Transactions on the Built Environment vol 8, 1994 WIT Press, ISSN

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1 Impact effects on the primary fragmentation generated by the HE81 mortar grenade on conventional concrete and steel fibers reinforced concrete M.F. Canovas," R.G. Pamies^ J.R. Simon del Potrcf & E.M. Almansa^ Department of Civil Engineering and Construction, ETSICCP, Universidad Politecnica de Madrid, Spain ^Department of Weapons, ESPOL, Ministry of Defence ^Department of Fluids and Heat, ICAI, Universidad Pontifica Commillas ABSTRACT The war and terrorist threat spectrum widened to include small arms, artillery, grenades mortar, rockets and car bombs. Herein we are going to study the effects of the primary fragmentation generated by the HE81 mortar grenade on conventional concrete and steel fibers reinforced concrete. The goal of the testing program was to developed validated penetration formulas on conventional concrete and steel fibers reinforced concrete, to choose them that provide superior performance to withstand the penetration effects generated by the HE 81 mortar grenade with additional characteristic of a reduced cost and weight. Lethaly of the fragments generated from a cylindrical spray pattern is compared with the experimental results, checking a significantly over predict penetration depth (lethality) on the steel fibers reinforced concrete. Finally we compare several penetration experimental formulae for concrete where it accounts for the parameter that characterizes this material such as the characteristic compression strength, quantity of steel fibers and for the parameter that characterizes the fragmentation such as the variability in fragments's mass (Mott's distribution), based on the raw arena test experimental distribution, stand-off and the fragment striking velocity.

2 38 Structures under Shock and Impact INTRODUCTION Significant damage from accidental explosions can be caused by the impact of fragments which were generated during the explosions and hurled against structures or other receivers at high speed. Fragments resulting from accidental explosions can be divided into two categories. The term "primary fragment" denotes a fragment from a casing or container of an explosive source or a fragment from an object in contact with an explosive. Primary fragments are characterized by very high initial velocities (in the order of thousand of feet per second), a large number of fragments, and relatively small sizes in comparison to secondary fragments and concrete fragments formed due to partial failure or total collapse of protective elements. PRIMARY FRAGMENTS AND PENETRATION Upon detonation of a mortar grenade, the casing breaks up into fragments with varying weigths and velocities. The destructive potential of this fragments is a function of their shapes, materials, momentum and cinetic energy distributions. The initial velocity and size of the fragments are functions of the thickness of the metal container, the shape of the explosive as a whole (spherical, cylindrical, prismatic), and the sections of the container (ends, middle, etc.) from which the fragments will depend greatly on the metallographic history of the casing, its physical condition (such as dents, grooves, bends, or internal cracks or flaws), and the condition of joints, most notably welded joints. Since a primary fragment can generally be categorized as a high-speed particle with a mass much smaller than the barrier or target wich it strikes, the interaction between local penetration effects and any overall structural response engendered by the impact is not significant. The effects of impact can then be broadly grouped into two classes, namely (a) "front face" effects which include deformation of the missile upon striking the surface, possible shatter or ricochet of the missile, spalling around the point of impact in a more or less conical crater and penetration of the missile into the barrier wall, and (b) "back face" effects including the possible formation of a back face crater with spalling and/or perforation wherein the missile completely penetrates the barriers and exits with a known residual velocity.

3 Structures under Shock and Impact 39 In the development of a desing equation for the concrete penetration, the "massive penetration" case is considered, i.e., the folowing conditions are assumed: (a) the angles of obliquity and yaw are zero, i.a., both the path of the missile and the missile axis are coincident wih the normal to the surface of the barrier; (b) the missile is an inert non-deforming armorpiercing (AP) projectile or fragment; (c) the barrier or wall constitutes a uniform target of sufficient thickness that this finite dimension does not influence the penetration, i.a., it is assumed initially that back-face phenomena do not influence penetration; and (d) the loss of fragment mass during penetration is not considered. Under these conditions, the penetration (Reference [1]) is obtained: For X<2d: Z=2\/^di'%^ (1) and for X > 2d: (2) where: k: concrete penetrability constant VA f ^concrete compressive strength (ksi) N= nose shape factor D = caliber density of the fragment Wf= fragment weight (Ib) d= diameter of the fragment (in) Kg= striking velocity (kft/s) X= depth of penetration (in) fc= concrete compressive strength (ksi) The procedure of Reference [3] calculates the penetration depth into massive concrete by using the following two ecuations.

