- Email: [email protected]
Cryogenic pullout behavior of steel fibers from ultra-high-performance concrete under impact loading
Construction and Building Materials 239 (2020) 117852
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Cryogenic pullout behavior of steel fibers from ultra-high-performance concrete under impact loading Min-Jae Kim, Doo-Yeol Yoo ⇑ Department of Architectural Engineering, Hanyang University, 222 Wangsimni-ro, Seongdong-gu, Seoul 04763, Republic of Korea
h i g h l i g h t s Bond strength of steel fibers in UHPC increases with increasing loading rate. Cryogenic temperature is effective on improving the bond strength of steel fibers in UHPC at both static and impact loads. The DIFs on the pullout resistance decrease at geometrically deformed and inclined conditions. Slip capacity is reduced in general at the impact loading condition regardless of temperature. Cryogenic temperature negatively affects the rate sensitivity of pullout resistance of steel fibers.
a r t i c l e
i n f o
Article history: Received 10 September 2019 Received in revised form 13 November 2019 Accepted 11 December 2019
Keywords: Ultra-high-performance concrete Steel fiber type Pullout response Impact loading Cryogenic condition
a b s t r a c t This study investigates the effect of impact loading condition on the pullout property of steel fibers from ultra-high-performance concrete (UHPC) under various temperatures. For this, static and impact loads, ambient and cryogenic ( 170 °C) temperatures, three steel fiber types, i.e., straight, half-hooked, and twisted, and two inclination angles of 0° and 45° were considered. From the test results and analysis, it was found that the steel fibers mostly had positive values of dynamic increasing factor (DIF) for the average bond strength at ambient temperature, but these were significantly reduced when the fibers were geometrically deformed, inclined, or tested under cryogenic temperature. Slip capacities generally decreased under the influence of the impact loading condition, and this effect was more severe at cryogenic temperature. There was no obvious impact loading rate effect on the probabilities of fiber and UHPC matrix damages at both ambient and cryogenic temperatures. The DIF values for pullout energies normally decreased or even became negative when the fibers were inclined, geometrically deformed, or tested at cryogenic temperature. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Ultra-high-performance concrete (UHPC) has been developed by a number of researchers since its first introduction in the 1990s [1–5]. UHPC has a very high compressive strength, tensile strength, and strain capacity of over 150 MPa, 8 MPa, and 0.5%, respectively, in general, when steel fibers are incorporated. These levels of performance are universally obtained from the optimized particle packing of reactive powders [6,7]. UHPC normally consists of Portland cement, silica fume (SF), silica sand, and silica flour, for more compact and homogeneous microstructures. Most of the time, 20–30% of the cement by weight is substituted with the same amount of SF [4,8,9]. The reactive powders undergo pozzolanic reactions with the calcium hydroxide (Ca(OH)2) in a hydrating
⇑ Corresponding author. E-mail address: [email protected] (D.-Y. Yoo). https://doi.org/10.1016/j.conbuildmat.2019.117852 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
mortar, forming dense calcium silicate hydrate (C-S-H) gels on the surfaces of hydrates. These gels enhance the mortar interfaces and increase the bond strength between the fiber and matrix [9]. Generally, 2% by volume of steel fibers are included into the UHPC mix to achieve excellent tensile performance. Lastly, water or steam curing at a high temperature above 90 °C is generally adopted to accelerate the cement hydration and pozzolanic reactions. It is significant that UHPC exhibits much better tensile performance than that of normal concrete mixes, but there still is a requirement for even better tensile performance, considering the practical uses of the material. Diverse studies have been performed to investigate and further improve the tensile performance of UHPC. For this, researchers have been trying to adjust the UHPC matrix components [5,8], interfacial transition zones [3,10], and fiber properties [2,4,11]. Despite previous research [2–5,8,10,11], steel fiber pullout properties and their influence on the UHPC matrix have not yet been fully investigated and developed.
2
M.-J. Kim, D.-Y. Yoo / Construction and Building Materials 239 (2020) 117852
In the meantime, many researchers have performed diverse research on the influence of cryogenic temperatures on the physical properties of normal strength concrete and reported enormous increases of compressive and tensile strength due to frozen and hardened moisture in the concrete [12–17]. In the case of the UHPC incorporating steel fibers, however, there was no published paper until Kim et al. [18] first reported the enhancement of compressive and tensile performances at the cryogenic temperature. They [18] examined the influence of cryogenic temperatures of about 162 °C, considering of the liquefied natural gas industry and found out that the tensile strength and even the strain capacity of UHPC remarkably improved at the cryogenic temperature. Based on micrographs for the surface scratches of each pulled out fiber, they [18] attributed these effects to variations in fiber–matrix interaction because of the hardening of matrix particles at the cryogenic condition. The tensile strength and strain capacity were both considerably enhanced because frozen and hardened matrix particles at the fiber–matrix interfaces could resist harder and longer against pullout of fibers at the cryogenic temperature than at the ambient temperature. Kim and Yoo [19] then focused on the pullout properties of single steel fibers embedded in the UHPC matrix at the cryogenic temperature and reported that cryogenic temperature significantly enhanced the average bond strength and slip capacity via the higher frictional resistance as reported in the previous study. They [19] also reported that the largely increased frictional resistance even caused higher chances of fiber fracture and matrix-spalling areas, especially for the inclined and geometrically deformed steel fibers.” ‘‘Furthermore, a number of studies have been conducted on the effect of loading rate on the mechanical behaviors of UHPC, including fiber pullout behavior [2,20–24]. It is generally known that increasing the loading rates changes load transfer mechanisms via the inertia effect, which influences the crack propagation or fiber–matrix deformation [20–22]. Some researchers have reported a positive dynamic increasing factor (DIF) regarding the fiber bond strength and tensile strength, although other researchers also reported that there was no positive influence on the bond strength [2,20,24]. However, there has been no published study investigating the influence of geometrical deformation and inclination of steel fibers and impact loading rate on the entire fiber pullingout process from the UHPC matrix, examined at the cryogenic condition. This study, therefore, focused on investigating the effects of the loading rate and cryogenic condition on the pullout behavior of various steel fibers from the UHPC matrix by considering their inclination angles and geometries.”
2.1. Mixture proportions The mix proportions for the UHPC matrix used in this study are given in Table 1. Type 1 Portland cement and SF with a mean particle size of 0.31 lm were used as binding materials. The SF replaced 25% of the cement by weight, for a denser microstructure and stronger fiber–matrix bond [8,25]. Silica sand and silica flour with average particle diameters of 337 and 4 lm, respectively,
Chemical composition (%)
Type 1 Portland Cement
SF
CaO Al2O3 SiO2 Fe2O3 MgO SO3 Specific surface area (cm2/g) Density (g/cm3)
61.33 6.40 21.01 3.12 3.02 2.30 3,413 3.15
0.38 0.25 96.00 0.12 0.10 – 200,000 2.10
Table 3 Physical properties of steel fibers.
Table 1 Mixture proportion of UHPC.
0.2
Fig. 1. Diffraction volume percentage of materials. Table 2 Chemical compositions and physical properties of cement and silica fume.
2. Experiments
W/ B
were incorporated as fine aggregate and filler, respectively. Coarse aggregates were eliminated from the mix, as shown in Table 1, for better homogeneity and a denser microstructure to improve fiber pullout properties. This is because for composites such as cement mortar or concrete mixes, the detrimental influence of interfacial transition zones (ITZ) between components leads to preexisting cracks and locally weaker structures. In addition, this phenomenon becomes severe when the sizes of the constituents are bigger. Moreover, the distribution and orientation of steel fibers become degraded, and the fiber pullout property deteriorates as the matrix shrinkage is resisted by the inclusion of coarse aggregates [26,27]. Fig. 1 shows the detailed diffraction percentages for the four kinds of powder ingredients. The SF used in this study was of the same type as the one used in a previous study [28]. Table 2 lists the chemical and physical properties of the Portland cement and SF. The type 1 Portland cement consisted of 61.3% CaO and 21.0% SiO2, whereas the SF comprised 96% SiO2. The specific surface areas of the cement and SF were 3,413 and 200,000 cm2/g, respectively. The mix consisted of a large amount of powdery materials with very high specific surface areas; the water-to-binder ratio (W/B) was controlled to as low as 0.2 to obtain higher mechanical properties. Thus, polycarboxylate superplasticizer (SP) was used to control the mortar flow value at over 210 mm, which was measured in accordance with ASTM C1437 [29]. Three types of steel fibers (i.e., straight (S), half-hooked (H), and twisted (T)) with equivalent diameters of 0.3, 0.375, and 0.5 mm, respectively, and with a length of 30 mm were used. Table 3 shows
Unit weight (kg/m3) Water
Cement
SF
Silica sand
Silica flour
Superplasticizer
160.3
788.5
197.1
867.4
236.6
52.6
3
[Note] Superplasticizer includes 30% solid (=15.8 kg/m ) and 70% water (=36.8 kg/ m3).
S H T
df [mm]
lf [mm]
Aspect ratio [lf/ df]
Density [g/ cm3]
ft [MPa]
Ef [GPa]
0.300 0.375 0.500
30.0 30.0 30.0
100.0 80.0 60.0
7.9 7.9 7.9
2,580 2,900 2,428
200 200 200
[Note] S = straight steel fiber, H = half-hooked steel fiber, T = twisted steel fiber, df = diameter, lf = length, ft = tensile strength, and Ef = elastic modulus.
M.-J. Kim, D.-Y. Yoo / Construction and Building Materials 239 (2020) 117852
the physical properties of the steel fibers. The S, H, and T fibers had tensile strengths of 2,580, 2,900, and 2,428 MPa, respectively. The H and T fibers had deformed geometries for applying mechanical anchorage effects to the cement matrix. The H fiber had the same round shape as that of the S fiber and had hooked ends, whereas the T fiber had a triangular section and was twisted three times along its longitudinal direction. These deformations led to mechanical anchoring effects in the cement matrix and stress concentration on the surrounding matrix. In this study, the H fiber was prepared by cutting each half of the end-hooks from previously used hooked-end steel fibers [30]. The mechanical anchorage of the deformed fibers began while the deformed fibers recovered their original geometry after the fiber yield strength was reached [2]. All the steel fibers were brass-coated mainly for better corrosion resistance, and also for a stronger chemical bond between the fiber surface and the matrix [31]. 2.2. Specimen fabrication Fiber pullout specimens were fabricated in accordance with the following procedures. The four dry ingredients, i.e., Portland cement, SF, silica flour, and silica sand, were premixed in a mixer for 5 min. Water-premixed SP was then poured into the mixer and mixed for another 10 min. Molds for pullout specimens and fibers were prepared as suggested in Fig. 2. Every fiber tested in this study was 30 mm long and embedded by 10 mm. This 10 mm embedded part of the fiber protruded from a PVC film, while the opposite 20 mm part was fixed onto an acryl block to keep the fresh mixture from flowing into the other side and to ensure the alignment of the fibers. The fixed end of the fiber was bent to form an anchoring hook in the matrix, to ensure that every fiber was pulled out only from the pullout side. The specimen had a cross-sectional area of 25 25 mm2. As described previously, two
3
experiment cases were prepared for each fiber; the cases were differentiated by the tilt angle, which was either 0° or 45°, to investigate the fiber inclining effect on the pullout behavior. The inclination angle of 45° was adapted from previous research, which reported that angles between 30° and 60° led to a higher fiber–matrix bond strength [2,32–34]. After the fresh UHPC mixture was mixed, the mix was placed into the pullout side, shown in Fig. 2, and the specimens were covered with plastic sheets to minimize abrupt drying and were cured at room condition for 48 h. The acryl blocks were then carefully removed, the same fresh UHPC mix was placed into the opposite fixed side, and the sealing and curing procedures were repeated. After another 48 h, the specimens were demolded and cured at 90 °C in a water tank for 72 h to accelerate the hydration process. Finally, the specimens were left in room condition for 4 days more before being tested. Three test variables, i.e., geometrical deformation of fiber, temperature condition, and fiber tilt angle, were set. Thus, 3 (fiber types) 2 (temperatures) 2 (fiber inclination angles) 5 (samples for each variable) = 60 specimens were tested and analyzed in this study. Results of the tests and microscopic analysis for the same specimens, which tested under the static loading condition, were adapted from a previous study [19], and they were compared with those of the impact pullout test results. Regarding the consistency of the pullout test results, the fiber inclination and cryogenic temperature increased the fiber rupturing possibility, showing irregular data patterns. Therefore, majority trends with more than three data for each variable were analyzed to obtain a reliable conclusion. The specimen-naming process was based on loading rates (‘‘ ” or ‘‘D” for static and impact loading rates, respectively), temperature conditions (‘‘ ” or ‘‘C” for ambient or cryogenic temperatures, respectively), fiber types (S, H, and T), and fiber inclination (‘‘ ” or ‘‘-I” for aligned or inclined states, respectively). For examples, if a specimen with an inclined T fiber was tested under the
Fig. 2. Specimen fabrication: (a) Fiber and mold setup and (b) placing and curing procedure.
