Keywords

1 Introduction

Reinforced concrete structures occupy most of the existing building structures in the world. Most reinforced concrete buildings have been in service for more than 30 years. At the same time, many reinforced concrete structures built before the 1950s are still in use in many European and American countries, including many transportation infrastructures [1]. Many reinforced concrete structures, especially those for civil infrastructure, have been in service beyond their usefulness. Reinforced concrete structures built for many years generally suffer from corrosion of steel reinforcement, cracking, and spalling of concrete [2]. These structures often carry loads higher than their maximum design-bearing capacity. The requirements for the seismic performance of structures also increase the need for reinforcement and repair buildings that have been in service for a long time [3,4,5,6]. Based on the above reasons, many scholars are devoted to exploring an economical technology capable of strengthening and repairing structures, which has made great progress in strengthening and repairing reinforced concrete structures in recent years. Since the beginning of the new century, infrastructure construction in China has grown rapidly, and reinforced concrete (RC) structure is one of the main structural forms. Due to their good mechanical properties and economy, RC components are widely used in bridges, urban underground spaces, and building structures. They are often used as load-bearing components of structures. During service and use, RC members may be impacted by automobiles, derailed trains, and airplanes. Suppose the impact resistance of RC members is weak or lacks impact resistance. In that case, the damage may have a catastrophic impact on the overall structure, resulting in huge economic losses and heavy casualties. In recent years, with the advancement of urban construction, more and more accidents have occurred due to the impact of building structures. For example, the JR train derailment accident in Japan [7] caused damage to buildings along the track, and many people died; the German high-speed rail derailment accident [8] caused the concrete bridge above the track to collapse. Accidents of ships hitting bridges have also occurred in Jiujiang Bridge piers, China, as shown in Fig. 1 according to the (http://www.chinadaily.com.cn/china/2007-06/15/content_895374.htm) Chinese net news website [9]. In impact accidents, RC members will show failure modes such as bending, shearing, and punching [10], as shown in Fig. 2.

Fig. 1
3 photographs of a collapsed bridge with debris floating in the water below.

Accidents of ship on RC piers of Jiujiang Bridge in China [9]

Full size image
Fig. 2
2 close-up photographs of a concrete pillar. The pillar on the left has a crushed vehicle behind it and the pillar on the left is destroyed and its steel columns are distorted.

Partial diagram of failure of RC member under impact [10, 11]

Full size image

Similar to static loading conditions, reinforced concrete (RC) members under impact loading will exhibit two failure modes: bending failure and shear failure [12]. After the plastic hinge is formed at the position where the member reaches the ultimate moment bearing capacity, the member undergoes bending failure. This failure mode has good flexibility and can absorb more impact energy during impact [13]. Contrary to the bending failure mode, the shear failure mode is brittle, and the component resistance cannot reach the ultimate bending moment capacity. This failure mode has poor flexibility, leading to premature failure of the component, which will seriously reduce the impact resistance of the component. However, the behavior of RC members under impact loading differs from that under static loading conditions. The impact load will cause the material’s strain rate effect, the component’s inertial force, and the local effect caused by the impact load. The static test cannot reflect the impact resistance of the RC component under the impact load. Experimental studies have shown that: RC beams fail in bending under static conditions, but shear failure occurs under impact loads [14]. RC beams under impact loads may exhibit more localized damage (such as punching shear damage) [15, 16]. Therefore, there have been some experimental studies at home and abroad to explore the behavior characteristics of RC components under impact loads. Khalil et al. (2021) [17] conducted experimental research on the bearing capacity of ordinary concrete and CFRP-strengthening concrete columns after the impact. The RC column and the other four reinforced columns were placed under the same impact load in the test.

Then the static axial loading test was carried out on the impact-damaged column to obtain the remaining bearing capacity of the impact-damaged column to evaluate the effectiveness of different strengthening measures. The test results show that adding CFRP layers of energy-dissipating materials can improve the post-impact bearing capacity. The strength, deformation, elasticity, and confining effect of members can be affected by changes in cross-sections, reinforcements, and width-to-thickness ratios [18,19,20,21]. Recently, there have been a lot of studies that have a deep discussion of the effect of asymmetrical lateral impact load on RC specimens with and without FRP-strengthening materials [22,23,24,25,26]. Consequently, research on the failure mode and force parameters of cross-section shape RC specimens is limited. Furthermore, the study investigated the dynamic behavior of RC specimens using asymmetrical-span drop hammer impact testing. The main focus of the study is on the typical failure mode caused by an asymmetrical span drop hammer, the time history curve of deflection and impact force, and the effect of different cross-section shapes on RC specimens.

