Research ArticleEffect of Garnet Characteristics on Abrasive Waterjet Cutting ofHard Granite Rock

Tae-Min Oh,1 Gun-Wook Joo,2 Yohan Cha,3 and Gye-Chun Cho 4

1Assistant Professor, Department of Civil and Environmental Engineering, Pusan National University (PNU), Busan 46241,Republic of Korea2Researcher, &e 4th R&D Institute-3rd Directorate, Agency for Defense Development, Daejeon 34186, Republic of Korea3Ph.D. Candidate, Department of Civil and Environmental Engineering,Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Republic of Korea4Professor, Department of Civil and Environmental Engineering, Korea Advanced Institute of Science and Technology (KAIST),Daejeon 34141, Republic of Korea

Correspondence should be addressed to Gye-Chun Cho; gyechun@kaist.edu

Received 2 October 2018; Revised 24 January 2019; Accepted 3 February 2019; Published 12 March 2019

Academic Editor: Giosue Boscato

Copyright © 2019 Tae-Min Oh et al. .is is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abrasive waterjet cutting technology has come back into use in the field of rock excavation (such as for tunneling) due to the needfor precision construction with low vibration. Because the abrasive particles play an important role in efficient erosion during thecutting process, the abrasive characteristics strongly affect the rock cutting performance. In this study, rock cutting tests wereperformed with five different coarse (40mesh) garnets to explore the effect of the abrasive feed rate, physical properties, andparticle size distribution on rock cutting performance. In addition, garnet particle disintegration was investigated with garnetcharacteristics for the abrasive waterjet. .e test results indicate that the particle size distribution, garnet purity, specific gravity,and hardness are the most important parameters for rock cutting performance. .is study offers better understanding of coarsegarnet performance and efficiency according to the garnet characteristics. .is should provide assistance in selection of the garnetneeded to achieve the desired performance for hard rock cutting.

1. Introduction

.e abrasive waterjet represents a suitable method that hasspecific benefits for the cutting, drilling, and turning process[1]. .e abrasive waterjet technology is applied for rockexcavation and drilling and for demolition of concretestructures. It is important because it can be used in the formof an efficient automated system that minimizes thermal andmechanical damage near the erosion zone due to its lowvibration during excavation [2–4]. In hard material cutting,abrasive particles play an important role in the efficienterosion of a target material [5, 6]. Abrasive particles areaccelerated within a high-velocity waterjet stream, anderosion of the target material occurs due to continuousimpact of the high-speed abrasive particles.

Garnet materials are a group of complex silicate mineralsthat are often used as abrasives for waterjet cutting. In the

garnet classification scheme based on composition, garnetsare grouped into two solid-solution series: pyralspite andugrandite [7]. .e pyralspite series includes almandine (Ferich), pyrope (Mg rich), and spessartine (Mn rich). .eugrandite series includes grossularite (Ca rich), andradite(Ca-Fe-Ti rich), and uvarovite (Ca-Cr rich).

Abrasive is characterized by its mineral composition(including garnet purity), particle shape, surface texture,hardness, specific gravity, and particle size distribution[8, 9]. .ere have been a number of studies on waterjetcutting, for example, at a short standoff distance with finegarnet for cutting thin materials and on the effect of thehardness of abrasive for cutting soft, brittle material(i.e., ceramics) [10]. In addition, the effect of the particle sizeof almandine garnet was explored in rock cutting tests: thehigher the content of larger particles was, the better thecutting performance was [11]. Abrasive particles with

HindawiAdvances in Civil EngineeringVolume 2019, Article ID 5732649, 12 pageshttps://doi.org/10.1155/2019/5732649

internal cracks were found to significantly worsen the in-fluence of the rock cutting performance [9].

Garnet with coarse particles (e.g., 40mesh) has beenincreasingly used to achieve efficient waterjet cutting of hardrock. .e larger abrasive particles can cut deeper and fasterwhen the water flow is sufficiently high and large enoughnozzles are provided in the field; the cutting efficiency with40mesh garnet is much better than 80mesh garnet. How-ever, further investigation into the influence of the prop-erties of coarse abrasives on hard rock cutting performanceis still needed for field applications. .is is because mostprevious studies used only fine abrasive particles (e.g., lessthan 80mesh) for more delicate cutting processes(e.g., precision machining).

