Dynamic Compressive and Tensile Characteristics of a New Type of Ultra-High-Molecular Weight Polyethylene (UHMWPE) and Polyvinyl Alcohol (PVA) Fibers Reinforced Concrete

,e dynamic mechanical properties of concrete materials are important parameters for evaluating the safety performance of concrete structures under dynamic loads. Fiber cement-based materials have been widely used in the construction projects due to their strength, toughening, and cracking resistance. In this study, we conducted experimental and theoretical studies on dynamic compression and tensile mechanical properties of different proportions of new-type fiber concrete. A Split-Hopkinson pressure bar equipment was used to determine the concrete behavior at different strain rates. ,e effects of strain rate and fiber content on the strength of the specimen, dynamic increase factor, and ultimate strain were analyzed. Based on the macrodamage factor, the traditional nonlinear viscoelastic constitutive model was simplified and improved. ,e four-parameter constitutive model was obtained, and the influence of these parameters on the performance of fiber concrete was analyzed. ,e experimental results were compared with those predicted from the available equations, and results were in accordance. Finally, an analytical equation for predicting the dynamic compression and tensile properties of these new-type fibers was proposed.


Introduction
Concrete as a common construction material used in the industry may often be subjected to dynamic loads during service, and concrete compression and tensile performances are important parameters for evaluating structures and building safety and stability.Tensile properties of the concrete material are significantly lower than the compression resistance, resulting in the structure being prone to tensile damage, and the dynamic direct tensile properties are difficult to measure directly by the test.Most researchers [1][2][3] have shown that the tensile test with Brazilian disc specimens yields the closest tensile strength value to the actual tensile value.Addition of fiber to concrete can effectively reduce cracks from rising and propagating, thus improving its strength and enhances its toughness and anti-impact properties [4].
Fiber cement-based composites have good reinforcement and toughening effects and have been widely used in civil engineering construction.Most of the mechanical properties of fiber-reinforced concrete under impact load are based on the experimental analysis [5,6].Due to the strain rate effect, the performance of concrete structures under static loading is different from those subjected to high strain rate loading conditions.Most research on properties of concrete subjected to high strain rate loadings has focused on compressive strength.It is generally accepted that there is a strength increase with the increase in strain rate [7][8][9].Mindess [10] found that an increase in the strain rate leads the elastic modulus to increase.Ultra-highmolecular weight polyethylene (UHMWPE) fiber is a new type of ultra-high-performance manufacturing industrial material, but there is little research on the dynamic tensile properties of UHMWPE fiber-based composites.Although the conclusions on the dynamic tensile properties of fiber cement-based materials are basically the same, the theoretical analysis needs to be further studied [11][12][13][14].
Research on polyvinyl alcohol (PVA) fiber cement composite material and its dynamic mechanical properties have not yet been agreed with credible conclusions, and the use of ultra-high-molecular weight polyethylene (UHMWPE) on the mechanical properties of fiberreinforced concrete under impact load is almost blank.
erefore, in this study, the dynamic tensile test of UHMWPE and PVA fiber concrete is carried out by a Split-Hopkinson pressure bar (SHPB) impact device, and the dynamic tensile effect of fiber concrete with different strain rates were obtained.Furthermore, dynamic compressive and tensile characteristics of UHMWPE and PVA fibers reinforced concrete in terms of failure mode of fiber concrete, the evaluation of the antisliding ability of the above fiber concrete, the influence of strain rate on the tensile strength, dynamic increase factor and dissipative energy of the test piece, and the establishment of uniformity between dynamic and static tensile strain and strain rate model are studied.

Test Materials.
e experimental materials are specified as follows: P.O 42.5 cement; natural river sand (MX � 2.8); tap water; coarse aggregate is limestone, in order to ensure that the size of the test specimen is greater than 3 times the maximum aggregate size.e aggregates before construction are screened to ensure the coarse aggregate size in the range of 5-10 mm; UHMWPE fiber is produced by Shanghai DISMAN Company, which is in the form of the white fiber bundle, as shown in Figure 1(a); the PVA fiber is in the form of a yellowish fiber bundle, as shown in Figure 1(b).e properties of the fibers are shown in Table 1.

Test Mixture Proportion.
e concrete quality of each group is shown in Table 2, where group No. 0 represents plain concrete as a control group, P-0.6 and U-0.6 represent a content of 0.6 kg/m 3 for PVA and UHMWPE fibers concrete, respectively, and the same for other groups.e ratios of 230, 440, 620, and 1100 for water, cement, sand, and aggregate were used, respectively.e proportions of the fibers are shown in Table 2.

