Compressive Behavior Characteristics of High-Performance Slurry-Inﬁltrated Fiber-Reinforced Cementitious Composites (Sifrccs) Under Uniaxial Compressive Stress

materials

Article Compressive Behavior Characteristics of High-Performance Slurry-Inﬁltrated Fiber-Reinforced Cementitious Composites (SIFRCCs) under Uniaxial Compressive Stress

Seungwon Kim 1 , Seungyeon Han 2, Cheolwoo Park 1,* and Kyong-Ku Yun 2,*

1 Department of Civil Engineering, Kangwon National University, 346 Jungang-ro, Samcheok 25913, Korea; [email protected] 2 KIIT (Kangwon Institute of Inclusive Technology), Kangwon National University, 1 Gangwondaegil, Chuncheon 24341, Korea; [email protected] * Correspondence: [email protected] (C.P.); [email protected] (K.-K.Y.); Tel.: +82-33-570-6515 (C.P.); +82-33-250-6236 (K.-K.Y.) Received: 11 October 2019; Accepted: 19 December 2019; Published: 1 January 2020

Abstract: The compressive stress of concrete is used as a design variable for reinforced concrete structures in design standards. However, as the performance-based design is being used with increasing varieties and strengths of concrete and reinforcement bars, mechanical properties other than the compressive stress of concrete are sometimes used as major design variables. In particular, the evaluation of the mechanical properties of concrete is crucial when using fiber-reinforced concrete. Studies of high volume fractions in established compressive behavior prediction equations are insufficient compared to studies of conventional fiber-reinforced concrete. Furthermore, existing prediction equations for the mechanical properties of high-performance fiber-reinforced cementitious composite and high-strength concrete have limitations in terms of the strength and characteristics of contained fibers (diameter, length, volume fraction) even though the stress-strain relationship is determined by these factors. Therefore, this study developed a high-performance slurry-infiltrated fiber-reinforced cementitious composite that could prevent the fiber ball phenomenon, a disadvantage of conventional fiber-reinforced concrete, and maximize the fiber volume fraction. Then, the behavior characteristics under compressive stress were analyzed for fiber volume fractions of 4%, 5%, and 6%.

Keywords: slurry-infiltrated fiber-reinforced cementitious composite; high-performance fiber-reinforced cementitious composite; fiber volume fraction; compressive stress; stress-strain relationship; filling slurry matrix

1. Introduction Conventional concrete, which is commonly used as a construction material, does not have an adequate resistance level in terms of collisions or explosive loads; thus, concerns are arising about human casualties from the brittle fracturing when reinforced concrete (RC) structures explode [1,2]. With the increasing threat of global terrorism, as well as safety concerns, there is a need for reinforced concrete structures that can withstand the sudden occurrence of dynamic loads like terrorist impact and blast. The energy absorbing capacity of the material plays an important role in developing protective structures. Fiber-reinforced concrete is being popular due to its greater impact resistance properties [3,4]. The possibility of high-performance ﬁber-reinforced cementitious composites (HPFRCCs) satisfying blast resistance design requirements for external blasts more eﬀectively than conventionally reinforced concrete. The HPFRCCs have been expected to improve such drawbacks of concrete and improve

Materials 2020, 13, 159; doi:10.3390/ma13010159 www.mdpi.com/journal/materials Materials 2020, 13, 159 2 of 13 the impact resistance [5]. From previous research, it has been found that the mechanical properties of the concrete can be improved by adding fibers into the mix. Otter and Naaman reported that the addition of fiber in the concrete had a significant effect on the strength and toughness of concrete [6]. The possibility of HPFRCCs satisfying blast resistance design requirements for external blasts more effectively than conventionally reinforced concrete. HPFRCC has been found to be an economical solution against the blast loadings [7]. As the compressive stress of concrete increases, the elastic area becomes larger, and the load-bearing capacity decreases sharply after the manifestation of the maximum strength [8]. The strengthening mechanism of fiber involves the transfer of stress from the matrix to the fiber by interlocking the fibers and matrices when the fiber surface is deformed. The stress is thus shared by the fibers and matrix in tension until the matrix cracks, and then the total stress is progressively transferred to the fibers [9]. These characteristics of high-strength concrete are determined from the size of strain and the shape and rising and falling curves at the manifestation of the maximum strength; these characteristics are also determined by analyzing compressive stress test results of concrete [8]. We examined established prediction equations and found that studies of high volume fractions remained insufficient compared to studies of conventional fiber-reinforced concrete. Furthermore, the mechanical properties of HPFRCCs and high-strength concrete revealed limitations in each prediction equation for the strength and characteristics of contained fibers (diameter, length, volume fraction) even though the stress-strain relationship was determined by these factors [10–17]. Slurry infiltrated fiber concrete (SIFCON) is one of the HPFRCCs. SIFCON is made by distributing short discrete fibers in the mold to the full volume or designed volume fraction and then infiltrated by a fine liquid cement-based slurry or mortar. The fibers can be sprinkled by hand or by using fiber-dispending units for large sections. [15]. Therefore, this study aimed to develop a high-performance slurry-infiltrated fiber-reinforced cementitious composite (SIFRCCs) that could prevent the fiber balling, a disadvantage of conventional fiber-reinforced concrete, and maximize the fiber volume fraction. Most fiber balling occurred during the fiber addition process due to excessive fibers in the mixing of HPFRCCs or fiber-reinforced concrete. Further, the compressive behavior of the developed SIFRCCs was analyzed for fiber volume fractions of 4%, 5%, and 6%. Cylindrical specimens were fabricated for each variable according to the mixing design of high-performance filling slurry, and an experimental study was conducted on the behavior characteristics under compressive stress to examine the mechanical properties of SIFRCCs with a high volume fraction.

