Effect of calcium sulfoaluminate-based expansive agent on rate dependent pullout behavior of straight steel fiber embedded in UHPC

Cement and Concrete Research 122 (2019) 196–211

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Effect of calcium sulfoaluminate-based expansive agent on rate dependent pullout behavior of straight steel fiber embedded in UHPC

T

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Doo-Yeol Yoo, Booki Chun, Jae-Jin Kim

Department of Architectural Engineering, Hanyang University, 222 Wangsimni-ro, Seongdong-gu, Seoul 04763, Republic of Korea

A R T I C LE I N FO

A B S T R A C T

Keywords: Ultra-high-performance concrete Calcium sulfo-aluminate expansive agent Straight steel fiber Pullout resistance Loading rate effect

To use a calcium sulfoaluminate (CSA) expansive agent (EA), beneficial for volume stability of concrete, its effect on the pullout behavior of straight steel fiber in ultra-high-performance concrete (UHPC) needs to be analyzed under various loads, from static to impact (0.018 to 1244 mm/s). Adding CSA EA enhanced the static pullout resistance of only inclined fibers, whereas the average bond strengths and pullout energies of both the aligned and inclined fibers improved by it under impact loading. Thus, adding the CSA EA is more effective in enhancing the dynamic pullout resistance. The fiber pullout resistance improved at a higher loading rate, and 45°-inclination of the fibers improved the dynamic pullout resistance. A higher rate sensitivity on the bond strength was observed with the CSA EA addition and fiber's inclination, and the pullout energy was more dependent on loading rate than the average bond strength as fiber rupture was prevented.

1. Introduction In the mid-1990s, reactive powder concrete (RPC), which is a precursor of ultra-high-performance fiber-reinforced concrete (UHPFRC) recently available worldwide [1–4], was first developed by Richard and Cheyrezy [5]. Based on a packing density theory and incorporation of large amounts of discontinuous micro straight steel fibers, the successfully developed RPC exhibited excellent mechanical strengths (i.e., a compressive strength > 150 MPa and design tensile strength of 8 MPa), energy absorption capacity (i.e., fracture energy of approximately 40 kJ/m3), durability, and fatigue resistance. Owing to these excellent material properties of UHPFRC, relative to commercially available several types of ordinary concretes, its practical applications to pedestrian bridges, highway bridge girders and decks, stadium, mega architectural buildings, etc. [6], have been triggered worldwide by pioneer engineers in the field of civil and architectural engineering. However, owing to its very low water-to-binder (W/B) ratio and large amounts of fineness cementitious materials, UHPFRC is known to be susceptible to early age shrinkage cracks [7]. From a previous study [8], the ultimate autogenous shrinkage of UHPFRC is approximately 800 με at curing temperatures of 20 and 90 °C, much greater than that of conventional concretes, and most importantly, a significant portion of its shrinkage is generated at very early ages [9]. Japan Concrete Institute (JCI) [10] recommended 80 με as the ultimate autogenous shrinkage of concrete with a W/B ratio > 0.5 and suggested the

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following equation, εc0 = 3070 exp [−7.2(W/B)], for that with a W/B ratio between 0.2 and 0.5, where εc0 is the ultimate autogenous shrinkage. From the JCI model, autogenous shrinkage strain of concrete increases with decreasing the W/B ratio, so that the UHPFRC exhibits very large autogenous shrinkage due to its low W/B ratio. Furthermore, because of its excellent mechanical characteristics, thin plate structures have been frequently fabricated using UHPFRC under high restraint degrees [7]. Due to a large amount of shrinkage, faster shrinkage development at early ages, and smaller cross-sectional area, early age shrinkage cracks were easily and often formed in structures made of UHPFRC although they were fabricated in a factory as precast types. Several researchers [7,9,11–14] have therefore studied ways to compensate for the shrinkage and early age cracking potential of UHPFRC mixtures. Ma et al. [12] investigated the feasibility of adding basalt split as coarse aggregate to ultra-high-performance concrete (UHPC) mixtures and reported that approximately 40% lower autogenous shrinkage could be achieved in UHPC containing coarse aggregates than the counterpart owing to its lower cement paste volume and the hindrance of stiffer basalt under volume reduction. Different methods to decrease the autogenous shrinkage strains of UHPC were also introduced by Dudziak and Mechtcherine [14] and Soliman and Nehdi [11]. To compensate for the amount of free autogenous shrinkage, they [11,14] adopted a shrinkage-reducing agent (SRA) and superabsorbent polymer (SAP), and successfully decreased the shrinkage of UHPC at various relative humidities. However, the reduction in compressive strength of

Corresponding author. E-mail address: [email protected] (J.-J. Kim).

https://doi.org/10.1016/j.cemconres.2019.04.021 Received 5 December 2018; Received in revised form 16 March 2019; Accepted 30 April 2019 Available online 20 May 2019 0008-8846/ © 2019 Elsevier Ltd. All rights reserved.

Cement and Concrete Research 122 (2019) 196–211

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UHPC due to the addition of the SRA and SAP was also observed, and they [11] thus insisted that the strength reduction needs to be considered in evaluating the overall field performance. Yoo et al. [9] and Park et al. [13] also examined the implications of SRA and expansive agent (EA) on both free and restrained shrinkage behaviors of UHPFRC. Yoo et al. [9] reported that approximately 15.2% and 28.4% of free autogenous shrinkage of UHPFRC after 30 days could be decreased by adding 1% and 2% SRA by cement weight, respectively, and its cracking potential was also decreased. Approximately 34% and 44% reductions in 28-day free shrinkage strains of UHPFRC were also achieved by including 5% and 7.5% EA by cement weight, respectively, as reported by Park et al. [13], and the mixture with 7.5% EA provided the best performance in terms of free shrinkage behavior compared with others incorporating SRA alone or hybrid EA and SRA. Likewise, several studies [9,11–14] have successfully decreased the amounts of shrinkage and degree of cracking potential in UHPFRC based on their own developed technologies. However, the incorporations of coarse aggregate, SRA, and SAP have a drawback in terms of deterioration of mechanical performance. The addition of coarse aggregate to a UHPFRC mixture can result in several advantages, i.e., low production cost, less shrinkage, high fluidity, and less mixing time, and no reduction in compressive strength [12,15]; however, it decreases the flexural strength of UHPC by reducing the bond strength of the fibers [16]. Incorporating the SRA and SAP deteriorates the compressive strength of UHPC [11] and pullout resistance of steel fibers, causing a poorer tensile performance of UHPFRC [17]. Thus, such a deterioration in the mechanical performance of UHPFRC needs to be considered as these shrinkage reducing methods are adopted. Meanwhile, even though the effect of EAs on the mechanical properties of UHPFRC has been examined very limitedly thus far [13,18], EAs have been reported to positively affect both the shrinkage and mechanical properties of UHPFRC: the compressive strength slightly increased by adding the EA [13], and a CaO-based EA is effective in reducing drying shrinkage and enhancing the flexural strength of UHPC with brass-coated steel fibers. In order to fundamentally understand a mechanism of how it enhances the mechanical properties, its impact on the pullout resistance of steel fiber in UHPC needs to be evaluated. However, no published studies are available on the effect of any type of EA, such as CaO-based or calcium sulfoaluminate (CSA)-based EA, on its pullout behavior yet. Accordingly, this study examined the effect of CSA EA on the shrinkage and fiber pullout behaviors of UHPC. For this, five different CSA EA contents, i.e., 0%, 2%, 4%, 6%, and 8% of cement weight, were adopted, and various loading rates ranging from 0.018 mm/s (quasistatic) to 1244 mm/s (impact) and fiber inclination angles of 0° and 45° were used. The effectiveness of adding the CSA EA to enhance the static and dynamic pullout resistance of straight steel fibers in UHPC was examined. Its implications on the early-age shrinkage behavior of UHPC matrix and the rate sensitivity of various pullout parameters were also evaluated.

Table 1 Chemical compositions and physical properties of cement, silica fume, and calcium sulfo-aluminate expansive agent. Composition % CaO Al2O3 SiO2 Fe2O3 MgO SO3 f-CaO Specific surface area [cm2/g] Density [g/cm3] a b

Cementa

Silica fume

CSA EAb

61.3 6.4 21.0 3.1 3.0 2.3 – 3413 3.15

0.4 0.3 96.0 0.1 0.1 – – 200,000 2.10

51.8 9.0 1.5 0.6 1.4 29.0 19.8 > 2000 3.10

Type I Portland cement. Calcium sulfo-aluminate expansive agent.

