| Ultra high toughness cementitious composites(UHTCC)are a special class of fiber reinforced cement-based composite that exhibits pseudo strain hardening and multiple cracking phenomena.The strain capacity of the UHTCC can be greater than 3%with an average crack width lower than 100 μm and the ultimate cracks spacing between 0.8 and 2.5 mm.The excellent deformation performance and very high energy absorption enable the application of UHTCC in the protection engineering.However,there are few studies on the dynamic mechanical properties of UHTCC at present.Meanwhile,the dynamic constitutive relationships and the mechanism for strain rate sensitivity of UHTCC are still unclear.In this thesis,the dynamic behavior of UHTCC was investigated by using an 80 mm diameter split Hopkinson pressure bar(SHPB).Specifically,our studies mainly focuse on the following aspects:The dynamic compressive behavior of high-strength and normal UHTCC were investigated in this study.The result showed that the high-strength UHTCC which was modified by nano-SiO2 possess higher dynamic compressive strength and energy absorption ability compared with those of normal UHTCC.Then,the dynamic compressive properties of high-strength UHTCC,including dynamic compressive strength,critical strain and specific energy absorption,were investigated systematically at strain rates ranging from 20 s-1 to 250 s-1.The relationship between the dynamic increase factor for compressive strength(DIF-a)and critical strain(DIF-ε),and strain rate were established in this study.The result revealed that the enhancement effect of strain rate on specific energy absorption of UHTCC was more pronounced than that on compressive strength.Based on the high-strength UHTCC,the influence of steel fiber on the dynamic compressive behavior of UHTCC was evaluated in terms of dynamic compressive strength,critical strain,specific energy absorption and failure patterns at different strain rates.The volume content of steel fiber was varied from 0.0%to 1.5%,whereas the volume content of polyvinyl alcohol(abbreviated as PVA)fiber remained constant at 2.0%.The result showed that steel-PVA fibers had synergy on the dynamic behavior of UHTCC and it was improved with the steel fiber content increasing.The specimens with 1.5%steel fiber showed the highest compressive strength and those with 1.0%steel fiber showed the highest critical strain and energy absorption capability.Besides,the result showed that the enhancement of steel fiber on toughness of UHTCC was more pronounced than that on compressive strength.The influences of steel fiber on the dynamic tensile behavior of UHTCC were evaluated by using Brazilian disk splitting test,and the results showed that steel fiber content has little effect on the strain rate sensitivity of dynamic tensile strength of UHTCC.When the strain rates range from 5 to 11.4 s-1,the dynamic tensile strength of HY-UT-1.5 was by 40%～50%,and the energy absorption ability were increased by 90%～112%,respectively,compared with those of normal UHTCC.SEM microscopic analysis showed the length of PVA fiber on the cross section was decreased with the increasing tensile strain rate,and the proportion of fracture failure of PVA fibers were increased.However,the failure mode of steel fiber was unchanged.Furthermore,the dynamic tensile capability of reactive powder concrete(RPC)was also studied,and the results indicated that the dynamic tensile strength of RPC was 1.5～1.6 times higher than that of HY-UT-1.5,but their energy absorption capability were similar.The multiple impact resistance of UHTCC was studied.By varying the impact velocity,different degrees of initial damage were first induced on the specimens.Then the multiple impact behavior of these specimens,including stress-strain relationship,energy absorption capability,damage degree and failure process of damaged material were investigated and discussed.The results reveal that hybrid fiber UHTCC specimens could maintain their integrity even though the dynamic peak stress was below 10.0 MPa,which significantly reduced the surface spalling of fragments.The specimens with a low-degree initial damage showed improved multiple impact resistance over those without initial damage,because the microstructure of specimens was densified under the low-velocity initial impact load.Through the comparison among four groups of specimens with 2.0%of PVA fiber and different volume fractions(0.0%,0.5%,1.0%,1.5%)of steel fiber,the specimens with 1.0%steel fiber showed the highest energy absorption capability and sustained the largest number of impact.This study investigates the combined effect of the strain rate and temperature on the compressive properties of UHTCC.Specimens were first heated to different exposure temperatures,e.g.ambient temperature,200,400,500,600 and 800℃,and subsequently,cooled to ambient temperature.Thereafter,the specimens were tested at strain rates ranging from 17.2 s-1 to 158.8 s-1.The test results show that the dynamic compressive strength of the UHTCC is enhanced at a temperature of 200℃,and subsequently,decreases with the increase in exposure temperature.The strain rate sensitivity of UHTCC is largely enhanced with the increase in exposure temperature.The possible mechanism of this phenomenon was discussed based on the high-speed photography of the crack propagation process on the surface of the specimens and microscopic observation of fibers condition on their fracture surfaces.Moreover,an empirical relationship is established to express the dynamic strength enhancement of fire-damaged UHTCC as a function of strain rate.Meanwhile,the dynamic compressive behavior of UHTCC at high temperature was investigated.The result found that the decreased ratio of dynamic compression strength and energy absorption with elevated temperature of UHTCC at high temperature and after high temperature is basically similar.The SHPB was utilized to impact UHTCC with three sets of impact pressure(0.35,0.45,0.55 MPa).The dynamic fragmentations of UHTCC after elevated temperature,e.g.ambient temperature,200,400,500,600 and 800℃ were collected to analysis the fractal characteristics.X-ray CT scanning and SEM scanning electronic microscope were utilized to observe the internal structures of the specimens after the high temperature,which explained the reason for the change of fractal characteristics of fragmentation with temperature.The results show that,the fractal dimension of UHTCC increases with impact pressure,and the dynamic compressive strength and energy dissipation capacity exponentially increases with the fractal dimension.The fractal dimension of UHTCC decreases with exposure temperature,which is in contrast with that of concrete.At 800℃,the fractal dimension of UHTCC is close to that of concrete.The PVA and steel fibers in UHTCC has a synergistic effect to change the propagating path of cracks,resulting in increased roughness of fracture surface and higher fractal dimension.With the increase of temperature in UHTCC,the weakened bridging effect of fibers,the formation of cracks due to temperature and the decomposition of hydration products lead to decrease of fractal dimension,which gradually close to that of concrete. |
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