Research On Dynamic Mechanical Behaviors Of Reactive Powder Concrete After Exposure In High Temperature

With the rapid development of modern large-span, high-rise, over-load and special environment (e.g. nuclear constructions, space facilities) engineering construction industry, just normal strength grade of concrete would not accommodate for the nowadays engineering requirements. However, reactive powder concrete, RPC for short, emerges as the times required. Its basic physical properties are greatly different from the traditional concretes. By mixing fine sands as aggregate and large amount of silica fume, high efficiency water reducing agent and minuteness steel fibers, its weak interface is substantially strengthened and fracture energy is improved to double orders of magnitude. Recently, RPC has become a kind of high strength, high toughness, low porosity super-performance concrete. Though, the RPC material has great promotional value and potential capability in bridge constructions, high-rise buildings, military shield structures and nuclear power engineering. Nevertheless, with the increases of engineering applications for RPC, this material always involves in more complex load conditions, like dynamic impact, fire burn and even load after burnt (e.g. diversion trench of rocket launching pad, experimental platform of engine ignitions). All of these require higher design rules, especially for impact resistance performances after high temperature burnt.Up to now, studies on impact resistance performances of RPC after exposure in fire or high temperature are less in open literatures. To investigate the mechanism properties after exposure in high temperature especially for the dynamic mechanical behaviors and influence of fiber mixture have effective and theoretical references on material development, application and design.Based on the background mentioned above, this paper firstly investigates the SHPB experimental techniques for concrete-like materials, to insure the reliability of experiments data; based on the experimental results, the quasi-static and dynamic properties of RPC after exposure in high temperature are investigated. And emphasis investigates the thermal effect, steel fibers and hybrid fibers reinforced effect on mechanical behaviors and temperature resistance detonation for RPC; combine with analysis of material micro-structures with scanning electron microscope, the macroscopical properties on micro-mechanism is discussed; combine with micron thermo-mechanical coupling simulation results, the steel fibers high temperature and detonation resistance mechanism is studied; a temperature modified ZWT constitutive model is published based on stress-strain curves; finally, based on the experiment data and by using a combine method of both experiment and simulation, the parameters of HJC constitutive model with strain rate (102s-1) is deduced. The investigation contents of this paper show below:Firstly, by using a SHPB experimental process simulation method, the methods for SHPB experimental techniques is investigated. Selection principle methods of input load pulses are discussed. Dispersion effect, stress uniformity, strain rate index are evaluated for three typical input waveforms (rectangular wave, semi-sine wave and triangular wave), the results show the semi-sine is an ideal input waveform which has less dispersion effect and also can fit the requirements of stress uniformity; The influence between parallelism on the double sides of specimens and experimental precision is investigated. An evident error is found with unparallel of two sides. The strain direct measurement method validity is discussed by numerical simulation; the selection method for wave head initialization is studied; finally, a self made PVDF is used to monitor the stress profiles on both sides of specimens in order to verify the experimental results.Secondly, quasi-static compression and bending experiments are preformed after exposure in high temperature RPC specimens by using compression and compress-bend testing devices. Based on the experimental results, the variety pattern between compression, bending strength and temperature is published, and corresponding empirical formulas are deduced; the influence among steel fiber ratio, compression and bending mechanical properties is discussed; the variety between inner micro-structures of RPC and temperature is discussed by a SEM micro-analysis, and connection between macro-mechanism and micro-structure is established; finally based on the micron thermo-mechanical coupling simulation results, the mechanism of steel fibers effect on detonation is investigated. The results show, the temperature gradient decreases with steel fibers, the tensile grade also decreases which could improve the RPC detonation properties after high temperature burnt.Thirdly, dynamic compression experiments under strain rate ranging from 75~85s-1 are preformed after exposure in high temperature RPC by using SHPB devices. Based on the experimental results, the influence between dynamic mechanical behaviors and temperature effect for RPC is discussed, the results show: material properties knee point with temperature is around 600℃; the influence between the dynamic mechanical behaviors and steel fibers mixture, hybrid fibers (steel fibers + PVA fibers) mixture ratio is also investigated, and hybrid effect is studied. It shows, the hybrid fibers reinforced material has better mechanical properties, a positive effect is found for hybrid fibers reinforced concrete by comparing with the same volume ratio steel fibers reinforced material; an optimum design is discussed. By the view of high temperature and impact resistance properties, and in the experiment capability of this paper, the superior hybrid reinforced ratio is: 2.0% of steel fibers and 0.1% of PVA fibers.Fourthly, dynamic compression experiments under large strain rate ranging from 75~150s-1 are preformed after exposure in high temperature RPC by using SHPB devices. Based on the experimental results, the stress-strain curves characteristic is discussed for RPC under different temperature burnt; the strain rate effect and temperature effect is analyzed based on a micro-structures and energy properties; based on the dynamic mechanical characterizes of RPC after exposure in high temperature, a temperature modification is discussed on ZWT constitutive model and the corresponding parameters are also deduced. And a best fit is got between modified model and experimental results which indicated the validation of this model for RPC material after exposure in high temperature.Fifty, by using a combined method of both experiment and simulation, and based on the analysis of physical significance for HJC parameters, the HJC constitutive parameters for RPC within SHPB strain rate range (102s-1) are determined. The validation of this model is confirmed by comparison with SHPB numerical simulation and experimental curves; a discussion of the applicability of this model, the results show, HJC model cannot get an accurate result for material fragmentation and compaction behavior, and this model only validate for initial proportion with little damage.In conclusion, the result of this paper is a basic technical knowledge for quasi-static and dynamic mechanical behaviors, effect of fibers for powder reactive concrete, and micro, meso-level recognition of its basic mechanism. The result can provide the basic experimental and theoretical ideas to RPC material development, application and fire (high temperature) resistance and has an effective and theoretical guidance values to concrete design.