4
M.-J. Kim, D.-Y. Yoo / Construction and Building Materials 239 (2020) 117852
Pressure Gauge Load cell (10 kN)
Air compressor Potentiometer
Air amplifier
Loading Piston
Impact load
Air storage tank
Fig. 3. Setup for fiber pullout test under impact loading condition.
cryogenic and the static loading condition, it is referred to as ‘‘CTI”, whereas the same specimen tested under the cryogenic and impact loading condition was referred to as ‘‘DCT-I.”
2.3. Impact fiber pullout test under cryogenic condition Fig. 3 shows a test setup for an impact fiber pullout test adapted for this study. Specially manufactured test equipment was used for this experiment. The impact loading rate was controlled by setting air pressure to be fixed at 31.9 kg/cm2, which resulted in the 2 kN load at the loading cylinder. This adjusted the loading rate to 480 mm/s without a dog-bond specimen as per a previous study [30]. Two steel jigs were separately installed. The lower one was completely fixed, whereas the upper one was pinned, allowing slight rotation of the specimen to minimize data noises caused by eccentric force. A load cell with a maximum capacity of about 10 kN was attached to the upper grip system to eliminate inertia effect at the start of the loading. A potentiometer was used to record the movements of the lower grip system, which was connected to a piston with an air-compressing system. The elastic deformations of the matrix, steel fibers, and steel jigs were expected to be negligible, and thus, the displacement of the potentiometer was used as the fiber end slip. Once a pullout specimen was held between the two jigs, an initial load was set at 2 kN to reduce data noises. A dynamic data recording system with a 20 kHz frequency was used to acquire sufficient data points from the quick impact loading test. For the cryogenic condition, the temperature of every specimen was kept below 170 °C. In previous research [18], the centroid temperature of Ø100 200 UHPC specimens surrounded by liquefied nitrogen (LN2) was drastically chilled to below 170 °C in about 15 min. After that, the decreasing rate of the centroid temperatures was significantly reduced and nearly constant. This study also used LN2 to rapidly freeze the fiber pullout specimens. The pullout specimens were also preliminarily chilled for 5 min, calculating the shorter distance to the centroid, to set the centroid temperature to below 170 °C. The completely chilled specimen was then moved into the grip system and immediately tested within approximately 60s.
3. Test results and analysis 3.1. Bond–slip curves Fig. 4 shows the general bond stress–slip curves of steel fibers tested under both the static and impact conditions. The impact pullout test results were compared with those for the static condition [19]. A steel fiber embedded in a UHPC matrix generally demonstrates linear increase of bond stress at the beginning of the test, until the partial de-bonding of the fiber–matrix interface begins. Subsequently, the partial de-bonding process lasts up to the peak point of the bond stress and slip curve, where the full de-bonding of the fiber and matrix occurs, and only the frictional pullout resistance is present [2,35–37]. The bond stress then decreases continuously until the fiber is completely pulled out from the matrix, but the pullout resistance is maintained because of the friction caused by the matrix debris at the fiber–matrix interface [38]. Kim and Yoo [19] found that, under the static loading condition, the initial stiffness increased when the fibers were geometrically deformed or pulled out at a cryogenic temperature. These test variables both enhanced the fiber–matrix interaction via their respective mechanisms. The deformed fibers had mechanical anchorage effects when a pullout load was applied through a large region of the matrix [30,38–40]. The UHPC matrix, meanwhile, incorporates a portion of water in diverse forms, such as pore water, C-S-H interlayer water, and water molecules in hydrates. Once the UHPC matrix becomes frozen, the various forms of included water molecules then provide additional strength [12,16]. This results in the higher strength of the UHPC matrix, including its debris at the fiber interface. This was the reason for the large improvement in steelfiber pullout performance under the cryogenic condition [18,19]. It was also reported that as the steel fibers were tilted from the pullout direction, the initial increasing phases of the steel fibers became smaller [2,33,34,37,41]. Fig. 5 shows SEM images taken for the surfaces of S-series fibers. The extent of surface damage of the pulled out S fibers became clearly more severe, when the fibers were pulled out under the inclined or cryogenic condition than under the aligned or ambient condition. This indicates that the UHPC matrix itself and particles at the interface resisted harder, under the inclined or cryogenic condition.
M.-J. Kim, D.-Y. Yoo / Construction and Building Materials 239 (2020) 117852
5
Fig. 4. General bond stress-slip curves of steel fibers.
Under the impact loading condition, however, pullout behaviors that were different from those under the static loading condition were observed. According to Fig. 4, the initial increasing phases were steeper under the impact loading condition than under the static condition, whether the fibers were aligned or tilted by 45° with respect to the pullout direction. Under the impact loading condition, the initial stiffness of the inclined steel fibers was also observed to be not discernably lower than that of the aligned fibers. These were also attributed to the inertia effect, which has been observed under the impact loading condition [20–22]. Based on previous research by Fu et al. [23], the mechanical strength and stiffness of concrete were clearly increased. In addition, under the impact loading condition, the fiber–matrix interface did not have enough time for a transfer of the applied stress, with cracks causing a bigger deformation of the surrounding matrix. In other words,
the fiber–matrix interface suffered concentrated stress under the impact loading condition. Furthermore, although the deformed fibers had higher bond strengths than that of the S fiber, the former generally showed more rapid reductions of bond stress than the latter did. This is attributed to the local stress concentration and damage on the fiber–matrix interfaces, caused by geometrical deformation, especially under the impact and cryogenic conditions. The S fiber without geometrical deformation exhibited a large improvement of the pullout strength and even the slip capacity, under the cryogenic temperature. This was because of a huge enhancement of pullout resistance provided from matrix as discussed in Fig. 4. For the deformed fibers, on the other hand, not only the pullout resistance was excessively increased because of the mechanical anchorage effect but also the tensile force applied to the fiber and matrix
6
M.-J. Kim, D.-Y. Yoo / Construction and Building Materials 239 (2020) 117852
Fig. 5. SEM images for the surface of S fibers pulled out under different test conditions.
was likely to be concentrated at certain points. The stress concentration also caused several fibers to be fractured during the pulling-out process, and this will be discussed later in this paper. 3.2. Increasing rates of fiber pullout parameters under cryogenic and impact loading conditions 3.2.1. Average bond strength Fig. 6 illustrates the average bond strengths and slip capacities obtained under the ambient and cryogenic temperatures, and under the static and impact conditions. Meanwhile, Fig. 7 visualizes DIF and cryogenic increasing factor (CIF), the latter of which denotes variations in the pullout parameters according to exposure to the cryogenic condition. Under the static loading condition, every aligned steel fiber could be observed to have a considerably higher bond strength at the cryogenic temperature as compared with at the ambient temperature. The S, H, and T fibers showed clear increases in bond strengths, from 10.9, 18.6, and 17.3 MPa, respectively, to 32.2 (CS), 23.7 (CH), and 34.2 (CT) MPa, respectively, after being exposed to the cryogenic condition. Regarding the bond strengths, the CIF values were 95%, 73%, and 98% for the S, H, and T fibers, respectively. This was mainly because of the enhancement of the fiber–matrix interaction, which was due to the hardened matrix particles at the fiber–matrix interface under the cryogenic temperature, as mentioned previously. It is generally known that inclined steel fibers exhibit higher bond strength than that of corresponding aligned fibers because of the stronger resistance of the matrix at the fiber exit. This is caused by the snubbing and bending effects of the fibers, which lead to stronger frictional resistance and additional damage to the matrix [37,38,40]. Kim and Yoo [18,19] reported that when the fibers were inclined, the damage on the matrix caused by the steel fibers, especially by the deformed fibers, reduced the CIF val-
ues of the bond strengths under the static condition. The average bond strengths of the S-I, H-I, and T-I fibers were measured to be at 12.0, 24.1, and 22.3 MPa, respectively, which were 11%, 29%, and 28%, respectively, higher than those of the aligned fibers. Under the cryogenic condition, however, these values were different, i.e., 20.2 (CS-I), 23.7 (CH-I), and 26.9 (CT-I) MPa, respectively. The cryogenic increasing rates were significantly smaller than those for the aligned condition, by only about 68%, 2%, and 21%, respectively. This result denotes that the inclination and deformation of steel fibers led to less effective cryogenic effects or even had a detrimental influence on the bond strength of the steel fibers. The DIF values for the average bond strengths of steel fibers with respect to change of loading rate from 0.018 mm/s (static) to 480 mm/s (impact) are generally known to range between 1.0 and 1.23, according to previous studies [2,20,24]. Figs. 6 and 7a show that the DS, DH, and DT fibers had higher average bond strengths of 14.4, 21.9, and 22.5 MPa, respectively, under the impact loading condition, corresponding to DIFs of 33%, 18%, and 30%, respectively. Even for the inclined fibers, the DIF values were generally positive at the ambient temperature. The average bond strengths of the DS-I, DH-I, and DT-I fibers were 19.0, 23.3, and 24.9 MPa, respectively, with DIFs of 58%, 3%, and 12%, respectively, based on the results for the S-I, H-I, and T-I fibers, respectively. As the temperature decreased to the cryogenic condition, on the other hand, the DIFs for the average bond strength became considerably lower. The average bond strengths of the CDS, CDH, and CDT fibers measured at 19.8, 27.1, and 37.0 MPa, respectively, corresponding to DIFs of 22%, 16%, and 8%, respectively. Subsequently, the CDS-I, CDH-I, and CDT-I fibers demonstrated average bond strengths of 19.8, 25.8, and 28.6 MPa, respectively, and thus the DIFs were 2%, 9%, and 6%, respectively. From a different perspective, it should also be noted that the CIF values with respect to the fiber bond strength under the impact
M.-J. Kim, D.-Y. Yoo / Construction and Building Materials 239 (2020) 117852
7
Fig. 6. Average bond strength and slip capacity.