A long standoff distance (e.g., 200mm) is also requiredfor field cutting (excavation) applications because of theuneven surface of the rock. Moreover, at short standoffdistances (e.g., 10mm), a nozzle head can easily be broken bycollision with protruding rock. Cutting efficiency at thelonger standoff distance causes results different from those atshort standoff distances. .is is because spreading of the jetleads to dissipative energy losses that eventually cause theenergy of the jet to be insufficient for eroding the targetmaterial [12].

.e aims of this study were to provide better un-derstanding of coarse garnet performance and efficiencyaccording to the garnet characteristics at a long standoffdistance and to provide assistance to those selecting abra-sives for hard rock cutting performance in constructionfields. In this study, performance according to the charac-teristics of garnet abrasives was explored for cutting intactgranite specimens. Cutting efficiency was analyzed usingdata on the abrasive feed rate, physical properties, andparticle size distribution at long standoff distance(i.e., 200mm).

2. Experimental Program

2.1. Test Setup. .e rock cutting tests were performed usinga high pressure waterjet pump (50 HP intensifier pump)..e pump generated water pressure up to 412MPa andwater flow rate up to 6 L/min. In the rock cutting test, waterpressure was fixed at 250MPa, and the water flow rate wasset as 2.59 L/min (inner diameter of the orifice: 0.33mm).For the constant cutting procedure, the nozzle was movedin one direction at the speed of 10mm/s. .e standoffdistance, between the tip of the focusing tube and the targetsurface of the specimens, was kept constant at 200mm;when the standoff distance is more than 300mm, thecutting performance can be decreased for this waterjetsystem. .e test setup of the abrasive waterjet is shown inFigure 1.

Abrasives were fed into a high-speed waterjet stream bythe Venturi suction effect. .e feed rate of the abrasive wasadjusted using an air control valve installed on the abrasivetank. .e abrasives were supplied through the feeding tube(inner diameter 6mm) from the abrasive tank. .e abra-sives were mixed with the jet stream in a mixing chamberand projected onto the target material through the focusing

tube. .e focusing tube selected was 1.27mm in innerdiameter and 101.6mm in length. A schematic illustrationof rock cutting with an abrasive waterjet is shown inFigure 2.

2.2. Preparation of Specimen and Abrasive. Intact granitespecimens of a type predominant in the Republic of Koreawere sampled at a quarry site in the Hwang-Deung region..e prepared rock specimens were cubic blocks(100×100×100mm). .e physical properties of the rockspecimens measured included density, porosity, absorptionratio, uniaxial compressive strength, tensile strength, andP-wave velocity. According to the ISRM classification, thespecimens were classified as very strong rock (R5). .ephysical properties of the rock specimens are summarized inTable 1.

Five different garnet products (all 40mesh) were orderedfrom abrasive suppliers in five different countries (India,Mongolia, China, Australia, and the United States ofAmerica). However, it was hard to discover in detail, thelocations of the garnet mining in each country. XRD (X-raydiffraction) tests were performed to analyze the mineralcomposition and garnet purity. Scanning electron micros-copy (SEM) was used to observe the particle surface textureand particle shape; Figure 3 presents an SEM image of somegarnet particles. Sieve analysis (ASTMD422) was performedto verify the particle size and distribution. Specific gravityvalues were measured according to the ASTM D854 stan-dard. Mohs hardness information was obtained from theabrasive suppliers. .e characteristics of the test garnetabrasives are summarized in Table 2.

2.3. Cutting Test Procedure. To analyze the effect of theabrasive feed rate on cutting performance, the feed rate wasinitially adjusted (and checked regularly) to a fixed value of5.4± 0.4 g/s and was increased approximately two times to10.7± 0.8 g/s. In addition, to eliminate the particle size effect,all the garnet particles were passed through a 40mesh sieveto obtain particles of uniform size (0.425mm)..ese filteredabrasive particles of uniform size were used for the cuttingexperiment.

After each cutting procedure, removal volume, cuttingdepth, and cutting width were measured to reveal cuttingperformances. Here, removal volume is defined as thevolume excavated after cutting 100mm. .is was de-termined by pouring water into the open space and mea-suring it.

