Design And Trans-Scale Mechanical Behaviour Studies Of 3D Printed Engineered Cementitious Composites

Applying 3D printing technology in the construction industry is one of the hot issues in current engineering field due to its automation and intelligentization.In the new era,3D printing technology in construction is an important way to develop the intelligence of the construction industry and an important part of intelligent construction.Recently,some cases have been reported that the 3D printing technology has been successfully adopted to personalized housing and bridge construction.Currently,extrusion-based 3D concrete printing(3DCP)has become the most widely used 3D printing technology for construction.A great deal of research has been carried out by researchers and industry professionals from all over the world in the areas of material design,basic mechanical properties,evaluation criteria and construction techniques.However,there are still many issues that need to be investigated in terms of design methods,printable materials,printing equipment and construction processes,especially in the area of structural reinforcement strategy for 3D printing engineered cementitious composites(ECC).ECC is a promising cementitious composite material exhibiting prominent strain-hardening behaviour along with steady-state multi-cracking mechanism,which is called“bendable concrete”for the layman.With tensile ductility typically exceeding 2%and average crack widths controlled below 100μm at ultimate tensile strain,ECC with 2%volume fraction of short-cut fibres offers significant advantages in improving the toughness,durability and sustainability of concrete infrastructure.The combination of ECC and extrusion-based 3DCP technology can reduce the consumption of traditional steel reinforcement and increase the durability of printed concrete structures.The macroscopic mechanical properties of ECC are highly correlated with its microstructural characteristics,and different fabrication processing methods can also affect its mechanical properties.Hence,it is important to carry out studies on the trans-scale mechanical behaviour of 3D printed ECC(3DP-ECC)and the intrinsic mechanisms associated with their microstructural characteristics.In this dissertation,method combining experimental studies,theoretical analysis and numerical simulations is adopted to carry out a systematic study of printable PE fibre reinforced ECC(PE-ECC)development,relationship between microstructure and strain-hardening behaviour,micromechanical modelling,tensile performance prediction of mould-cast and printed PE-ECC,anisotropy analysis of hardened mechanical properties,and bending properties of 3D printed reinforcement-free PE-ECC beams.The main work is as follows:(1)Development and preparation of printable PE-ECCConsidering the effect of different viscosity modifying agent content,sand to binder ratios and PE fibre dosages,a series of orthogonal rheological tests were designed to investigate the yield stress and thixotropy of the fresh PE-ECC mixtures.The results showed that the order of influence of the different factors on the static yield stress and thixotropy of the printable ECC was fibre dosage>HPMC dosage>sand to binder ratio.The final preferred mixture was determined by combining printability and the requirements of the strain hardening characteristics for ECC.Then,the time-dependent rheological and flow properties,and the drying shrinkage of the optimal mixture were tested.The results indicated that the optimal mixture has good extrudability,shape retention and dry shrinkage resistance close to that of normal concrete.Finally,the printability of the preferred mixture was verified in field printing tests.(2)Relationship between microstructure and strain-hardening behaviour of printed PE-ECCA systematic investigation on the relationship between tensile strain-hardening behaviour and microstructural characteristics of 3D printed PE-ECC was carried out.First,the effects of fabrication method(normal mould-cast and extrusion-based 3DCP),sample thickness(15 and 30 mm)and fibre length(6 and 12 mm)on the mechanical properties of PE-ECC,including compressive strength,uniaxial tensile properties,and crack patterns,were studied.X-ray computed tomography(X-ray CT)and Backscattered electron(BSE)imaging along with image processing and analysis was then undertaken to gain an in-depth understanding of the pore structure and fibre orientation and dispersion of normal ECC and printed PE-ECC.Afterwards,the effects of pore structure characteristics including porosity,pore size distribution and pore shape on the first cracking strength as well as the effects of fibre orientation and dispersion on the fibre bridge capacity were estimated to understand the underlying mechanisms of strain-hardening behaviour of printed PE-ECC compared to normal PE-ECC in relation to the microstructural characteristics.Results indicated that it is desirable to use block specimens for mould-casting fabrication as contrast to printed ECC samples.The printed PE-ECC containing 1.5 vol%6 mm and 0.5 vol%12 mm fibres exhibits unique tensile ductility of over 5%,crack width control capability(less than 100μm),and compressive strength of over 50MPa.Normal PE-ECC tended to have more large pores compared to printed PE-ECC,leading to larger variations in tensile properties.Short fibres in printed PE-ECC exhibited a higher fibre alignment in the printing direction.There did not exist a close correlation between fibre distribution characteristics and manufacturing method.Both strength and energy criteria for tailoring ECC were successfully satisfied,which guarantees the tensile strain-hardening behaviour in normal and printed PE-ECC.