Table 2 Mix proportion of UHPC. W/Bb

0.2

Mix design [kg/m3] Water

Cement

Silica fume

Silica sand

Silica flour

CSA EAc

SPa

160.3

788.5

197.1

867.4

236.6

0.00–63.08

52.6

[Note] W/B = water-to-binder ratio, CSA EA = calcium sulfo-aluminate expansive agent, and SP = superplasticizer. a Superplasticizer includes 30% solid (=15.8 kg/m3) and 70% water (=36.8 kg/m3). b W/B is calculated by dividing total water content (160.3 kg/m3 + 36.8 kg/ 3 m ) by total amount of binder (788.5 kg/m3 + 197.1 kg/m3). c Unit weights of 0%, 2%, 4%, 6%, and 8% CSA EA are 0.00, 15.77, 31.54, 47.31, and 63.08 kg/m3, respectively.

mixture could be fabricated with a flow value of approximately 250 mm, as per ASTM C1437 [19]. The detailed mixture proportion of UHPC is presented in Table 2. To examine the effects of matrix shrinkage and expansive behaviors on the pullout behavior of straight steel fibers in UHPC, a CSA EA, produced in Japan (DENKA CSA #20), was incorporated into the UHPC mixture at various amounts, ranging from 0% to 8% by weight of cement. The EA is mainly composed of CSA and calcium oxide (CaO), about 51.8%, and it can expand concrete by producing expansive ettringite [20,21]. Because the calcium oxide mainly leads to the volume expansion of concrete, it is a major chemical composition of most of the expansive agents available, such as CSA EA, CaO-based EA, and CSA cement. The chemical compositions of CSA EA is presented in Table 1. The diameter (df) and length (lf) of the straight steel fiber used were 0.3 and 30 mm, respectively, leading to an aspect ratio (lf/df) of 100. To prevent fiber breakage before a complete pullout, high-strength steel fiber with a tensile strength of approximately 2580 MPa and a density of 7.9 g/cm3 was used. To mix the UHPC, a Hobart-type mixer was used. First, all dry ingredients, including cement, SF, silica sand, silica flour, and powder type EA, were added into the mixer and pre-mixed for 10 min to ensure sufficient dispersion. Then, water premixed with a certain amount of SF was added into the mixer with dry ingredients and mixed for another 10 min to fabricate a flowable UHPC mixture. Subsequently, the fresh UHPC mixture was used for making pullout dog-bone and prismatic shrinkage specimens.

2. Test program 2.1. Fabrication of flowable UHPC mixture Type I Portland cement and silica fume (SF) were used as cementitious materials to fabricate the UHPC mixture. Their chemical compositions and physical properties are summarized in Table 1, and the mean grain sizes were 22 and 0.31 μm, respectively. To improve homogeneity, coarse aggregate was not included, and fineness additives, such as silica sand and silica flour, were incorporated as fine aggregate and filler, respectively. The mean grain sizes of silica sand and flour were 337 and 4.2 μm, respectively, and the silica flour was composed of 98% silicon dioxide (SiO2). A very low W/B ratio of 0.2 was adopted, and thus, in order to achieve a proper fluidity, a highrange water reducing admixture called polycarboxylate superplasticizer (SP) was included. Owing to the addition of SP, a flowable UHPC

2.2. Compressive strength measurement The compressive strength of the UHPC mortar was measured according to ASTM C109 [22] using 50-mm cubic specimens. Three cubic specimens were tested to obtain an average compressive strength value for all types of specimens. The compressive tests were conducted at 197

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2 days (before applying the steam-heat curing) and at the fiber pullout testing date (after the steam-heat curing) to evaluate the effect of a steam-heat curing on the strength development of UHPC mortar. Several cubic molds with a dimension of 50 × 50 × 50 mm3 were prepared, and a fresh UHPC mixture was cast. Then, the casting surface was made flat, and it was immediately covered with plastic sheet to prevent excessive water evaporation. After initial curing at room temperature, the specimens were all demolded; next, half of the specimens were tested and another half were stored in a water tank at a high temperature of 90 °C (steam-heat curing) for 2 days to promote strength development.

for additional 13 days to measure their shrinkage strains without any restraints. The strain and temperature data were recorded using a data logger, TDS 540, at an interval of 30 min. 2.4. Quasi-static and impact fiber pullout tests To evaluate the fiber pullout behaviors of straight steel fiber embedded in UHPC with various amounts of EA, dog-bone specimens with a length of 75 mm and a cross-sectional area of 25 × 25 mm2 at the center were fabricated and tested, as shown in Fig. 2a. Five specimens for each variable were used, and the average test data were used for reliable data analysis. Before casting fresh concrete, a straight steel fiber was located at the center using a very thin PVC sheet with an embedment length of 10 mm. During concrete casting, the initial embedment length is likely to be changed, so that to minimize its variation, a foam board was additionally used to fix the fiber in an appropriate location. As the pullout behavior of straight steel fiber in UHPC is strongly influenced by the fiber's inclination angle [25] and most of the fibers in UHPC composites are inclined from the tensile load direction [26], two different inclination angles of 0° and 45° were considered. The potential pullout side of the dog-bone mold was cast and cured for 2 days in the same room where the shrinkage tests were performed. To prevent surface cracking by rapid water evaporation, the casting surface was immediately covered with a plastic sheet. Two days later, another side of the mold was also cast using the same UHPC mixture after a removal of form board and cured in the same room for another 2 days. Subsequently, the specimens were all demolded and stored in a water tank with a high temperature of 90 °C for 2 days to promote strength development. Then, the specimens were stored in the laboratory until the testing date. A quasi-static fiber pullout test machine used to evaluate the pullout behavior of a straight steel fiber in UHPC under static loading conditions is shown in Fig. 2b. The dog-bone specimen was inserted into a specially fabricated steel grip system, which is affixed to the machine, and a uniaxial load was applied monotonically with a stroke speed of 0.018 mm/s. The applied load was measured through an installed load cell, and the fiber slip was measured from the vertical displacement of the crosshead by assuming that elastic deformation of the specimen and steel grip were both negligible. The maximum capacity of the test machine is 3 kN. As given in Fig. 2c, a specially fabricated impact pullout test machine was applied to analyze the dynamic pullout behaviors. The adopted dog-bone specimen and steel grip system were similar to those of the static pullout tests. The incident dynamic pullout load was applied to the specimen using air pressure, ranging from 0.5 kN to 10 kN.

2.3. Shrinkage measurement To investigate the implications of EA content on the shrinkage behavior of UHPC, prismatic molds each with a cross-sectional area of 50 × 50 mm2 and a length of 250 mm were prepared. Three specimens were fabricated for each variable to obtain reliable average test data. To eliminate any constraints due to friction between the inner surface of the mold and UHPC, a Teflon sheet was first applied inside the molds. Aïtcin [23] mentioned that high-performance concrete generates autogenous shrinkage steeply at an early age so that the measurement of shrinkage needs to be initiated at the time of concrete casting or at least from its setting time. As the UHPC is a type of self-consolidating concrete without coarse aggregate, it is difficult to properly locate a strain gauge in it during its casting. Therefore, a dumbbell-shaped embedded strain gauge made of a special plastic with almost zero stiffness was located at the center of the molds using a nylon line before concrete casting. A thermocouple was also located at the center to measure internal temperature variations mainly due to hydration heat to determine the measurement starting point and exclude the thermal strain from the measured shrinkage strain. The thermal strain was eliminated from the measured strain of the specimens using the following equation [24]: εsh = εm − αΔT, where εsh is the actual shrinkage, εm is the measured strain excluding temperature effect of gauge, α is the coefficient of thermal expansion, and ΔT is the temperature variation. In the case of dumbbell-shaped gauge, its temperature effect was excluded by subtracting the apparent strain, given in a manual, from the measured strain. To prevent sudden water evaporation immediately after concrete casting, a vinyl was also applied. The detailed test setup for the shrinkage measurement of UHPC is shown in Fig. 1. After casting a fresh UHPC mixture, the mixture was covered with the vinyl and cured for 3 days in a room with a constant temperature and humidity of 20 ± 0.5 °C and 60 ± 5%, respectively. After 3 days of initial curing, the prismatic specimens were demolded and exposed in the same room

Data logger (TDS 540) Teflon sheet Vinyl

Wood mold (50œ50œ250 mm3) Computer

250 mm 50 mm Thermocouple Nylon line

Electric wire

Plastic strain gauge

(a)

(b)

Fig. 1. Test setup for free shrinkage measurement of UHPC: (a) details of mold preparation and gauge location and (b) data acquisition equipment. 198

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P 40 mm

Cross head 7 5 mm

UHPC matrix a

Load cell (3 kN)

a θ (0‚, 45‚) LE

PVC Sheet Dog-bone Specimen

25 mm Single steel fiber Grip system

Fixed support

[a-a section]

(a)

(b) Gauge

Load cell (10 kN) Grip system Air compressor

Specimen

Potentiometer

Steel bar Air amplifier

Piston

Air storage tank

Impact load

(c) Fig. 2. Test setup for static and impact fiber pullout tests: (a) schematic description of dog-bond specimen, (b) static pullout test, and (c) impact pullout test.

Com pressive strength, f c' (M Pa)

A higher air pressure can provide a higher loading rate. To measure the dynamic pullout load precisely without specimen inertia, a load cell with a maximum capacity of 10 kN was affixed to the test machine at the opposite side of a piston applying the pullout load. A potentiometer was affixed to the machine using a magnetic base and connected to a thick steel plate above the piston for fiber slip measurement. By assuming that the elastic deformation of specimen and steel grip are minor, the displacement measured from the installed potentiometer was used for the fiber slip value. In this study, two different air pressures of 2 kN and 8 kN, respectively, were applied to provide fiber pullout impact loads and various loading rates.

3. Test results and discussion

180 150

140.6

151.1

148.2

140.6

128.1

120 90

60

49.3

54.1

56.6

30

59.3

60.3

Aer 2 days Aer heat curing

0 0%

2%

4%

6%

8%

CSA EA content (%)

3.1. Compressive strength

Fig. 3. Comparative compressive strengths of UHPC matrices with various CSA EA contents and ages.

