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Static and dynamic compressive properties of ultra-high performance concrete (UHPC) with hybrid steel fiber reinforcements
Accepted Manuscript Static and dynamic compressive properties of ultra-high performance concrete (UHPC) with hybrid steel fiber reinforcements Zemei Wu, Caijun Shi, Wen He, Dehui Wang PII:
S0958-9465(17)30162-2
DOI:
10.1016/j.cemconcomp.2017.02.010
Reference:
CECO 2786
To appear in:
Cement and Concrete Composites
Received Date: 13 January 2016 Revised Date:
17 January 2017
Accepted Date: 19 February 2017
Please cite this article as: Z. Wu, C. Shi, W. He, D. Wang, Static and dynamic compressive properties of ultra-high performance concrete (UHPC) with hybrid steel fiber reinforcements, Cement and Concrete Composites (2017), doi: 10.1016/j.cemconcomp.2017.02.010. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Static and dynamic compressive properties of ultra-high
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performance concrete (UHPC) with hybrid steel fiber
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reinforcements Zemei Wu a, b, Caijun Shi a, *, Wen He a, Dehui Wang a a
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College of Civil Engineering, Hunan University, Changsha 410082, China; Department of Civil, Architectural and Environmental Engineering, Missouri University of Science and Technology, Rolla, MO, 65401, USA
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Abstract: Ultra-high performance concrete (UHPC) is promising in construction of concrete structures
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that suffer impact and explosive loads. In order to make UHPC structures more ductile and cost-effective,
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hybrid fiber reinforcements are often incorporated. In this study, a reference UHPC mixture with no fiber
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reinforcement and five mixtures with a single type of fiber reinforcement or hybrid steel fiber
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reinforcements of 6 and 13 mm in length at a total dosage of 2%, by the volume of concrete, were
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prepared. Quasi-static compressive and flexural properties of those mixtures were investigated. Split
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Hopkinson press bar (SHPB) testing was adopted to evaluate their dynamic compressive properties under
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three impact velocities. Test results indicated that UHPC with 1.5% long fiber reinforcements and 0.5%
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short fiber reinforcements demonstrated the best static and dynamic mechanical properties. The static
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compressive and flexural strengths of UHPC with 2% long fiber reinforcements were greater than those
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with 2% short fiber reinforcements, whereas comparable dynamic compressive properties were observed.
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Strain rate effect was observed for the dynamic compressive properties, including peak stress, dynamic
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increase factor, and absorbed energy. The reinforcing mechanisms of hybrid fiber reinforcements in UHPC
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were eventually discussed.
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*Corresponding author. Tel./fax: +86 731 8882 3937. E-mail address: [email protected]; [email protected] (C. Shi)
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Keywords: Ultra-high performance concrete; Dynamic compressive properties; Hybrid fiber
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reinforcements; Static mechanical properties; Split Hopkinson press bar
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1. Introduction
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Ultra-high performance concrete (UHPC) was introduced in the mid-1990s [1]. Use of high content
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of cementitious materials, superplasticizer, steel fiber reinforcement, and special curing, such as heat and
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autoclave curing, and even nano-particles, are often mentioned in order to achieve high strength and
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toughness [1-4]. Compared to ordinary concrete, UHPC has a very high compressive strength in the order
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of 150 MPa and a tensile strength in the order of 10 MPa along with a strain hardening behavior [5, 6]. Its
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superior performance allows its potential applications in military and protective structures that suffer
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impact and explosive loads with high impact velocity [7, 8]. However, the cost of steel fiber
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reinforcements used in UHPC is generally higher than that of the matrix. With the emphasis on
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sustainability of UHPC, minimizing the fiber reinforcement volume without sacrifice of its performance is
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of great significance [9]. Blending different types, shapes and/or sizes of fiber reinforcement has been
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proven as one of the effective methods because of a combination of various features from those different
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fiber reinforcements [10-12]. For example, the short fiber reinforcements can effectively restrict the
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development of micro-cracks while the long fiber reinforcements can bridge macro-cracks [9]. Research
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on the mechanical properties of hybrid fiber-reinforced UHPC has attracted a lot of attention, but most of
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the literature thus far was focused on quasi-static properties, such as compressive, tensile, and flexural
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properties, rather than dynamic behavior [5, 12, 13].
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There are two common methods, drop weight and split Hopkinson pressure bar (SHPB) testing, to
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measure the dynamic behavior of a composite material. The drop weight testing has the ability to 2
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replication of testing arrangement is possible by using this method. Habel et al. [6] employed this method
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to evaluate the impact bending performance of ultra-high performance fiber reinforced concrete
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(UHPFRC) and found that the strength increased with the increase of strain rate. Although this method is
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simple and easy to be implemented, the experimental results are greatly influenced by the specimen size
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and configuration, such as drop weight and speed, support stiffness, etc. [15, 16]. The SHPB testing is the
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most commonly used one for evaluating dynamic properties [17, 18]. Rong et al. [19] used the SHPB
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testing to investigate the dynamic compression behavior of UHPFRC with different fiber reinforcement
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volumes and found that the impact resistance of UHPFRC was improved with the increase of fiber
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reinforcement volume. Under the same compressive loading, the control specimens were crushed, while
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those with 3% or 4% of steel fiber reinforcements remained essentially intact. Lai et al. [20] indicated that
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the peak stress, peak strain, and the area under strain-stress curve of UHPFRC increased with the increase
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of strain rate. Zhang et al. [21] reported that the dynamic tensile strength of C200 green reactive powder
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concrete increased markedly with the increase of impact velocity, ranging from 4 to 14 m/s. Until now, no
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information can be found about the effect of fiber hybridization on the dynamic mechanical properties,
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especially on the dynamic compressive properties of UHPC.
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This research aims at investigating the influences of hybrid steel fiber reinforcements with lengths of
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13 and 6 mm on flowability, quasi-static compressive and flexural properties, as well as dynamic
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compressive behavior of UHPC. Split Hopkinson pressure bar testing was conducted to characterize the
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dynamic compressive behavior, including failure pattern, load-deflection curve, peak stress, and fracture
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energy, under three high impact velocities. Dynamic increase factor was introduced to compare the
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quasi-static properties with the dynamic compressive properties. Finally, the reinforcing mechanisms of
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hybrid fiber reinforcements in UHPC were discussed.
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2. Materials and experimental program
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2.1. Materials Portland cement (P.I 42.5) complying with the Chinese Standard GB175-2007 was used [22]. Table 1
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summarizes the chemical composition and density of the cement, silica fume, and slag. Silica fume has an
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average particle size of 0.1 to 0.2 µm and a specific surface area of 18,500 m2/kg. Slag has a specific area
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of 410 m2/kg. Two straight smooth steel fibers with diameters of 0.2 mm but one with a length of 6 mm
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and the other one of 13 mm were used. The tensile strength is 2,800 MPa. River sand with a maximum
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particle size of 2.36 mm was used. A polycarboxylate-based superplasticizer (SP) with a solid content of
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20% was used.
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Table 1 Chemical composition and density of cementitious materials Chemical CaO SiO2 Al2O3 Fe2O3 SO3 MgO Na2Oeq K2O C composition (%) (%) (%) (%) (%) (%) (%) (%) (%) Cement Silica fume Slag
62.49 21.18 1.85
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39.11 33.00 13.91
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2.2. Mixture proportion
Density (kg/m3)
3.41
2.83
2.53
0.56
-
-
3,110
0.59
-
0.27
0.17
0.86
1.06
2,150
0.82
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10.04
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-
-
2,900
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93.90
4.73
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Based on the previous study [23, 24], UHPC with a water to binder ratio (w/b) of 0.18 and silica
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fume content of 20% to 25%, by mass of cementitious materials, can obtain a satisfactory bond to steel
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fiber reinforcement and mechanical properties. Therefore, UHPC mixture with a w/b of 0.18 and silica
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fume content of 25% was used in this study. The SP dosage was fixed at 2% by the mass of the binder. To
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investigate the hybridization of short and long fiber reinforcements on quasi-static and dynamic properties
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of UHPC, a reference mixture with no fiber reinforcement and five mixtures with a single type of fiber
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reinforcement or hybrid fiber reinforcements at a total content of 2%, by volume of the concrete, were
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used. Table 2 shows the five mixture proportions. L0S0 was the reference mixture with no fiber 4
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reinforcement. L1.5S0.5 was referred to the mixture with 1.5% long fiber reinforcements and 0.5% short
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fiber reinforcements. Table 2 Mixture proportions of UHPC Silica Superplasticizer Cement Slag Sand Water No. fume 3 3 (kg/m3) (kg/m3) (kg/m ) (kg/m ) (kg/m3) (kg/m3) L0S0 472 315 262 1,049 178 21 L2S0 472 315 262 1,049 178 21 L1.5S0.5 472 315 262 1,049 178 21 L1S1 472 315 262 1,049 178 21 L0.5S1.5 472 315 262 1,049 178 21 L0S2 472 315 262 1,049 178 21
Long fiber (%) 0 2.0 1.5 1.0 0.5 0
Short fiber (%) 0 0 0.5 1.0 1.5 2.0
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2.3. Mixing procedure and sample preparation
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During mixing, dry powders, including cement, silica fume, slag, and river sand, were first mixed for
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3 min at a high speed. Water and superplasticizer were then added and mixed for approximately 6 min at a
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low speed. Steel fiber reinforcements were slowly added and mixed for another 6 min until achieving a
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uniform distribution.
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All the mixtures were cast into molds for the testing of flowability and mechanical properties.
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Specimens were then kept in a room at 20°C and relative humidity (RH) greater than 95% for
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approximately 24 h. They were then demolded and cured in lime-saturated water at 20°C for 7 and 28 d.
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2.4. Experimental methods
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2.4.1. Flowability
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The flowability of all mixtures was measured in accordance with the Chinese Standard GB/T
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2419-2005 [25]. The mixtures were cast into a mini cone mold and jolted for 25 times. Two diameters
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perpendicular to each other were determined and mean values were reported.