Samples Preparation.
e test was carried out by using a 75 mm diameter PVC pipe.After the concrete was compacted using a vibrating table, the surface of the samples was covered with a plastic film to cover the moisturizing and curing.After 48 hours, the mold was removed and the sample was cured by the saturated water method.After 28 days, the cutting was polished to a height of 74 mm for the static compression test, and an additional height of 37 mm test was carried out for the dynamic compression test.e samples with a thickness of 37 mm are shown in Figure 2.

Static Compression Test.
e concrete test specimens were statically loaded with a universal testing machine, with the maximum capacity of 1000 kN. e samples dimensions of 74 mm diameter and 148 mm height were tested.Loading and test procedure were carried out in accordance with ASTM C192 specification, and the static compressive strength of each group of fiber concrete is shown in Table 3.

SHPB Dynamic Compression Test.
e SHPB device consists of a launching system, striker, incident bar, transmitted bar, shock absorber, and a data acquisition system.During the test, the striker was launched by compressed gas towards the incident bar.e schematic of the device is shown in Figure 3. Adjust the impact air pressure load, where striker impacts the incident rod at different speeds, thus generating the corresponding stress wave [15].Figure 4 shows a three-wavelength map obtained from the impact test.e SHPB test principle shows that the reflected stress wave can be unique.e strain rate of the sample is determined, and a "platform zone" appears on the reflected wave curve in the figure.At this time, the constant strain rate loading is realized inside the test [16].e SHPB test is based on two assumptions: (1) calculation based on onedimensional stress wave propagation; and (2) stress wave propagation in the specimen is axial and the stress is evenly distributed.e stress, strain, and strain rate of the specimens can be calculated from Equations ( 1)-(3) [17].e schematic of the strain rate determination is shown in Figure 5.
where A b ，E b ，and C 0 are the cross-sectional area of the pressure bar, the elastic modulus, and the propagation velocity of the waveform, respectively; l and A s are the thickness and cross-sectional area of the specimens, respectively.Each group of fiber concrete was loaded with five different (0.30 MPa, 0.35 MPa, 0.40 MPa, 0.45 MPa, and 0.50 MPa) impact air pressures, and each group was repeatedly tested in the same working condition for five times.
e strain rate was considered to be approximately constant during the test loading [16].

Samples Preparation.
e sample preparation of the tensile test is the same as that of the compression test.e 2   Commercial PVA   After the concrete was compacted using a vibrating table, the surface of the samples was covered with a plastic lm to cover the moisturizing and curing.After 48 hours, the mold was removed and the sample was cured by the saturated water method.After 28 days, the cutting was polished to a height of 74 mm for the static Brazilian disc test, and an additional height of 37 mm test was carried out for the dynamic Brazilian disc test.

Static Brazilian Disc
Test. e static Brazilian disc test was carried out using a universal testing machine.According to the ASTM speci cations [18], the specimen sizes were 74 mm and 37 mm for diameter and thickness, respectively.e principle of the Brazilian disc test is shown in Figure 6.
Based on the theory of elasticity, the Brazilian condition of the diameter-compressed disk specimen is expressed in Equation (4) [19], while Equation (5) showed the stress at the center of the disc: where L and D are the thickness and diameter of the sample, respectively, while σ y is the tensile strength in the normal direction, and σ x is the tensile strength.It can be seen from Equation (5), the compressive stress at the center of the circle is three times the tensile stress.Generally, the tensile strength of cement-based materials is 1/20-1/10 of the compressive pressure.

Dynamic Brazilian Disc Test.
For the dynamic Brazilian disc test, each group of ber concrete was carried out by ve kinds of air pressure (0.20 MPa, 0.25 MPa, 0.30 MPa, 0.35 MPa, and 0.40 MPa), and the impact test was carried out for 5 times in the same working condition.
For dynamic Brazilian disc test, a 74 mm diameter variable cross section SHPB pressure bar device is used, and the test sample is sandwiched between the incident bar and the transmitted bar, as shown in Figure 7.By controlling the impact air pressure, the striker produces di erent impact velocities and strikes the incident bar, thereby generating corresponding stress waves.e time course of the incident wave, re ected wave, and transmitted wave is shown in Figure 8. 4

Shock and Vibration
According to [20], the test tensile strength can be expressed by using Equation ( 6): where σ t is the dynamic tensile strength and ε max t is the maximum strain measured on the transmitted bar.e dynamic tensile strain rate of cement-based materials can be de ned as in Equation (7) [21]: where E s is a static tensile modulus and t is the duration of the transmitted wave from the beginning to the climax.

