Investigation On Design Of Expansive UHPC Filled Steel Tube And Its Mechanical Behavior Of Short Column

The ultra-high,large-span and highly-durable buildings and structures are important developing directions of building industry.The material combining ultrahigh-performance concrete with steel pipes,can take advantages of the two ingredients,which would strengthen each other,and prepare ultra-high strength building materials.Buildings using this material will have significantly improved durability and reduced self-weight.Thus,it is proposed to be widely used in the defense engineering,marine engineering,protective engineering and bridge engineering in the future.But for the ultra-high-performance concrete,the characteristics of high-content superfine binders usage,very low water to cement ratio and absent coarse aggregate,result in its ultrahigh autogenous shrinkage.This ultra-high shrinkage leads to low bonding strength between the two ingredients,and further hinders the development and application of ultra-high-performance concrete(UHPC)filled steel tube.This Thesis is funded by the National Key R&D Program of China and the National Natural Science Foundation of China.A self-consolidating UHPC,which has elevated composition and shrinkage characteristics,was prepared by a new method proposed by the authors.The source and mechanism of the shrinkage of UHPC was revealed,the mechanism that realizing expansion was difficult for UHPC was proposed.Based on the investigation,a new theory obtaining expansive UHPC was put forward.The expansion of expansive agent and internal curing of lightweight aggregate were combined.After that,a kind of expansive UHPC was prepared,which has excellent mechanical properties and durability.And the action mechanisms of expander and enhancer were revealed which further guides the preparation of expansive UHPC.This work solved the high shrinkage problem of UHPC,and improved the bonding strength between steel pipes and UHPC,leading to excellent mechanical properties.The main research work and innovations are as follows:(1)Based on the characteristics of the concrete using in steel filled concrete,a design method relying on wet packing density was proposed.The packing density model proposed by Andreasen and Andersen was investigated,and the results show that the reliability of this model is not good as expected.On the basis of both mechanical property and workability of UHPC,the response surface method ology was used to find out the composition of binders in UHPC.The effect of the water to binder ratio,steel fiber and water reducing agent on rheological behavior and mechanical properties of UHPC were studied.Based on these results,a quickly design method preparing the UHPC was proposed and a self-compacted UHPC was prepared.(2)The shrinkage performance of UHPC was systematically investigated.And the process and mechanism of shrinkage were revealed,which guided the control of autogenous shrinkage.It is low water to binder ratio that makes the shrinkage performance different from that of normal and high-performance concrete.In UHPC mix,the free water is not enough,and the relative humidity is low before and after setting.Additionally,the number of pores related to shrinkage is significantly increased at early age.The average capillary negative pressure can be up to-1.95 MPa before the setting.At the same time,the strength of concrete is very low,so the autogenous shrinkage of UHPC shows a high early shrinkage,and relatively low shrinkage at later age.(3)The mechanism that realizing expansion was difficult for UHPC was revealed,and a theory of expansion design of UHPC was proposed.Based on the investigation,a kind of expansive UHPC was prepared.The evolution of shrinkage of concrete,expansive window and internal relative humidity development were not unified in UHPC,which leaded to the difficulty of realizing expansion.In view of the shrinkage characteristics of UHPC,the two steps of "expansion" were studied: the extra water was introduced to reduce the autogenous shrinkage,which could also promote the hydration of expansive agent.Then the expansion window and the internal curing water release window were combined,and the expansive UHPC with excellent mechanical properties and durability were prepared.(4)The expansion and property enhancement mechanisms of UHPC were revealed.From the viewpoint of hydration products composition,morphology,microstructure,pore structure evolution and micro/nano mechanical properties,the expansive UHPC was studied,and the release of internal curing materials and formation of enhancing area around it were investigated.The results showed that the internal curing water were usually released at early age,and a higher density area around internal curing material could be found.The calcium hydroxide crystal increased and surrounded the lightweight aggregate.The calcium hydroxide crystals tended to gather around,leading to an excellent expansive effect.Compared with normal UHPC,the amount,mean chain length of C-S-H and chemical bonding of all element were significant improved.And the porosity of cement matrix was more than 50% reduced.Meanwhile,the dense area around lightweight aggregate about 100 microns hindered the disadvantages of porous lightweight aggregate.All these improvements of microstructure make UHPC had excellent volume stability as well as excellent mechanical performance and durability.(5)According to the achievements above,it was the first time that a kind of expansive UHPC filled steel tube was designed.The expansion design and mechanical properties of cylinder steel tube filled expansive UHPC were systematically investigated.For normal UHPC,because of the giant shrinkage,the interface between the steel pipe with UHPC was very weak.The steel pipe cannot give full play to the role of constraining on inner UHPC,leading to low mechanical properties of UHPC filled steel tube.When the expansive UHPC was used,the bonding strength between two materials and mechanical properties was significantly improved.These two materials together play a role of the load-displacement curve.Based on the experimental study,the damage model of expansive UHPC filled steel tube was put forward,and the ultimate bearing capacity was analyzed and calculated,which provided the basis for the practical application of UHPC.This work is meaningful to promote the development of shrinkage expansion theory and control technology of UHPC,which accelerates the design and application of UHPC and steel filled UHPC.