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2.4.2. Static mechanical properties Specimens of 40 × 40 × 160 mm were cast for determination of static compressive and flexural
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strengths. Three samples of each batch were tested. The flexural and compressive strengths measurements
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were conducted according to GB/T 17671-1999 (similar to ISO 697:1989) [26]. Three-point bending
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testing was first performed to obtain flexural properties. Six broken specimens measuring 40 × 40 × 40
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mm were then tested for compressive strength. Two non-casting surfaces of the specimen were used as a
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bottom and a top surface to ensure complete contact with the platen of the machine. The loading rate was
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set at 2.4 kN/s. Averages over three samples for flexural strength and over six samples for compressive
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strength were reported.
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2.4.4. Split Hopkinson pressure bar (SHPB) testing
A schematic illustration of SHPB testing equipment is shown in Fig. 1. The testing is based on the
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theory of one-dimensional wave propagation in an elastic bar [27]. The specimen is placed between the
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ends of the input and output bars. A stress wave is created and propagates through the bar toward the
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specimen at the end of the input bar. This incident wave then splits into a transmitted wave and a reflected
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wave upon reaching the specimen. The transmitted wave travels through the specimen and goes into the
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output bar, causing plastic deformation in the specimen. The reflected wave is reflected away from the
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specimen and travels back to the input bar. The whole process is recorded through a computer.
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The SHPB equipment used in this research is made of superior alloy steel with a diameter of 100 mm.
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The lengths of the input and output bars are 6,000 and 4,000 mm, respectively. The distances of strain
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gauge on the input and output bars to the specimen are 3,000 and 1,000 mm, respectively. The length of
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the striker bar is 1,500 mm. Cylinder samples with a diameter of 92 mm and a height of 46 mm were used.
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Before testing, the two ends of each specimen were ground to make them parallel so as to ensure the 6
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(a) Schematic illustration of SHPB equipment [28]
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(b) Picture of SHPB equipment Fig. 1 SHPB testing equipment 3. Results and discussion
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3.1. Effects of hybrid steel fiber reinforcements on flowability of UHPC
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Figure 2 shows the effects of hybrid fiber reinforcements on flowability of the UHPC mixtures. The
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UHPC mixture with no steel fiber reinforcement had the highest flowability of 248 mm. With the
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incorporation of a single type of fiber reinforcement or hybrid fiber reinforcements, the flowability
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decreased. This agrees well with the results reported by Yu et al. [29]. The reduced flowability associated
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with the fiber reinforcements was attributed to the following reasons [30, 31]: (1) fiber reinforcements are
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much more elongated and have higher surface area compared to aggregates, which can increase the
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cohesive force; (2) steel fiber reinforcements can alter the granular skeleton structure and hinder the flow
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of fresh paste; (3) steel fiber reinforcements tends to be perpendicular to the flow direction, generating
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resistance force and moment to the flow velocity of fresh mixtures. It can be also observed from Fig. 2, the UHPC mixtures with the hybrid fiber reinforcements
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demonstrated higher flowability than those with the single type of fiber reinforcement. The higher the
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short fiber reinforcement volume in the UHPC mixture with hybrid fiber reinforcements was, the better
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the flowability was. However, when all the 2% long fiber reinforcements were replaced by the short fiber
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reinforcement, the mixture (L0S2) obtained the worst flowability. In the case of hybrid fiber
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reinforcements, the mutual effects between the short and long fiber reinforcements can restrict each
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other’s rotation to avoid the perpendicular orientation to the flow direction [29-31]. This can reduce the
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resistance force to some extent, and eventually enhance the flowability when compared to those with the
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single type of fiber reinforcement (L2S0 and L0S2 mixtures). The worst flowability of the L0S2 mixture
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might be attributed to the increase in fiber reinforcement volume associated with short length.
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Flowability (mm)
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235 230
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167 168 169 170
L0S0
L2S0 L1.5S0.5 L1S1 L0.5S1.5 L0S2 No.
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Fig. 2 Effects of hybrid fiber reinforcements on flowability of UHPC
3.2. Effects of hybrid fiber reinforcements on static compressive and flexural strengths
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The effects of hybrid fiber reinforcements on static compressive strength of UHPC are illustrated in
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Fig. 3. The 7- and 28-d static compressive strengths of the reference UHPC mixture (L0S0) were 81.1 and
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98.3 MPa, respectively. It increased obviously with the incorporation of steel fiber reinforcements. The 8
ACCEPTED MANUSCRIPT compressive strengths of the L2S0 mixture at 7 and 28 d were increased by 35% and 28%, respectively, in
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comparison to the reference mixture. For the mixtures with hybrid fiber reinforcements, the compressive
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strength increased first but then decreased with the increase of the short fiber reinforcement volume.
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When 1.5% long fiber reinforcements and 0.5% short fiber reinforcements were used, the highest
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compressive strength was achieved. Besides, the static compressive strength of the L2S0 mixture was
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obviously greater than that of the L0S2 mixture. The decrease in compressive strength with the increase of
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short fiber reinforcement content might be due to the lower efficiency of short fiber reinforcements in
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restricting the development of cracks [11, 12, 29]. Therefore, the use of 2% hybrid fiber reinforcements
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could increase the static compressive strength by 20% to 40% compared to the reference mixture.
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Compressive strength (MPa)
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60
L0S0
L2S0 L1.5S0.5 L1S1 L0.5S1.5 L0S2 No.
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7d 28d
Fig. 3 Effects of hybrid fiber reinforcements on static compressive strength of UHPC
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Figure 4 depicts the effects of hybrid fiber reinforcements on static flexural behavior of UHPC. The
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UHPC reference specimens with no fiber reinforcement showed a sudden drop of load when reaching the
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initial cracking point at a deflection of 0.25 mm, as observed from Fig. 4(a). UHPC specimens with fiber
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reinforcement could continue to sustain load after the initial cracking until they reached the peak load. The
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volume of long and short fiber reinforcements had a significant effect on the peak load. The L1.5S0.5
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mixture was shown to exhibit the greatest peak load, followed by the L2S0 mixture. As can be also
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observed from Fig. 4(a), the load-deflection curves showed obvious zigzag patterns after the peak load.
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This is consistent with the results in literature [11]. It was observed that there were tearing sounds after the
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peak load, which indicates that the fiber reinforcements were continuously pulled out from the matrix. The change in static ultimate flexural strength of UHPC with hybrid fiber reinforcements showed a
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similar trend to that of the static compressive strength, as illustrated in Fig. 4(b). The L1.5S0.5 mixture
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showed the highest flexural strength of 32 MPa at 28 d, which increased by 108% compared to the
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reference mixture. This might be due to the combined strengthening effects from the short and long fiber
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reinforcements, which can restrict the crack development in two length scales, both in a scale of 6 mm and
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13 mm length [12]. However, when more long fiber reinforcements were replaced by short fiber
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reinforcements, the flexural strength decreased. The L0S2 mixture obtained the worst static flexural
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strength among the five UHPC mixtures with fiber reinforcement.
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15000
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Load (N)
12000 9000 6000
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Flexural strength (MPa)
L0S0 L2S0 L1.5S0.5 L1S1 L0.5S1.5 L0S2
2
3
4
5
204 205 206
20
10
L0S0
L2S0 L1.5S0.5 L1S1 L0.5S1.5 L0S2 No.
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203
30
0
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7d 28d
(a) Flexural load-deflection curves at 28 d (b) Flexural strengths at 7 and 28 d Fig. 4 Effects of hybrid fiber reinforcements on flexural behavior of UHPC
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3.3. Effects of hybrid fiber reinforcements on dynamic compressive properties of UHPC
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3.3.1. Effects of hybrid fiber reinforcements on dynamic failure pattern of UHPC
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Figure 5 illustrates the dynamic failure pattern of UHPC with and without fiber reinforcement at 28 d.
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At the impact velocity of 8.9 m/s, all the specimens exhibited a failure mode of pulverization. The 10
ACCEPTED MANUSCRIPT reference mixture (L0S0) with no fiber reinforcement showed a large quantity of small and irregular
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broken concrete fragments. The use of steel fiber reinforcements could effectively restrict the transverse
213
deformation of specimens. As can be seen from Fig. 5(b-d), the sizes of the broken fragments of the
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UHPC specimens varied with the hybrid fiber reinforcement volumes. The L2S0 mixture showed relative
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large and round broken fragments. For the L1.5S0.5 mixture, balanced broken fragments with both large
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and small parts could be observed. Many embedded fiber reinforcements were observed in the fracture
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surface, bridging the small fragments. However, relatively small and elongated broken fragments were
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observed for the L0S2 mixture, as can be observed from Fig. 5(d).
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(a) L0S0 mixture
(b) L2S0 mixture
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(c) L1.5S0.5 mixture (d) L0S2 mixture Fig. 5 Effects of hybrid fiber reinforcements on dynamic failure patterns of UHPC at an impact velocity of 8.9 m/s
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3.3.2. Effects of strain rates on dynamic compressive load-deflection relationship of UHPC The effects of different strain rates on dynamic compressive load-deflection curves of UHPC with and
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without fiber reinforcement at 28 d are illustrated in Fig. 6. For all UHPC mixtures, the stress changed
230
almost linearly with the strain up to the peak stress. Afterward, the stress-strain curves showed a
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decreasing branch. When the strain reached approximately 0.04, a constant stress was observed.
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From Fig. 6, it can be also seen that the dynamic compressive behavior of UHPC was sensitive to
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strain rate. All the mixtures showed a strain rate effect, that is, the peak stress increased with the increase
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of strain rate. The peak stress for the L2S0 mixture was 141.2 MPa at the strain rate of 115.5 s-1, and
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increased to 160.6 and 177.4 MPa, respectively, when the strain rate increased to 158.2 and 195.8 s-1. Li
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and Cotsovos [17, 32, 33] proposed that this strain rate effect was due to lateral inertia associated with
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friction to the contact surface under rapid impact loading. Besides, the increasing rate for peak stress
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decreased gradually with the increase of strain rate. This might be attributed to the Stefan effect associated
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with the presence of free water in concrete [34].