Compressive Characteristics
3.1.1.Failure Mode. Figure 9 shows the failure modes samples of each group under static load, while the damage pattern of typical ber concrete under dynamic loading is shown in Figure 10.
It can be seen from Figure 9 that the surface of the ordinary concrete is seriously damaged, and a large number of microcracks are formed on the surface of the ber concrete.
ere is no wide crack, and the surface of the 2.4 kg/m 3 UHMWPE ber concrete specimen showed tinier cracks.Generally, the concrete expands outward from the top portion or the middle portion at ultimate.When the tensile stress of the outward expansion reaches the peak tensile stress of the concrete material, the concrete specimen is damaged; the concrete crack is weakened from the joint by the matrix.At the same place, the addition of bers increases the bonding strength of the matrix and increases the path of concrete cracking.ere is a negative correlation between crack width and ber content during ber concrete failure.e ber and bridging and lateral restraint can e ectively suppress the deformation of the specimen when it is compressed under pressure, so that the ber concrete specimen can maintain good integrity when subjected to large damage, thus UHMWPE and PVA ber restraint effects are gradually enhanced.From the crack width and its surface damage morphology, the compressive performance of the equivalent UHMWPE ber concrete is better than that of PVA ber, because the UHMWPE ber elastic modulus and tensile strength are higher than PVA ber, but the density is lower than that of PVA ber, so the UHMWPE ber concrete specimens has better integrity and surface microcracks, and UHMWPE antistatic compression ability is stronger.
As the strain rate increased (Figure 10), the damage degree of each group of concrete specimens is increased, and the plain concrete starts breaking; and with the increase of ber content, the strength of the ber concrete specimens improved and showed a certain ductile damage.e UHMWPE ber concrete showed better performance compared with the same amount of PVA ber.It indicates that the ber bridging e ect and lateral restraint function increase the path of crack development and can e ectively inhibit the crack propagation in the impact process, thereby improving the concrete compression resistance.
Under the impact load, the failure mode of the concrete specimen and ber cracking are shown in Figure 11.At the lower strain rate, the crack expands along the joint surface of the cement mortar matrix and the coarse aggregate and nally failed as shown in Figure 11(a).e impact time of the high strain rate is very short, the internal crack is not extended by the path with the least resistance, and the stress has reached the critical stress throughout the coarse aggregate.e expansion path is basically linear, which leads to an increase in the amount of coarse aggregate fracture of concrete on the failure surface than in the static test, as shown in Figure 11(b).As the strain rate continues to increase, the concrete material cannot be dissipated and absorb excess energy.According to the principle of energy conservation, at this time, the material must generate more strain energy; this means, the damage degree of the test specimen will be further aggravated and will break into more fragments [22,23], as shown in Figure 11(c).e schematic of ber cracking in concrete materials is shown in Figure 11(d).
e bridging e ect of ber can inhibit the expansion of cracks during impact and restrain the crack width to develop.Furthermore, it can enhance the lateral restraint performance of concrete when it is damaged.Impact damage occurs under pressure conditions, which can improve the dynamic compressive strength of the concrete; in addition, the ber can dissipate and absorb a large amount of impact energy and improve the threshold value of the energy dissipation of the concrete material at a high strain rate to generate strain energy.erefore, the integrity of ber concrete is stronger than that of plain concrete under impact compression, while the compressive strength and elastic modulus of UHMWPE ber are higher than those of PVA ber.e bridging and limiting e ect of the equivalent UHMWPE ber is stronger.Also, UHMWPE ber is better Shock and Vibration than PVA ber concrete in terms of energy absorption, reinforcing the concrete, toughening and crack-resisting e ect, and dynamic compressive performance.

Stress-Strain Curve.
Figure 12 shows that the dynamic elastic modulus of ber concrete increases with the strain rate increasing.With the increase of ber content, the ultimate strain of concrete is also positively correlated, but the increase under the same working condition is obviously smaller than the strain rate e ect.Ultimate stress and ultimate strain of ber concrete is larger than that of plain concrete, and the increasing amplitude of UHMWPE ber concrete is larger than that of PVA ber.UHMWPE ber can increase the compressive strength and ultimate strain up to 23%, and 17.5%, respectively; thus, the addition of ber can e ectively enhances the concrete performance.

