Electrical and microstructural analysis of UHPC containing short PVA fibers

Construction and Building Materials 235 (2020) 117448

Contents lists available at ScienceDirect

Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat

Electrical and microstructural analysis of UHPC containing short PVA fibers S.H. Ghasemzadeh Mosavinejad a,⇑, Mir Alimohammad Mirgozar Langaroudi b, Jalil Barandoust a, Ardalan Ghanizadeh c a b c

Department of Civil Engineering, University of Guilan, Rasht, Iran Department of Civil Engineering, Fouman and Shaft Branch, Islamic Azad University, Fouman, Iran Department of Civil Engineering, University of New Brunswick, Fredericton, Canada

h i g h l i g h t s
Different properties of UHPC specimens containing PVA fibers were studied.
Electrical properties and microanalysis of UHP with different silica fume replacement ratios were investigated.
PVA fibers could increase flexural strength while the effect of fibers on compressive strength was inconsiderable.
IT was shown that electrical resistivity, RCPT and RCMT results are properly correlated in UHPC.
Silica fume replacement ratio of 0.25–0.3 was the optimum amount based on the microanalysis of the specimens.

a r t i c l e

i n f o

Article history: Received 15 January 2019 Received in revised form 10 October 2019 Accepted 2 November 2019

Keywords: Ultra-high performance concrete PVA fiber Scanning electron microscope Microstructure Electrical properties

a b s t r a c t Steel fibers are the most widely used fibers in making Ultra-high performance concrete. Under extreme weather conditions the embedded steel fibers may be corroded and the risk of serious deterioration will be high. Moreover, while the influence of steel fibers has been thoroughly studied, the addition of synthetic fibers and their effect on properties of UHPC has not completely investigated throughout the years. In this paper, mix proportions containing different volume percentages of PVA fibers were fabricated. Mechanical, electrical and microstructural properties were evaluated. Results showed that unlike steel fibers, chloride diffusion and penetration are reduced by the addition of PVA fibers. Different fractions of cement were replaced with silica fume to determine the optimum value by processing the scanning electron microscope images and Energy Dispersive Spectroscopy analysis. SEM and EDS analysis indicated that a uniform cement paste with a Ca/Si ratio of near 1 which means a harder matrix is obtained when the replacement ratio of silica fume increases. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction Initiated in the mid90s, Ultra-high performance concrete with exceptional mechanical properties has turned to an acceptable structural material among engineers. Very high compressive strength (Up to 200 MPa), leads to a significant weight loss of the structure and makes it possible to build slender structural elements [1,2]. To obtain a dense cement matrix with minimum porosity which results in the low permeability and high durability [3–5]. Cheyrezy and Charron [2,6] eliminated coarse aggregate from the constituent materials of UHPC to achieve compressive ⇑ Corresponding author. E-mail addresses: [email protected] (S.H. Ghasemzadeh Mosavinejad), ali. [email protected] (M.A.M. Langaroudi), [email protected] (A. Ghanizadeh). https://doi.org/10.1016/j.conbuildmat.2019.117448 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

strength higher than 150 MPa. High costs of casting was a roadblock in the way of UHPC to be commercially developed in a mass scale. To overcome this, some investigators used different types of pozzolans as partial replacement of cement. Moreover, since the compressive strength of UHPC stems from the failure in interfacial zone between cement and fine aggregate, using fillers such as silica fume and fly ash might strengthen the interface and enhance the composite’s mechanical properties [7]. Park et al used high percentages of silica fume as cement replacement. They suggested 25–30 vol% replacement to obtain the optimum compressive strength [7]. Zhao et al used silica sand instead of natural sand and also mixed fly ash as partial replacement of cement into mix proportions. They gained a UHPC with compressive strength exceeding 170 MPa, while the costs of casting were significantly reduced [8]. Although high compressive strength was the superior

2

S.H. Ghasemzadeh Mosavinejad et al. / Construction and Building Materials 235 (2020) 117448

characteristic of UHPC, low tensile and flexural strength as well as brittle fracture were the concerning drawbacks. Researchers came up with the idea of dispersing different types of fibers such as steel in different shapes and lengths to improve UHPC’s flexural strength and tensile properties such as energy absorption capacity and strain capacity [9–15]. While steel fiber is the most popular fiber among researchers, some of studies have hybridized fibers to gain higher mechanical properties in UHPC. Skazlic and Bjegovic [16] studied the flexural properties of UHPC containing straight and hooked steel fibers. Kim et al. [17] dispersed both micro and macro steel fibers into the UPC mixtures and concluded that deflection capacity and toughness improved by hybridization of fibers. Regarding the use of PVA fiber, limited studies including the one by Hannawi et al. [18] tested the addition of different types of fibers and their effect on mechanical properties of UHPC. Fixed volume percentage of PVA fiber (1%) with the diameter of 100 mm was employed in the study. Kang et al. [19] used hybrid PVA and steel fiber and investigated the tensile behavior of UHPC. Previous literatures have also reviewed the effect of high temperatures and pressure on the mechanical behavior of UHPC. Wu et al [20] studied the mechanical properties of UHPC containing different supplementary cementing materials exposed to hot water and steam curing. Compared to the specimens with standard curing, hot water and steam curing specimens exhibited higher compressive and flexural strength. Other investigators also concluded that steam and autoclave curing improve the mechanical and physical properties of UHPC [21,22]. But limited studies have been conducted on the effect of hot water curing on long-term durability and mechanical properties of UHPC. UHPC is known as a durable structural material with a very low permeability. High density and low air content greatly decrease the transport of ions through the matrix and in turn improve durability properties. Rapid chloride ion penetration in UHPC was tested by Graybeal and Hartmann [23] and 360 and 40 C were recorded as the total charge passed at the age of 28 days for specimens with different curing regimes, both of which are considered negligible penetration rate according to ASTM C1202 [24]. Abbas et al. [25] reported that since steel fibers prevent the early formation of microcracks the total charge passed in RCPT is considerably lower compared to that of plain concrete. Likewise, the air permeability of UHPC has been investigated by Roux et al. [26] and it was found that it is two magnitudes of order lower than normal concrete. Ghafari et al. [27] incorporated nano-silica in UHPC mixtures and measured the rate of corrosion of steel reinforcement by accelerated corrosion test. It was found that UHPC containing nano-silica exhibited the best performance compared to plain UHPC and HPC specimens. While lots of studies have been carried out concerning the chloride ion penetrability of UHPC, very limited studies have measured the electrical resistivity of these composites which is a simple means of obtaining information about the durability of UHPC specimens. Also, majority of these studies have been conducted at the ages up to 28 days and little references are available about the long-term durability properties of UHPC. According to aforementioned literatures the aims of this study is twofold. First, to investigate whether high volume of silica fume (up to 40%) could be added to UHPC mixtures without sacrificing long-term mechanical and durability properties. Secondly, the effect of different volume percentage of PVA fiber as the only fiber dispersed in UHPC mixtures on mechanical and durability properties was evaluated. Also all of the tests were performed on two group of specimens exposed to two different curing regimes. Finally, SEM and EDS analysis uncovered the role of silica fume in UHPC.

