ABSTRACT INTRODUCTION

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1 Improvement on impact resistance of reinforced concrete panels against projectile impact T. Ohno," T. Uchida," N. Ishikawa," Y. Kasai,& H. Tsubota,& M. Ueda," A. Kambayashi^ & T. Shira^ ^Department of Civil Engineering, National Defense Academy, Yokosuka, 239 Japan Kajima Corporation, Technical Research Institute, Tokyo, 106 Japan ^Takenaka Corporation, Technical Research Institute, Chiba, , Japan ABSTRACT To investigate and improve the impact resistance of reinforced concrete panels, the effects of lining with a steel liner on the rear face of the concrete panel, double-layering of RC panels and the use of absorber were examined. Based on test results, the effect of a steel liner on both scabbing and perforation was evaluated quantitatively by converting steel liner with a certain thickness into the equivalent concrete thickness. And how to improve the impact resistance of double-layered RC panels is discussed. INTRODUCTION The establishment of a design method for reinforced concrete (RC) structures subjected to impact load has recently become a topic of high priority in the nuclear power plant facility [1], chemical and heavy industries, especially for which high-safety requirements have to be applied besides the conventional structural design methods. It has been well known that local damage consists of spalling of concrete from the impacted area and scabbing of concrete from the back face of the target together with projectile penetration into the target. There are several techniques for improving the impact resistance of RC slabs, that is, lining with a steel plate on the impacted and/or rear face of the slab of concrete[2]; making the slab a double-layered composite slab

2 254 Structures under Shock and Impact with an elastic absorber[3] and employing a fiber reinforced concrete or a high-strength concrete as the slab materials. To improve impact resistance of RC panels, it is important to prevent concrete scabbing. For this purpose, it is necessary to design structures that could reduce or isolate the propagation of stress waves. Of the many measures available for withstanding impact loads, in this study, two types of targets; RC panels with thin steel liner on the back face and double-layered RC panels with absorber, were employed for impact tests. TESTING METHOD (1) Impact Testing Apparatus A schematic view of the impact test setup is shown in Figure 1. The impact testing facility employed in the tests consisted of a high-speed loading machine, an accumulator and a launching tube. The high-speed loading machine had a loading capacity of 490kN and a maximum loading velocity of 400cm/sec. In the tests, the air in the accumulator was instantaneously compressed to the specified high pressure by using the high-speed loading machine. The projectile was placed at the front end of the launching tube by the projectile's press ring and was ejected when the air pressure in the accumulator exceeded the restricting pressure of the press ring. In the tests, the impact velocity was kept constant at 170m/sec. To enable the target panel to move freely after impact, it was suspended by steel slings 1.2m from the muzzle of the launching tube. The projectile used in the tests was made of solid mild steel with a diameter of 35mm and had aflatnose shape. High-speed Loading Connecting Hose Optical / Velocity \ Sensor Launching Tube /^, \ Sling Figure 1 Schematic view of impact test setup. (2) Target Specimen Figure 2 outlines the dimension of the RC panels employed for the target specimen. They were 0.Cm-square RC panels with thicknesses ranging from 5.0 to 1cm. The average compressive strength of the concrete was 2MPa. Four different thin steel liners with thicknesses of 0.8, and 2.0mm were attached to the rear face of concrete panels. Steel liner was

3 Transactions on the Built Environment vol 8, 1994 WIT Press, ISSN Structures under Shock and Impact 255 attached to the concrete by stud bolts of 35mm long and mm in diameter, which were welded to the steel liner at 50mm intervals as shown in Figure 3. A total of 40 panels with various combinations of concrete panel thicknesses and steel liner thicknesses were examined Stud 175 4> ^~ 75 -t Figure 2 Dimensions of test panel. J [35 Steel Liner Figure 3 Arrangement of stud bolts. Figure 4 outlines the double-layered RC target panels. Two of RC panels with the different three thicknesses of, 4.0 and cm were combined as to sum in 9.0cm. As shown in Figure 5, three types of double-layered RC targets (Cl, C2 and C3-types) with or without absorber between two panels, were employed. To examine the effects of double-layering, the standard RC panels (Ml, M2 and M3) with the thicknesses of 9.0cm were also tested. A total of 28 specimens were used for impact tests. 55 I T"! 7(3) ^ 55 I "t! in f N R T) 0>e Cl d (cm) & 3 (X C2 C3 % i _.-U. i *- I j.-i I 160 :» >: ' " «'» *" t N Figure 4 Outlines of double-layered RC panels. p a Figure 5 Types of double-layered RC targets. TEST RESULTS AND CONSIDERATIONS (1) Quantitative Evaluation of the Effect of Steel Liner Figure 6 indicates the damage to the test panels with and without the steel