2. Mechanical Properties of Fiber-Reinforced Concrete Previously proposed stress-strain relationship equations have been modified based on the research results of Propovics [10] and Sargin [11]. Propovics [10] defined the stress-strain relationship through the rate of decrease of the elastic modulus, as shown in Equation (1), and Sargin [11] reflected the properties of a section through the changes of the components and the strength of concrete, as shown in Equation (2). fc β(ε/ε0) = (1) β fck β 1 + (ε/ε ) − 0 2 fc A(ε/ε0) + B(ε/ε0) = (2) f 2 ck 1 + C(ε/ε0) + D(ε/ε0)

Here, fc is the stress acting on the concrete; fck, the compressive stress of concrete; ε, the compressive strain; ε0, the strain at peak stress; and β, a coefficient that determines the slope and shape of the curve. Furthermore, A~D are coefficients determined by the boundary condition. A review of existing studies’ results revealed that the mechanical properties of concrete, including the maximum strength, elastic modulus, and strain at maximum strength, need to be defined before the stress-strain relationship of concrete under compressive stress can be defined. Of these, the elastic modulus is an important design variable for a concrete structure and has a significant effect on the Materials 2020, 13, 159 3 of 13 stress-strain relationship. The elastic modulus tends to increase with the strength of the matrix and is proportional to the square root or cube root of the compressive stress of the matrix [1]. The strain at peak stress is usually fixed at 0.002 or 0.0022 for conventional concrete (average strength) [8]. However, the measured compressive stress of high-strength concrete has been reported to have a value exceeding 0.002, and many studies have proposed prediction equations of the strain at peak stress that reflect this value [8]. Steel fibers are mixed for increasing the tensile strength of concrete; however, they also change the mechanical properties in compressive stress measurements [8]. The mixing of steel fibers, generally, increases the compressive stress and the strain and elastic modulus at peak stress [8]. The strengthening effect of fibers is realized as an algebraic sum or a magnification of the fiber reinforcing effects for the elastic modulus of the matrix alone [8,17]. The prediction equations for the stress-strain relationship that reflect changes in the mechanical properties with the mixing of fibers revealed that the effect of fibers was reflected only for conventional concrete with no fiber reinforcement [8–17]. Furthermore, a prediction equation for estimating the elastic modulus of concrete is generally proposed on the basis of an empirical formula using regression analysis of experimental data of various ranges. The elastic modulus of conventional concrete can be estimated using the compressive stress, unit weight of concrete, etc. [8–11,15]. However, HPFRCCs, such as SIFRCCs, contain steel fibers and other materials in conventional concrete, depending on the mixing conditions, and these added materials can greatly affect the estimation of the elastic modulus. For HPFRCCs, such as SIFRCCs, the volume fraction of each composite material can be calculated according to the composite theory [18–21]. We examined established prediction equations and found that studies of high volume fractions remained insufficient compared to studies of conventional fiber-reinforced concrete. Therefore, we conducted an experimental study of the behavior characteristics under compressive stress to examine the mechanical properties of SIFRCCs with a high volume fraction of fibers.

3. Experimental

3.1. Materials

3.1.1. Cement This study used type 1 ordinary Portland cement (OPC), the physical and chemical properties of which are listed in Table1[21].

Table 1. Physical and chemical properties of the used cement.

Physical Properties Setting Time (min) Speciﬁc Gravity Blaine (cm2/g) Stability (%) Loss on Ignition (%) Initial Final 3.15 3400 0.10 230 410 2.58 Chemical Composition (%, mass)

SiO2 CaO MgO SO3 Al2O3 21.95 60.12 3.32 2.11 6.59

3.1.2. Silica Fume To realize high-performance and high-strength ﬁlling slurry, this study used silica fume (Elkem Korea, South Korea), the physical and chemical properties of which are listed in Table2. Materials 2020, 13, 159 4 of 13

Table 2. Physical and chemical properties of the used silica fume.

Physical Properties Speciﬁc Gravity Blaine (cm2/g) Materials 2020, 12, x FOR PEER REVIEW2.10 200,000 4 of 13 Chemical Composition (%, mass) Chemical Composition (%, mass) SiO2 CaO MgO SO3 Al2O3 SiO2 CaO MgO SO3 Al2O3 96.00 0.38 0.10 - 0.25 96.00 0.38 0.10 - 0.25

3.1.3. Aggregates 3.1.3. Aggregates To improve the filling performance of the slurry and reduce material separation, fine aggregates To improve the filling performance of the slurry and reduce material separation, fine aggregates with diameters of 0.5 mm or smaller were used. No coarse aggregates were used. with diameters of 0.5 mm or smaller were used. No coarse aggregates were used. 3.1.4. Admixture 3.1.4. Admixture To improve the filling performance of the slurry, a polycarboxylic acid, the high-performance To improve the filling performance of the slurry, a polycarboxylic acid, the high-performance water reducing agent with high dispersion performance, was used. The admixture used in this water reducing agent with high dispersion performance, was used. The admixture used in this experiment had high strength and high flow characteristics, as well as excellent unit water quantity experiment had high strength and high flow characteristics, as well as excellent unit water quantity reduction property and material separation resistance. Table3 lists the characteristics of the used reduction property and material separation resistance. Table 3 lists the characteristics of the used high-performance water-reducing agent. high-performance water-reducing agent. Table 3. Characteristics of the used high-performance water reducing agent. Table 3. Characteristics of the used high-performance water reducing agent. Principal Component Specific Gravity pH Alkali Content (%) Chloride Content (%) Principal Component Specific Gravity pH Alkali Content (%) Chloride Content (%) Polycarboxylate 1.05 0.05 5.0 1.5 <0.01 <0.01 Polycarboxylate 1.05± ± 0.05 5.0± ± 1.5 <0.01 <0.01 3.1.5. Steel Fibers 3.1.5. Steel Fibers Double-hook steel fibers for conventional concrete with a diameter of 0.75 mm, length of 60 mm, and anDouble aspect-hook ratio steel of 80 fibers were for used. conventional These steel concrete fibers had with a density a diameter of 7.8 of g 0.75/cm3 mm,and alength tensile of strength 60 mm, 3 ofand 1200 an aspect MPa. Figureratio of1 80shows were the used. shape These of thesteel used fibers steel ha fibers.d a density of 7.8 g/cm and a tensile strength of 1200 MPa. Figure 1 shows the shape of the used steel fibers.