Dynamic Increase Factor.
e dynamic increase factor (DIF) can be determined by Equation (8).Based on existing researches [24,25] and experimental results, the logarithmic relationship is used to characterize the relationship between strain rate and DIF, as shown in Equation ( 9). e obtained results using equation ( 9) are shown in Table 4: where σ d and σ s are the dynamic strength and static strength of concrete materials, respectively.
As shown in Table 4, the logarithmic function relationship can well t the relationship between the DIF of the UHMWPE and PVA ber concrete and the strain rate.Positive values of m indicate that the dynamic compressive strength of the ber concrete has positive correlation rate e ect.With the increase of ber content, m value led the concrete to fail gradually.e m value of the same amount of UHMWPE ber concrete is larger than that of PVA ber, indicating that UHMWPE ber improves the concrete performance.Furthermore, higher strength proves that UHMWPE ber is better in strengthening, toughening, and delaying the failure point.

Energy Absorption.
e energy absorption of ber cement-based composites can be characterized by speci c energy absorption (SEA), determined by the strength and degree of deformation of the material [26], and SEA can be expressed by Equation ( 10) [27]: e relationship between SEA and strain rate of di erent PVA and UHMWPE bers concrete at di erent strain rates is shown in Figure 13.
Figure 13 shows that the SEA of each group of berreinforced concrete increases gradually with the increase of strain rate, indicating that there is an upper limit threshold for the energy absorption of ber concrete.
e SEA of concrete with an equal amount of UHMWPE ber is greater than that of PVA ber, indicating that UHMWPE ber is better than PVA ber in improving the impact compression performance of concrete.Since the side surface of the SHPB uniaxial compression test left free, lateral deformation occurs under impact load, and the bridging and lateral limitation of the ber can delay the expansion of the crack and suppress the lateral shape of the cracks.UHMWPE ber concrete can absorb and dissipate more impact energy, so its SEA is greater than PVA ber.

Dynamic Damage Constitutive Model.
e Zhu-Wang-Tang (ZWT) [28] constitutive model was originally a constitutive relationship proposed for polymer materials.Because cement-based materials have rate sensitivity and the test curve under impact load exhibits hysteresis viscoelasticity, this model is also commonly used to describe the rate constitutive of ber-reinforced concrete materials [29].
e ZWT constitutive model consists of a nonlinear spring and two Maxwell bodies with di erent characteristic  Shock and Vibration times in parallel, as shown in Figure 14(a), and its constitutive equation is given as follows: where E 0 , α, and β are the corresponding elastic constants and E 1 and θ 1 represent the low-frequency Maxwell elastic modulus and characteristic time, respectively.Two integral expressions were used to characterize the viscoelastic behavior of cement-based materials at low and high strain rates.According to research conducted by [30]; the low-frequency Maxwell unit can be equivalently converted into an E 1 spring, as shown in Figure 14(b).en the ZWT constitutive model can be simpli ed to the following equation: Hu et al. [31] considered that the strain of ber concrete material is very small, the damage factor (D) is introduced, and the improved constitutive equation is as follows: where σ is the true stress in the test specimen and σ r is the stress when the material is not damaged.
According to the damage evolution rate of cement-based materials by [32], the rate of damage change can be expressed by Equation ( 14) (simpli ed rate constitutive model): Integrate the time t to Consider the actual boundary condition: one gets D 1 0, therefore With k σ − 1 and substituting the damage factor into Equation ( 13) and considering the SHPB test approximate constant strain rate test, let D c D 0 (_ ε) k , then the nal dynamic compression constitutive equation of ber concrete is expressed as where E 3 E 1 + E 2 ; D c is the damage evolution factor; E 3 is the elastic modulus independent of strain rate; E 2 is the highfrequency Maxwell elastic modulus; and θ 2 is the characteristic time.e tting results for each group of ber concrete are shown in Table 5.
As shown in Table 5, the strain rate increases, the damage evolution rate (D c ) value of each group of ber concrete decreases gradually.Under the impact load, the internal stress of the specimen rises rapidly and the crack does not expand enough.With stress increasing, the critical strength throughout the coarse aggregate is reached; as the ber content increases, the D c value decreases gradually.is revealed that the bridging and con nement of the ber can e ectively increase the stress threshold required for the coagulation crack initiation and reduces the crack propagation.e damage evolution rate (D c ) of the UHMWPE ber-concrete with same working conditions is smaller than that of the PVA ber, Figure 13: Relationship between strain rate and SEA of ber-reinforced concrete.10 Shock and Vibration which indicates that the UHMWPE ber is better than the PVA ber in strengthening, toughening, and crack resistance.
In addition, with the increase of the strain rate, the berreinforced concrete specimens and strains of each group and rate-independent elastic modulus E 3 basically increases rst and then decreases.Similarly, the elastic modulus E 2 of the high-frequency Maxwell body also increases rst and then decreases.e characteristic time θ 2 is positively correlated with strain rate and ber content.e characteristic time θ 2 of the equivalent UHMWPE ber concrete specimen is larger than that of the PVA ber, and the characteristic time θ 2 of the 2.4 kg/m 3 UHMWPE ber concrete is the largest.
e comparison between the experimental values of various groups of ber concrete and the theoretical values are shown in Figure 15.It can be seen from Figure 15, the simpli ed four-parameter constitutive model is simple in form and clear in physical meaning and can e ectively simulate the stress-strain relationship of the dynamic compression process of each group of ber-reinforced concrete specimens.