The compressive strength test data measured after 2 days and heat curing are summarized in Fig. 3. The addition of CSA EA improved the compressive strength of plain UHPC, regardless of the curing age and method. For example, the early age (2-day) compressive strength of plain UHPC, 49.3 MPa, was improved as much as 10–22% by adding the CSA EAs. Furthermore, the compressive strength, 128.1 MPa, of heat-

cured plain UHPC was improved by approximately 10–18% due to the addition of CSA EAs. The higher early-age strengths of EA-included UHPC matrices are attributed to a hardening process that is more 199

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previous study [7], and the initial measured strains before the point where the strain and internal temperature exhibit different trends need to be excluded from the data analysis. In the same vein, the technical committee on autogenous shrinkage at the Japan Concrete Institute (JCI) [32] recommended measuring the autogenous shrinkage strain of concrete from the initial setting time to exclude the volume change generated as the concrete is still fresh. Furthermore, Darquennes et al. [33] and ASTM C1698 [34] have suggested the final setting time as the zeroing point of shrinkage measurement of cement mortar. Close to 22 h after concrete casting (plain UHPC without EA), the measured strain started to increase steeply in a negative direction and differed from the behavior of temperature, which increased owing to the hydration heat. This is because, as the fresh UHPC mixture initiated hardening and shrunk at this point, it effectively deformed the embedded dumbbell-shaped strain gauge. Therefore, the zeroing point of shrinkage in this study was determined as the point where the measured strain exhibits a different trend with that of the internal temperature, similar to the previous studies [7,9] and found to be about 21.5, 17, 9, 8, and 8 h for the UHPC mixtures with CSA EAs of 0, 2, 4, 6, and 8%, respectively.

accelerated than that of plain UHPC. According to previous studies [27,28], CSA-based cementitious binders resulted in higher early-age strength and fast setting than the counterpart owing to the rapid reaction of Ye'elimite (C4A3Ŝ). Lee et al. [29] have reported that, although the effectiveness of CSA-based cement in improving the compressive strength is higher at an early age, it still provides higher strengths at later ages due to the reduced porosity caused by the filling effect of ettringite hydrates than ordinary Portland cement. In the same vein, the compressive strengths of heat-cured UHPCs with CSA EA were higher than that of plain UHPC, and the strength increase rate by adding the CSA EA was remotely lower than that of early-age strength and, beyond 4% addition of CSA EA, the heat-cured strength slightly decreased. The highest compressive strength was 151.1 MPa and was found in the specimen with 4% EA, but all the others exhibited compressive strengths slightly below 150 MPa, which is recommended by ACI committee 239 for UHPC with steel fibers [2]. Lee et al. [30] reported that a significantly lower compressive strength was obtained in plain UHPC as compared to UHPFRC with various steel fibers because the fiber bridging effect, limiting the crack formation, propagation, and widening, could not be obtained [31]. In their study [30], the highest compressive strength was found in UHPFRC (172.9 MPa), while the lowest strength was observed in plain UHPC (133.4 MPa). Therefore, although the compressive strengths of UHPC matrices tested in this study were slightly lower than the recommended value (150 MPa), they can be considered as UHPC materials since we used and adopted the ingredients, mix proportions, and curing conditions, identical to those in a previous study [26], satisfying the strength requirements of the ACI committee 239 [2], as steel fibers were incorporated.

3.2.2. Effect of CSA EA content on shrinkage behavior of UHPC The shrinkage strain versus time curves of all the UHPC mixtures with various EA amounts are shown in Fig. 5. To precisely evaluate the effect of EA content on the shrinkage behaviors of UHPC, the thermal strain was excluded from the measured strain [24], indicating that the strains shown in Fig. 5 are mainly caused by drying and self-desiccation of concrete. The shrinkage behavior of UHPC is influenced by the CSA EA content. The zeroing point was accelerated by incorporating the EA and increasing its amount. For instance, the zeroing point of plain UHPC was approximately 22 h, whereas the fastest zeroing point of UHPC with 8% EA was found to be approximately 8 h. This is caused by the fact that the addition of CSA EA accelerates the setting of concrete [35–37]. Similarly, Glasser and Zhang [28] and Hargis et al. [27] noted that CSA-based binders exhibit high early strength, rapid setting, selfstressing, and shrinkage compensating properties due to the fast reaction of Ye'elimite (C4A3Ŝ) and expansive nature of ettringite. The early age compressive strength of plain UHPC was also improved by adding the CSA EA, and the effectiveness increased with its amounts, attributed to the accelerated hardening of UHPC mixture. Irrespective of the EA content, all UHPC mixtures showed a very steep increase in shrinkage strain immediately after the zeroing point up to nearly 30 h after casting, which is similar to the findings of an autogenous shrinkage development of ultra-high-strength mortar with EA under a curing temperature of 20 °C given by Zhang et al. [38]. The initial increase rate of shrinkage strain seemed to be not affected by the EA content in this study. Interestingly, the magnitude of the first peak shrinkage strain at approximately 30 h was higher for the case of UHPC mixtures with EA as compared to that without EA. However, there was no clear trend on the effect of EA amount on the magnitude of the first peak shrinkage strain, which is inconsistent with the findings of Zhang et al. [38], who reported a decrease in the first peak autogenous shrinkage strain with a higher EA content. Specimen with 4% EA provided the highest value of about −730 με, followed by the 8% EA, 6% EA (or 2% EA), and plain UHPC without it. A potential explanation for this observation is that as the addition of EA accelerated the setting and hardening of fresh UHPC, the embedded strain gauge deformed earlier and more significantly by its shrinkage. However, to rationally understand the greater early age shrinkage strain observed in the specimens with EA, further studies need to be conducted. After reaching the first peak shrinkage strain, all tested UHPC specimens expanded shortly and then shrank again. The magnitude and duration of this expansion are affected by the amount of added CSA EA: the magnitude and duration increased with the CSA EA amount. Similar results were also found by Nagataki and Gomi [20], who reported that the rate of expansion increases with increasing the EA content.

3.2. Shrinkage behavior 3.2.1. Determination of the zeroing point Fig. 4 depicts typical measured strain- and internal temperaturetime curves of UHPC without EA at the initial stage. At the point when the fresh UHPC mixture was cast into the shrinkage molds, both the temperature and measured strain values instantly and significantly increased since the temperature of the fresh mixture, which ranged from 27 °C to 29 °C, was much higher than the room temperature of 20 °C. Owing to the coefficient of thermal expansion (CTE) of the embedded strain gauge, the gauge expended greatly and recorded a very high strain value of approximately 250 με in the positive direction (expansion). At an early age, the internal temperature of the UHPC mixture steeply decreased to nearly 20 °C, which is the room temperature, and remained constant up to a certain age (23.5 h), just before the increase of internal temperature. At this stage, there was no obvious increase in strain in the negative direction (shrinkage) as the mixture was still fresh, and the measured strain exhibited a very similar trend to the internal temperature behavior. This is consistent with the findings of a 300

30

Zeroing point

Internal temp.

S train, ε (με)

100

25

0

20

-100

Meaningless strains

-200

15 Measured strain

-300

10

-400

Temperature (ƒC)

200

5

-500

Shrinkage strain

-600 0

10

20

30

40

0 50

60

70

Age (hour) Fig. 4. Initial strain and temperature variations of plain UHPC without CSA EA. 200

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0

0 w/o EA (plain) w/ EA (2%) w/ EA (4%) w/ EA (6%) w/ EA (8%)

-400

Strain, ε (με)

S train, ε (με)

-200

Demolding -600 -800

-1000 0

20

40

60

80

100

-400 -600 -800

-1000

Exposure cond.

Sealed cond.

w/o EA (plain) w/ EA (2%) w/ EA (4%) w/ EA (6%) w/ EA (8%)

-200

-1200 120

0

50

100

150

200

Age (hour)

Age (hour)

(a)

(b)

250

300

Fig. 5. Average free shrinkage behaviors of UHPC matrices with various CSA EA contents: (a) initial behavior up to 5 days [demolded after 3 days] and (b) overall behavior.

20

S train rate (με/h)

The overall shrinkage behaviors of UHPC mixtures with and without EA are shown in Fig. 5b. It can be seen that specimen with 2% EA exhibited the highest shrinkage. The UHPC mixtures including 2% and 4% CSA EAs exhibited higher shrinkage strains than that of plain UHPC without it, because they experienced accelerated shrinkage development at the initial stage from the zeroing point to about 30 h. Furthermore, the addition of 2% and 4% CSA EAs was insufficient to expand the UHPC noticeably immediately after reaching the first peak strains, as shown in Fig. 5a. The shrinkage strain differences between the UHPC including 4% EA and plain UHPC became smaller at later ages although it exhibited a much higher first peak shrinkage strain at approximately 30 h, because the former with 4% EA exhibited the higher volume expansion at an early age (after the first peak strain), and its shrinkage strain increased more gradually than that of plain UHPC after the initial expansion, which is similar to an observation of Nagataki and Gomi [20]. They reported that the shrinkage rate of concrete with EA is smaller than that of ordinary concrete without it. Owing to a higher volume expansion and slower increase in shrinkage for the cases of UHPCs with 6% and 8% EAs, they provided smaller shrinkage strains than that of plain UHPC after about 2 weeks (Fig. 5). As the shrinkage strain of UHPC significantly decreased with the addition of 6% EA or more, it is concluded that a minimum CSA EA content of 6% must be added to effectively compensate for the volume reduction (shrinkage) in UHPC. The prismatic specimens were all demolded approximately 3 days (72 h) after concrete casting. As they were exposed to atmosphere from this point on, the rate of shrinkage development noticeably increased immediately after demolding, as shown in Fig. 5a. This is caused by evaporation of pore water in UHPC by differences in relative humidity, leading to drying shrinkage. In agreement with Yoo et al. [24], even though UHPC exhibited a much smaller drying shrinkage relative to autogenous shrinkage, it normally increased up to roughly 10 days after casting. In addition, the shrinkage strains of all UHPCs were measured only up to about 2 weeks, as shown in Fig. 5b, because their increase rate became almost zero after this point. Fig. 6 shows the shrinkage rate (in με/h) versus time relation of plain UHPC. At the initial stage, the shrinkage rate increased steeply and reached its maximum value of approximately 92 με/h. After reaching the maximum value, the shrinkage rate greatly decreased and showed a positive value, due to the initial volume expansion by the formation of ettringite. At approximately 3 days, the shrinkage rate suddenly increased due to the demolding and exposure of prismatic specimens, and gradually converged to zero. Thus, 2 weeks after concrete casting, a shrinkage rate close to zero was obtained, indicating an insignificant increase in shrinkage strain.