Figure 1. TheThe s shapehape of the used steel fiber fiber.. 3.2. SIFRCCs 3.2. SIFRCCs SIFRCCs that can accommodate a high volume fraction of steel fibers were developed in this study SIFRCCs that can accommodate a high volume fraction of steel fibers were developed in this to prevent the fiber ball phenomenon, the main disadvantage of conventional fiber-reinforced concrete, study to prevent the fiber ball phenomenon, the main disadvantage of conventional fiber-reinforced and maximize the fiber volume fraction [22–26]. Unlike conventional fiber-reinforced concrete, SIFRCCs concrete, and maximize the fiber volume fraction [22–26]. Unlike conventional fiber-reinforced is a type of HPFRCC that can contain a high volume of steel fibers. It is fabricated by filling steel concrete, SIFRCCs is a type of HPFRCC that can contain a high volume of steel fibers. It is fabricated fibers by dispersing them and then filling high-performance slurry. SIFRCCs affords the advantages of by filling steel fibers by dispersing them and then filling high-performance slurry. SIFRCCs affords preventing the fiber ball phenomenon and allowing a high fiber volume fraction [23,24,26]. the advantages of preventing the fiber ball phenomenon and allowing a high fiber volume fraction [23,24,26].

3.3. Mixing and Fabrication of Specimens OPC, fine aggregate, water, superplasticizer, and additional silica fume were mixed to prepare a slurry. The water binder ratio was 0.35. First, the cylindrical mold with diameters of 100 mm and heights of 200 mm were filled with steel fiber with respect to volume fraction. Randomly sprinkled

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3.3. Mixing and Fabrication of Specimens OPC, fine aggregate, water, superplasticizer, and additional silica fume were mixed to prepare Materials 2020, 12, x FOR PEER REVIEW 5 of 13 a slurry. The water binder ratio was 0.35. First, the cylindrical mold with diameters of 100 mm and heightssteel fibers of 200in the mm mo wereld shoul filledd not with overfill steel fiber the depth with respectof mold to and volume level up fraction. as much Randomly as possible sprinkled [25,26]. steelThe s fiberslurry inas theprepared mold should after mixing not overfill the contents the depth was of moldpoured and until level no up more as much bubbles as possible were seen [25,26 to]. Theensure slurry infiltration as prepared of slur afterry mixinginto the the fibers contents because was the poured void has until negative no more effects bubbles on were the strength seen to ensure of the infiltrationconcrete. The of slurryamount into of thehigh fibers-performance because the water void reducing has negative agent e wasffects set on to the 2.5% strength of the ofbinder the concrete. weight The[25,26 amount]. To reduce of high-performance the material separation water reducing and achieve agent the was required set to 2.5%strength, of the fine binder aggregates weight (50% [25,26 of]. Tobinder reduce weight) the material and silica separation fume (15% and of achieve cement theweight) required were strength, added [2 fine5,26 aggregates]. Table 4 shows (50% ofSIFRCCs binder weight)mixing. and silica fume (15% of cement weight) were added [25,26]. Table4 shows SIFRCCs mixing.

Table 4. 4. SIFRCCs ( (high-performancehigh-performance slurryslurry-inﬁltrated-infiltrated fiberﬁber-reinforced-reinforced cementitious composite)composite) mixingmixing proportion.proportion. 3 W/B Unit Material Quantity (kg/m ) 3 Fiber W/B Unit Material Quantity (kg/m ) (% vol.) (Water-Binder Fiber Fine Silica Steel (WaterRatio)-Binder Water Cement Fine Silica Superplasticizer Steel (% vol.) Water Cement Aggregate Fume Superplasticizer Fiber Ratio) Aggregate Fume Fiber 4% 312 4% 312 5% 0.35 407.4 962.8 566.4 169.9 28.3 390 5% 0.35 407.4 962.8 566.4 169.9 28.3 390 6% 468 6% 468

To analyzeanalyze thethe compressivecompressive behavior characteristics of the SIFRCCs for ﬁberfiber volumevolume fractions of 4% (312 kg per cubic meters),meters), 5% (390 kg per cubic meters) meters),, and 6% (468 kg per cubic meters) meters),, cylindrical specimens with diameters of 100 mm and heights of 200 mm w wereere fabricated with respect to the mixing ratio of each variable, as listed inin TableTable4 4.. Figure Figure2 2shows shows the the fabrication fabrication of of specimens. specimens.

