Toughening Mechanism And Control Method Of Concrete Based On Synergistic Modification Of PVA Fiber And Cement-based Materials

Concrete is the basic material for the construction of major projects,especially with the continuous development of the country’s major strategy to the western,severe cold,ocean and other harsh environments,which puts forward higher requirements for the performance of concrete materials.Traditional concrete,as a brittle material,has low ductility,high cracking risk and uncontrollable crack development after failure,which leads to premature failure of the structure and reduced durability.Therefore,it is urgent to improve the toughness of concrete materials,control the occurrence and development of cracks,and improve the comprehensive mechanical properties and durability of concrete structures,so as to provide technical support for the safety,stability and long-term service of concrete structures.Engineering Cementitious Composites（ECC）is a kind of composite material with high ductility and toughness with tensile strain rate exceeding 3%,which is based on micromechanics and fracture mechanics.At present,most of the domestic tests on ECC materials focus on the optimization of water-binder ratio,mineral admixture and aggregate,therefore,the basic research on the properties of fiber,matrix and interface,which can stably improve the strain hardening characteristics of ECC materials,has become an urgent task to solve this problem.In this paper,aiming at the core problem of improving the toughness of polyvinyl alcohol fiber ECC（PVA-ECC）materials,firstly,by using the methods of weakening the chemical bonding properties of different fiber-matrix interfaces,the influence of different PVA fiber interface modification methods on the working properties and basic mechanical properties of ECC materials was analyzed,and the more suitable fiber surface treatment methods were determined.Secondly,the different concentrations in effective fiber modification methods are screened and analyzed,and the matrix with different nano calcium carbonate content to enhance ECC is selected,the working performance,static mechanical properties,dynamic mechanical properties and micro-morphology changes of PVA-ECC materials with different modification methods are studied,and the influence of bidirectional modification methods on the drawing performance of PVA fiber monofilament is explored.The corresponding micro-mechanical correction model is established and the appropriate PVA fiber modification concentration and nano calcium carbonate content are determined.Finally,based on the study of PVA modification and nano-materials on the properties of ECC materials,the influence law of ECC materials’durability was studied and its long-term service performance was verified.The main contents and achievements of this paper are as follows:（1）The fluidity of ECC paste was adjusted by using water reducing agent and thickener,and its working performance was studied.It was found that polycarboxylate water reducing agent had a significant positive effect on improving the fluidity of ECC paste,while hydroxypropyl methylcellulose（HPMC）absorbed free water in the paste,in addition,because HPMC with high viscosity can significantly increase the viscosity of ECC slurry and have a great influence on the fluidity of slurry,when the content of polycarboxylate superplasticizer reaches 2%,HPMC with lower viscosity is selected to act synergistically with it,which significantly improves the expansibility and stability of ECC slurry.At the same time,it is also found that HPMC and polycarboxylate superplasticizer have adsorbability to cement particles,and adsorption competition will occur when the content of both reaches 10%and 2%.The water reducing efficiency of polycarboxylate superplasticizer is seriously reduced because a large number of cement particles are coated by HPMC.（2）The matrix structure and fiber-matrix interface of ECC were adjusted,and the influence of single modification method on the compressive strength of ECC materials was studied,the results showed that boric anhydride would affect the hydration process of cement particles in ECC materials and destroy the homogeneity of hydration products;However,rubber powder weakens the essence of aggregate,which leads to the fragile interface of aggregate matrix;The use of VAE emulsion modified PVA fiber has the smallest decrease in the compressive strength of ECC material,mainly because the weak acid film introduced by VAE emulsion weakens the interface performance,but its weakening degree is not as good as that of boric anhydride and rubber powder,so the compressive strength of ECC material has little loss.（3）On the basis of ensuring the structural strength of ECC matrix,the influence of single modification method on the flexural and tensile properties of ECC materials was studied,and it was concluded that the results of four-point bending and tensile test were consistent,the VAE emulsion modification of PVA fiber significantly increased the ultimate strain of ECC materials,followed by rubber powder,and boric anhydride was the weakest,all three modification methods reduced the initial cracking strength of ECC materials.The pulling test results of PVA fiber monofilaments with different dip angles and buried depths show that the modification of 50#VAE emulsion will reduce the ultimate tensile strength of PVA fiber,but it can significantly reduce the sensitivity of PVA fiber to dip angle and greatly improve the effective participation efficiency of PVA fiber under the tensile or bending load of ECC material.（4）The VAE emulsion modification of PVA fiber and the addition of nano calcium carbonate as matrix reinforcement were studied to compensate for the influence of fiber modification on matrix strength.The two mechanisms are opposite,but they are consistent in improving the toughness of ECC.The results show that VAE emulsion modification on the surface of PVA fiber has an adverse effect on the compressive strength of ECC material,and the degree of influence increases with the increase of the concentration of modified VAE emulsion.Under certain dosage,nano calcium carbonate can obviously improve the compressive strength of ECC,at the same time,when nano calcium carbonate acts on VAE emulsion modified PVA-ECC,it can reduce the influence of VAE emulsion concentration on the strength of ECC.The interaction of modified fiber and nano-CaCO₃can make ECC absorb more energy and show stronger toughness when bending at four points;among them,the effect of nano calcium carbonate is more reflected in the initial cracking strength,and the effect of modified fiber is more reflected in the final cracking strength.With the combination of the two,the toughness of ECC can be significantly improved under a suitable mixture ratio.（5）The tensile properties of bi-directional modified PVA-ECC were analyzed,the initial cracking strength of ECC was obviously improved by adding 1%and 2%nano-CaCO₃,when nano-CaCO₃and VAE emulsion modified PVA fiber with specific concentration acted synergistically,the tensile properties of ECC could be effectively improved.The monofilament drawing test of PVA-ECC shows that the single action of nano-materials has an adverse effect on the toughness improvement of the material.However,when nano-materials and VAE emulsion modify synergistically,it is considered that there is a slip activation concentration of PVA fiber in this system,which can significantly improve the slip characteristics of PVA fiber.（6）Based on the experimental analysis of bi-directional modified PVA-ECC,the micro-parameter elastic modulus model was established,and the chemical adhesive force,interfacial friction force and slip hardening coefficient in the classical design model were modified,and the PVA-ECC strain hardening and steady-state cracking model was established,the model was mainly used to predict whether the nano-reinforced modified PVA-ECC could meet the requirements of strain hardening and high ductility,and the design direction was given.（7）The dynamic compression performance of PVA-ECC material was tested by Hopkinson bar,and it was found that PVA-ECC had obvious strain rate effect.With the increase of strain rate,the peak stress of ECC increased.Because the bi-directional modification technology mainly affects the ductility of PVA-ECC,but has little influence on its compressive strength,there is little difference in dynamic mechanical properties among different mixing ratios,and the peak stress can reach81.5～95.3MPa at high strain rate.The finite element numerical model of PVA-ECC dynamic impact compression was established by using LS-DYNA software,and the constitutive parameters of PVA-ECC dynamic compression were determined.（8）Through the study on the durability of bi-directional modified PVA-ECC,it was found that when the content of nano calcium carbonate was 2%or less,the durability of ECC was significantly improved,and when the content of nano calcium carbonate exceeded 2%,the durability of ECC material was adversely improved.In the durability analysis of ECC,the modification of PVA fiber showed adverse effects except the shrinkage performance and freeze-thaw cycle,which can be played by fiber bridging.Based on the research in this paper,the mechanism of synergistic effect of fiber interface and matrix characteristics on the overall properties of ECC was explored,and a fiber slip activation method was proposed,which provided a new way for further popularization and application of ECC.