Role of Shear Reinforcements on the Punching Shear Resistance of Two-Way RC Slab subjected to Impact Loading

 Abstract 

The possibility of acting impulsive loads such as blast and impact on gravity load-carrying structural members such as slabs during their construction and service life cannot be ruled out. Studies on the response of RC slabs under impact loading are available in the literature. However, the role of the shear reinforcements in the slab under impact is yet to be investigated. The slab subjected to concentrated impact loading in the form of falling construction equipment, landslip hazards, and other similar events may cause punching or perforation. Under quasi-static concentrated loading, the design parameters that contribute to the shear resistance against punching are (1) concrete strength, (2) flexural steel, and (3) shear reinforcement with vertical legs. In this work, a threedimensional finite element model of a square concrete slab of normal strength concrete (30.10MPa), 1000mm x 1000mm x 75mm, with tension reinforcement only (0.88%) subjected to low-velocity drop-weight impulsive impact is developed in ABAQUS/Explicit computer code considering Concrete Damage Plasticity and Johnson-Cook models with strain rate effects and validated with the experimental results in the open literature. The drop weight is a hard steel cone frustum-headed striker/impactor with a flat impact face of a total mass of 105kg allowed to fall from 2500mm height, impacting the slab at its centroid. To study the effect of the vertical shear reinforcement on the impact shear resistance of the slab, the shear reinforcement has been varied by varying the stirrups’ width, spacing, and the number of compression reinforcement bars while keeping the impact load and other design parameters such as concrete/steel strength, support conditions, slab thickness, and flexural tension steel constant. Numerical simulations are executed and the outcomes are presented in terms of displacement-time plots, damage/stress profiles, damage modes, and damage dissipation energy. The results so extracted are discussed and compared.

1. Introduction 

In general, reinforced concrete (RC) slabs are not designed for extreme impulsive loadings but nevertheless, they are subjected to the loadings such as projectile impact and blast. Researchers had investigated the response of RC slabs subjected to impulsive loadings by hard impactors. To minimize the damage of the slabs, it seems necessary to understand their dynamic response and corresponding load-carrying mechanism under low- and high-velocity impact loadings. The degree of local and global failure modes of response depends upon the design parameters of the slab and impact. The local failure mode of response includes localized damage in the form of scabbing/spalling of the concrete, perforation, and plugging while global failure mode may include deformation of the impactor and/or deformation of the target structure through flexure, shear, or combination thereof. A large-scale laboratory test setup to conduct experimental investigations on the response of RC members under impulsive loadings (blast and impact) is of prime importance. Nevertheless, conducting such experiments in this field demands a lot of physical work in addition to the cost, time, and complexity involved with the instruments of the test is a herculean task. Numerical techniques simplify and overcome the aforesaid difficulties and give reasonable acceptable solutions. High-fidelity FEM-backed dynamic code, ABAQUS/Explicit, in vogue for solving complex problems of high strain rate loadings for numerical modeling has been used in the present work to collaborate the experimental results in the open literature.

2. Objectives

 Objectives of the research work presented herein are: 

  • To study the dynamic behavior of RC slab without shear reinforcement under low-velocity impact (<15m/sec). 
  • To investigate the influence of shear reinforcement on the punching shear resistance of the slab subjected to impact loading by varying the stirrups’ width, spacing, flexural compression reinforcement within the stirrups, and compression steel with shear stirrups under the impacted area. 

3. Review of latest literature 

In literature, a limited number of investigations have been carried out, experimentally and analytically/numerically, on the dynamic response of RC slabs subjected to impulsive impact loading. Kojima [31] tested twelve square RC slabs of five different thicknesses (60, 90, 120, 180, and 240mm) subjected to impact velocities of 100, 150, and 200m/sec. Localized damage in the form of reinforcement rupturing, concrete scabbing, and perforation was observed. It was found that the damaged area increases with the increase of the slab thickness under similar impact load. Chen and May [22] experimentally analyzed six square RC slabs of two different dimensions: (1) 760mm x 760mm x 76mm, and (2) 2300mm x 2300mm x 150mm, under low velocity impact loads (between 6.50 and 8.70m/sec). Few researchers [2, 19, 22, 25, 29, 30, 36-37, 39-40] had conducted parametric investigations to study the influence of support conditions, steel ratio, and slab thickness on the behavior of the concrete slabs under impact loading. In an experimental study, Zineddin and Krauthammer [46] evaluated the damage modes of RC slabs with different steel reinforcement ratios subjected to concentric impact load of 2608kg from different heights of 152.40, 304.80, and 609.60mm. It was found that more reinforcement in the slab resulted in localized punching failure while less reinforcement resulted in brittle concrete failure. The slab behavior was found dominated by the local failure modes with increased drop height. Berthet-Rambaud et al. [21] experimentally and numerically investigated the impact resistance of 12000mm x 4400mm x 280mm simply supported RC road shed of concrete strength 32.50MPa subjected to impact load from two different heights of 15 and 30m. The deflection values were compared and found similar to those obtained with the ABAQUS computer program. Researchers namely; Dancygier et al. [23] and Sugano et al. [42], conducted experiments to investigate the damage modes of RC slabs subjected to high-velocity impact loading. ร…gรฅrdh and Laine [3] numerically analyzed the effect of high-velocity impact on RC slabs using the ABAQUS software. In these investigations, localized behaviors such as concrete spalling, scabbing, penetration, and shear plugging were found to dominate the response of the slabs.

