LU Juan,ZHANG Yafang

(School of Civil Engineering,Guangzhou University,Guangzhou 510006,China)

Abstract: The influence of carbon fiber reinforced plastic (CFRP) on dynamic mechanical properties of reinforced concrete (RC) beam was studied by drop hammer impact test system.The impact behaviors of beam,including failure mode,impact force peak value and peak deflection were analyzed.The experimental results show that bonding CFRP can reduce the crack width and change the failure mode of the beam.The length of CFRP has a certain influence on the impact force and deflection,and the peak inertia force of most beams is roughly in the range of 1/2-5/6 of the peak impact force.In addition,dynamic increase factor (DIF) increases with the increase of CFRP length,and its maximum value can reach 2.11.

Key words: CFRP;RC beam;failure mode;impact resistance;DIF

1 Introduction

As one of the main load-bearing components of building structures,reinforced concrete (RC)beams bear not only routine static and live loads,but also dynamic loads such as earthquakes,shocks and explosions.Dynamic loading is characterized by its large amplitude within a short duration,causing strain rate effect of materials (such as concrete and steel) and inertia effect of structure[1,2].These effects can lead to significant differences in the performance of concrete structures under impact loading compared with that under static loading.Previous studies have found that the inherent brittleness and poor fracture energy dissipation capacity of concrete are the fundamental reasons for its damage degree under dynamic loads.Such damage is actually much more serious than that caused by static loads[3,4].Improving the ability of concrete structures or components to resist impact loads and evaluating accurately the damage of concrete structures under impact have gradually become the focus of research at home and abroad[5,6].

Due to high tensile strength and elastic modulus,high strength-weight ratio,corrosion resistance,chemical stability,and good bonding with concrete,CFRP is widely used in concrete structure reinforcement[7-10].Sang HKet al[11]studied the improvement of ductility and bending capacity of RC beam with different amounts of CFRP through experiments.Saleemet al[12]carried out four-point bending tests of seven RC beams.They believed that increasing the CFRP reinforcement above certain levels does not necessarily increase the bearing capacity,and bond property at the CFRP-concrete interface play a vital role.Zhanget al[13]conducted quasi-static loading tests on eight RC T-section beams strengthened with CFRP sheets.The effects of U-wrap spacing,flexural reinforcing ratio,and concrete compressive strength on the flexural behavior of the CFRP strengthened RC T-section beams were analyzed and discussed.

The research on the performance of CFRP reinforced structure under static load is relatively mature,however,the research on it under impact load is still in the preliminary stage[14-16].Jeromeet al[17]and Rosset al[18]researched the flexural behavior of CFRP strengthened RC beams under impact loading.The drop-weight impact test results showed that the CFRP can improve the flexural bearing capacity of RC beams,and the peak flexural bearing capacity increased with the increase of impact height.Tanget al[19]found that CFRP can improve the bearing capacity under impact load by preventing cracks,and can effectively reduce the width and number of cracks.Erfanet al[20]applied CFRP wraps to retrofit RC beams.The impact test results showed that the CFRP reduced the deflection,restrained the crack,limited the damage,and enhanced the absorbed impulse ratios.

In summary,the existing researches are mostly based on the qualitative research of the performance of CFRP reinforced structures.Given that studies on the dynamic failure mechanism of CFRP reinforced RC beams are still limited,how to reasonably evaluate the strength contribution of CFRP under impact load remains to be further studied.A total of 4 RC/CFRP-RC beams were designed and tested under impact loading.A drop hammer device was used in the impact test.The main purpose was to study the failure mechanism of CFRP reinforced RC beams and the contribution of CFRP to the resistance of reinforced concrete beams by changing the length of CFRP,in terms of crack pattern,failure mode and dynamic time history.

2 Experimental

2.1 Materials

The full-scale beam had a length of 2000 mm and a cross section of 150 mm×300 mm.The bars in tension zone were 2φ18 (HRB400),in compression area were 2φ10 (HPB235),and the stirrups wereφ8@140(HPB235).The elasticity modulus of hot-rolled ribbed was 2.06×105MPa in the test.The thickness of concrete protective cover was 20 mm.