(a) Placing steel fibers (b) Filling slurry

Figure 2. Fabrication of specimens specimens.. 3.4. Compression Test of SIFRCCs 3.4. Compression Test of SIFRCCs A universal testing machine (UTM) was used to study the compressive behavior of specimens. A universal testing machine (UTM) was used to study the compressive behavior of specimens. The specimens were placed centrally between the two compression plates, such that the center of The specimens were placed centrally between the two compression plates, such that the center of moving head was vertically above the center of specimen, as shown in Figure3, then the load was moving head was vertically above the center of specimen, as shown in Figure 3, then the load was applied on the specimens by moving the movable head. The load and corresponding contraction applied on the specimens by moving the movable head. The load and corresponding contraction were measured at different intervals. A review of existing studies on the mechanical properties of were measured at different intervals. A review of existing studies on the mechanical properties of high-strength concrete and fiber-reinforced concrete found that there was an applicable strength limit high-strength concrete and fiber-reinforced concrete found that there was an applicable strength limit for each prediction equation [8,10–17]. If this strength limit is exceeded, structural design problems for each prediction equation [8,10–17]. If this strength limit is exceeded, structural design problems can be generated by estimation of the unsafe side. Furthermore, major mechanical properties that can be generated by estimation of the unsafe side. Furthermore, major mechanical properties that define the stress-strain relationship have been found to be determined by the compressive stress of define the stress-strain relationship have been found to be determined by the compressive stress of concrete and the fiber volume and shape [8,10–17]. Therefore, this study also used the high fiber concrete and the fiber volume and shape [8,10–17]. Therefore, this study also used the high fiber volume fraction as a major variable. In addition, an experimental study on the elastic modulus under volume fraction as a major variable. In addition, an experimental study on the elastic modulus under compressive stress, strain at peak strength, and Poisson’s ratio was performed. Figure3 shows the compressive stress, strain at peak strength, and Poisson’s ratio was performed. Figure 3 shows the experimental setup for compression tests. Compressive strength, modulus of elasticity, and Poisson’s ratio were determined according to ASTM (American Society for Testing and Materials) specifications (ASTM C873 [27] and ASTM C469 [28], respectively).

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experimental setup for compression tests. Compressive strength, modulus of elasticity, and Poisson’s ratio were determined according to ASTM (American Society for Testing and Materials) speciﬁcations Materials(ASTM 2020, C87312, x FOR [27] PEER and ASTM REVIEW C469 [28], respectively). 6 of 13

FigureFigure 3. Experimental 3. Experimental setup setup for for compressive compressive stressstress and and elastic elastic modulus modulus tests. tests. 4. Experiment Results and Analysis 4. Experiment Results and Analysis 4.1. Compressive Stress 4.1. Compressive Stress Figure4 shows the compressive stress experiment results with respect to the fiber volume fraction forFigure SIFRCCs. 4 shows For fiber the volume compressive fraction stress of 6%, experiment the average compressive results with stress respect was ~83 to MPa. the fiber For fiber volume fractionvolume for SIFRCCs. fraction of 5%,For thefiber average volume compressive fraction of stress 6%, wasthe average ~75 MPa; compressive this was ~10% stress lower was than ~83 that MPa. For fiberfor the volume fiber volume fraction fraction of 5%, of the 6%. average Furthermore, compressive for the fiberstress volume was ~75 fraction MPa; of this 4%, was the average~10% lower compressive stress was ~66 MPa; this was ~12% and ~21% lower than that for fiber volume fractions of than that for the fiber volume fraction of 6%. Furthermore, for the fiber volume fraction of 4%, the 5% and 6%, respectively. The compressive stress tended to increase in proportion to the fiber volume average compressive stress was ~66 MPa; this was ~12% and ~21% lower than that for fiber volume fraction. It seemed that the increased amount of steel fibers with increasing fiber volume fraction Materials 2020, 12, x FOR PEER REVIEW 7 of 13 fractionsproduced of 5% a restrainingand 6%, respectively. effect on the specimen The compressive itself, thereby stress affecting tended the to increase increase in compressive in proportion stress. to the fiber volume fraction. It seemed that the increased amount of steel fibers with increasing fiber volume fraction produced a restraining effect on the specimen itself, thereby affecting the increase in compressive stress. Many studies have shown that the mixing of fibers in fiber-reinforced concrete generally does not influence the compressive stress of the matrix itself. The compression experiment results of Shah and Rangan [29] reported that although the compressive strain at fracture increased significantly when fibers were mixed, the compressive stress did not clearly improve. As the fiber volume fraction increased, the compressive stress gradually increased at first; however, above a certain fraction (4% or more), this tendency changed somewhat. Within a certain mixing level of fibers, the compressive stress increased because the resistance was increased by crack suppression; however, above this level, the compressive stress decreased owing to additional defects that appeared with the improvement effect. The compressive fracture of conventional fiber-reinforced concrete was caused not by the yield Figure 4. Compressive stress experiment results with respect to fiber volume fraction. or drawing Figureof fibers 4. Compressive but by the fracturestress experiment of a matrix results with with relatively respect to low fiber strength volume beforefraction fibers. play a structuralMany role. studiesAccordingly, have shown the compressive that the mixing stress of fiberswas considered in fiber-reinforced to improve concrete with generally the fiber does volume 4.2.fraction Elasticnot influenceas Msufficientodulus the compressiveadhesion occurred stress of between the matrix the itself. fiber The and compression the high-strength experiment cementitious results of Shah matrix usedand in this Rangan study. [29 Micro] reported-cracks that we althoughre caused the in compressive conventional strain concrete at fracture and increasedfiber-reinforced significantly concrete The elastic modulus experiment results indicated that the elastic modulus showed insignificant by thewhen interface fibers wereproperties mixed, of the the compressive coarse aggregates stress did notand clearly the cement improve. paste. As the For fiber SIFRCCs, volume a fraction composite differencesincreased, with the the compressive fiber volume stress fraction; gradually its increased value atremained first; however, ~28 GPa. above This a certain was fractionin contrast (4% orto the made of fine particles with no coarse aggregates, cracks at the interface could be reduced, and the compressivemore), this stress tendency experiment changed somewhat. results. However, Within a certain as shown mixing in level Figure of fibers, 5, the the compressivestrain at peak stress stress adhesion performance of the filling slurry and steel fibers could be improved. Therefore, the effect of showed significant differences with increasing fiber volume fraction. These results implied that the steel fibers, such as the fiber volume fraction, rather than the effect of the compressive stress caused energy absorption capacity improved as the fiber volume fraction increased. by matrix fracture was considered to be reflected.