Mabrouk et al. [33] tested seven square RC slabs having different shear reinforcement layouts and flexural steel subjected to quasi-static load. The punching shear resistance of the slab was greatly enhanced with close stirrups’ spacing (50mm) and higher flexural tension reinforcement. Mohamed and Khattab [35] carried out dynamic analyses using the ABAQUS code to calculate the punching shear resistance of RC slab provided with shear stirrups subjected to concentric static load. It was observed that the larger diameter of the shear stirrups in the slab greatly improved its ultimate punching resistance. In addition, close stirrups’ spacing with a higher diameter of the stirrups contributed further to excels the punching capacity of the slab.

Numerous strength-enhancing techniques were utilized by several researchers [28, 38] to ameliorate the punching shear resistance of the slabs under concentrated quasi-static loading such as screw anchors, C-FRP laminates, shear heads/studs, and steel fiber reinforced concrete.

 RC slabs usually undergo punching failure mode under impulsive impact loadings. In general, the punching resistance of the slab is influenced by (1) concrete strength, (2) thickness of slab, (3) flexural tension/compression reinforcement, and (4) shear reinforcement. Literature reveals that no study to investigate the effects of the shear reinforcements on the punching shear resistance of the RC slab subjected to low-velocity impact had been carried out. This intrigues the investigators to deepen the understanding of the influence of shear reinforcement on the impact resistance of the slab.

4. Modeling of RC slab

3-D numerical model of the slab, impactor, and support system is shown in Fig. 1. The details of the flexural tension steel are given in Fig. 2. In addition to the control model (i.e., slab S0 without shear stirrups), a total of eight finite element models of RC slab with different shear reinforcement layouts as per Fig. 3 are generated in the ABAQUS computer code. It is worth noting that shear reinforcements details shown in Fig. 3 are from Mabrouk et al. [33] experimental study on quasi-static punching response of RC flat slabs. All the models are having identical tension steel with a total percentage of 0.88%, Fig. 2. The stirrups and the flexural compression bars in each direction are identical. Also, the size and spacing of the stirrups in the two directions are the same. Models S1-S6 are embedded with shear reinforcement in each direction along the middle of the slab while the last two models namely S7 and S8 are having stirrups only under/around the impacted area of the slab. In general, slabs S1 and S2 are used to investigate the influence of changing the stirrups’ spacing from 100mm to 55mm with identical tension and compression steel. Slabs S3 and S4 are intended to examine the effect of increasing the number of compression reinforcement bars with identical stirrups’ spacing and width, shown in Fig. 3. Slabs S5 and S6 are considered to analyze the influence of stirrups’ width and compression reinforcement ratio while keeping the spacing of the stirrups constant. The clear span of the slab is 900mm [19, 40]. The used stirrups and compression steel are of the same strength as the flexural tension steel but of 6mm diameter, Fig. 3.
It is noted that the repetitive impact of the drop weight has not been considered in the present analysis. The impactor is a hardsteel cylindroconical shape with a flat impacting face of 40mm diameter with 105kg mass allowed to drop (free-fall) from a height of 2500mm at rest and is arranged to have a concentric impact on the slab. Self-weight of the slab is also taken into account along with the impact load using the *LOAD_Y_GRAVITY keycard option in [1]. A little higher time step (1.0sec) than the time of freefall (0.71sec) is considered to take into account the self-weight of the impactor. All the degrees of freedom of the impactor are restrained except the translation in Y-direction [40]. The slab is supported on a support system consisting of I-beams and columns, reported in [40]. The columns have their base fixed. Default interactions in the ABAQUS code have been used to define the contact between beams and columns and between the impactor and the slab. Hinges have been introduced between the common nodes of the slab and supporting beams through connecting bolts used by Sadraie et al. [40] for stable boundary conditions of the slab (see Fig. 1).
 While generating FE models, the 8-node solid element is chosen to discretize the concrete, impactor (striker), supporting beams, and columns; and steel reinforcing bars with 2-node beam elements. Default *GEN_CONTACT interaction keycard in [1] is used to model the contacts between the impactor and the slab, and between the slab and the support system. ABAQUS consists of a material library with ample plasticity-based material models to simulate the damage and non-linear behavior [4-20, 41, 43, 45]. As per the literature, material models with supposed “simple-input” requirements are usually considered by a lot of scholars and engineers owing to the available test data for material properties. The plastic behavior of the concrete is defined in ABAQUS by Concrete Damage Plasticity (CDP) model [1, 24, 32, 34] considering strain rate effects, and the damage of the slab simulated using an erosion parameter (*ERODE keycard) based on the maximum principal strain of the concrete (i.e., 0.00357). It is noted that the original CDP model has been modified as per fib MODEL CODE 2010 (R2010) [27, 34] to incorporate the effects of strain rate on the concrete subjected to impulsive impact loading. Detailed information on the CDP can be found in [1, 24, 34]. For clarity to show the damage, the concrete elements are deleted from the FE simulation when ERODE has a value greater than 0.99 in ABAQUS [1, 10, 19, 40]. In addition, the isotropic and kinematic hardening behaviors of the reinforcement steel are defined using the Johnson-Cook approach considering a dynamic increase factor of 1.25 as suggested in UFC-3-340-02 [44]. The classical elastoplastic idealization in [1] is considered for modeling the support system as well as the striker. For acceptable accuracy and efficient mesh size, different element sizes viz. 20, 15, and 10mm were tried in ABAQUS. The static and dynamic properties of the materials are given in Table 1.
The present study on the investigation of the influence of shear reinforcements on the punching resistance of RC slab is concentric impact loading specific.
 Referring to Fig. 4, the mode of damage with punching shear including failure of the bond between embedded steel and surrounding concrete and formation of diagonal cracks along with maximum displacement, are closely matched and in excellent agreement with experimental results of Sadraie et al. [40] with 10mm finite element size.
Table 1. Static and dynamic properties of the materials, reported in [19, 40].