Fig. 7. (a) DIF (b) CIF values for the average bond strength and slip capacity.
loading condition were not as effective as under the static condition, as shown in Figs. 6 and 7b. The average bond strengths of the CDS, CDH, and CDT fibers measured at 16.5, 27.1, and 37.0 MPa, respectively, and their corresponding CIFs were 15%, 23%, and 64%, respectively, significantly lower than the CIFs of the S, H, and T fibers, which were 95%, 73%, and 98%, respectively. The CDS-I, CDH-I, and CDT-I fibers provided average bond
strengths of 19.8, 25.8, and 28.6 MPa, respectively, and their CIF values were measured at only 5%, 10%, and 15%, respectively. 3.2.2. Slip capacity Slip capacity, shown in Fig. 6, denotes the fiber-end slip values measured at the peak points of the bond–slip curve. The S, H, and T fibers demonstrated slip capacities of 0.9, 1.6, and 3.1 mm, respec-
8
M.-J. Kim, D.-Y. Yoo / Construction and Building Materials 239 (2020) 117852
tively, and the inclined S-I, H-I, and T-I fibers exhibited 2.1, 2.9, and 3.0 mm, respectively. This was coincident with previous reports noting that under the static loading condition, the slip capacities are generally increased as steel fibers are tilted from the pullout direction [40,41]. It was mentioned [19] that under cryogenic temperature and static loading condition, the slip capacity of the S fiber was largely increased because of enhanced fiber pullout resistance, whereas the deformed fibers underwent overly high pullout resistance and even fracture. The slip capacities of the S and S-I fibers increased from 0.9 and 1.3 mm, respectively, to 1.7 and 2.65 mm, respectively, corresponding to CIFs of 89% and 29%, respectively, whereas those for the deformed, H (1.6 mm) and H-I (2.9 mm) fibers or T (3.1 mm) and T-I (3.0 mm) fibers, were measured at only 3% and 26%, or 54% and 7%, respectively. Based on the investigation on fiber surface, matrix tunnel, and fiber exit with a scanning electron microscope and an optical microscope, it was found that the matrix damage became more severe at the cryogenic temperature, specifically for the deformed or inclined fibers, leading to lower bond strengths or more rapid losses of bond stress [19]. Figs. 6 and 7a also show that the impact loading condition influenced the slip capacity of the steel fibers. The increased loading rate generally decreased the slip capacities of the steel fibers. Every steel fiber tested under the impact loading condition, except for the DT fiber, had lower slip capacities as compared with those tested under the static condition. The slip capacities of the DS, DH, and DT fibers were measured at only 0.9, 0.8, and 4.0 mm, respectively, and thus the DIFs for the slip capacities were 0%, 52%, and 30%, respectively. Moreover, the DS-I, DH-I, and DT-I fibers also demonstrated 1.3, 1.6, and 2.9 mm slip capacities. These were significantly lower than those of the S-I, H-I, and T-I fibers with DIFs, which are all measured negatively to be –32%, –43%, and –2%, respectively. The cryogenic temperature noticeably decreased the slip capacity of steel fibers under the impact loading condition, unlike under the static loading condition. The CDS, CDH, and CDT fibers exhibited 0.5, 0.9, and 0.7 mm, respectively, slip capacity values, which were the lowest for among all of the experimental conditions. For the cases involving inclined fibers, i.e., CDS-I, CDH-I, and CDT-I, the slip capacities measured were 1.0, 1.1, and 1.8 mm, respectively, which were much lower than those of the S-I (1.3 mm), H-I (2.9 mm), and T-I (3.0 mm) fibers tested under ambient temperature and static condition. 3.3. Maximum fiber tensile stress and failure mode In Fig. 8, the maximum fiber tensile stress and the probability of fiber fracture are summarized. It is shown that the fracture of inclined steel fibers was not thoroughly attributed to the fact that
the tensile stress received by each fiber exceeded the tensile strength. Under the static loading condition, the maximum fiber stresses of the ruptured H-I, CH-I, T-I, and CT-I fibers were 2,573, 2,533, 1,780, and 2,154 MPa, respectively, while the tensile strengths of the S, H, and T fibers were 2,580, 2,900, and 2,428 MPa, respectively, as described in Table 3. Kim and Yoo [19] explained this phenomenon based on the mechanism of the bending actions of the inclined fibers, which finally causes a higher frictional resistance and a stress concentration at the exit of the fibers [37,40]. The inclined fibers, thus exhibited larger chances of fracture than the aligned fibers. For the cases of the deformed fibers, however, the local stress concentration was more critical because the fibers already had higher pullout resistance than the S fiber via the mechanical anchorage effect. The inclined deformed fibers, therefore, had a 100% chance of being ruptured, under the static condition, whereas none of the S-I fiber was failed. Under the impact condition, the DS-I fiber also showed 40% chance of being ruptured unlike under the static condition (S-I). This was also be attributed to the inertia effect mentioned previously, which increases local stress concentration resulted from not enough time for stress dispersion. The chance of being ruptured for the DCT fiber, however, was observed to be only 40% which was 60% lower than that of the CT fiber. Except for these two cases, no notable variation in fiber fracture mode derived from the impact loading rate was found. In the meantime, the cryogenic temperature also considerably enhanced the pullout resistance provided to each fiber via the hardened matrix debris at the fiber–matrix interface, according to the previously explained mechanism. This phenomenon even caused fractures of the aligned CH (20%) and CT (100%) fibers: the maximum tensile stresses were 3,372 and 2,738 MPa, respectively, about 16.2% and 12.7%, respectively, higher than the respective tensile strengths provided by manufacturers. As illustrated in Fig. 8, under the impact loading rate, the maximum fiber tensile stresses measured at the ambient temperature generally increased. The maximum fiber tensile stresses of the DS, DH, and DT fibers were increased by 35.3%, 22.7%, and 29.8%, respectively, measuring at 1,731, 2,404, and 1,799 MPa, respectively. Those of the inclined cases were also increased by 62.9%, – 3.2%, and 11.8%, respectively, and measured to be 2,271 (DS-I), 2,490 (DH-I), and 1,990 (DT-I) MPa, respectively. Under the cryogenic and static condition, the maximum fiber stresses were considerably increased, as compared to those under the ambient and static condition. The maximum fiber stresses of the CS, CH, CT, CS-I, CH-I, and CT-I fibers were observed to be 2,829, 3,372, 2,738, 2,320, 2,533, 2,154 MPa, respectively. Furthermore, those values also generally increased under the impact load-
Fig. 8. Maximum fiber tensile stress and the probability of fiber fracture.
M.-J. Kim, D.-Y. Yoo / Construction and Building Materials 239 (2020) 117852
ing, except for the cases of the CDS and CDH fibers. The maximum tensile stresses of the CDS, CDH, CDT, CDS-I, CDH-I, and CDT-I fibers varied by –23.0%, –16.0%, 8.0%, 9.0%, 8.5%, and 6.3%, respectively, under the cryogenic temperature, as compared to those under the static condition, and the practical values measured at 2,178, 2,831, 2,958, 2,530, 2,748, and 2,291 MPa, respectively. It is normal for the tensile strength of most irons to increase at low temperatures. More specifically, high-carbon steel fibers with 0.7% carbon content were used in this study. Hong et al. [42] reported that the tensile strength of a high-carbon steel including a 0.65–0.76% carbon increased by 40%, when the temperature decreased from 25 °C to 192 °C. Similar to this result, the maximum fiber stress values measured at the cryogenic temperature could exceed the nominal tensile strength of the steel fibers. The maximum tensile stresses of the CDS and CDS-I fibers, on the other hand, were specifically lower than those of the CS and CS-I fibers, having differences of 651 and 518 MPa, respectively. The other deformed fibers, meanwhile, demonstrated relatively smaller differences resulting from the increase in loading rate. The tensile strengths of the H and T fibers were 2,900 and 2,428 MPa, respectively. Thus, the CH and CDH fibers, which were measured to have maximum tensile stresses of 3,372 and 2,831 MPa, respectively, were noted to have 20% and 0%, respectively, chances of failing, but the CH-I and CDH-I fibers with maximum tensile stresses of 2,533 and 2,748 MPa, respectively, were determined to both have a 100% probability of being fractured. Moreover, it was also noted that the CT fiber, with a maximum fiber stress of 2,738 MPa, had a 100% chance of fracture, whereas the aligned CDT fiber, with a higher maximum fiber stress of 2,958 MPa, had only a 40% chance of fracture, although the maximum fiber stresses of the CT and CDT fibers exceeded the tensile strength by 12.8% and 21.8%, respectively. This characteristic might be caused by both the stronger matrix splitting action of the T-fiber under the impact loading rate. 3.4. Degree of matrix spalling The matrix-spalling areas that have been calculated are summarized in Fig. 9 with their respective probabilities of fiber fracture. Kim and Yoo [19] have taken optical microscopic images of fiber–matrix interfaces and fiber exits, and they have found that while the inclined or deformed steel fibers caused significantly larger damage to the matrix, even the aligned steel fibers caused the matrix around the fiber exit to crack. In addition, the damage received by the matrix was stronger at a cryogenic temperature. In this study, similar microscopic images were taken to investigate
9
the effect of impact loading rate on matrix damage around the fiber exit points. According to microscopic analysis, however, no discernable difference regarding the magnitude of matrix damage resulted from the impact loading rate.
3.5. Fiber pullout energy Fig. 10 shows normalized pullout energy values measured up to the peak point and to 5 and 10 mm of slips, which were denoted as Ep, E5, and E10, respectively. These values indicate the absorbed energy per unit bonding area (mJ/mm2). As was previously reported [19], under the static and ambient conditions, the aligned, deformed fibers absorbed higher energies than did the S fiber. The pullout energies of the deformed fibers, however, showed clear inefficiency when they were inclined. The deformed fibers also had high Ep/E10 and E5/E10 ratios, which indicate brittle pullout behaviors of the fibers. These trends became more significant as the fibers were inclined or pulled out at the cryogenic temperature and attributed to a stronger pullout resistance, stress concentration, and deterioration of fiber–matrix interface. As already mentioned, it is clearly seen that the deformed fibers had higher Ep/ E10 and E5/E10 ratios than those of the S fiber (Fig. 11). This trend became more significant under the inclined and cryogenic conditions. The slip capacities were generally lower under the impact loading condition because of the inertia effect. On the other hand, regarding the E5/E10 ratios, there was no discernable and typical influence by the impact loading rate. Fig. 12a illustrates DIFs with respect to the pullout energy. The deformation and inclination of fiber and the cryogenic temperature normally caused enhancement of the pullout resistance and stress concentration, and thus, a larger matrix-spalling area or a higher chance of fiber fracture was observed. The E10 values of the aligned S and T fibers largely increased by 24% and 63%, respectively, while that of the H fiber decreased by 7%. It is noted that the DIFs for the pullout energies tended to decrease or even became negative as the fibers were inclined, geometrically deformed, or tested at the cryogenic temperature. Except for the three aforementioned fibers, most of the other fibers exhibited negative DIFs, as can be seen in Fig. 12a. Although the DIF values of the CT and CH-I fibers were recorded to be 51% and 118%, respectively, it should be considered that these fibers failed right after the peak point and had very small E10 values of 24.7 and 29.5 mJ/mm2, respectively. Only the S-I fiber had a positive DIF value of 29% for the E10 parameter, among all the other fibers that were inclined or tested under the impact loading condition.
Fig. 9. Matrix spalling area and probability of fiber fracture.
