Dynamic split tensile tests of reinforced concrete were carried out using the split Hopkinson pressure bar experimental technique to determine the failure modes of reinforced concrete at different strain rates, and the effect of reinforcement ratio and reinforcement layouts on the dynamic performance. The specimens with nine reinforcement ratios were used in the tests. Experimental results show that the tensile strength of reinforced concrete exhibits a critical strain rate, beyond which larger  increases in dynamic strength of specimens occur. The dynamic split tension strength of reinforced concrete is demonstrated to be greater than the plain concrete with the same strength grade over the range of tested strain rate. The results also indicate that the dynamic split tension strength of specimens enhances with the increase of reinforcement ratio. These findings are instrumental to guide the structural design of reinforced concrete in engineering constructions

Jing, L.

Ma, H.W.

Pei, Q.

Wang, Z.H.

Zhao, Z.

Наукова електронна бібліотека періодичних видань НАН України (Vernadsky National Library of Ukraine)

UDC 539.4An Experimental Study of the Dynamic Split Tension Properties of ReinforcedConcreteZ. Zhao,aL. Jing,bQ. Pei,aH. W. Ma,cand Z. H. Wangd,1a Yuncheng Polytechnic College, Yun Cheng, Shan Xi, Chinab State Key Laboratory of Traction Power, Southwest Jiaotong University, Chengdu, Sichuan, Chinac College of Science and Engineering, Jinan University, Guangzhou, Chinad Institute of Applied Mechanics and Biomedical Engineering, Taiyuan University of Technology,Taiyuan, Shanxi, China1 wangzh@tyut.edu.cnDynamic split tensile tests of reinforced concrete were carried out using the split Hopkinson pressurebar experimental technique to determine the failure modes of reinforced concrete at different strainrates, and the effect of reinforcement ratio and reinforcement layouts on the dynamic performance.The specimens with nine reinforcement ratios were used in the tests. Experimental results show thatthe tensile strength of reinforced concrete exhibits a critical strain rate, beyond which largerincreases in dynamic strength of specimens occur. The dynamic split tension strength of reinforcedconcrete is demonstrated to be greater than the plain concrete with the same strength grade over therange of tested strain rate. The results also indicate that the dynamic split tension strength ofspecimens enhances with the increase of reinforcement ratio. These findings are instrumental toguide the structural design of reinforced concrete in engineering constructions.Keywords: reinforced concrete, dynamic splitting tension, the split Hopkinson pressure bar,reinforcement ratio.Introduction. Reinforced concrete has wide applications in structural engineering ofmilitary and civilian applications, due to its unique advantages including high compressivestrength and simple processing. However, its relatively lower tensile strength and fragilecharacter are major weaknesses for application. Rebar, as an auxiliary material, can partlyimprove the strength and bearing the capacity of concrete. Besides the basic load behavior,dynamic loading, including blast and shock loads, are currently a hot research topic [1].Recently, investigations have focused on compressive performance [2–4]. There arescarce data on the dynamic tensile properties of the reinforced concrete. The splitHopkinson pressure bar (SHPB) technique was used to perform the direct tensile testing[5], while the split tensile test [6–8], and the compressive test [9–12] were used todetermine the mechanical properties of the concrete cylinder. The experimental resultsshow that under the tensile tests, the strain rates of the reinforced concrete vary from 0.1 to20 s1; and the dynamic enhancement factor is 6.47 under the strain rate of 17.8 s1.Antoun [13] also determines the splitting tensile properties, the corresponding range of thestrain rate is from 5 10 7  to 70 s1, using the SHPB; and the experimental results reveal4.8 is the corresponding dynamic enhancement factor of the reinforced concrete. In [14,15], perform the dynamical tensile test under the strain rates from 1 10 5  to 2 10 2  s1,and the corresponding results show the tensile strength of concrete is increasinglyreinforced