Avraham N. Dancygier

High strength concrete response to hard projectile impact

Abstract High strength concrete (HSC) becomes more common in practice and may have advantageous implementations. According to existing penetration formulae HSC is expected to enhance the performance of structural elements that are designed to resist projectile impacts. However, scabbing at the rear face is expected to be more severe in elements that are made of HSC, because of the relatively high material brittleness. Therefore, it is important to enhance the ductility of HSC elements, and one possible direction is to use fibers or wire mesh reinforcement. In order to study the influence of the concrete strength and of the reinforcement type on the elements response, penetration tests were conducted on regular strength concrete (RSC) and on HSC plates, with the following types of reinforcement: 5 mm steel mesh, steel fibers, small diameter steel wire mesh, and woven steel fence mesh of various diameters. The plates were subjected to an impact of a cylindrical hard steel projectile, weighing 120 g, with a conical nose and a 1.5 aspect ration. The projectiles were accelerated by a laboratory gas gun to velocities that ranged between 85 and 230 m/sec, which were measured by an electro-optical device. By comparing the response of these plates to an impacting projectile, the effects of concrete strength and of the reinforcement were studied. Major trends of the elements behavior were studied, their responses were compared and are described herein.

Effect of reinforcement ratio on the resistance of reinforced concrete to hard projectile impact

This paper discusses the effect of the reinforcement ratio on the perforation resistance of reinforced concrete elements, and proposes a way to evaluate it quantitatively. A review of the works that refer to the reinforcement effect on the perforation resistance, is followed by a theoretical quantitative evaluation of this effect. This theoretical expression is then used to modify existing perforation formulae, such that they include the reinforcement ratio as a variable. The theoretical results are compared to experimental results of tests that were planed to observe the resistance of concrete plates with different reinforcement ratios, to hard projectile impact.

Effects of Reinforced Concrete Properties on Resistance to Hard Projectile Impact

Common design of reinforced concrete elements to resist hard projectile impact is carried out by calculating their proper thickness utilizing existing formulas. The reinforced concrete element resistance to the impact is expressed by its concrete compressive strength. The current design formulas do not allow the design of high-strength concrete (HSC) barriers, as they are limited to normal strength concrete (NSC), nor do they include other parameters that may affect the barriers resistance. These other parameters include the aggregate type and size and the reinforcement ratio, as well as the reinforcing bar diameter and spacing. Some of these parameters and their effect on the barriers performance were examined in a laboratory experimental study. The specimens in this study were made of NSC, HSC, polymer concrete (PC), and concrete with a front face layer of basalt aggregates. The reinforcement consisted of steel mesh near the plates rear faces and included smooth and deformed steel reinforcing bars, small diameter (0.5-mm) steel wires, woven and nonwoven steel meshes. A limited number of specimens included hooked steel fibers. Different reinforcement ratios and reinforcement spacings have been examined. At the end of each test, measurements of the rear face damage (crater size and volume) were documented, and the specimens perforation resistance (and its dependence on projectile velocity) was evaluated. These results indicate the advantage of HSC and PC in enhancing perforation resistance and the effect of the different concrete and reinforcement properties on the rear face damage level of the perforated barriers.

An analytical model to evalulate the static soil pressure on a buried structure

Abstract A discrete-continuous model to analyze a buried structure response to static surface loading as well as the soil gravitational load at ‘service-state’ conditions is presented. A two-degree-of-freedom model represents the structure above which a continuous vertical column represents the soil. The proposed model simulates the soil-buried structure interaction affected by the structure’s roof displacement as well as the rigid body displacement of the whole structure relative to that of the free field. The model can represent positive and negative arching and provides an understanding of the effects that various variables have on the arching type and on the structure response. Other soil-structure parameters that are included in the model are the soil and structure material properties, roof span and thickness, the structure’s height, and the depth of burial and external pressure. Simulations of a rectangular buried conduit performed by both the proposed model and by a finite element analysis yielded similar interface loads and similar influence of the problem parameters on the results. This example demonstrates the effect of the structure’s stiffness and height on the soil arching above it and on the average interface load acting on its roof. Thus, the proposed model can be used in preliminary stages of the design process to easily evaluate the effect of variables such as the structure properties on the response.