10
M.-J. Kim, D.-Y. Yoo / Construction and Building Materials 239 (2020) 117852
Fig. 10. Normalized pullout energy.
Fig. 11. Ep/E10 and E5/E10 ratios.
The CIFs are also visualized in Fig. 12b. It is clearly seen that all of the deformed fibers, except for the H fiber, demonstrated negative CIFs. The H fiber had a high CIF of 64%, but this drastically decreased to 23%, 13%, and 4% for the H-I, DH, and DHI fibers, respectively. The CIF for the E10 of the S, S-I, DS, and DS-I fibers, on the other hand, were measured at 72%, 44%, 22%, and 6%, respectively. It is noted that even for the inclined S fiber, which involves no geometrical deformation and severe local matrix damage, exhibited a decrease in CIF for pullout energy. When the impact loading rate and cryogenic temperature were overlapped, a negative influence on the pullout energy of deformed or inclined steel fibers were clearly observed. This result was also attributed to the fact that performance deterioration at the fiber–matrix interface, caused by locally concentrated stress, became more severe under both the impact loading and cryogenic conditions.
4. Conclusion The influences of impact loading rate, cryogenic temperature, geometrical deformation, and inclination of steel fibers in UHPC on fiber pullout behavior were investigated. The bond–slip curves, bond strengths, slip capacities, and normalized pullout energy were analyzed, and the factors that increased according to the impact loading rate and cryogenic temperature were evaluated. For better understanding of the influences on the fiber pullout properties and for more reliable data analysis, optical micrographs
on the fiber–matrix interfaces and fiber exit were used. From the aforementioned test results and discussion, the following conclusions were obtained: The initial stiffness of the inclined steel fibers in UHPC largely increased under the impact loading condition. Even differences in the stiffness between the aligned and inclined fibers were not as discernable as the difference in the stiffness under the static loading condition. At the ambient temperature, the average bond strengths highly increased under the impact loading rate, exhibiting DIFs ranging from 3% to 58%. At the cryogenic temperature, however, the DIFs were considerably decreased and ranged from 30% to 9%. The slip capacities generally decreased at the impact loading condition regardless of the temperature but were further deteriorated by the cryogenic temperature. The DIFs for the slip capacities were generally recorded as negative, reaching down to 72%. Stress concentration due to enhanced pullout resistance at the cryogenic temperature or deformation and inclination of steel fibers caused premature fracture of fibers and larger matrix damage. However, there was no discernable influence of the loading rate on the chances of fiber failure mode and on the magnitude of matrix damage at both the ambient and cryogenic temperatures. The DIFs for the pullout energy normally decreased when the inclined, geometrically deformed, or tested at the cryogenic temperature. Only few specimens, i.e., S, T, and S-I fibers, had positive DIFs with respect to the pullout energy.
M.-J. Kim, D.-Y. Yoo / Construction and Building Materials 239 (2020) 117852
11
Fig. 12. (a) DIF and (b) CIF for the pullout energy.
CRediT authorship contribution statement Min-Jae Kim: Conceptualization, Methodology, Data curation, Writing - original draft. Doo-Yeol Yoo: Supervision, Conceptualization, Writing - review & editing. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. 2017R1C1B2007589). References [1] P. Richard, M. Cheyrezy, Composition of reactive powder concretes, Cem. Concr. Res. 25 (1995) 1501–1511.
[2] N. Banthia, J.-F. Trottier, Deformed steel fiber - cementitious matrix matrix bond under impact, Cem. Concr. Res. 21 (1991) 158–168. [3] A. Bentur, M.D. Cohen, Effect of Condensed Silica Fume on the Microstructure of the Interfacial Zone in Portland Cement Mortars, J. Am. Ceram. Soc. 70 (1987) 738–743. [4] D.Y. Yoo, M.J. Kim, S.W. Kim, J.J. Park, Development of cost effective ultra-highperformance fiber-reinforced concrete using single and hybrid steel fibers, Constr. Build. Mater. 150 (2017) 383–394. [5] K. Wille, A.E. Naaman, S. El-Tawil, G.J. Parra-Montesinos, Ultra-high performance concrete and fiber reinforced concrete: Achieving strength and ductility without heat curing, Mater. Struct. Constr. 45 (2012) 309–324. [6] H.H.C. Wong, A.K.H. Kwan, Packing density : a key concept for mix design of high performance concrete, Mater. Sci. Technol. Eng. Conf. Hong Kong. (2005) 1–15. [7] U. Particles, F.O.R. The, M. Of, V. High, S. Concretes, Ultrafine Particles for the Making of, 19 (1989) 161–172. [8] Y.W. Chan, S.H. Chu, Effect of silica fume on steel fiber bond characteristics in reactive powder concrete, Cem. Concr. Res. 34 (2004) 1167–1172. [9] A. Korpa, T. Kowald, R. Trettin, Principles of development, phase composition and nanostructural features of multiscale Ultra High Performance Concrete modified with pyrogenic nanoparticles – A review article, Am. J. Mater. Sci. Appl. 2 (2014) 17–30. [10] W. Sun, J.A. Mandel, S. Said, Study of the Interface Strength in Steel FiberReinforced Cement-based Composites, ACI J. Proc. 83 (2014) 597–605. [11] D.Y. Yoo, J.J. Park, S.W. Kim, Fiber pullout behavior of HPFRCC: Effects of matrix strength and fiber type, Compos. Struct. 174 (2017) 263–276. [12] A.L. Marshall, Cryogenic concrete, Cryogenics (Guildf) 22 (1982) 555–565. [13] T. Miura, The properties of concrete at very low temperatures, Mater. Struct. 22 (1989) 243–254.
12
M.-J. Kim, D.-Y. Yoo / Construction and Building Materials 239 (2020) 117852
[14] G.C. Lee, T.S. Shih, K.C. Chang, Mechanical properties of concrete at low temperature, J. Cold Reg. Eng. 2 (1988) 13–24. [15] L.A. Montejo, J.E. Sloan, M.J. Kowalsky, T. Hassan, Cyclic Response of Reinforced Concrete Members at Low Temperatures, J. Cold Reg. Eng. 22 (2008) 79– 102. [16] P.J.M. Monteiro, A.I. Rashed, J. Bastacky, T.L. Hayes, Ice in cement paste as analyzed in the low-temperature scanning electron microscope, Concrete. 19 (1989) 306–314. [17] Z. Yang, W.J. Weiss, J. Olek, Water transport in concrete damaged by tensile loading and freeze-thaw cycling, J. Mater. Civ. Eng. 18 (2006) 424–434. [18] M.J. Kim, D.Y. Yoo, S. Kim, M. Shin, N. Banthia, Effects of fiber geometry and cryogenic condition on mechanical properties of ultra-high-performance fiberreinforced concrete, Cem. Concr. Res. 107 (2018) 30–40. [19] M.-J. Kim, D.-Y. Yoo, Analysis on enhanced pullout resistance of steel fibers in ultra-high performance concrete under cryogenic condition, Constr. Build. Mater. (2019). submitted. [20] M. Xu, B. Hallinan, K. Wille, Effect of loading rates on pullout behavior of high strength steel fibers embedded in ultra-high performance concrete, Cem. Concr. Compos. 70 (2016) 98–109. [21] P. Rossi, F. Toutlemonde, Effect of loading rate on the tensile behaviour of concrete: description of the physical mechanisms, Mater. Struct. 29 (2006) 116–118. [22] A. Brara, J.R. Klepaczko, Experimental characterization of concrete in dynamic tension, Mech. Mater. 38 (2006) 253–267. [23] B.H.C. Fu, M.A. Erki, M. Seckin, Review o f effects of loading rate on reinforced concrete. Tech. Rep. No. R642, U.S. Naval Civ. Engrg. Lab, Calif. J. Struct. Eng. 117 (1991) 3660–3679. [24] K. Wille, A.E. Naaman, Effect of ultra-high-performance concrete on pullout behavior of high-strength brass-coated straight steel fibers, ACI Mater. J. 110 (2013) 451–461. [25] Y.-W. Chan, V.C. Li, Effects of Transition Zone Densification On Fiber/Cement Paste Bond Strength Improvement, Mater. Res. 7355 (1997) 8–17. [26] J. Ma, M. Orgass, F. Dehn, D. Schmidt, N. V. Tue, Comparative Investigations on Ultra-High Performance Concrete with or without Coarse Aggregates, Proc. Int. Symp. Ultra High Perform. Concr. Kassel. (2004) 205–212. [27] S. Collepardi, L. Coppola, R. Troli, M. Collepardi, Mechanical properties of modified reactive powder concrete, ACI Spec. Publ. 173 (1997) 1–22.
[28] N.K. Lee, K.T. Koh, M.O. Kim, G.S. Ryu, Uncovering the role of micro silica in hydration of ultra-high performance concrete (UHPC), Cem. Concr. Res. 104 (2018) 68–79. [29] ASTM C 1437, Standard test method for flow of hydraulic cement mortar, ASTM Int. West Conshohocken, PA. (2007) 1–2. [30] D.Y. Yoo, S. Kim, Comparative pullout behavior of half-hooked and commercial steel fibers embedded in UHPC under static and impact loads, Cem. Concr. Compos. 97 (2019) 89–106. [31] M.N. Al Khalaf, C.L. Page, Steel/mortar interfaces: Microstructural features and mode of failure, Cem. Concr. Res. 9 (1979) 197–208. [32] J. Morton, G.W. Groves, The cracking of composites consisting of discontinuous ductile fibres in a brittle matrix - effect of fibre orientation, J. Mater. Sci. 9 (1974) 1436–1445. [33] Y. Lee, S.T. Kang, J.K. Kim, Pullout behavior of inclined steel fiber in an ultrahigh strength cementitious matrix, Constr. Build. Mater. 24 (2010) 2030–2041. [34] C. Ouyang, A. Pacios, S.P. Shah, Pullout of Inclined Fibers From Cementitious Matrix, Engineering. 120 (1995) 2641–2659. [35] J. Alwan, A.E. Naaman, P. Guerrero, Effect of mechanical clamping on the pullout response of hooked steel fibers embedded in cementitious matrices, Concr. Sci. Eng. 1 (1999) 15–25. [36] A.E. Naaman, G.G. Namur, J.M. Alwan, H.S. Najm, Fiber pullout and bond slip. I: Analytical study, 117 (1992) 2769–2790. [37] N. Banthia, J.F. Trottier, Concrete reinforced with deformed steel fibers part II: toughness characterization, ACI Mater. J. 92 (1995) 146–154. [38] Y.S. Tai, S. El-Tawil, T.H. Chung, Performance of deformed steel fibers embedded in ultra-high performance concrete subjected to various pullout rates, Cem. Concr. Res. 89 (2016) 1–13. [39] K. Wille, A.E. Naaman, Pullout behavior of high-strength steel fibers embedded in ultra-high-performance concrete, ACI Mater. J. 109 (2012) 479–488. [40] Y.S. Tai, S. El-Tawil, High loading-rate pullout behavior of inclined deformed steel fibers embedded in ultra-high performance concrete, Constr. Build. Mater. 148 (2017) 204–218. [41] V.M.C.F. Cunha, J.A.O. Barros, J.M. Sena-Cruz, Pullout Behavior of Steel Fibers in Self-Compacting Concrete, J. Mater. Civ. Eng. 22 (2009) 1–9. [42] Z. Zhao, S.Y. Hong, Cooling strategies for cryogenic machining from a materials viewpoint, J. Mater. Eng. Perform. 1 (1992) 669–678, https://doi.org/10.1007/ BF02649248.