Tensile Characteristics
3.2.1.Failure Mode. Figure 16 shows that the crack width of each group of ber concrete is smaller than that of plain concrete, and the crack width decreases with the increase of the ber concrete specimens.
e crack width of the 2.4 kg/m 3 UHMWPE ber concrete is less.Due to ber bridging and lateral restraint, it not only increases the stress threshold required for cracking of concrete specimens but also prolongs the path of concrete failure and e ectively reduces the crack growth rate, thus inhibiting crack propagation of specimens.With the increase of the dosage, the inhibition e ect is gradually enhanced, while the UHMWPE ber elastic modulus and tensile strength are higher than those of PVA ber.So, the limitation and inhibition e ect is obviously stronger than that of the equivalent PVA ber.
e typical dynamic tensile failure modes of each group of ber concrete are shown in Figure 17.
Figure 17 shows that the failure of each group of berreinforced concrete specimens is caused by the central penetrating crack, and the splitting damage occurs along the loading direction.e degree of cracking of the ber concrete of each group gradually increases with the increase of the strain rate, with partial cracking failures in the triangular region at both ends.
is is because the specimens are partially broken at both ends of the specimen when the stress is greater than the compressive strength before the center crack penetrates.As the strain rate increases, the degree of fracture in the central region is aggravated (from the splitting into two pieces and gradually becoming part of the fracture in the test specimen) and the degree is more serious than the static tensile failure of the plain concrete.e  Shock and Vibration impact time of the impact load is very short because the internal stress of the test specimen rises very quickly, the specimen cannot be destroyed by the minimum path resistance, and the stress has reached the critical stress required to penetrate the coarse aggregate.As the strain rate continues to increase, the concrete specimen cannot absorb and dissipate excess energy.According to the principle of conservation of energy, the specimen must offset the additional impact energy in the form of greater strain energy.e failure modes of UHMWPE and PVA fiber concrete are mainly center cracks, but UHMWPE fiber concrete can maintain better relative integrity than PVA fiber, which indicates that UHMWPE fiber with equal volume fiber has better crack resistance, reinforcement, and toughening effect.Verification of fiber elastic modulus and tensile strength are the main factors affecting fiber toughening and cracking resistance, while elongation at break is a secondary factor [33].Since the fiber itself can absorb more energy, the strain energy of the test specimen is greatly reduced to offset the threshold of excess energy.e dynamic antipull ability of UHMWPE fiber concrete is obviously better than that of PVA fiber concrete.

