Evaluation Of UHTCC Bending Toughness And Its Bonding Properties With Concrete

UHTCC(Ultra-High Toughness Cementitious Composites)has excellent deformability,impermeability,corrosion resistance,freeze-thaw resistance and self-healing properties of cracks,which can effectively control the development of cracks and improve The durability of the structure meets the needs of materials for different projects.Therefore,the development of cement-based composite materials with such properties has far-reaching significance for the development of new structures.In addition,the special properties of UHTCC make it a condition for becoming an ideal repair material,that is,using its crack control ability to improve Repair the load carrying capacity and durability of the structure.Based on the research status of ultra-high toughness cement-based composite materials at home and abroad,this paper draws on the existing research results.Firstly,the combination of fiber blending amount,fly ash type and fly ash content is designed and the flow ratio and pressure resistance are designed.Str ength,uniaxial tension and four-point bending test,the UHTCC uniaxial tensile crack development model was studied by means of DIC method.Secondly,for the UHTCC large deformation characteristics,the existing toughness evaluation method was improved,an d the four-point bending toughness was comprehensive.Finally,the bonding properties of UHTCC and concrete were studied by bonded splitting tensile test and concrete-UHTCC composite beam bending test.The strain in the interface zone was studied by combining strain gauge and DIC method.The main findings are as follows:(1)The fiber volume fraction has two opposite effects on the compressive strength,the fiber volume fraction increases,the compressive strength increases first,then decreases and then increases,and the 1.7% fiber volume fraction exhibits better compressive properties;The amount of coal ash increased and the compressive strength decreased.The uniaxial tensile ultimate strain can reach 6.38%,and the DIC results show that it has a good dispersion crack development mode.The results of four-point bending test show that UHTCC exhibits obvious deformation hardening characteristics when bent,and the specimens with 2.0% fiber volume fraction and 2.4(fly ash/cement)fly ash content show better bending performance.(2)The improved two methods of bending toughness evaluation have obtained similar conclusions: the flexural toughness of the specimens with 2.0% fiber content and 2.4 fly ash content is the best.It is indicated that reasonable fibe r content and fly ash content can improve the toughness of UHTCC,and also prove the effectiveness of the two methods of bending toughness evaluation.Compared with the existing bending toughness evaluation method,the improved method simplifies the calcul ation and can effectively evaluate the bending toughness in the whole section of the bending,and makes up for the deficiency of the existing method for the evaluation of the bending toughness after UHTCC ultimate load.(3)The bond tensile test results show that the interface can be obtained with high tensile strength after immersion treatment,but if the interface coarse aggregate is exposed or the interface is damaged,the tensile strength will be greatly reduced.The interface microstructure shows that the interface area is loose and there are a lot of micro cracks.Compared with the equivalent concrete beam,the flexural strength and deflection of the concrete-UHTCC composite beam are greatly improved,showing ductile failure,and the flexural strength of the composite beam increases with the increase of UHTCC thickness;the setting of the transition layer reduces the deflection value of the test piece only In the case of 0.47 mm,the strength is increased by 31%.(4)The increase in the thickness of the UHTCC and the setting of the transition layer reduce the strain difference at the interface to some extent.The DIC method obtains the crack development mode of the composite beam: the crack is generated from the bottom section of the beam and expands upw ard.When it is extended to the interface,the vertical tensile stress is generated at the interface,which causes the beam to generate microcracks in the interface direction and keep moving forward.At the same time,the UTCC layer continues to generate n ew cracks,in which the crack propagation on the concrete is slowly suppressed by the suppression of the underlying UHTCC.