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Cryogenic pullout behavior of steel fibers from ultra-high-performance concrete under impact loading Min-Jae Kim, Doo-Yeol Yoo ⇑ Department of Architectural Engineering, Hanyang University, 222 Wangsimni-ro, Seongdong-gu, Seoul 04763, Republic of Korea
h i g h l i g h t s Bond strength of steel fibers in UHPC increases with increasing loading rate. Cryogenic temperature is effective on improving the bond strength of steel fibers in UHPC at both static and impact loads. The DIFs on the pullout resistance decrease at geometrically deformed and inclined conditions. Slip capacity is reduced in general at the impact loading condition regardless of temperature. Cryogenic temperature negatively affects the rate sensitivity of pullout resistance of steel fibers.
a r t i c l e
i n f o
Article history: Received 10 September 2019 Received in revised form 13 November 2019 Accepted 11 December 2019
Keywords: Ultra-high-performance concrete Steel fiber type Pullout response Impact loading Cryogenic condition
a b s t r a c t This study investigates the effect of impact loading condition on the pullout property of steel fibers from ultra-high-performance concrete (UHPC) under various temperatures. For this, static and impact loads, ambient and cryogenic ( 170 °C) temperatures, three steel fiber types, i.e., straight, half-hooked, and twisted, and two inclination angles of 0° and 45° were considered. From the test results and analysis, it was found that the steel fibers mostly had positive values of dynamic increasing factor (DIF) for the average bond strength at ambient temperature, but these were significantly reduced when the fibers were geometrically deformed, inclined, or tested under cryogenic temperature. Slip capacities generally decreased under the influence of the impact loading condition, and this effect was more severe at cryogenic temperature. There was no obvious impact loading rate effect on the probabilities of fiber and UHPC matrix damages at both ambient and cryogenic temperatures. The DIF values for pullout energies normally decreased or even became negative when the fibers were inclined, geometrically deformed, or tested at cryogenic temperature. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Ultra-high-performance concrete (UHPC) has been developed by a number of researchers since its first introduction in the 1990s [1–5]. UHPC has a very high compressive strength, tensile strength, and strain capacity of over 150 MPa, 8 MPa, and 0.5%, respectively, in general, when steel fibers are incorporated. These levels of performance are universally obtained from the optimized particle packing of reactive powders [6,7]. UHPC normally consists of Portland cement, silica fume (SF), silica sand, and silica flour, for more compact and homogeneous microstructures. Most of the time, 20–30% of the cement by weight is substituted with the same amount of SF [4,8,9]. The reactive powders undergo pozzolanic reactions with the calcium hydroxide (Ca(OH)2) in a hydrating
⇑ Corresponding author. E-mail address: [email protected] (D.-Y. Yoo). https://doi.org/10.1016/j.conbuildmat.2019.117852 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
mortar, forming dense calcium silicate hydrate (C-S-H) gels on the surfaces of hydrates. These gels enhance the mortar interfaces and increase the bond strength between the fiber and matrix [9]. Generally, 2% by volume of steel fibers are included into the UHPC mix to achieve excellent tensile performance. Lastly, water or steam curing at a high temperature above 90 °C is generally adopted to accelerate the cement hydration and pozzolanic reactions. It is significant that UHPC exhibits much better tensile performance than that of normal concrete mixes, but there still is a requirement for even better tensile performance, considering the practical uses of the material. Diverse studies have been performed to investigate and further improve the tensile performance of UHPC. For this, researchers have been trying to adjust the UHPC matrix components [5,8], interfacial transition zones [3,10], and fiber properties [2,4,11]. Despite previous research [2–5,8,10,11], steel fiber pullout properties and their influence on the UHPC matrix have not yet been fully investigated and developed.
2
M.-J. Kim, D.-Y. Yoo / Construction and Building Materials 239 (2020) 117852
In the meantime, many researchers have performed diverse research on the influence of cryogenic temperatures on the physical properties of normal strength concrete and reported enormous increases of compressive and tensile strength due to frozen and hardened moisture in the concrete [12–17]. In the case of the UHPC incorporating steel fibers, however, there was no published paper until Kim et al. [18] first reported the enhancement of compressive and tensile performances at the cryogenic temperature. They [18] examined the influence of cryogenic temperatures of about 162 °C, considering of the liquefied natural gas industry and found out that the tensile strength and even the strain capacity of UHPC remarkably improved at the cryogenic temperature. Based on micrographs for the surface scratches of each pulled out fiber, they [18] attributed these effects to variations in fiber–matrix interaction because of the hardening of matrix particles at the cryogenic condition. The tensile strength and strain capacity were both considerably enhanced because frozen and hardened matrix particles at the fiber–matrix interfaces could resist harder and longer against pullout of fibers at the cryogenic temperature than at the ambient temperature. Kim and Yoo [19] then focused on the pullout properties of single steel fibers embedded in the UHPC matrix at the cryogenic temperature and reported that cryogenic temperature significantly enhanced the average bond strength and slip capacity via the higher frictional resistance as reported in the previous study. They [19] also reported that the largely increased frictional resistance even caused higher chances of fiber fracture and matrix-spalling areas, especially for the inclined and geometrically deformed steel fibers.” ‘‘Furthermore, a number of studies have been conducted on the effect of loading rate on the mechanical behaviors of UHPC, including fiber pullout behavior [2,20–24]. It is generally known that increasing the loading rates changes load transfer mechanisms via the inertia effect, which influences the crack propagation or fiber–matrix deformation [20–22]. Some researchers have reported a positive dynamic increasing factor (DIF) regarding the fiber bond strength and tensile strength, although other researchers also reported that there was no positive influence on the bond strength [2,20,24]. However, there has been no published study investigating the influence of geometrical deformation and inclination of steel fibers and impact loading rate on the entire fiber pullingout process from the UHPC matrix, examined at the cryogenic condition. This study, therefore, focused on investigating the effects of the loading rate and cryogenic condition on the pullout behavior of various steel fibers from the UHPC matrix by considering their inclination angles and geometries.”
2.1. Mixture proportions The mix proportions for the UHPC matrix used in this study are given in Table 1. Type 1 Portland cement and SF with a mean particle size of 0.31 lm were used as binding materials. The SF replaced 25% of the cement by weight, for a denser microstructure and stronger fiber–matrix bond [8,25]. Silica sand and silica flour with average particle diameters of 337 and 4 lm, respectively,
Chemical composition (%)
Type 1 Portland Cement
SF
CaO Al2O3 SiO2 Fe2O3 MgO SO3 Specific surface area (cm2/g) Density (g/cm3)
61.33 6.40 21.01 3.12 3.02 2.30 3,413 3.15
0.38 0.25 96.00 0.12 0.10 – 200,000 2.10
Table 3 Physical properties of steel fibers.
Table 1 Mixture proportion of UHPC.
0.2
Fig. 1. Diffraction volume percentage of materials. Table 2 Chemical compositions and physical properties of cement and silica fume.
2. Experiments
W/ B
were incorporated as fine aggregate and filler, respectively. Coarse aggregates were eliminated from the mix, as shown in Table 1, for better homogeneity and a denser microstructure to improve fiber pullout properties. This is because for composites such as cement mortar or concrete mixes, the detrimental influence of interfacial transition zones (ITZ) between components leads to preexisting cracks and locally weaker structures. In addition, this phenomenon becomes severe when the sizes of the constituents are bigger. Moreover, the distribution and orientation of steel fibers become degraded, and the fiber pullout property deteriorates as the matrix shrinkage is resisted by the inclusion of coarse aggregates [26,27]. Fig. 1 shows the detailed diffraction percentages for the four kinds of powder ingredients. The SF used in this study was of the same type as the one used in a previous study [28]. Table 2 lists the chemical and physical properties of the Portland cement and SF. The type 1 Portland cement consisted of 61.3% CaO and 21.0% SiO2, whereas the SF comprised 96% SiO2. The specific surface areas of the cement and SF were 3,413 and 200,000 cm2/g, respectively. The mix consisted of a large amount of powdery materials with very high specific surface areas; the water-to-binder ratio (W/B) was controlled to as low as 0.2 to obtain higher mechanical properties. Thus, polycarboxylate superplasticizer (SP) was used to control the mortar flow value at over 210 mm, which was measured in accordance with ASTM C1437 [29]. Three types of steel fibers (i.e., straight (S), half-hooked (H), and twisted (T)) with equivalent diameters of 0.3, 0.375, and 0.5 mm, respectively, and with a length of 30 mm were used. Table 3 shows
Unit weight (kg/m3) Water
Cement
SF
Silica sand
Silica flour
Superplasticizer
160.3
788.5
197.1
867.4
236.6
52.6
3
[Note] Superplasticizer includes 30% solid (=15.8 kg/m ) and 70% water (=36.8 kg/ m3).
S H T
df [mm]
lf [mm]
Aspect ratio [lf/ df]
Density [g/ cm3]
ft [MPa]
Ef [GPa]
0.300 0.375 0.500
30.0 30.0 30.0
100.0 80.0 60.0
7.9 7.9 7.9
2,580 2,900 2,428
200 200 200
[Note] S = straight steel fiber, H = half-hooked steel fiber, T = twisted steel fiber, df = diameter, lf = length, ft = tensile strength, and Ef = elastic modulus.
M.-J. Kim, D.-Y. Yoo / Construction and Building Materials 239 (2020) 117852
the physical properties of the steel fibers. The S, H, and T fibers had tensile strengths of 2,580, 2,900, and 2,428 MPa, respectively. The H and T fibers had deformed geometries for applying mechanical anchorage effects to the cement matrix. The H fiber had the same round shape as that of the S fiber and had hooked ends, whereas the T fiber had a triangular section and was twisted three times along its longitudinal direction. These deformations led to mechanical anchoring effects in the cement matrix and stress concentration on the surrounding matrix. In this study, the H fiber was prepared by cutting each half of the end-hooks from previously used hooked-end steel fibers [30]. The mechanical anchorage of the deformed fibers began while the deformed fibers recovered their original geometry after the fiber yield strength was reached [2]. All the steel fibers were brass-coated mainly for better corrosion resistance, and also for a stronger chemical bond between the fiber surface and the matrix [31]. 2.2. Specimen fabrication Fiber pullout specimens were fabricated in accordance with the following procedures. The four dry ingredients, i.e., Portland cement, SF, silica flour, and silica sand, were premixed in a mixer for 5 min. Water-premixed SP was then poured into the mixer and mixed for another 10 min. Molds for pullout specimens and fibers were prepared as suggested in Fig. 2. Every fiber tested in this study was 30 mm long and embedded by 10 mm. This 10 mm embedded part of the fiber protruded from a PVC film, while the opposite 20 mm part was fixed onto an acryl block to keep the fresh mixture from flowing into the other side and to ensure the alignment of the fibers. The fixed end of the fiber was bent to form an anchoring hook in the matrix, to ensure that every fiber was pulled out only from the pullout side. The specimen had a cross-sectional area of 25 25 mm2. As described previously, two
3
experiment cases were prepared for each fiber; the cases were differentiated by the tilt angle, which was either 0° or 45°, to investigate the fiber inclining effect on the pullout behavior. The inclination angle of 45° was adapted from previous research, which reported that angles between 30° and 60° led to a higher fiber–matrix bond strength [2,32–34]. After the fresh UHPC mixture was mixed, the mix was placed into the pullout side, shown in Fig. 2, and the specimens were covered with plastic sheets to minimize abrupt drying and were cured at room condition for 48 h. The acryl blocks were then carefully removed, the same fresh UHPC mix was placed into the opposite fixed side, and the sealing and curing procedures were repeated. After another 48 h, the specimens were demolded and cured at 90 °C in a water tank for 72 h to accelerate the hydration process. Finally, the specimens were left in room condition for 4 days more before being tested. Three test variables, i.e., geometrical deformation of fiber, temperature condition, and fiber tilt angle, were set. Thus, 3 (fiber types) 2 (temperatures) 2 (fiber inclination angles) 5 (samples for each variable) = 60 specimens were tested and analyzed in this study. Results of the tests and microscopic analysis for the same specimens, which tested under the static loading condition, were adapted from a previous study [19], and they were compared with those of the impact pullout test results. Regarding the consistency of the pullout test results, the fiber inclination and cryogenic temperature increased the fiber rupturing possibility, showing irregular data patterns. Therefore, majority trends with more than three data for each variable were analyzed to obtain a reliable conclusion. The specimen-naming process was based on loading rates (‘‘ ” or ‘‘D” for static and impact loading rates, respectively), temperature conditions (‘‘ ” or ‘‘C” for ambient or cryogenic temperatures, respectively), fiber types (S, H, and T), and fiber inclination (‘‘ ” or ‘‘-I” for aligned or inclined states, respectively). For examples, if a specimen with an inclined T fiber was tested under the
Fig. 2. Specimen fabrication: (a) Fiber and mold setup and (b) placing and curing procedure.