Figure 5. Strain at peak stress.

Figure 6 shows the elastic modulus estimation curve using the compressive stress as presented in KCI 2012 [30] and ACI 318 [31] and the compressive stress and elastic modulus with respect to the fiber volume fraction. The elastic modulus with respect to the fiber volume fraction measured in this study satisfied the lower limit of the elastic modulus estimation curve. This seemed to be because the elastic modulus prediction equation suggested in KCI 2012 [30] and ACI 318 [31] estimated the elastic modulus using only the compressive stress and unit weight.

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increased because the resistance was increased by crack suppression; however, above this level, the compressive stress decreased owing to additional defects that appeared with the improvement effect. The compressive fracture of conventional fiber-reinforced concrete was caused not by the yield or drawing of fibers but by the fracture of a matrix with relatively low strength before fibers play a structural role. Accordingly, the compressive stress was considered to improve with the fiber volume fraction as sufficient adhesion occurred between the fiber and the high-strength cementitious matrix used in this study. Micro-cracks were caused in conventional concrete and fiber-reinforced concrete by the interface properties of the coarse aggregates and the cement paste. For SIFRCCs, a composite made of fine particles with no coarse aggregates, cracks at the interface could be reduced, and the adhesion performance of the filling slurry and steel fibers could be improved. Therefore, the effect of steel fibers, such asFigure the fiber 4. Compressive volume fraction, stress rather experiment than the results effect with of the respect compressive to fiber volume stress caused fraction by. matrix fracture was considered to be reflected. 4.2. Elastic Modulus 4.2. Elastic Modulus The elastic modulus experiment results indicated that the elastic modulus showed insignificant differencesThe with elastic the modulus fiber volume experiment fraction; results its indicated value remained that the elastic ~28 GPa. modulus This showed was in insignificantcontrast to the compressivedifferences stress with experiment the fiber volume results. fraction; However, its value as remained shown in~28 Figure GPa. This5, the was strain in contrast at peak to the stress showedcompressive significant stress differences experiment with results. increasing However, fiber as shown volume in Figure fraction.5, the These strain atresults peak stressimpl showedied that the significant differences with increasing fiber volume fraction. These results implied that the energy energy absorption capacity improved as the fiber volume fraction increased. absorption capacity improved as the fiber volume fraction increased.

Figure 5. Strain at peak stress. Figure 5. Strain at peak stress. Figure6 shows the elastic modulus estimation curve using the compressive stress as presented in FigureKCI 2012 6 shows [30] and the ACI elastic 318 [ 31modulus] and the estimation compressive curve stress using and elasticthe compressive modulus with stress respect as presented to the fiber volume fraction. The elastic modulus with respect to the fiber volume fraction measured in this in KCI 2012 [30] and ACI 318 [31] and the compressive stress and elastic modulus with respect to the study satisfied the lower limit of the elastic modulus estimation curve. This seemed to be because the fiber volume fraction. The elastic modulus with respect to the fiber volume fraction measured in this elastic modulus prediction equation suggested in KCI 2012 [30] and ACI 318 [31] estimated the elastic studymodulus satisfied using the lower only the limit compressive of the elastic stress modulus and unit estimation weight. curve. This seemed to be because the elastic modulus prediction equation suggested in KCI 2012 [30] and ACI 318 [31] estimated the elastic modulus using only the compressive stress and unit weight.

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Figure 6. Correlations of compressive stress and elastic modulus between KCI 2012 [30 ] and ACI 318 Figure 6. [31]. Correlations of compressive stress and elastic modulus between KCI 2012 [30] and ACI Figure318 [31 6.]. Correlations of compressive stress and elastic modulus between KCI 2012 [30] and ACI 318 [31]. 4.3. Poisson’s Poisson’s R Ratioatio 4.3. Poisson’sPoisson’s R atioratio ratio of of conventional conventional concrete concrete is known is known to be to 0.16 be– 0.16–0.20;0.20; that of that fiber of-reinforced fiber-reinforced high- strength concrete is expected to be higher. Figure 7 shows Poisson’s ratio with respect to the fiber high-strengthPoisson’s ratio concrete of conventional is expected concrete to be higher. is known Figure to be7 shows0.16–0.20; Poisson’s that of ratiofiber-reinforced with respect high to- volume fraction of SIFRCCs. As with the elastic modulus experiment results, Poisson’s ratio showed strengththe fiber concrete volume fractionis expected of SIFRCCs. to be higher. As with Figure the elastic7 shows modulus Poisson’s experiment ratio with results, respect Poisson’s to the fiber ratio insignificant differences with the fiber volume fraction. The average Poisson’s ratio was ~0.3; this was volumeshowed fraction insignificant of SIFRCCs. differences As with with the the elastic fiber volume modulus fraction. experiment The average results, Poisson’sPoisson’s ratioratio wasshowed ~0.3; much higher than that of conventional concrete and was similar to that of steel. This was because the insignificantthis was much differences higher than with that the offiber conventional volume fraction. concrete The and average was Poisson’s similar to ratio that wa of steel.s ~0.3; Thisthis wa wass high-strength filling slurry matrix improved the interface adhesion between the matrix and the steel mucbecauseh higher the high-strengththan that of conventional filling slurry concrete matrix improvedand was similar the interface to that of adhesion steel. This between was because the matrix the fibers. SIFRCCs showed homogeneous behavior owing to its high fiber volume fraction; this could highand- thestrength steel fibers.filling SIFRCCsslurry matrix showed improve homogeneousd the interface behavior adhesion owing between to its high the fiber matrix volume and the fraction; steel suppress brittle fracture, a disadvantage of conventional concrete, and provide sufficient energy fibers.this could SIFRCCs suppress show brittleed homogeneous fracture, a disadvantage behavior owing of conventional to its high fiber concrete, volume and fraction; provide this suffi couldcient absorption capacity. suppressenergy absorption brittle fracture, capacity. a disadvantage of conventional concrete, and provide sufficient energy absorption capacity.