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206.1/s 169.8/s
150
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Stress (MPa)
150
180
Stress (MPa)
180
210
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120
130.0/s
90
120
60
30
30
0.02
0.04 0.06 Strain
0.08
0.10
158.2/s 115.5/s
90
60
0.00
195.8/s
0.00
0.02
0.04
0.06 Strain
240 241
(a) L0S0
(b) L2S0
12
0.08
0.10
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Stress (MPa)
150
155.7/s 114.8/s
90
107.8/s
120 90
60
60
30
30 0.02
158.7/s
150
120
0.00
195.4/s
180
0.04 0.06 Strain
0.08
0.10
0.00
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210
194.8/s
Stress(MPa)
210
0.02
210 194.7/s 155.6/s
150
Stress(MPa)
Stress (MPa)
111.8/s
90
120
0.08
0.10
156.1/s
109.0/s
90 60
60
30
0.02
0.04
0.00
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0.08
0.10
0.02
0.04
Strain
244
0.06
194.8/s
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0.00
0.10
(d) L1S1
210
120
0.08
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(c) L1.5S0.5
180
0.06
Strain
242 243
0.04
Strain
(e) L0.5S1.5 (f) L0S2 Fig. 6 Effects of strain rates on dynamic compression stress-strain curves of UHPC at 28 d
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The dynamic increase factor (DIF) was introduced to give a comparision between the quasi-static and
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dynamic compressive properties of UHPC , as summerized in Table 3. DIF is defined as the ratio of the
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dynamic compressive strength (peak stress) to the static compressive strength at curing age of 28 d. An
251
accurate DIF can ensure an appropriate stucture dimension design, reduced maximum amount of materials,
252
and eventually enhanced energy efficiency and environmental protection [28].
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As can be seen from Table 3, all the DIFs were larger than 1. This suggests the dynamic compressive
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strength was greater than the static compressive strength. Besides, the DIFs increased with the increase of 13
ACCEPTED MANUSCRIPT strain rate. It is known that weak zones like pores and micro-cracks intrinsically exist in the UHPC
256
specimens. Under static loading, there is enough time for only a few micro-cracks in local area to develop
257
into macro-cracks, and eventually lead to failure of concrete. However, the loading time is extremely short
258
under impact loading. Before one or two micro-cracks slowly propagate propogates into macro-cracks,
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more new micro-cracks have been nucleated and enlarged [35, 36]. Such nucleation of a large number of
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cracks can consume more energy, and hence results in higher ultimate dynamic compressive strength.
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Moreover, the lateral confinement from both contact surface restriction and lateral inertia can also cause
262
enhancement in dynamic strength during the rapid loading process [36].
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The increased rate for DIF decreased with further increase of strain rate. This is consitent with the
264
results in Ref. [36, 37]. Under low strain rate, cracks in concrete are limited in size and quantity, which
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can be efficiently prevented by the fiber reinforcements. However, a large quantity of cracks with
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increasing size would be induced at high strain rate, which is less efficient for fiber reinforcement to exert
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their influence.
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Table 3 Summarization of static and dynamic compressive properties of UHPC at 28 d Dynamic Static Strain at Dynamic Fracture Impact Strain compressive compressive No. velocity rate peak stress increase energy strength strength -1 -3 (m/s) (s ) (×10 ) factor (MJ/m3) (MPa) (MPa) 8.9 130.0 100.4 6.01 98.3 1.02 1.76 L0S0 11.6 169.8 128.4 5.24 98.3 1.31 1.94 14.0 206.1 146.8 7.54 98.3 1.49 3.10 8.9 115.5 141.2 5.32 134.3 1.05 2.62 L2S0 11.7 158.2 160.6 6.63 134.3 1.20 3.76 13.9 195.8 177.4 6.89 134.3 1.32 3.90 8.9 114.8 159.7 4.06 143.6 1.11 2.72 L1.5S0.5 11.7 155.7 184.3 4.22 143.6 1.28 3.29 13.9 194.8 204.8 7.38 143.6 1.43 4.15 8.9 107.8 151.5 4.28 127.7 1.19 2.59 L1S1 11.7 158.7 177.2 4.67 127.7 1.39 3.27 14.0 195.4 192.0 6.17 127.7 1.50 3.88 8.9 111.8 124.0 3.76 123.2 1.01 2.41 L0.5S1.5 11.6 155.6 171.8 4.50 123.2 1.39 3.17
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L0S2
13.9 8.8 11.6 13.9
194.7 109.0 156.1 194.8
190.4 128.8 161.5 177.9
6.87 3.69 6.48 6.17
123.2 120.5 120.5 120.5
1.55 1.07 1.34 1.48
3.61 2.36 3.06 3.50
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3.3.3. Effects of hybrid fiber reinforcements on dynamic compressive load-deflection relationship of
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UHPC
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Figure 7 compares the dynamic compression stress-strain curves of UHPC with hybrid steel fiber
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reinforcements under the same impact velocity. The reference mixture L0S0 showed the lowest dynamic
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compressive strength and toughness, whereas the L1.5S0.5 mixture exhibited the highest corresponding
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values followed by the L1S1 mixture and then the L0.5S1.5 mixture. This agrees well with the results of
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static compressive properties. Moreover, the UHPC mixtures with the single type of fiber reinforcement
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(L0S2 and L2S0 mixtures) had comparable dynamic peak stress. Besides, the dynamic compressive peak
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stress was higher than that of the static compressive strength, especially at higher impact velocity.
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As we know, the impact energy that needed to break the concrete includes two parts: the energy to
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break the matrix and the energy to pull out the fiber reinforcements. Under impact loading, the impact
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velocity was extremely fast and the loading time was extremely short. A failure mode of pulverization
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could occur under impact loading at high impact velocity, which is different from a local failure mode
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under static compressive loading. Randomly oriented fiber reinforcements can form a network structure in
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concrete. The internal structure of concrete, including fiber geometry, spacing, and quantity, can have a
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combined effect on the dynamic properties. A good balanced effect associated with these parameters can
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provide satisfactory mechanical properties for the composite materials. Generally, for a given amount of
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fiber reinforcement with single type, the longer the fiber reinforcement is, the less the fiber number is and
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the larger the fiber spacing is [38]. The contribution from the longer length of long fiber reinforcements in
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the L2S0 mixture would compensate those from the higher fiber spacing and the less fiber number, and
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vice versa for the L0S2 mixture. This eventually made the dynamic properties of the L2S0 and L0S0 15
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L0S0
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Strain
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Strain
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(a) 8.9 m/s
(b) 11.7 m/s
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Stress (MPa)
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Stress (MPa)
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(c) 13.9 m/s Fig. 7 Effects of hybrid fiber reinforcements on dynamic compression stress-strain curves of UHPC at 28 d
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Table 3 also summarizes the toughness or fracture energy of UHPC, which can be expressed as the
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area surrounded by the stress-strain curve to a strain of 0.04. The use of steel fiber reinforcement
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significantly increased the dynamic compressive toughness. The L1.5S0.5 mixture again showed the
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highest toughness, which increased by 54.5% and 33.9% at the impact velocities of 8.9 and 13.9 m/s,
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respectively. With the increase of short fiber reinforcement volume, the toughness decreased.
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4. Discussion The results above showed that the use of single type of fiber reinforcement and hybrid fiber
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reinforcements significantly improved the static compressive and flexural properties, as well as the
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dynamic compressive properties of UHPC. The L1.5S0.5 mixture obtained the highest static and dynamic
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compressive properties. However, with the increase of short fiber reinforcement content, these properties
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decreased.
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As we know, there existed intrinsically weak zones, such as micro-cracks and pores, in UHPC, as
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shown in Fig. 9. The propagation of micro-cracks can be initiated at the weak zones. This can lead to
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crack tip stress concentration under external forces. To explain the reason for enhanced strength of a
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composite material associated with fiber reinforcements, Romualdi et al. [38, 39] proposed a model of
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crack restraining mechanism, as shown in Fig. 8. It was assumed that continuous fiber reinforcements are
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evenly distributed in the matrix along the loading direction. A crack with radius of a is surrounded by four
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fiber reinforcements with a fiber spacing of S. Under tensile loading, the region of the crack around the
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fiber reinforcements can produce cohesive force distribution, as shown in Fig. 8(b). The cohesive force at
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the crack tip can yield reverse stress field. This can reduce the degree of stress concentration and restrain
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the propagation of cracks, and hence improving the strength. However, with further increase in the stress,
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the micro-cracks can propagate into macro-cracks and eventually lead to the failure of concrete members.
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S
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(a) (b) Fig. 8 Crack restraining mechanism of fiber reinforcements [38, 39] 17
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long fiber reinforcements at different length levels could be exerted, as shown in Fig. 9. The initiated
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micro-cracks can be restrained by short fiber reinforcements. When the micro-cracks propagate into
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macro-cracks under further loading, the short fiber reinforcements around the cracks are gradually pulled
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out and the load is mainly sustained by the long fiber reinforcements. This ensures that more time and
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energy are needed to damage the concrete.
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Stage II - action of short fibers. Propagation of micro-cracks.
Stage I - no fiber action. Microcracking initiated at weak zones of concrete.
Stage III - action of long fibers. Formation of macro-cracks and eventually cracking of concrete.
Fig. 9 Crack control mechanism for UHPC with hybrid steel fiber reinforcements
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The L2S0 and L0S2 mixtures with the single type of fiber reinforcement demonstrated similar
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dynamic compressive properties, which were different from static compressive properties. This is because
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the impact velocity was extremely fast and the loading time was extremely short under impact loading. A
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large number of micro- and macro-cracks can be nucleated and enlarged before the limited micro-cracks
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developed into macro-cracks under impact loading. Moreover, the quality of the structure, such as fiber
345
geometry, spacing, and quantity, can have a combined effect on the mechanical properties of concrete. For
346
a given volume of single type of fiber reinforcement at a certain fiber reinforcement orientation, the longer
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the fiber reinforcement is, the less the fiber number is and the larger the fiber spacing is. It was reported
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that the tensile strength decreasd with the increase of the fiber spacing, but increased with the increase of
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to the internal strucutre of the composite materials in such a very short time when compared to the static
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properties. Therefore, only when the combined effect from these parameters reached an optimal state, can
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the maximum dynamic compressive properties be obtained.