3. Analysis of Garnet Characteristics

3.1. Mineral Composition. Regarding the garnet abrasives,those in the pyralspite series are broadly used as commercialabrasives because they show higher erosion performancethan the members of the ugrandite series. Vasek et al. [9]reported that cutting efficiency with almandine (pyralspiteseries) for rock specimens was better than with grossulariteand andradite (ugrandite series). Almandine garnet is theprincipal abrasive for industrial uses because of its high

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specific gravity and hardness [13, 14]. However, spessartineand pyrope (pyralspite series) have also been used asindustrial-grade garnet abrasives [7].

All the garnet samples were determined to be membersof the pyralspite series through mineral composition anal-ysis, more specifically, almandine and spessartine. XRDanalysis results showed that the Indian garnet (G1) waspyrope; the Mongolian (G2) and Australian garnets (G4)

were almandine; and the Chinese (G3) and American (G5)garnets were spessartine.

.e chemical composition of garnet within the pyr-alspite series is generally described as [A3B2(SiO4)], inwhich A is a divalent metal (Ca, Fe, Mg, or Mn) and B is atrivalent metal (Al, Cr, Fe, or Mn) [14]. .e specificchemical composition of almandine, pyrope, and spes-sartine is [Fe3Al2(SiO4)3], [Mg3Al2(SiO4)3], and[Mn3Al2(SiO4)3], respectively. .e purity of the garnetmineral compositions was 99.8% for G1, 95.4% for G2,65.6% for G3, 97.9% for G4, and 90.3% for G5. Table 2 liststhe mineral composition and specific formulas of the ex-perimental garnets.

3.2. Specific Gravity. .e magnitude of kinetic (impact)energy is determined by the density and velocity of theabrasive particles at a specific time. Higher specific gravity(i.e., density) of the particles improves their cutting per-formance. To generate greater impact energy on the targetmaterial, abrasives of higher specific gravity are required,assuming the water flow rate is sufficient [12].

According to the specifications of the InternationalOrganization for Standardization [15], the specific gravity ofgarnet ranges from 3.5 to 4.2. In this study, the specificgravity of the garnets used ranged from 3.66 to 4.05. .egarnets G1 and G4 had specific gravities >4.0 (4.05 for G1and 4.02 for G4). G2 garnet had specific gravity of 3.85. .e

Table 1: Properties of the rock specimen.

Property ValueRock type GraniteDensity (kN/m3) 25.8Porosity (%) 0.68Absorption ratio (%) 0.25Uniaxial compressive strength (MPa) 208.5Tensile strength (MPa) 10.2P-wave velocity (m/s) 3394

Observation window

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Figure 1: Test setup for abrasive waterjet rock cutting: (a) cutting test tank with a traverse speed controller; (b) rock cutting test setup in thecutting test tank.

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Figure 2: Schematic illustration of rock cutting using an abrasivewaterjet.

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spessartine garnets had lower specific gravity (3.66 for G3and 3.74 for G5) than the other types did.

3.3. Hardness. For efficient rock erosion, sufficient hardnessof the abrasive material is required. At the least, the hardnessof the abrasive must be greater than the hardness of thetarget rock specimen. An efficiency improvement of thewaterjet removal process occurs when the hardness ratio(abrasive/workpiece) is 1.0∼1.1 [10]. Beyond the properabrasive hardness ratio (i.e., >1.1), a further increment inhardness does not substantially improve removal perfor-mance when using an abrasive waterjet.

According to the IOS [15] specifications, Mohs hardnessof the garnet abrasive should exceed 6.5. .e range of Mohshardness of the garnets used in this study was 6.5–8.5, which

meets the IOS specifications. G5 abrasive was the hardest(7.5–8.5), while G3 was the softest (7.5–7.8).

Generally, the hardness of rock is less than 6.5 on theMohs scale because rock is generally a mixture of variousminerals [16]. .e maximum theoretical hardness of graniterock can be 7.0 if the rock consists only of quartz minerals.Quartz has the greatest hardness (Mohs 7.0) among theminerals composed of granite rock.

