Flexural Crack Monitoring For Fiber Reinforced Concrete Beams Based On Electric Potential Change

The durability problems of concrete structures are closely related to concrete cracks.If the crack can be detected timely and the damage can be repaired properly,the concrete structures can serve for longer time and thus the economical benefits can be achieved.In the recent years scholars proposed some crack monitoring methods,such as optical fiber sensors,acoustic emission method and self-sensing concrete as so on.The self-sensing concrete can sense the strain and cracks via the conductivity of composite material composed by concrete and functional fillers.In order to improve the strain sensing ability of self-sensing concrete,high dosage of functional fillers are needed to be added into concrete.The high dosage of functional fillers can be used to make small size strain sensors,but they are not suitable for making large size structural members because concrete with high dosage of functional fillers exhibits bad workability.What is more,using high dosage of functional fillers increases the construction cost.Because the resistivity of air is much higher than that of concrete,the concrete resistance can be greatly increased when crack occurs in concrete.If we make full use of this property and lay out the electrodes appropriately,the crack monitoring of concrete beams can be achieved without adding high dosage of functional fillers.In order to achieve the aforementioned goal,a new crack monitoring method is proposed in this paper.The basic element consists of six symmetrically placed electrodes,which locate at the edge of the bottom surface.One basic element can monitor two regions.To verify the validity of the novel method,the crack monitoring test for concrete beams was performed.The beams were subjected to four-point bending.Before the four point bending test was carried out,the workability test and cubic compressive strength test had been performed.Steel fibers(SF)and polypropylene fibers(PP)are used in this paper.Both types of fibers have two kinds of lengths:35mm and 60mm(35SF,60SF,35PP,60PP).There are two kinds of diameters for PP fibers:0.1mm and 0.6mm.In order to investigate the efficiency of this method for single crack monitoring and multiple crack monitoring,three types of SF dosages(10,35,and 60kg/m~3)and one type of PP dosage(10kg/m~3)are used in this study.During the loading process,two linear variable displacement transducers were used to measure the midspan deflection and an extensometer was used to measure the crack opening displacement(COD).The load-time curves and voltage-time curves were drawn to study the crack monitoring behavior.The load-deflecion curves were drawn to study the flexural behaviors of all types of FRC beams.The results of bending test were compared with those obtained in crack monitoring test.What is more,the relationship between deflection and COD was investigated in order to use the value of COD to represent the deflection for the experiments which are not convenient to monitor the delfection.The relationship between monitoring voltage and COD was also studied to analyze the effectiveness of crack development monitoring of the novel method.Adding fibers to concrete can decrease the slump of fresh concrete.The fibers have no clear impact on the cube compressive strength of concrete.The results of the four-point bending test show that fibers can improve the flexural strength,the residual flexural strength and the flexural toughness.The flexural toughness can be promoted most among the aforementioned three flexural parameters.The improvement is closely related to the fiber type,the fiber length,the fiber diameter and the fiber content.The fibers with high modulus,tensile strength and greater aspect ratio are good at improving the flexural behavior of concrete beams.The more fibers are added to concrete the more improvement of flexural strength and toughness can be gained on condition that the good workability can be garanteed.The experimental results show that the load/voltage-time curves can be divided into three stages:before cracking stage,cracking stage and post cracking stage.The monitoring voltage is almost unchanged in the first stage,which means the novel method has a function of diminishing the negative effect of electric polarization on the crack monitoring.The monitoring voltage shows obvious change at the moment of cracking.The voltage drop indicates the crack occurs in monitoring region 1 while the voltage rise indicates the crack occurs in monitoring region 2.The monitoring voltage in the third stage continues to increase or decrease as the crack propagates and the voltage becomes almost stable finally.Under the circumstance of single crack monitoring the voltage and COD exhibits exponential relationship.Under the circumstance of multiple crack monitoring the crack location can be determined by the change of voltage(from increasing to decreasing or from decreasing to increasing).If the final trend of voltage variation is decrease,the critical crack locates at monitoring region 1;if the final trend of voltage variation is increase,the critical crack locates at monitoring region 2.The novel method in this thesis can be applied to the practical engineering structures such as beams,griders and slabs by connecting the basic monitoring elements and expanding the number of monitoring regions.