Study On Self-Monitoring Of Crack In Concrete Member

As one of the most widely used building materials in the world,concrete shows many advantages like adequate in raw materials,low cost and good compression behaviour.However,concrete is a brittle material with low tension capacity.The crack shows negative effect on the durability and safety of the concrete structure.Therefore,it is necessary to monitor the cracking position andcrack propagation in real time.This paper presents a self-monitoring method which can be widely used in existing concrete sturcture members likeone-way slab,two-way slab and beam.Compared with smart concrete self monitoring methods,the proposed method can achieve the crack monitoring goal without the addition of conductive materials.Compared with those crack and strain sensing techniquesby using the optic fiber sensor and piezoelectric sensor,there is no need to embedthe sensor to the fresh concretein site for this method.Instead,this method allows the propbes to be attached onthe surface of the existing structure.It provides a new idea and method for health monitoring of existing concrete structures.The main contents are as follows:(1)The effects of different kinds of conductive materials and their content on the compressive strength and conductivity of concrete were studied.The resluts show that nanocarbon black can significantly improve the concrete conductivitywith high content,but it demostrates negative impact on the compressive strength of concrete.Steel fiber can improve the conductivity of concrete to some extent,and improve the compressive strength of concrete slightly.(2)The self-monitoring method which can be used in existing concrete one-way slab specimens has been determined.The effect of steel fiber and polypropylene fiber on the crackself-monitoring behavior of one-way concrete slab under bendingis studied.The results show that the self-monitoring method with the electrode spacing of 100 mm can effectively monitor the cracking time and crack propagation of the crack in one-way concrete slab.The signal-to-noise ratio and the sensitivity of the self-monitoring signal of polypropylene fiber reinforced concrete one-way slab are higher than that of steel fiber ones.(3)Six kinds of crackself-monitoring methods for two-way concrete slabs with simple supported on four sides under bending have been proposed.The optimum electrode arrangement for the existing two-way concrete slab is determined.The results show that the self-monitoring method based on the principle of potential redistribution can effectivelymonitor the crack information of two-way concrete slab,including cracking time,crack propagation,crack direction,crack location,cracking sequence and crack number,etc.Compared with the four-electrode method,the signal stability is higher,and the noise can be reduced siginificantly.Even when the electrode spacing is extended to 300 mm,effective monitoring signals can still be obtained.(4)The crack self-monitoring method which can be used in the existing concrete beam specimens is determined.The effect of steel fiber and polypropylene fiber on thecrack self-monitoring behavior of the pre-cracked concrete beamsubjected tothree-point bending is studied.The results show that the self-monitoring method based on the principle of potential redistribution can effectively monitor the crack propagation of concrete beams.The monitoring signals have response tothe microcracks in the tensile zone before the macrocrack occurs.The signal to noise ratio and sensitivity of the self-monitoring signal of polypropylene fiber concrete beam are higher than that of steel fiber ones.