2 Materials and Methods

In the impact test, eight reinforced concrete specimens will have square and circular cross-sections. The test was conducted using a DHR-9401 drop hammer impact tester from the Taiyuan University of Technology. Using Chinese Pinyin initials, “F” represents square members, “Y” represents circular members, and “H” represents concrete specimens. Each test specimen is poured with the same material to achieve interoperability. Asymmetrical lateral impact forces are applied to specimens with different impact heights, longitudinal reinforcement ratios, and stirrup ratios to determine impact resistance. The specimen is supported at both ends, has a design length of 1500 mm, and supports lengths of 200 mm in the test. This collision is designed to mimic a collision between a high-speed railway locomotive and an RC column member. The impact point is located at 2/9 of the clear span, 200 mm from the right and 700 mm from the left. [27]. The cross-sectional dimension of the square specimen is 120 * 120 mm, while the diameter of the circular specimen is 114 mm. Figure 3 shows that square specimens contain 4 HRB235 steel bars, whereas circular specimens have 6 HRB235 steel bars evenly distributed along the ring direction. The stirrup has a diameter of 4 mm. The concrete cover thickness for square specimens is 20 mm, while for circular specimens is 12 mm.

Fig. 3
3 diagrams with the dimensions of a long specimen with fixed supports on both ends, longitudinal reinforcement, transverse reinforcement, and hammer indicated, along with the square cross-section, and circular cross-section.

The specimen’s schematic illustration

Full size image

The specimens’ dimensions, reinforcing details, and parameter factors are included in Table 1. Frictional energy loss is included in the impact energy depicted in the Table, which was calculated using energy conservation principles. Under the asymmetrical impact scenario, this study investigates the RC members’ failure mode, impact force, and deflection. Since the impact, members’ responses have been related to strain rate, confinement effects, and other train-specific factors. As a consequence, two impact scenarios and several reinforcement configurations were considered.

Table 1 Test specimen’s design information
Full size table

2.1 Test Devise, Loading, and Measurement Scheme

The authors completed this impact test on the DHR9401 drop hammer impact-testing machine, as shown in Fig. 4. The DHR9401 drop hammer slideway height is 13.47 m, and the effective drop distance is 12.60 m. The test frame is a portal frame composed of two vertical lattice steel columns and rigid beams. The body falls smoothly, and the impact velocity of the drop hammer is very repeatable. The lower part of the support is fixed on the rigid platform, and a linear bearing is installed in the support. This bearing can restrain the displacement of the members in all directions except the axial direction to realize the boundary condition of fixed support. The impact hammer is made of (45# forged steel) and consists of three main elements, and the first element is a stiffened flat head constructed of large-strength chromium 15 (64HRC), which is a rectangular parallelepiped face, and the size is 80 mm × 30 mm × 80 mm. The striker’s second element is the weight portion; this element contains the striker’s main mass and provides a weight of (270) kg. The third element is that of load cells, which are used to calculate the impact force record. The impact hammer’s total weight is 65 kg and could be lifted to the requisite height to generate varied velocities of impact (up to 15.7 m/s) and energy (see Fig. 4). A load cells sensor is placed between the impact head and the drop hammer to measure the impact force. The test specimen support is made of (Q235) steel and divided into upper and lower parts. Figure 4 shows a bolt connection between the upper and lower parts. The bolts provide pre-tightening force for the two parts of the support, effectively improving the friction between the support and the test specimen. It can restrain the specimens from moving in the axial direction to a certain extent. A force sensor connects an impact head and drops a hammer for measuring the impact time-time history curve.

Fig. 4
A schematic diagram and 2 photographs of the test equipment, with the specimen, track points, fixed support, rigid base, high-speed camera shooting range, bolt hole, weight support, upper and lower half support, releasing mechanisms, drop hammer, and slide rail labeled.

Test equipment photos

Full size image

Data is collected at a million per second. The TDS420 dynamic data storage oscilloscope records and saves the strain amplifier signal. Furthermore, with a shooting frequency of 2500 frames/s, the high-speed camera in front of the specimen’s impact site records the impact deformation and destruction process. A high-speed camera captures a deformation shot of the test specimen each 0.4 ms. Three-track white spots were marked on the specimen at 150 mm intervals to capture deformations (see Fig. 4). A collection of Microsoft Excel spreadsheets, Origin 19 pro, CAD design applications, and picture editing software handles the specimens’ white spots data. Both qualitative and quantitative data were evaluated.