2. Materials and methods 2.1. Materials High packing density and tailoring the particle size distribution of cementitious materials and aggregate are the key parameters in determining mix proportions of ultra-high performance concrete [28,29]. The constituent materials of this study included Cement type I in accordance with standard specifications of ASTM-C150 [30], Silica fume compatible with the standard specifications of ASTM-C1240 [31] as a partial replacement for cement., silica sand which passed sieve number 30 (smaller than 600 mm) as fine aggregate and Polycarboxylate based superplasticizer as a waterreducing agent and retarder in accordance with ASTM C-494 [32]. Chemical composition of silica fume and cement are presented in Table1. Short PVA fibers were used in this study. Physicomechanical properties of PVA fiber are given in Table 2. 2.2. Mix designs Fabricating, molding and curing were performed in accordance with the requirements of ASTM C109 [33] test method. First of all, cementitious materials and silica sand were mixed until silica fume was properly distributed and a mixture with a light grey color was obtained. This was followed by the addition of the water and superplasticizer blend. Mixing was continued for about 1 min at a low speed. After the formation of the paste PVA fiber was added and to be properly dispersed, mixing continued for another 2 min at a high speed. In all mix designs water to cementitious material ratio was kept constant. According to different fractions of silica fume and PVA fiber in mix proportions, both of which considerably affect the consistency, and keeping superplasticizer dosage constant to eliminate its effects on different properties, flowability of mixtures was measured to determine whether the fresh UHPC shows thixotropic behavior. Table3 shows the mix designs for this study. After the full dispersion of fibers, the mixtures were poured into the molds and desired compaction was gained by tamping the mixtures by a steel rod. After 24 h the specimens were demolded. Regarding the curing of specimens, two different curing regimes were considered. For this purpose, specimens divided into two groups and each group was exposed to a different curing regime. First group was cured conventionally. The specimens were immersed in a 20 °C limewater tank after demolding. Second group of specimens were immersed in a 70 °C hot-water tank for 72 h and then after cooling in air they were put in a limewater tank until the date of testing. 2.3. Testing procedure 2.3.1. Flowability The flowability of mixes was assessed using ASTM C1437 [34] and modifications described in ASTM C1856 [35]. Fresh UHPC

Table 1 Chemical composition of cement and silica fume (%). Compound

Cement

Silica fume

CaO SiO2 Al2O3 Fe2O3 MgO K2O Na2O L.O.I

64.38 21.08 5.36 3.64 2 0.82 0.5 0.9

1.87 89.22 1.2 2.12 1.61 1.056 0.556 2.6

3

S.H. Ghasemzadeh Mosavinejad et al. / Construction and Building Materials 235 (2020) 117448 Table 2 Properties of PVA fiber. Material

Geometry

L (mm)

D (mm)

Specific gravity (kg/m3)

Elastic Modulus (GPa)

Synthetic

Micro

6

13

1300

29.5

Table 3 Mix designs. Batch designation

Cementitious Materials (CM) content (kg/m3)

Silica fume (vol. %)

Water/cementitious material

Superplasticizer/cementitious material

PVA(vol. %)

S15P0 S20P0 S25P0 S30P0 S35P0 S40P0 S15P03 S20P03 S25P03 S30P03 S35P03 S40P03 S15P06 S20P06 S25P06 S30P06 S35P06 S40P06 S15P09 S20P09 S25P09 S30P09 S35P09 S40P09 S15P12 S20P12 S25P12 S30P12 S35P12 S40P12

1060 1060 1060 1060 1060 1060 1060 1060 1060 1060 1060 1060 1060 1060 1060 1060 1060 1060 1060 1060 1060 1060 1060 1060 1060 1060 1060 1060 1060 1060

15 20 25 30 35 40 15 20 25 30 35 40 15 20 25 30 35 40 15 20 25 30 35 40 15 20 25 30 35 40

0.17 0.17 0.17 0.17 0.17 0.17 0.17 0.17 0.17 0.17 0.17 0.17 0.17 0.17 0.17 0.17 0.17 0.17 0.17 0.17 0.17 0.17 0.17 0.17 0.17 0.17 0.17 0.17 0.17 0.17

0.015 0.015 0.015 0.015 0.015 0.015 0.015 0.015 0.015 0.015 0.015 0.015 0.015 0.015 0.015 0.015 0.015 0.015 0.015 0.015 0.015 0.015 0.015 0.015 0.015 0.015 0.015 0.015 0.015 0.015

0 0 0 0 0 0 0.3 0.3 0.3 0.3 0.3 0.3 0.6 0.6 0.6 0.6 0.6 0.6 0.9 0.9 0.9 0.9 0.9 0.9 1.2 1.2 1.2 1.2 1.2 1.2

was poured into cone-shaped mold on top of the 254-mm diameter flow table. Once filled and screeded, the mold was slowly removed and mixes were allowed to flow until no more movement was detected. Initial measurements were performed and reported as static flow. Subsequently, 20 drops were applied to the flow table and the average diameter was determined again and reported as dynamic flow.

2.3.4. Rapid chloride penetration test The test was carried out in accordance with ASTM C1202 [38] test method. Cylindrical specimens with a diameter of 95 mm were fabricated and subjected to RCPT. According to the procedure described in the test method for the specimens with a diameter of 95 mm the total charge passed (coulombs) can be calculated using Eq. (1).

Q ¼ 900ðI0 þ 2I30 þ 2I60 þ 2I90 þ . . . þ 2I330 þ 2I360 Þ 2.3.2. Flexural strength The test was performed based on the ASTM C 348 [36] test method. The specimens were subjected to flexural test at the age of 28 days and 6 months. Six samples of each mix proportion (3 standard curing and 3 hot-water curing) were tested. Because of the large number of mix proportion designs the results were displayed as a contour so the variations of the strength with regard to the portions of constituent materials can be analyzed easily.

2.3.3. Compressive strength The test was performed based on the ASTM C349 [37] test method. Portions of the same specimens which were previously used to determine flexural strength were subjected to compressive loading. Two 40 * 50.8 mm hard steel plates were placed at the top and bottom of each specimen in a way that a cube with the dimension of 40 mm placed between the bearing heads of the universal testing machine. This test was also carried out at the ages of 28 days and 6 months.

ð1Þ

where Q is the charge passed, I0 is the current immediately after the voltage is applied, It is the current at the time t min after voltage is applied. 2.3.5. Rapid chloride migration test The test was performed in accordance with the procedure described in NT Build 492 [39]. Cylinders with the diameter of 100 mm were fabricated and subjected to RCMT test as is schematically shown in Fig. 1. The specimen was placed at the bottom of the rubber barrel and on the plastic stud to avoid direct contact between the specimen and cathode. The rubber barrel was fixed with steel clamp to keep the flank of the specimen in a sealed state. After that, the specimen was positioned on the plastic support which was immersed in an aqueous catholyte (10% NaCl) on one side and an aqueous anolyte (3% NaOH) on the other. The operation was carried out in the room temperature (23 ± 1 °C). According to NT Build 492 To determine the duration of the test and the applied voltage during the test, an initial 30 V voltage was applied to the specimens. Due to the very low permeability of the UHPC speci-

4

S.H. Ghasemzadeh Mosavinejad et al. / Construction and Building Materials 235 (2020) 117448

Fig. 1. Schematic presentation of RCMT.

mens the initial current was below 5 mA which means that the duration of the test should be fixed at 96 h and applied voltage will be 60 V. Non-steady state migration coefficient 1012 m2/s can be calculated by equation (2).