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5. Conclusions
This study investigated the effects of hybrid steel fiber reinforcements on flowability, static
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compressive and flexure properties, as well as dynamic compressive behavior of UHPC. A reference
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UHPC mixture with no fiber reinforcement and five mixtures with a single type of fiber reinforcement or
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hybrid steel fiber reinforcements at a total content of 2%, by the volume of concrete, were prepared. Based
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on the results presented in this study, the following conclusions can be drawn:
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(1) The use of steel fiber reinforcement significantly reduced the flowability of UHPC mixtures.
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UHPC with the single type of fiber reinforcement demonstrated lower flowability than those with hybrid
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fiber reinforcements. Among the five UHPC mixtures with fiber reinforcements, the mixture with 2%
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single type of short fiber reinforcements exhibited the worst flowability. For UHPC with hybrid fiber
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reinforcements, the flowability gradually increased with the increase of the short fiber reinforcement
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volume.
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(2) Fiber hybridization significantly improved the static compressive and flexural properties of UHPC,
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as well as the dynamic compressive behavior. The use of steel fiber reinforcement could increase the static
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compressive strength by 20% to 40%. The L1.5S0.5 mixture demonstrated both the best static and
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dynamic compressive behaviors. The static compressive and flexural properties of UHPC with 2% long
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fiber reinforcements were significantly higher than those with 2% short fiber reinforcements, while
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similar dynamic compressive properties were observed. The dynamic compressive properties, including
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peak stress, dynamic increase factor, and fracture energy of UHPC, increased gradually with the increase
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of the strain rate. (3) The reinforcing mechanisms of hybrid fiber reinforcements in UHPC were due to the combined
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strengthening effects from both the short and long fiber reinforcements, which can restrict the crack
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development at two length scales. Initially, the initiated micro-cracks were restrained by the short fiber
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reinforcements. With the micro-cracks propagated further, the short fiber reinforcements were pulled out
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from the matrix while the long fiber reinforcements began to sustain load. This means that more time and
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energy are needed to crack the concrete compare to that with the single type of fiber reinforcement. Under
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impact loading, the impact velocity was extremely fast and the loading time was extremely short. The
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nucleation and enlargement of cracks, lateral confinement from both the contact surface restriction and
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lateral inertia could contribute to the dynamic compressive behavior.
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Acknowledgements
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Financial support from National Science Foundation of China under contract Nos. 51378196 and U1305243 is greatly appreciated.
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References
389
[1] Richard P, Cheyrezy M. Reactive powder concretes with high ductility and 200-800 MPa compressive
391 392 393 394 395
strength. ACI Mater J 1994; 144(3): 507-518.
AC C
390
EP
386
[2] Shi C, Wu Z, Wang D, Xiao J, Huang Z, Fang Z. A review on ultra high performance concrete: Part I. Raw materials and mixture design. Constr Build Mater 2015; 101: 741-751. [3] Zhang J, Zhao Ya. The mechanical properties and microstructure of ultra-high-performance concrete containing various supplementary cementitious materials. J Sustain Cem Mater 2016: 1-13. [4] Wu Z, Shi C, Khayat KH, Wan S. Effects of different nanomaterials on hardening and performance of
20
ACCEPTED MANUSCRIPT
399 400 401 402 403 404 405 406
reinforced concrete. Cem Concr Compos 2012; 34(2): 172-184. [6] Habel K and Gauvreau P. Response of ultra-high performance fiber reinforced concrete (UHPFRC) to
RI PT
398
[5] Park SH, Kim DJ, Ryu GS, Kohb KT. Tensile behavior of ultra high performance hybrid fiber
impact and static loading. Cem Concr Compos 2008; 30(10): 938-946.
[7] Wang D, Shi C, Wu Z, Xiao J, Huang Z, Fang Z. A review on ultra high performance concrete: Part II. Hydration, microstructure and properties. Constr Build Mater 2015; 96: 368-377.
[8] Tuan NV, Ye G, Breugel KV, Copuroglu O. Hydration and microstructure of ultra high performance
SC
397
ultra-high strength concrete (UHSC). Cem Concr Compos 2016; 70: 24-34.
concrete incorporating rice husk ash. Cem Concr Res 2011; 41(11): 1104-1111.
M AN U
396
[9] Kim DJ, Park SH, Ryu GS, Koh KT. Comparative flexural behavior of hybrid ultra-high performance fiber reinforced concrete with different macro fibers. Constr Build Mater 2011; 25:4144-4155. [10] Hannawi K, Bian H, Prince-Agbodjan W, Raghavan B. Effect of different types of fibers on the
408
microstructure and the mechanical behavior of ultra-high performance fiber-reinforced concretes.
409
Composites Part B: Eng. 2016; 86: 214-220.
412 413
high performance concrete. Constr Build Mater 2016; 103: 8-14.
EP
411
[11] Wu Z, Shi C, He W, Wu L. Effects of steel fiber content and shape on mechanical properties of ultra
[12] Wu Z, Shi C, He W, Wang D. Uniaxial compression behavior of ultra-high performance concrete with hybrid steel fiber. Journal of Materials in Civil Engineering; 2016: 06016017-1-7.
AC C
410
TE D
407
414
[13] Kwon S, Nishiwaki T, Kikuta T, Mihashi H. Development of ultra-high-performance hybrid
415
fiber-reinforced cement-based composites (with Appendix). ACI Materials Journal 2014; 111(3):
416
309-318.
417 418
[14] Ku H, Cheng YM, Snook C, Baddeley D. Drop weight impact test fracture of vinyl ester composites: micrographs of pilot study. Journal of composite materials 2005; 39(18): 1607-1620. 21
ACCEPTED MANUSCRIPT
425 426 427 428 429 430 431 432 433 434 435 436
RI PT
424
[17] Li Q, Meng H. About the dynamic strength enhancement of concrete-like materials in a split Hopkinson pressure bar test. International Journal of Solids and Structures 2003; 40: 343-360. [18] Mohammadi Y, Carkon-Azad R, Singh SP, Kaushik SK. Impact resistance of steel fibrous concrete containing fibres of mixed aspect ratio. Constr Build Mater 2009; 23: 183-189.
SC
423
reinforced lightweight aggregate concrete. Constr Build Mater 2013; 38: 1146-1151.
[19] Rong Z, Sun W, Zhang Y. Dynamic compression behavior of ultra-high performance cement based
M AN U
422
[16] Wang H, Wang L. Experimental study on static and dynamic mechanical properties of steel fiber
composites. International Journal of Impact Engineering 2010; 37(5): 515-520. [20] Lai J, Sun W, Lin W, Jin Z. Static and dynamic mechanical behavior of ECO-RPC. Journal of Southeast University. 2005: 2.
[21] Zhang Y, Sun W, Liu S, Jiao C. Preparation of C200 green reactive powder concrete and its
TE D
421
concrete material with different fibres. Materials & Design 2012; 33: 42-55.
static-dynamic behaviors. Cem Concr Compos 2008; 30(9): 831-838. [22] Chinese national standard. Chinese cement: common portland cement, GB175-2007. Beijing, China, 2007.
EP
420
[15] Xu Z, Hao H, Li H. Experimental study of dynamic compressive properties of fibre reinforced
[23] Shi C, Wang D, Wu L, Wu Z. The Hydration and microstructure of ultra high-strength concrete with cement-silica fume-slag binder. Cem Concr Compos 2015; 61: 44-52.
AC C
419
437
[24] Wu Z, Shi C, Khayat KH. Influence of silica fume content on microstructure development and bond
438
to steel fiber in ultra-high strength cement-based materials (UHSC). Cem Concr Compos 2016; 71:
439
97-109.
440 441
[25] Chinese national standard. Test methods for flowability of cement paste, GB2419-2005, Beijing, China, 2005. 22
ACCEPTED MANUSCRIPT 442 443
[26] China National Standards. Method of testing cements - determination of strength, GB/T 17671-1999, Beijing, China (in Chinese), 1999. [27] Yang H, Li J, Huang Y. Study on the mechanical properties of high performance hybrid fiber
445
reinforced cementitious composite (HFRCC) under impact Loading. in Key Engineering Materials.
446
2015: Trans Tech Publ.
447 448
RI PT
444
[28] Zhang W. Investigation of microstructure formation mechanism and dynamic mechanical behavior of UHPCC. Ph.D. thesis, Southeast University. Nanjing, December 2012.
[29] Yu R, Spiesz P, Brouwers HJH. Static properties and impact resistance of a green ultra-high
450
performance hybrid fibre reinforced concrete (UHPHFRC): experiments and modeling. Constr Build
451
Mater 2014; 68: 158-171.
453
M AN U
452
SC
449
[30] Grünewald S. Performance-based design of self-compacting fiber reinforced concrete. Delft, the Netherlands: Delft University of Technology; 2004.
[31] Boulekbache B, Hamrat M, Chemrouk M, Amzianec S. Flowability of fibre-reinforced concrete and
455
its effect on the mechanical properties of the material. Constr Build Mater 2010; 24(9): 1664-1671.
456
[32] Cotsovos D, Pavlović M. Numerical investigation of concrete subjected to compressive impact
457
loading. Part 1: A fundamental explanation for the apparent strength gain at high loading rates.
458
Computers & structures 2008; 86(1): 145-163.
EP
TE D
454
[33] Cotsovos D, Pavlović M. Numerical investigation of concrete subjected to compressive impact
460
loading. Part 2: Parametric investigation of factors affecting behaviour at high loading rates.
461
Computers & structures 2008; 86(1): 164-180.
462 463 464
AC C
459
[34] Bischoff P, Perry S. Compressive behaviour of concrete at high strain rates. Materials and Structures 1991; 24(6): 425-450. [35] Lee MK, Barr BIG. An overview of the fatigue behavior of plain and fiber reinforced concrete. Cem 23
ACCEPTED MANUSCRIPT
466 467 468 469 470 471
Concr Compos 2004; 26 (4): 299-305. [36] Wang Y, Wang Z, Liang X, An M. Experimental and numerical studies on dynamic compressive behavior of reactive powder concretes. Acta Mechanica Solida Sinica, 2008; 21(5): 420-430. [37] Tai Y. Uniaxial compression tests at various loading rates for reactive powder concrete. Theoretical and Applied Fracture Mechanics 2009; 52(1): 14-21.
