Multi-investigation On Fiber-matrix Interfacial Bond Properties In Ultra-high Performance Concrete

Ultra-high performance concrete(UHPC),as a new cement-based material,has a great potential application due to characteristics of extremely dense micro structure,high strength and toughness,and superior durability.The use of high strength steel fibers is often mentioned for its application.The steel fibers can enhance the strength and toughness and reduce the structure size to render more flexible and lighter structures with high esthetic value.However,high dosages of fibers can significantly enhance the cost of UHPC because of high cost associated with steel fiber.In addition,the use of fiber can introduce a new interfacial transition zone(ITZ).Because this ITZ has higher water-to-cement ratio and porosity and contains larger preferentially calcium hydroxide(CH),it is usually recognized as the weakest zone in a composite material.When loads are acted on the composite material,the fiber does not sustain load initially.The load is transferred from the matrix to the fiber through fiber-matrix ITZ.Therefore,the fiber-matrix interface plays a vital role in determining the load efficiency and the strengthening and reinforcing effects of the whole system.There are three main strategies to improve the bond properties at ITZ between the UHPC matrix and the embedded fibers.These include densification of the cementitious matrix by use of supplementary cementitious materials(SCMs)or high temperature curing,enhancement of mechanical anchorage through use of deformed fibers,and improvement of fiber-matrix friction by surface treatment.Among these,the first one is the most fundamental one.This study focuses on the multiple investigations of UHPC based on fiber-matrix interfacial properties,microstructure,mechanical properties,and their interrelationships.It aims at preparing cost-effective UHPC with dense structure and superior strength and toughness.The study mainly includes the following four parts:1)influence of different types and dosages of SCMs,including silica fume,slag,fly ash,nano-particles,on properties of UHPC matrix.Such properties include flowability,heat of hydration,matrix strength,and fiber-matrix bond properties;2)investigation on micro structure of matrix and fiber-matrix interface by using ad-vanced techniques,such as TG,MIP,XRD,SEM,and micro-hardness;3)improvement on fiber-matrix bond properties by adopting deformed fibers,including corrugated and hooked fibers,and clarification of bond strengthening and toughening mechanisms associated with fiber shape;4)study on effect of fiber shape and fiber content on mechanical properties of UHPC.Based on the corresponding results,the relationship between fiber-matrix bond strength and flexural strength of UHPC is established based on the composite theory.In terms of the results,the following results can be drawn:(1)Through investigating the properties of specimens made with different types and contents of SCM,the effect of different factors on the properties of matrix after different curing times are discussed.A statistical model relating either bond strength or pullout energy to silica fume content and curing time is established.Test results indicate that 28 d normal curing can render stable strength of the UHPC matrix and fiber-matrix bond properties.Silica fume accelerates the heat of hydration,however,there exists an optimal silica fume content from 15%to 25%,in terms of flowability,matrix strength,and interfacial bond properties.The incorporation of fly ash and slag delays the hydration and decreases the matrix strength and interfacial bond properties at early age,but increases the corresponding properties after 28 d normal curing.A small amount of nano-CaCO3 and nano-SiO2 accelerates the heat of hydration and significantly improves the properties of matrix and interface.There exhibits optimal dosages,however,which is 1.6%-4.8%for nano-CaCO3 and 0.5%-1.5%for nano-SiO2.When exceeding these values,the matrix strength decreases.Besides,nano-CaCO3 shows better toughening effect than that of nano-SiO2,especially at 28 d.(2)TG,MIP,XRD,SEM,and micro-hardness analyses are used to characterize CH content,pore structure,hydration product,and morphology of the matrix and the fiber-matrix interface,respectively.The interrelationship between microstructure and previous macro-properties is discussed.Extensive microstructural analyses confirm the change in matrix strength and fiber-matrix bond relating with SCMs and curing time.UHPC matrix with 15%-25%silica fume exhibits a very low calcium hydroxide content and low porosity after 28 d curing,which are 2%and 6.8%,respectively.The CH content in samples with incorporation of slag or fly ash is very low at early ages,but the bond properties of hydration products are very weak.Nano-particles can consume CH and improve the C-S-H structure.This is proven by the XRD analysis with appearance of a new and strong peak at 28.2°,which corresponds to C-S-H.Therefore,the micro-hardness of matrix and fiber-matrix bond properties are improved.(3)Effect of deformed fibers,such as corrugated and hooked fibers,on fiber-matrix bond properties is studied and compared with that of straight fiber.The strengthening,toughening,and pullout mechanisms associated with fiber shape are clarified.Fiber shape significantly enhances the fiber-matrix bond properties.The bond strengths of corrugated and hooked fibers are approximately three and seven times higher than those with straight fibers,while three and four times greater for pullout energies.The use of deformed fibers can efficiently enhance bond given additional mechanical interlocking and enhanced friction with surrounding matrix provided by the fiber geometry.(4)Effect of fiber shape and fiber content on compressive and flexural properties of UHPC is studied.Based on the experimental results,the relationship between fiber-matrix bond strength and flexural strength of UHPC is established based on the composite theory.Finally,the fiber strengthening and toughening mechanisms to UHPC is discussed.Test results indicate that fiber shape has limited effect on the initial cracking point of flexural load-deflection curve but a significant influence on the peak load(ultimate flexural strength).The change in peak load associated with fiber shape follows this order:hooked fiber>corrugated fiber>straight fiber.With the increase of steel fiber content,the strength and toughness of UHPC are greatly enhanced.When compared to UHPC with straight fibers,the flexural strengths of samples with 1%to 3%corrugated and hooked fibers are increased by 4%-10%and 10%-27%,respectively.According to the fiber-matrix bond strength and the flexural strength of matrix without any fiber,the flexural strength of UHPC can be efficiently predicted based on the composite theory.The ratios of predicted to experimental flexural strengths are in the range from 0.8 to 1.2.The proposed model takes into account the fiber content,fiber length,fiber diameter,and coefficient related to fiber orientation in 3D direction.The coefficient related to fiber orientation is determined at 0.5.