4 256 Structures under Shock and Impact liner by the relation between concrete thickness and steel liner thickness. Three limit- thickness lines are plotted. The most important limit-thickness is the perforation one, defined as the minimum concrete thickness required to prevent perforation by the projectile. As shown in Figure 6, the perforation limit thickness decreases exponentially with increase in steel liner thickness. The splitting limit thickness, defined as the minimum concrete thickness required to prevent splitting of the steel liner, can be evaluated as the boundary between the splitting and the bulging damage mode(steel liner bulged and residual deformation occurred). Since a large opening must occur in the concrete under the splitting damage mode as discussed before, the damage state of this limit line may be the same as that of the boundary between the perforation and scabbing damage mode of the test panels without steel liner. 160 B 140 [] 40 Perfection Limit [Eq.(D] I S Steel Liner Thickness (mm) Figure 6 Test results and limit thicknesses Steel Liner Thickness (mm) Figure 7 Relation between equivalent cocnrete thickness and steel liner thickness. As shown in Figure 6, the splitting limit decreases linearly with increase in steel liner thickness. The bulging limit thickness, defined as the minimum concrete thickness required to prevent bulging of steel liner, is evaluated as the boundary between the penetration and the bulging damage mode. Though it was very hard to inspect concrete scabbing for the test panels with steel liner, scabbing was assumed to occur in the inner concrete. Thus the damage state of this limit thickness is the same as that of the boundary between the penetration and scabbing damage mode of the test panel without steel liner. As shown, the bulging limit also decreases linearly with increase in steel liner thickness. The decrease in limit thicknesses of the test panels with steel liner compared to those without steel liner is regarded as

5 Structures under Shock and Impact 257 the effect of the steel liner, and it can be converted quantitatively to an equivalent concrete thickness for restraining local damage. Figure 7 shows the relationship between the equivalent concrete thickness and the steel liner thickness evaluated from the decrease in limit concrete thickness indicated in Figure 6. The three curves plotted in Figure 7, for predicting the equivalent concrete thicknesses are given by the following formulae; for Perforation Thickness: t^ = 30^ (1) for Splitting Thickness: t^. = 21*, (2) for Bulging Thickness: f^ = 156, (3) where teq,p^ ^eq,s and t^ are the equivalent concrete thicknesses(mm) for perforation, splitting and bulging, respectively, and tg is the thickness(mm) of steel liner. (2) Reduction of Local Damage by Double-layering RC panels As a typical example of test results, post-test figures of the impacted and rear surfaces in C2 and C3 types, double-layered RC panels without and with an absorber, are shown in Figure 8 and the extent of local damage in double-layered RC panels are summarized in Table 1. C2E C2F C2B C3E C3F C3B PQ v// (V *^ i/\ \/// /// ///~7A\/// /ir /// \\JL /// /// //I Figure 8 Damage state of RC panels after impact.

6 258 Structures under Shock and Impact Table 1 Summary of extent of local damage in RC panels. Name of specimen C1E GIF C1B C2E C2F C2B C3E C3F C3B Ml M2 M3 Thickness ofconcrete(cm) Front Rear normal- 9.0 high- Heavy-* Perforation O Old** 00= OO:#i#2 0 OiO=O3 03 Extent of local damage Scabbing Size of scabbing 40(cm) # # # #2 #2 #2#3 #^ O # 1#^ o= 0*0= lightweight- #2 4 [ Note/O: Front plate, # : Rear plate, Number: No. ^of specimen ] -*- Light Scabbinglimit 0 #2 #* * * > #1 #*#2 (a) Effect of concrete strength on local damage Only a few difference in the extent of local damage of RC panel cast in either Ml panels: a normal-concrete(35mpa) or M3 panels: a lightweightconcrete(40mpa), is observed. Local damage of M2 panels cast in a highstrength concrete (57MPa) were all resulted in the scabbing limit: a concrete debris is not come off from the back face but circular cracks are clearly observed around the center of the panel. Thus it can be said that the use of the higher strength concrete is effective to reduce the extent of local damage. (b) Double-layered RC panels without absorber and just contacted two panels(cl-types) Local damage on the impacted face is reduced in size by a thickness increase. Scabbing was occurred on the rear face of every panels, and as the thickness of concrete panel increases the size of scabbing increases. It can be said from the size of local damage on the rear face that it is somewhat advantageous when the thicker panel is used for the impacted face. (c) Double-layered RC panels without absorber and with 1.5cm interval between two panels(c2-types) The impacted face of every panels were perforated. Scabbing was resulted only in the panels having the thickest concrete panel for the rear. Thus, when the impacted panel of double-layered panel without absorber is easily perforated, it may be effective to use the thicker concrete panel for the rear. (d) Double-layered RC panels with rubber as an absorber(c3-types)