with the strain rate.In this study, the dynamic split tensile properties of the reinforced concrete wereinvestigated experimentally using the variable cross-sectional straight taper SHPB apparatus.The relationships between the strain rate and the tensile strength of the reinforced concrete,and the effects of strain rate on the failure mode of specimens are analyzed. How the© Z. ZHAO, L. JING, Q. PEI, H. W. MA, Z. H. WANG, 2016ISSN 0556-171X. Ïðîáëåìû ïðî÷íîñòè, 2016, ¹ 1 75dynamical properties of the reinforced concrete are influenced by the reinforcement ratioand the reinforcement method is also discussed.1. Materials and Methods.1.1. Specimens. Concrete/reinforced concrete materials were selected as the specimens.Cloth muscle was used by the circumference reinforcement and the axial reinforcement twomethods. Heights of the internal and the external longitudinal reinforcements equal to 60 mm,whereas outer and inner stirrup diameters are 60 and 30 mm, respectively. Ninereinforcement ratios were used, i.e., 0, 0.998, 1.374, 1.621, 1.749, 1.996, 2.372, 2.619, and2.995%. The reinforced concrete specimens were simply numbered by Rabc . Here Rdenotes that the base material is concrete, a is the number of the stirrups, b is the numberof longitudinal bars, and c is the serial number of the specimen. For example, R692 meansthat this reinforced concrete specimen is the second specimen with six stirrups and ninelongitudinal bars. The concrete specimens were numbered by the alphabet of c.1.2. Experimental Protocol. The static experiment was performed to determine thetensile strength of specimens using a servo-hydraulic test machine, and the test wasterminated when the load achieved to 100 kN. The mean values of the tensile strength ofthe concrete and the reinforced concrete are 3.86 and 7.96 MPa, respectively.The dynamic split tensile experiment was performed using the device of SHPB (Fig. 1).The diameter and the length of the SHPB used here are 74 and 800 mm, respectively. In theexperiment, trip rod strikes the incident bar and then produces the compression stress wave,whose amplitude is dependent on the impact velocity and the wave impedance. This actingtime is dependent on the length of the trip rod and the wave velocity. Adjusting the airpressure of the pistol, the different impact velocities were obtained and then the differentstrain rates were obtained. To get a more accurate data, the semiconductor gauge was usedin this experiment. The corresponding incident, reflect, and transmission waves of the stresspulse of a specimen are shown in Fig. 2.Z. Zhao, L. Jing, Q. Pei, et al.76 ISSN 0556-171X. Ïðîáëåìû ïðî÷íîñòè, 2016, ¹ 1Fig. 1. The SHPB device scheme.Fig. 2. Data trace from the SHPB dynamic split tension test.1.3. Analysis Method. The specimens were produced as the additional strap testspecification of American Society for Testing and Materials. The additional strap caneffectively reduce the stress concentration of the specimen. The tensile strength is definedasf P LDtd  2  , (1)whereP R T  2 , (2)where P is the maximum load measured through the test, L and D are the length and thediameter of the specimen, respectively, R is the SHPB diameter, and T is the peak stressof the output bar.The stress rate and the strain rate are respectively defined as ,  ftd (3)  ,  E (4)where  is the rising time of the transmission wave and E is the Young modulus of thereinforced concrete specimen.In this paper, the dynamic increase factor (DIF), which is defined as Eq. (5), is used todescribe the enhancement effect of the concrete or reinforced concrete,DIF f ftd ts , (5)where ftd is the dynamic tensile strength and fts is the quasistatic tensile strength.2. Results and Discussion.2.1. Strain Rate Effect. The corresponding experimental results of the dynamicsplitting tensile tests are summarized in Table 1. The results as shown in Table 1 reveal thatthe strain rate effect is obvious in the specimens with different reinforcement ratios, i.e., thedynamic split tensile strength of concrete/ reinforced concrete is increasingly improvedwith the strain rate. The possible reason is the microcracks, which largely exist in the innerof the concrete/ reinforced concrete. Under the action of the external loads, crack in theinner core of