Initial expansion

0

-20

Exposure by demolding

-40

-60 Initial shrinkage development -80 -100 0

50

100

150

200

250

300

350

Age (hour) Fig. 6. Shrinkage rate behavior of plain UHPC without CSA EA.

Koh et al. [8] and Yoo et al. [24] have identically reported that similar ultimate autogenous shrinkage strains of UHPC were obtained at both the ambient and heat curing conditions even though the shrinkage development was accelerated in the case of heat curing. However, the effect of steam curing with heat on the shrinkage behavior of exposed UHPC samples is still controversial. Graybeal [39] noted that the ultimate shrinkage of UHPC increased by applying steam curing, and its shrinkage development also accelerated. On the contrary, Yoo et al. [24] reported that the total shrinkage of exposed UHPC under steam curing with heat was slightly smaller than that under ambient curing because the magnitude of drying shrinkage was decreased by wet condition maintained for the former. However, owing to the very low W/B ratio of UHPC, causing minor drying shrinkage, the difference of shrinkage strains of UHPC at the ambient and heat curing conditions was insignificant of approximately 60 με. Therefore, we analyzed the test data by assuming that the magnitude of total shrinkage of UHPC measured in the constant temperature and humidity room (20 ± 0.5 °C and 60 ± 5%) was similar to that under heat curing, applied for making fiber pullout dog-bone specimens, regardless of the EA content.

3.3. Fiber pullout behavior 3.3.1. Pullout parameters To evaluate the pullout resistance at the interface between the fiber and matrix, the average bond strength was calculated based on Eq. (1). The equation was derived by assuming that the interfacial bond stress is 201

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with the surrounding matrix in this stage, the fiber could not be pulled out of the matrix, causing a smaller slip and steep increase in the pullout load–slip curve until near the maximum loading point. Therefore, the slip generated in this ascending zone was mainly attributed to the elastic deformation of the steel fiber and matrix. Once the fiber was fully debonded from the matrix, it could then be pulled from the matrix; because of the decreased embedment length (or bonding area), the pullout load decreased, as shown in Fig. 7a. As the UHPC mixture consists of high fineness ingredients, the interface between the fiber and matrix is densely filled with hydration products and fineness ingredients [42]. For this reason, a sudden decrease in the pullout load immediately after reaching the peak point and a steeper decrease in the pullout load by Poisson effect were not observed in the case of steel fibers embedded in UHPC (Fig. 7a), which is inconsistent with the trends of conventional cement mortar [41]. The abrasion of very fine adherent components at the interface and the damaging and scratching of the fiber coating also prevented the steep decrease in pullout load in the descending branch. According to previous studies [43,44], the frictional shear stress of a straight steel fiber at an interface increases with normalized slip, which is caused by fiber end deformation called flattening that occurs during the cutting process in fiber production. This deformed fiber end additionally resists the pullout load, leading to a gradual decrease in the load in the descending part. Once the fiber is almost completely pulled out from the matrix at near the slip of 9 mm, a steeper decrease in the pullout load was obtained due to its abrupt pullout. As the fibers were inclined at 45° to the direction of the pullout force, the initial pullout stiffness decreased, and a considerable gradual load decrease in the descending branch was observed (Fig. 7b). In addition, higher maximum pullout loads of inclined straight steel fibers in UHPC were observed, consistent with the findings of a previous study [25]. Owing to the lower stiffness and higher maximum pullout load, a higher slip capacity corresponding to the maximum pullout load was also obtained in the inclined fiber specimens. The reduced pullout stiffness might be caused by the movement of the inclined fiber owing to a minute gap between the fiber and matrix, and crushing debonded fine particles at the interface, which led to greater measured slip values. An additional reaction (R) activated at the fiber exit by the inclined pullout force provided an additional frictional resistance at the interface so that a higher maximum pullout load and long-lasting high pullout resistance in the descending branch were obtained in the inclined fiber specimens under the quasi-static pullout loads (Fig. 7). The dynamic pullout behaviors of straight steel fibers in UHPC are presented in Fig. 8. It can be seen that the maximum pullout loads of both aligned and inclined steel fibers increased with the loading rates. This is consistent with the findings of Yoo and Kim [45] and is caused by the end deformation formed by the cutting process, which provides a moderate mechanical anchorage effect relative to those of deformed steel fibers, and its curly shape. For these reasons, more obvious matrix spalling was observed in the impact test specimens than in the static test specimens, as shown in Fig. 9. It is well known that the strength of concrete is dependent on the loading rate, caused by enhanced crack growth resistance and an altered and shortened crack path [46–48]. A rate dependent parameter, matrix spalling occurs due to the formation of tensile cracks in the matrix, becoming more obvious at impact, which verifies the rate sensitive pullout behaviors of straight steel fibers in UHPC. By applying the impact loads, the pullout stiffness was slightly enhanced, regardless of the inclination angle of the fibers and EA content. The pullout loads decreased more steeply in the descending part under impact loading, owing to the increased maximum pullout load and stiffness but identical embedment length of 10 mm. Although the pullout parameters, such as maximum load, initial stiffness, and softening slope, were influenced by the loading rate, the shape of the pullout load-fiber slip curve was similar for both static and impact loading conditions. For instance, triangular PeS curve shapes were observed in the aligned fiber specimens under static and impact loads,

uniformly distributed along the entire embedment length.

P τav = max πdf LE

(1)

where τav is the average bond strength in MPa, Pmax is the maximum pullout load in N, df is the fiber diameter in mm, and LE is the initial embedment length in mm. One of main reasons for adding discontinuous fibers to a concrete mixture is to enhance the concrete's toughness. Enhanced toughness can be achieved by the fiber bridging effect at the crack surface and the pulling out process of the fibers. Therefore, it is important to design fiber-reinforced concrete (FRC) that exhibits a fiber pullout failure mode instead of a fiber rupture failure mode. Before fabricating the FRC composites, the fiber failure mode can be determined by comparing the maximum tensile stress applied to the fiber and its ultimate tensile strength. Thus, even though the maximum fiber tensile stress is determined based on the maximum pullout load and geometrical dimension of fiber like the average bond strength, it was analyzed in this study. The maximum fiber tensile stress was thus analyzed based on the following equation.

σf ,max =

4Pmax πdf2

(2)

where σf,max is the maximum fiber tensile stress in MPa. The toughness of FRC is closely related to the energy absorbed by the pulling out process of fibers. Thus, the pullout energy was also evaluated by calculating the area under the pullout load versus slip curves, as follows:

WP =

S = LE

∫S=0

P (S ) dS

(3)

where WP is the pullout energy in N·mm, S is the fiber slip in mm, and P (S) is the pullout energy applied at a certain slip value of S in N. Based on an assumption that a shear stress is uniformly distributed over the entire embedment length of fiber, the equivalent bond strength can be calculated using the pullout work energy, as follows:

τeq =

2Wp πdf LE2

(4)

where τeq is the equivalent bond strength. The equivalent bond strength is useful and simply considered as a parameter to infer a degree of strain-hardening behavior of the composites. Kim et al. [40] reported that the higher value of τeq can lead to better strain-hardening tensile behavior of the composites at a smaller fiber volume fraction. 3.3.2. General pullout load–slip responses under quasi-static and impact loads Figs. 7 and 8 show the pullout load versus slip (PeS) curves of straight steel fibers in UHPC with various EA contents, fiber inclination angles, and loading rates. It is obvious that the pullout responses of the steel fiber are influenced by several factors, i.e., EA content, inclination angle of the fiber, and loading rate. In the case of aligned fiber specimens (Fig. 7a), the pullout load increased steeply up to near the maximum load, and then gradually decreased until a slip of approximately 9 mm. The steep increase in pullout load at the initial stage was attributed to the pullout mechanism, i.e., the chemical bond and partial debonding processes. At the very initial stage, the fiber was chemically bonded to the surrounding matrix, and its chemical adhesion was destroyed by only a small magnitude of pullout load. Because the chemical adhesion was not slip-induced [41], the magnitude of slip measured at this stage was nearly zero in general. Owing to the continuously increased pullout force, the frictional bond stress activated at the interface also increased, and once the maximum bond stress at the fiber exit exceeded the maximum fiber's bond strength, the fiber started to debond from the matrix toward the fiber end, a process called partial debonding. As the end of the fiber was still fully bonded 202

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Normalized slip, S/LE (mm/mm) 0.6

0.8

Pullout load, P (N)

120 w/o EA (plain) w/ EA (2%) w/ EA (4%) w/ EA (6%) w/ EA (8%)

100 80

Normalized slip, S/LE (mm/mm)

1

60

0 12 10 8 6

40

4

20

2

0

2

4

6

Slip, S (mm)

8

0.4

0.6

0.8

1 12

100

10

80

8

60

6 w/o EA (plain) w/ EA (2%) w/ EA (4%) w/ EA (6%) w/ EA (8%)

40 20

0

0

0.2

120

4 2 0

0

10

Avg. bond stress, τ (MPa)

0.4

Pullout load, P (N)

0.2

Avg. bond stress, τ (MPa)

0

0

(a) Aligned fiber (0ƒ)

2

4

6

Slip, S (mm)

8

10

(b) Inclined fiber (45ƒ)

Fig. 7. Average static pullout load versus slip responses: (a) aligned fiber and (b) inclined fiber [Note: V = 0.018 mm/s and V = loading rate].