RI PT
465
[38] Romualdi JP, Mandel JA. Tensile strength of concrete affected by uniformly dispersed and closely spaced short length of wire reinforcement. ACI J Proc 1964; 61(6):657-671. [39] Romualdi JP, Batson GB. Mechanics of crack arrest in concrete, 1963.
473
[40] Mangat PS. Tensile strength of steel fiber reinforced concrete. Cem Concr Res 1976; 6(2): 245-252.
474
[41] Shah SP, Rangan BV. Fiber reinforced concrete properties. In ACI Journal Proceedings 1971; 68(2):
M AN U
475
SC
472
126-137.
AC C
EP
TE D
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S0958-9465(17)30162-2
DOI:
10.1016/j.cemconcomp.2017.02.010
Reference:
CECO 2786
To appear in:
Cement and Concrete Composites
Received Date: 13 January 2016 Revised Date:
17 January 2017
Accepted Date: 19 February 2017
Please cite this article as: Z. Wu, C. Shi, W. He, D. Wang, Static and dynamic compressive properties of ultra-high performance concrete (UHPC) with hybrid steel fiber reinforcements, Cement and Concrete Composites (2017), doi: 10.1016/j.cemconcomp.2017.02.010. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Static and dynamic compressive properties of ultra-high
3
performance concrete (UHPC) with hybrid steel fiber
4
reinforcements Zemei Wu a, b, Caijun Shi a, *, Wen He a, Dehui Wang a a
b
College of Civil Engineering, Hunan University, Changsha 410082, China; Department of Civil, Architectural and Environmental Engineering, Missouri University of Science and Technology, Rolla, MO, 65401, USA
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Abstract: Ultra-high performance concrete (UHPC) is promising in construction of concrete structures
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that suffer impact and explosive loads. In order to make UHPC structures more ductile and cost-effective,
14
hybrid fiber reinforcements are often incorporated. In this study, a reference UHPC mixture with no fiber
15
reinforcement and five mixtures with a single type of fiber reinforcement or hybrid steel fiber
16
reinforcements of 6 and 13 mm in length at a total dosage of 2%, by the volume of concrete, were
17
prepared. Quasi-static compressive and flexural properties of those mixtures were investigated. Split
18
Hopkinson press bar (SHPB) testing was adopted to evaluate their dynamic compressive properties under
19
three impact velocities. Test results indicated that UHPC with 1.5% long fiber reinforcements and 0.5%
20
short fiber reinforcements demonstrated the best static and dynamic mechanical properties. The static
21
compressive and flexural strengths of UHPC with 2% long fiber reinforcements were greater than those
22
with 2% short fiber reinforcements, whereas comparable dynamic compressive properties were observed.
23
Strain rate effect was observed for the dynamic compressive properties, including peak stress, dynamic
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increase factor, and absorbed energy. The reinforcing mechanisms of hybrid fiber reinforcements in UHPC
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were eventually discussed.
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*Corresponding author. Tel./fax: +86 731 8882 3937. E-mail address: [email protected]; [email protected] (C. Shi)
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Keywords: Ultra-high performance concrete; Dynamic compressive properties; Hybrid fiber
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reinforcements; Static mechanical properties; Split Hopkinson press bar
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1. Introduction
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Ultra-high performance concrete (UHPC) was introduced in the mid-1990s [1]. Use of high content
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of cementitious materials, superplasticizer, steel fiber reinforcement, and special curing, such as heat and
33
autoclave curing, and even nano-particles, are often mentioned in order to achieve high strength and
34
toughness [1-4]. Compared to ordinary concrete, UHPC has a very high compressive strength in the order
35
of 150 MPa and a tensile strength in the order of 10 MPa along with a strain hardening behavior [5, 6]. Its
36
superior performance allows its potential applications in military and protective structures that suffer
37
impact and explosive loads with high impact velocity [7, 8]. However, the cost of steel fiber
38
reinforcements used in UHPC is generally higher than that of the matrix. With the emphasis on
39
sustainability of UHPC, minimizing the fiber reinforcement volume without sacrifice of its performance is
40
of great significance [9]. Blending different types, shapes and/or sizes of fiber reinforcement has been
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proven as one of the effective methods because of a combination of various features from those different
42
fiber reinforcements [10-12]. For example, the short fiber reinforcements can effectively restrict the
43
development of micro-cracks while the long fiber reinforcements can bridge macro-cracks [9]. Research
44
on the mechanical properties of hybrid fiber-reinforced UHPC has attracted a lot of attention, but most of
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the literature thus far was focused on quasi-static properties, such as compressive, tensile, and flexural
46
properties, rather than dynamic behavior [5, 12, 13].
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There are two common methods, drop weight and split Hopkinson pressure bar (SHPB) testing, to
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measure the dynamic behavior of a composite material. The drop weight testing has the ability to 2
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50
replication of testing arrangement is possible by using this method. Habel et al. [6] employed this method
51
to evaluate the impact bending performance of ultra-high performance fiber reinforced concrete
52
(UHPFRC) and found that the strength increased with the increase of strain rate. Although this method is
53
simple and easy to be implemented, the experimental results are greatly influenced by the specimen size
54
and configuration, such as drop weight and speed, support stiffness, etc. [15, 16]. The SHPB testing is the
55
most commonly used one for evaluating dynamic properties [17, 18]. Rong et al. [19] used the SHPB
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testing to investigate the dynamic compression behavior of UHPFRC with different fiber reinforcement
57
volumes and found that the impact resistance of UHPFRC was improved with the increase of fiber
58
reinforcement volume. Under the same compressive loading, the control specimens were crushed, while
59
those with 3% or 4% of steel fiber reinforcements remained essentially intact. Lai et al. [20] indicated that
60
the peak stress, peak strain, and the area under strain-stress curve of UHPFRC increased with the increase
61
of strain rate. Zhang et al. [21] reported that the dynamic tensile strength of C200 green reactive powder
62
concrete increased markedly with the increase of impact velocity, ranging from 4 to 14 m/s. Until now, no
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information can be found about the effect of fiber hybridization on the dynamic mechanical properties,
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especially on the dynamic compressive properties of UHPC.
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This research aims at investigating the influences of hybrid steel fiber reinforcements with lengths of
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13 and 6 mm on flowability, quasi-static compressive and flexural properties, as well as dynamic
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compressive behavior of UHPC. Split Hopkinson pressure bar testing was conducted to characterize the
68
dynamic compressive behavior, including failure pattern, load-deflection curve, peak stress, and fracture
69
energy, under three high impact velocities. Dynamic increase factor was introduced to compare the
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quasi-static properties with the dynamic compressive properties. Finally, the reinforcing mechanisms of
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hybrid fiber reinforcements in UHPC were discussed.
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2. Materials and experimental program
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2.1. Materials Portland cement (P.I 42.5) complying with the Chinese Standard GB175-2007 was used [22]. Table 1
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summarizes the chemical composition and density of the cement, silica fume, and slag. Silica fume has an
77
average particle size of 0.1 to 0.2 µm and a specific surface area of 18,500 m2/kg. Slag has a specific area
78
of 410 m2/kg. Two straight smooth steel fibers with diameters of 0.2 mm but one with a length of 6 mm
79
and the other one of 13 mm were used. The tensile strength is 2,800 MPa. River sand with a maximum
80
particle size of 2.36 mm was used. A polycarboxylate-based superplasticizer (SP) with a solid content of
81
20% was used.
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Table 1 Chemical composition and density of cementitious materials Chemical CaO SiO2 Al2O3 Fe2O3 SO3 MgO Na2Oeq K2O C composition (%) (%) (%) (%) (%) (%) (%) (%) (%) Cement Silica fume Slag
62.49 21.18 1.85
-
39.11 33.00 13.91
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2.2. Mixture proportion
Density (kg/m3)
3.41
2.83
2.53
0.56
-
-
3,110
0.59
-
0.27
0.17
0.86
1.06
2,150
0.82
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10.04
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-
-
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Based on the previous study [23, 24], UHPC with a water to binder ratio (w/b) of 0.18 and silica
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fume content of 20% to 25%, by mass of cementitious materials, can obtain a satisfactory bond to steel
88
fiber reinforcement and mechanical properties. Therefore, UHPC mixture with a w/b of 0.18 and silica
89
fume content of 25% was used in this study. The SP dosage was fixed at 2% by the mass of the binder. To
90
investigate the hybridization of short and long fiber reinforcements on quasi-static and dynamic properties
91
of UHPC, a reference mixture with no fiber reinforcement and five mixtures with a single type of fiber
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reinforcement or hybrid fiber reinforcements at a total content of 2%, by volume of the concrete, were
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used. Table 2 shows the five mixture proportions. L0S0 was the reference mixture with no fiber 4
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reinforcement. L1.5S0.5 was referred to the mixture with 1.5% long fiber reinforcements and 0.5% short
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fiber reinforcements. Table 2 Mixture proportions of UHPC Silica Superplasticizer Cement Slag Sand Water No. fume 3 3 (kg/m3) (kg/m3) (kg/m ) (kg/m ) (kg/m3) (kg/m3) L0S0 472 315 262 1,049 178 21 L2S0 472 315 262 1,049 178 21 L1.5S0.5 472 315 262 1,049 178 21 L1S1 472 315 262 1,049 178 21 L0.5S1.5 472 315 262 1,049 178 21 L0S2 472 315 262 1,049 178 21
Long fiber (%) 0 2.0 1.5 1.0 0.5 0
Short fiber (%) 0 0 0.5 1.0 1.5 2.0
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2.3. Mixing procedure and sample preparation
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During mixing, dry powders, including cement, silica fume, slag, and river sand, were first mixed for
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3 min at a high speed. Water and superplasticizer were then added and mixed for approximately 6 min at a
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low speed. Steel fiber reinforcements were slowly added and mixed for another 6 min until achieving a
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uniform distribution.
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All the mixtures were cast into molds for the testing of flowability and mechanical properties.
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Specimens were then kept in a room at 20°C and relative humidity (RH) greater than 95% for
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approximately 24 h. They were then demolded and cured in lime-saturated water at 20°C for 7 and 28 d.
