Improving the Impact Resistance of Reinforced Concrete

Transcription

1 Advanced Materials Research Vols (2014) pp (2014) Trans Tech Publications, Switzerland doi: / Improving the Impact Resistance of Reinforced Concrete Husain Abbas 1,a, Tarek Almusallam 1,b and Yousef Al-Salloum 1,c 1 Department of Civil Engineering, MMB Chair for Research and Studies in Strengthening and Rehabilitation of Structures, King Saud University, Riyadh 11421, Saudi Arabia a abbas_husain@hotmail.com, b musallam@ksu.edu.sa, c ysalloum@ksu.edu.sa Keywords: Impact, Concrete, Slab, Fibers, SHPB, FRP. Abstract. The strategic concrete structures are often required to resist impact loads arising from the projectile strike, falling weight, blast generated missile etc. The existing structures found deficient in resisting these loads are required to be retrofitted whereas the upcoming structures are required to be designed for expected impact loads. This paper explores the ways of strengthening existing reinforced concrete (RC) structures using externally bonded carbon fiber reinforced polymer (CFRP) sheets and improving the impact resistance of concrete by mixing hybrid fibers in its production. The impact response of concrete structures is assessed using experiments involving the impact of projectiles of different nose shapes on slab specimens. The material behavior at high strain rate is established using split Hopkinson pressure bar (SHPB) testing at varying strain rates. Analytical models are developed for predicting penetration depth, scabbing thickness, ballistic limit velocity and ejected mass. The experimental results were also validated through numerical modeling using LS-DYNA. Introduction Structural impact problems, which include accidental loads such as dropped objects, collisions, explosions and penetration of fragments, are common in construction industries. These loads are also pertinent in the design of protective structures which are mainly of reinforced concrete (RC). The strategic RC structures found deficient in impact resistance are often required to be strengthened for which different techniques may be employed. The carbon fiber reinforced polymer (CFRP) sheets commonly employed for the retrofitting of RC structural elements have been used in the present study for strengthening the existing RC slab panels. For new designs, the target of achieving improved impact response was investigated by mixing varying proportions of hybrid fibers in the production of concrete. The experimental response of RC against the impact of projectiles has been studied extensively [1-7]. In these studies, the striking velocity of steel projectile varied from 150 to 1100 m/s and the unconfined compressive strength of concrete varied from 30 to 235 MPa. Though conflicting results are reported but in general most of these studies showed that a higher compressive strength of concrete enhances resistance against dynamic punching to some extent, although it also increased target brittleness. The quasi-static behavior of flexural members strengthened with carbon fiber reinforced polymer (CFRP) laminates has been well documented covering flexure, shear and bond studies [8] and lately their application for impact situation has also been investigated [9-13]. The use of externally bonded CFRP sheets has been found to significantly improve the impact response of RC beams [9-13]. Abbas et al. [14] presented experiments involving drop hammer loading on circular plain and skin reinforced concrete plates. The analysis of slabs employed strain rate sensitive elasto visco-plastic two surface model possessing the capability of predicting cracking of concrete and yielding of steel [15, 16]. The results of analysis were in good agreement with experiments of the authors and also with some other experiments taken from literature. Several studies [2, 5, 17-18] reported reduced brittleness of concrete with fibers under impact loads. However, the incorporation of fibers had insignificant effect on the penetration depth though it does reduce the visible damage. The fibers tend to arrest crack development and thus minimize the size of damaged area. Chen et al. [19] developed an analytical model on the normal perforation of RC All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, (ID: /04/14,10:24:49)