Normalized slip, S/LE (mm/mm) 0.8

Pullout load, P (N)

180 w/o EA (plain) w/ EA (2%) w/ EA (4%) w/ EA (6%) w/ EA (8%)

150

120

Normalized slip, S/LE (mm/mm)

1

0 18 15 12

90

9

60

6

30

3 0

0 0

2

4

6

Slip, S (mm)

8

120

160

6

30

3 0

20 16

8

40

4

0

Slip, S (mm)

8

4

6

Slip, S (mm)

0.2

0.4

0.6

8

10

0.8

1

w/o EA (plain) w/ EA (2%) w/ EA (4%) w/ EA (6%) w/ EA (8%)

200

0 6

2

Normalized slip, S/LE (mm/mm) 0

80

4

12

60

0

12

2

15

9

1

120

0

18

(b) Inclined fiber (45ƒ) V = 551.0 mm/s

w/o EA (plain) w/ EA (2%) w/ EA (4%) w/ EA (6%) w/ EA (8%)

200

Pullout load, P (N)

0.8

1

0

Pullout load, P (N)

0.6

0.8

90

10

Avg. bond stress, τ (MPa)

0.4

0.6

w/o EA (plain) w/ EA (2%) w/ EA (4%) w/ EA (6%) w/ EA (8%)

Normalized slip, S/LE (mm/mm) 0.2

0.4

150

(a) Aligned fiber (0ƒ) V = 401.3 mm/s 0

0.2

180

Avg. bond stress, τ (MPa)

0.6

160 120 80

8

40

4 0 0

(c) Aligned fiber (0ƒ) V = 665.5 mm/s

16 12

0

10

20

Avg. bond stress, τ (MPa)

0.4

Pullout load, P (N)

0.2

Avg. bond stress, τ (MPa)

0

2

4

6

Slip, S (mm)

8

10

(d) Inclined fiber (45ƒ) V = 789.5 mm/s

Fig. 8. Average impact pullout load versus slip responses: (a) aligned fiber with avg. loading rate of 401.3 mm/s, (b) inclined fiber with avg. loading rate of 551.0 mm/s, (c) aligned fiber with avg. loading rate of 665.5 mm/s, and (d) inclined fiber with avg. loading rate of 789.5 mm/s [Note: V = loading rate]. 203

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(a) Aligned fiber (0ƒ) Quasi-static

(b) Aligned fiber (0ƒ) Impact (2 kN)

(c) Inclined fiber (45ƒ) Quasi-static

(d) Inclined fiber (45ƒ) Impact (2 kN)

Fig. 9. Comparison of pictures of matrix spalling. Table 3 Summary of static and impact pullout test results. Fiber type

Inclination angle [°]

CSA EA contents [%]

Loading rate [mm/s]

Avg. bond strength, τav [MPa]

Max. fiber stress, σf,max [MPa]

Pullout energy, WP [×10−3 J]

Equ. bond strength, τeq [MPa]

Straight steel fiber

0

0

0.018 346.0 487.3 0.018 573.4 810.8 0.018 362.0 863.9 0.018 342.1 409.9 0.018 383.1 755.8 0.018 683.5 908.3 0.018 611.6 744.2 0.018 532.1 786.4 0.018 477.8 691.7 0.018 449.7 816.9

10.4 (3.125) 13.3 (2.423) 17.3 (4.690) 10.1 (2.691) 12.5 (2.823) 16.9 (1.880) 8.4 (1.758) 13.8 (2.804) 15.7 (3.710) 8.9 (2.460) 15.7 (4.521) 19.3 (2.973) 8.6 (1.260) 15.8 (2.394) 18.6 (3.275) 9.9 (2.248) 15.9 (2.044) 16.4 (1.913) 13.1 (1.876) 21.2 (2.370) 21.3 (3.041) 12.2 (2.342) 18.4 (2.086) 22.8 (2.429) 12.7 (2.431) 20.4 (1.993) 23.2 (1.422) 11.3 (1.419) 19.5 (0.990) 19.8 (2.149)

1249.0 (290.73) 1718.8 (182.9) 2231.9 (229.51) 1278.8 (343.07) 1612.9 (441.07) 2483.1 (231.32) 1072.0 (47.44) 1688.8 (343.00) 2106.7 (196.55) 1111.6 (390.30) 2095.9 (439.96) 2918.0 (363.70) 1017.2 (169.95) 2004.2 (129.26) 2592.9 (388.36) 1241.9 (179.36) 2151.3 (291.66) 2478.7 (269.31) 1374.3 (129.50) 2296.1 (88.87) 2505.5 (218.44) 1383.9 (252.89) 2451.1 (278.09) 3039.4 (323.92) 1700.3 (263.62) 2720.3 (265.68) 3092.1 (187.59) 1441.5 (155.18) 2602.7 (191.99) 2569.5 (348.93)

457.0 (98.09) 785.6 (26.22) 822.9 (213.7) 600.0 (130.99) 870.2 (335.88) 1236. 6 (200.03) 373.0 (85.78) 854.1 (212.16) 1075.2 (65.61) 399.0 (96.80) 1022.7 (148.23) 1300.7 (179.24) 414.6 (74.35) 856.0 (200.70) 996.6 (147.39) 627.5 (90.49) 1063.2 (111.00) 1258.8 (186.85) 616.7 (65.24) 992.3 (71.22) 1070.5 (22.90) 669.9 (155.03) 396.4 (111.31)a 590.5 (302.76)a 916.6 (272.045) 267.0 (61.87)a 334.7 (78.45)a 814.1 (100.55) 263.7 (125.29)a 1290.6 (184.44)

11.6 (2.105) 17.6 (2.180) 18.1 (5.447) 14.3 (3.609) 19.0 (2.910) 21.6 (4.798) 8.45 (2.189) 21.3 (3.479) 22.2 (5.059) 9.64 (0.721) 21.0 (5.285) 21.3 (2.560) 11.2 (1.953) 20.4 (6.185) 19.4 (4.725) 14.9 (3.286) 21.9 (1.981) 20.9 (3.518) 17.9 (1.782) 31.6 (2.378) 28.9 (2.908) 19.4 (4.157) 8.4 (2.362)a 12.5 (6.425)a 18.7 (4.359) 5.7 (1.313)a 7.1 (1.665)a 18.9 (2.204) 5.6 (1.037)a 29.0 (2.755)

2

4

6

8

45

0

2

4

6

8

[Note] CSA EA = calcium sulfoaluminate expansive agent and () = standard deviation. a Fiber is ruptured.

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fiber is related to the compactness of the fiber-matrix interface. If there is no fiber in the UHPC matrix, the inner radius of the hole will decrease by matrix shrinkage. On the contrary, if there is a steel fiber, the volume reduction in the matrix due to its shrinkage will be restrained and will cause a radial confinement pressure to be applied to the fiber, which mainly generates the frictional shear resistance. The magnitude of confinement pressure is related to the difference between the fiber radius and the hole radius after matrix shrinkage. As the frictional shear strength is affected by the magnitude of matrix shrinkage, Yoo et al. [17] reported that the pullout resistance of straight steel fibers in UHPC decreases by adding a SRA, leading to poorer postcracking tensile performance. For this reason, the aligned straight fiber specimens in UHPC with 6% and 8% EA exhibited approximately 15% and 17% lower average bond strengths, respectively, than those in UHPC without EA (Fig. 10a). However, even though the UHPC mixtures with EA contents of 2% and 4% showed higher shrinkage strains than the plain UHPC mixture without the EA, they exhibited poorer pullout resistance in terms of the bond strength than the latter as they were aligned. This might be due to their compactness, which is slightly poorer than that of the plain UHPC matrix. We could not identify a decrease in flowability of fresh UHPC when 2% EA was added to it, whereas beyond that amount of EA, its flowability became slightly lower because the dry powder type EA was additionally included into the UHPC mixture. The fresh UHPC mixture with 8% EA was thus denser than the plain mixture, and consequently, the average bond strengths of the aligned straight fibers in UHPC greatly decreased beyond the CSA EA content of 4%. On the contrary, the average bond strength of inclined straight steel fibers in UHPC was improved by addition of the CSA EA to UHPC, as shown in Fig. 10a, which is caused by the increased matrix strength of the UHPC. Given our preliminary test results on direct tensile specimens, the initial cracking strength of UHPC reinforced with 2% by volume of straight steel fibers, which is related to matrix cracking rather than the fiber bridging capability, increased by adding the EA and increasing its amounts, as shown in Fig. 12. Therefore, it can be noted that the addition of the CSA EA is beneficial to improving the cracking strength of a matrix under tension. When the straight steel fibers are inclined in the direction of the pullout load, a local interaction force between the fiber and matrix at the exit point is generated [52], leading to matrix spalling. From a sound investigation during the fiber pullout tests, matrix spalling generally occurs before the ultimate pullout strength of inclined fiber specimens is achieved. Therefore, the fibers in a matrix with a higher resistance to spalling can achieve higher bond strengths because there are a delay in the generation of matrix spalling and an increase in the maximum pullout load applied. The UHPC matrix with the CSA EA attained the higher cracking strength (Fig. 12), so that the specimens in UHPC with the EA exhibited higher average bond strengths, as shown in Fig. 10a, even though they had smaller frictional shear resistances at the fiber-matrix interfaces. From these observations, it can be inferred that, if matrix spalling is prevented or delayed by some micro (or nano) reinforcements, the bond strength of the inclined straight steel fiber in the UHPC matrix can be enhanced. Fig. 10b summarizes the maximum fiber stresses (σf,max) of aligned and inclined straight steel fibers in UHPC matrix. Similar to the average bond strength results in Fig. 10a, the maximum fiber stress of aligned fiber specimens generally reduced with the addition of the CSA EA, whereas it increased with the addition of the EA as the fibers were inclined to the pullout load direction due to the reasons (slightly reduced flowability and increased matrix strength) mentioned above. For example, the highest maximum fiber tensile stress was found to be approximately 1700 MPa for the 45°-inclined fiber in UHPC containing 6% of the EA, which was 37% greater than that in UHPC without it. Since the tensile strength of the straight steel fiber used was 2580 MPa, the maximum fiber stress was almost 66% of the fiber's tensile strength. The fiber efficiency is considered to be higher as its maximum fiber stress approaches the fiber's tensile strength (around 2580 MPa).