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2.4. Experimental methods
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2.4.1. Flowability
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The flowability of all mixtures was measured in accordance with the Chinese Standard GB/T
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2419-2005 [25]. The mixtures were cast into a mini cone mold and jolted for 25 times. Two diameters
112
perpendicular to each other were determined and mean values were reported.
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2.4.2. Static mechanical properties Specimens of 40 × 40 × 160 mm were cast for determination of static compressive and flexural
116
strengths. Three samples of each batch were tested. The flexural and compressive strengths measurements
117
were conducted according to GB/T 17671-1999 (similar to ISO 697:1989) [26]. Three-point bending
118
testing was first performed to obtain flexural properties. Six broken specimens measuring 40 × 40 × 40
119
mm were then tested for compressive strength. Two non-casting surfaces of the specimen were used as a
120
bottom and a top surface to ensure complete contact with the platen of the machine. The loading rate was
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set at 2.4 kN/s. Averages over three samples for flexural strength and over six samples for compressive
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strength were reported.
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2.4.4. Split Hopkinson pressure bar (SHPB) testing
A schematic illustration of SHPB testing equipment is shown in Fig. 1. The testing is based on the
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theory of one-dimensional wave propagation in an elastic bar [27]. The specimen is placed between the
127
ends of the input and output bars. A stress wave is created and propagates through the bar toward the
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specimen at the end of the input bar. This incident wave then splits into a transmitted wave and a reflected
129
wave upon reaching the specimen. The transmitted wave travels through the specimen and goes into the
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output bar, causing plastic deformation in the specimen. The reflected wave is reflected away from the
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specimen and travels back to the input bar. The whole process is recorded through a computer.
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The SHPB equipment used in this research is made of superior alloy steel with a diameter of 100 mm.
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The lengths of the input and output bars are 6,000 and 4,000 mm, respectively. The distances of strain
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gauge on the input and output bars to the specimen are 3,000 and 1,000 mm, respectively. The length of
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the striker bar is 1,500 mm. Cylinder samples with a diameter of 92 mm and a height of 46 mm were used.
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Before testing, the two ends of each specimen were ground to make them parallel so as to ensure the 6
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(a) Schematic illustration of SHPB equipment [28]
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(b) Picture of SHPB equipment Fig. 1 SHPB testing equipment 3. Results and discussion
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3.1. Effects of hybrid steel fiber reinforcements on flowability of UHPC
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Figure 2 shows the effects of hybrid fiber reinforcements on flowability of the UHPC mixtures. The
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UHPC mixture with no steel fiber reinforcement had the highest flowability of 248 mm. With the
150
incorporation of a single type of fiber reinforcement or hybrid fiber reinforcements, the flowability
151
decreased. This agrees well with the results reported by Yu et al. [29]. The reduced flowability associated
152
with the fiber reinforcements was attributed to the following reasons [30, 31]: (1) fiber reinforcements are
153
much more elongated and have higher surface area compared to aggregates, which can increase the
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cohesive force; (2) steel fiber reinforcements can alter the granular skeleton structure and hinder the flow
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of fresh paste; (3) steel fiber reinforcements tends to be perpendicular to the flow direction, generating
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resistance force and moment to the flow velocity of fresh mixtures. It can be also observed from Fig. 2, the UHPC mixtures with the hybrid fiber reinforcements
158
demonstrated higher flowability than those with the single type of fiber reinforcement. The higher the
159
short fiber reinforcement volume in the UHPC mixture with hybrid fiber reinforcements was, the better
160
the flowability was. However, when all the 2% long fiber reinforcements were replaced by the short fiber
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reinforcement, the mixture (L0S2) obtained the worst flowability. In the case of hybrid fiber
162
reinforcements, the mutual effects between the short and long fiber reinforcements can restrict each
163
other’s rotation to avoid the perpendicular orientation to the flow direction [29-31]. This can reduce the
164
resistance force to some extent, and eventually enhance the flowability when compared to those with the
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single type of fiber reinforcement (L2S0 and L0S2 mixtures). The worst flowability of the L0S2 mixture
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might be attributed to the increase in fiber reinforcement volume associated with short length.
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L0S0
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Fig. 2 Effects of hybrid fiber reinforcements on flowability of UHPC
3.2. Effects of hybrid fiber reinforcements on static compressive and flexural strengths
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The effects of hybrid fiber reinforcements on static compressive strength of UHPC are illustrated in
172
Fig. 3. The 7- and 28-d static compressive strengths of the reference UHPC mixture (L0S0) were 81.1 and
173
98.3 MPa, respectively. It increased obviously with the incorporation of steel fiber reinforcements. The 8
ACCEPTED MANUSCRIPT compressive strengths of the L2S0 mixture at 7 and 28 d were increased by 35% and 28%, respectively, in
175
comparison to the reference mixture. For the mixtures with hybrid fiber reinforcements, the compressive
176
strength increased first but then decreased with the increase of the short fiber reinforcement volume.
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When 1.5% long fiber reinforcements and 0.5% short fiber reinforcements were used, the highest
178
compressive strength was achieved. Besides, the static compressive strength of the L2S0 mixture was
179
obviously greater than that of the L0S2 mixture. The decrease in compressive strength with the increase of
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short fiber reinforcement content might be due to the lower efficiency of short fiber reinforcements in
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restricting the development of cracks [11, 12, 29]. Therefore, the use of 2% hybrid fiber reinforcements
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could increase the static compressive strength by 20% to 40% compared to the reference mixture.
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7d 28d
Fig. 3 Effects of hybrid fiber reinforcements on static compressive strength of UHPC
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Figure 4 depicts the effects of hybrid fiber reinforcements on static flexural behavior of UHPC. The
187
UHPC reference specimens with no fiber reinforcement showed a sudden drop of load when reaching the
188
initial cracking point at a deflection of 0.25 mm, as observed from Fig. 4(a). UHPC specimens with fiber
189
reinforcement could continue to sustain load after the initial cracking until they reached the peak load. The
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volume of long and short fiber reinforcements had a significant effect on the peak load. The L1.5S0.5
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mixture was shown to exhibit the greatest peak load, followed by the L2S0 mixture. As can be also
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observed from Fig. 4(a), the load-deflection curves showed obvious zigzag patterns after the peak load.
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This is consistent with the results in literature [11]. It was observed that there were tearing sounds after the
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peak load, which indicates that the fiber reinforcements were continuously pulled out from the matrix. The change in static ultimate flexural strength of UHPC with hybrid fiber reinforcements showed a
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similar trend to that of the static compressive strength, as illustrated in Fig. 4(b). The L1.5S0.5 mixture
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showed the highest flexural strength of 32 MPa at 28 d, which increased by 108% compared to the
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reference mixture. This might be due to the combined strengthening effects from the short and long fiber
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reinforcements, which can restrict the crack development in two length scales, both in a scale of 6 mm and
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13 mm length [12]. However, when more long fiber reinforcements were replaced by short fiber
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reinforcements, the flexural strength decreased. The L0S2 mixture obtained the worst static flexural
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strength among the five UHPC mixtures with fiber reinforcement.
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Flexural strength (MPa)
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(a) Flexural load-deflection curves at 28 d (b) Flexural strengths at 7 and 28 d Fig. 4 Effects of hybrid fiber reinforcements on flexural behavior of UHPC
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3.3. Effects of hybrid fiber reinforcements on dynamic compressive properties of UHPC
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3.3.1. Effects of hybrid fiber reinforcements on dynamic failure pattern of UHPC
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Figure 5 illustrates the dynamic failure pattern of UHPC with and without fiber reinforcement at 28 d.
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At the impact velocity of 8.9 m/s, all the specimens exhibited a failure mode of pulverization. The 10
ACCEPTED MANUSCRIPT reference mixture (L0S0) with no fiber reinforcement showed a large quantity of small and irregular
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broken concrete fragments. The use of steel fiber reinforcements could effectively restrict the transverse
213
deformation of specimens. As can be seen from Fig. 5(b-d), the sizes of the broken fragments of the
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UHPC specimens varied with the hybrid fiber reinforcement volumes. The L2S0 mixture showed relative
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large and round broken fragments. For the L1.5S0.5 mixture, balanced broken fragments with both large
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and small parts could be observed. Many embedded fiber reinforcements were observed in the fracture
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surface, bridging the small fragments. However, relatively small and elongated broken fragments were
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observed for the L0S2 mixture, as can be observed from Fig. 5(d).
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(a) L0S0 mixture
(b) L2S0 mixture
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(c) L1.5S0.5 mixture (d) L0S2 mixture Fig. 5 Effects of hybrid fiber reinforcements on dynamic failure patterns of UHPC at an impact velocity of 8.9 m/s
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3.3.2. Effects of strain rates on dynamic compressive load-deflection relationship of UHPC The effects of different strain rates on dynamic compressive load-deflection curves of UHPC with and
229
without fiber reinforcement at 28 d are illustrated in Fig. 6. For all UHPC mixtures, the stress changed
230
almost linearly with the strain up to the peak stress. Afterward, the stress-strain curves showed a
231
decreasing branch. When the strain reached approximately 0.04, a constant stress was observed.
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From Fig. 6, it can be also seen that the dynamic compressive behavior of UHPC was sensitive to
233
strain rate. All the mixtures showed a strain rate effect, that is, the peak stress increased with the increase
234
of strain rate. The peak stress for the L2S0 mixture was 141.2 MPa at the strain rate of 115.5 s-1, and
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increased to 160.6 and 177.4 MPa, respectively, when the strain rate increased to 158.2 and 195.8 s-1. Li
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and Cotsovos [17, 32, 33] proposed that this strain rate effect was due to lateral inertia associated with
237
friction to the contact surface under rapid impact loading. Besides, the increasing rate for peak stress
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decreased gradually with the increase of strain rate. This might be attributed to the Stefan effect associated
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with the presence of free water in concrete [34].
