Study On Performance And Strengthening Design Of Electrothermal Curing Concrete Under Negative Temperature Environment

Concrete is the most widely used building material.However,due to the concrete can not hydrate in the negative temperature environment,the construction of concrete in the cold region in winter has been affected,which brings negative impact on the economical construction in cold region of our country.The strength of concrete is closely related to its environment,and the specific requirements of concrete strength vary with different environments.In the construction of concrete structures such as Bridges,tunnels and culverts under the harsh natural conditions in cold regions in winter,concrete needs to have both sufficient strength and excellent working performance.Ordinary concrete is difficult to meet these requirements.High performance concrete has good construction performance,high strength and high early strength.Obviously,these characteristics of high performance concrete can meet the needs of concrete structure in cold winter.Therefore,this paper proposes to use electrothermal curing method to realize the construction of high performance concrete in cold area.Firstly,it was found that nickel-plated carbon fiber(Ni CF)and carbon nanofibers(CNFs)can enhance the electrical conductivity of the specimen.Ni CF was taken as the main conductive fiber,and the percolation threshold of Ni CF was determined by experiments and numerical simulation.On the basis of the conductive network constructed by Ni CF,the effect of small-scale CNFs on electrical resistance stability of electrothermal curing concrete was studied.At the same time,the system of electrothermal curing was optimized,and the concept of gradient power Electrothermal curing was proposed to ensure that the temperature difference between the inside and outside of the specimen was reasonably controlled in the early curing stage,and to prevent structural cracking caused by large thermal stress.The development rules of mechanical properties of high performance concrete under different electrothermal curing systems were compared,and the mechanical property results were further explained and analyzed from the aspects of microscopic pore structure and hydration products.Then,nickel-plated carbon fiber/carbon nanofiber concrete matrix(Ni CF/CNF-CM)was modified with different contents of mekaolin(MK),mixed mekaolin concrete(MK-CC),and the performance of MK-CC matrix under Electrothermal curing was studied,and the influence of the use of MK on the macroscopic mechanical strength of the specimen was studied.And from micro performance to macro performance verification.At the same time,the fracture characteristics of MK-CC under electrothermal curing were studied,and the loadcrack opening displacement(P-CMOD)curves of MK-CC specimens under different metakaolin contents were obtained.The fracture energy and double k fracture parameters of the specimen were analyzed.The Hopkinson bar test was carried out on MK-CC specimens with different metakaolin contents under electrothermal curing,and the mechanical parameters under different strain rates were obtained.Therefore,the dynamic mechanical properties of MK-CC specimens with different metakaolin contents were analyzed.In addition,based on three characteristics of nanomaterials: filling effect,nucleation effect and volcanic ash effect.Nanomaterials can not only reduce the porosity of concrete,but also promote the formation of C-S-H inside the structure,thus improving the early strength of concrete.Different kinds of nanomaterials were applied to the preparation of electrothermal curing concrete,and nanomaterials reinforced concrete(NM-CC)was obtained.The nanomaterials selected in this chapter include nano-silica(NS),nano-titanium dioxide(NT)and nano-calcium carbonate(NC),which are added to NM-CC at different mixing amounts.Among them,the compressive strength of 3%NC-CC specimens reaches108.5MPa,reaching ultra-high strength grade.The mechanical properties evolution of different nanomaterial matrix under electrothermal curing and standard curing were compared.Meanwhile,MIP,TG-DSC,The effects of different nanomaterials on the microstructure of electrothermal curing NM-CC matrix were studied by XRD and SEM.Finally,based on the conclusions of the previous chapter,the electric-heating conversion and heat transfer principle of electro-induced cured NM-CC under the condition of different nanomaterial content were studied in detail.Firstly,COMSOL simulation was used to judge the temperature development of NM-CC board under different curing conditions,and a large size NM-CC board was prepared for experiment,and the experimental results were verified.In order to clarify the use of nano-materials to study the temperature development and distribution of NM-CC board under the effect of electrothermal curing,to clarify the temperature development of NM-CC specimen under the influence of different nanomaterials.