Dnssm ¼

rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi! 0:0239ð273 þ T ÞL ð273 þ T ÞLxd xd 0:0238 ðU  2Þt U2

ð2Þ

where Dnssm: non-steady state migration coefficient 1012 m2/s U: absolute value of the applied voltage T: Average value of the initial and final temperatures in the anolyte solution L: Thickness of the specimen Xd: average value of the penetration depth t: Test duration

2.3.6. Electrical resistivity Electrical resistivity test was conducted at the age of 6 months in order to investigate the long-term durability-related properties of UHPFRC. Since, the result of the test is highly dependent on the amount of water present, ion concentration of pore solution and porosity of the composite, this test can be a decisive criterion for the durability of the concrete structures. There are two conventional methods for determining the electrical resistivity values of cement composites called two-probe and four-probe (Wenner) method. Both methods have been used by investigators of the field frequently [40–43]. Although two probe method has a simple measurement circuit, but due to the elimination of contact resistance between electrode and cement matrix, four probe method is preferred. In this study four probe method was used. Electrode material is another important parameter that needs to be determined in order to obtain reliable results. Due to the low noise magnitude and negligible fluctuations, in this study brass electrodes were used [44]. Electrode dimension and spacing were determined in accordance with the information given in [45]. Diameter of the electrodes were 12 mm and spacing was 15 mm.

2.3.7. SEM micrographs Scanning electron microscope was employed to investigate the microstructure of UHPC specimens containing different replacement values of silica fume. For the first time SEM micrographs with magnification of higher than 100 kx were taken from the specimens to uncover the role of silica fume in the microstructure of UHPC. To develop an analytical method in order to quantify and measure the reacted and non-reacted silica fume particles, cement grains and the filler effect of abandoned silica fume inside C-S-H matrix, SEM micrographs with high magnification were taken so that these very fine particles can be observed at the state of semi hydrated covered with C-S-H. This can pave the way for studying the initiation of the chemical reaction between silica fume and cement grains or portlandite crystals.

2.3.8. EDS analysis Energy dispersive spectroscopy analysis was employed to evaluate the Ca/Si ratio in C-S-H when different silica fume replacement values are present in mix designs. Since silica fume to cement ratio was different in mix designs it is expected that Ca/ Si ratio varies significantly through the proportions. Three EDS analysis were conducted from each specimen. To reach a precise overview of Ca/Si ratio in UHPC specimens, EDS analysis was extracted from SEM micrographs with the same magnification (8 kx). Three spot on cement paste were chosen and the average value was reported as the Ca/Si ratio.

3. Results and discussion 3.1. Flowability Table 4 shows static and dynamic flow of all UHPC mixtures. Since all mixture have shown static flow lower than 178 mm, these UHPC mixes can be considered as thixotropic [46]. Also, increasing the content of silica fume and PVA fiber led to the decrease of both static and dynamic flow. On average increasing silica fume from 15% to 40% reduced static flow by 13% and dynamic flow by 6%. Furthermore, addition and increasing the amount of PVA fiber to 1.2 vol% reduced static flow by 8% and dynamic flow by 11% (Table 5).

5

S.H. Ghasemzadeh Mosavinejad et al. / Construction and Building Materials 235 (2020) 117448 Table 4 Results of flow table test. Mix

Static flow (mm)

Dynamic flow (mm)

S15P0 S20P0 S25P0 S30P0 S35P0 S40P0 S15P03 S20P03 S25P03 S30P03 S35P03 S40P03 S15P06 S20P06 S25P06 S30P06 S35P06 S40P06 S15P09 S20P09 S25P09 S30P09 S35P09 S40P09 S15P12 S20P12 S25P12 S30P12 S35P12 S40P12

128 123 122 119 117 113 127 123 119 118 114 111 124 119 114 109 108 108 121 116 115 113 109 106 117 114 113 110 108 105

214 209 211 209 207 204 209 205 201 199 200 196 203 199 200 196 195 192 200 197 195 192 193 189 194 192 189 187 183 179

Fig. 2a shows the results of compressive strength test at the age of 28 days on normal-cured samples. As the silica fume replacement ratio increases, the compressive strength increases as well. Highest values have been obtained in replacement ratios of 0.25 and 0.35. As the replacement values decreases to lower than 0.25 or increases to beyond 0.35, the compressive strength decreases significantly. According to the roles of silica fume in cement composites namely pozzolanic reaction with cement grains and filler effect, these observations can be explained. As a high reactive pozzolan, silica fume with its satisfying particle size distribution is able to react with portlandite crystals at early ages. These chemical reactions lead to a dense cement matrix with minimum porosity. When the concentration of silica fume particles is much below the optimum amount, a great proportion of cement grains are not included in the chemical reactions and remain in a non or semi-hydrated state. On the other hand, the addition of silica fume beyond the optimum amount also leads to a detrimental effect. While a number of particles remain unhydrated, the agglomeration

of very fine silica fume particles may result in the formation of microcrack at the boundaries of these agglomerated particles. Thus, as is seen in Fig. 2a, the decrease in compressive strength by increasing silica fume to more than 35% is consistent with the theory hydration in cement composites. Regarding the effect of short PVA fiber on compressive strength, it is observed that when the replacement ratio of silica fume is minimum, the addition of PVA fiber has led to an increase in compressive strength, while in the specimens with 40% silica fume replacement, highest compressive strength has been obtained when the fiber is concentration is 0.9%. Fig. 2b shows the compressive strength values of the specimens with hot water curing at the age of 28 days. The replacement ratio of 0.35 has resulted in the best performance. In general, comparing the results at the age of 28 days indicate that the compressive strength of standard-cured specimens is slightly higher than those of hot water cured specimens. While the studies have shown the positive effect of hot water curing on compressive strength at early ages, it seems that normal cured specimens exhibit better performance at later ages. The results of compressive strength for specimens with standard curing at the age of 6 months is similar to those of 28 days as is shown in Fig. 2c. With the passage of time the specimens with 0.25 and 0.35 silica fume replacement ratio still have the highest compressive strength. The effect of fiber is rather unclear and a constant trend cannot be detected. Similar to the results of hot water cured specimens at the age of 28 days, the best performance belongs to the specimens with 0.35 silica fume replacement at the age of 6 months. The only difference is the area of green zone which represents the specimens with highest strength. The relationship between compressive strength and silica fume replacement ratio is shown in Fig. 3. As is observed a polynomial trend line has been satisfactorily fitted to the results of standardcurd specimens at the age of 28 days as shown in Fig. 3a. This indicates that silica fume addition has positively affected the values up to the optimum amount. By increasing the silica fume content to beyond the optimum level a substantial decrease can be detected which is clearly observed at the down ward zone of the curves. It should be noted that the maximum strength belongs to the specimens with 0.3 and 0.35 silica fume replacement ratio. Increasing the pozzolan content from 15% to 30% has led to an average 9.5% increase in strength. A reduction between 4.2% and 19% has occurred by increasing the silica fume content from 30% to 40%. Likewise, at the age of 28 days a similar trend can be seen for the samples with hot water curing. Again, the specimens with 30 and 35% silica fume content exhibited the highest compressive strength. In the specimens with different PVA content, increasing

Table 5 RCPT results (coulombs passed). Silica fume (%)

PVA fiber (%) 0

15 20 25 30 35 40

Standard curing

15 20 25 30 35 40

Hot water curing

0.6

0.9

1.2

Charge passed (Coulombs) 62 60 57 52 51 54 48 49 43 46 40 41

0.3

54 48 46 40 39 33

50 44 43 37 33 30

51 45 40 36 32 29

60 58 54 46 47 41

54 51 47 43 40 35

50 46 42 40 35 31

46 43 38 35 30 33

55 51 51 48 45 41

6

S.H. Ghasemzadeh Mosavinejad et al. / Construction and Building Materials 235 (2020) 117448