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206.1/s 169.8/s
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Stress (MPa)
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130.0/s
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158.2/s 115.5/s
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(a) L0S0
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Stress (MPa)
150
155.7/s 114.8/s
90
107.8/s
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158.7/s
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194.8/s
Stress(MPa)
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210 194.7/s 155.6/s
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Stress(MPa)
Stress (MPa)
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Strain
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0.04
Strain
(e) L0.5S1.5 (f) L0S2 Fig. 6 Effects of strain rates on dynamic compression stress-strain curves of UHPC at 28 d
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The dynamic increase factor (DIF) was introduced to give a comparision between the quasi-static and
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dynamic compressive properties of UHPC , as summerized in Table 3. DIF is defined as the ratio of the
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dynamic compressive strength (peak stress) to the static compressive strength at curing age of 28 d. An
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accurate DIF can ensure an appropriate stucture dimension design, reduced maximum amount of materials,
252
and eventually enhanced energy efficiency and environmental protection [28].
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As can be seen from Table 3, all the DIFs were larger than 1. This suggests the dynamic compressive
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strength was greater than the static compressive strength. Besides, the DIFs increased with the increase of 13
ACCEPTED MANUSCRIPT strain rate. It is known that weak zones like pores and micro-cracks intrinsically exist in the UHPC
256
specimens. Under static loading, there is enough time for only a few micro-cracks in local area to develop
257
into macro-cracks, and eventually lead to failure of concrete. However, the loading time is extremely short
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under impact loading. Before one or two micro-cracks slowly propagate propogates into macro-cracks,
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more new micro-cracks have been nucleated and enlarged [35, 36]. Such nucleation of a large number of
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cracks can consume more energy, and hence results in higher ultimate dynamic compressive strength.
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Moreover, the lateral confinement from both contact surface restriction and lateral inertia can also cause
262
enhancement in dynamic strength during the rapid loading process [36].
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The increased rate for DIF decreased with further increase of strain rate. This is consitent with the
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results in Ref. [36, 37]. Under low strain rate, cracks in concrete are limited in size and quantity, which
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can be efficiently prevented by the fiber reinforcements. However, a large quantity of cracks with
266
increasing size would be induced at high strain rate, which is less efficient for fiber reinforcement to exert
267
their influence.
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Table 3 Summarization of static and dynamic compressive properties of UHPC at 28 d Dynamic Static Strain at Dynamic Fracture Impact Strain compressive compressive No. velocity rate peak stress increase energy strength strength -1 -3 (m/s) (s ) (×10 ) factor (MJ/m3) (MPa) (MPa) 8.9 130.0 100.4 6.01 98.3 1.02 1.76 L0S0 11.6 169.8 128.4 5.24 98.3 1.31 1.94 14.0 206.1 146.8 7.54 98.3 1.49 3.10 8.9 115.5 141.2 5.32 134.3 1.05 2.62 L2S0 11.7 158.2 160.6 6.63 134.3 1.20 3.76 13.9 195.8 177.4 6.89 134.3 1.32 3.90 8.9 114.8 159.7 4.06 143.6 1.11 2.72 L1.5S0.5 11.7 155.7 184.3 4.22 143.6 1.28 3.29 13.9 194.8 204.8 7.38 143.6 1.43 4.15 8.9 107.8 151.5 4.28 127.7 1.19 2.59 L1S1 11.7 158.7 177.2 4.67 127.7 1.39 3.27 14.0 195.4 192.0 6.17 127.7 1.50 3.88 8.9 111.8 124.0 3.76 123.2 1.01 2.41 L0.5S1.5 11.6 155.6 171.8 4.50 123.2 1.39 3.17
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13.9 8.8 11.6 13.9
194.7 109.0 156.1 194.8
190.4 128.8 161.5 177.9
6.87 3.69 6.48 6.17
123.2 120.5 120.5 120.5
1.55 1.07 1.34 1.48
3.61 2.36 3.06 3.50
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3.3.3. Effects of hybrid fiber reinforcements on dynamic compressive load-deflection relationship of
272
UHPC
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Figure 7 compares the dynamic compression stress-strain curves of UHPC with hybrid steel fiber
274
reinforcements under the same impact velocity. The reference mixture L0S0 showed the lowest dynamic
275
compressive strength and toughness, whereas the L1.5S0.5 mixture exhibited the highest corresponding
276
values followed by the L1S1 mixture and then the L0.5S1.5 mixture. This agrees well with the results of
277
static compressive properties. Moreover, the UHPC mixtures with the single type of fiber reinforcement
278
(L0S2 and L2S0 mixtures) had comparable dynamic peak stress. Besides, the dynamic compressive peak
279
stress was higher than that of the static compressive strength, especially at higher impact velocity.
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As we know, the impact energy that needed to break the concrete includes two parts: the energy to
281
break the matrix and the energy to pull out the fiber reinforcements. Under impact loading, the impact
282
velocity was extremely fast and the loading time was extremely short. A failure mode of pulverization
283
could occur under impact loading at high impact velocity, which is different from a local failure mode
284
under static compressive loading. Randomly oriented fiber reinforcements can form a network structure in
285
concrete. The internal structure of concrete, including fiber geometry, spacing, and quantity, can have a
286
combined effect on the dynamic properties. A good balanced effect associated with these parameters can
287
provide satisfactory mechanical properties for the composite materials. Generally, for a given amount of
288
fiber reinforcement with single type, the longer the fiber reinforcement is, the less the fiber number is and
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the larger the fiber spacing is [38]. The contribution from the longer length of long fiber reinforcements in
290
the L2S0 mixture would compensate those from the higher fiber spacing and the less fiber number, and
291
vice versa for the L0S2 mixture. This eventually made the dynamic properties of the L2S0 and L0S0 15
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(a) 8.9 m/s
(b) 11.7 m/s
210 180
Stress (MPa)
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Stress (MPa)
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Strain
(c) 13.9 m/s Fig. 7 Effects of hybrid fiber reinforcements on dynamic compression stress-strain curves of UHPC at 28 d
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Table 3 also summarizes the toughness or fracture energy of UHPC, which can be expressed as the
301
area surrounded by the stress-strain curve to a strain of 0.04. The use of steel fiber reinforcement
302
significantly increased the dynamic compressive toughness. The L1.5S0.5 mixture again showed the
303
highest toughness, which increased by 54.5% and 33.9% at the impact velocities of 8.9 and 13.9 m/s,
304
respectively. With the increase of short fiber reinforcement volume, the toughness decreased.
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4. Discussion The results above showed that the use of single type of fiber reinforcement and hybrid fiber
308
reinforcements significantly improved the static compressive and flexural properties, as well as the
309
dynamic compressive properties of UHPC. The L1.5S0.5 mixture obtained the highest static and dynamic
310
compressive properties. However, with the increase of short fiber reinforcement content, these properties
311
decreased.
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As we know, there existed intrinsically weak zones, such as micro-cracks and pores, in UHPC, as
313
shown in Fig. 9. The propagation of micro-cracks can be initiated at the weak zones. This can lead to
314
crack tip stress concentration under external forces. To explain the reason for enhanced strength of a
315
composite material associated with fiber reinforcements, Romualdi et al. [38, 39] proposed a model of
316
crack restraining mechanism, as shown in Fig. 8. It was assumed that continuous fiber reinforcements are
317
evenly distributed in the matrix along the loading direction. A crack with radius of a is surrounded by four
318
fiber reinforcements with a fiber spacing of S. Under tensile loading, the region of the crack around the
319
fiber reinforcements can produce cohesive force distribution, as shown in Fig. 8(b). The cohesive force at
320
the crack tip can yield reverse stress field. This can reduce the degree of stress concentration and restrain
321
the propagation of cracks, and hence improving the strength. However, with further increase in the stress,
322
the micro-cracks can propagate into macro-cracks and eventually lead to the failure of concrete members.
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(a) (b) Fig. 8 Crack restraining mechanism of fiber reinforcements [38, 39] 17
ACCEPTED MANUSCRIPT When hybrid fiber reinforcements were incorporated, the combined effect associated with short and
327
long fiber reinforcements at different length levels could be exerted, as shown in Fig. 9. The initiated
328
micro-cracks can be restrained by short fiber reinforcements. When the micro-cracks propagate into
329
macro-cracks under further loading, the short fiber reinforcements around the cracks are gradually pulled
330
out and the load is mainly sustained by the long fiber reinforcements. This ensures that more time and
331
energy are needed to damage the concrete.
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Stage II - action of short fibers. Propagation of micro-cracks.
Stage I - no fiber action. Microcracking initiated at weak zones of concrete.
Stage III - action of long fibers. Formation of macro-cracks and eventually cracking of concrete.
Fig. 9 Crack control mechanism for UHPC with hybrid steel fiber reinforcements
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The L2S0 and L0S2 mixtures with the single type of fiber reinforcement demonstrated similar
341
dynamic compressive properties, which were different from static compressive properties. This is because
342
the impact velocity was extremely fast and the loading time was extremely short under impact loading. A
343
large number of micro- and macro-cracks can be nucleated and enlarged before the limited micro-cracks
344
developed into macro-cracks under impact loading. Moreover, the quality of the structure, such as fiber
345
geometry, spacing, and quantity, can have a combined effect on the mechanical properties of concrete. For
346
a given volume of single type of fiber reinforcement at a certain fiber reinforcement orientation, the longer
347
the fiber reinforcement is, the less the fiber number is and the larger the fiber spacing is. It was reported
348
that the tensile strength decreasd with the increase of the fiber spacing, but increased with the increase of
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350
to the internal strucutre of the composite materials in such a very short time when compared to the static
351
properties. Therefore, only when the combined effect from these parameters reached an optimal state, can
352
the maximum dynamic compressive properties be obtained.
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5. Conclusions
This study investigated the effects of hybrid steel fiber reinforcements on flowability, static
356
compressive and flexure properties, as well as dynamic compressive behavior of UHPC. A reference
357
UHPC mixture with no fiber reinforcement and five mixtures with a single type of fiber reinforcement or
358
hybrid steel fiber reinforcements at a total content of 2%, by the volume of concrete, were prepared. Based
359
on the results presented in this study, the following conclusions can be drawn:
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(1) The use of steel fiber reinforcement significantly reduced the flowability of UHPC mixtures.