while quadrangular-shaped PeS curves were observed in the inclined fiber specimens, owing to the increased slip capacity and gradual postpeak softening. For the aligned fiber specimens (Fig. 8a and c), all of the fibers embedded in UHPC matrices with and without CSA EA were completely pulled out without any rupture. Meanwhile, some inclined fiber specimens exhibited fiber rupture failure modes under the impact loading conditions, as shown in Fig. 8b and d. Generally, the specimens with higher amounts of the CSA EA showed fiber rupture failure modes in the inclined condition. For instance, most of the inclined fibers in UHPC matrices with > 4% CSA EA were ruptured before their complete pullout under impact loading, except for the specimen with 8% EA at a higher loading rate of about 789.5 mm/s. Among five tested samples, three samples with 4% CSA EA were ruptured (thus they were averaged), and the inclined fibers in the matrices with 6% and 8% CSA EAs were all ruptured and pulled out, respectively. On the contrary, the inclined fiber specimens with and without 2% EA exhibited complete pulling out failure under impact loading conditions. This is caused by the higher rate sensitivity of the cracking strength of the matrix compared with the tensile strength of steel fiber. The split tensile strength of UHPC matrix exhibits higher loading rate sensitivity than the tensile strength of steel fibers, leading to the change in failure mode. According to previous studies [49,50], the dynamic increase factors (DIFs) of the tensile and yield strengths of UHPFRC and steel rebars can be calculated, and Yoo and Kim [45] have calculated the DIFs of tensile and yield strengths of UHPFRC and steel rebar as 3.98 and 1.28, respectively, at a strain rate of 102 s−1. Thus, matrix spalling was prevented at the impacts, but instead the steel fiber was ruptured due to its less rate sensitivity. As shown in Fig. 8b and d, the pullout load of inclined fiber specimens showing fiber rupture instantly reduced just after reaching the peak load, indicating a brittle failure mode. Owing to the premature rupture failure of steel fibers, the full development of interfacial bond strength was also limited. 3.3.3. Effect of CSA EA content on the pullout parameters of straight steel fibers in UHPC 3.3.3.1. Static pullout parameters. Several important static pullout parameters, i.e., τav, σf,max, WP, and τeq, are summarized in Table 3 and given in Fig. 10. The average bond strengths of straight steel fibers in UHPC improved and deteriorated owing to the addition of the CSA EA at 45°-inclined and aligned conditions, respectively. For instance, the highest value of τav of aligned straight fibers was found to be 10.4 MPa in UHPC without EA, while the lowest value of τav was obtained as 8.4 MPa in UHPC with an EA content of 4%. On the contrary, the highest and lowest values of τav of inclined straight fibers were found to be approximately 13.1 MPa and 9.9 MPa in UHPC with 2% EA and without it, respectively. Even though the average bond strength was influenced by the presence of CSA EA, there was no clear trend regarding the effect of EA content on the bond strength of aligned and inclined straight steel fibers in UHPC: the lowest and highest strengths were observed for aligned and inclined fibers in UHPC with CSA EA content of 4% and 2%, respectively. In order to support these explanations, the relationships between the average bond strengths and the 15-day shrinkage strains were compared as shown in Fig. 11. The average bond strengths of both the aligned and inclined fibers in UHPC seemed to be increased with increasing the absolute shrinkage strains, but their correlations were fairly low: the coefficient of determination (R2) was about 0.16 and 0.09 for the aligned and inclined fiber cases, respectively. The frictional shear resistance of a straight steel fiber is affected by various factors, i.e., the compactness, stiffness, strength of the fiber-matrix interface, and the fiber-matrix misfit after chemical debonding [41,44]. In accordance with Jewell [51], steel fibers embedded in CSA-based cement paste exhibited higher bond strengths compared to those embedded in ordinary Portland cement paste due to the dense microstructure and acicular nature of ettringite crystals. These observations indicate that the pullout resistance of steel 205

Cement and Concrete Research 122 (2019) 196–211

Aligned fiber (0ƒ)

25

Exp. results Linear fiing curve

V = 665.5 mm/s

V = 401.3 mm/s

20 15 V = 0.018 mm/s 10 5

Quasi-static

Impact

V = 551.0 mm/s

Exp. results Linear fiing curve

20 V = 0.018 mm/s

15 10 5

0

Quasi-static

0 0 2 4 6 8

0 2 4 6 8

V = 789.5 mm/s

Inclined fiber (45ƒ)

25

Avg. bond strength, τa v (MPa)

Avg. bond strength, τa v (MPa)

D.-Y. Yoo, et al.

0 2 4 6 8

Impact

0 2 4 6 8

CSA EA content (%)

0 2 4 6 8

0 2 4 6 8

CSA EA content (%)

(a) V = 665.5 mm/s

3000

Exp. results Linear fiing curve

V = 401.3 mm/s

2500 2000 V = 0.018 mm/s 1500 1000 500 Quasi-static

Impact

V = 789.5 mm/s

Inclined fiber (45ƒ) 3000

Max. fiber stress, σf,max (MPa)

Max. fiber stress, σf,max (MPa)

Aligned fiber (0ƒ)

V = 551.0 mm/s

Exp. results Linear fiing curve

2500 2000

V = 0.018 mm/s

1500 1000

500 Quasi-static

0

Impact

0 0 2 4 6 8

0 2 4 6 8

0 2 4 6 8

0 2 4 6 8

CSA EA content (%)

0 2 4 6 8

0 2 4 6 8

CSA EA content (%)

(b)

1200

V = 665.5 mm/s

Aligned fiber (0ƒ) Exp. results Linear fiing curve

V = 401.3 mm/s

1000

800

V = 0.018 mm/s

600

400 200 Quasi-static

Impact

1400

Pullout energy, WP (Ő10-3 J)

Pullout energy, WP (Ő10-3 J)

1400

1200 1000

0

V = 789.5 mm/s

Inclined fiber (45ƒ) Exp. results V = 551.0 Linear fiing curve

mm/s

V = 0.018 mm/s

800 600 400 200 Quasi-static

Impact

0

0 2 4 6 8

0 2 4 6 8

0 2 4 6 8

0 2 4 6 8

CSA EA content (%)

0 2 4 6 8

0 2 4 6 8

CSA EA content (%)

: Fiber rupture

(c) V = 551.0 mm/s

30

Exp. results Linear fiing curve

V = 401.3 mm/s

25 20

Inclined fiber (45ƒ) Equ. bond strength, τeq (MPa)

Equ. bond strength, τeq (MPa)

Aligned fiber (0ƒ) V = 665.5 mm/s

V = 0.018 mm/s

15 10 5 Quasi-static

Impact

0

30 25

V = 789.5 mm/s

Exp. results Linear fiing curve

V = 0.018 mm/s

20

15 10 5 Quasi-static

Impact

0 0 2 4 6 8

0 2 4 6 8

0 2 4 6 8

0 2 4 6 8

CSA EA content (%)

0 2 4 6 8

CSA EA content (%)

0 2 4 6 8 : Fiber rupture

(d) (caption on next page) 206

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Fig. 10. Summary of static and impact pullout parameters of aligned and inclined steel fibers in UHPC: (a) average bond strength, (b) maximum fiber tensile stress, (c) pullout energy, and (d) equivalent bond strength.

= 0.

Avg. b ond strength, τav (M Pa)

14

+ 6.60,

= 0.0942

+ 7.00,

= 0.1558

UHPC with 6% EA, whereas the lowest pullout energy of 373.0 N·mm was found in the aligned fiber specimen in UHPC with 4% EA, which is only 41% of the highest energy value although an identical fiber, matrix, and embedment length of 10 mm were applied. This means that if the amount of EA and inclination angle of steel fiber are properly designed and controlled, the energy absorption capacity by fiber bridging can be doubled.

12 10 8

= 0.

