Impact Testing of Concrete Using a Drop-weight Impact Machine by N. Banthia, S. Mindess, A. Bentur and M. Pigeon ABSTRACT--A detailed description of the instrumented drop- weight impact machine is presented. The instrumentation, the calibration, the inertial loading correction, and the dynamic analysis of a concrete beam specimen undergoing three-point impact flexural loading are described. Some results, using such an impact testing machine, obtained from tests done on plain concrete, fiber-reinforced concrete, and conventionally reinforced concrete are presented. It is concluded that the use of such a testing machine may be successfully made in order to test cementitious materials under )mpact. Introduction The low strains associated with concrete failure place it in the category of brittle materials. Like other ceramics, concrete also exhibits stress-rate sensitivity in all the three loading configurations, viz. compression, '-3 tension',' and flexure.': This implies that the statically determined properties of concrete in the laboratory may not be used to predict the behavior of concrete Subjected to high stress rates, those associated with impact, blast, or earth- quake, Since the conventional testing machines may not be used to generate such high rates of loading, special apparatus are required. Unfortunately, a standard tech- nique for testing concrete under impact does not exist. Although various investigators '-7 have used various testing techniques, results often cannot be compared. The main reasons behind the incomparable nature of these testing techniques are the different methods of loading, the different energy-loss mechanisms and the different ways of analyzing the results. Consequently, little general agree- ment exists over the magnitude of the observed effects. Nevertheless, a general agreement exists over the necessity of a standard testing technique for testing concrete under high stress rates associated with impact. In this paper, a drop-weight impact machine, its construction, instrumenta- tion and calibration, analysis of the results, and the problems associated with its use in generating impact flexural loading are outlined. Some results obtained with normal-strength plain, high-strength plain, fiber-reinforced, and conventionally reinforced concrete are also presented. N. Banthia is Attache de Recherche, Department of Civil Engineering, Laval University, Ste-Foy. Quebec, G1K7P4, Canada. S. Mindess is Professor, Department of Civil Engineering, University of British Colum- bia, Vancouver, British Columbia V6T1 WS, Canada. A. Bentur is Profes- sor, Building Research Station, Technion-Israel Institute of Technology, Technion City, Haifa 32000, Israel. M. Pigeon is Professor, Department of Civil Engineering, Laval University, Ste-Foy, Quebec, GIK7P4, Canada. Original manuscript submitted: September 25, 198Z Final manuscript received: June 2, 1988. Experiment The Drop-weight Impact-testing Machine The drop-weight impact machine is shown in Fig. I. It has a frame 3.5-m tall mounted on a reinforced-concrete pedestal 1.5 m • 1.5 m in area and 0.9-m high. The frame is rigidly secured on top of the pedestal using 37- ram bolts. A hammer weighing 3.38 kN slides up and down the vertical posts upon being attached to a hoist. The hammer has pneumatic brakes in its body by which it can 'grab on' to the vertical posts. Once the brakes are applied, the hoist may be detached from the hammer. Upon releasing the brakes, the hammer falls freely on a beam specimen supported on two support anvils as shown in Fig. 1. The striking end of the hammer (called the 'tup') is shown in Fig. 2. The hammer may be raised to heights of up to 2.4 m above the specimen. By dropping the hammer through different heights, the applied stress rate may be varied. Instrumentation THE TUP As the hammer strikes the beam, the contact load between the hammer and the beam develops. Load measurements are made by the eight bonded strain gages placed in two 25-mm diameter holes (Fig. 2). This procedure resulted in an amplification (by a factor of three in this case) in the signals by making use of the stress concentration at the boundaries of the holes. 8'9 The circuit of the tup is shown in Fig. 2(b). THE SUPPORT ANVIL The support anvil [Fig. 3(a)] is capable of reading the vertical as well as the horizontal support reaction. These two reactions are read separately by the imbalance generated in two separate Wheatstone bridges. The vertical reaction is read from the strain gages mounted in the circular holes [Fig. 3(b)], while the horizontal reaction is read from the strain gages mounted in between the two holes [Fig. 3(c)]. The independent nature of the horizontal and the vertical reaction channels in the support anvil should be noted. ACCELEROMETERS The accelerometers (Fig. 1) mounted along the length of the beam were piezoelectric sensors with a resonant fre- quency of 45 kHz. With a resolution of 0.01 g, the accelerometers can read up to • 500 g and have an over- load protection of up to 5000 g (where g is Earth's gravita- tional acceleration). The calibration for the accelerometers was supplied by the manufacturer. Experimental Mechanics ~ 63