5. Results and discussion 

The following sub-sections discuss the numerical results obtained from the explicit finite element analyses conducted in the present work: 

5.1. Damage profile 

In general, all the slabs fail due to the crushing of concrete exhibiting brittle failure mode. Fig. 5 and Fig. 6 show the damage profiles of the slabs at their bottom and top faces. Table 2 presents the results computed for all the slabs. Referring to Fig. 5(a) and Fig. 6(a), the damage caused to slab S0 at its top surface due to the impulsive load is localized at the contact area in the form of punching shear and it propagates in flexure mode diagonally towards the corners but with reduced severity. The reduction in the severity of damage in flexure mode is evident from the fact that the concrete does not crush along the diagonals. As can be seen from Fig. 5(a) that the damage due to punching makes the reinforcing bars exposed. The slab S0 experiences total damage (DDE) of 191.23J, Table 2. To control the punching failure of the slab, shear reinforcement in the two directions is considered. 

The shear reinforcement provided in slab S1 not only decreases the punching but also flexural bond failure and formation of diagonal cracks at the bottom surface in comparison to slab S0 without shear reinforcement, Fig. 5. Application of the shear reinforcement in slab S1 decreases the total damage by 26% with respect to slab S0. 

Reduction of the stirrups’ spacing from 100mm (S1) to 55mm (S2) with identical flexural compression reinforcement lessens the severity of damage i.e., perforation and cracking both, and makes a significant decrease of the damage dissipation energy (DDE) by 41% with respect to that of slab S1. Higher shear reinforcement changes the mode of failure from punching (S1) to scabbing (S2) and controls the flexural bond failure emanated from the critical punching section in slab S1, thus minimizing the DDE of slab S2 much less than all the other slabs (S3-S8), Table 2 and Fig. 5.

The effect of increasing the compression steel from two bars (S1) to three bars (S3) with identical spacing (100mm) of the stirrups reduces the severity of diagonal cracking at the bottom surface of slab S3 but unaffected punching of the slab shows that increase in the compression reinforcement contributes only to flexural resistance of the slab subjected to low-velocity impact, Fig. 5. Further increase of the compression bars from 3 (S3) to 5 (S4) is found to slightly excels the flexural resistance without affecting the punching of the concrete supports the above inference.

350mm width of the stirrups in slab S5 with 100mm stirrups’ spacing and two bars in the compression zone, chosen in accordance with the experimental study by Mabrouk et al. [33], slightly reduces the severity of diagonal cracking without improving the concrete-steel bond failure as well as concrete perforation as compared to 240mm width of the stirrups in slab S1 with same stirrups’ spacing and compression steel, Fig. 5. A marginal decrease in the DDE of 1.80% is recorded as can be observed from Table 2. Similar observations on cracking and perforation have been found with regards to slab S6 consisting of five bars in the compression zone with 350mm width of the stirrups at 100mm spacing, Fig. 5. 

