106 ACI Structural Journal/January-February 2009 ACI Structural Journal, V. 106, No. 1, January-February 2009. MS No. S-2007-252.R2 received January 4, 2008, and reviewed under Institute publication policies. Copyright © 2009, American Concrete Institute. All rights reserved, including the making of copies unless permission is obtained from the copyright proprietors. Pertinent discussion including author’s closure, if any, will be published in the November- December 2009 ACI Structural Journal if the discussion is received by July 1, 2009. ACI STRUCTURAL JOURNAL TECHNICAL PAPER The presented research is part of a wider research project involving the study of the dynamic behavior under extreme loads of the Tenza Bridge, a concrete arch bridge located in southern Italy. The dynamic behavior of the concrete of the bridge under tensile loads is herein investigated. Several dynamic tensile tests under different strain rates were performed on concrete specimens at the DynaMat laboratory of the University of Applied Sciences of Southern Switzerland using modified Hopkinson bars. The results were then processed in terms of strength dynamic increase factor-strain rate relationships. These are fundamental to assess constitutive laws of concrete to be implemented in analytical models of the bridge under dynamic loads. The results are compared with existing analytical formulations that attempt to predict the dynamic tensile strength of concrete. The comparisons show that, even though tested concrete was taken from an existing structure, the relationships found in the literature accurately describe its tensile dynamic behavior. Keywords: dynamic behavior; loads; strain; tensile strength. INTRODUCTION Nowadays, structural design of some critical infrastructures has to allow for events such as blasts, impacts, or strong earthquakes, which force such structures to withstand severe dynamic load conditions, characterized by high intensity and short duration. Therefore, dynamic properties of materials play a fundamental role in the evaluation of the structural behavior of critical infrastructures under such particular events. The activities presented in the current paper, part of a wider research project—namely, the Tenza project—are related to this topic. The objective of the Tenza project is to study the effect of high dynamic loads on a reinforced concrete (RC) arch bridge that is part of the abandoned path of the Salerno- Reggio Calabria highway. The bridge was built in the 1960s and retrofitted in the 1990s. Four types of materials can be identified in the Tenza Bridge: 1) the original concrete used in the 1960s when the bridge was built—this accounts for most of the concrete in the structure; 2) the concrete used to strengthen the piers and arches; 3) the original lightly ribbed steel used for the reinforcement of the original concrete; and 4) the new ribbed steel used as reinforcement of the more recent RC portions. In the first phase of the project, the structure was charac- terized through static analysis under gravity and live loads and a complete seismic assessment. The finite element method (FEM) model used to perform these analyses was validated by comparing the numerical vibration modes with the results of a vibrodyne test. During such test, an evaluation of the vibration modes was performed processing the acceleration fields acquired using accelerometers distributed on the structure. 1 The objective of the second phase of the project was to perform an assessment of the structure under severe dynamic loads, such as impact or blast, through numerical analysis and in-place tests. For this purpose, a dynamic characterization of both concrete and steel of the Tenza Bridge, subjected to severe dynamic load conditions, was performed; the results of the activity conducted on concrete represents the specific object of the current article. In particular, high-strain-rate tensile failure tests were conducted at the DynaMat Laboratory of the University of Applied Sciences of Southern Switzerland, and stress-strain relationships were then evaluated. The strain rate range of interest for concrete was identified between 1 s –1 and 50 s –1 because these values are related to impact and blast load condi- tions. 2 The obtained results, widely discussed in the following paragraphs, are fundamental to define the influence of dynamic loads on the constitutive behavior of materials aged in a real structure and provide a reliable point of reference to be used in structural analyses accounting for strain rate effects. Indeed, knowledge of the dynamic behavior of both structure and materials is essential to perform a complete assessment of the bridge under impact or blast loads. In particular, under such load conditions, two different types of failure can be distinguished 3 : 1) local failure; and 2) global failure. The former can be due to an impact or an explosion occurring close to structural elements; the characteristics of this failure depend on the dynamic properties and ductility of the element concerned and its constituent materials. By contrast, global failure occurs after local failure and it is related to the ability of the structure to withstand the loss of elements without activating progressive collapse. It depends on global ductility properties of the structure and on the quality and frequency of connections between its elements. Obviously, the more severe the local failure, the more likely global failure is to occur. Local failure can be distinguished into local failure of materials and local failure of structural elements. The first type of failure occurs when the explosion is so close to the structure that the consequent shockwave in the air, impacting on the surface of the element, causes a high field of compression and propagates a tensile wave inside the material. Hence, these stresses can cause the concrete to crack and, consequently, a projection of debris. By contrast, the second type can occur when the failure of one or more sections within the element is activated. 4 Instead, global failure takes place after severe damage to one or more structural elements, which can lead to progressive Title no. 106-S12 Experimental Analysis on Tensile Dynamic Behavior of Existing Concrete under High Strain Rates by Domenico Asprone, Ezio Cadoni, and Andrea Prota