Impact resistance of laterally confined fibre reinforced concrete plates

Materials and Structures/Matériaux et Constructions, Vol. 34, December 2001, pp 612-618 SCIENTIFIC REPORTS Impact resistance of laterally confined fibre reinforced concrete plates P. Sukontasukkul 1, S. Mindess 2, N. Banthia 2 and T. Mikami 3 (1) On-leave from King Mongkut Institute of Technology-North Bangkok, Thailand (2) University of British Columbia, Canada (3) Senshu University, Hokkaido College, Japan. Paper received: December 11, 2000; Paper accepted: April 17, 2001 A B S T R A C T The effect of both biaxial and uniaxial lateral confinement on fibre reinforced concrete (FRC) plates under transverse impact loading was investigated. It was found that confinement had a major effect on the failure patterns as well as on the mechanical properties of both plain and fibre reinforced concretes. With different types and degrees of confinement, the failure changed from a flexural mode to a punching shear mode or a combined flexural-shear mode. With confinement, the apparent strength of concrete increased by up to two or three times and the inertial load decreased to a small enough fraction of the measured impact load that it could be ignored in the analysis. The ability of the FRC to absorb energy also increased with increasing confinement. R É S U M É Les effets du confinement bi-axial et uni-axial sur les propriétés de plaques de béton renforcé de fibres ont été examinés en conditions de choc. Il a été trouvé que le confinement a des effets majeurs sur le mode de rupture ainsi que sur les propriétés mécaniques du béton normal et celles du béton de fibres. Avec un changement de confinement, le mode de rupture a également changé, passant de la flexion au cisaillement. En raison du confinement, la résistance du béton a doublé voire triplé, et le chargement inertiel a été réduit substantiellement. L absorption d énergie a aussi augmenté à cause du confinement. 1. INTRODUCTION In most standard tests, concrete specimens are subjected to a simple loading condition, such as four-point bending. However, in practice, concrete may be subjected to much more complicated multiaxial loading. In this study, the impact behaviour of FRC under uniaxial and biaxial confinement was examined, with a focus on the failure pattern, response of the material, mechanical properties and energy absorption capacity. 2. EXPERIMENTAL PROCEDURE Concrete with mix proportion of 1.0:0.5:2.0:2.5 (cement:water:sand:coarse aggregate), providing an average 30-days compressive strength of 44.5 MPa for plain concrete and 45.1 MPa for fibre reinforced concrete, was prepared in the form of square plates of dimensions 400 x 400 x 75 mm. Three different types of steel fibres were used (Table 1) at volume fractions of 0.5% and 1.0%. An instrumented, drop-weight impact apparatus designed and constructed in the Department of Civil Engineering, University of British Columbia (UBC) with the capacity of dropping a 578 kg hammer from heights of up to 2500 mm on the target specimen, was used to carry out the impact tests [1]. A load cell, 100 mm in diameter, with strain gauges mounted on it was rigidly connected to the impact hammer. An instrumented confinement apparatus was used to apply the lateral confinement stresses (Fig. 1). This apparatus consisted of two 50-ton hydraulic jacks and two load cells containing four strain gauges each, oriented in the x- and y-axis directions. The load cells were placed opposite Editorial Note Prof. Sidney Mindess is a RILEM Senior Member and a Member of the Board of Editors of Materials and Structures. Prof. N. Banthia is a Senior Member. He participates in the work of RILEM TCs 162-TDF (Test and design methods for steel fibre reinforced concrete) and 181-EAS (Early age shrinkage induced stresses and cracking in cementitious systems). 1359-5997/01 RILEM 612

