Study On Mechanical Properties And Microstructure Of Ultra-high Performance Cementitious Composite

Ultra-high performance cementitious composite(UHPCC)is a new type of civil engineering materials which developed in recent years.Compared with conventional concrete,UHPCC exhibits exceptional mechanical properties and durability,and thus can satisfy the development of modern concrete structures such as high-rise buildings,long-span structures and light-weight buildings,etc.,all of which are designed with a long service life.Therefore,UHPCC is considered a promising construction material in the future.As an important characteristic of UHPCC,the ductility of the material is mainly derived from the addition of high-strength fibers.The underlying mechanisms that contribute to the macro toughness have been well established,and include fiber pullout,fiber fracture,fiber deformation and matrix multiple cracking,etc.What is not clear,however,is quantitative analysis that can isolates the relative contributions of those toughening mechanisms.Concerning preparation of UHPCC,the value of using mineral admixtures such as fly ash have been increasingly recognized,while there is a lack of study on mechanisms that govern their macro performance.In this dissertation,toughening mechanisms of fibers in UHPCC are quantitatively studied,and the influence of mineral admixtures,especially fly ash,on the microstructure of UHPCC at low water to binder ratios(w/b)is analyzed.In addition,to promote greening of UHPCC,the feasibility of using iron ore tailings as fine aggregate to prepare UHPCC is investigated.Using a ternary system that contains cement,fly ash and silica fume as binder,a UHPCC with excellent properties was prepared and studied.The results indicate that combined addition of fly ash and silica fume compensates the flexural strength loss that happens when fly ash is individually used.Compared with standard curing to a mature age,the compressive strength is increased while the flexural strength is almost not affected after steam curing,and high-temperature dry heat curing can significantly improve the compressive strength.River sand and quartz sand give comparable strength while using bauxite as fine aggregate shows higher strength.The strength of the UHPCC increases with the increase of fiber content,and the enhancing effect of fibers on flexural strength is higher than that on compressive strength.Compared to a high performance concrete(HPC)with a w/c=0.3,the abrasion resistance of the UHPCC is 54%higher,which can be mainly attributed to the high wear resistance of the UHPCC paste.Introduction of silica fume in the material shortens the induction period at early hydration age while addition of fly ash shows a significant retarding effect on hydration.Under semi-adiabatic condition,the maximum temperature rise in the UHPCC is 6.8?higher than that in the HPC.The drying shrinkage of the UHPCC is about 300 micro strains when tested after long-term water curing,which is lower than that of the HPC.In addition,the UHPCC has a much higher resistance to the penetration of chloride ions than the HPC.The toughness of the UHPCC was characterized through conducting a three-point bending test on a notched beam,and the consumed energies by fiber pullout and matrix cracking during fracture of a specimen were determined by virtue of fiber pullout tests and X-ray computed tomography,respectively.The results show that with the addition of 1%and 2%fiber,the fracture energies of the UHPCC are 230 times and 290 times as high as that of the UHPCC matrix,respectively.Through quantitative analysis,it is found that matrix cracking contributes a small proportion to toughness,while fiber pullout takes a large part.With fiber content of 1%and 2%,the energy dissipated through matrix cracking only accounts for about 4%and 5.5%of the total input energy,respectively,while the energy dissipated by fiber pullout accounts for 50%and 70%of the total energy,respectively.The evolution of the microstructure of the UHPCC was studied by scanning electron microscopy,Rietveld X-ray diffraction analysis and mercury intrusion porosimetry.It is concluded that under standard curing,a relatively dense microstructure can be observed after 3 days,and most of the hydration completes after 7 days.At 28th day,the degree of hydration of cement in the UHPCC is close to 50%.The formation of a dense microstructure at early age can be delayed if fly ash is added individually,which is detrimental to early age strength,while this impairment can be much lowered with the combined addition of silica fume and fly ash.Compared with standard curing to 28 days,the degree of hydration of cement is slightly lower after steam curing,but the pozzolanic reaction is increased,and a very dense microstructure can be created.Besides,the micro mechanical properties of the UHPCC paste and the interfacial zone were studied by grid nano-indentation tests.The results show that the hydration products in the UHPCC are mainly high-stiffness phases.Specifically,after standard curing to a mature age,high density(HD)calcium silicate hydrate(C-S-H)and the ultra-high density(UHD)phase account for 80%of.the hydration products;under steam curing,the content of low density(LD)C-S-H is decreased while the content of the UHD phase is greatly increased,and HD C-S-H and the UHD phase account for 90%of the hydration products,which can help to understand the strength improvement of the UHPCC when subjected to steam curing.Significant quantities of unreacted cement and fly ash are present in the UHPCC paste,and as they have much higher mechanical properties than the hydration products,they can function as micro-aggregates to strengthen the UHPCC paste.The mechanical properties of the paste near the aggregate or fiber surfaces are similar to those of the bulk paste,which indicates that the UHPCC has a strong and efficient bond at the interfacial zone.From the perspective of micro mechanical behavior,these findings help to unveil the underlying mechanisms that govern the macro mechanical properties of the UHPCC.Moreover,the nano-scratch behaviors of the UHPCC paste and the HPC paste were studied.The results show that in contrast to the HPC paste,the instantaneous scratch depth and the residual scratch depth of the UHPCC paste decrease by 35%and 38%,respectively,which is consistent with the abrasion test results.To promote greening of UHPCC and sustainability of cementitious materials,the feasibility of utilizing iron ore tailings as fine aggregate in UHPCC was investigated.It is found that 100%replacement of natural sand by the tailings significantly decreases the workability and compressive strength of the material.However,when the replacement level is no more than 40%,for both 90 days standard cured specimens and steam cured specimens,the mechanical properties of the tailings mixes are comparable to that of the control mix.Therefore,from the perspective of mechanical behavior,it is feasible to use iron ore tailings to partially replace natural sand to produce UHPCC.