Research On Strain Hardening Behavior Of Ultra-high Performance Concrete Based On Fiber Surface Modification

Ultra-high performance concrete is a new type of building material with ultra-high strength and ultra-high durability,which has wide applications in engineering practice.But as the strength of concrete increases,the brittleness of concrete becomes more and more significant.To tackle this defect,fibers are often added to the concrete matrix to improve the ductility of the concrete.However,because steel fibers and polyethylene fibers,which are commonly used to improve the ductility of the concrete matrix,are hydrophobic surfaces,it is difficult to establish a reliable bond with the concrete matrix,which might form a weak layer of the composite material.When the loads acted on the concrete,concrete transfers the load to the fiber through the fiber-concrete interface,and then bears the load together through the interface bond between the fiber and the concrete.Therefore,the fiber-concrete interface bonding performance plays a significant role in the stress transmission.Modifying the fiber surface is one of the most effective method to improve the fiber-matrix interface performance.In this paper,through the use of surface modifiers to modify the surface of the fiber,the effect of the concrete on the direct tensile performance,strain hardening behavior and energy dissipation ability was studied to obtain high ductility and high energy consumption concrete.This paper first started with PE-SHCC without fiber surface modification,studied the effect of different water-binder ratios and different fiber sizes on the concrete's direct tensile performance and strain hardening behavior,and clarified the goal and direction of further research;Based on the former experiment,the effect of using surface modifiers on the surface modification of PE fibers was studied;further,the scope of the study was extended to steel fibers which is more commonly used in UHPC,and the effect of surface modifier on the surface modification of steel fiber was studied.The main conclusions of this paper are as follows:(1)In concrete specimens incorporated with PE fibers,as the water-binder ratio of the concrete matrix decreases,the straight tensile strength of the concrete increases,but the toughness of the matrix will increase first and then decrease,especially in ultra-low water.In the ultra-high-performance concrete matrix,the toughness of concrete has not been fully enhanced.The low water-binder ratio makes the concrete matrix more dense and the porosity is significantly reduced,so the tensile strength of the concrete is improved.However,in the low water-to-binder ratio test piece,due to the presence of a large amount of unhydrated gelling material particles,the interface bond between the fiber and the matrix cannot be fully established,and the fiber is easily pulled out of the matrix,so that the toughness of the test piece cannot be obtained Effective enhancement(2)The use of surface modifiers can effectively improve the adhesion of PE fibers to the concrete matrix.The surface modifier is coupled between the fiber and the matrix by forming covalent bonds,hydrogen bonds,and electrostatic adsorption,which significantly improves the chemical adhesion between the fiber and the matrix.When the3%concentration treatment is used,the strain hardening section of the direct tensile stress-strain curve of the test piece has been fully developed.The test piece shows obvious multi-cracking behavior,the cracks are fine,and the cracks are uniformly cracked in a large range without localized development.The crack strength,ultimate tensile strength,ultimate tensile strain,and energy dissipation per unit volume of the specimen reached7.26MPa,8.01MPa,5.06%,and 337.6 k J/m~3,which were 39.6%,31.1%,and 295.3%,2178.2%higher than the control group.However,there is an optimal concentration of surface modifier(3%),within this range,it has the best effect on improving the mechanical properties and strain hardening behaviors of the concrete.A too high concentration of the surface modifier will have an adverse effect.(3)The use of surface modifiers can effectively improve the bonding between steel fibers and concrete matrix.On the one hand surface modifiers improves the chemical adhesion between the fiber and the matrix by forming of covalent bonds(such as Surface Modifier B),and on the other hand by making the fiber surface hydrophilic and rough increase the energy dissipation when the steel fiber is being pulled out(Surface Modifier C).The difference between the two surface modification methods is mainly reflected in the strain hardening section of the stress-strain curve.The former has a large zigzag curve,while the latter has a fuller development and a smoother curve.The final failure form of the specimen is one main crack and several fine micro cracks.By using 1%Surface Modifier B to treat the steel fiber surface,the tensile strength of the concrete is 9.80MPa,the ultimate tensile strain is 0.55%,and the energy dissipation capacity is 49.44k J/m~3,which is respectively increased by 15%,17 times,and 45 times,almost reaching the level of high energy absorbing concrete(energy dissipation?50k J/m~3)in strain hardened concrete classification.By using 3%Surface Modifier C to treat the steel fiber surface,the cracking strength and tensile strength of the concrete were increased by 15%and 17%.(4)Through surface modification of the fiber,the toughness,crack development and the mechanical properties of UHPC have been significantly improved.Therefore,the fiber surface modification treatment is an effective way to strengthen the fiber-matrix interface.At the same time,this treatment method has many advantages such as simple process and low environmental pollution and has the potential for industrialization.