Sukontasukkul, Mindess, Banthia, Mikami Table 1 Geometry of fibres The energy absorbed by the specimen at any particular displacement, E ab, is equal to: Eab() t = Pb () t du0 (2) Fig. 1 An instrumented confinement apparatus. to the jacks on each axis. Four heavy steel blocks, rigidly connected to a base plate to prevent horizontal displacement, were used to hold both the jacks and the load cells. Steel plates, 400 x 75 x 50 mm, were placed between the jacks and the specimen edges, in order to distribute the load as uniformly as possible along the loaded edges. The entire apparatus was mounted on the base of the impact machine, along with the 300 x 300 mm support anvil. Three series of impact tests were carried out: 1) without confinement, 2) with biaxial confinement and 3) with uniaxial confinement. In the latter two series, stresses were varied from 0 to 5 MPa, and both symmetric and asymmetric confining stresses were provided. The specimens were placed on a support anvil, which provided simple support along the four edges. The hammer was dropped from 250 mm to provide a striking velocity of 0.49 m/s and an impact energy of 1290 J. 3. IMPACT DATA ANALYSIS The energy lost by the falling hammer at any time, E(t), is equal to [1]: 2 1 1 E() t = m h 2gh 2gh Ptdt t() 2 m (1) h m h = mass of the hammer h = hammer drop height g = gravitational acceleration. If the area under the plot of tup load vs. time is known, the energy lost by the hammer at any time can be calculated. P b (t) = corrected or true bending load u 0 = mid-plate displacement. The generalized inertial load (P i ) was subtracted from the measured tup load (P t ) in order to obtain the true bending load (P b ), which is then given as [1]: Pb() t = Pt() t Pi() t (3) P t (t) = measured impact load (output from tup load cell) P i (t) = generalized inertial load. The inertial force depends on the failure mode. Several different failure modes were observed in these tests, depending on the confining pressures. Based on data both from this study and from a previous study [3], four different methods of calculating the inertial load were used. a) When the specimen failed in a flexural mode [2], the acceleration distribution from the centre of the specimen to the corners was assumed to be linear. The generalized inertial load is then given as: Pt (4) i() = 320 2 ρ. Tul. 0 d 243 b) When the specimen was biaxially confined and failed in a punching shear mode, in which pieces of the specimen separated completely, the inertial force is given as: Pt i() = mu b 0 (5) c) When the specimen was under uniaxial confinement, it failed in a flexural mode along the edge of the support anvil. The specimen was then treated as a simply supported beam under three-point bending with a width equal to 300 mm (clear span length). The generalized inertial load is then given as (ignoring the overhang): l Pt i( ) = 0. 0225ρ. u (6) 0() t 3 d) When the specimen did not fail completely or showed no sign of significant failure, the entire specimen remained in the elastic region. The inertial force is then assumed to be of a sinusoidal shape [3], and is given as: 2 l Pt i( ) =ρ. hu. t (7) 0() 4 l d = half of the clear span length of the concrete plate in the diagonal direction l = clear span length ü 0 = acceleration at the centre of the plate ρ = density of the concrete T = thickness of the plate m b = mass of the broken out piece of concrete. 613

Materials and Structures/Matériaux et Constructions, Vol. 34, December 2001 4. FAILURE PATTERN VS LOAD DEFLECTION CURVES As mentioned above, several different types of failure pattern were observed in this study: 1) Flexural Failure (Fig. 2). This type of failure occurred primarily in the specimens tested without confinement. The specimens were free to bend under the applied load without any restraint at the ends. Several cracks occurred, running randomly from one edge to the other. 2) Punching Shear Failure (Fig. 3). With biaxial confinement, the failure mode changed gradually from flexural to punching shear. With passive confinement (0 MPa), the failure was a combination of punching shear and flexure (a punching shear crack formed along the supports and several flexural cracks ran from one edge to the other). As the confining stress increased (became more active), the flexural cracks began to disappear and the failure was dominated by a punching shear crack. In some cases, failure did not occur; no large crack was found but several hairline cracks on the bottom surface or a very small plastic deflection were noted (Fig. 3-FRC at 5MPa and Fig. 4). This mode only occurred in high volume fraction (V f = 1.0%) FRC or FRC specimens tested under high confinement (i.e., 5 MPa Biaxial). 3) Flexural, or 3-point Bending Mode (Fig. 5). Under uniaxial confinement, the failure was in a flexural mode, like a beam subjected to three point bending. At both sides of the confined plane, cracks occurred along the edge of the supports; a main crack causing failure formed in the middle of plate parallel to the unconfined plane. Typical load-deflection curves of plain concrete and 1% hooked end FRC plates are shown in Figs. 6 and 7, respectively. In all cases, it was found that the response of the material changed with the degree of confinement and with the type of fibre reinforcement. In plain concrete, the load-deflection response did not change much except for the increase in peak load and toughness (Fig. 6). For FRC plates, the response of material with and without confinement was quite different. In particular, for the FRC tested under high confinement (5 MPa) or for the FRC plates with higher fibre contents, the response of the material was found to be elastic-plastic (Fig. 7). The load increased up to the peak, Fig. 3 The developpement of punching shear failure with the confined stress (a) Plain concrete and (b) FRC. (a) Plain concrete (b) FRC Fig. 2 Flexural failure patterns of unconfined plates. and then, without the specimen fracturing, fell back to zero with only a small amount of damage (or plastic deformation). It was found that both types of confinement affected the measured peak load of the specimen. The effect of biaxial confinement seemed to be more pronounced than that of uniaxial confinement. The biaxially confined specimens exhibited peak loads about 2-3 times higher than those of unconfined specimens, depending on the type of material and the degree of confinement. 614