Fig. 2. Compressive strength (a) standard curing, 28 days (b) hot water curing, 28 days (c) standard curing, 6 months (d) hot water curing, 6 months.

silica fume from 15% to 30% led to 19, 15, 18.2, 19 and 11% increase in strength. Further increase in silica fume content resulted in strength reduction. Agglomeration of very fine silica fume particles which leads to the formation of defects in matrix can be a reason for strength decrease. These microcracks propagate at the presence of a external load and eventually cause the failure of the specimen. At the age of 6 months the behavior of the specimens with both curing regimes shown in Fig. 3c and d do not change considerably compared to the age of 28 days. Based on the results obtained from these curves silica fume replacement ratio of 0.3 can be an optimum amount when compressive strength is of concern. The results of flexural strength for specimens with standard curing at the age of 28 days is seen in Fig. 4a. The effect of PVA fiber is evidently clear. The highest values have been obtained from the specimens containing 0.9 and 1.2% fiber. On the other hand, the increase in silica fume content up to 25% has also led to increase in strength. This can be attributed to the better fiber dispersion at the presence of silica fume. Other investigators have reported the better fiber dispersion when mineral admixtures are used in concrete mix proportions [45]. A satisfactory dispersion is achieved by gradation, adsorption and separation effects [46]. Further increase in silica fume significantly reduced the flexural strength. As previous literatures have shown [47,48] the addition of silica fume up to the optimum value densifies the cement matrix which leads to the enhancement of flexural strength. But high concentration of these particles may detrimentally affect the strengths especially flexural strength. A quantitative comparison between the

corresponding specimens with the high-fiber specimens and nonfibrous specimens shown a difference of about 22.3% in flexural strength. While the increase in silica fume to 30% has resulted in an average increase of 8.2%. The flexural strength of the specimens with hot water curing at the age of 28 days is shown in Fig. 4b. The green and red zones clearly demonstrate the effect of PVA fiber on flexural strength. By increasing the fiber content flexural strength has also continuously increased. Highest values belong to the specimens with 25 and 30% silica fume which again show the effect of silica fume on fiber dispersion in UHPC. It seems that the effect of fiber on flexural strength is more notable compared to the standard-cured specimens. On average the difference between the specimens containing 1.2% fiber and non-fibrous specimens is 37.5% which uncover the role of PVA on mechanical strengths of UHPC. At the age of 6 months the pattern of strength change is similar to that of the specimens at the age of 28 days for standard curing regime. The average difference in strength is 19% for mix designs with 1.2 and 0% PVA fiber. When silica fume content is 30%, the flexural strength is maximum. Similar to compressive strength results, further increase in silica fume leads to a descending strength trend. Comparing the values of specimens with 40% and 30% silica fume replacement percentage shows a difference about 9.1%. It can be concluded that both compressive and flexural strengths are negatively affected by high concentration of silica fume particles.

S.H. Ghasemzadeh Mosavinejad et al. / Construction and Building Materials 235 (2020) 117448

7

Fig. 3. Relationship between compressive strength and silica fume content for (a) standard-cured specimens at the age of 28 days (b) hot-water cued specimens at the age of 28 days (c) standard-cured at the age of 6 months (d) hot-water cured at the age of 6 months.

Fig. 4d shows the results of flexural strength test for the hotwater cured specimens at the age of 6 months. Compared to Fig. 4b, it seems that the focus of the green zone which represents the specimens with high flexural strength is on 0.35 silica fume replacement ratio. According to the results of compressive and flexural strength it can be said that if hot water is the curing regime chosen for UHPC specimens, even higher silica fume replacement values can be considered without sacrificing the mechanical strengths. An average difference of 37% between the specimens with 1.2% PVA fiber and non-fibrous specimens highlights the role of fiber in flexural strength. Regardless of the curing regime, PVA fiber substantially affects the flexural strength of UHPC. Fig. 4e compares the flexural strength values of the specimens with different curing regime at the ages of 28 days and 6 months. According to the trend line equation and its slope, the specimens with hot water curing have shown a higher increase rate between the two ages. Shown in Fig. 5a is the electrical resistivity of the specimens containing different fractions of silica fume and PVA fiber and subjected to standard curing regime. The results indicate that introducing silica fume ha a remarkable effect on resistivity as it was expected. As the replacement ratio increases, the resistivity increases as well. In non-fibrous specimens substituting 40% of the cement with silica fume results in 10% resistivity increase compare to that of the specimen with 15% replacement. The corresponding difference for the specimens with 0.3, 0.6, 0.9 and 1.2% PVA is 13.7%, 39.9%, 12.8% and 24.6% respectively. It seems that by incorporating fibers into the mixtures the difference in resistiv-

ity increases. This increase can be attributed to the effects of silica fume on cement matrix. As a high reactive pozzolan, silica fume enhances the chemical reactions and densifies the microstructure of cement composites. Plus, the filler effect is also known as a decisive factor in increasing both mechanical and durability properties of cement composites. Satisfying size distribution and great surface area of these very fine particles allow the growth of hydration products on their surface resulting in a denser microstructure. The results of Teng et al. [49] also confirmed the remarkable effect of silica fume on electrical resistivity. By the addition of silica fume main hydration product namely C-S-H fills the connected micropores and consequently increases the resistivity. The electrical resistivity of cement composites is highly influenced by the presence of conductive fibers such as steel fibers. The incorporation of steel fibers in UHPC mixtures decreases resistivity and even at high concentration of these fibers the resistivity drops to the values of ordinary concrete. But in the case synthetic fibers such as PVA, resistivity remains high even at high concentration of fibers. Reviewing the results in the Fig. 5a shows that the addition of PVA fiber has led to the decrease of resistivity but the decrease is considerably lower than that of the steel fiber reinforced UHPC. It seems that resistivity is reversely proportional to the PVA content. For example, for the specimens with 15% silica fume the addition of fiber in the amount of 0.3, 0.6, 0.9 and 1.2% led to the 6.4, 17.2, 19.3 and 24.4% decrease in resistivity. Although PVA is a non-conductive fiber but the incorporation of it has led to the reduction of resistivity. This is attributable to the micropores formed around the fibers. As the concentration of fibers goes up the number of thee micropores increases and thus the resistivity

8

S.H. Ghasemzadeh Mosavinejad et al. / Construction and Building Materials 235 (2020) 117448

Fig. 4. (a) and (c): Flexural strength of standard-cured specimens at the ages of 28 days and 6 months respectively. (b) and (d): Flexural strength of hot-water cured specimens at the ages of 28 days and 6 months respectively.

Fig. 5. Electrical resistivity of specimens with (a) standard curing (b) hot water curing.