4
M.-J. Kim, D.-Y. Yoo / Construction and Building Materials 239 (2020) 117852
Pressure Gauge Load cell (10 kN)
Air compressor Potentiometer
Air amplifier
Loading Piston
Impact load
Air storage tank
Fig. 3. Setup for fiber pullout test under impact loading condition.
cryogenic and the static loading condition, it is referred to as ‘‘CTI”, whereas the same specimen tested under the cryogenic and impact loading condition was referred to as ‘‘DCT-I.”
2.3. Impact fiber pullout test under cryogenic condition Fig. 3 shows a test setup for an impact fiber pullout test adapted for this study. Specially manufactured test equipment was used for this experiment. The impact loading rate was controlled by setting air pressure to be fixed at 31.9 kg/cm2, which resulted in the 2 kN load at the loading cylinder. This adjusted the loading rate to 480 mm/s without a dog-bond specimen as per a previous study [30]. Two steel jigs were separately installed. The lower one was completely fixed, whereas the upper one was pinned, allowing slight rotation of the specimen to minimize data noises caused by eccentric force. A load cell with a maximum capacity of about 10 kN was attached to the upper grip system to eliminate inertia effect at the start of the loading. A potentiometer was used to record the movements of the lower grip system, which was connected to a piston with an air-compressing system. The elastic deformations of the matrix, steel fibers, and steel jigs were expected to be negligible, and thus, the displacement of the potentiometer was used as the fiber end slip. Once a pullout specimen was held between the two jigs, an initial load was set at 2 kN to reduce data noises. A dynamic data recording system with a 20 kHz frequency was used to acquire sufficient data points from the quick impact loading test. For the cryogenic condition, the temperature of every specimen was kept below 170 °C. In previous research [18], the centroid temperature of Ø100 200 UHPC specimens surrounded by liquefied nitrogen (LN2) was drastically chilled to below 170 °C in about 15 min. After that, the decreasing rate of the centroid temperatures was significantly reduced and nearly constant. This study also used LN2 to rapidly freeze the fiber pullout specimens. The pullout specimens were also preliminarily chilled for 5 min, calculating the shorter distance to the centroid, to set the centroid temperature to below 170 °C. The completely chilled specimen was then moved into the grip system and immediately tested within approximately 60s.
3. Test results and analysis 3.1. Bond–slip curves Fig. 4 shows the general bond stress–slip curves of steel fibers tested under both the static and impact conditions. The impact pullout test results were compared with those for the static condition [19]. A steel fiber embedded in a UHPC matrix generally demonstrates linear increase of bond stress at the beginning of the test, until the partial de-bonding of the fiber–matrix interface begins. Subsequently, the partial de-bonding process lasts up to the peak point of the bond stress and slip curve, where the full de-bonding of the fiber and matrix occurs, and only the frictional pullout resistance is present [2,35–37]. The bond stress then decreases continuously until the fiber is completely pulled out from the matrix, but the pullout resistance is maintained because of the friction caused by the matrix debris at the fiber–matrix interface [38]. Kim and Yoo [19] found that, under the static loading condition, the initial stiffness increased when the fibers were geometrically deformed or pulled out at a cryogenic temperature. These test variables both enhanced the fiber–matrix interaction via their respective mechanisms. The deformed fibers had mechanical anchorage effects when a pullout load was applied through a large region of the matrix [30,38–40]. The UHPC matrix, meanwhile, incorporates a portion of water in diverse forms, such as pore water, C-S-H interlayer water, and water molecules in hydrates. Once the UHPC matrix becomes frozen, the various forms of included water molecules then provide additional strength [12,16]. This results in the higher strength of the UHPC matrix, including its debris at the fiber interface. This was the reason for the large improvement in steelfiber pullout performance under the cryogenic condition [18,19]. It was also reported that as the steel fibers were tilted from the pullout direction, the initial increasing phases of the steel fibers became smaller [2,33,34,37,41]. Fig. 5 shows SEM images taken for the surfaces of S-series fibers. The extent of surface damage of the pulled out S fibers became clearly more severe, when the fibers were pulled out under the inclined or cryogenic condition than under the aligned or ambient condition. This indicates that the UHPC matrix itself and particles at the interface resisted harder, under the inclined or cryogenic condition.
M.-J. Kim, D.-Y. Yoo / Construction and Building Materials 239 (2020) 117852
5
Fig. 4. General bond stress-slip curves of steel fibers.
Under the impact loading condition, however, pullout behaviors that were different from those under the static loading condition were observed. According to Fig. 4, the initial increasing phases were steeper under the impact loading condition than under the static condition, whether the fibers were aligned or tilted by 45° with respect to the pullout direction. Under the impact loading condition, the initial stiffness of the inclined steel fibers was also observed to be not discernably lower than that of the aligned fibers. These were also attributed to the inertia effect, which has been observed under the impact loading condition [20–22]. Based on previous research by Fu et al. [23], the mechanical strength and stiffness of concrete were clearly increased. In addition, under the impact loading condition, the fiber–matrix interface did not have enough time for a transfer of the applied stress, with cracks causing a bigger deformation of the surrounding matrix. In other words,
the fiber–matrix interface suffered concentrated stress under the impact loading condition. Furthermore, although the deformed fibers had higher bond strengths than that of the S fiber, the former generally showed more rapid reductions of bond stress than the latter did. This is attributed to the local stress concentration and damage on the fiber–matrix interfaces, caused by geometrical deformation, especially under the impact and cryogenic conditions. The S fiber without geometrical deformation exhibited a large improvement of the pullout strength and even the slip capacity, under the cryogenic temperature. This was because of a huge enhancement of pullout resistance provided from matrix as discussed in Fig. 4. For the deformed fibers, on the other hand, not only the pullout resistance was excessively increased because of the mechanical anchorage effect but also the tensile force applied to the fiber and matrix
6
M.-J. Kim, D.-Y. Yoo / Construction and Building Materials 239 (2020) 117852
Fig. 5. SEM images for the surface of S fibers pulled out under different test conditions.
was likely to be concentrated at certain points. The stress concentration also caused several fibers to be fractured during the pulling-out process, and this will be discussed later in this paper. 3.2. Increasing rates of fiber pullout parameters under cryogenic and impact loading conditions 3.2.1. Average bond strength Fig. 6 illustrates the average bond strengths and slip capacities obtained under the ambient and cryogenic temperatures, and under the static and impact conditions. Meanwhile, Fig. 7 visualizes DIF and cryogenic increasing factor (CIF), the latter of which denotes variations in the pullout parameters according to exposure to the cryogenic condition. Under the static loading condition, every aligned steel fiber could be observed to have a considerably higher bond strength at the cryogenic temperature as compared with at the ambient temperature. The S, H, and T fibers showed clear increases in bond strengths, from 10.9, 18.6, and 17.3 MPa, respectively, to 32.2 (CS), 23.7 (CH), and 34.2 (CT) MPa, respectively, after being exposed to the cryogenic condition. Regarding the bond strengths, the CIF values were 95%, 73%, and 98% for the S, H, and T fibers, respectively. This was mainly because of the enhancement of the fiber–matrix interaction, which was due to the hardened matrix particles at the fiber–matrix interface under the cryogenic temperature, as mentioned previously. It is generally known that inclined steel fibers exhibit higher bond strength than that of corresponding aligned fibers because of the stronger resistance of the matrix at the fiber exit. This is caused by the snubbing and bending effects of the fibers, which lead to stronger frictional resistance and additional damage to the matrix [37,38,40]. Kim and Yoo [18,19] reported that when the fibers were inclined, the damage on the matrix caused by the steel fibers, especially by the deformed fibers, reduced the CIF val-
ues of the bond strengths under the static condition. The average bond strengths of the S-I, H-I, and T-I fibers were measured to be at 12.0, 24.1, and 22.3 MPa, respectively, which were 11%, 29%, and 28%, respectively, higher than those of the aligned fibers. Under the cryogenic condition, however, these values were different, i.e., 20.2 (CS-I), 23.7 (CH-I), and 26.9 (CT-I) MPa, respectively. The cryogenic increasing rates were significantly smaller than those for the aligned condition, by only about 68%, 2%, and 21%, respectively. This result denotes that the inclination and deformation of steel fibers led to less effective cryogenic effects or even had a detrimental influence on the bond strength of the steel fibers. The DIF values for the average bond strengths of steel fibers with respect to change of loading rate from 0.018 mm/s (static) to 480 mm/s (impact) are generally known to range between 1.0 and 1.23, according to previous studies [2,20,24]. Figs. 6 and 7a show that the DS, DH, and DT fibers had higher average bond strengths of 14.4, 21.9, and 22.5 MPa, respectively, under the impact loading condition, corresponding to DIFs of 33%, 18%, and 30%, respectively. Even for the inclined fibers, the DIF values were generally positive at the ambient temperature. The average bond strengths of the DS-I, DH-I, and DT-I fibers were 19.0, 23.3, and 24.9 MPa, respectively, with DIFs of 58%, 3%, and 12%, respectively, based on the results for the S-I, H-I, and T-I fibers, respectively. As the temperature decreased to the cryogenic condition, on the other hand, the DIFs for the average bond strength became considerably lower. The average bond strengths of the CDS, CDH, and CDT fibers measured at 19.8, 27.1, and 37.0 MPa, respectively, corresponding to DIFs of 22%, 16%, and 8%, respectively. Subsequently, the CDS-I, CDH-I, and CDT-I fibers demonstrated average bond strengths of 19.8, 25.8, and 28.6 MPa, respectively, and thus the DIFs were 2%, 9%, and 6%, respectively. From a different perspective, it should also be noted that the CIF values with respect to the fiber bond strength under the impact
M.-J. Kim, D.-Y. Yoo / Construction and Building Materials 239 (2020) 117852
7
Fig. 6. Average bond strength and slip capacity.