All beams were produced with commercial concrete in a single batch.The average cube strengthfcuat 28 days was 42.8 MPa for the cubical concrete specimens (150 mm×150 mm×150 mm),and the elastic modulus of concrete is 3.3×104MPa.The average measured value offcuat the time of the impact tests was 43.7 MPa.

CFRP and concrete needed to be bonded by impregnating adhesive in the test.The CFRP was produced in Nanjing Haituo Composites Co.,Ltd,with elastic modulus of 2.3 × 105MPa and tensile strength of 3585.6 MPa.The impregnating adhesive produced in Shanghai Zhinuo Decorative Materials Co.,Ltd.The impregnating adhesive is composed of A and B adhesive with a mass ratio of 2:1.

2.2 Specimen preparation

The impurities were removed by sand grinding to bond CFRP and the surface of the beam was prepared to be smooth.The two sides of CFRP sheet were also smoothed and wiped with anhydrous ethanol.Then,the CFRP was cut into the size shown in Table 1,and the impregnated adhesive was uniformly coated on the bonding area between CFRP and concrete beams.Finally,the CFRP-RC beams with fully hardened adhesive layer would be tested,after curing at room temperature for more than 7 days.

Table 1 Details of beams and test results

2.3 Impact test setup and instrumentations

In this test,the drop hammer was consisted of a cylindrical steel hammer with a plane impact surface,a weighing sensor based on a strain gauge (measuring range 4000 kN for impact force measurement) and some adjustable steel counterweight blocks to achieve the total target mass.The hammer head was a circular plane with a diameter of 200 mm and the total weight of the drop hammer was 575 kg.The data collected by the transducer were obtained by the dynamic digital data acquisition system with a sampling rate of 50 kHz in the impact test.

The net span (S) of beam was fixed at 1750 mm.When testing,the drop hammer was lifted to a predetermined height and the hook was released through the electrical control system.Before the drop hammer contacted beam,the laser scanning sensor fixed on the transverse support frame was used to measure the drop hammer speed,which was defined as the drop hammer impact speed.The impact speedvof each beam is shown in Table 1.Furthermore,the mid-span deflection of the beam was recorded by a deflection meter during the test.

In order to record the whole process of impact test,the high resolution camera of type FASTCAM SA-Z was set in place in front of the beam.Two highpower LED lights were placed beside the two cameras to improve the lighting throughout the process.

3 Results and discussion

In this section,the impact/inertial force time history curves and deflection-time history curves of the three-point flexural beams with different bonded CFRP lengths will be presented.At the same time,the crack patterns and failure modes of the test beams are discussed in details.Finally,effect of the CFRP length on the dynamic property are discussed.

3.1 Crack patterns and failure modes

Fig.1 shows the crack pattern on one face of the beam after impact test.It can be seen that the concrete in compression zone of all beams fractured under impact load.It is obvious that the plain beam H1.4 suffered severe flexural damage due to the full penetration of a major flexural crack under the impact with the speed of 5.22 m/s,and the maximum crack width is 4.11 mm.

Fig.1 The crack pattern and crack width of each beam

For the beam reinforced with CFRP,in H1.4L0.8,more shear cracks develop and the separation of concrete blocks of the CFRP wraps adjacent to the loading position.The width of maximum flexural crack is approximately 3.25 mm.At the same impact height,there is the similar phenomena in other CFRPRC beams,i e,more shear cracks develop and shear failure appears mainly,and the maximum width of shear crack is 1.31 mm.However,it is worth noting that the maximum crack width of beam H1.4L1.0 is 4.05 mm,and that of beam H1.4L1.2 is 3.63 mm.The crack width of beam H1.4L1.2 is smaller than that of H1.4L1.0 beam,and the damage degree is weaker.It indicates that the CFRP has positive effect on the damage control of RC beam under impact load and can change the failure mode of the beam.Moreover,the longer the CFRP is,the more obvious the strengthening effect on the beam is.In this study,the beam bonded CFRP failed in shear mode under ultimate impact loading.Therefore,on the basis of this study,it is recommended to use the CFRP full package scheme for reinforcement to prevent the shear failure of RC beams under impact load.

