Advances in experimental assessment of dynamic tensile strength of concrete by the spalling technique

Abstract

Abstract. An experimental method to test concrete in dynamic tension by spalling with a Hopkinson bar as loading and measuring tool was developed in 1999. The dynamic strength of concrete specimen and strain rate were indirectly derived from an accurate data processing of the signals measured on the Hopkinson bar surface. This method suggested by late Prof. Klepaczko, allowed for reaching the highest strain rate reported in literature for which an intriguing tensile strength increase was highlighted. This simple and efficient technique has been adopted by many researchers around the world. Some significant improvements in terms of definition and reproducibility of the incident loading pulse travelling along the bar and direct and/or contactless measurements on concrete specimens have been introduced. The very high rate sensitivity of concrete tensile strength was corroborated by the additional experimental data obtained with this experimental technique during the last fifteen years. IntroductionThe most important phenomenon highlighted by the experimentation on concrete dynamics undertaken during the last decades is the rate sensitivity of the material, which means that its mechanical properties -notably the tensile strength, the Young's modulus, the Poisson ratio, the critical strain -depend on the applied loading rate. The phenomenon of increase of tensile strength with the loading rate, observed in the early sixties of the last century has been recognized as the "rate effect", and is typically represented as a dynamic increase factor (DIF), i.e., the ratio of dynamic to quasi-static strength versus strain rates. Globally, the rate dependency of concrete tensile strength can be subdivided into two regimes; a regime starting from the quasi-static range of strain rates up to about 1.0 s −1 with moderate rate effects and a regime with extensive rate effects for higher strain rates for which a DIF increased from 2 to about 7 [1]. The highest strain rates were essentially obtained with techniques based on the spalling phenomenon. Spall is defined as the ejection of fragments of a material specimen from the opposite side from which the specimen was impacted and/or impulsively loaded.The earliest experiment at very high strain rate reported by the literature was conducted on large plain concrete specimens (51 mm diameter and 838 mm length) impacted at one end by a steel sphere. The signals were recorded by means of strain gauges cemented on concrete specimen surface. In these tests, DIFs of 2.5 and 6 for 2.0 s −1 and 23 s −1 were respectively highlighted [2].This considerable strength increase was also observed about two decades later for large concrete instrumented wall panels (4,57 m × 1,65 m), each which were mounted on the side of a rectangular box reaction structure, loaded a Corresponding author: ahmedbrara@hotmail.com by surface explosion of TNT charge [3]. Pullback velocity of the ejected fragments measured during the test by means of high speed photography (10 3 pictures/s) allowed estimating the spall stren...

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