Fig. 7. (a) DIF (b) CIF values for the average bond strength and slip capacity.
loading condition were not as effective as under the static condition, as shown in Figs. 6 and 7b. The average bond strengths of the CDS, CDH, and CDT fibers measured at 16.5, 27.1, and 37.0 MPa, respectively, and their corresponding CIFs were 15%, 23%, and 64%, respectively, significantly lower than the CIFs of the S, H, and T fibers, which were 95%, 73%, and 98%, respectively. The CDS-I, CDH-I, and CDT-I fibers provided average bond
strengths of 19.8, 25.8, and 28.6 MPa, respectively, and their CIF values were measured at only 5%, 10%, and 15%, respectively. 3.2.2. Slip capacity Slip capacity, shown in Fig. 6, denotes the fiber-end slip values measured at the peak points of the bond–slip curve. The S, H, and T fibers demonstrated slip capacities of 0.9, 1.6, and 3.1 mm, respec-
8
M.-J. Kim, D.-Y. Yoo / Construction and Building Materials 239 (2020) 117852
tively, and the inclined S-I, H-I, and T-I fibers exhibited 2.1, 2.9, and 3.0 mm, respectively. This was coincident with previous reports noting that under the static loading condition, the slip capacities are generally increased as steel fibers are tilted from the pullout direction [40,41]. It was mentioned [19] that under cryogenic temperature and static loading condition, the slip capacity of the S fiber was largely increased because of enhanced fiber pullout resistance, whereas the deformed fibers underwent overly high pullout resistance and even fracture. The slip capacities of the S and S-I fibers increased from 0.9 and 1.3 mm, respectively, to 1.7 and 2.65 mm, respectively, corresponding to CIFs of 89% and 29%, respectively, whereas those for the deformed, H (1.6 mm) and H-I (2.9 mm) fibers or T (3.1 mm) and T-I (3.0 mm) fibers, were measured at only 3% and 26%, or 54% and 7%, respectively. Based on the investigation on fiber surface, matrix tunnel, and fiber exit with a scanning electron microscope and an optical microscope, it was found that the matrix damage became more severe at the cryogenic temperature, specifically for the deformed or inclined fibers, leading to lower bond strengths or more rapid losses of bond stress [19]. Figs. 6 and 7a also show that the impact loading condition influenced the slip capacity of the steel fibers. The increased loading rate generally decreased the slip capacities of the steel fibers. Every steel fiber tested under the impact loading condition, except for the DT fiber, had lower slip capacities as compared with those tested under the static condition. The slip capacities of the DS, DH, and DT fibers were measured at only 0.9, 0.8, and 4.0 mm, respectively, and thus the DIFs for the slip capacities were 0%, 52%, and 30%, respectively. Moreover, the DS-I, DH-I, and DT-I fibers also demonstrated 1.3, 1.6, and 2.9 mm slip capacities. These were significantly lower than those of the S-I, H-I, and T-I fibers with DIFs, which are all measured negatively to be –32%, –43%, and –2%, respectively. The cryogenic temperature noticeably decreased the slip capacity of steel fibers under the impact loading condition, unlike under the static loading condition. The CDS, CDH, and CDT fibers exhibited 0.5, 0.9, and 0.7 mm, respectively, slip capacity values, which were the lowest for among all of the experimental conditions. For the cases involving inclined fibers, i.e., CDS-I, CDH-I, and CDT-I, the slip capacities measured were 1.0, 1.1, and 1.8 mm, respectively, which were much lower than those of the S-I (1.3 mm), H-I (2.9 mm), and T-I (3.0 mm) fibers tested under ambient temperature and static condition. 3.3. Maximum fiber tensile stress and failure mode In Fig. 8, the maximum fiber tensile stress and the probability of fiber fracture are summarized. It is shown that the fracture of inclined steel fibers was not thoroughly attributed to the fact that
the tensile stress received by each fiber exceeded the tensile strength. Under the static loading condition, the maximum fiber stresses of the ruptured H-I, CH-I, T-I, and CT-I fibers were 2,573, 2,533, 1,780, and 2,154 MPa, respectively, while the tensile strengths of the S, H, and T fibers were 2,580, 2,900, and 2,428 MPa, respectively, as described in Table 3. Kim and Yoo [19] explained this phenomenon based on the mechanism of the bending actions of the inclined fibers, which finally causes a higher frictional resistance and a stress concentration at the exit of the fibers [37,40]. The inclined fibers, thus exhibited larger chances of fracture than the aligned fibers. For the cases of the deformed fibers, however, the local stress concentration was more critical because the fibers already had higher pullout resistance than the S fiber via the mechanical anchorage effect. The inclined deformed fibers, therefore, had a 100% chance of being ruptured, under the static condition, whereas none of the S-I fiber was failed. Under the impact condition, the DS-I fiber also showed 40% chance of being ruptured unlike under the static condition (S-I). This was also be attributed to the inertia effect mentioned previously, which increases local stress concentration resulted from not enough time for stress dispersion. The chance of being ruptured for the DCT fiber, however, was observed to be only 40% which was 60% lower than that of the CT fiber. Except for these two cases, no notable variation in fiber fracture mode derived from the impact loading rate was found. In the meantime, the cryogenic temperature also considerably enhanced the pullout resistance provided to each fiber via the hardened matrix debris at the fiber–matrix interface, according to the previously explained mechanism. This phenomenon even caused fractures of the aligned CH (20%) and CT (100%) fibers: the maximum tensile stresses were 3,372 and 2,738 MPa, respectively, about 16.2% and 12.7%, respectively, higher than the respective tensile strengths provided by manufacturers. As illustrated in Fig. 8, under the impact loading rate, the maximum fiber tensile stresses measured at the ambient temperature generally increased. The maximum fiber tensile stresses of the DS, DH, and DT fibers were increased by 35.3%, 22.7%, and 29.8%, respectively, measuring at 1,731, 2,404, and 1,799 MPa, respectively. Those of the inclined cases were also increased by 62.9%, – 3.2%, and 11.8%, respectively, and measured to be 2,271 (DS-I), 2,490 (DH-I), and 1,990 (DT-I) MPa, respectively. Under the cryogenic and static condition, the maximum fiber stresses were considerably increased, as compared to those under the ambient and static condition. The maximum fiber stresses of the CS, CH, CT, CS-I, CH-I, and CT-I fibers were observed to be 2,829, 3,372, 2,738, 2,320, 2,533, 2,154 MPa, respectively. Furthermore, those values also generally increased under the impact load-
Fig. 8. Maximum fiber tensile stress and the probability of fiber fracture.
M.-J. Kim, D.-Y. Yoo / Construction and Building Materials 239 (2020) 117852
ing, except for the cases of the CDS and CDH fibers. The maximum tensile stresses of the CDS, CDH, CDT, CDS-I, CDH-I, and CDT-I fibers varied by –23.0%, –16.0%, 8.0%, 9.0%, 8.5%, and 6.3%, respectively, under the cryogenic temperature, as compared to those under the static condition, and the practical values measured at 2,178, 2,831, 2,958, 2,530, 2,748, and 2,291 MPa, respectively. It is normal for the tensile strength of most irons to increase at low temperatures. More specifically, high-carbon steel fibers with 0.7% carbon content were used in this study. Hong et al. [42] reported that the tensile strength of a high-carbon steel including a 0.65–0.76% carbon increased by 40%, when the temperature decreased from 25 °C to 192 °C. Similar to this result, the maximum fiber stress values measured at the cryogenic temperature could exceed the nominal tensile strength of the steel fibers. The maximum tensile stresses of the CDS and CDS-I fibers, on the other hand, were specifically lower than those of the CS and CS-I fibers, having differences of 651 and 518 MPa, respectively. The other deformed fibers, meanwhile, demonstrated relatively smaller differences resulting from the increase in loading rate. The tensile strengths of the H and T fibers were 2,900 and 2,428 MPa, respectively. Thus, the CH and CDH fibers, which were measured to have maximum tensile stresses of 3,372 and 2,831 MPa, respectively, were noted to have 20% and 0%, respectively, chances of failing, but the CH-I and CDH-I fibers with maximum tensile stresses of 2,533 and 2,748 MPa, respectively, were determined to both have a 100% probability of being fractured. Moreover, it was also noted that the CT fiber, with a maximum fiber stress of 2,738 MPa, had a 100% chance of fracture, whereas the aligned CDT fiber, with a higher maximum fiber stress of 2,958 MPa, had only a 40% chance of fracture, although the maximum fiber stresses of the CT and CDT fibers exceeded the tensile strength by 12.8% and 21.8%, respectively. This characteristic might be caused by both the stronger matrix splitting action of the T-fiber under the impact loading rate. 3.4. Degree of matrix spalling The matrix-spalling areas that have been calculated are summarized in Fig. 9 with their respective probabilities of fiber fracture. Kim and Yoo [19] have taken optical microscopic images of fiber–matrix interfaces and fiber exits, and they have found that while the inclined or deformed steel fibers caused significantly larger damage to the matrix, even the aligned steel fibers caused the matrix around the fiber exit to crack. In addition, the damage received by the matrix was stronger at a cryogenic temperature. In this study, similar microscopic images were taken to investigate
9
the effect of impact loading rate on matrix damage around the fiber exit points. According to microscopic analysis, however, no discernable difference regarding the magnitude of matrix damage resulted from the impact loading rate.
3.5. Fiber pullout energy Fig. 10 shows normalized pullout energy values measured up to the peak point and to 5 and 10 mm of slips, which were denoted as Ep, E5, and E10, respectively. These values indicate the absorbed energy per unit bonding area (mJ/mm2). As was previously reported [19], under the static and ambient conditions, the aligned, deformed fibers absorbed higher energies than did the S fiber. The pullout energies of the deformed fibers, however, showed clear inefficiency when they were inclined. The deformed fibers also had high Ep/E10 and E5/E10 ratios, which indicate brittle pullout behaviors of the fibers. These trends became more significant as the fibers were inclined or pulled out at the cryogenic temperature and attributed to a stronger pullout resistance, stress concentration, and deterioration of fiber–matrix interface. As already mentioned, it is clearly seen that the deformed fibers had higher Ep/ E10 and E5/E10 ratios than those of the S fiber (Fig. 11). This trend became more significant under the inclined and cryogenic conditions. The slip capacities were generally lower under the impact loading condition because of the inertia effect. On the other hand, regarding the E5/E10 ratios, there was no discernable and typical influence by the impact loading rate. Fig. 12a illustrates DIFs with respect to the pullout energy. The deformation and inclination of fiber and the cryogenic temperature normally caused enhancement of the pullout resistance and stress concentration, and thus, a larger matrix-spalling area or a higher chance of fiber fracture was observed. The E10 values of the aligned S and T fibers largely increased by 24% and 63%, respectively, while that of the H fiber decreased by 7%. It is noted that the DIFs for the pullout energies tended to decrease or even became negative as the fibers were inclined, geometrically deformed, or tested at the cryogenic temperature. Except for the three aforementioned fibers, most of the other fibers exhibited negative DIFs, as can be seen in Fig. 12a. Although the DIF values of the CT and CH-I fibers were recorded to be 51% and 118%, respectively, it should be considered that these fibers failed right after the peak point and had very small E10 values of 24.7 and 29.5 mJ/mm2, respectively. Only the S-I fiber had a positive DIF value of 29% for the E10 parameter, among all the other fibers that were inclined or tested under the impact loading condition.
Fig. 9. Matrix spalling area and probability of fiber fracture.
10
M.-J. Kim, D.-Y. Yoo / Construction and Building Materials 239 (2020) 117852
Fig. 10. Normalized pullout energy.
