penetration, were investigated. Haddadi et al. [11
] performed laser cutting on an extruded polystyrene sheet using CO2
laser. They found that the width of the heat affected zone (HAZ) decreases when the cutting speed is increased using the maximum laser power and compressed air.
Because of these advantages, various studies have been conducted on the use of lasers to process cement-based materials. Muto et al. [12
] used a fiber laser system (YLR-5000 MM, IPG) with fiber optic beam delivery to cut concrete. They used a 4 kW ytterbium multimode continuous wave laser and a 1-km-long fiber optic cable, and focused the laser to a 0.75 mm spot through a 120 mm lens. The thickness of the tested concrete slab was 100 mm, and the scanning speed was 5 mm/s. Their research demonstrated the feasibility of remotely using a laser-based concrete cutting technique by specifying the core diameter of the fiber and the fiber length as parameters. Lee et al. [13
] used a 1 kW multimode continuous fiber laser (YLS-1000 MM, IPG) emitting at 1070 nm to evaluate the variation of physical and chemical composition of cement-based materials after the laser cutting performance. In particular, they measured compositional variations using energy-dispersive X-ray spectroscopy to study the compositional change before and after the laser interaction with cement-based materials. Furthermore, they determined the laser parameters that were suitable for the complete cutting of cement-based materials. However, the addition of silica sand, silica fume, and silica powder to cement paste resulted in the degradation of the cut quality during laser beam cutting. Crouse et al. [14
] used a 1.2 kW CO2
laser and a 1.5 kW diode laser to cut a concrete slab. They measured the number of laser cutting passes required to achieve a concrete penetration depth of 120 mm and determined the penetration depth and kerf width according to the number of laser passes. In addition, they performed a numerical simulation to investigate the effect of laser beam characteristics on kerf convergence during multipass processing. The rectangular beam shape of the diode laser yielded higher transmittance and provided a wider kerf during deep-section concrete cutting. In addition, the kerf formed by a CO2
laser experienced a higher conductive loss at the edge of the laser spot and a higher temperature in the central region. Because of this influence, the kerf formed by the CO2
laser was narrower and more tapered than that formed by the diode laser. Consequently, a high-power diode laser was considered more suitable than a CO2
laser for practical concrete cutting applications. Based on the literature review, it can be concluded that most of the previous studies used less than 4 kW of laser power, and a multi-scan technique is applied to cut thick concrete. Furthermore, the interaction mechanism between laser and cement-based materials has not been tested in detail.
Therefore, laser cutting parameters need to be optimized to develop the laser irradiation technique for the efficient removal of cement-based materials. In addition, an understanding of material removal mechanisms is necessary to control and enhance the performance of laser irradiation on cement-based materials. Thus, this study aims to investigate the feasibility of the laser cutting technique to cement-based materials using multimode fiber lasers. Experimental results are presented to show the effect of laser interaction on cement-based materials. Experimental variations are introduced using the laser scanning speed, water-to-cement ratio, and composition of materials such as cement paste, cement mortar, and ultra high performance concrete (UHPC). Cement-based materials without coarse aggregate were chosen in this research to systematically investigate the effects of various forms of silica-based materials on the laser cutting quality. It should be noted that UHPC is one of the most advanced cement-based materials, and active research has been carried out to explore its material characteristics [15
]. A multimode fiber laser is used for the experiment. Furthermore, the surface and cross section of the cement-based materials are observed, and the kerf width and penetration depth of the materials are analyzed. Furthermore, the calculated values obtained using elliptic equations are compared with the measured values to explain the elliptical holes observed in the cement paste series. In addition to these observations, a mechanism for the interaction between the laser and cement-based materials is proposed.
2. Raw Materials and Mix Design
In this study, various material compositions were designed to investigate the effect of mechanical reactions on cement-based materials according to the laser cutting speed. Table 1
lists the mix proportions of the cement-based materials used in this study. The materials used in the experiment were ordinary Portland cement, silica sand containing approximately 93 wt % of SiO2
, undensified silica fume (grade 940U, Elkem) containing approximately 95 wt % of SiO2
, silica powder containing approximately 98% of SiO2
with a median diameter of 3.15 μm, and a polycarboxylate-based superplasticizer with 25 wt % solid content by weight.