Figure 7. Correlation between compressive stress and Poisson’s ratio with respect to fiber Figurevolume 7. fraction. Correlation between compressive stress and Poisson’s ratio with respect to fiber volume fraction. 4.4. Stress-StrainFigure 7. Correlation Relationship between compressive stress and Poisson’s ratio with respect to fiber volume fraction. 4.4. StressThe stress-strain-Strain Relationship experiment results of SIFRCCs with respect to the fiber volume fraction showed 4.4.relatively StressThe stress- highStrain- strainstrain Relationship valuesexperiment for all results variables of SIFRCCs compared with to that respect of conventional to the fiber volume concrete, fraction with a showed strain of relatively0.008 or higher high strain at peak values stress. for Figure all variables8 shows compared the results to forthat the of fiberconventional volume fractionconcrete, of with 6%. a In strain this The stress-strain experiment results of SIFRCCs with respect to the fiber volume fraction showed ofcase, 0.008 the or post-peak higher at behavior peak stress. exhibited Figure a 8 strain shows hardening the results behavior. for the fiber Even volume after the fraction elastic of section 6%. In in this the relatively high strain values for all variables compared to that of conventional concrete, with a strain case,stress-strain the post curve,-peak behavior it showed exhibited ductile behavior a strain hardening characteristics behavior. like those Even of after metal. the Thiselastic showed section that in of 0.008 or higher at peak stress. Figure 8 shows the results for the fiber volume fraction of 6%. In this theasin stress the experimental-strain curve, resultsit showed obtained ductile with behavior high Poisson’s characteristics ratio, like the specimenthose of metal. exhibited This su showfficiented case, the post-peak behavior exhibited a strain hardening behavior. Even after the elastic section in thatenergy as absorption in the experimental capacity even results after obtainedthe peak stress with owing high Poisson’s to its high ratio, fiber volume the specimen fraction. exhibit Figureed9 the stress-strain curve, it showed ductile behavior characteristics like those of metal. This showed sufficientshows the energy results absorption for the fiber capacity volume even fraction after of the 5%. peak As stress in the owing previous to its case, high the fiber post-peak volume behavior fraction. that as in the experimental results obtained with high Poisson’s ratio, the specimen exhibited sufficient energy absorption capacity even after the peak stress owing to its high fiber volume fraction.

Materials 2020, 12, x FOR PEER REVIEW 9 of 13 Materials 2020, 12, x FOR PEER REVIEW 9 of 13 MaterialsFigure 92020 shows, 12, x theFOR results PEER REVIEW for the fiber volume fraction of 5%. As in the previous case, the post-9peak of 13 FigureMaterialsbehavior 92020 shows exhibited, 12, x theFOR resultsa PEER strain REVIEW for hardening the fiber behavior volume fractionin the plastic of 5%. section As in theof the previous stress- straincase, the curve. post Figure-9peak of 13 behaviorFigure10 showsMaterials 9 showsexhibited the2020 results, 13the, 159 aresults strainfor the forhardening fiber the volumefiber behavior volume fraction fractionin theof 4%.plastic of Unlike 5%. section As in in the ofthe theprevious previous stress- twostrain case, cases, thecurve. post 9the of Figure 13- postpeak- 1behaviorFigurepeak0 shows behavior 9 shows exhibitedthe results exhibited the aresults strainfor athe strain forhardening fiber the hardening volumefiber behavior volume fraction behavior fractionin theof to4%. plastic some of Unlike 5%. degreesection As in in the intheof the theprevious previous plastic stress - twosection straincase, cases, thecurve. of postthe the Figurestress -postpeak-- peak1behavior0 shows behavior exhibitedthe results exhibited a strainfor athe strain hardening fiber hardening volume behavior fraction behavior in theof to 4%. plastic some Unlike degreesection in thein of the theprevious plastic stress -sectiontwostrain cases, curve. of the the Figurestress post- strainexhibited curve and a strain a strain hardening-softening behavior behavior in the plastic in the section end. of This the stress-strain was because curve. the Figure low fiber10 shows volume peak10 shows behavior the results exhibited for athe strain fiber hardening volume fraction behavior of to 4%. some Unlike degree in thein the previous plastic sectiontwo cases, of the the stress post- strainfractionthe curve resultsof 4% and forproduced the a strain fiber only volume-softening a small fraction behavior amount of 4%. Unlikeof in fiber the in end.reinforcement the previous This was two thatbecause cases, decreased the the post-peak low the fiber resistance behavior volum toe peak behavior exhibited a strain hardening behavior to some degree in the plastic section of the stress- frastrainthection expansionexhibited curve of 4% and a producedforce strain a strainin hardening the only- softeningvertical a behaviorsmall direction behavior amount to some of of inthe degree fiber the specimen end.reinforcement in the Thisplastic axis. was Figure section thatbecause 1decreased of1 theshows the stress-strain lowthe the results fiberresistance curve volum for the toe thefrastrainstressction expansionand -strain curve of a strain-softening4% curve and forceproduced a with instrain the respect only behavior-verticalsoftening a smallto fiberdirection in behavior theamount volume end. of This ofthe in f raction. fiber thespecimen was end.reinforcement because This axis. the was Figure low thatbecause fiber 1 decreased1 volumeshows the lowthe fraction the results fiberresistance of volum 4%for the toe stressthefraction expansionproduced-strain of 4% curve onlyforceproduced awith in small the respect only amountvertical a tosmall of fiberdirection fiber amount volume reinforcement of ofthe f raction.fiber specimen thatreinforcement decreased axis. Figure the that resistance 1decreased1 shows to the the the results expansion resistance for the to stressthe expansionforce-strain in thecurve force vertical with in the direction respect vertical to of fiber thedirection specimen volume of the axis.fraction. specimen Figure 11 axis. shows Figure the results 11 shows for the the stress-strain results for the stresscurve-strain with curve respect with to respect fiber volume to fiber fraction. volume fraction.