3.4. Particle Shape and Surface Condition. .e shape of theabrasive particles can influence the material removal regimein solid-particle erosion. .e removal mechanisms are de-termined by particle shape: microploughing for sphericalparticles and microcutting for angular particles [17]. How-ever, regarding its influence on hard rock, microcutting action

Figure 3: SEM images of garnet particles: (a) G1, (b) G2, (c) G3, (d) G4, and (e) G5.

Table 2: Characteristics of garnets.

Garnet symbol G1 G2 G3 G4 G5Type Pyrope garnet Almandine garnet Spessartine garnet Almandine garnet Spessartine garnetFormula Mg3Al2(SiO4)3 Fe3Al2(SiO4)3 Mn3Al2(SiO4)3 Fe3Al2(SiO4)3 Mn3Al2(SiO4)3

Mineralcomposition

(i) Pyrope (99.8%) (i) Almandine (95.4%) (i) Spessartine (65.6%) (i) Almandine(97.9%) (i) Spessartine (90.3%)

(ii) Calcite (0.2%) (ii) Quartz (2.9%) (ii) Hornblende (14.0%) (ii) Calcite (1.7%) (ii) Hornblende (9.7%)(iii) Magnetite (1.7%) (iii) Augite (10.3%) (iii) Quartz (0.4%)

(iv) Albite (6.5%)(v) Rutile (3.6%)

Specific gravity (—) 4.05 3.85 3.66 4.02 3.74Hardness (Mohs) 8.0 7.5–7.8 6.5–7.5 7.5–8.0 7.5–8.5Mean particlesize (mm) 0.340 0.505 0.475 0.280 0.510

Coefficient ofuniformity (—) 1.95 1.22 3.03 1.45 1.20

Source India Mongolia China Australia United States of America

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contributes but does not play a major role, in the erosionprocess..is is because of brittle failure behavior, in which theaction of stress waves generated by the particle impact is morelikely to fracture the more brittle material [18].

Bad conditions of abrasive particles (e.g., cracks or de-fects) can decrease their cutting performance [9]. Particles ofG1 and G4 included diverse shapes represented by com-binations of polygonal angular grains and abraded roundgrains (Figures 3(a) and 3(d)). In addition, the primarysurface of garnet particles was observed to be almost roundin G1 and G4. Particles of G2 were observed in variousshapes: some particles were polygonal, and some were ir-regular, with surfaces that were smooth or rough(Figure 3(b)). Particle shapes of G3 were observed to besharp-edged and polygonal with very rough surfaces. Aftercrushing, the surfaces of new particles were rough andjagged. .e particles also showed damage in the form of finenetworks of cracks (Figure 3(c)). Particles of G5 werepolyhedral particles (Figure 3(e)). Moreover, the crushing ofprimary particles generated new angular particles, and newsurfaces were exposed by separation of stable parts at naturalfracture cracks.

3.5. Particle Size andDistribution. Even though the productswere labeled as 40mesh as provided by the abrasive sup-pliers, the particle size distributions were found to differ.Using sieve analysis, particle size distributions were analyzedfor each garnet sample. .e results of the particle sizedistribution analysis are shown in Figure 4..e coefficient ofuniformity decreases with increase in the uniformity ofparticle size (i.e., 1.00 is the lowest value and indicates aperfectly uniform distribution). .e results showed that G2and G5 had almost uniform particle size; their coefficients ofuniformity were the lowest (1.22 for G2 and 1.20 for G5). G4also had a good uniformity level (1.45). G1 had a higher value(1.95) than G2, G4, and G5. Meanwhile, for G3, the co-efficient of uniformity was the highest (3.03). .is meansthat G3 was composed of particles in a variety of sizes.Uniform particle size is advantageous for smoother flow inthe abrasive tube [19].

Mean particle size (D50) is an important factor for de-termining the cutting performance because larger particleshave greater kinetic (impact) energy. For this reason, cuttingperformance tends to increase with increase in the particlesize of abrasives, up to a peak value [17, 20]. .e size dis-tribution results showed that G2 and G5 had large values ofD50 (0.505 and 0.510mm, respectively) because they hadhigher levels of uniformity in particle size, compared withmost of the other garnets (G1, G3, and G4). Meanwhile, G4had the lowest value (0.280mm) for mean particle size. .emean particle sizes and coefficients of uniformity are listedin Table 2.