Sukontasukkul, Mindess, Banthia, Mikami Fig. 4 Unfractured specimens: a) High fibre content FRC and b) Tested under high biaxial confinement stress. Fig. 8 Effect of 5 MPa biaxial and uniaxial confinement on the measured peak load (HE: Hooked end, CP: crimped, FE: flattened end fibre). Table 2 Measured and inertial loads of plain and FRC plates Without Confinement Fig. 5 Flexural failure of specimen tested under uniaxial confinement: a) Plain concrete and b) FRC. Table 3 Measured and inertial loads of plain and FRC plates with Uniaxial Confinement Fig. 6 Impact load-deflection curve of plain concrete plate with/without confinement. Fig. 7 Impact load-deflection curve of 1% Hooked end FRC plate with/without confinement. At 0.5% volume fraction and 5 MPa biaxial confinement, all three types of fibres exhibited similar behaviour, though the flattened end fibres seemed to be slightly better than the other two (Fig. 8). However, at high volume fractions (1%), the efficiency of all of the fibres decreased slightly, with the crimped fibre showing the largest decrease (20%). In the case of the inertial loads (Tables 2-4), which are used in correcting the measured loads, the unconfined specimens exhibited considerably higher inertial force as compared to the peak load (about 29-49% of the 615

Materials and Structures/Matériaux et Constructions, Vol. 34, December 2001 Table 4 Measured and inertial loads of plain and FRC plates with Biaxial Confinement Fig. 9 Effect of fibre content on energy lost by hammer and fracture energy of unconfined plates (FE: Fracture energy and ELH: Energy lost). measured peak load). However, when the confinement stresses were applied, the inertial forces decreased. Since confinement prevents the specimen from moving freely under impact loading, the increase in confinement stress from 0 to 5 MPa led to a significant drop in the inertial force from 27% to 8% for plain concrete and 16% to 7% for 0.5% hooked end FRC plates. In addition, it was also observed that the inertial force not only decreased with increasing confinement but also with increasing fibre content. In both unconfined and confined tests, the inertial loads of FRC with 1% V f were less than those of FRC with 0.5%V f by 1 to 15%. This is due to the fibres acting as passive confinement inside the specimen. 5. ENERGY LOST BY THE HAMMER VS FRACTURE ENERGY OF THE SPECIMEN a) Unconfined specimens For plain concrete (Fig. 9), the energy lost by the hammer (to the machine, specimen vibration, sound, heat etc.) was very small as was the fracture energy (about 50 J and 100 J for fracture energy and energy lost, respectively). For the 0.5%V f FRC (Fig. 10), the energy lost by the hammer was higher (about 800 J) and the fracture energy also increased up to 400 J. The gap between energy lost by the hammer and fracture energy was still wide, which means that a considerable amount of energy was lost during the impact. By increasing the fibre content to 1%, the energy absorption capacity of the concrete was enhanced to 770 J, and the gap between energy lost and energy absorption was reduced. The increase in the fracture energy of FRC results from the effects of fibres bridging across the cracks. Plain concrete is a brittle material; FRC, on the other hand, due to the effects of fibres stretching and bridging over the cracks, is able to sustain much higher deflections Fig. 10 Effect of fibre type on energy lost by hammer and fracture energy of unconfined plates (FE: Fracture energy and ELH: Energy lost). prior to failure. Since the fracture energy is the function of load and deflection, and if the peak loads are similar for most types of specimens, the increase in failure deflection is the principle factor that can increase the fracture energy. In FRC with higher fibre contents, as the number of fibres increased, the reduction in fibre spacing helps to promote the fibre bridging effect, leading to the occurrence of multiple peaks in the load vs deflection response [2] and longer post-peak response, and increases in fracture energy. The smaller energy loss and energy absorption of plain concrete are a direct result of the shorter impact event and smaller failure deflection than for FRC. For FRC plates, comparing 0.5% and 1.0% hooked end FRC, the impulses (given by the area under the loadtime curves) of both FRC plates were very much the same. Since the energy loss is proportional to the impulse, there were similar energy losses in both 0.5% and 1.0% FRC (750 J and 850 J, respectively). However, in term of fracture energy or energy absorption, the two different fibre volumes yielded different behaviors. The peak loads were very much the same, but the deflection at failure of the 1.0% FRC plate was 616

