The Mechanical Properties Of Nanomaterials Reinforced Reactive Powder Concrete

Reactive Powder Concrete(RPC)with ultra-high compressive strengths,durability and toughness is made by eliminating large aggregate completely,carefully controlling the size of the fine aggregates to ensure the best possible packing,and incorporating steel fibers into the matrix.RPC exhibits excellent resistance to the penetration of aggressive agents,so structures built with RPC are expected to significantly outlast those built with ordinary concrete.At present,RPC have broad application prospects in the field of long-span bridges,concrete penstock and containment structures due to its excellent performance,however,the premature cracking of matrix can hardly be avoided in these structures owing to excessive tensile loading,environmental conditions,or improper constructions.Therefore,a new type of RPC with the functions of both high strength and high toughness has been developed in this study,which can reduce the weight of the concrete structure,improve the durability of the structure and reducing the project cost.Carbon nanotubes(CNTs)have great potential to improve the strength and toughness of cement-based materials.After effective dispersion in water with a surfactant,followed by separation using a centrifuge,MWCNTs can be stored stably for over 1 month.With the addition of 0.1 wt%MWCNT,the 7-day and 28-day compressive strengths increased by 22%and 15%,respectively.With the increasing addition of MWCNTs,strength index increasing first and decreasing afterwards.With teh addition of MWCNTs and graphene,the fracture energy and the fracture toughness of composite increased significantly.The porosity and pore size distribution results indicated that the presence of MWCNTs and graphene reduced micropore size.Scanning electron microscopy(SEM)and energy dispersive x-ray analysis(EDS)analysis showed that the nanofillers were well dispersed in the cement hydration products,debonding,bridging,and mesh filling were also observed.Reactive powder concrete consisted of many kinds of cementitious material,and it was prepared by domestically produced Portland cement and supplementary cement materials.The high fluidity RPC was obtained by adjusting the different stirrer modes.Meanwhile,the influences of curing method(standard curing,hot water curing,dry heating curing)on the mechanical properties of RPC were studied,and the characteristics of shrinkage,hydration products and hydration were also analysied.In order to improve the toughness of RPC,most researchers used high amount of steel fiber.However,the workability of RPC was reduced with the increasing addition of steel fiber,and the cracking strength was not improved significantly.MWCNTs dispersed using surfactants have been incorporated into RPC for improving the cracking strength,microstructure,and toughness.With the addition of 0.025 wt.%F-MWCNTs,the compressive strength and initial-cracking flexural strength increased by 7.2%and 36%,respectively.The uniaxial direct tensile strength of RPC can reache 10MPa,and its tensile strain was not less than 0.4%.Energy absorption capability indices(Ein,Eu,E15,E25)were formulated based on tensile load-displacement curves,and results showed that the energy absorption capabilities improved significantly as a result of incorporation of MWCNTs.The hydrated calcium silicate gel of composites produced a high bonding strength between the MWCNTs and RPC matrix,and fiber debonding were the dominant fracture processes at the interfaces.We inferred that there was synergism between the two types of fibers:the MWCNTs inhibited the growth of microcracks and the steel fiber effectively restrained macroscopic crack propagation.Such dimensional hybrids produced composites with high strength and toughness.A dynamic splitting strength test of RPC cylinder specimens was also carried out by using ?80 Split Hopkinson Pressure Bar(SHPB),and the splitting stress-time curve and damage process was recorded by using high speed camera and data synchronous acquisition system.In addition,the dynamic tensile damage was measured by the energy dissipation,and the energy change dissipation was analyzed.The experimental results showed the addition of CNTs improved the splitting tensile strength and energy absorption of RPC under dynamic loading.The enhancement of CNTs on the dynamic tensile properties of RPC was more significant when the impact pressure was low.Moreover,the influence of CNTs on the dynamic tensile properties of RPC decreased with the increase of CNTs content.Steel fibers delayed crack growth and propagation,improved the tensile damage tolerance of RPC obviously under the high speed splitting loading.The dynamic splitting tensile strength of RPC was up to 35MPa,which was roughly 26.1%higher than that of matrix.Moreover,the failure patterns of RPC remained integrity under dynamic splitting loading.The bond strength and shear strength between SF and matrix were the main factors to determine the dynamic splitting tensile properties of RPC,rather than the tensile strength of steel fibers.The Digital Image Correlation(DIC)method was used to analyze the strain distribution of RPC under dynamic splitting loading.The effectiveness of flattened Brazilian tests was verified by DIC method.The analysis results showed that the initiation time of crack was about 40 ?s before the time of peak loads.Under dynamic splitting loading,the peak strain of RPC in Y direction was up to 1%.Moreover,the addition of CNT improved the tensile strength and energy dissipating capacity of RPC in certain extent.