the concrete/ reinforced concrete gradually evolves to damage. The shockload, in case of higher strain rates, acts during a short time period for a crack to bypass theaggregate and to extend in the weaker direction, and then the damage of the aggregatehappens. This phenomenon can also be observed in the rupture surfaces of the specimens.The relationship between the DIF and the strain rate, as shown in Fig. 3, reveals thatthe reinforced concrete has some effects of the strain rate under the dynamic loads. The DIFis correlated with the strain rate; its growth factor is approximately linear with the strainrate and the corresponding slope is nearly two. When the strain rate exceeds a certain value,which is defined as the critical strain rate, the tensile strength will sharply rise. In thispaper, the fitted curve reveals this critical value is ~ 2 1s . Also, the experimental data arediscrete due to the defects of materials, e.g., the inhomogeneity and the microcrack. Theconcrete/reinforced concrete are brittle materials. Prior to loading, the variation of themortar volume happens due to the cement hydration induces the chemical and physicalshrinkages. This is the reason for the disorder cracks appearing on the surface between thecoarse aggregate and the mortar. Under the external loads, the stress concentration earlyhappens in the crack defect and then the microcrack propagates across the specimen untilthe specimen damages.An Experimental Study of the Dynamic Split Tension Properties ...ISSN 0556-171X. Ïðîáëåìû ïðî÷íîñòè, 2016, ¹ 1 772.2. Reinforcement Ratio Effect. The failure mode and the quantitative experimentalresults were used to analyze how the reinforcement ratio influences the tensile strength ofthe reinforced concrete. Under the same strain rate, the concrete specimens completelybreak into two segments; while the reinforced concrete specimens crack rather than breakdue to the dynamic loads being counteracted by the reinforcements. The damage degree isgradually weakened with the reinforcement ratio (Fig. 4).78 ISSN 0556-171X. Ïðîáëåìû ïðî÷íîñòè, 2016, ¹ 1T a b l e 1Collection of Experimental DataSpec. ftd ,MPaStress rate(GPa/s)Strain rates(s–1)DIF Reinforcedconcrete DIFC3 4.38 31.79 1.33 1.13 1.00C9 5.87 64.53 2.08 1.52 1.00R23 8.35 96.55 1.87 1.05 1.62R23 10.97 68.76 1.33 1.38 2.14R26 9.48 80.02 1.99 1.19 1.85R26 12.16 113.62 2.82 1.53 2.37R29 11.78 106.58 2.78 1.48 2.30R29 13.80 166.21 4.33 1.73 2.69R43 10.90 113.34 2.88 1.38 2.14R43 13.75 150.25 3.82 1.73 2.68R46 12.74 126.12 3.71 1.60 2.48R46 13.00 68.95 4.81 1.63 2.53R49 14.56 168.23 3.88 1.83 2.84R49 15.28 252.58 5.83 1.92 2.98R66 12.26 139.35 3.61 1.54 2.39R69 9.32 97.63 1.76 1.17 1.82Fig. 3. Strain rate effects on the DIF of concrete.Z. Zhao, L. Jing, Q. Pei, et al.Different reinforcements have different influences on the tensile strength of thereinforced concrete. As the images shown in Fig. 5 at v 9 m/s, the cooperation betweenstirrups and longitudinal bars directly influences the failure mode of the reinforcedconcrete. Setting the same number of the longitudinal bars, the specimen with two stirrupsdamages like that the concrete completely breaks from the center and the reinforcementdoes not break; while the specimen with four stirrups and six stirrups slightly crack ratherthan break. Therefore, the dynamical tensile strength of the reinforced concrete can beeffectively reinforced by selecting the advisable cooperation between stirrups andlongitudinal bars.In order to quantitatively analyze how the reinforcement ratio influences the tensilestrength of the reinforced concrete, the reinforced factor K , which is defined as the ratiobetween the dynamic tensile strengths of the reinforced concrete and the concrete, i.e.,Eq. (6), is introduced to demonstrate the enhancement effect (Fig. 6),ISSN 0556-171X. Ïðîáëåìû ïðî÷íîñòè, 2016, ¹ 1 79Fig. 4. Reinforcement ratio effects on failure pattern.Fig. 5. Reinforcement form effects on concrete failure pattern.Fig. 6. Effect of strain rate on the reinforcement factor of concrete.An Experimental Study of the Dynamic Split Tension Properties ...K f ftr tc , (6)where ftr and ftc are the dynamic tensile strength of the reinforced concrete and theconcrete, respectively.The results shown in Fig. 6 reveal that the