Effect Of High Temperature On The Bonding Performance Of Ultra-high Performance Concrete-steel Fiber

As a new cement-based composite material that has been promoted and developed rapidly in the world in recent years,steel fiber-UHPC has attracted extensive attention for its excellent mechanical properties.It is particularly important to explore how to exert its physical and mechanical properties after fire and high temperature.The bonding performance between the steel fiber and the matrix directly affects the reinforcing effect of the fiber on the concrete.The bond between the steel fiber and the matrix is the result of the joint action of friction and mechanical occlusal force.Different steel fiber types,buried depths,strengths and Geometric features and other factors make the energy dissipation mechanism of concrete failure slightly different.In order to study the bonding performance between steel fiber and UHPC after high temperature,this paper set up 144 test pieces with the type of matrix,fiber type,fiber burial depth and temperature as the test variables,and carried out the test after high temperature from 20°C to 800°C.Fiber pullout test.The surface of the extracted fiber was observed by using the micro-scanning technology of the ring field electron microscope（SEM）.The hydrated product was analyzed by phase.According to the above test results,the following main research conclusions are drawn:（1）Compared with the specimens at room temperature,the appearance of the specimens after high-temperature heating has changed significantly.As the temperature increases,the color depth of the matrix appearance gradually becomes lighter.The change of base is the most obvious.After800°C,the appearance changes from dark gray at room temperature at 20°C to white.In addition,when Portland cement base is at 400°C,some specimens burst.The aluminate cement-based specimens maintained their integrity at all temperature ranges,and the surface did not peel off,which proved that the UHPC with this mix ratio had good resistance to high temperature peeling.（2）The pull-out force of the straight round steel fiber in the drawing process is mainly composed of chemical bonding force and interface friction force,while the pull-out force of the end-hook steel fiber is mainly composed of the mechanical anchoring force provided by the tail hook at the end The chemical bonding force and interfacial friction only play a secondary role.What is reflected in the drawing load-displacement curve is that the drawing load-displacement curve of the straight round steel fiber has a good integrity compared with the end hook steel fiber.The trend of the curve shows obvious elastic stage,interface debonding stage and friction pull-out stage,while the end-hook steel fiber is broken after sliding for a short distance due to its strong anchoring force.（3）By increasing the fiber burial depth,the bonding performance between the steel fiber and the matrix cannot be improved,but it will have a negative effect.The greater the burial depth,the greater the ultimate load when the steel fiber is pulled out,but the interface is sticky.On the contrary,the knot strength is smaller,which has nothing to do with the type of steel fiber.The interfacial bonding strength between straight round and end-hook steel fibers and Portland cement-based UHPC showed a gradual decline trend with the increase of fiber burial depth and temperature.The bonding strength is greater than that of straight round steel fibers.Because the high temperature promotes the hydration reaction of aluminate cement,the bond strength between the straight round steel fiber and aluminate cement-based UHPC interface is higher than that of other temperature ranges after the high temperature of 400°C.The interfacial bonding strength of UHPC-based UHPC presents a gradually decreasing trend with the increase of temperature when the burial depth is 10mm and 15mm.（4）With the increase of temperature,the ultimate load of the straight round and end-hook steel fibers showed a gradually decreasing trend when they were pulled out from the Portland cement matrix,and the ultimate load showed a gradually increasing trend with the increase of the buried depth.The trend shows that in all temperature ranges,the ultimate load of end-hook steel fibers at the same buried depth is greater than that of straight round steel fibers.In the temperature range of 20°C to 400°C,the change law of the ultimate load of the two types of steel fibers is quite different when they are pulled out from the aluminate cement matrix,while in the temperature range of 400°C to 800°C,the ultimate load changes with As the temperature increased,it showed a gradually decreasing trend.（5）At room temperature of 20°C,when the burial depth is 20mm,the drawing energy of straight round and end-hook steel fibers and Portland cement is the largest.After a high temperature of 400°C,straight round steel fibers and aluminate cement The drawing energy of the base is the largest,and the drawing energy of the end-hook steel fiber and the aluminate cement base is the largest at 20°C at room temperature.After the straight round steel fiber and aluminate cement base are subjected to high temperatures of 600°C and 800°C,the drawability decreases the most.77.7%,80.6%and 83%.（6）The Portland cement base is affected by high temperature,and the C-S-H gel content decreases,resulting in the instability of the gel structure in the matrix,and the decrease in the matrix debris density bonded to the steel fiber surface.The hydration products C₂AH₈,C₃AH₆ and CAH₁₀ of aluminate cement-based UHPC at room temperature of 20°C will disappear at 200°C with the increase of temperature,and when the temperature rises to 400°C～600°C,the aluminate In the hydration reaction of salt cement,three new hydration products,CAS₂H₆,C₄A₃H₃ and C₆S₆H,appeared,and at the same time,part of CA was consumed,and the matrix debris density bonded to the steel fiber surface was greater than that of the bonded matrix when it was pulled out after a high temperature of 200°C Crumb density.The high temperature of 800℃causes the hydration product to decompose to form CA,and at the same time,the steel fiber undergoes oxidation reaction under the action of high temperature,and the surface layer of the fiber becomes rough and peels off.