Static and Dynamic Tensile Strength.
e static tensile properties of each group of fiber concrete are shown in Table 6, and the relationship between fiber content and static tensile strength of concrete is shown in Figure 18.
e results from Table 6 and Figure 18 show that, as the dosage increases, the tensile strength increases gradually, but the increasing amplitude decreases gradually.According to Table 6, the elastic modulus and fiber content of concrete was a negative correlation.e bridging and lateral restraint of the fiber can effectively restrain the lateral deformation of the specimen during the tension, thereby improving the tensile properties of the fiber concrete.e dynamic tensile performance parameters of each group of fiber concrete are shown in Table 7.
It can be seen from Table 7 the tensile strength of the equivalent UHMWPE fiber concrete is larger than that of the PVA fiber concrete; the dynamic tensile strength of UHMWPE fiber concrete can be increased by at least 10% compared to that of PVA concrete.Because under the impact load, the internal stress rise time of the test specimen is very short and the material does not have sufficient time to deform, showing a viscous mechanism.According to the functional principle, the test specimen can only offset the excess impact energy by increasing the stress [34].e fiber bridging and lateral restraint function can not only prolong the crack development path of the test specimen but also limit the lateral deformation when it is damaged, so it can effectively delay the crack growth rate and improve the toughness of the test specimen.

Relationship between Tensile Strength and Strain Rate.
e fiber concrete DIF can be calculated from the ratio of dynamic to static tensile strength.Some researchers have used a piecewise function to express the relationship between strain rate and dynamic tensile strength.For example, [35] proposed that the relationship between cement-based material DIF and strain rate can be expressed by the following equation:

⎧ ⎨ ⎩ (19)
In Equation (19), _ ε d is a dynamic strain rate.CEB [36] proposed a concrete DIF relation (Equation (20)) for the highest strain rate of 300 s −1 : where c � 10 (7.11a−2.33), a � 1/(10 + 6f c /10)(f c is static compressive strength, MPa), and _ ε s � 3 × 10 −6 s −1 .Equations ( 19) and (20) are complex in the current form, have segmentation points, and the physical meaning is not clear enough.So, based on previous studies and combined with experimental results, a new equation for calculating the dynamic and static uniform strain rate and tensile strength of cement-based materials is proposed in this paper: where _ ε s is a static strain rate and _ ε s � 3 × 10 −6 s −1 , and p, q, and r are all undetermined coefficients.14

Shock and Vibration
Correlation was applied to each group of fiber concrete, the abscissa was measured by logarithmic coordinates, and the ordinate was linear.Taking the relative strain rate (_ ε d /_ ε s ) as the horizontal axis variable, the fitting parameter results are shown in Table 8. e comparison between the experimental and the theoretical values is shown in Figure 19.
Figure 19 shows that the empirical fitting equation proposed in this paper has clear physical meaning and simple form.Furthermore, it can easily be used to simulate the relationship between dynamic and static tensile strength and strain rate of each group.According to Equation ( 21), the tensile strength of the test specimen is composed of two parts: static and rate type effect enhancement part.In Table 8, the parameter p is close to the static compressive strength of concrete.Under static conditions, the strain rate changes very little.At this time, the tensile strength of the specimen depends mainly on the static tensile strength; the parameter q indicates an increase with the strain rate.e degree of tensile strength increases the q value, which is positive, indicating that the tensile strength of each group of fiber concrete is positively correlated with the impact strain rate and fiber content.
e positive correlation of the equivalent amount of UHMWPE fiber is higher than PVA fiber which indicates that the reinforcement effect of UHWMPE fiber is stronger; the parameter r characterizes the degree of strength-type effect of fiber concrete of each group.

Energy Dissipation.
e energy principle is used to study the mechanical properties of fiber concrete during dynamic tensioning.e input, reflected, and transmitted waves energies are defined by Equations ( 22)-( 24), respectively [37]: According to the principle of energy conservation, the energy dissipation W s (t) can be obtained by the following equation: e time-history relationship of four energy changes of plain concrete with an impact load of 0.30 MPa using Equations ( 22)-( 25) is shown in Figure 20(a).
e time-history  20(b); the relationship between the maximum energy consumption and strain rate of ber concrete with di erent contents is shown in Figure 20(c).As shown in Figure 20(b), the dissipative energy of all specimens shows the same tensile failure threshold time period: initial a-section (0-50 μs), the ber concrete specimen is in the compaction stage, and no damage e ect has been produced; accumulating b-segment (50-275 μs), the dissipative energy of di erent ber concrete specimens increases linearly with time.With the increase of the dosage, the energy dissipation rate of ber concrete is gradually increased, and the energy consumption rate of the equivalent UHMWPE concrete is faster than that of PVA ber concrete; stable c-section (after 275 μs) at this time, the ber concrete specimen has been destroyed, and the peak dissipated energy has stabilized and will not continue to grow.
e above three-stage characteristics show whether the dynamic tensile energy consumption of ber-reinforced concrete has a small di erence in duration, re ecting that the ber is e ective for improving the concrete failure mode but has little e ect in preventing the crack-tip opening.
Figure 20(c) shows that the maximum dissipative energy of the ber concrete specimen is positively correlated with the dosage.Similarly, the maximum dissipative energy of the specimen also increases nonlinearly with the increase of the strain rate, and there is an upper threshold; the equivalent amount of UHMWPE ber concrete dissipates more than PVA bers.After the strain rate exceeds 25 s −1 , the maximum dissipative energy of ber concrete tends to be rough and basically no longer increases.e ber's maximum energy dissipation performance is no longer signi cant, and the ber has reached the dissipation and energy absorption.At the same time, it is veri ed that the strength of the concrete material is enhanced under the impact load due to the addition of ber, and the degree of reinforcement is positively correlated with the dosage.Compared with plain concrete, UHMWPE ber-concrete with 2.4 kg/m 3 can increase the maximum dissipative energy W s , by up to 50%.