7 Structures under Shock and Impact 259 The front panel with the thickness of cm were all perforated, but both perforation and scabbing were occurred in the front panel with the thicknesses of 4.0 and cm. Of the local damage in the rear panel, cracks were produced in the panels having the rear panel thickness of cm and scabbing and/or scabbing limit were occurred in both the panels having the rear panel thicknesses of and cm. In this case, it can be said that in order to reduce local damage, it is more effective to set up the thicker concrete panel as the rear panel. Table 2 Comparison between test results and estimated damage states. Estimation of Local Damage by Damage Index ^^^ Severe Damage -^ ^- Light Damage Naine of Panel Perforatioṇ Spc cimen Thickness s Scabbing Scabbing 8. (cm) Limit Cl C2 C3 C1E GIF C1B C2E C2F C2B C3E C3F C3B Front Rear 0# cm cm # O # s& ^ # # ] # Test results, O : Estimated o # # # (e) Estimation of the damage state in double-layered RC panel Since there are many properties to have direct effects upon the damage mode in double-layered RC panels and their influence is very complicated, it is hard to predict the damage mode and the extent of local damage under projectile impact. Variables to be considered in estimating the damage state are concrete strength and density, the dimensions of concrete panel and projectile velocity etc.. In this study, the multiple regression analysis was employed to evaluate the damage state of double-layered RC panel. The damage state is denned as the damage index given in Equation (4). D = (di + di)/t (4) where di is the depth of penetration(cm), d^ is the depth of scabbing(cm) and t is the thickness of concrete panel. When the damage of perforation was occurred, the damage index D becomes a unity. The value of zero indicates no damage. As concrete material used in this study is the same for all specimens, such two parameters as the thickness of rear panel t^ and the residual velocity of projectile after passing through the front panel %. (calculated by using one of the existing prediction formulae of local damage: e.g., the modified NDRC formula) were chosen as the multivariate for

8 260 Structures under Shock and Impact analysis. By the multivariate analysis, the damage indices D are obtained respectively for the different types of double-layering as follows; for 01 type: D = 0.429* % (5) for C2 type: D = * % (6) for C3 type: D = 0.418*% %. -f (7) The comparison between the test results and the estimated damage states by using the expressions given in Equations (5)-(7) is presented in Table 2. The agreement between the damage states thus tested (indicated by black mark) and estimated(indicated by white mark) is remarkably good except for the C3B- specimen. CONCLUDING REMARKS The following conclusions were obtained through the tests; (1) The effect of the steel liner attached to the rear face of the concrete panel was remarkable. It prevented not only scattering of scabbed concrete debris but also perforation by the projectile impact. (2) To evaluate the effect of steel liner quantitatively, new formulae for predicting equivalent concrete thicknesses for perforation, splitting and bulging were proposed. (3) It may be concluded that double-layering and having an absorber had a considerable effect on an increase in impact resistance of RC panel. (4) In order to reduce local damage, it is more effective to set up the thicker concrete panel as the rear panel. (5) By using the multivariate analysis, it can be well estimated the damage state of double-layered RC panel. The obtained results are limited to the specified testing conditions, and further data acquisition is required to improve the accuracy and reliability of the proposed evaluation formulae. REFERENCES 1. Barr,P. et.al: UKAEA Guidelines for the Design and Assessment of Concrete Structures Subjected to Impact-1987 Edition, Kasai,Y., Tsubota,H., Ohno,T., Kogure,K. and Uchida,T. 'Experimental Study on Impact Resistance of Reinforced Concrete Slabs with Thin Steel Plates against Projectile Impact', Proc. of The International Symposium on Impact Engineering, Vol.1, pp , Ohno,T. et.al 'Impact Resistance of Double-layered RC Beam with Shock- absorber and Impact Response Analysis by Multi-mass Model', Proc. of Structural Engineering, Vol.38A, JSCE, pp , 1992(in Japanese).