2.2 Material Properties

The concrete properties were measured using the “Standard for Test Methods of Mechanical Properties of Concrete” (GB50081-2010) on concrete cubes to determine the average compressive concrete strength. The quasi-static tensile tests on steel reinforcement bars were performed according to the “Metal Material Tensile Test Method” (GB/T228-2010) [28]. Table 1 summarizes the materials’ properties and figures of testing procedures of materials (Table 2).

Table 2 Details on the materials of the test specimen
Full size table

2.3 Details of Test and Analysis

The specimens are positioned in the impact test machine to demonstrate the structural specimen’s dynamic response properties under the asymmetrical lateral impact (see Fig. 4). The specimen’s compression zone concrete was shattered. Large bending fractures approach the compression zone. Once the ultimate deflection of the specimen exceeds 1.1% of the clear span, it is indicated that the specimen has failed related to bending.

3 Failure Modes

Figure 5 shows the final failure mode of specimens after impact load. It seems that all RC specimens have suffered a shear failure. Damaged concrete looks to have lost its bearing ability at impact. Arrows indicated the left support has flexural fractures, while the dotted line has a brittle shear failure. Observe how the drop height affects the RC specimens’ failure patterns. The 1 m hammer specimen had a shear failure at the impact point and a vertical fracture at the left support. The impact point’s bottom vertical crack did not propagate, but the support’s left and right vertical cracks developed. Figure 5-a shows brittle shear failure with significant concrete cracking and buckling of steel reinforcing bars in the diagonal crack zone. A larger longitudinal and transverse reinforcement ratio improve the impact resistance of specimens.

Fig. 5
2 photogrpahs of specimens after impact tests. The square specimens labeled F H 1 to F H 4, have fewer cracks indicating flexure failure and breakage points indicating shear failure, than 4 circular specimens labeled Y H 1 to Y H 4.

Failure modes for specimens after the end of the impact test

Full size image

On the other hand, Fig. 5-b shows the total damage of circular cross-section specimens is rather high. Compared to square shape specimens, the steel bar is stretched whenever longitudinal reinforcement fails between the impact point and the failure surface. In the final failure mode, square specimens seem to have more bearing ability than circular specimens. In summary, the cross-sectional shape affects the distribution of contact damage, which controls the pattern of specimens’ failure and results in the difference between failure surfaces [29, 30].

4 Test Results and Discussion

4.1 Time History of Impact Load Analysis

Figure 6 illustrates an impact force-time history curve that may be separated into different stages; a triangle pulse is produced by contacting the impact body with the specimen at zero time. In the stable stage, the impact force varies within a particular range and lasts for a significant duration. The impact force begins to attenuate after a steady stage and reaches zero. The peak impact force for square shape specimens is focused between 294 and 486kN in less than a second (4 ms), while circular shape specimens are concentrated between 209 and 305kN in less than a second (2 ms). FH1 and YH1 have the least impact force once compared to other specimens due to their lower impact height. The deformation of the specimens transfers and dissipates a large amount of kinetic energy. The peak impact force for specimen FH2, notable compared to other specimens, increases as impact velocity increases with stable fluctuations on the plateau value. This observation is consistent with prior impact test patterns in previous studies [31, 32]. Simultaneously, increasing impact height for YH2 might result in a lower plateau value and shorter impact duration. In the plateau stage, increasing the longitudinal reinforcement had a limited effect on the plateau impact force for FH3. Still, it enhanced the peak impact force and improved the plateau value for YH3. With extended impact duration in consideration, increasing the stirrup ratio causes a violently fluctuating plateau value for FH4 with a minimal influence on the peak impact value, which differs from prior studies [33, 34]. By increasing the peak impact force and decreasing the plateau value for YH4, the stirrup ratio has a greater influence on strengthening the specimen’s stiffness than the reinforcement ratio.

Fig. 6
4 multiline graphs plot impact force versus time. The first, second, third, and fourth graphs plot F H 1 and Y H 1, F H 2 and Y H 2, F H 3 and Y H 3, and F H 4 and Y H 4, respectively, with dense variations indicating declining trends.