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UHPC with the single type of fiber reinforcement demonstrated lower flowability than those with hybrid
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fiber reinforcements. Among the five UHPC mixtures with fiber reinforcements, the mixture with 2%
363
single type of short fiber reinforcements exhibited the worst flowability. For UHPC with hybrid fiber
364
reinforcements, the flowability gradually increased with the increase of the short fiber reinforcement
365
volume.
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(2) Fiber hybridization significantly improved the static compressive and flexural properties of UHPC,
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as well as the dynamic compressive behavior. The use of steel fiber reinforcement could increase the static
368
compressive strength by 20% to 40%. The L1.5S0.5 mixture demonstrated both the best static and
369
dynamic compressive behaviors. The static compressive and flexural properties of UHPC with 2% long
370
fiber reinforcements were significantly higher than those with 2% short fiber reinforcements, while
371
similar dynamic compressive properties were observed. The dynamic compressive properties, including
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peak stress, dynamic increase factor, and fracture energy of UHPC, increased gradually with the increase
373
of the strain rate. (3) The reinforcing mechanisms of hybrid fiber reinforcements in UHPC were due to the combined
375
strengthening effects from both the short and long fiber reinforcements, which can restrict the crack
376
development at two length scales. Initially, the initiated micro-cracks were restrained by the short fiber
377
reinforcements. With the micro-cracks propagated further, the short fiber reinforcements were pulled out
378
from the matrix while the long fiber reinforcements began to sustain load. This means that more time and
379
energy are needed to crack the concrete compare to that with the single type of fiber reinforcement. Under
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impact loading, the impact velocity was extremely fast and the loading time was extremely short. The
381
nucleation and enlargement of cracks, lateral confinement from both the contact surface restriction and
382
lateral inertia could contribute to the dynamic compressive behavior.
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Acknowledgements
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Financial support from National Science Foundation of China under contract Nos. 51378196 and U1305243 is greatly appreciated.
387 388
References
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[1] Richard P, Cheyrezy M. Reactive powder concretes with high ductility and 200-800 MPa compressive
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strength. ACI Mater J 1994; 144(3): 507-518.
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[2] Shi C, Wu Z, Wang D, Xiao J, Huang Z, Fang Z. A review on ultra high performance concrete: Part I. Raw materials and mixture design. Constr Build Mater 2015; 101: 741-751. [3] Zhang J, Zhao Ya. The mechanical properties and microstructure of ultra-high-performance concrete containing various supplementary cementitious materials. J Sustain Cem Mater 2016: 1-13. [4] Wu Z, Shi C, Khayat KH, Wan S. Effects of different nanomaterials on hardening and performance of
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reinforced concrete. Cem Concr Compos 2012; 34(2): 172-184. [6] Habel K and Gauvreau P. Response of ultra-high performance fiber reinforced concrete (UHPFRC) to
RI PT
398
[5] Park SH, Kim DJ, Ryu GS, Kohb KT. Tensile behavior of ultra high performance hybrid fiber
impact and static loading. Cem Concr Compos 2008; 30(10): 938-946.
[7] Wang D, Shi C, Wu Z, Xiao J, Huang Z, Fang Z. A review on ultra high performance concrete: Part II. Hydration, microstructure and properties. Constr Build Mater 2015; 96: 368-377.
[8] Tuan NV, Ye G, Breugel KV, Copuroglu O. Hydration and microstructure of ultra high performance
SC
397
ultra-high strength concrete (UHSC). Cem Concr Compos 2016; 70: 24-34.
concrete incorporating rice husk ash. Cem Concr Res 2011; 41(11): 1104-1111.
M AN U
396
[9] Kim DJ, Park SH, Ryu GS, Koh KT. Comparative flexural behavior of hybrid ultra-high performance fiber reinforced concrete with different macro fibers. Constr Build Mater 2011; 25:4144-4155. [10] Hannawi K, Bian H, Prince-Agbodjan W, Raghavan B. Effect of different types of fibers on the
408
microstructure and the mechanical behavior of ultra-high performance fiber-reinforced concretes.
409
Composites Part B: Eng. 2016; 86: 214-220.
412 413
high performance concrete. Constr Build Mater 2016; 103: 8-14.
EP
411
[11] Wu Z, Shi C, He W, Wu L. Effects of steel fiber content and shape on mechanical properties of ultra
[12] Wu Z, Shi C, He W, Wang D. Uniaxial compression behavior of ultra-high performance concrete with hybrid steel fiber. Journal of Materials in Civil Engineering; 2016: 06016017-1-7.
AC C
410
TE D
407
414
[13] Kwon S, Nishiwaki T, Kikuta T, Mihashi H. Development of ultra-high-performance hybrid
415
fiber-reinforced cement-based composites (with Appendix). ACI Materials Journal 2014; 111(3):
416
309-318.
417 418
[14] Ku H, Cheng YM, Snook C, Baddeley D. Drop weight impact test fracture of vinyl ester composites: micrographs of pilot study. Journal of composite materials 2005; 39(18): 1607-1620. 21
ACCEPTED MANUSCRIPT
425 426 427 428 429 430 431 432 433 434 435 436
RI PT
424
[17] Li Q, Meng H. About the dynamic strength enhancement of concrete-like materials in a split Hopkinson pressure bar test. International Journal of Solids and Structures 2003; 40: 343-360. [18] Mohammadi Y, Carkon-Azad R, Singh SP, Kaushik SK. Impact resistance of steel fibrous concrete containing fibres of mixed aspect ratio. Constr Build Mater 2009; 23: 183-189.
SC
423
reinforced lightweight aggregate concrete. Constr Build Mater 2013; 38: 1146-1151.
[19] Rong Z, Sun W, Zhang Y. Dynamic compression behavior of ultra-high performance cement based
M AN U
422
[16] Wang H, Wang L. Experimental study on static and dynamic mechanical properties of steel fiber
composites. International Journal of Impact Engineering 2010; 37(5): 515-520. [20] Lai J, Sun W, Lin W, Jin Z. Static and dynamic mechanical behavior of ECO-RPC. Journal of Southeast University. 2005: 2.
[21] Zhang Y, Sun W, Liu S, Jiao C. Preparation of C200 green reactive powder concrete and its
TE D
421
concrete material with different fibres. Materials & Design 2012; 33: 42-55.
static-dynamic behaviors. Cem Concr Compos 2008; 30(9): 831-838. [22] Chinese national standard. Chinese cement: common portland cement, GB175-2007. Beijing, China, 2007.
EP
420
[15] Xu Z, Hao H, Li H. Experimental study of dynamic compressive properties of fibre reinforced
[23] Shi C, Wang D, Wu L, Wu Z. The Hydration and microstructure of ultra high-strength concrete with cement-silica fume-slag binder. Cem Concr Compos 2015; 61: 44-52.
AC C
419
437
[24] Wu Z, Shi C, Khayat KH. Influence of silica fume content on microstructure development and bond
438
to steel fiber in ultra-high strength cement-based materials (UHSC). Cem Concr Compos 2016; 71:
439
97-109.
440 441
[25] Chinese national standard. Test methods for flowability of cement paste, GB2419-2005, Beijing, China, 2005. 22
ACCEPTED MANUSCRIPT 442 443
[26] China National Standards. Method of testing cements - determination of strength, GB/T 17671-1999, Beijing, China (in Chinese), 1999. [27] Yang H, Li J, Huang Y. Study on the mechanical properties of high performance hybrid fiber
445
reinforced cementitious composite (HFRCC) under impact Loading. in Key Engineering Materials.
446
2015: Trans Tech Publ.
447 448
RI PT
444
[28] Zhang W. Investigation of microstructure formation mechanism and dynamic mechanical behavior of UHPCC. Ph.D. thesis, Southeast University. Nanjing, December 2012.
[29] Yu R, Spiesz P, Brouwers HJH. Static properties and impact resistance of a green ultra-high
450
performance hybrid fibre reinforced concrete (UHPHFRC): experiments and modeling. Constr Build
451
Mater 2014; 68: 158-171.
453
M AN U
452
SC
449
[30] Grünewald S. Performance-based design of self-compacting fiber reinforced concrete. Delft, the Netherlands: Delft University of Technology; 2004.
[31] Boulekbache B, Hamrat M, Chemrouk M, Amzianec S. Flowability of fibre-reinforced concrete and
455
its effect on the mechanical properties of the material. Constr Build Mater 2010; 24(9): 1664-1671.
456
[32] Cotsovos D, Pavlović M. Numerical investigation of concrete subjected to compressive impact
457
loading. Part 1: A fundamental explanation for the apparent strength gain at high loading rates.
458
Computers & structures 2008; 86(1): 145-163.
EP
TE D
454
[33] Cotsovos D, Pavlović M. Numerical investigation of concrete subjected to compressive impact
460
loading. Part 2: Parametric investigation of factors affecting behaviour at high loading rates.
461
Computers & structures 2008; 86(1): 164-180.
462 463 464
AC C
459
[34] Bischoff P, Perry S. Compressive behaviour of concrete at high strain rates. Materials and Structures 1991; 24(6): 425-450. [35] Lee MK, Barr BIG. An overview of the fatigue behavior of plain and fiber reinforced concrete. Cem 23
ACCEPTED MANUSCRIPT
466 467 468 469 470 471
Concr Compos 2004; 26 (4): 299-305. [36] Wang Y, Wang Z, Liang X, An M. Experimental and numerical studies on dynamic compressive behavior of reactive powder concretes. Acta Mechanica Solida Sinica, 2008; 21(5): 420-430. [37] Tai Y. Uniaxial compression tests at various loading rates for reactive powder concrete. Theoretical and Applied Fracture Mechanics 2009; 52(1): 14-21.
RI PT
465
[38] Romualdi JP, Mandel JA. Tensile strength of concrete affected by uniformly dispersed and closely spaced short length of wire reinforcement. ACI J Proc 1964; 61(6):657-671. [39] Romualdi JP, Batson GB. Mechanics of crack arrest in concrete, 1963.
473
[40] Mangat PS. Tensile strength of steel fiber reinforced concrete. Cem Concr Res 1976; 6(2): 245-252.
474
[41] Shah SP, Rangan BV. Fiber reinforced concrete properties. In ACI Journal Proceedings 1971; 68(2):
M AN U
475
SC
472
126-137.
AC C
EP
TE D
476
24