International Concrete Abstracts Portal

This paper investigates the use of structural damping in blast response calculations. In recently published literature, there are many examples of structural damping being used in computational models with little or no experimental or theoretical justification. The use of even small amounts of damping in computational models involving nonlinear plastic response can significantly influence the response calculations. For example, for a given blast loading, a reinforced concrete slab with only 48 kPa maximum capacity and 25 percent of critical damping (a value typically recommended) will deflect the same as (i.e., provide the same level of protection as) a slab with 690 kPa maximum capacity and no damping. Clearly a fictitious damping term that provides as much as 93 percent of the resistance is problematic. Structural damping during plastic response cannot be clearly defined or verified experimentally. Therefore, the use of damping in plastic response calculations should be avoided.

Department of Defense (DOD) facilities which may be subjected to blast effects from accidental explosions are required to satisfy the safety requirements delineated in DOD 6055.9-STD, "DOD Ammunition and Explosives Safety Standards."(l) In the safety standard, Army Technical Manual 5-1300, "Structures to Resist the Effects of Accidental Explosions, "(2) is referenced for specific criteria to be used in the analysis, design, and construction of blast resistant structures. Design procedures for concrete elements are provided in chapter 4 of the manual. According to chapter 4 of TM 5-1300, mechanical splices must be capable of developing the ultimate dynamic strength of the reinforcement without reducing its ductility before they can be used in blast resistant concrete elements. Unfortunately, no mechanical splicing system is currently available which can fully satisfy these requirements. Numerous splicing systems can develop the ultimate dynamic strength of the reinforcement but none can do so without some reduction in ductility. An effort is currently underway to more accurately define the performance of mechanical splices under rapid dynamic loading. It is hoped that the results of this research will permit the use of mechanical splices in blast resistant concrete structures. Preliminary investigations have indicated that some splicing systems may be safely used in low ductility regions. In this paper, available data from dynamic tests i of mechanical splicing systems will first be reviewed. The current research effort will then be outlined, and I

The Naval Facilities Engineering Service Center (NFESC) is developing a new ordnance storage magazine that will reduce encumbered land and improve operational efficiency. Energy absorbing walls using lightweight concrete are being developed to prevent sympathetic detonation between cased munitions stored in adjacent cells. Design loads, wall response, and wall effectiveness are predicted and compared to test results from one-third scale development tests and full scale demonstration and certification tests. Specially designed lightweight concretes (or chemically bonded ceramics, CBC’s) with high porosities in excess of 50% were used in the development program. The most efficient (cost and performance) barrier wall design utilizes a composite wall consisting of an exterior reinforced concrete cover and a heavy granular fill material. The CBC which makes up the cover has a strength of 2500 psi, a unit weight of 65 pcf, and a porosity over 50%. This CBC cover mitigates initial shock on impact with acceptors while the heavy granular fill reduces wall velocity (and kinetic energy), disperses momentum, and stops fragments. The exterior magazine walls, also constructed with lightweight concrete, reduce shock loads on impact by acceptor munitions.

The bond between deformed reinforcing bars and concrete under pull-out and push-in loading was studied under dynamic loading for plain concrete, polypropylene fibre reinforced concrete, and steel fibre reinforced concrete. A universal testing machine and an instrumented drop weight impact machine were used to generate static, medium rate, and impact loading, which covered a bond stress rate ranging from 0.5 x 1 O-8 to 0.5x 10-2 MPa/s. The stress distributions in both the steel and the concrete, the bond stresses and slips, the bond stress-slip relationships, and the fracture energy in bond failure were investigated. It was found that loading rates had a significant influence on these parameters.

This paper is concerned with the description and explanation of Hardened Cement Paste (HCP) response to dynamic (explosive) loading. An experimental testing technique had been developed to study the dynamic cracking of HCP samples, using cylindrical explosive microcharges. Following the initiation of the microcharge, radial cracks propagate and measurements of their growth may be conducted. Procedures to predefine the crack path have been investigated, like preparation of linear grooves along the sample. Predefining the crack path enabled relatively simple measurements of its propagation velocity. The dynamic crack propagation velocity was found to be relatively low, within the range of 70-200 m/sec. (about an order of magnitude lower than the theoretical value). The dynamic HCP failure process was found to be usually of multicrack type. Studies of michrocharge initiation near a samples boundary provided insight into the development of scabbing cracks and of their interaction with the radial cracks propagating towards the boundary. It has been found that that crack interaction is strongly dependent on the relationship between the stress wave velocity and the crack propagation velocity.