| High-performance engineered cementitious composites（ECC）,a type of fiber reinforced material with high toughness and good energy absorption capacity,can possess a higher ultimate tensile strain than normal concrete.It can effectively solve the cracking problem of the concrete structure.In hydraulic and hydro-power engineering and underground engineering,the structure requires building materials to be tough and impermeable.The ultimate tensile strain and permeability coe fficient are important indicators to evaluate the toughness and impermeability of materials.Most of the exsiting research on ECC focuses on its tensile toughness,very few of them focuses on both of the requirements.In this paper,sample statistics,uniform design experiment method,as well as non-parametric and parametric regression analyses have been used in search of the mix proportion range to produce ECC with high toughness and impermeability.Based on the mix proportion range of ECC,which the ultimate tensile strain was between 3.8%and 6.7%,the W/B,S/B,and F/B ranges were 0.35–0.42,0.25–0.3,and 0.02,respectively.The paper found the mix proportion range to produce impervious ECC that are impermeable enough for hydraulic structures（W/B is 0.23～0.26,S/B is 0.22～0.36,F/B is 0.2）.Based on the mix proportion range result,three samples are randomly selected to verify both their tensile toughness and impermeability by uniaxial tensile tests and water permeability test,respectively.The ultimate tensile strains were all over 3%,which was 300 times that of normal concrete（0.01%）.The permeability coefficients were all lower than2.61×10-1 3 m/s,and equivalent to impermeability level P8(at which the permeability coefficient is 2.61×10-11m/s).Test results showed that all the randomly adopted mix proportions had high ultimate tensile strains and low permeability coefficients,so that the applicability of impervious ECC mix proportion range in this paper was verified.One sample displayed very high performance.The compressive strength was over 30 MPa,and ultimate compressive strain was higher than 11%,which was 30times that of normal concrete（0.33%）.The tensile strength was over 3MPa,and ultimate tensile strain was higher than 3%.The fracture energy was over 2800N/m,which was 10 times that of same strength grade concrete.Based on the test results,a piecewise function is used to fit the measured stress-strain curve,thus the uniaxial compressive stress-strain empirical model was proposed.The empirical model based on the cubic specimen with 150mm side length.This empirical model could apply to the impervious ECC with compressive strength about 30 MPa and ultimate compressive strain about 11%.According to the characteristics of linear elastic stage,strain hardening stage and strain softening stage from the tensile stress-strain curve test results and the fracture energy,the uniaxial tensile stress-strain empirical model was proposed,which is based on the dumbbell-shaped specimen with 50mm tested section.The empirical model could apply to the impervious ECC with tensile strength about 30 MPa and ultimate tensile strain about 11%.As ECC is commonly used to strengthen and repair existing concrete structures,the impermeability of interface is the key to the durability and impermeability of the strengthened structures.T he interfacial bonding behaviour,on the other hand,is also crucial for the strengthened structures to bear loads.In the paper,permeability coefficient test,split tensile strength test and bi-surface shear test were used to study the impermeability,tensile and shear properties of the ECC-concrete interface.The effects of interface form,interface agent,curing time,and interface roughness on the impermeability and splitting tensile strength of the interface were analyzed.The interface forms include surface without treatment（P）,flat surface with brushing（S）and W-type surface（W）.The interface agents include cement paste（J）,cement expansion paste（U）and epoxy resin interface agent（H）.The curing time is 14 days and 28 days,respectiverly.The i nterface roughness is described with 2～3mm and0.5～1mm depth of sand filling.The test results show that with the same interface agent,the interface with flat surface with brushing has the best impermeability and tensile strength.The permeability coefficient of the interface can reach 1.7×10-12m/s and the splitting tensile strength can reach 1.73 MPa on average.With the same interfacial roughness,the bonding interface with ce ment expansion paste has the best impermeability with an average of 1.69×10-12m/s,while that with epoxy resin interface agent has the largest splitting tensile strength with an average of 1.71 MPa.The impermeability and the splitting tensile strength of the brushed flat-cement expansion paste interface（S-U）for the specimens with 28 days curing time increased by 75%and 35%respectively compared with the interfaces for the specimens with 14days curing time.The higher the interface roughness,the impermeability and tensile strength of the interface were enhanced.The impermeability of ECC-concrete interface with 2～3mm sand filling depth has a permeability of 5.7 times that of the interface with 0.5～1mm sand filling depth.The former has a 37%higher tens ile strength than the latter.According to the comparison of interface impermeability and splitting tensile strength under different interface treatment,the Bi-surface shear test was completed with the selective preference of interface treatment.The shear strength and shear-slip relationship were used as indicators to evaluate the shear performance of the ECC-concrete interface with 28 days curing time and 2～3mm sand filling depth.The test results were compared with the ECC-concrete interface without treatment and the effect of the interface treatment on shear performance were evaluated.The test results were also compared with concrete-concrete interface with the same interface treatment.The effects of impervious ECC or concrete as the reinforcement material on shear performance were compared.The test results show that the average shear strength of the ECC-concrete interface can reach 3.69MPa when the curing time is 28 days and the sand filling depth is 2～3mm,and the average slip corresponding to the shear peak stress can reach 0.4mm during loading.Compared with the P-U interface treatment with 28 days curing time,the shear performance of the interface showed a significant improvement under the S-U interface treatment.The average shear strength increases by 141%and the average slip corresponding to the shear peak stress increases by43.1%.The shear performance of impervious ECC-concrete interface is better than that of concrete-concrete under the same interface treatment,the average shear strength was improved 1 time and the average slip corresponding to the shear peak stress was 75.9%higher.According to the characteristics of load-slip curve in ECC-concrete interface shear test,the exponential load-slip empirical model of impervious ECC-C25 concrete interface was fitted by Matlab and the interface shear fracture energy was obtained. |
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