3.2 Impact/inertial force-time history analysis

Suariset al[21]and Weddinget al[22]believed that the impact load would produce inertial oscillation in the contact area between the beam and the hammer,so it was necessary to consider the influence of the beam inertia.According to Suaris,the inertia load of the beam may be much larger than the load caused by the initial impact stage,and the distribution of acceleration and deflection along the length of RC beam was basically linear.However,the inertial load results were not sensitive to the assumption of acceleration distribution along the beam.Therefore,the generalized inertial load is calculated based on the linear distribution of deflection in this paper.The deflection along the beam length direction can be calculated by Eqs.(1) and (2):

where,s(x,t) is the the deflection of the beam between the bracket,s(y,t) the deflection of the beam outside the bracket,ands0(t) the deflection of the beam in the span,as shown in Fig.2.

Fig.2 Definition of terms for calculation of the generalized inertial load

Since the virtual work of distributed inertial force is equal to the virtual work of generalized inertial load,it is necessary to introduce a virtual displacement δs0,so that the inertial load along the length direction of the beam can be represented by the generalized inertial load at the center of the beam,as follows:

where,ρ represents the density of beam,Athe crosssectional area of beam,andthe acceleration of the beam central point.Given that the density ρ and cross section areaAof the beam maintain constant within the whole length range of the beam,Eq.(3) can be simplified as:

The impact/inertial force-time history curves of each beam are shown in Fig.3,where,the positive number on the longitudinal coordinate expresses the downward impact force and the upward inertia force.The peak impact force is shown in Table 1.

As shown in Fig.3,the impact force rapidly increases to the peak value due to the abruptly interaction between the beam and the drop hammer.Then,the drop hammer and beam are separated instantaneously due to the beam stiffness,so the impact force quickly dropped to zero.It shows an approximate triangular response with short duration and high amplitude.Then,the downward speed of the beam decreases quickly and is less than that of the drop hammer.The drop hammer impacts the beam again,resulting in the increase of impact force again and reaching the second peak,but the amplitude is less than the first peak.After several damped sinusoidal vibrations,due to the multiple rebounds of the drop hammer,the energy is gradually consumed,and the impact force is gradually reduced to 0 kN.All beams show similar shapes.

Fig.3 Impact/ inertial force-time history curves of RC/ CFRP-RC beams

According to the law of momentum,the shorter the impact time,the greater the impact force.It can be seen that with the increase of CFRP length,the peak impact force increases slightly,indicating that CFRP has a certain impact on the peak impact force.The fitted relationship between the peak impact force and the CFRP length can be obtained by Eq.(5),

where,Fpis the peak impact force,andLthe length of CFRP.From Eq.(5),Fpwill not always increase with the increase of CFRP length,that is,when the length of CFRP exceeds a certain threshold,its impact resistance ability to RC beam almost does not increase.The curves ofFpversus CFRP length are shown in Fig.4.

The response of inertial force is similar to that of impact force,but there is a short time lag because the stress wave needs time when it passes through the beam.After reaching the peak,the inertial force is reversed to a negative peak,and gradually decreases to 0kN after several low-frequency damping oscillations.In addition,it can be found that the peak inertia force of most beams is roughly in the range of 1/2-5/6 of the impact force peak.Some literatures have the same research results[22,23].It shows that the influence of inertial force must be considered when analyzing the dynamic behavior of beams.

According to the motion equation,the force equilibrium should be satisfied at any time in the vertical direction of the beam under impact load.Therefore,if the damping force is ignored,the force equilibrium equation during impact is as follows:

where,FR(t) is the resisting force of the beam,which is used as the dynamic flexural strength capacity of the beam.

In order to understand the influence of CFRP on the dynamic flexural strength of the beam,the dynamic increase factor (DIF) is defined to reflect the dynamic flexural strength and static flexural strength of the beam,as in Eq.(7):

where,PUis the dynamic flexural strength of beam,andPSthe static flexural strength.The research group has carried out relevant static flexural tests and obtained the flexural strength of CFRP reinforced RC beams.TheDIFof each beam is shown in Fig.4.The maximum value ofDIFcan reach 2.11.It can be seen thatDIFgradually increases with the increase of CFRP length,that is,the flexural strength of the beam is increasing,indicating that the longer CFRP has a more obvious effect on the improvement of the dynamic flexural capacity of the beam.