4. Rock Cutting Results and Analysis

.e cutting performance indexes used included the removalvolume, cutting depth, and cutting width. Among theseindexes, the removal volume is the most useful index for

estimating the overall erosion performance. Figure 5 showsthe removal values according to garnet type at the fixed feedrate of 10.7 g/s. Cutting with G4 garnet achieved the largestremoval volume. .e comparison result shows better per-formance in the order G3<G1<G2<G5<G4. In theperformance comparison between G4 (best performance)and G3 (worst performance) at the 10.7 g/s of abrasive feedrate, the removal volume with G4 (9.0 cm3) was 56.4% largerthan with G3 (5.7 cm3).

In the comparison of cutting depth according to thegarnet type, the cutting depths were obtained at the fixedabrasive feed rate (Figure 6). .e cutting depth for G4 wasthe deepest (12.0mm). .e cutting results for G5 and G1were both good (G5: 11.4mm and G1: 11.2mm), while thecutting depths were shallower for G3 (9.4mm) and G2(9.0mm). .e deepest cutting depth with G4 was 33.1%larger than the shallowest depth with G2.

In the comparison of cutting width (Figure 6), thecutting result with G5 had the greatest width (13.8mm),while G4 had a width of 12.3mm. Meanwhile, the cuttingresults with G2 and G3 were lesser widths (G2: 11.5mm andG3: 11.8mm). .e greatest cutting width for G5 was 20.3%larger than the least width for G2.

4.1.AbrasiveFeedRateEffects. As the number of particles fedinto the jet stream increases, the kinetic energy of theabrasive jet increases until a critical feed rate is reached. Ifthe critical feed rate is exceeded, the kinetic energy is re-duced by the high momentum transfer needed to acceleratemore abrasive particles [12].

For the waterjet system used in these experiments, if theabrasive feed rate exceeded 12.0 g/s, the abrasive-feedingcondition was changed to an oversupply state (i.e., higherthan the critical feed rate). Considering the appropriate feedrate, the abrasive feed rate was initially set at 5.4 g/s; then, thefeed rate was increased by two times (10.7 g/s) to explore theeffects of the abrasive feed. Figure 5 shows changes in theremoval volume according to the abrasive feed rate. Whenthe abrasive feed rate was increased two times from 5.4 to10.7 g/s (near 100%), the removal volumes increased by only20.5–37.0%, except for the G3 garnet. .is result indicatesthat cutting performance was clearly improved but that theefficiency could not reach the expected values for most of thegarnets.

Given the experimental data, it seems likely that thebetter performing garnet may work with lower efficiencysensitivity according to the increasing feed rate. For ex-ample, the removal volume with G4 (the best performinggarnet) increased only by 20.5% with increase in theabrasive feed rate (5.4⟶ 10.7 g/s), whereas the removalvolume with G2 (the medium performance garnet) in-creased by 37.0%. Meanwhile, G3 (the worst performinggarnet) showed dramatic increase in the removal volume(109.9%) due to increase in the abrasive feed rate. .eseresults indicate that the use of larger amounts is advan-tageous for increasing the cutting performance efficiency ofpoorer quality garnet (e.g., G3) than for high quality garnet(e.g., G4).

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4.2. Physical Property Effects. To explore the effects ofonly the physical properties (i.e., specific gravityand hardness), all the abrasive particles were passedthrough a 40mesh sieve to obtain particles of uni-form size (0.425 mm). .e cutting performance results(i.e., volume, depth, and width) with uniform particle sizeat the constant abrasive feed rate (10.7 g/s) are shown inFigure 7. .e performance was found to be more efficientin the order G3 <G2 <G5 <G1 <G4 based on the removalvolume (Figure 7(a)) and cutting depth (Figure 7(b)). .ecutting performance with G4 showed the greatest re-moval volume (10.0 cm3) and cutting depth (12.0 mm);meanwhile, the cutting results with G3 showed the lowestperformance of removal volume (7.0 cm3) and cuttingdepth (9.5 mm). .e highest performance of G4 showedefficiency of 42.9% better for removal volume and 26.3%for cutting depth, compared with the lowest performanceof G3.

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Figure 5: Comparison of removal volume: the removal volumeat 5.4 g/s of the abrasive feed rate and removal volume at 10.7 g/sof abrasive feed rate.

