The Effect Of Fiber Orientation On The Mechanical Properties And Durability Of Engineered Cementitious Composite

Engineered cementitious composite(ECC)is a type of short fiber-reinforced composite.It exhibits pseudo-strain-hardening behavior accompanied by multiple cracks and a controlled crack width lower than 0.1 mm.The tensile strain capacity of ECC is greater than 3%.Typically,fibers are randomly distributed during mixture production,leading to fiber agglomeration in some regions and divergence in others,which negatively affects their mechanical properties and durability.Therefore,controlling fiber orientation and distribution in ECC is the key for achieving its unique characteristics.In pursuit to gain a fundamental understanding of the relationship between fiber orientation and ECC performance,this thesis proposes a flow-induced casting approach to enhance the fiber alignment with the flow direction using an extrusion machine and compare it with conventional casting.The influence of ECC rheology on fiber orientation was examined.Afterwards,the mechanical properties and durability of ECC were investigated at different fiber orientation degrees.Fiber characteristics,including orientation,distribution,and number at cutting cross-section,were quantified using microscopy and image processing analysis.The failure processes of specimens under flexural,compression,and tensile tests were analyzed using the digital image correlation(DIC)technique.Durability tests,including water absorption,sorptivity,freeze-thaw,chloride penetration,and sulfate attack of specimens with fiber oriented horizontally(O-0),vertically(O90),and randomly(R),corresponding to the direction of the exposure surface were assessed.Also,the effect of long-term exposure to water,Na2SO4,and Na2SO4+NaCl solutions on the compressive behavior of ECC with different fiber orientations was studied.The results showed that the flow-induced casting effectively improved the fiber alignment in the flow direction despite the use of different viscosity modified agents dosages when compared to conventional casting.Compared to specimens with random fiber distribution,specimens with fibers oriented with loading direction exhibited the highest flexural and tensile strengths,while those with fibers oriented perpendicular to the loading had the lowest strength.Therefore,both flexural and tensile strength decrease as the fiber orientation angle relative to the direction of the principal tensile load increases.This is accompanied by greater deflection hardening,number of cracks,and deformation ability.It was also found that the fiber alignment produced by flow-induced casting can contribute to restricting crack width growth under loading.Besides,increasing the number of oriented fibers horizontally or vertically improves the compressive strength and ductility of the post-peak behavior.Furthermore,the fiber orientation is critical to the response of ECC matrix under splitting load,where the splitting tensile strength increases as the fiber orientation angle perpendicular to the load decreases.Water absorption and sorptivity measurements revealed that the fiber oriented perpendicular to the water path delays water migration into the ECC matrix.The sorptivity of the O-0 specimens was 35%and 13%lower than that of the O-90 and R specimens,respectively.After 180 days of exposure,the chloride penetration depth of the O-0 specimens was 5.7 mm,which is 13.6%and 20.8%lower than that of the O-90 and R specimens,respectively.Furthermore,the sulfate ingress profile analysis indicates that the fiber-matrix interface oriented perpendicular to the penetration path can effectively delay sulfate migration.Additionally,the fiber orientation also influences the compressive strength gain under immersion conditions(Na2SO4 solution,Na2SO4+NaCl solution,and water).Compared with the O-0 and R specimens,the O-90 specimens are more sensitive to the immersion condition.In conclusion,employing flow-induced casting through an extrusion machine can be adopted as an effective approach for improving fiber bridging efficiency via fiber alignment with the direction of the principal tensile load,decreasing the crack width,and improving the performance of ECC.