2 Advanced Materials Research Vols slabs. The effect of reinforcing bars was hybridized in a general three-stage model consisting of initial crater, tunneling and shear plugging. Simpler solutions of ballistic performances of normal perforation of RC slabs were proposed. There is a limited work available on impact behavior of concrete slabs reinforced with hybrid-fibers (i.e. fibers containing steel and plastic fibers together in different proportions). There is a need for the modifications in the available formulation for the prediction of the local impact damage in hybrid-fiber RC. The present study investigates the material behavior of concrete at high strain rate and impact performance of CFRP strengthened RC panels. The control and CFRP strengthened RC slabs were tested under the impact of projectiles fired using gas gun. The numerical model developed using LS-DYNA [20] has been validated with the experimental results. The impact response of hybrid fiber RC slabs was also investigated. The main purpose of the inclusion of hybrid fibers was to arrest the crack propagation and increase the energy absorption capacity and ductility. The prediction of local damage in hybrid-fiber RC was done by modifying the existing formulation. A simple formulation was also proposed for the prediction of the ejected mass from the front and the rear faces of the concrete slabs. SHPB Testing of Concrete In the present research work, dynamic compression testing of concrete samples was carried out on solid cylindrical and annular concrete samples of varying aspect ratios (length to outer diameter ratio = 0.25 to 1). Recently installed Split Hopkinson Pressure Bar (SHPB) compression tester (Longwin, Taiwan) was employed for the tests. The inner diameter of annular samples was 20 mm. Large variation of strain-rate (32 to 300 s -1 ) was achieved by using striker bars of varying lengths and varying pressure. The diameter of the input and output bars was 75 mm. The DIF of annular specimen was lower than the DIF of solid specimens at high strain-rates. This phenomenon strengthens the fact reported by several researchers that the lateral inertia plays an important role in rapid increase in DIF at high strain-rates. The dynamic behavior of concrete seems to be independent of the quasi-static strength of concrete. The mode of failure of concrete was a typical ductile failure at high strain-rates and brittle at low strain-rates. No significant influence of strain-rate on the initial elastic modulus of concrete was observed. CFRP Strengthened Slabs The experiments involve testing of RC slabs under the impact of projectiles at varying strike velocities. RC slabs of mm size were reinforced with φ8@100 mm c/c rebars provided on the back face of slab. There was no fiber added to this normal strength concrete. The slabs were strengthened using one layer of CFRP. The slabs were tested under the impact of projectile at varying striking velocity using the gas gun (Longwin, Taiwan). The yield stress of rebars was 575 MPa. The two opposite edges of slabs were clamped and the slabs were centered so that the projectile strikes the center of slab. The hemispherical nosed steel projectile used in the study was 40 mm in diameter and 0.8 kg in weight. The measurements included penetration depth, crater sizes, rebar strains, ejected weights and residual velocity, if any. As the craters were not exactly circular, the damaged area in plan was approximated by elliptical shape for which major and minor diameters were measured. However, the equivalent diameter of circle, Deq, representing same area as that of the elliptical shape of damaged area was calculated. The ejected weight of concrete from the back face and back face crater diameters for the strengthened slabs were quite less than the unstrengthened slabs. Thus the use of externally bonded CFRP sheet on the back face was found to be effective in containing the flying fragments. The RC slab panels were modeled numerically to obtain complete damage pattern in the slab specimens. The numerically obtained response was then compared with the experimental results. The concrete and the projectile were discretized using 8-node reduced integration solid hexahedron elements having three degrees of freedom at each node. CFRP sheets were modeled using 4-node

3 1926 Advanced Construction Technologies Belytschko-Tsay shell elements with 6 degrees of freedom at each node. The main reinforcing bars in both directions were modeled using 2-node Hughes-Lui beam elements. A mesh size of 5 mm was used for concrete elements and the corresponding reinforcement as well as the CFRP shell elements. The concrete slab and the projectile were meshed using 265,680 solid 8-node elements. The steel reinforcement was meshed using 1,440 two-node beam elements. Another 14,400 quad elements were utilized to mesh the CFRP sheet for the strengthened slabs. A total of 283,342 nodes were incorporated in the entire model including the projectile, the CFRP sheet and the steel reinforcement. The dynamic loading of the slab being penetrated by a projectile having a particular impact velocity was accomplished using the INITIAL_VELOCITY card of LS-DYNA. A comparison of numerical and experimental results for one of the CFRP strengthened RC slabs is shown in Fig. 1. The figure illustrates good validation of the numerical modeling with experiments. CFRP tearing observed in experiments (a) Front face (b) Rear face Fig. 1: Slab SA-C3 after impact with projectile velocity of m/s showing pressure contours with fringe levels in Pa at the time of maximum force. Fiber Reinforced Concrete (FRC) Slabs Experimental damaged area Aforementioned RC slabs were cast using FRC in which steel and polyolyphene plastic fibers in different proportions were mixed (Table 1). Crimped steel fibers with a length of 33 mm and a diameter of 0.20 mm were used. Plastic fibers had a diameter of 15 mm and length of 50 mm. The impact penetration tests were carried out with a gas-gun (Longwin, Taiwan). The projectiles were made of hardened steel with bi-conic nose; 0.8 kg in mass, 40 mm in diameter of aft body, and 115 mm long. The slab specimens were mounted on a stationary stiff steel frame in front of the gun. In order to place the specimen on the frame, its two opposite edges were clamped through bolts to the rigid frame. The projectile was then fired to strike the specimen with desired velocity. The measurements and observations were taken for the failure pattern, the crater diameter on the front and the rear faces, the penetration depth, residual velocity, if any, and the ejected mass of the concrete from the front and the rear faces of the specimen. There are several empirical equations available in literature for the prediction of local damage of RC targets [21-26] but these equations do not consider the effect of fibers in their formulations. It is due to this reason that the impact function term of NDRC equation was modified to incorporate the effect of hybrid-fibers. The choice of NDRC equation for the modification was based on its popularity with practicing structural designers. The impact function term, G, of NDRC equation was modified, by dividing it by exp(αp1+βp2), to incorporate the effect of hybrid fibers. Here, p1 and p2 are the volume percentages of the steel and the plastic fibers respectively; α and β are the empirical constants to be determined from the experimental data. For the present set of data, these parameters were obtained as 0.10 and 0.13 respectively. For the prediction of perforation thickness, NDRC equation was adopted. The experimentally observed penetration depths and perforation thickness were in good agreement with the predictions.