Advances in Civil Engineering 7

For the cutting width results, G1 showed the highestcutting width (14.9mm). G4 and G5 garnet types alsoshowed good width results (G4: 14.3mm and G5: 14.4mm)(Figure 7(c)). Meanwhile, G3 showed the lowest width result(12.0mm) and worse efficiency (19.4%) in cutting width,compared with G1.

Because the removal volume index is suitable for esti-mating the overall cutting performance, the removal volume

data with uniform-size abrasive (0.425mm) can be used todirectly compare the effects from the physical properties ofthe abrasives (garnet mineral purity, specific gravity, andhardness). Figure 8 shows the relationship between removalvolume and (a) garnet purity, (b) specific gravity, and (c)hardness (averaged) of the abrasives. When the relationshipswere assumed to be exponential functions, the relationshipswere well matched (Figure 8). For all garnet types, the

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Figure 6: Comparison of cutting depth and width results at 10.7 g/s of abrasive feed rate.

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removal volume tended to increase with increase in thegarnet purity, specific gravity, and hardness. Given theexperimental data, it seems that G1 and G4 garnets have thebest properties for achieving high removal performance;meanwhile, G3 garnet has to be considered a low qualityabrasive.

In the analysis of G3 (the worst performer), the garnetpurity seems to be important for effectiveness of an abrasive.Normally, impurities in the garnet mineral reduce thecutting performance [8]. G3 garnet has significant quantitiesof other minerals (purity just 65.6%); thus, it showed worsecutting performance than the other types of garnet (thepurities of the other garnets were in the range 90.3–99.8%).

In addition, given the physical property data, specificgravity and hardness were also shown to be importantparameters for improving the cutting efficiency. G3 garnethad the lowest specific gravity (4.02) and lowest level ofhardness (7.0 average) among the abrasive types (Table 2).Meanwhile, G1 and G4 showed much better cutting per-formance due to their high specific gravity and hardness.

4.3. Particle Size Distribution Effects. In the comparison ofperformance between the original size distribution anduniform size distribution of abrasive particles, all the rockcutting results with uniform abrasive size showed betterperformance for all erosion indexes (volume, depth, andwidth) than with abrasives of varied size (i.e., size distribution

as provided), as shown in Figure 7. It seems likely that thehomogenous size distribution of particles minimizes colli-sions between particles in the high-speed jet stream.

Efficiency in volume removal increased 32.3% for G1,1.1% for G2, 21.9% for G3, 11.4% for G4, and 1.4% for G5after switching to abrasive particles of uniform size. .eseresults indicate that uniform abrasive particles erode thetarget material more efficiently, and the cutting efficiency isdirectly related to the cutting cost in the field.

It was especially observed in Figure 7 that the incrementof performance depends on the original distribution of theabrasive particle size. In this study, the rate of increase in theremoval volume due to the uniform-size effect is defined asthe improvement rate [%]. In addition, the uniformity indexof abrasive size is defined to be 1/Cu, which has the range 0-1.When the size distribution of an abrasive is perfectly uni-form, 1/Cu is equal to “1.” Decreasing 1/Cu means increasingthe dispersion of the size distribution (decreasing unifor-mity). If an abrasive is originally composed of particles ofsimilar size, performance in the removal volume will not bemuch changed by the uniformity effect because there wouldnot be much change in the particle size distribution.

Because of the limited experimental data sets in thisstudy, it is difficult to determine the exact relationship be-tween the increment rate and 1/Cu. However, the re-lationship can be shown to be a reasonable match (R2 � 0.60)when the relationship between the increment rate and 1/Cuis assumed to be a logarithmic function (Figure 9). In this

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6.6 6.8 7 7.2 7.4 7.6 7.8 8 8.2 8.4

Rem

oval

vol

ume (

cm3 )

Hardness (Mohs)

y = 0.13x2.06

(R2 = 0.62)

G3G2

G4 G1

G5

(c)

Figure 8: Relationship between removal volume and (a) garnet purity, (b) specific gravity, and (c) hardness at a uniform particle size of abrasive.

Advances in Civil Engineering 9

relationship, if 1/Cu becomes 0.4, approximately 25% per-formance improvement can be expected when the abrasive isused with particles of uniform size.