tensile strength of the reinforced concreteobviously stronger than that of the concrete. Under the same strain rate, the value of Kincreasingly grows with the reinforcement ratio. The corresponding reason is that rebar caneffectively prevent the propagation of the crack and delay the damage time, and thenreinforce the tensile strength.Conclusions. In this study, the dynamic splitting tensile tests were performed on thenine different reinforcements to determine the tensile strength of the concrete/reinforcedconcrete. The corresponding experimental results reveal that the dynamic tensile strength ofthe reinforced concrete increasingly reinforces with the strain rate, and that will sharplyreinforce after the strain rate beyond the critical value. Implanting rebar can effectivelyprevent the propagation of the crack and delay the damage time, and then availablyreinforce the tensile strength of the reinforced concrete. Different reinforcements havedifferent influences on the damage modes of specimens. Under the same strain rate, thedamaged condition of specimens depends on the number of the stirrups. More stirrups canprovide a better protection of the specimen.Acknowledgments. The authors wish to acknowledge the financial support providedby the China National Natural Science Funding (Grant No. 11390362) and opening foundationfor State Key Laboratory of Explosion Science and Technology (Grant No. 33810005).1. Zh. H. Guo, Theory of Reinforced Concrete Members, Tsinghua University Press,Beijing (1999).2. D. A. Abrams, “Effect of rate of application of load on the compressive strength ofconcrete,” in: Proc. of 20th Ann. Meeting, Vol. 17, ASTM, West Conshohocken, PA(1917), pp. 366–374.3. D. Watstein, “Effect of straining rate on the compressive strength and elasticproperties of concrete,” ACI J., 49, No. 8, 729–744 (1953).4. J. H. Lu, “Influence of reinforcement on the dynamic behavior of concrete,” Trans.Beijing Inst. Technol., 27, No. 6, 473–476 (2007).5. C. A. Ross, D. M. Jerome, J. W. Tedesco, and M. L. Hughes, “Moisture and strainrate effects on concrete strength,” ACI Mater. J., 93, No. 3, 293–300 (1996).6. C. A. Ross, J. W. Tedesco, and S. T. Kuennen, “Effects of strain rate on concretestrength,” ACI Mater. J., 92, No. 1, 37–47 (1995).7. F. Yang, H. W. Ma, L. Jing, et al., “Dynamic compressive and splitting tensile tests onmortar using split Hopkinson pressure bar technique,” Lat. Am. J. Solids Struct., 12,No. 4, 730–746 (2015).8. J. H. Yon, N. M. Hawkins, and A. S. Kobayashi, “Strain-rate sensitivity of concretemechanical properties,” ACI Mater. J., 89, No. 2, 146-153 (1992).9. J. W. Tedesco, C. A. Ross, and S. T. Kuennen, “Experimental and numerical analysisof high strain rate splitting tensile test,” ACI Mater. J., 90, No. 2, 162–169 (1993).10. D. W. Harris, E. M. Caroline, and P. D. Timothy, “Dynamic properties of massconcrete obtained from dam cores,” ACI Mater. J., 97, No. 2, 290–296 (2000).11. L. E. Malvern, D. A. Jenkins, T. X. Tang, and S. A. Ross, “Dynamic compressivetesting of concrete,” in: Proc. of 2nd Symp. on the Interaction of Non-NuclearMunitions with Structures, US Department of Defense, Florida (1985), pp. 194–199.80 ISSN 0556-171X. Ïðîáëåìû ïðî÷íîñòè, 2016, ¹ 1Z. Zhao, L. Jing, Q. Pei, et al.12. C. A. Ross, P. Y. Thompson, and J. W. Tedesco, “Split-Hopkinson pressure bar testson concrete and motar in tension and compression,” ACI Mater. J., 86, No. 5,475–481 (1989).13. T. H. Antoun, Constitutive/Failure Model for the Static and Dynamic Behaviors ofConcrete Incorporating Effects of Damage and Anisotropy, The University of Dayton,Dayton, OH (1991).14. R. J. Shang, Investigation of Dynamic Constitutive Behavior of Concrete, DalianUniversity of Technology, Dalian (1994).15. L. J. Malvar and C. A. Ross, “Review of strain rate effects for concrete in tension,”ACI Mater. J., 95, No. 6, 735–739 (1998).Received 03. 08. 2015ISSN 0556-171X. Ïðîáëåìû ïðî÷íîñòè, 2016, ¹ 1 81An Experimental Study of the Dynamic Split Tension Properties ...

An Experimental Study of the Dynamic Split Tension Properties of Reinforced Concrete

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An Experimental Study of the Dynamic Split Tension Properties of Reinforced Concrete

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