Study Of Piezoresistivity Of Smart Concrete And Mechanism On Reinforcement Corrosion And Protection

As a kind of cement-based composites, smart concrete presents many characteristics, such as high durability and excellent compatibility with concrete structures themselves, as well as cheap and large-scale-distributed embeded and so on. Therefore, it has been more and more attractive for structural health monitoring in civil infrastructures due to its promising performance in long-term service, and has become an attractive topic and cutting-edge research in this field.Cement-based conductive composite materials with strain self-sensing performance is based on the corresponding relationship between electrical resistance and applied strain, that is, piezoresistivity. At the existing problems on the smart concrete, this thesis focuses on the research of cement-based composite materials containing cementitious capillary crystalline waterproofing materials (CCCW), the electrical conductive phase of carbon fiber and graphite. The detailed contents include fabrication method and optimization of proportion, piezoresistivity and its mechanism under one-way and multiaxial stress state, and the effects of CCCW and short-cut carbon fiber on the corrosion and protection of reinforcement in concrete. The following conclusions and innovation can be obtained:(1) Adding CCCW material in the preparation of smart concrete was proposed for improving the stability of electrical, pressure-sensing and mechanical properties of smart concrete for the first time. By improving the dispersion technology of carbon fiber, carbon fiber cement paste and mortar with CCCW were prepared, in which PAN-based carbon fibers were dipped by concentrated nitric acid. And their microstructure, electrical properties, the piezoresistivity under one-way cyclic loading and mechanical properties were carried out experimental research and theoretical analysis. At the same time, the above properties of graphite cement-based composites (GCC) have also investigated. The results have shown that the dispersion of carbon fiber after treatment by concentrated nitric acid in the block of cement paste has been enhanced. Carbon fibers were dispersed uniformly in the matrix. The active chemical material in CCCW has a kind of ability of self-repairing to micro-cracks in concrete, that may improve the stability of electrical properties and mechanical properties of smart concrete.(2) The electrical and mechanical properties of carbon fiber graphite cement-based composites (CFGCC) with CCCW were studied. By coplexing long-range conductive carbon fiber and short-range conductive graphite, and adding CCCW to partially offset the decline in mechanical properties of CFGCC caused by graphite, the new-type smart materials were prepared. The results show that the percolation threshold of CFGCC with 4%(mass fraction of cement, the same below) CCCW and 1% carbon fiber is about 20%. The piezoresitivity of CFGCC with 20% to 30% graphite is the best. Mixing carbon fiber and graphite in smart materials improves the stability of electrical performance.(3) The use of hydroxypropyl methyl cellulose (HPMC) as a dispersant of chopped carbon fiber in the preparation of smart concrete was proposed. The result shows that HPMC can replace carboxymethyl cellulose (CMC) in order to avoid its easy to moldy and non-water.(4) The strain gage factor K (defined as the fractional change in resistance per unit strain) of smart materials GCC and CFGCC were studied. And the effect of multi-cycle on the electrical properties of CFGCC test pieces were discussed.Based on the theory of tunneling effect, the electric-mechanical theory was analyzed under multiaxial strain. The results show that, the blocks of GCC and CFGCC present a good piezoresistive effect. Under the action of 6.25MPa compressive stress amplitude cyclic loading, the measured gage factor of GCC with graphite content of 20% and 50% are 37 and 22 respectively, and the measured gage factor of CFGCC with 1% carbon fiber and 20% and 30% graphite are 37 and 22 respectively, which are higher than gage factor 2 of resistance-strain gauge. The self-sensing properties of GCC and CFGCC block embedded in concrete columns were also studied under the conditions of multiaxial stress. In the monotonic loading conditions, the resistance value of the block embedded in concrete column can clearly reflect its elastic range, plastic scope and failure.(5) By the use of fresh mortar method, hardened mortar method and macro-cell technology, the effects of adding CCCW and carbon fiber, dipped in salt solution and wet-dry cycle on reinforcement corrosion in concrete were analyzed. And the mechanism of steel bar corrosion and protection was discussed. The results show that, the chopped carbon fiber is to promote steel bar corrosion, and the greater the content, the more obvious role in the promotion of steel bar corrosion. Structural system protection methods can be effectively implemented on the reinforcement of protection.v...