(3)Micromechanical model of 3D printed PE-ECC under uniaxial tensionA micromechanical modified model accounting for the pore structure characteristics and fibre-matrix interaction was proposed to predict the tensile properties of mould-cast ECC and printed PE-ECC.A series of Monte Carlo simulations were conducted to predict the single crack tensile and tensile behaviour of PE-ECC,which was validated with experimental data.This was followed by a systematic parametric study to assess the effects of crucial parameters on tensile properties.Finally,the tensile properties of printed PE-ECC were predicted based on the proposed model,and the mechanism underlying the improved mechanical properties was elaborated.Results showed that the new factor(ξ_n)and interfacial frictional bond strength reduction factor(γ)proposed facilitate the calculations of composite crack strength and fibre bridging strength.The good agreement between the predicted crack-bridging relationships and experimental results suggests that the developed model can provide a reliable prediction of tensile properties of PE-ECC,including cracking sequence and tensile stress-strain curves.The interfacial frictional bond strength played a dominant role in the tensile ductility of PE-ECC,followed by fibre location,orientation and diameter.The long half-axis of the equivalent ellipse selected as the pore size was more representative for predicting the tensile performance of printed PE-ECC,and thus the predicted cracking strength is closer to the measured value.Possible factors underlying the higher tensile ductility of printed PE-ECC compared to cast-in-place PE-ECC,in addition to the lower fibre inclination,include the increased frictional bond strength at the interface due to the extrusion-based printing process and the reduced fracture toughness of the matrix.(4)Analysis of basic mechanical properties of hardened printed PE-ECCThe effects of different preparation methods,loading directions and curing environments on the hardened mechanical properties of printed PE-ECC were investigated.Uniaxial compression tests,flexural tests and splitting tensile tests were conducted to investigate the basic mechanical properties of printed PE-ECC.In addition,the microstructural characteristics of the mould-cast and printed PE-ECC,such as fibre orientation and pore distribution and shape,were analysed qualitatively and quantitatively by means of digital image correlation(DIC)technology,BSE image analysis tests and X-ray CT tests,and the mechanisms underlying the differences in mechanical properties were analysed on the basis of experimental phenomena and data.The results showed that the recommended size for printed PE-ECC uniaxial compression specimens(Ф100×200 mm cylindrical specimens)reduces the dimensional effects of internal defects in the specimen and end restraint effect,as well as the operational difficulties of preparation.Owing to the good printing process,the anisotropy of the printed cylindrical specimens in terms of compressive strength under standard curing conditions is not obvious.The printed PE-ECC specimens show an anisotropy pattern of X≈Z>Y.A uniaxial compression stress-strain constitutive model suitable for cast-in-place and printed PE-ECC was established,and the accuracy of the model was verified by comparing with the experimental measurement results.The ultimate flexural strength of the printed PE-ECC showed an anisotropic pattern of Y>Z>X,and the ultimate mid-span displacement and fracture energy showed Z>Y>X.The interface splitting tensile test exhibited typical brittle damage characteristics.The strength of the vertical interface between two adjacent filaments was higher than that of the horizontal interface between two adjacent filaments,but weaker than that of the cast-in-place specimen.Moreover,the direction of crack initiation and expansion induced by pore defects of different configurations is one of the key factors leading to differences in the mechanical properties of PE-ECC based on different fabrication methods.(5)Bending performance of 3D printed reinforcement-free PE-ECC beam componentThe feasibility of the practical application of printed PE-ECC beams was experimentally verified in terms of both the printing process optimisation and the mechanical properties of the components.Further,comparisons were made with mould-cast PE-ECC beams as well as reinforced concrete(RC)beams to verify the effect of the preparation method on the mechanical properties.Results showed that the whole printed PE-ECC beams presented a ductile failure mode and showed good shape integrity with no block peeling or detachment.The ultimate bending load of 3D printed beam was greatly affected by geometric configuration,indicating that the printing path of structural members should be tailored to achieve the best mechanical performance.For different print angles,the specimen with the angle of 0-degree had the best bending capacity,followed by 90-degree and 45-degree,and the maximum difference of bending capacity can reach twice.For different superposition forms,0-45-90-degree was the best,followed by 0-45-degree,0-90-degree,and 45-90-degree.The printed PE-ECC beams showed obvious anisotropy under bending and the one with the best bending resistance was equivalent to RC beams with a reinforcement ratio of 1.51%,which indicated the feasibility of printed ECC to substantially reduce or replace the steel bars for structural application of 3DCP.