9

S.H. Ghasemzadeh Mosavinejad et al. / Construction and Building Materials 235 (2020) 117448

the standard-cured specimens have exhibited higher resistivity and consequently less permeability. Hot water curing considerably increase the rate of hydration at early ages thus better performance is obtained when testing is performed at the ages below 28 days, but at later ages the slow but continuous hydration of standard curing leads to the better performance of the specimens. Permeability is considered as the most important factor in long-term assessment of concrete structures. Distribution, interconnection and size of the microcracks are the main parameters influencing permeability [50]. To measure the chloride penetration of concrete which is an aspect of its permeability Rapid Chloride Penetration Test is performed based on the procedure prescribed in ASTM C1202. Table 3 shows the results of RCPT test on the UHPC specimens of this study. According to the classification of the standard which is shown in Table 4, all the specimens tested in this study have shown negligible penetrability. This indicates that UHPC specimens have a satisfactory performance and can be used in structures that are designed to have a very long service life. The results show that increasing substitution ratio of silica fume significantly reduces the chloride penetration. The difference between

decreases. It should be noted that the incorporation of silica fume greatly improves this situation but since the percentage of dispersed fibers is high the decrease in resistivity occurs. Fig. 5b shows the resistivity of the specimens with hot water curing. Similar to the standard-cured specimens increasing silica fume content leads to the increase of resistivity. Increasing the silica fume replacement from 15% to 40% for the specimen with different PVA content resulted in 11.2, 21.4, 23.1, 8.2 and 19.11% increase in resistivity. Regardless of the curing regime, the addition of silica fume substantially increases the resistivity. The addition and increasing fiber content decreased resistivity. Thus, it seems that curing regime does not affect the formation of pores around PVA fibers. Both standard and hot water cured specimens showed reduction in resistivity as the fiber content went up. A comparison between the results of standard and hot water cured specimens reveals that the specimens cured in standard conditions has 4.01% more resistivity than that of hot water curing. Although it has been mentioned by numerous researchers that hot water curing leads to a better and durable performance of UHPC at early ages, but the results indicate that at the age 180 days

900 850

R² = 0.9595

Electrical resisvity(kΩ.cm)

R² = 0.8684 800 PVA0

750

PVA0.3 700

R² = 0.9855

PVA0.6

R² = 0.9422

650

R² = 0.7057

PVA0.9 PVA1.2

600 550 500 20

25

30

35

40

45

50

55

60

65

Total charge passed (Columbs)

(a) 800 R² = 0.8209 R² = 0.6044

Electrical resisvity(kΩ.cm)

750

R² = 0.8648

700

PVA0 PVA0.3

650

R² = 0.875

PVA0.6 PVA0.9

600

R² = 0.8282

PVA1.2

550

500 25

30

35

40

45

50

55

60

65

Total charge passed(Columbs)

(b) Fig. 6. Correlation between resistivity and chloride penetration for (a) standard curing (b) hot water curing.

10

S.H. Ghasemzadeh Mosavinejad et al. / Construction and Building Materials 235 (2020) 117448

Area

Ca/Si

Area

Ca/Si

Spot 1

1.5

Spot 1

1.13

Spot 2

1.32

Spot 2

1.05

Spot 3

1.23

Spot 3

1.11

Average

1.35

Average

1.09

(d)

(a)

Area

Ca/Si

Area

Ca/Si

Spot 1

1.08

Spot 1

1.3

Spot 2

1.21

Spot 2

1.19

Spot 3

1.11

Spot 3

1.17

Average

1.13

Average

1.22

(b)

(e)

Area

Ca/Si

Area

Ca/Si

Spot 1

1.2

Spot 1

1.27

Spot 2

1.24

Spot 2

1.25

Spot 3

1.11

Spot 3

1.18

Average

1.18

Average

1.23

(c)

(f) Fig. 7. SEM micrographs and EDS analysis.

Table 6 Chloride ion penetrability based on charge passed [38]. Charge passed (coulombs)

Chloride ion penetrability

>4000 2000–4000 1000–2000 100–1000 <100

High Moderate Low Very low Negligible

the specimens with 0.15 and 0.4 silica fume replacement ratio and different PVA content is 44.2% in average. This shows that incorporation of mineral admixtures especially silica fume reduces the chloride penetration to a considerable degree. This can be due to the pore size refinement and stronger transition zone between aggregate and cement matrix. Also, the addition of PVA fiber has

also led to a decrease in penetrability. Since the RCPT results is highly dependent on the current that passes through the matrix, introducing non-conductive fibers such PVA causes a decrease in penetration values. Nevertheless, the rate of decrease is considerably lower when compared with increasing silica fume replacement value. In a study by Afroughsabet et al. [51] it was reported that the addition of steel fibers caused an increase in penetration value due to the conductivity of steel fibers but the addition of PP fibers reduced the chloride penetrability of the specimens. The penetration values for hot water-cured specimens is comparable to that of standard cured specimens. But as is shown in Table 3 the values are slightly higher. This shows that hot water curing may result in a less durable UHPC specimens at later ages. It has been reported that all properties of UHPC at early ages are improved by hot water curing. But the results of this study shows

11

S.H. Ghasemzadeh Mosavinejad et al. / Construction and Building Materials 235 (2020) 117448 Table 7 RCMT results. Silica fume (%)

PVA fiber (%)

15 20 25 30 35 40

Standard curing

15 20 25 30 35 40

Hot water curing

0

0.3

0.6

0.9

1.2

D28  1012 m2/s 2.54 2.43 2.35 2.2 2.12 2.06

2.53 2.44 2.34 2.22 2.15 2.05

2.5 2.47 2.32 2.18 2.11 2.03

2.55 2.41 2.3 2.18 2.11 2.04

2.52 2.44 2.35 2.19 2.13 2.07

2.61 2.5 2.33 2.29 2.18 2.1

2.58 2.48 2.32 2.29 2.19 2.11

2.57 2.5 2.35 2.26 2.17 2.1

2.59 2.43 2.29 2.26 2.16 2.11

2.60 2.49 2.32 2.28 2.19 2.09

Table 8 Resistance to chloride ingress based on the diffusivity [49]. Chloride diffusivity D28  1012 m2/s

Resistance to chloride ingress

>15 10–15 5–10 2.5–5 <2.5

Low Moderate High Very high Extremely high

Table 9 Variation of image histograms. Silica fume content

Variation of image histogram

15% 20% 25% 30% 35% 40%

0.02 0.034 0.0183 0.0169 0.0229 0.0253

Compressive strength(MPa)

125

120

115

110 R² = 0.6268 105

100 1

1.05

1.1

1.15

1.2

1.25

1.3

1.35

1.4

Ca/Si rao Fig. 8. Relationship between Ca/Si ratio and compressive strength.

that to obtain better performance at later ages standard curing could be a better option. Comparing the results of the specimens with 0.15 and 0.4 silica fume replacement ratio and different PVA content shows an increase of 48.8%. Again, similar to the values of standard curing regime, hot water-cured specimens exhibited higher resistance to chloride penetration by increasing the silica fume content. Even high concentration of silica fume has not detrimentally affect the penetrability. This shows that even high replacement ratios can be employed when the chloride ion penetration through the concrete element is the main concern.