4 40 Structures under Shock and Impact 0.95 WF* V \ *=,025 ^ X* Wf (3) X=-l L_ Wf** for X>\A Wf (4) ; / ; \/ The general expression for the maximum penetration X (Reference [2]) of an armor-piercing fragment, is derived in terms of the fragment weight Wf and striking velocity %': Z = (10^) Wf ^ (5) Equation (5) is based on a concrete compression strength f% equal to 5,000 psi. Maximum penetrations of fragments in concrete of other strengths may be obtained by multiplying the value of X of ecuation (5) by the square root of the ratio of 5,000 psi to the compressive strength of the concrete in question. EXPERIMENTAL PROGRAM The test were begin in July and comprised a total number of six test: five of them were field test and the other was raw arena test. The raw test was performed in accordance to NM-2276-EMA in order to determine the number, and size of fragments and the homogeneity of the fragmentation. The field test were carried out with an HE 81 mm M51B mortar round surrounded by twelve concrete slabs. The grenade was located 1.5 m above the ground and the concrete slabs were secured to the structures (Fig 1 & 2) leaving the center of each slab at 1.5 m above the ground. Two distances from the grenade to the slabs were selected: 5 m (one test) and 2.8 m (four test). Sensors were located on the front face of each slab in order to measure the average velocity of the fragments on impact. Concrete has been designed to have 40 and 25 MPa of compressive strength at 28 days, without fibers, in cilindrical 0 15x30 cm specimen.

5 Structures under Shock and Impact 41 Cement is a 1-45 type in accordance with Spanish cement reception standard (RC-92). Siliceous sand and crushed siliceous gravel has been used throughout this work such as to have an overall aggregate size of 10 mm. Another test performed on 40 MPa concrete grave us the following results: -splitting tensile test: 3.18 MPa at 28 days. -third point loading flexural test: 4.87 MPa -clastic modulus: GPa at 28 days. at 28 days From 25 MPa concrete only elastic modulus are available, being its value about 25.1 GPa. Concrete mix proportions in weightused in this work are, for 40 MPa concrete: 1 : 0.45 : 2.49 : 2.16, and for 25 MPa concrete: 1 : 0.61 : 2.89 : 2.49, both expressed as weight-to-cement-weight ratios in the form: cement : water : sand : gravel. A high range superplastifier was used too. Round steel crimped-end fibers were used in this work. Its aspect ratio, defines as the fiber length divided by an equivalent fiber diameter, was equal to 100. To facilitate handling and mixing, fibers have been collated with water-soluble glue into bundles of fibers. Fibers have been added to concrete into three proportions: 0, 80 and 120 kg/m^, which imply 0, 1 and 1.5 in volume percentage. Moreover, plate thickness was varied from 8 to 16 cm as showed in the next table. The other dimensions of plates were 100x100 cm. TEST RESULTS Figure 3 shows the following features found on 25 MPa compression strength concrete: characteristic -experimental results are in good agreement with equations (3) and (4) (TM formula in Ref[3]). -a 50% increase in characteristic compression strength concrete reduces penetration in about 7%. Figure 4 shows the following features found on 40 MPa compression strength concrete: characteristic -for small fragments, the formula in eq. (3) and (4) slightly overpredicts the depth of peneration.

6 42 Structures under Shock and Impact -on the other hand for fragments of weight over 2 grams gives penetration depth values below those found experimentally. SUMMARY AND CONCLUSIONS Fiber dosification over 80 kg/nf does not produce substantial increase in the concrete resistance to penetration. For an optimal dosification it is recommended to increase the characteristic compression strength concrete yields average penetration reductions of around 10%. Concrete with characteristic compression strength over 40 MPa show a substantial increase in their penetration resistance. do not According to the experimental results, formula TM is recommended for predicting the penetration depths for primary fragmentation. This confirm the fact that, since the usual formulae consistently give larger penetration depth values than those found experimentally, the actual penetration resistance of the current conventional concrete is better than expected. ACKOWLEDGMENTS We would like to acknowledge Colonel JL CABANES CIP from LABINGE (MOD Spain), Captain JC FERNANDEZ CIP from LQCA La Maranosa and Captain F.RAMIREZ SANTA-PAU from POLIGONO CARABANCHEL for their efforts and contribution in making this survey possible. In addition we would like to thank those who's provided the funds that allowed this basic research to be pursued such as a DIGENIN-MOD Spain. REFERENCES 1. Book; 1 "Primary Fragment Characteristics and Penetration of Steel Concrete and Other Materials". Picatinny Arsenal, Technical Report, Draft. 2 "Structures to Resist the Effects of Accidental Explosions". TM Department of the Army "Fundamentals of Protective Desingfor Conventional Weapons". TM Department of the Army. US Army Corps of Engineers. 1984

7 Structures under Shock and Impact 43 FIGURE 1 FIGURE 2

8 44 Structures under Shock and Impact MORTAR ROUND HE 81 mm M51B FRAGMENTS PENETRATION IN CONCRETE 25 MPa EFFECT OF STEEL FIBER PENETRATION (mm) TM5-05b-1 -PICATINNY - TM * TEST 25/00 1 TEST 25/1 20 0,5 1,0 1,5 2,0 2,5 FRAGMENT WEIGHT (g) 3,0 STRIKING VELOCITY 1190,8 m/s FIGURES MORTAR ROUND HE 81 mm M51B FRAGMENTS PENETRATION IN CONCRETE 40 MP,i EFFECT OF STEEL FIOEFt PENETRATION (mm) ' TM5-1300» TEST 40/00 1,0 1,5 2,0 2,5 FRAGMENT WEIGHT (g) STRIKING VELOCITY 1190,8 m/s I FIGURE 4