Figure 8. Stress-strain curve for the fiber volume fraction of 6%. Figure 8. Stress-strain curve for the fiber volume fraction of 6%. Figure 8. Stress-strain curve for the fiber volume fraction of 6%. Figure 8. Stress-strain curve for the ﬁber volume fraction of 6%. Figure 8. Stress-strain curve for the fiber volume fraction of 6%.

Figure 9. Stress-strain curve for the fiber volume fraction of 5%. Figure 9. Stress-strain curve for the fiber volume fraction of 5%. Figure 9. Stress-strain curve for the ﬁber volume fraction of 5%. Figure 9. Stress-strain curve for the fiber volume fraction of 5%. Figure 9. Stress-strain curve for the fiber volume fraction of 5%.

Figure 10. Stress-strain curve for the fiber volume fraction of 4%. Figure 10. Stress-strain curve for the ﬁber volume fraction of 4%. Figure 10. Stress-strain curve for the fiber volume fraction of 4%. Figure 10. Stress-strain curve for the fiber volume fraction of 4%. Figure 10. Stress-strain curve for the fiber volume fraction of 4%.

Figure 11. The stress-strain curve with respect to ﬁber volume fraction. Figure 11. The stress-strain curve with respect to fiber volume fraction. Figure 11. The stress-strain curve with respect to fiber volume fraction . Figure 11. The stress-strain curve with respect to fiber volume fraction . Figure 11. The stress-strain curve with respect to fiber volume fraction.

Materials 2020, 13, 159 10 of 13

Table5 lists the experimental results for maximum strength, elastic modulus, and strain at peak stress; these are generally used to define the stress-strain curve of the SIFRCCs. The relative error of compressive strength in the average value of the experimental results for each variable with respect to the fiber volume fraction was lower than 10%. The stress-strain experiment results revealed insignificant differences in the elastic modulus with the fiber reinforcement amount. This suggested that fiber reinforcement had no effect on the elastic modulus owing to the use of the same filling slurry matrix regardless of the fiber volume fraction.

Table 5. Experimental results for compressive behavior characteristics with respect to the ﬁber volume fraction.

6 Variables (Vf) f (MPa) Ec (MPa) εco ( 10 ) ν c × − 69.9 28,430 5.74 0.289 70.6 25,098 16.08 0.233 4% 62.8 31,826 5.86 0.333 64.0 27,667 7.40 0.336 60.2 24,493 6.67 0.239 Average 65.5 27,503 8.37 0.286 73.7 29,853 8.16 0.360 68.5 26,548 11.52 0.231 5% 79.5 26,617 21.36 0.343 75.4 24,804 3.84 0.305 75.4 28,481 29.16 0.358 Average 74.5 27,261 14.81 0.319 88.0 28,551 18.18 0.237 78.0 26,967 15.02 0.253 6% 86.4 24,886 27.38 0.267 85.9 32,182 38.02 0.373 76.3 29,071 13.20 0.367 Average 82.9 28,331 22.36 0.300

However, the strain at peak stress also increased with the fiber volume fraction. In contrast to the above-mentioned compressive stress experiment results with respect to the fiber volume fraction, the compressive stress for the fiber volume fraction of 6% increased slightly by approximately 1.11 and 1.27 times compared to those for fiber volume fractions of 5% and 4%, respectively. However, the strain at peak stress for the fiber volume fraction of 6% increased significantly by 1.51 and 2.67 times compared to those for fiber volume fractions of 5% and 4%, respectively. The compressive fracture patterns obtained by performing the compressive behavior experiment, shown in Figure 12, indicated that the strain increased further as axial cracks in the specimen were restrained. This characteristic of the strain increase rate was considered to be caused by the restraining effect of the high fiber volume fraction, in which many steel fibers resisted the expansion force generated vertically to the specimen axis and thereby caused longitudinal split cracks. Moreover, the steel fiber reinforcement effect served to increase the size of the peak stress and the strain at peak stress in all compressive stress areas. Materials 2020, 13, 159 11 of 13 Materials 2020, 12, x FOR PEER REVIEW 11 of 13