Sukontasukkul, Mindess, Banthia, Mikami about twice as high as that of the 0.5% FRC plate. This means that the increase in fibre content enhances the ability of concrete to carry loads at high deflection, and hence the larger energy absorption or fracture energy of higher content FRC was found. b) Confined specimens For the confined tests (Figs. 11-13), the energy lost by the hammer as it passed through the specimen increased to well above 1000 J. The confinement apparatus played an important role in this. The confined specimens allowed more energy to dissipate through them and were able to absorb more energy than the unconfined specimens (Fig. 11). Except for the case of specimens for which the impact energy was insufficient to cause rupture, the response of the material was elasticplastic: the load increased up to the peak then fell back to zero. In this case, a portion of the energy was returned to the system; the measured energy was actually elastic energy, and not fracture energy; hence, lower energy absorption was observed. For both confined plain concrete and FRC plates, the gap between energy absorption and energy lost by the hammer was still relatively large. However, the gap began to narrow as the confinement increased except in the case of high confinement and high fibre content (Fig. 12) where failure did not occur and some of the energy was then returned to the system. Comparing the three types of fibres, the performance of each fibre type also depended on the degree of confinement, for example: at 2.5 MPa biaxial confinement, hooked end and flattened end fibres performed better than crimped fibres (Fig. 13). Comparing biaxial and uniaxial confinement, both at 2.5 MPa (Fig. 14), the energy absorption of biaxially confined specimens was higher than that of uniaxially confined specimens. Comparing plain concrete and FRC, the uniaxial confinement increased the energy absorption ability by very little while with the biaxial confinement, the energy absorption increased significantly. Fig. 11 Effect of confinement on energy lost by hammer and fracture energy of plain concrete (FE: Fracture energy and ELH: Energy lost). Fig. 13 Energy lost by hammer and fracture energy of specimens tested at 2.5 MPa biaxial confinement-comparing 3 types of fibres (FE: Fracture energy and ELH: Energy lost). Fig. 12 Effect of confinement on energy lost by hammer and fracture energy of 1% FRC concrete (FE: Fracture energy and ELH: Energy lost). Fig. 14 Energy absorption: comparison between biaxial and uniaxial confinement. 617

Materials and Structures/Matériaux et Constructions, Vol. 34, December 2001 6. CONCLUSIONS 1. Both uniaxial and multiaxial confinements have a large effect on the failure patterns and mechanical properties of plain and FRC plates. 2. The failure pattern gradually changes from flexural to punching shear failure with increasing confinement. 3. Confinement improves the load carrying capacity of plain concrete and FRC, and in some cases prevents failure from occurring. Except for the crimped fibres, both hooked end and flattened end fibres exhibited higher measured peak loads than plain concrete in both confined and unconfined tests. 4. Under confinement, the inertial force decreased, depending on the fibre type and degree of confinement. 5. Higher confinement and higher fibre contents permitted more impact energy to dissipate through the specimens, and at that same time, permitted the specimens to absorb more energy. Hooked fibres performed best, in terms of energy absorption, in both unconfined and confined tests. 6. Biaxial confinement improved the mechanical properties of concrete more than uniaxial confinement. REFERENCES [1] Banthia, N., Mindess, S., Bentur, A. and Pigeon, M., Impact testing of concrete using a drop-weight impact machine, Experimental Mechanics 29 (2) 1989 63-69. [2] Sukontasukkul, P., Mindess, S. and Banthia, N., Fibre reinforced concrete plates under impact loading, 2nd Asia-Pacific Specialty Conference on Fibre Reinforced Concrete, Singapore, 1999 189-196. [3] Gupta, P., Impact behaviour of fiber reinforced wet-mix shotcrete, MA. Sc Thesis. (University of British Columbia, Canada, 1998). 618