6

3.3.3.2. Impact pullout parameters. The dynamic pullout parameters are also presented in Table 3 and Fig. 10. The average bond strengths of both aligned and inclined fiber specimens were improved by incorporating the CSA EA under impact loading. Although the impact test results on the average bond strength of inclined fiber specimens were consistent with the findings of static pullout tests that the bond strength increases by adding the CSA EA, the aligned fiber specimens' impact test results were contrary to the static test results. The static average bond strength of the aligned straight steel fibers in UHPC decreased owing to the addition of the CSA EA, whereas their impact average bond strength generally improved, as shown in Fig. 10a. This is due to the eccentric effect becoming more obvious under impact loading than under static loading [53]. To minimize the eccentric effect during the process of pulling out the steel fibers from the UHPC matrix, a hinge-type grip system, which can allow slight rotation of a specimen, was used for the impact test. However, in the impact loading condition, the rotation of the dog-bone specimens was limited because an instantaneous load was applied at a very high speed. For this reason, the steel fibers were observed to be more curly after their complete pullout from the matrix under impact loading than under static loading [53]. If the aligned straight steel fibers are pulled out from the matrix under an eccentric condition, their pullout mechanism becomes similar to that of inclined fibers. Thus, matrix spalling became more obvious under impact loading, as shown in Fig. 9, indicating that the area of the spalled matrix was higher for the aligned fiber specimens under impact loading than under static loading. Matrix spalling depends on the splitting cracking strength of the matrix, so that the UHPC matrices with CSA EA provided higher average bond strengths than the plain matrix without it due to their higher strengths. As the fibers are pulled out of the matrix after the formation of matrix spalling, the higher strength matrix more effectively resisted the pullout force and resulted in a higher bond strength under impact loading. Although both the aligned and inclined fiber specimens exhibited higher dynamic average bond strengths when the CSA EA was incorporated, the bond strength of inclined fiber case more greatly increased with the addition of CSA EA than the aligned one in general at the identical air pressure. This might be due to the fact that the pullout resistance of the inclined fiber is more greatly influenced by the matrix strength, which is related to the higher spalling area in Fig. 9, than that of the aligned fiber and the higher loading rates obtained in the former. For example, the highest average bond strengths of aligned and inclined fibers in the UHPC matrices with 6% EA were found to be approximately 15.7 MPa and 20.4 MPa, respectively, at 2-kN impact, which were 22% and 26% higher than those in the plain UHPC without EA, respectively. The maximum fiber tensile stresses also exhibited similar trends with those of the average bond strengths. The maximum fiber stresses of both the aligned and inclined fiber specimens increased by adding the CSA EA under impact loading. Because of the enhanced bond strength with higher loading rates, the maximum tensile stress obtained in the fiber exceeded its tensile strength, and thus, fiber rupture failure modes were observed in some inclined fiber specimens with the CSA EAs (Fig. 8). The highest maximum fiber stress was found to be approximately 3093 MPa in the inclined fiber specimen with 6% EA and under the loading rate of about 789.5 mm/s (8-kN impact), which is

Test data (A) Test data (I) fiing curve (A) fiing curve (I)

4 2 0 800

850

900

950

1000 1050 1100 1150

Absolute shrinkage strain at 15 days (με) Fig. 11. Relationship between the average bond strength and 15-day shrinkage strain [Note: A = aligned fiber and I = inclined fiber].

16

20

Exp. results Linear fiing curve

= 0.

12

+ 8.92, 10.6

= 0.5964 11.4

9.6

10.1

8.4

8 4 0

Tensile strength, f t (MPa)

Tensile strength, f t (MPa)

20

16

= 0. 15.9

+ 15.53,

= 0.1026 17.7

16.8

15.7 14.2

12 8 4 0

0

2

4

6

8

0

2

4

6

CSA EA content (%)

CSA EA content (%)

(a) Initial cracking

(b) Post cracking

8

Fig. 12. Effect of CSA EA content on (a) initial and (b) post cracking tensile strengths of UHPFRC.

Therefore, the addition of CSA EA is effective in increasing the fiber efficiency, due to the increased σf,max value, at the 45°-inclined conditions. The pullout energy (WP) and corresponding equivalent bond strength (τeq) of all tested specimens are given in Fig. 10c and d. After the generation of matrix cracks, most of the tensile stress at the crack surface is resisted by the reinforcing fibers. Therefore, the pullout energy of the fiber from the matrix is closely related to the post-cracking energy absorption capacity of the composites. As shown in Fig. 10c and d, the pullout energy and equivalent bond strength of the aligned straight steel fibers in UHPC decreased increasing CSA EA in general, whereas they increased with increasing CSA EA for the inclined fiber specimens. For the case of the aligned fiber specimens, the maximum pullout load (Pmax) generally decreased with increasing EA content, but the post-peak softening curve was not influenced by the content of EA, as shown in Fig. 7a. For these reasons, the pullout energy, which is the area under the pullout load and slip curve, also reduced with increasing EA content. Moreover, the maximum pullout load of inclined fiber specimens increased with an increase in the EA content, and their postpeak softening slope was mitigated by the EA content, leading to a more gradual decrease in pullout load carrying capacity versus fiber slip, as shown in Fig. 7b. Thus, the pullout energy of 45°-inclined steel fibers in UHPC improved according to the EA amount. The highest pullout energy of 916.6 N·mm was obtained in the 45°-inclined fiber specimen in 207

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with the findings of average bond strength. Since the 8% EA-included specimens resulted in different failure modes according to the loading rate, their DIF-loading rate relations were not analyzed. For the case of inclined fiber specimens with the CSA EA content > 4%, the DIFs were lower than 1 at higher loading rates owing to the fiber rupture failure. Therefore, to achieve a reinforcing effect of steel fibers that can enhance the dynamic energy absorption capacity of UHPC matrix, the fiber breakage before completely pulling the fibers out needs to be prevented at higher EA content. 3.3.4. Effect of the inclination angle on the static and impact pullout parameters of straight steel fiber in UHPC As summarized in Table 3, the pullout parameters of straight steel fibers embedded in UHPC are affected by their inclination angle. Similar to a previous study [25], the 45°-inclined fiber provided higher average bond strength than the aligned fiber regardless of the existence of CSA EA. This is because the reaction force (R) activated at the exit of the inclined fiber provides additional frictional resistance, called a snubbing effect [54]. For the case of a straight steel fiber in UHPC, it has been reported [25] that the average bond strength decreases at a highly inclined condition of 60° because the matrix spalling phenomenon becomes prominent. However, its highest average bond strength has been obtained at an inclination angle of 30° or 45° because, at these angles, the snubbing effect is more dominant than the matrix spalling effect [25,45]. Thus, approximately 29% of the average bond strengths of the aligned fibers in all UHPC matrices with and without CSA EAs were improved by inclining the fibers to 45° on average. Furthermore, the maximum fiber stresses of all inclined fiber specimens were approximately 26% higher than those of all aligned fiber specimens on average owing to the same reasons mentioned above. The actual embedment lengths varied slightly from the target value of 10 mm by several uncertainties during specimen manufacturing. The bond strength is the interface property which does not depend on any measurement taken, whereas the maximum fiber stress is dependent on the bond strength and the embedment length. Thus, the average improvement in the percentage of maximum fiber stress was not consistent with that of the average bond strength but slightly lower. Although the maximum fiber stresses increased in the 45°-inclined condition, all the fibers were stably pulled out from the matrix without any fiber rupture because their maximum stress was much lower than its tensile strength. The pullout energies and equivalent bond strengths of the aligned fibers in UHPC were also improved by inclining them, as given in Table 3. For instance, the pullout energies of the inclined fiber specimens were approximately 69% greater than those of the aligned ones regardless of the EA content. However, the average improvement in pullout energy in percentage was much higher (> 2 times higher) than those of the

3.5

DIF on avg. bond strength, τav

DIF on avg. b ond strength, τav

approximately 20% greater than its static tensile strength of 2580 MPa. This means that the tensile strength of steel fiber is also rate sensitive, but its sensitivity is much less than the strength of the matrix, which can be supported by a previous study [45]. As shown in Fig. 10c and d, the pullout energy and equivalent bond strength of aligned and inclined fibers in UHPC were affected by the presence of CSA EA under impact loading. For the case of aligned fiber specimens, their dynamic pullout energies and equivalent bond strengths were improved by adding the CSA EA, mainly caused by the enhanced pullout resistance. It is obvious that poorer fiber pullout energy is obtained if the fiber rupture failure occurs. Thus, much smaller dynamic pullout energies and equivalent bond strengths were obtained in the inclined fiber specimens with CSA EA contents > 4%, compared to those with 2% EAs and without it. For instance, the dynamic pullout energies of inclined fiber specimens showing a fiber rupture failure were approximately 59–66% less than those showing a pulling out failure on average. The highest pullout energy of 1.30 J (N·m) was found in the aligned fiber specimen with 6% EA under the loading rate of about 665.5 mm/s, which is approximately 3.3 times greater than the static pullout energy of the identical specimen (0.40 J). Therefore, it can be concluded that the pullout energy of straight steel fibers embedded in UHPC matrix is influenced by the CSA EA content and loading rate: the incorporation of CSA EA into a UHPC mixture is effective in increasing the pullout energy of straight steel fibers under the impacts if the fiber's rupture failure is prevented, and the higher loading rate generally leads to a higher pullout energy. The loading rate sensitivity can be adopted to quantitatively evaluate the effectiveness of reinforcing fiber type for concrete under high loading rate conditions, i.e., impact and blast. The rate sensitivity of the average bond strength (τav) and pullout energy (WP) were thus compared according to the CSA EA content, as given in Figs. 13 and 14. The maximum fiber stress (σf,max) and equivalent bond strength (τeq) intimately depend on the maximum pullout load and pullout energy, respectively, so that they gave similar trends to those of the average bond strength and pullout energy and were not considered to avoid duplicative analyses. However, this does not mean that the parameters, σf,max and τeq, are unimportant and meaningless. In general, both the aligned and inclined straight steel fibers in UHPC containing CSA EA exhibited higher loading rate sensitivity to the average bond strength as compared to their counterparts without it, as shown in Fig. 13. This phenomenon is more obvious in the inclined fiber specimens than in the aligned specimens. It is thus concluded that the UHPC matrix with CSA EA can be more effectively reinforced with the straight steel fibers under impact loading as the fibers are inclined to the direction of the load. There was no clear trend on the effect of CSA EA content on the rate sensitivity of the pullout energy in Fig. 14, which is inconsistent

w/o EA (plain) w/ EA (2%) w/ EA (4%) w/ EA (6%) w/ EA (8%) fiing curve [w/o EA (plain)] fiing curve [w/ EA (2%)] fiing curve [w/ EA (4%)] fiing curve [w/ EA (6%)] fiing curve [w/ EA (8%)]

3 2.5 2 1.5 1 0.5

3.5 w/o EA (plain) w/ EA (2%) w/ EA (4%) w/ EA (6%) w/ EA (8%) fiing curve [w/o EA (plain)] fiing curve [w/ EA (2%)] fiing curve [w/ EA (4%)] fiing curve [w/ EA (6%)] fiing curve [w/ EA (8%)]

