Study On Grading Effects Of Ultra-high Performance Concrete

Ultra-high Performance Concrete (UHPC) has become the focus on developing cement-based composite materials due to its ultra-high strength and outstanding durability. As a brittle material, it is still difficult to overcome its inherent lower tensile strength in the engineering. Thus, to improve the tensile strength, compressive strength and toughness of UHPC has attracted increasing attention of concrete researchers.Gradation of commonly used micro-powder (usually finer than Portland cement) and Portland cement in UHPC was studied in this Dissertation. With the guidance of Compressible Packing Model (CPM), the dense packing effect of micro-powder gradation on cement-based composite paste at a low water-binder ratio was researched, and the grading effect of micro-powder on rheological and hardening properties of cement paste was also discussed. Different types of fibers were added and their mix ratio was changed to cope with the poor tensile strength of UHPC under the premise of meeting the modeling technology. The grading effect of fibers was investigated via the study on its splitting tension strength and direct tensile strength.Through the systematic experimental research and analysis to the grading effect of micro-powder on cement-based composite binder of UHPC and the grading effect of steel fiber, the major research achievements were summarized as follows:(1) Theoretical basis and fundamental assumption of CPM were discussed and key equations related to CPM were deduced in detail, and meanwhile, two important problems concerning its application in powder packing density calculation were discussed: 1) Minimum solution of the nonlinear equation was proved mathematically correct for solving the packing densityФand its calculation was realized by MATLAB programming; 2) Parameter setting, forward and inverse computation methods for multi-phase particulate system with different powder size distributions was determined.(2) Enhancement mechanism and synergetic effect of fibers were analyzed and its synergetic effect was deduced. The results show that, under the condition of meeting the modeling technology, the synergetic effect is reflected mainly in two aspects: first, the presence of fiber improves tensile strength and strain of concrete matrix for incipient crack; second, it enhances the interfacial situation between fibers and concrete matrix significantly, thus increasing the ultimate tensile stress after fracture.(3) Gradation of micro-powder is related closely to rheological property of freshly mixed cement paste both initially and at an elapsed time. The better the gradation, the lower the yielding shear stress and plastic viscosity, and the smaller the loss of rheological performance.(4) The lower the water-binder ratio, the faster the increase of yielding shear stress and plastic viscosity of cement paste. The improvement of gradation however may be in favor of relieving the influence of low water-binder ratio on the rheological property at elapsed time, which may lead to indistinctive change of rheological parameters at elapsed time.(5) Lower water-binder ratio brings the dense packing effect of micro-powder into full play. At a lower water-binder ratio, cement-micropowder gradation is related closely to compressive strength of its hardened cement paste. Desirable gradation means a denser packing density of the solid particulate system and thus a well compacted packing structure, which will lead to a higher compressive strength. The dense packing effect will not be the major factor affecting the compressive strength any more in case of a relatively higher water-binder ratio because the particles do not contact closely each other any longer for this moment.(6) Combination of ultra-fine fly ash (UFA) and silica fume (SF) may bring a denser packing structure than the single application of UFA, thus, with a desirable gradation. At an extremely lower water-binder ratio, the dense packing effect of micro-powder influences significantly performance of cement paste with a denser packing structure, hence playing a significant role in increasing the compressive strength of cement paste.(7) Gradation of micro-powder affects the dry shrinkage rate of hardened cement paste to a certain extent. The dry shrinkage rate of hardened cement paste depends partially on the packing density of solid powder system when the particle size of the mineral micro-powder is within a certain range. It decreases with the increase of packing density and the improvement of gradation. In case of too fine micro-powder, the dry shrinkage rate of hardened cement paste may however be increased due to the subsequent large amount of assumption of free water.(8) Maximum volume fraction of micro-fiber, short fiber and long fiber was determined to be 6.5%, 6.0% and 7.0%, respectively, under the premise of meeting the successful manufacture of specimens. There exists a fiber grading effect in case of a combination of different fibers at the same time. The maximum volume fraction was higher when two types of fibers were added than one type of fibers was added. The maximum volume fraction was 7.7% in case of an incorporation of short fiber and long fiber. It went up to 8.0% when both micro-fiber and long fiber were added at the same time and the value peaked as 8.5% when micro-fiber and short fiber were added together into the mixture.(9) Mechanical properties of UHPC were tested in the presence of two different types of steel fibers. The test result of its splitting tension strength shows that optimum mix ratio of steel fibers is that: short fiber to long fiber, micro-fiber to short fiber and micro-fiber to long fiber is 2:1, 1:2 and 2:1, respectively. The test result of direct tensile strength however indicates a common optimum mix ratio of 1:1 for steel fibers.(10) The results also support the fiber grading effect when short fiber and micro-fiber (or long fiber) are added into UHPC at the same time. No obvious fiber grading effect was observed in case of an incorporation of micro-fiber and long fiber. Combination of two types of fibers in UHPC seems more effective in enhancing the tensile strength of UHPC than addition of mono-type fibers when the total volume fraction of fibers keeps constant. Simultaneously, by calculating the theoretical tensile strength of UHPC for initial crack and comparing the theoretical value with the testing result, the theoretical value was found to be identical basically to the experimental results. These further validated the assumption of fiber synergetic effect.(11) The complete axial tensile stress-strain curve of UHPC prepared with an addition of 3% micro-fiber, short fiber or long fiber was plotted and analyzed. The result shows that all fibers contribute to higher ultimate tensile strength of UHPC no matter what type it is. Short fiber is the best effective one to enhance its direct tensile strength. Micro-fiber ranks second but it does not bring any help to the improvement of tensile ductility.(12) Based upon complete axial stress-strain curve analysis of UHPC with addition of two types of fibers, its direct tensile strength was found to be extremely enhanced when a desirable fiber mix ratio was kept. And meanwhile, the multiple cracking of UHPC was realized and its fake strain hardening was observed successfully. Compared with the complete stress-strain curve of UHPC without fibers, the result shows that not only is the direct tensile strength of UHPC greatly improved, but also its tensile ductility is significantly increased during the cracking course of UHPC, which has thus further verified the grading effect of fibers in the combined addition.