(a) 4% (b) 5% (c) 6%

Figure 12. CompressiveCompressive fracture pattern with respect to fiberfiber volume fraction.fraction. 5. Conclusions 5. Conclusions A high-performance SIFRCCs that can prevent the fiber ball phenomenon, a disadvantage of A high-performance SIFRCCs that can prevent the fiber ball phenomenon, a disadvantage of conventional fiber-reinforced concrete, and maximize the fiber volume fraction was developed, and conventional fiber-reinforced concrete, and maximize the fiber volume fraction was developed, and the compressive behavior characteristics with respect to the fiber volume fraction were analyzed. The the compressive behavior characteristics with respect to the fiber volume fraction were analyzed. The following conclusions were derived from this study. following conclusions were derived from this study. (1) TheThe static static compressive compressive behavior behavior characteristics characteristics with with respect respect to to the fiberfiber volume fraction of SIFRCCsSIFRCCs showed showed that that the the compressive compressive stress stress increased increased in in proportion to the incrincreasingeasing fiberfiber volumevolume fraction. fraction. The The micro micro-cracks-cracks in in conventional conventional concrete concrete and and fiber fiber-reinforced-reinforced concrete werewere causedcaused by by the the interface interface properties properties of coarse aggregates and cement paste. By contrast, SIFRCCs couldcould reduce reduce cracks cracks at at the the interface interface because because it itwa wass a composite a composite made made of offine fine particles particles with with no coarseno coarse aggregates. aggregates. Furthermore, Furthermore, the the filling filling slurry slurry and and steel steel fibers fibers improve improvedd the the adhesion performanceperformance and and reflect reflecteded the the effects effects of the fiber fiber volume fraction and steel fibers fibers instead of the compressivecompressive stress stress characteristics characteristics caused caused by matrixby matrix fracture. fracture. In addition, In addition, the high the volume high volumefraction fractionof the steel of the fibers steel generated fibers generated a restraining a restraining effect in effect the compressive in the compressive stress test stress specimen, test specimen, thereby therebyaffecting affecting the increase the increase in compressive in compressive stress. stress. (2) TheThe elastic modulus experimentexperiment resultsresults showed showed that that the the elastic elastic modulus modulus did did not not increase increase with with the theincreasing increasing fiber fiber volume volume fraction, fraction, unlike unlike the the compressive compressive stress stress experiment experiment results. results. However, However, for forstrain strain under under compressive compressive stress, stress, the increased the increase rated of rate the strain of the at strain peak stress at peak showed stress a showed difference a differenof up toce 2.7 of times up to depending 2.7 times on depending the fiber volume on the fraction. fiber volume This means fraction. that This with means increasing that withfiber increasingvolume fraction, fiber volume the post-peak fraction, behavior the post after-peak peak behavior stress showed after peak a strain stress hardening showedbehavior, a strain hardeningimplying that behavior, the energy implying absorption that the capacity energy improved absorption at acapacity higher fiberimprove volumed at fraction.a higher fiber (3) volumeThe characteristics fraction. of the strain increase rate were attributed to the restraining effect of the high (3) Thefiber characteristics volume fraction of the in which strain many increase steel rate fibers were resisted attributed the expansion to the restraining force generated effect of vertically the high fiberto the volume specimen fraction axis thatin which caused many longitudinal steel fibers split resist cracks.ed the Furthermore, expansion force the generated reinforcing vertically effect of tosteel the fibers specimen was consideredaxis that ca touse increased longitudinal the peak split stress cracks. and strainFurthermore, at peak stressthe reinforcing in all compressive effect of steelstress fibers areas. was considered to increase the peak stress and strain at peak stress in all compressive (4) stressPoisson’s areas. ratio experiment results showed insignificant differences in the elastic modulus with (4) Poisson’sthe fiber volumeratio experiment fraction. results Similarly, showed Poisson’s insignificant ratio showed differences insignificant in the elastic differences modulus with with the thefiber fiber volume volume fraction. fraction. However, Similarly, Poisson’s Poisson’s ratio ratio was showed ~0.3 for insignificant every variable; differences this was with similar the fiberto that volume of steel. fraction. This was However, caused Poisson’s by the improved ratio was interface ~0.3 for adhesion every variable; performance this wa betweens similar the to thathigh-strength of steel. This filling was slurrycaused matrix by the andimproved the steel interface fibers. adhesion Therefore, performance the SIFRCCs between with a high the high fiber- strengthvolume fraction filling slurry could matrix suppress and brittle the steel fracture, fibers. a disadvantage Therefore, the of SIFRCCs conventional with concrete, a high fiber and volumeexhibit sufractionfficient could energy suppress absorption brittle capacity fracture, owing a disadvantage to its homogeneity. of conventional concrete, and exhibit sufficient energy absorption capacity owing to its homogeneity.

Author Contributions: Conceptualization, S.K., C.P., and K.-K.Y.; methodology, S.K., C.P., and K.-K.Y.; writing—original draft preparation, S.K., S.H., C.P., and K.-K.Y.

Materials 2020, 13, 159 12 of 13

Author Contributions: Conceptualization, S.K., C.P., and K.-K.Y.; methodology, S.K., C.P., and K.-K.Y.; writing—original draft preparation, S.K., S.H., C.P., and K.-K.Y. All authors have read and agreed to the published version of the manuscript. Acknowledgments: This research was supported by a grant (19SCIP-B146646-02) from Construction Technology Research Project and was conducted under research project “Development of High-Performance Concrete Pavement Maintenance Technology to Extend Roadway Life (19TLRP-B146707-01)” funded by the Ministry of Land, Infrastructure, and Transport (MOLIT) and the Korea Agency for Infrastructure Technology Advancement (KAIA). Conﬂicts of Interest: The authors declare no conﬂict of interest.

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