5. Abrasive Crush Characteristics

Verifying the crush characteristics (size distribution change)is important when considering abrasive recycling after hardrock cutting for cost savings in the cutting process. For thisreason, these test results regarding crush characteristicsshould help future studies to find the best recycling rate ofgarnet, even though recycling experiments were not in-cluded in this study.

Particles are crushed by the kinetic energy of theirimpact against the hard target surface; thus, the crushcharacteristics depend on the physical characteristics of theabrasives and the target rock. .e diameter reduction forgarnet particles is caused by internal cracks during abrasivewaterjet cutting [21]. In addition, the crush characteristics ofabrasive particles are also affected by the inner shape of themixing chamber and by interaction with the high-speedwaterjet [22].

Because the same rock specimens and same nozzle head(provides the inner shape of the mixing chamber) were usedfor the experiments, only the physical characteristics of theabrasive were considered to analyze the abrasive crushcharacteristics. .e crush characteristics of the abrasiveparticles were analyzed based on variation of the size dis-tribution and mean particle size. .e variation in the sizedistribution between the inlet abrasive particles and outletabrasive particles after cutting was analyzed using theuniformity index (1/Cu) and mean particle size (D50).

5.1. Variation of Size Distribution. Figure 10 shows com-parison of the particle size distribution of inlet abrasiveparticles (case A) and outlet abrasive particles (case B) afterrock cutting. For the uniformity index (1/Cu), the sizedistributions of G1, G2, G4, andG5 were changed to bemoredispersive (better distributed). .e uniformity index waschanged from 0.51 to 0.43 for G1, 0.82 to 0.53 for G2, 0.69 to0.48 for G4, and from 0.83 to 0.53 for G5 (Figure 10).

Meanwhile, the size distribution of G3 was rather uni-form, and the original uniformity index (0.33) increased to

0.59 after rock cutting. .is reverse phenomenon might beexplained by the fact that the G3 garnets had defects on theparticle surfaces and a low uniformity index, originally. Veryrough surfaces with fine networks of cracks were moreclearly observed with G3 than for the other garnet types(Figure 3).

Of special interest is that the overall uniformity indexchanged to similar values (0.43–0.59 for case B) for allabrasive types; the average uniformity index for all types wasestimated to be 0.51 with a standard deviation of 0.06.

5.2. Variation of Mean Particle Size. .e mean particle sizefor all the abrasive types was smaller after hard rock cutting,compared with the inlet abrasive size (Figure 11). Abrasiveparticles are fractured by impact at the natural cracks thatcreate preferred planes for breaking up in the cuttingprocess. Based on the mean particle size (D50), the particlesize significantly dropped by 61.8% for G1, 72.3% for G2,70.5% for G3, 46.4% for G4, and 72.5% for G5. .e ex-perimental results after cutting indicate that the meanparticle sizes tended to become similar after the particleswere crushed, as shown in Figure 11. .e average meanparticle size for all abrasive types was 140.0 μm with astandard deviation of 6.3 μm.

6. Conclusions

Waterjet cutting tests were performed with garnets from fivedifferent origins to explore the effects of the abrasive feedrate, physical properties, and particle size distribution. .isstudy can help others make reasonable choices about garnetabrasives when waterjet cutting is applied for cutting hardgranite rock at long standoff distance. In addition, it can helpthose conducting future studies to analyze the recycling rateaccording to the garnet types. .e main findings are asfollows:

(i) Australian garnet (G4) shows the best performancefor removal volume; Chinese garnet (G3) showedthe worst performance for removal volume. .eremoval volume with G4 (9.0 cm3) was 56.4% largerthan the performance with G3 (5.7 cm3) under thesame test conditions. .is result indicates thatperformance can differ even though the coarse

0

10

20

30

40

50

60

0.00 0.20 0.40 0.60 0.80 1.00Im

prov

emen

t rat

e, I (

%)

1/Cu (—)

25%G1

G4G3

G5

G2

I = –26.6 ln(1/Cu) (R² = 0.60)

Figure 9: Rate of increase in the rate of removal volume according to 1/Cu.

10 Advances in Civil Engineering

garnets used look similar, due to their geometricproperties (particle size and shape) and physicalproperties (specific gravity and hardness).