Fig. 11. Ep/E10 and E5/E10 ratios.
The CIFs are also visualized in Fig. 12b. It is clearly seen that all of the deformed fibers, except for the H fiber, demonstrated negative CIFs. The H fiber had a high CIF of 64%, but this drastically decreased to 23%, 13%, and 4% for the H-I, DH, and DHI fibers, respectively. The CIF for the E10 of the S, S-I, DS, and DS-I fibers, on the other hand, were measured at 72%, 44%, 22%, and 6%, respectively. It is noted that even for the inclined S fiber, which involves no geometrical deformation and severe local matrix damage, exhibited a decrease in CIF for pullout energy. When the impact loading rate and cryogenic temperature were overlapped, a negative influence on the pullout energy of deformed or inclined steel fibers were clearly observed. This result was also attributed to the fact that performance deterioration at the fiber–matrix interface, caused by locally concentrated stress, became more severe under both the impact loading and cryogenic conditions.
4. Conclusion The influences of impact loading rate, cryogenic temperature, geometrical deformation, and inclination of steel fibers in UHPC on fiber pullout behavior were investigated. The bond–slip curves, bond strengths, slip capacities, and normalized pullout energy were analyzed, and the factors that increased according to the impact loading rate and cryogenic temperature were evaluated. For better understanding of the influences on the fiber pullout properties and for more reliable data analysis, optical micrographs
on the fiber–matrix interfaces and fiber exit were used. From the aforementioned test results and discussion, the following conclusions were obtained: The initial stiffness of the inclined steel fibers in UHPC largely increased under the impact loading condition. Even differences in the stiffness between the aligned and inclined fibers were not as discernable as the difference in the stiffness under the static loading condition. At the ambient temperature, the average bond strengths highly increased under the impact loading rate, exhibiting DIFs ranging from 3% to 58%. At the cryogenic temperature, however, the DIFs were considerably decreased and ranged from 30% to 9%. The slip capacities generally decreased at the impact loading condition regardless of the temperature but were further deteriorated by the cryogenic temperature. The DIFs for the slip capacities were generally recorded as negative, reaching down to 72%. Stress concentration due to enhanced pullout resistance at the cryogenic temperature or deformation and inclination of steel fibers caused premature fracture of fibers and larger matrix damage. However, there was no discernable influence of the loading rate on the chances of fiber failure mode and on the magnitude of matrix damage at both the ambient and cryogenic temperatures. The DIFs for the pullout energy normally decreased when the inclined, geometrically deformed, or tested at the cryogenic temperature. Only few specimens, i.e., S, T, and S-I fibers, had positive DIFs with respect to the pullout energy.
M.-J. Kim, D.-Y. Yoo / Construction and Building Materials 239 (2020) 117852
11
Fig. 12. (a) DIF and (b) CIF for the pullout energy.
CRediT authorship contribution statement Min-Jae Kim: Conceptualization, Methodology, Data curation, Writing - original draft. Doo-Yeol Yoo: Supervision, Conceptualization, Writing - review & editing. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. 2017R1C1B2007589). References [1] P. Richard, M. Cheyrezy, Composition of reactive powder concretes, Cem. Concr. Res. 25 (1995) 1501–1511.
[2] N. Banthia, J.-F. Trottier, Deformed steel fiber - cementitious matrix matrix bond under impact, Cem. Concr. Res. 21 (1991) 158–168. [3] A. Bentur, M.D. Cohen, Effect of Condensed Silica Fume on the Microstructure of the Interfacial Zone in Portland Cement Mortars, J. Am. Ceram. Soc. 70 (1987) 738–743. [4] D.Y. Yoo, M.J. Kim, S.W. Kim, J.J. Park, Development of cost effective ultra-highperformance fiber-reinforced concrete using single and hybrid steel fibers, Constr. Build. Mater. 150 (2017) 383–394. [5] K. Wille, A.E. Naaman, S. El-Tawil, G.J. Parra-Montesinos, Ultra-high performance concrete and fiber reinforced concrete: Achieving strength and ductility without heat curing, Mater. Struct. Constr. 45 (2012) 309–324. [6] H.H.C. Wong, A.K.H. Kwan, Packing density : a key concept for mix design of high performance concrete, Mater. Sci. Technol. Eng. Conf. Hong Kong. (2005) 1–15. [7] U. Particles, F.O.R. The, M. Of, V. High, S. Concretes, Ultrafine Particles for the Making of, 19 (1989) 161–172. [8] Y.W. Chan, S.H. Chu, Effect of silica fume on steel fiber bond characteristics in reactive powder concrete, Cem. Concr. Res. 34 (2004) 1167–1172. [9] A. Korpa, T. Kowald, R. Trettin, Principles of development, phase composition and nanostructural features of multiscale Ultra High Performance Concrete modified with pyrogenic nanoparticles – A review article, Am. J. Mater. Sci. Appl. 2 (2014) 17–30. [10] W. Sun, J.A. Mandel, S. Said, Study of the Interface Strength in Steel FiberReinforced Cement-based Composites, ACI J. Proc. 83 (2014) 597–605. [11] D.Y. Yoo, J.J. Park, S.W. Kim, Fiber pullout behavior of HPFRCC: Effects of matrix strength and fiber type, Compos. Struct. 174 (2017) 263–276. [12] A.L. Marshall, Cryogenic concrete, Cryogenics (Guildf) 22 (1982) 555–565. [13] T. Miura, The properties of concrete at very low temperatures, Mater. Struct. 22 (1989) 243–254.
12
M.-J. Kim, D.-Y. Yoo / Construction and Building Materials 239 (2020) 117852
[14] G.C. Lee, T.S. Shih, K.C. Chang, Mechanical properties of concrete at low temperature, J. Cold Reg. Eng. 2 (1988) 13–24. [15] L.A. Montejo, J.E. Sloan, M.J. Kowalsky, T. Hassan, Cyclic Response of Reinforced Concrete Members at Low Temperatures, J. Cold Reg. Eng. 22 (2008) 79– 102. [16] P.J.M. Monteiro, A.I. Rashed, J. Bastacky, T.L. Hayes, Ice in cement paste as analyzed in the low-temperature scanning electron microscope, Concrete. 19 (1989) 306–314. [17] Z. Yang, W.J. Weiss, J. Olek, Water transport in concrete damaged by tensile loading and freeze-thaw cycling, J. Mater. Civ. Eng. 18 (2006) 424–434. [18] M.J. Kim, D.Y. Yoo, S. Kim, M. Shin, N. Banthia, Effects of fiber geometry and cryogenic condition on mechanical properties of ultra-high-performance fiberreinforced concrete, Cem. Concr. Res. 107 (2018) 30–40. [19] M.-J. Kim, D.-Y. Yoo, Analysis on enhanced pullout resistance of steel fibers in ultra-high performance concrete under cryogenic condition, Constr. Build. Mater. (2019). submitted. [20] M. Xu, B. Hallinan, K. Wille, Effect of loading rates on pullout behavior of high strength steel fibers embedded in ultra-high performance concrete, Cem. Concr. Compos. 70 (2016) 98–109. [21] P. Rossi, F. Toutlemonde, Effect of loading rate on the tensile behaviour of concrete: description of the physical mechanisms, Mater. Struct. 29 (2006) 116–118. [22] A. Brara, J.R. Klepaczko, Experimental characterization of concrete in dynamic tension, Mech. Mater. 38 (2006) 253–267. [23] B.H.C. Fu, M.A. Erki, M. Seckin, Review o f effects of loading rate on reinforced concrete. Tech. Rep. No. R642, U.S. Naval Civ. Engrg. Lab, Calif. J. Struct. Eng. 117 (1991) 3660–3679. [24] K. Wille, A.E. Naaman, Effect of ultra-high-performance concrete on pullout behavior of high-strength brass-coated straight steel fibers, ACI Mater. J. 110 (2013) 451–461. [25] Y.-W. Chan, V.C. Li, Effects of Transition Zone Densification On Fiber/Cement Paste Bond Strength Improvement, Mater. Res. 7355 (1997) 8–17. [26] J. Ma, M. Orgass, F. Dehn, D. Schmidt, N. V. Tue, Comparative Investigations on Ultra-High Performance Concrete with or without Coarse Aggregates, Proc. Int. Symp. Ultra High Perform. Concr. Kassel. (2004) 205–212. [27] S. Collepardi, L. Coppola, R. Troli, M. Collepardi, Mechanical properties of modified reactive powder concrete, ACI Spec. Publ. 173 (1997) 1–22.
[28] N.K. Lee, K.T. Koh, M.O. Kim, G.S. Ryu, Uncovering the role of micro silica in hydration of ultra-high performance concrete (UHPC), Cem. Concr. Res. 104 (2018) 68–79. [29] ASTM C 1437, Standard test method for flow of hydraulic cement mortar, ASTM Int. West Conshohocken, PA. (2007) 1–2. [30] D.Y. Yoo, S. Kim, Comparative pullout behavior of half-hooked and commercial steel fibers embedded in UHPC under static and impact loads, Cem. Concr. Compos. 97 (2019) 89–106. [31] M.N. Al Khalaf, C.L. Page, Steel/mortar interfaces: Microstructural features and mode of failure, Cem. Concr. Res. 9 (1979) 197–208. [32] J. Morton, G.W. Groves, The cracking of composites consisting of discontinuous ductile fibres in a brittle matrix - effect of fibre orientation, J. Mater. Sci. 9 (1974) 1436–1445. [33] Y. Lee, S.T. Kang, J.K. Kim, Pullout behavior of inclined steel fiber in an ultrahigh strength cementitious matrix, Constr. Build. Mater. 24 (2010) 2030–2041. [34] C. Ouyang, A. Pacios, S.P. Shah, Pullout of Inclined Fibers From Cementitious Matrix, Engineering. 120 (1995) 2641–2659. [35] J. Alwan, A.E. Naaman, P. Guerrero, Effect of mechanical clamping on the pullout response of hooked steel fibers embedded in cementitious matrices, Concr. Sci. Eng. 1 (1999) 15–25. [36] A.E. Naaman, G.G. Namur, J.M. Alwan, H.S. Najm, Fiber pullout and bond slip. I: Analytical study, 117 (1992) 2769–2790. [37] N. Banthia, J.F. Trottier, Concrete reinforced with deformed steel fibers part II: toughness characterization, ACI Mater. J. 92 (1995) 146–154. [38] Y.S. Tai, S. El-Tawil, T.H. Chung, Performance of deformed steel fibers embedded in ultra-high performance concrete subjected to various pullout rates, Cem. Concr. Res. 89 (2016) 1–13. [39] K. Wille, A.E. Naaman, Pullout behavior of high-strength steel fibers embedded in ultra-high-performance concrete, ACI Mater. J. 109 (2012) 479–488. [40] Y.S. Tai, S. El-Tawil, High loading-rate pullout behavior of inclined deformed steel fibers embedded in ultra-high performance concrete, Constr. Build. Mater. 148 (2017) 204–218. [41] V.M.C.F. Cunha, J.A.O. Barros, J.M. Sena-Cruz, Pullout Behavior of Steel Fibers in Self-Compacting Concrete, J. Mater. Civ. Eng. 22 (2009) 1–9. [42] Z. Zhao, S.Y. Hong, Cooling strategies for cryogenic machining from a materials viewpoint, J. Mater. Eng. Perform. 1 (1992) 669–678, https://doi.org/10.1007/ BF02649248.