Research On Mechanical Properties And Micro Structure Analysis Of High Toughness Engineered Cementitious Materials

Traditional concrete materials are inherently brittle and easy to crack,which make them not so satisfactory applied in urban railway system.Engineered cementitious composite reinforced with PVA fibers possesses excellent crack-controlling ability and ultrahigh toughness two orders of magnitude higher than the concrete materials.Therefore,replacing concrete with PVA-ECC would significantly increase the durability of concrete structure and service life,and lower the cost for maintenance.The uniaxial tensile,compressive and four-point bending tests were carried out to characterize the macro mechanical behavior of corresponding mixture.What's more,Scanning Electron Microscopy(SEM)image analysis and X-ray CT scan method were used to study the micro mechanism through microstructures and pores distributions.Test results showed that:(1)Compared to the dog-bone shaped specimens,their rectangle counterparts are more suitable adopted to test the tensile performance.Both increasing the percentage of fly ash content and water to binders ratio improve the tensile strain and the former works better,but they are detrimental to ultimate tensile strength.The first cracking strain is independent of the two indexes,fluctuating between 0.01% and 0.08%.(2)Thickener combined with superplasticizer plays an important role in dispersing the PVA fibers during mixing process,which further enhance the stability of the tensile performance.(3)The tensile performance is influenced by the sliding distance before PVA fibers lose their bridging function.Thus,the interfacial bonding strength should matches well with mechanical and physical properties of PVA fibers.High tensile strength and strain could be obtained if PVA fiber fully slide and then snap.In this research,the maximum tensile strain achieved is 4.46% with the tensile strength being 4.11 MPa,the maximum tensile is 4.81 MPa with the tensile strain being 3.8%.(4)Compressive toughness of cementitious materials can be improved by PVA fibers.The peak strain under compression of PVA-ECC is several times higher than ordinary concrete and the strain-softening period is smoother.The completeness of the specimens after reaching peak load is also better.(5)Partly displacing PVA fibers with same volume of steel or basalt fibers has little effects on compressive strength,but steel fiber is proved to be beneficial to the residual strength.Meanwhile,5% calcium sulfate can improve the compressive strength by 52.3%(6)Steel and basalt fibers addition increase the first cracking strength,especially basalt fibers which improve it by 26.4%,but the ultimate tensile train slashes almost linearly to the ratio of the ultimate tensile strength to the first cracking strength.Calcium sulfate improve the ultimate tensile strength by 8.7%while maintain the inherent high ductility of PVA-ECC with less PVA fibers incorporated.(7)The SEM pictures at the fractured plane indicates that steel fibers are intactly pulled out and cause serious spalling of the surrounding matrix,which makes these PVA fibers there premature and then the tensile strain decreases.In the case of basalt fiber,the mechanism is that the basalt fibers reach their strength much earlier than PVA fibers in the propagation of crack,and the stress transferred to PVA fibers immediately and make the PVA fibers exhaust their bridging strength with a sudden rupture or pull-out.(8)Calcium sulfate accelerates the hydration cement and stimulates the volcanic activity of fly ash,which forms large amount of C-S-H and ettringite filling pores and enhance fiber-matrix bonding strength.Also,the production of ettringite in matrix generates pressure and induces additional micro cracking in matrix,so the strength is improved and strain is inherited.(9)The introduction of steel fiber and calcium sulfate improve both bending strength and toughness,while basalt fibers shows completely opposite effects.The way of calculating bending toughness from ASTM is highly depends on the picking of the first cracking point,which could give misguiding information.