Compaction and Spall of UHPC Concrete Under Shock Conditions

Abstract

Several dozen impact experiments were conducted utilizing gun-driven, parallel-plate impacts in several complimentary configurations to produce weak shock waves in an ultra-high performance concrete (UHPC) known as “cortuf.” This UPHC variant was developed and fabricated at the U.S. Army Engineer Research and Development Center (ERDC) in Vicksburg, MS with no fiber reinforcement or coarse aggregate. The Hugoniot of the material in the range of 0–21 GPa is reported, although at lower shock pressures the material is highly dispersive and does not transmit a shock, leading to some ambiguity when conveying the results in the construct of a shock. The results, when compared with similar studies on other concrete variants, suggest that Portland cement based concretes without fiber reinforcement display shock behavior below 3 GPa which is dependent on the formulation and curing, but above 3 GPa, most can be represented by the empirical shock relation U_S = 2.35 km/s + 1.66u_P up to at least the maximum pressure investigated in this work (21 GPa). Furthermore, it is shown that of the three types of porosity present in most concrete, in the concrete examined, only the entrapped air acts as porosity that is compacted during the shock. This investigation shows that although different grades of concrete vary widely in quasi-static compressive and tensile strength, under dynamic shock loading (uniaxial strain), most have similar yield points that can be described as a Hugoniot elastic limit of ~ 0.5 GPa or a yield strength of 0.4 GPa. In cortuf, the dynamically determined compressive yield point agrees closely with the quasi-statically determined yield (in conditions of uniaxial strain), implying very little strain-rate strengthening in UHPC, but in low strength (conventional) concrete, the dynamically determined yield is much higher than the corresponding quasi-static yield point. Therefore, the yield point of concrete in high-rate uniaxial strain is found to be independent of unconfined yield strength, and limited to an upper bound of ~ 0.4 GPa (HEL = 0.5 GPa). Post-yield compaction is strain-rate dependent in cortuf as well as other formulations for which literature data is available. In uniaxial-strain tension, cortuf has a spall strength of ~ 50 MPa, in rough agreement with the limited spall data available elsewhere, and the work shows that the spall and tensile behavior of cortuf loosely follows trends recently proposed by others.