Fig.4 DIF and peak impact force of each beam

In order to investigate the CFRP contribution to impact resistance,the impact force calculated by subtracting the force of pure RC beam from that of CFRP-RC beam is regarded as the contribution of CFRP,and the impact resistance of beam H1.4L0.8,H1.4L1.0,and H1.4L1.2 is 74.05,81.87,172.41 kN,respectively.It indicates that the longer the CFRP,the greater the contribution of CFRP to the impact resistance,that is,the longer CFRP can improve the impact resistance of RC beams.

3.3 Deflection-time history of mid-span

The initial 0.06 s time histories of midspan deflection are shown in Fig.5.The peak value of midspan deflection is shown in Table 1.From Fig.5,the deflection of the middle span of beam H1.4 under impact loading is the largest (40.39 mm).

Fig.5 The mid-span deflection time history

Through comparative analysis,it can be found that bonding CFRP can improve the deflection of the beam.When the impact height is constant,the deflection of the beam H1.4,H1.4L0.8,H1.4L1.0,H1.4L1.2 is 40.39,38.15,37.55,and 35.02 mm,respectively.The deflection of beam H1.4L1.2 is 15.33% lower than that of beam H1.4.

Since the impact energy is mainly consumed by the crack formation and plastic deformation of the beam,the elastic deformation energy stored in the beam member itself is less,and CFRP will consume part of the energy in the process of concrete stripping.Therefore,the longer the CFRP is,the more the energy is consumed,and the smaller the deflection is.

3.4 Impact-resistance energy analysis

The impact energy is mainly consumed by the crack development and plastic deformation of the beam.CFRP can consume a part of energy in the process of stripping from the concrete.The value of energy dissipated by the specimen can be approximately estimated from the area under the impact force versus midspan deflection curve[24].For CFRPRC beam,assuming that the bonding zone between beam surface and CFRP is not destroyed during the impact process,the total impact energy includes two parts: impact energy absorbed by RC beam and by CFRP[55].Under the same impact conditions,the impact energy absorbed by the RC beam is considered to be constant,so the impact energy absorbed by CFRP is the difference between the total impact energy and the energy absorbed by the RC beam,as shown in Eq.(8):

where,Eb,RCis the dissipation energy absorbed by the RC beam,andEb,fthe dissipation energy contributed by the CFRP.Hence,the dissipation energy contributed by CFRP in each beam can be calculated by Eq.(8),and the energy dissipation of each beam is shown in Fig.6.

Fig.6 The dissipation energy of each beam

As shown in Fig.6,the beam with CFRP bonded length of 1400 mm has the maximum impact energy.When the CFRP length is 800,1000,and 1200 mm,theEb,fis 355.74,404.16,and 535.96 J,respectively.It can be seen thatEb,fincreases with the increase of CFRP length,showing a significant positive correlation,and again indicate that CFRP reinforcement has a significant impact on the impact resistance of RC beams.

4 Conclusions

The dynamic impact resistance of RC beams and CFRP-RC beams is investigated by carrying out drop hammer tests.The impact/ inertial force time history,displacement time history and failure mode of RC beams with different lengths of CFRP under same impact height are obtained,the following conclusions can be drawn:

a) Bonding CFRP has a significant effect on the impact resistance of RC beams and can transform the failure mode of RC beam from flexural failure to shear one.In addition,the crack width of RC beam decreases when the length of CFRP increases.

b) Bonding CFRP has certain influence on the peak load,deflection and impact energy of RC beam.The peak inertia force of most beams is roughly in the range of 1/2-5/6 of the peak impact force,so it is necessary to consider the influence of inertial force when analyzing the dynamic behavior of beams.

c)DIFis a useful index to evaluate the influence of CFRP on dynamic flexural strength of beams.The longer CFRP length,the lager theDIFvalue,and the more obvious the effect on the improvement of the dynamic flexural capacity of the beam.