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.

Reliable analitical methods are needed to aid the analysis and design of concrete structures under impact and blast loading. Calculated results from such an analysis are often compared and fitted with physical test results to validate the method of analysis employed. Material models used in the analysis must, account for strain-rate sensitive behavior, and these material models are also based on results from experiments. Hence, reliable development of material models and analytical techniques is contingent upon correct, observation of experimental results. This paper focuses on effects which could alter the test results and influence their subsequent interpretation, such as testing machine characteristics, inertia, time delays in measured signals caused be analogue filters, and vibrational energy. All of these effects can lead to incorrect, measurement of a test response under high strain-rate loading. Examples are given of incorrect measurement, of the compressive stress-strain response of concrete at strain-rates in the order of 0.1 s -1 , where results from such tests have been obtained with hydraulic testing machines. Failure to account. for inadequacies in the testing technique affected conclusions about changes in deformation behavior (such as stiffness and axial strain at peak stress). and also led to an apparent loss of ductility. Results from impact tests on a flexural member demonstrate how vibrational effects from a falling mass can lead to incorrect conclusions about the measured contact. load.