Fig. 6a shows the relationship between electrical resistivity and chloride ion penetration for standard-cured specimens. As is observed a linear trend line has been properly fitted the values which shows that the chloride penetration increases as the resistivity decreases. This is consistent with the findings of other investigators. Since both resistivity and chloride penetration are dependent on permeability, porosity and the transition zone, thus it is reasonable to find a linear correlation between the results of these two experiments. Fig. 6b shows the relationship between resistivity and chloride penetration for hot water-cured specimens. Similar correlation has been obtained by hot water curing. Regardless of curing regime, it seems that with a satisfactory estimation, resistivity and chloride ion penetration are reversely proportional. Since the tests related to the permeability and porosity is highly affected by the cement matrix and whether or not it contains mineral admixtures, rapid chloride migration test was performed on the non-fibrous specimens with different silica fume replacement ratio. Based on the procedure prescribed in NT-Build 492 a 30 V initial voltage was applied and initial current was measured. Due to the dense microstructure of the UHPC specimens all current magnitudes were below 5 mA. According to the standard and the initial current magnitude, the applied voltage and test duration were set to 60 V and 96 h respectively. Fig. 7 shows the results of RCMT. Based on a study by Nilsson et al. [52] the resistance to chloride penetration based on the chloride diffusivity can be classified as is shown in Table 6. Fig. 7 shows that approximately all the UHPC specimens has shown extremely high resistance to chloride penetration. Again, by increasing silica fume the chloride diffusivity has declined. This test also confirms the important role of silica fume in penetrability of UHPC specimens. Due to the pore refinement, stronger transition zone and less permeability, by increasing silica fume content, chloride diffusivity decreases. The decrease for non-fibrous specimens at standard curing condition was 4.5, 8, 15.4, 19.8 and 23.3% as the silica fume content increases from 15 to 40%. For hot water curing condition, the rate of decrease was 4.4, 12, 13.9, 19.7 and 24.2%. Comparing different curing regimes indicated that hot water curing leads to a lower resistance to chloride penetration at the age of 180 days. It seems that while the rate of hydration is significantly high at the first three days for hot water curing specimens, after three days of hot water curing the rate of hydration significantly decreases which lead to a less dense microstructure compare to standard-cured specimens at later ages. Nevertheless, the difference between the results for different curing conditions is between 1 and 3% which shows that the performance of specimens subjected to different curing regimes is comparable (Tables 7–9). An important point of the table is that the addition and increasing the percentage of PVA fiber has led to reduction in chloride ion diffusion. It has been reported that the addition of fibers may result

12

S.H. Ghasemzadeh Mosavinejad et al. / Construction and Building Materials 235 (2020) 117448

in a better performance of UHPC specimens subjected to chloride penetration [53]. But in the case of steel fiber not only the method of dispersion is of great importance but also the corrosion of fibers may arise another problematic issue for the UHPC specimens. Addition of PVA fibers improves the chloride diffusivity and do not corrode due to their synthetic nature. Furthermore, the dispersion of PVA fibers into the UHPC mixtures is much easier compared to that of steel fibers.

In order to investigate the Ca/Si ratio of the cement matrix in UHPC specimens with different silica fume replacement percentage, EDS analysis was taken from three distinct spot on each specimens. The spots were chosen from cement paste and away from aggregates surface. In a study by Pellenq et al. [54] it was reported that Ca/Si ratio in C-S-H, an ingredient of concrete fundamental to performance of concrete, is a major parameter in mechanical strengths of cement composites. In molecular simulations it was

Fig. 9. SEM micrographs and their corresponding image histograms.

S.H. Ghasemzadeh Mosavinejad et al. / Construction and Building Materials 235 (2020) 117448

predicted that Ca/Si of about 1 can produce a cement composite 60 to 90% harder than a concrete with Ca/Si of 1.7. In a silica-rich cement composite such as UHPC it is expected that Ca/Si of about 1 can be obtained. To gain an approximate value of Ca/Si ratio in UHPC and to relate it to the compressive strength of the specimens, EDS analysis was employed. Fig. 8 shows the EDS analysis of the specimens with different silica fume replacement ratios. As the substitution ratio increases to 35% the Ca/Si decreases. In Fig. 4a the specimen with 15% silica fume has a ratio of 1.35 while this values decreases to 1.09 in the specimens with 30% substitution

13

of cement. This shows that a Ca/Si ratio of near 1 can be obtained in a silica-rich cement composite. This means that harder and stiffer cement composites can be made by the replacement of cement with silica fume even at high dosages. By further increasing the silica fume percentage to 35 and 40% an increase in the ratio is observed which can be attributed to the agglomeration of the particles and an increase in the number of non-reacted silica fume particles. Fig. 9 shows the relationship between Ca/Si ratio and compressive strength of the specimens. A linear trend line has been fitted to

Fig. 9 (continued)

14

S.H. Ghasemzadeh Mosavinejad et al. / Construction and Building Materials 235 (2020) 117448

the dots. The correlation value of 0.62 among the results shows that a relationship between the Ca/Si ratio and compressive strength of UHPC specimens can be obtained but further research in this field is required to confirm a strong relationship between these two properties of cement composites. Scanning electron microscope was employed to compare the quality of the cement paste of the specimens with different silica fume replacement ratios. Since high cementitious content is used in UHPC, it is expected that when silica fume content is low, the

amount of unhydrated or semi-hydrated cement grains are high, and this can be evaluated by SEM observations through identifying C2S and C3S particles. On the other hand, when the concentration of silica fume particles exceeds the optimum amount, the number of unreacted silica fume particles can be investigated by SEM micrographs. To qualitatively evaluate the cement pastes, the SEM micrographs are interpreted, to quantitatively evaluate the SEM micrographs, the image histograms were extracted based on the number of pixels for each bar in the grayscale picture where 0 rep-

Fig. 10. Image processing using edge detection function for specimens with silica fume content of (a) 15% (b) 20% (c) 25% (d) 30% (e) 35% (f) 40%

S.H. Ghasemzadeh Mosavinejad et al. / Construction and Building Materials 235 (2020) 117448

15

Fig. 11. SEM micrographs.

resents the black color and 256 represents the white color. By moving from 0 to 256 the color shifts from black to white. Since the SEM micrographs are grayscale and the format of pictures in MATLAB is uint8, the numbers of color levels equals to 256. Since the cement paste and unhydrated particles are different in colors in SEM micrographs, the variations of histograms can be an index to determine the quality of paste and optimum amount of silica fume percentage based on the variation of the image histograms. All SEM micrographs for paste evaluation were taken with identical magnification (8000) and they all contain 500 * 500 pixels. Fig. 9a shows the SEM image of the specimen with 15% silica fume replacement. As is seen the unhydrated cement grains are clearly observed on the surface of the cement paste. The histogram shows that the intensity of the bars representing gray color is considerably higher than black and white bars and the variation of histogram is 0.02. The SEM micrograph of the specimen with 20% silica fume is shown in Fig. 9b and similar to that of previous specimen the semi-hydrated cement particles are seen. The histogram

has a variation of 0.034 which is higher compared to that of the specimen with 15% silica replacement. SEM of the specimen with 25% silica fume replacement shows a uniform cement paste with no or little unhydrated particles and this uniformity results in a histogram variation of 0.0183 which is considerably lower than the two previous specimens. Since unhydrated particles are rather white, so their absence leads to the uniformity of the histogram. Fig. 9d which shows the SEM of the specimen with 30% silica fume also has a uniform cement paste and 0.0162 histogram variation. The further decrease in variation means that the most uniform paste has been obtained in this specimen. By increasing silica fume content to 35% as is shown in Fig. 9e the semi-hydrated or unhydated silica fume particles can be seen and the histogram variation increases to 0.0229. The SEM of the most silica rich cement paste with 40% silica fume replacement percentage is shown in Fig. 9f. The amount of unhydrated particles has increased compared to the specimen with 35% silica fume. Histogram variation also increased to 0.0253.