Three representative series of cement-based materials were prepared with a thickness of 50 mm and were named as LP, LM, and LU, indicating the use of cement paste, cement mortar, and UHPC for laser cutting, respectively. Furthermore, the LP and LM series names were suffixed with the water-to-cement ratio; for example, LP 0.4 indicates that a water-to-cement ratio of 0.4 is used. Different combinations of materials were set as variables to investigate their effects on laser cutting. The variables of the LU series were named in terms of the weight ratio of silica sand, silica powder, and silica fume. In addition, a superplasticizer was added to LM 0.25 and the LU series to ensure proper mixing by following previous research [19
In this study, specimens were prepared by a planetary mixer. A fresh mixture was poured into molds (50 × 50 × 50 mm3
) when the mixture showed proper viscosity. The specimens were covered with plastic sheets, and those were stored at room temperature for 24 h. Subsequently, the specimens were demolded and cured in a water tank maintained at 23 °C. After 24 h of drying in a laboratory environment, the compressive strength was measured following the ASTM C109 [21
] at the age of 28 days. The compressive strengths listed in Table 1
were obtained by averaging the results calculated by measuring three specimens. In addition, in the laser cutting experiment, each specimen was used thrice at different laser cutting speeds. Figure 1
shows the experimental setup and an example of a tested specimen after laser cutting.
The experiment was performed using a 10 kW multimode fiber laser (IPG YLS-10000, IPG Photonics, Oxford, MS, USA) operating at a 1070 nm wavelength. The laser had a spot size of 150 μm, and N2
assist gas was maintained at a pressure of 7 bar. The specimen was placed on the testbed and fixed using a vice. The spacing of the testbed was set to 6.5 mm to facilitate the removal of the material discharged during the laser cutting. The focal point was set on the specimen surface, and the laser beam was irradiated vertically onto the specimen during the laser head movement. To collect dust particles during the laser irradiation, a ventilation duct was placed on the left side of laser device. The laser power was set at 9 kW, and the laser cutting speed was designated as the only controllable parameter in laser settings so as to simplify the experimental parameters. The laser cutting speeds were set as 0.25 and 0.5 m/min; in addition, speeds from 1 to 4 m/min in increments of 1 m/min were considered. Table 2
lists the laser cutting speeds and line energies used in the experiment.
The line energy, which was obtained by dividing the laser output power by the laser cutting speed and the laser spot size, was used to continuously monitor the irradiated laser energy per unit volume. Furthermore, the line energy was an important parameter used to understand the interaction between the laser and specimens. After the laser cutting (see Figure 2
), specimens’ kerf width and penetration depth were observed using a digital microscope (AnMo Electronics Corp., New Taipei, Taiwan).
The kerf width and penetration depth were used to indicate the amount of materials that were removed by the laser processing. That is, the effect of the laser on the materials can be effectively evaluated according to the parameters set in the experiment. The kerf width and penetration depth represented the completely cut width of the specimens at the surface (top surface) and the measured depth of the specimen, respectively, under laser processing. The kerf width was measured at nine points for each case, and the maximum, minimum, and average values of the measured widths were evaluated. In addition, the penetration depth was measured using an additional specimen cut using a mechanical saw cutter with a blade thickness of 3 mm.
This study was conducted to investigate the applicability of laser cutting technology to cement-based materials using multimode fiber lasers. The experimental variation parameters were the material composition and laser cutting speed, which was set to 0.25, 0.5, 1, 2, 3, and 4 m/min. After laser irradiation, the top and cross-sectional views of cement-based materials were observed. Trends were analyzed by measuring the kerf width and penetration depth of the cement-based materials. To elucidate the elliptical hole observed in cross sections of the LP series, the focal and measured widths (top, middle, and bottom widths) were compared through graphs. The major observations and results of this study can be summarized as follows:
The kerf width of the LP series showed good cutting quality, but partially cracked and burnt areas were observed in the LM series. In the LU series, not only was the molten pool not completely evaporated in the material removal zone, but impurities were also observed on the surface;
Under the considered parameters, the 50-mm-thick cement-based material was completely cut only for the LP series when the cutting speed was 0.25 m/min at 9 kW. To completely cut the 50-mm-thick cement paste (LP series) material, a line energy of 1.22 × 1014 J/m3 or more was required;
Single and multiple elliptical holes were generated by the heat transfer at the sidewalls of the material removal zone and the effect of the molten concrete on the laser absorption rate, respectively.
In the future, as an extension of the present study, the surface temperature should be measured during fast laser cutting for an accurate analysis of the interaction mechanisms between the laser and cement-based materials.