3 2.5 2 1.5 1 0.5

0.01

0.1

1

10

100

1000

0.01

0.1

1

10

100

Loading rate (mm/s)

Loading rate (mm/s)

(a) Aligned fiber (0ƒ)

(b) Inclined fiber (45ƒ)

1000

Fig. 13. Effect of CSA EA content on the DIF of average bond strength and loading rate relationships: (a) aligned fiber and (b) inclined fiber. 208

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3.5 w/o EA (plain) w/ EA (2%) w/ EA (4%) w/ EA (6%) w/ EA (8%) fiing curve [w/o EA (plain)] fiing curve [w/ EA (2%)] fiing curve [w/ EA (4%)] fiing curve [w/ EA (6%)] fiing curve [w/ EA (8%)]

3 2.5 2 1.5

DIF on pullout energy, WP

DIF on pullout energy, WP

3.5

1 0.5

w/o EA (plain) w/ EA (2%) w/ EA (4%) w/ EA (6%) w/ EA (8%) fiing curve [w/o EA (plain)] fiing curve [w/ EA (2%)] fiing curve [w/ EA (4%)] fiing curve [w/ EA (6%)]

3 2.5 2 1.5

1 0.5

0.01

0.1

1

10

100

1000

0.01

0.1

1

10

100

1000

Loading rate (mm/s)

Loading rate (mm/s)

(a) Aligned fiber (0ƒ)

(b) Inclined fiber (45ƒ)

Aligned fiber Inclined fiber fiing curve [Aligned fiber] fiing curve [Inclined fiber]

3

2.5 2 1.5

1 0.5

3.5

DIF on avg. bond strength, τav

3.5

DIF on avg. bond strength, τav

Aligned fiber Inclined fiber fiing curve [Aligned fiber] fiing curve [Inclined fiber]

3

2.5 2 1.5

1 0.5

0.1

1

10

100

1000

3.5 Aligned fiber Inclined fiber fiing curve [Aligned fiber] fiing curve [Inclined fiber]

3

2.5 2 1.5

1 0.5

0.01

0.1

1

10

100

1000

0.01

0.1

1

10

100

Loading rate (mm/s)

Loading rate (mm/s)

Loading rate (mm/s)

(a) CSA EA 0% (plain)

(b) CSA EA 2%

(c) CSA EA 4%

DIF on avg. bond strength, τav

0.01

3.5

DIF on avg. bond strength, τav

DIF on avg. b ond strength, τav

Fig. 14. Effect of CSA EA content on the DIF of pullout energy and loading rate relationships: (a) aligned fiber and (b) inclined fiber.

Aligned fiber Inclined fiber fiing curve [Aligned fiber] fiing curve [Inclined fiber]

3

2.5 2 1.5

1 0.5

1000

3.5 Aligned fiber Inclined fiber fiing curve [Aligned fiber] fiing curve [Inclined fiber]

3

2.5 2 1.5

1 0.5

0.01

0.1

1

10

100

1000

0.01

0.1

1

10

100

Loading rate (mm/s)

Loading rate (mm/s)

(d) CSA EA 6%

(e) CSA EA 8%

1000

Fig. 15. Effect of fiber inclination angle on the DIF of average bond strength and loading rate relationships of steel fiber in UHPCs with various CSA EA contents: (a) CSA EA 0% (plain), (b) CSA EA 2%, (c) CSA EA 4%, (d) CSA EA 6%, and (e) CSA EA 8%.

with the findings of the static pullout tests explained above because of the snubbing effect. Although the enhanced pullout resistances of 45°inclined fibers in UHPC were obtained at the impact loads, the effectiveness of inclining the fibers on the pullout resistance was higher for the slower loading rates (2-kN impact). For example, the average bond strengths of all the inclined fiber specimens with and without CSA EAs were averaged and found to be about 19.1 and 20.7 MPa at the slower and faster loading rates, respectively, and these values were approximately 34% and 18% higher than the averaged values of all the aligned fiber specimens at identical loading rates, respectively. The dynamic pullout energy and equivalent bond strength are dependent on the entire behavior of the pullout load–slip curve. Thus,

average bond strength and maximum fiber stress. This is caused by the much gradual decrease in the post-peak pullout load versus slip of the inclined fiber cases than the aligned cases, as shown in Fig. 7. As the pullout energy is calculated from the area under the pullout load-slip curve, it was amplified by the higher maximum pullout load and the more gradual post-peak softening curve. Therefore, inclining the straight steel fiber to 45° is effective in enhancing the pullout resistance and most efficient at improving the pullout energy absorption capacity compared to the aligned fiber case. Given impact loading conditions, the inclined fiber specimens provided higher average bond strengths and maximum fiber stresses compared to the aligned fiber specimens in general. This is consistent 209

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3.5 Aligned fiber Inclined fiber fiing curve [Aligned fiber] fiing curve [Inclined fiber]

3 2.5

DIF on pullout energy, WP

DIF on pullout energy, WP

3.5

2 1.5 1 0.5

Aligned fiber Inclined fiber fiing curve [Aligned fiber] fiing curve [Inclined fiber]

3 2.5 2 1.5 1 0.5

0.01

0.1

1

10

100

1000

0.01

0.1

1

10

100

Loading rate (mm/s)

Loading rate (mm/s)

(a) CSA EA 0% (plain)

(b) CSA EA 2%

1000

Fig. 16. Effect of fiber inclination angle on the DIF of pullout energy and loading rate relationships of steel fiber in UHPCs with various CSA EA contents: (a) CSA EA 0% (plain) and (b) CSA EA 2%.

analyses, the following conclusions can be drawn:

these parameters were strongly influenced by the failure mode (pullout vs. rupture), as shown in Fig. 10: some inclined fiber specimens in UHPC with CSA EAs exhibited much poorer pullout energies under impact loading because of their fiber rupture failure. Given an identical pulling out failure mode, higher dynamic pullout energy and equivalent bond strength were obtained in the inclined fiber specimens than in the aligned ones. In order to quantitatively support this explanation, the pullout energies of all inclined fiber specimens with various CSA EA contents from 0 to 8% were averaged and found to be 1.03 and 1.21 J at the slower and faster impact loads, respectively, when they were pulled out without any rupture. These values were approximately 17% and 11% higher than the averaged pullout energies of the aligned fiber specimens at the identical loading rates, respectively, which implies that the effectiveness of inclining the fiber 45° to the direction of the load on enhancing the pullout energy is higher at the slower loading rate than at the faster rate in general. As shown in Fig. 15, the inclined steel fibers led to generally higher or at least similar loading rate sensitivity of the average bond strengths than the aligned ones, except for the case of plain UHPC because the bond strength of the former was more greatly dependent on the matrix strength than that of the latter, which is verified by the greater matrix spalling area in Fig. 9. For both aligned and inclined conditions, the pullout energy was more strongly dependent on the loading rate than the average bond strength if the steel fibers were completely pulled out from the UHPC matrix without any rupture. This indicates that the energy absorption capacity of UHPC can be more efficiently strengthened by incorporating straight steel fibers under impact loading than the post-cracking tensile strength. Since the inclined steel fibers embedded in the UHPC matrices with higher EA content were ruptured, the effect of the inclination angle on the rate sensitivity of the pullout energy was only compared in the specimens with 0 and 2% CSA EAs, as shown in Fig. 16. Interestingly, it was observed that the rate sensitivity of the pullout energy of straight steel fibers in UHPC is insignificantly affected by the inclination angle of the fibers.

1) Both the early age and ultimate compressive strengths of UHPC were improved by approximately 10–22% due to the addition of CSA EA. 2) The shrinkage strain of UHPC could be reduced by including 6% EA or more. Therefore, the minimum content of CSA EA is suggested as 6% to effectively compensate for the volume reduction in UHPC. 3) Some inclined steel fibers in UHPC matrices with the CSA EAs > 4% led to changes in the failure mode from the pullout at static loading to the rupture at impact loading, due to the higher rate sensitivity of the matrix cracking strength than the fiber tensile strength. 4) The average bond strength and pullout energy of aligned fiber in UHPC decreased, whereas those of inclined fiber increased by including the CSA EA under the quasi-static loads. 5) The dynamic bond strengths and pullout energies of aligned and inclined fibers in UHPC improved with the addition of the CSA EA if the fiber breakage is prevented. The dynamic pullout energies of inclined, ruptured fibers were approximately 59–66% less than those with complete pullout failures. 6) Inclining the straight steel fiber to 45° according to the direction of load was efficient in improving the dynamic bond strength and pullout energy, approximately 29% and 69% higher than the aligned fibers, respectively. 7) The fiber pullout resistances improved at higher loading rates. In general, a higher rate sensitivity of the average bond strength of straight steel fiber in UHPC was obtained by adding the CSA EA and inclining the fiber. The rate sensitivity of the pullout energy of the steel fibers that were pulled out from the matrix was higher than the average bond strength, indicating that the energy absorption capacity of UHPC can be more effectively reinforced with the addition of straight steel fibers as compared to its post-cracking tensile strength under the impact loading conditions. Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. 2017R1C1B2007589).

4. Conclusions In this study, the shrinkage behaviors of UHPC matrices with various CSA EA contents were measured to evaluate their impact on the matrices' volume stabilities. To examine the effects of CSA EA content, inclination angle, and loading rate on the pullout behaviors of straight steel fiber embedded in UHPC matrix, five different EA contents ranging from 0% to 8%, two different inclination angles of 0° and 45°, and various loading rates ranging from 0.018 mm/s (static) to 1244 mm/s (impact) were also considered. From the experimental results and

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