4 Advanced Materials Research Vols Table 1: Concrete mixes and their compressive strength Percent of fiber by volume (by weight) Compressive strength (MPa) Normal High Mix strength strength Steel Plastic concrete concrete (NSC) (HSC) M0 (0) (0) M1 0.6 (2.05) (0) M2 0.4 (1.37) 0.2 (8) M3 0.2 (0.68) 0.4 (0.16) M4 (0) 0.6 (0.23) M5 0.9 (3.07) (0) M6 0.6 (2.05) 0.3 (0.12) M7 0.3 (1.02) 0.6 (0.23) M8 (0) 0.9 (0.35) The perforation thickness of the slabs for the perforated and the non-perforated slabs are plotted in Fig. 2 against the impact parameter G. The lines corresponding to the non-dimensional thickness of slab (t/d) along with a band of ±5% of t/d is also plotted in these figures. For conformance with the experiments, all the data points corresponding to the perforated slabs shown in Fig. 2(a) should lie above the t/d line, whereas, the non-perforated data points plotted in Fig. 2(b) should fall below the t/d line. Figure 2(a) clearly illustrates that except one specimen (which is also very close to the slab thickness) all the perforated specimens had higher perforation thickness than the thickness of the slab (±5%). This is an expected trend because in such situations the available thickness is less than the desired thickness (= perforation thickness) to resist the perforation. Figure 2(b) also shows a success in the prediction as only two data points are above the +5% error band Perforation No perforation Perforation thickness, e /d t /d + 5 t /d - 5 Perforation thickness, e /d t /d + 5 t /d Impact function, G Impact function, G (a) Perforation data (b) No perforation data Fig. 2: Prediction of the perforation thickness using the proposed model (t = slab thickness; d = diameter of the projectile aft body). Ballistic limit velocity of the projectile can be defined as the minimum initial impact velocity to perforate the target. If the initial impact velocity is less than the ballistic limit, perforation of the target is not expected. The modified UKAEA formula [21] normally employed for the estimation of the ballistic limit was further modified for the hybrid-fibers by incorporating the same exponential function given above in the estimation of the impact parameter, G.

5 1928 Advanced Construction Technologies Figure 3 shows the variation of the ejected concrete mass from the front and the rear face of HSC slab for the same strike velocity of the projectile. The study of ejected mass derives its importance as severe material and life injuries may cause due to the ejection of the concrete fragments. The figure highlights the effect of the hybrid-fibers on the ejection of concrete mass from the front and the rear faces of the specimens. The addition of fibers (whether steel or plastic or their hybrid) reduces the detached mass from front and rear faces of concrete slabs which illustrate the role of fibers in limiting the ejection of the fragments from the concrete mass. The performance of steel fibers is found to be better than the plastic fibers. In hybrid fibers concrete, the ejected mass reduces initially when the proportion of plastic fibers is small and increases when the proportion of plastic fibers is increased. In general, increasing the quantity of fibers from 0.6% to 0.9% reduces the detached mass. Front face detached mass (kg) Front face Rear face HM0 HM1 HM2 HM3 HM4 Concrete mix Back face detached mass (kg) Front face detached mass (kg) Front face Rear face HM0 HM5 HM6 HM7 HM8 Concrete mix (a) Total volume of fibers = 0.6% (b) Total volume of fibers = 0.9% Fig. 3: The ejected concrete mass in specimens impacted by the projectile at 125 m/s (H in concrete mix legends of X-axis stands for HSC). A formula was also proposed for the mass of concrete, M, ejected from the front face of singly reinforced slabs (reinforcement on back face) [17]: 2 [ 4λd + D { 2(5D 4d ) 3 ( D d )}] πx M = ρ c e e π (1) e 24 Where, x = penetration depth; De = diameter of the equivalent crater; d = diameter of the tunnel; ρc = density of the concrete. The parameter λ is zero when the slab gets perforated else its value is unity. The shape of the front face crater was assumed to be elliptical and the shape of crater at the penetration depth was considered circular which was assumed to be equal to the diameter of the aft body of the projectile. The variation in the shape of the crater section from the front face to the depth of the penetration was assumed to be elliptical. For the purpose of simplification, the elliptical shape of the front face crater was replaced by an equivalent circle of the same area. Conclusions The dynamic behavior of concrete seems to be independent of the quasi-static strength of concrete. The mode of failure of concrete was a typical ductile failure at high strain-rates and brittle at low strain-rates. The ejected weight of concrete from the back face and back face crater diameters for the strengthened slabs were quite less than the unstrengthened slabs. Thus CFRP strengthening is effective in containing the flying fragments. The hybrid-fibers arrest the crack development and thus minimize the size of the damaged area effectively. The addition of hybrid-fibers in RC leads to a considerable reduction in the concrete mass ejection from the specimens. A simple modification was proposed to incorporate the effects of fibers in the prediction of the penetration depth, perforation thickness and ballistic limit velocity. A simple formulation was proposed for the prediction of ejected concrete mass from the front face of the RC Back face detached mass (kg)

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