A simple model to assess the effect of soil shear resistance on the response of soil-buried structures under dynamic loads

The response of a buried structure to a surface loading is analyzed by a relatively simplistic model, yet comprehensive enough to delineate both wave propagation phenomena and effects of soil arching. Analysis of a buried circular plate response under surface impulsive loading, according to this model (which shows a good agreement with the theoretical predictions to the experimental results) enables an insight to the problems full range, from very short impulse load to long, quasi-static loading. Application of this solution to a study case shows that both wave propagation related phenomena and the arching phenomenon (related to relative displacements in the soil above the structure) may be involved in the systems response to surface impact loading. Hence, a general analysis of a buried structures response to dynamic surface loading should consider not only the wave propagation effects, but also take into account the soil arching effect.

Scaling of non-proportional non-deforming projectiles impacting reinforced concrete barriers

Abstract This paper discusses the effects of an impact on reinforced concrete barriers caused by similarly shaped but not necessarily proportional, non-deforming projectiles. Laboratory simulations of reinforced concrete barriers’ response to non-deforming projectiles’ impact that are conducted with heavier and slower projectiles (compared to the simulated ones) are considered. Based on the known formulae, most of which representing experimental results, the expressions for the velocities ratios between the simulator and the simulated projectiles that are required in order to yield equal penetration depths and equal perforation limit depths, are developed. Application of these expressions to a study case shows that in order to obtain equal results by both projectiles the heavier projectiles energy should be higher than that of the smaller one, however the energies ratio should be smaller than that which is required by perfect similitude considerations. The influences of the projectiles’ masses and diameters ratios are also examined.

A Modified Energy Method to Assess the Residual Velocity of Non-Deforming Projectiles That Perforate Concrete Barriers

In assessing vulnerability of a concrete barrier it is important to know the projectiles residual velocity, in case of perforation. The residual velocity is a measure of the resistance level provided by the barrier in case it is breached. It is also an important parameter in studying the response of barriers, whether experimentally or analytically. This paper proposes a formula to evaluate the residual velocity of a perforating non-deforming projectile. It is based on a semi-empirical approach that consists of theoretical assumptions regarding the damaged barrier under striking velocities that are higher than the perforation velocity and on empirically-calibrated parameters. The main hypothesis is that once a concrete barrier is perforated part of the projectiles striking energy is dissipated through fracture of the ejecting crater into concrete fragments, as well as additional cracking of the barrier. A parameter denoted γd is introduced as a function of the perforation-limit and striking velocities, and thus a common regression formula is deduced. A suitable energy balance and additional pertinent physical assumptions lead to formulation of an expression for the residual velocity, based on the knowledge of the perforation velocity. This approach yields a non-dimensional expression for the residual velocity, which agrees well with published experimental results (different than the ones used for calibration of the models parameters).

Response of a buried structure to surface repetitive loading

Analysis of the dynamic response of a buried structure subjected to steady-state vibrations at the soil surface is analyzed by a relatively simple model, yet comprehensive enough to delineate both wave propagation phenomena and effects of soil arching. Application of the model to a study case shows that there exists a range of external load periods which may cause significant increase of the structures deflection, and that as the frequency of the surface repetitive load increases it is more likely that it would match one of the natural periods of the soil-structure system, and special care would be needed in the design process, either to change the systems properties or to verify a proper amount of damping for the system. The analysis demonstrates the effect on the response, of the surface load period, of the depth of burial, and of the soil arching, and it shows the importance of the soil-structure systems parameters to form a basis for the design procedure of similar problems.

A soft layer to control soil arching above a buried structure

The load on a buried structures flat roof, under static conditions, is influenced by a soil-structure interaction phenomenon, known as arching. The possibility of controlling this mechanism by application of a soft layer in the soil above the roof is investigated and described herein. New definitions, that better describe various aspects of the phenomenon, are proposed, and used in a quantitative analysis of the problem. The effects of the soft layer are discussed and it is shown that there is an optimal length of the soft layer that yields minimal loading on the buried roof.

Behavior of High Ductility Cement Composite Beams under Low Impact

This paper describes part of a study that examines possible material compositions aiming at increasing and controlling the ductility of cementitious composities under dynamic loading. The work described herein focuses on the influence of adding micro or macro scale steel fibers to the cementitious composite mix on the hardened specimens mechanical properties. Small beam specimens made of 14 types of mixes were tested under static and dynamic loads, where the low-velocity dynamic loading was generated in a free-fall hammer. Variables that were examined included compression strength, single and hybrid usage of different types of steel fibers and the fibers volume ratios. The main conclusions indicate enhanced mechanical properties of the hybrid HSC specimens under dynamic loading. The results show that an enhanced material behavior, and therefore also structural response, can be engineered by designing different fiber reinforced cement composite mixes with similar compressive strengths.