Impact force time history curves

Full size image

4.2 Time History of Deflection Analysis

Deflection time history curves were calculated by tracking the specimen’s middle three spots with the high-speed camera (see Sect. 2.1, Fig. 4). The deflection data collected by the impact body starts once it contacts the specimen and ends when it stops falling. The deflection time history curves for specimens are shown in Fig. 7. There are two distinct stages in the vertical deflection curve of square shape specimens. The deflection increases from zero to the maximum value during the loading stage, then drops from the maximum to the residual value during the unloading stage. While there is just one stage (loading stage) in which the deflection goes from zero to the maximum value for circular shape specimens, data will be lost in the later stage owing to the concrete fragments collapsing. Table 3 summarize the impact, deflection, and duration findings for square and circular shape specimens.

Fig. 7
4 multiline graphs plot deflection versus time. The first, second, third, and fourth graphs plot increasing lines for F H 1 and Y H 1, F H 2 and Y H 2, F H 3 and Y H 3, and F H 4 and Y H 4, where Y H 1, F H 2, F H 3, and F H 4 are higher in their respective graphs.

Deflection time history curves

Full size image
Table 3 Test specimen results collected from experimental outcomes
Full size table

For the FH2 specimens, both the peak and residual deflection increase with the drop hammer height. With the same reinforcing configuration, impact height affects deflection rates for YH2. As shown on the FH3 specimen, the reinforcement ratio improves the members’ lateral stiffness, which was observable during the loading stage. Deflection change for YH3 is decreased and slowed by increasing the reinforcement ratio. The maximum deflection is slowed when the stirrup ratio is increased. The YH4 deflection value is increased by damaged concrete between the impact point and the right-end support. Compared to other specimens, the negative bending fracture of the YH4 distant bearing is practically penetrated, which differs from prior studies [35, 36]. Compared to previous specimens, the FH4 and YH4 impact specimens deflect significantly.

4.3 Effect of Cross-Section Shape

Figure 8 shows the impacts of various shapes cross-sections on peak impact force, plateau, duration, and deflection for the specimen parameters listed in Table 1 (see Sect. 2). The square specimen FH2 with an impact height of 2 m has the largest peak impact force because its squaring shape protects the concrete core body, as shown in Fig. 8-a. Since it transmits and dissipates much kinetic energy, the circular specimen YH1 with an impact height of 1 m has the least peak impact force. Simultaneously, Fig. 8-b shows the effect of plateau force on the specimens’ shape cross-section. Although increasing the longitudinal reinforcement ratio had a limited influence on the plateau force, square specimen FH3 with a high longitudinal reinforcement ratio had the least plateau force. YH3’s peak impact force and plateau value were improved as a result. At the same time, the plateau values for square specimen FH2 improve as the impact velocity increases. Due to the concrete collapsing of circular specimens, which have a smaller peak impact force and bearing capacity than square specimens. As seen in Fig. 8-c, the impact duration, square-shaped cross-section specimens consume longer than circular specimens during the impact period [37, 38]. As Fig. 8-d illustrates for specimen FH2, increasing impact velocity creates maximum deflection. The YH4 and FH4 distant bearings’ negative bending fractures have been almost penetrated. As a result, increasing the stirrups ratio does not affect the deflection value for square and circular specimens [39].

Fig. 8
4 grouped bar graphs plot peak impact force, plateau impact force, impact duration, and deflection versus specimen parameters, with bras for circular and square shapes. The square shapes mostly have higher values than circular shapes.

Effect of specimen’s shape cross-section

Full size image

5 Conclusion

The experimental work on RC square and circular shape cross-section specimens under asymmetrical lateral impact loads was summarized in this study. The impact height, reinforcement, and stirrups ratio were investigated and explored. These factors have been compared in terms of (impact force and deflection record). Both square and circular-shaped RC specimens subjected to asymmetrical impact loading conditions tend to fail brittle, with diagonal cracks extending from the impact point down the right support. The drop hammer height affects crack width early in the impact stages for square shape specimens. The stirrups reinforcement ratio was one of the most important factors influencing square RC specimens’ performance and failure mode. The results of this experiment demonstrated that decreasing the stirrups reinforcement spacing from 100 to 50 mm improves the stiffness of the circular shape specimen. The increased passive pressure from the stirrups then decreases the concrete’s overall and local failure. Thereby minimizing the maximum deflection of the square shape specimens by 8.5%. In addition, longitudinal reinforcement greatly protects the concrete core and mitigates the specimen’s maximum deflection of both shapes. The experimental outcomes presented in this work showed an increase in longitudinal reinforcement for square shape specimens from (2.01%) to (6.16%), indicating an increase in crack numbers. Simultaneously, these cracks’ width is smaller in square shape specimens than in circular shapes containing a high longitudinal reinforcement ratio, resulting in a decrease in the maximum deflection of the square RC specimens by 41.5%.