| Ultra-high performance concrete(UHPC)is a new generation of building materials with compressive strength greater than 150 MPa and tensile strength greater than 7 MPa.UHPC has excellent performance of Ultra-high strength,high toughness and high corrosion resistance,so it is widely used in a variety of engineering conditions with extremely high requirements on mechanical properties and extremely cruel environment requirements,such as long-span Bridges,super high-rise buildings,military protection facilities,nuclear power plant containment,oil and gas pipelines,etc..Although the compressive strength of UHPC is as high as 150 MPa at present,it is still unable to get rid of the characteristics of the low ratio of tensile strength to compression strength of traditional concrete,and it is still difficult to reach 1/10 of the compressive strength even if a large number of micro-steel fibers are added.Therefore,how to improve the tensile and flexural performance of UHPC is an important research direction to further explore the performance potential of UHPC.In this paper,a fiber directional forming technology is proposed,which can significantly improve the tensile and flexural properties of UHPC and achieve the following innovative achievements:(1)This paper puts forward the idea of fiber orientation and verifies the effectiveness and operability of fiber orientation printing casting UHPCBased on the analysis of fiber orientation both at home and abroad research,the combination of 3 d printing technology of concrete at the same time,the 3 d printing pouring fiber orientation enhancement UHPC beam plate to demonstrate the feasibility,finally it is concluded that fiber directional UHPC efficient molding method,obtained the optimum proportion of Ultra-high performance concrete low water-binder ratio,optimum fiber directional slurry liquidity,The optimal fiber type and fiber size were selected and the optimal fiber dosage was obtained.The compressive and bending test block,axial tensile test block and dynamic i MPact compression test block with three kinds of fiber content were formed by this casting process.(2)The statics properties of fiber directional reinforced UHPC are systematically studiedBased on the above-made multi-fiber directional reinforced UHPC test block,its statics properties are systematically studied.The compressive and flexural tests were carried out on a universal testing machine,and the following conclusions were drawn :1.with the increase of fiber content,the compressive strength of UHPC rose linearly,but the increase of fiber orientation on the compressive strength of the test block was small;2.With the increase of fiber content,the flexural strength of UHPC test block is significantly improved,from 20.8MPa of the benchmark mix to 42.13 MPa of 4% fiber content,an increase of 102.5%;3.Under the condition of the same fiber content,the fiber oriented test block improved more obviously than the undirected test block.When the fiber content was2%,the flexural strength increased by 45.6%.When the fiber content was 4%,the flexural strength of the fiber undirected test block increased from 42.13 MPa to 83.19 MPa,increasing by97.46%.In the thin plate bending test,the load-deflection curves of the test block at four points under seven working conditions were obtained.By analyzing the stress-strain curves and other parameters,the following conclusions were obtained: 1.In the slightly larger bending test block,fiber incorporation is still the most efficient and direct way to improve the bending strength.With the increase of fiber content,the bending strength of the test block also continues to rise,from10.42 MPa of the benchmark mix to 26.4MPa of 4% fiber incorporation,an increase of 2.53 times.However,there is an upper limit to the improvement of the performance of fiber incorporation.When the fiber content is increased from 4% to 6%,the strength does not increase.2.Fiber orientation has the same effective and significant improvement in bending strength of thin plate test block.The highest strength improvement of 4% fiber test block with the same fiber content is also from 26.4MPa to 53.39 MPa,which is an Ultra-high 100% improvement.In order to test the axial tensile performance of fiber oriented UHPC,a "dog bone" shaped test block was made.The axial tensile test was carried out by custom universal rotating fixture,and seven groups of axial tensile stress-strain curves were calculated and processed.Under the condition of undetermined direction,the axial tensile strength of the test block increases with the increase of fiber content.Under the condition of directional fiber,the axial tensile strength of test block increases firstly and then decreases with the increase of fiber content.From the perspective of transverse co MParison,when fiber content is 2% and 4%,the tensile strength is highly sensitive to fiber orientation.For example,when fiber content is 4%,the tensile strength of fiber orientation test block is 65.25% higher than that of undirected test block,but when fiber content is 6%,the tensile strength is less sensitive to fiber orientation.The tensile strength of fiber oriented test block is only 19.67% higher than that of undirected test block.(3)The effect of fiber orientation on dynamic mechanical properties of UHPC was analyzedFirst of all,like ordinary Ultra-high performance concrete,fiber-oriented UHPC test blocks show typical strain rate effect in dynamic i MPact compression test,that is,with the increase of strain rate,the peak value of dynamic i MPact stress of test blocks also increases.In addition,the addition of steel fiber greatly improves the dynamic i MPact mechanical properties of the benchmark concrete.By co MParing the failure pattern of fiber oriented UHPC test block after i MPact compression test,it is confirmed that fiber oriented concrete makes the material more anisotropic,and can get a great strength improvement in the preset direction.It should be mentioned that in the analysis of statics performance,the test block with 4% content shows the optimal mechanical properties.When the fiber content continues to increase,all mechanical properties decrease due to poor fluidity and matrix density.However,with the increase of fiber content in dynamic i MPact compression test,the peak value of i MPact compression stress,the ability to resist multiple i MPact compression and the energy absorption performance increase continuously,showing the important contribution degree of fiber toughening in dynamic i MPact compression test.(4)Fiber orientation characterizationThe fiber orientation UHPC thin plate test block was scanned by X-ray CT,and a 3D model of fiber center line in the matrix was simulated by software,and the fiber orientation performance was quantitatively analyzed by data.It is confirmed that the fiber dispersion in all the proportions is uniform and good,and there is no phenomenon such as sinking and agglomeration,which confirms the successful control of slurry workability in the design of the proportions.Under directional printing pouring,each ratio shows a very high fiber orientation law,and the Angle between fiber and pouring direction is strictly controlled within 20 degrees. |
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