(ii) Under a fixed particle size distribution condition,garnet purity, specific gravity, and hardness areimportant parameters for determining the cuttingefficiency for hard rock. .e cutting performanceincreases with increase in the garnet purity, specificgravity, and hardness.

(iii) When the original particle distribution was changedto particles of uniform size (for all garnet types), thecutting efficiency indicated by removal volumeincreased in the range 1.1–32.3% due to effects of theartificially uniform size..e increment rates dependon the coefficient of uniformity of the originalparticle distribution.

(iv) .e particle size of the garnet abrasives decreased46.4–72.5% due to their breaking up during the

0

20

40

60

80

100

0.01 0.1 1Particle size (mm)

1/Cu = 0.511/Cu = 0.43

A (G1)B (G1)

Perc

ent p

assin

g

(a)

1/Cu = 0.821/Cu = 0.53

0

20

40

60

80

100

0.01 0.1 1Particle size (mm)

A (G2)B (G2)

Perc

ent p

assin

g

(b)

1/Cu = 0.331/Cu = 0.59

A (G3)B (G3)

0

20

40

60

80

100

0.01 0.1 1

Perc

ent p

assin

g

Particle size (mm)

(c)

1/Cu = 0.48 1/Cu = 0.69

Particle size (mm)

A (G4)B (G4)

Perc

ent p

assin

g

0

20

40

60

80

100

0.01 0.1 1

(d)

1/Cu = 0.53 1/Cu = 0.83

A (G5)B (G5)

Perc

ent p

assin

g

0

20

40

60

80

100

0.01 0.1 1Particle size (mm)

(e)

Figure 10: Change of particle size distribution after rock cutting (A: condition before cutting, B: condition after cutting) for the fivegarnet types.

Advances in Civil Engineering 11

cutting process. Analysis of the variation in particlesize for all garnet types after the rock cuttingprocess showed that the mean particle sizes werereduced to a similar size of 140.0 μm. Moreover,the uniformity index values also changed to similarvalues (0.51).

Data Availability

.e experimental data (figures and tables) used to supportthe findings of this study are included within the article.

Conflicts of Interest

.e authors declare that they have no conflicts of interest.

Acknowledgments

.is research was supported by a grant (19SCIP-B105148-05) from the Construction Technology Research Programfunded by the Ministry of Land, Infrastructure, andTransport of the Korean government.

References

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[9] J. Vasek, P. Martinee, J. Foldyna, and L. Hlavac, “Influence ofproperties of garnet on cutting process,” in Proceedings of 7thAmerican Waterjet Conference, pp. 375–387, Seattle, WA,USA, August 1993.

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[15] International Organization for Standardization, Preparationof Steel Substrates Prior to the Application of Paints and Re-lated Products—Surface Preparation Methods—Part Two:Abrasives for Blast Cleaning, International Organization forStandardization, Geneva, Switzerland, 1992.

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[18] A.W.Momber and R. Kovacevic, Principles of AbrasiveWaterJet Machining, Springer Science & Business Media, Berlin,Germany, 1998.

[19] G. W. Joo, T. M. Oh, and G. C. Cho, “Influencing factors forabrasive flow rate and abrasive flow quality of abrasive in-jection waterjet systems for tunnel excavation,” Journal ofKorean Tunnelling and Underground Space Association,vol. 16, no. 4, pp. 417–430, 2014, in Korean.

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[21] A. Nag, J. Scucka, P. Hlavacek et al., “Hybrid aluminiummatrix composite AWJ turning using olivine and Bartongarnet,” International Journal of Advanced ManufacturingTechnology, vol. 94, no. 5–8, pp. 2293–2300, 2018.

[22] L. M. Hlavac, I. M. Hlavacova, P. Jandacka et al., “Commi-nution of material particles by water jets—influence of theinner shape of the mixing chamber,” International Journal ofMineral Processing, vol. 95, no. 1–4, pp. 25–29, 2010.

0

100

200

300

400

500

600

G1 G2 G3 G4 G5

Mea

n pa

rtic

le si

ze, D

50 (μ

m)

A

B

A

B

A

B

A

B

A

B

A: Condition before cutting B: Condition a�er cutting

Figure 11: Mean particle size comparison before and after cuttingfor the five garnet types.

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