16

S.H. Ghasemzadeh Mosavinejad et al. / Construction and Building Materials 235 (2020) 117448

According to the SEM micrographs and histograms it should be noted that the 30% silica fume replacement percentage seems to be the optimum amount. The most uniform cement paste with the lowest histogram variation was obtained when the replacement ratio reached to 0.3. This result was also confirmed by mechanical strengths where the corresponding specimens exhibited the best performance. To evaluate the quality of cement paste in each specimen the edge detection of image processing toolbox in MATLAB was used. By distinguishing the edge of features with other parts of the image the amount of unhydrated cement grains in C-S-H can be qualitatively investigated. Fig. 10 shows the SEM images after processing by edge detection functions. The white colors represent the edge of features (in this case unhydrated particles). Since image spatial filtering differs can differ the features by the color of the edges, brighter white color means greater difference between the frequency of those areas in the picture. Thus, brighter white color appears when the hydration has not commenced or has been remained incomplete at early stages. As is observed the specimen with 15% silica fume seems to have a great proportion of unhydrated particles. Increasing silica fume content leads to a more uniform cement paste and hence the less feature edges can be detected as is shown in Fig. 10c and 10d. Bright white color in Fig. 10e and f indicates that increasing the silica fume ratio to more than 30% leads to a paste with high percentage of non or semihydrated particles. It has been reported by numerous researchers that an important role of mineral admixtures in concrete is filler effect. The surface of pozzolans became an area for hydration products to grow and this results in the densification of the cement matrix. In a study by Scrivener et al. [55] it was mentioned that SEM images have taken from the concretes containing fly ash showing the filler effect on the surface of particles. Due to the fineness of silica fume particles it is hard to observe the filler effect on the surface of these particles and SEM graphs with high magnification needs to be taken in order to observe this effect on the surface of silica fume particles. In this study with the help of a powerful electron microscope the SEM images u to 400 kx were taken to observe the filler effect on the surface of silica fume particles. Fig. 11a shows a silica fume particle with a diameter of about 400 nm which have been partially coated by the hydration products. The hydrates have been attached to the particle from bottom and it seems that the chemical reaction between the particle and hydrates has begun. A completely coated particle is shown in Fig. 11b. The hydrates seem to completely occupy the surface of the particle. Fig. 11c shows two silica fume particles next to each other and completely coated with hydration products. It seems that that the spherical shape of the particles is changing due to the chemical reactions. Fig. 11d shows the silica fume particles remained uncoated and unreacted due to the high concentration of silica fume in some specimens. These figures clearly show that the high surface area of silica fume particles leads to the growth of hydration products on the surface of these particles. 4. Conclusion Ultra-high performance concrete specimens with high volume of silica fume and short PVA fiber were fabricated and subjected to different tests. The following conclusions can be drawn from the results obtained: - As the silica fume replacement percentage reaches to the optimum level the compressive strength increases. On average 8% increase in compressive strength was measured as the silica content changed from 15% to 30%. For compressive strength

-

-

-

-

-

-

the optimum silica fume to cement ratio is 0.3. Further increase in silica fume content leads to the decrease of the compressive strength PVA fiber content has a substantial effect on the flexural strength. As the fiber content went up, the flexural strength increased as well. About 30% difference was observed between the results of control mix and the mix containing 1.2% PVA. The electrical resistivity seemed to be directly proportional to the silica fume content. Even high concentration of silica fume particles did not detrimentally affect the resistivity of the specimens. On average 19% increase was measured as the silica fume content went up from 15% to 40%. Despite steel fiber, PVA fiber did not significantly decreased the resistivity of the UHPC sample due to its high electrical resistance. Both silica fume and PVA content led to a decrease in total charge passed during the rapid chloride penetration test. The effect of silica fume on RCPT results was significantly high. An average difference of 51.4% was observed between the results of specimens containing 15% and 40% silica fume. Similar to the results of RCPT, increasing silica fume content also led to a decrease in chloride diffusivity in RCMT. Both curing regimes resulted in the satisfactory performance of the specimens in both mechanical and durability tests. Although it was concluded that when long-term performance has to be assessed, the standard curing condition may excel the hot water curing due to its slow and uniform hydration with the passage of time. The EDS analysis showed that the ratio of Ca/Si can be related to the compressive strength of UHPC. A silica-rich cement paste can lead to the Ca/Si ratio of about 1 which results in a harder cement paste. SEM micrographs and their corresponding image histograms showed that a uniform and dense microstructure with the lowest amount of unhydrated and unreacted particles can be obtained when the silica fume replacement ratio is between 0.25 and 0.3.

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. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.conbuildmat.2019.117448. References [1] D. Yoo, N. Banthia, Mechanical properties of UHPFRC: a review, Cem. Concr. Compos. 73 (2016) 267–280. [2] J.P. Charron, Permeability of UHPFRC under high stresses, Mater. Struct. (2005). [3] P. Rechard, M. Cheyrezy, H. Marcel, Reactive powder concrete with high ductility, and 200–800 MPa compressive strength, American Concrete Spring Convention, San Francisco, 1994, pp. 507–518. [4] M.M. Reda, N.G. Shrive, J.E. Gillott, Microstructural investigation of innovative UHPC, Cem. Concr. Res. 29 (3) (1999) 323–329. [5] T. Ahlborn, D. Harris, D. Misson, E. Peuse, Characterization of strength and durability of ultra-high-performance concrete under variable curing conditions, Transp. Res. Rec.: J. Transp. Res. Board 2251 (2011) 68–75. [6] P. Rechard, M. Cheyrezy, Composition of reactive powder concrete, Cem. Concr. Res. 25 (7) (1995) 1501–1511. [7] J.J. Park, S. Kang, K.T. Koh, S.W. Kim, Influence of the ingredients on the compressive strength of UHPC as a fundamental study to optimize the mixing proportion, Proceedings of the international symposium on Ultra High Performance Concrete, Kassel, Germany, march 05–07, 2008.