 The damage behavior of slabs S7 and S8 with the shear reinforcements provided under/around the impacted area is found comparable to that of slabs S1 and S2 with-through shear reinforcements, Table 2. It is worth mentioning that the above response of the slabs with shear reinforcements considered in this study is for concentric impact on two-way slabs with isotropic flexure reinforcements

Table 2. Computational results of the slabs (S0-S8).


5.2. Slab displacement and stresses 

The displacement profiles are shown in Fig. 7. The displacement time plot of the slabs subjected to impact load is presented in Fig. 8. 

The profiles of normal stresses in the steel reinforcement and concrete are illustrated in Fig. 9 and Fig. 10, respectively. Referring to Table 2, the nominal shear reinforcement provided in the form of stirrups does not contribute to minimizing the slab displacement except in the case of slabs S2 and S8 with close stirrups’ spacing (55mm) having ๐ด๐‘ ๐‘ฃ ๐‘“๐‘ฆ ๐‘ ๐‘†๐‘ฃ = 1.81 [26] under the applied impact load; where Asv = Area of 2-legged stirrups (56.54mm2 ), fy = 423MPa, b = stirrups’ width (240mm), Sv = stirrups’ spacing (55mm). The peak Y-displacement of slab S2 is found much less than that of the other considered slabs. The results reveal that close stirrups’ spacing (55mm) with ๐ด๐‘ ๐‘ฃ ๐‘“๐‘ฆ ๐‘ ๐‘†๐‘ฃ = 1.81 greatly improves the punching shear resistance of the slabs (S2 and S8) subjected to concentric impact. The displacement response of slabs S7 and S8 is found comparable to that of slabs S1 and S2, however, the normal stresses developed in the materials of slabs S1 and S2 are much less than in slabs S7 and S8, Fig. 8 and Fig. 9. 
 It is noted that the flexural tension reinforcement of slabs S1, S2, S4 and S8 does not yield i.e., maximum tensile stress is less than the yield stress level (423MPa) of the steel. The presence of compression steel makes the slabs respond as under-reinforced under concentric impact. The slabs provided with shear reinforcement with wide stirrups (350mm) make them respond in a similar fashion. Yielded tension reinforcement for slabs S0, S3 and S5-S7 can be seen in Fig. 9

Comparing the stresses of concrete in slabs S2 and S8, the concrete at the impacted area on the bottom surface of slab S2 gets crushed at higher compressive strength (34.03MPa > fc=30.10MPa) than the concrete of slab S8 (25.83MPa) shows that the slab S2 with-through shear reinforcements responds excellent performance with regards to displacement and stresses developed in the material(s), Fig. 10. Flexural reinforcements and shear stirrups in the two directions in the slab under the impact load seem to have supported the concrete, and make to develop the compressive stresses before its crushing. Consequently, the flexural steel and bottom horizontal part of the shear stirrups get develop tensile stresses while the concrete remains in compression.

6. Conclusions 

Altogether nine two-way reinforced concrete slabs provided with identical flexural tension steel but having different compression steel and shear stirrups layouts are analyzed under concentric impact using the ABAQUS/Explicit code in this research work. Conclusions are as follows: 


  1. (1) Ultimate failure of the slab under the impact load is essentially similar to the failure under quasi-static load [33] by crushing of concrete. 
  2. (2) Application of shear stirrups improves the punching resistance of the slabs thereby reducing the damage (DDE) by 26% in case of 100mm stirrups’ spacing having ๐ด๐‘ ๐‘ฃ ๐‘“๐‘ฆ ๐‘ ๐‘†๐‘ฃ = 0.99 and 41% in case of 55mm spacing of stirrups with ๐ด๐‘ ๐‘ฃ ๐‘“๐‘ฆ ๐‘ ๐‘†๐‘ฃ = 1.81. The slabs provided with-through shear reinforcements are found more effective than the slabs with the shear stirrups at the impacted area only with regards to displacement, damage (perforation and cracking), and lower level of stresses in the flexural steel. 
  3. (3) The effect of increasing the compression steel reduces the severity of the diagonal cracking at the bottom surface of the slabs without influencing the punching resistance shows the increase in the compression steel contributes only to flexural resistance of the slabs under impact loading. 
  4. (4) The cracking resistance of the slabs improves by increasing the width of the stirrups from 240mm to 350mm with insignificant influence on flexural bond failure and punching of the slabs. 
This work recommends the investigations on the response of reinforced cement concrete slabs with shear stirrups subjected to eccentric impact loading. To have deep insight into the dynamic response, a parametric study of varying materials, span, flexural and shear reinforcements, seems to be of considerable interest subjected to concentric and eccentric impacts.

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