Conclusion
In this study, the experimental and theoretical studies on dynamic compression and tensile mechanical properties of di erent proportions of new-type ber concrete (UHMWPE and PVA) were carried out by using 74 mm variable cross section SHPB pressure bar device.e main conclusions are as follows: (1) e reinforcing, toughening, and crack-resisting e ects of UHMWPE ber are better than those of PVA ber concrete.UHMWPE ber concrete can increase the compressive strength by up to 23% and increase the ultimate strain to at least 17.5%.Furthermore, the semilogarithmic function relationship can well characterize the relationship between the strain rate of UHMWPE and PVA ber concrete and DIF.
(2) e simpli ed four-parameter constitutive models which is proposed in this paper has a simple form and clear physical meaning, which can easily be used to simulate the stress-strain, dynamic and static tensile strength, and the strain rate relationship of each group of ber-reinforced concrete materials.us, it can be used as an alternative to experimental test which is typically costly and time-consuming.(3) e contents of the ber have great e ect on the concrete properties such as dynamic compressive strength, ultimate strain, and the speci c energy of berconcrete.(4) e dynamic tensile strength, DIF, and dissipative energy of each group increase nonlinearly with the increase of strain rate and ber content, and there was an attenuation of increase, but the strain rate had stronger e ect than ber content.(5) UHMWPE ber concrete has the highest tensile strength, which increased by at least 10% compared with the plain concrete.Furthermore, blending 2.4 kg/m 3 UHMWPE ber concrete can increase the maximum dissipative energy (W s ) approximately 50% compared with the plain mortar concrete.
(6) e logarithmic function expression can be used to simulate the relationship between DIF and the strain rate of each group of ber concrete.
Data Availability e data used to support the ndings of this research are available from the corresponding author upon request.

Figure 12 :
Figure 12: Stress-strain curves of various groups of ber concrete.

Figure 18 :
Figure 18: Relationship between the amount of different fiber concrete and the tensile strength of each group.

Figure 19 :
Figure 19: comparison between the experimental and the theoretical values by using the proposed equation.

Figure 20 :
Figure 20: (a) Energy time-history variation in dynamic Brazilian disc test; (b) W s (t) time-history curve of each group of ber concrete; (c) relationship between strain rate and maximum dissipation energy of ber concrete of each group.

Table 2 :
Mix proportion of ber concrete in each group (measured in m3

Table 3 :
Compressive strength (MPa) of each group of ber concrete.
Figure 10: Failure modes of concrete specimens of di erent groups at di erent strain rates.
Aggregate Crack FiberFigure11: Schematic of the failure mode of concrete specimens and ber cracking in concrete materials at di erent strain rates.

Table 4 :
values of m and n as obtained from Equation (9).

Table 5 :
Parameter results of each group of concrete constitutive models.Figure 16: Static collapse behavior of each group of concrete.
Figure 15: Comparison of the experimental values and the tted values of each group of ber concrete.6.75/s 11.27/s 16.34/s 21.98/s 17.06/s Figure 17: Dynamic collapse of fiber-reinforced concrete in each group.

Table 6 :
Static tensile properties of fiber concrete of each group.

Table 7 :
Performance parameters of fiber concrete of different groups under different strain rates.

Table 8 :
Results of fitting parameters of each group of fiber concrete using the proposed equation.
s (t) of each group of ber concrete under the impact pressure of 0.30 Mpa is shown in Figure