S.H. Ghasemzadeh Mosavinejad et al. / Construction and Building Materials 235 (2020) 117448 [8] S. Zhao, J. Fan, W. Sun, Utilization of iron ore tailings as fine aggregate in ultrahigh performance concrete, Constr. Build. Mater. 50 (2014) 540–548. [9] S.H. Park, D.J. Kim, G.S. Ryu, K.T. Koh, Tensile behavior of ultra high performance hybrid fiber reinforced concrete, Cem. Concr. Compos. 34 (2012) 172–184. [10] K. Wille, S. El-Tawil, A.E. Naaman, Properties of strain hardening ultra-high performance fiber reinforced concrete (UHP-FRC) under direct tensile loading, Cem. Concr. Compos. 48 (2014) 53–66. [11] S. Kang, J. Kim, The relation between fiber orientation and tensile behavior in an ultra high performance fiber reinforced cementitious composites (UHPFRCC), Cem. Concr. Res. 41 (2011) 1001–1014. [12] M. Kim, D. 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. [13] D.L. Nguyen, G.S. Ryu, K.T. Koh, D.J. Kim, Size and geometry dependent tensile behavior of ultra-high-performance fiber reinforced concrete, Compos. B Eng. 58 (2014) 279–292. [14] D. Yoo, S. Kang, Y. Yoon, Enhancing the flexural performance of ultra-highperformance concrete using long steel fibers, Compos. Struct. 147 (2016) 220– 230. [15] D.J. Kim, S.H. Park, G.S. Ryu, K.T. Koh, Comparative flexural behavior of Hybrid Ultra High Performance Fiber Reinforced Concrete with different macro fibers, Constr. Build. Mater. 25 (2011) 4144–4155. [16] M. Skazlic´, D. Bjegovic´, Toughness testing of ultra high performance fibre reinforced concrete, Mater. Struct. 42 (8) (2009) 1025–1038. [17] D.J. Kim, S.H. Park, G.S. Ryu, K.T. Koh, Comparative flexural behavior of hybrid ultra high performance fiber reinforced concrete with different macro fibers, Constr. Build. Mater. 25 (11) (2011) 4144–4155. [18] K. Hannawi, H. Bian, W. Prince-Agbondjan, B. Raghavan, Effect of different types of fibers on the microstructure and the mechanical behavior of UltraHigh Performance Fiber-Reinforced Concretes, Compos. B Eng. 86 (2016) 214– 220. [19] S. Kang, J. Choi, K. Koh, K.S. Lee, B.Y. Lee, Hybrid effects of steel fiber and microfiber on the tensile behavior of ultra-high performance concrete, Compos. Struct. 145 (2016) 37–42. [20] Z. Wu, C. Shi, W. Hi, Comparative study on flexural properties of ultra-high performance concrete with supplementary cementitious materials under different curing regimes, Constr. Build. Mater. 136 (2017) 307–313. [21] L. Massidda, U. Sanna, E. Cocco, P. Meloni, High pressure steam curing of reactive-powder mortars, ACI 200 (2001) 447–464. SP200-27. [22] Y.S. Zhang, W. Sun, S.F. Liu, C.J. Jiao, J.Z. Lai, Preparation of C200 green reactive powder concrete and its static-dynamic behaviors, Cem. Concr. Compos. 30 (9) (2008) 831–838. [23] B.A. Graybeal, J.L. Hartmann, Strength and durability of ultra-high performance concrete, Concr. Bridge Conference (2003) 1–20. [24] ASTM C1202-12, Standard Test Method for Electrical Indication of Concrete’s Ability to Resist Chloride Ion Penetration. ASTM International, West Conshohocken, PA, 2012. [25] S. Abbas, A.M. Soliman, M.L. Nehdi, Exploring mechanical and durability properties of ultra-high performance concrete incorporating various steel fiber lengths and dosages, Constr. Build. Mater. 75 (2015) 429–441. [26] N. Roux, C. Andrade, M.A. Sanjuan, Experimental study of durability of reactive powder concretes, J. Mater. Civ. Eng. 8 (1) (1996) 1–6. [27] E. Ghafari, M. Arezoumandi, H. Costa, E. Júlio, Influence of nano-silica addition on durability of UHPC, Constr. Build. Mater. 94 (2015) 181–188. [28] H.Y. Sohn, C. Moreland, The effect of particle size distribution on packing density, Can. J. Chem. Eng. 46 (1968) 162–167. [29] R. Yu, P. Spiesz, H.J.H. Brouwers, Mix design and properties assessment of ultra-high performance fibre reinforced concrete (UHPFRC), Cem. Concr. Res. 56 (2014) 29–39. [30] ASTM C150/C150M-16, Standard Specification for Portland Cement, ASTM International, West Conshohocken, PA, USA. [31] ASTM C1240-15, Standard Specification for Silica Fume Used in Cementitious Mixtures, ASTM International, West Conshohocken, PA, USA.

17

[32] ASTM C494/C494M-16, Standard Specification for Chemical Admixtures for Concrete, ASTM International, West Conshohocken, PA, USA [33] ASTM C109/C109M – 16a, Standard Test Method for Compressive Strength of Hydraulic Cement Mortars, ASTM International, West Conshohocken, PA, USA. [34] ASTM C1437-15, Standard Test Method for Flow of Hydraulic Cement Mortar, ASTM International, West Conshohocken, PA, USA [35] ASTM C1856-17, Standard Practice for Fabricating and Testing Specimens of Ultra-High Performance Concrete, ASTM International, West Conshohocken, PA, USA [36] ASTM C 348-14, Standard Test Method for Flexural Strength of HydraulicCement Mortars, ASTM International, West Conshohocken, PA, USA. [37] ASTM C 349-14, Standard Test Method for Compressive Strength of HydraulicCement Mortars (Using Portions of Prisms Broken in Flexure), ASTM International, West Conshohocken, PA, USA. [38] ASTM C 1202-08, Standard Test Method for Electrical Indication of Concrete’s Ability to Resist Chloride Ion Penetration, ASTM International, West Conshohocken, PA, USA [39] NT Build 492, Chloride migration coefficient from non-steady-state migration experiments, Nordtest, Nordic Council of Ministers, Finland [40] B. Chen, K. Wu, W. Yao, Conductivity of carbon fiber reinforced cement-based composites, Cem. Concr. Compos. 26 (2004) 291–297. [41] J. Cao, D.D.L. Chung, Electric polarization and depolarization in cement-based materials, studied by apparent electrical resistance measurement, Cem. Concr. Res. 34 (2004) 481–485. [42] P. Xie, P. Gu, Electrical percolation phenomena in cement composites containing conductive fibers, J. Mater. Sci. 31 (1996) 4093–4097. [43] K.R. Gowers, S.G. Millard, Measurement of concrete resistivity for assessment of corrosion severity of steel using wenner technique, ACI Mater. J. 96 (5) (1999). [44] S.H. Ghasemzadeh Mosavinejad, J. Barandoust, A. Ghanizadeh, M. Sigari, Crack detection of a HPCFRCC thin plate using electrical resistivity method, Constr. Build. Mater. 193 (2018) 255–267. [45] B. Han, S. Ding, X. Yu, Intrinsic self-sensing concrete and structures: a review, Measurement 59 (supplement C) (2015) 110–128. [46] S. Bhanja, B. Sengupta, Influence of silica fume on the tensile strength of concrete, Cem. Concr. Res. 35 (4) (2005) 743–747. [47] A. Anwar, S.A. Ahmad, An experimental investigation on strength behaviour of concrete by partial replacement of fine aggregate with copper slag and cement with silica fume, Int. J. Comput. Sci. Eng. 6 (1) (2018). [48] H.A. Toutanji, Z. Bayasi, Effect of curing procedures on properties of silica fume concrete, Cem. Concr. Res. 29 (1999) 497–501. [49] S. Teng, V. Afroughsabet, C.P. Ostertag, Flexural behavior and durability properties of high performance hybrid-fiber-reinforced concrete, Constr. Build. Mater. 182 (2018) 504–515. [50] M.H. Zhang, H. Li, Pore structure and chloride permeability of concrete containing nano-particles for pavement, Constr. Build. Mater. 25 (2) (2011) 608–616. [51] V. Afroughsabet, L. Biolzi, P.J. Monteiro, The effect of steel and polypropylene fibers on the chloride diffusivity and drying shrinkage of high-strength concrete, Compos. B Eng. 139 (2018) 84–96. [52] L. Nilsson, M.H. Ngo, O.E. Gjørv, High-performance repair materials for concrete structures in the port of Gothenburg, in: Second International Conference on Concrete Under Severe Conditions: Environment and Loading, 1998, pp. 1193–1198. [53] J.P. Vincler, T. Sanchez, V. Turgeon, D. Conciatori, L. Sorelli, A modified accelerated chloride migration tests for UHPC and UHPFRC with PVA and steel fibers, Cem. Concr. Res. 117 (2019) 38–44. [54] J. Ronald, M. Pellenq, A. Kushima, R. Shahsavari, J. Krystyn, V. Vliet, M.J. Buehler, S. Yip, F.J. Ulm, A realistic molecular model of cement hydrates, PNAS 106 (38) (2009) 16102–16107. [55] K.L. Scrivener, P. Juilland, P.J.M. Monteiro, Advances in understanding hydration of Portland cement, Cem. Concr. Res. 78 (2015) 38–56.