Study On Self-Monitoring Of Cracks Of Concrete Beam With Multiphase Conductive Materials

During the service life,the concrete structural members are inevitably affected by different loads and environmental conditions.Due to which the damage and crack resistance of concrete structure is reduced.It will accelerate the formation of cracks,and allow the moisture and other harmful ions enter into the concrete matrix which will lead the corrosion of steel re-bars.Structural cracks significantly reduce the durability and serviceability of structure.If the crack is detected in its early age,then the repair and strengthening work can be carried out as soon as possible,which will greatly reduce the unexpected accidents and maintenance cost in later stage.Therefore,it is necessary to produce a real-time monitoring system for cracks and damages.Structural health monitoring is a new concept which aims to real-time monitoring of damage and crack through different embedded and attached sensors.These sensors are expensive and have short life time.Therefore,a new kind of intrinsic self-sensing cementitious sensor have developed which can not only self-monitor the strain,stress and cracks but also improve the mechanical properties of concrete.In this research work,steel fiber（SF）,carbon fiber（CF）and nano carbon black（NCB）are used as a diphasic and triphasic conductive admixtures to improve self-sensing and mechanical properties of concrete beam.The main contents of this research work are as follows:Firstly,the effect of conductive materials on workability and air content of the concrete are investigated.It can be concluded that the conductive admixtures reduce the workability and increase the air content with respect to plain concrete.Secondly,the cubic specimens of the conductive concrete are tested for compressive strength with diphasic and triphasic conductive admixture.The results showed that with the increment of nano carbon black,the compressive strength of concrete is increased.Moreover,the effect of 50 kg/m³ and 70 kg/m³ SF for the flexural toughness of the beam with four-point bending test are also explored.As the steel fiber content increased,the toughness parameters(the equivalent flexural strength/f_eqq and energy absorption capacity/D^f_BZ)are greatly increased with compare to plain concrete.Deflection hardening（multiple cracking）behaviour is obtained with all the beams having 70 kg/m³ SF.However,some of the beams with the 50kg/m³SF have also shown deflection hardening behaviour but no multiple cracking.Furthermore,the improvement of toughness behaviour with an increment of NCB is insignificant both in diphasic and triphasic conductive admixture.And finally,the effect of conductive admixture on fractional change in resistance（FCR）and crack opening displacement（COD）values at 3 mm were measured.It can be concluded that SF50（conductive admixtures with 50 kg/m³SF）have better FCR₃ value as compare to SF70.Moreover,the triphasic conductive admixture has higher values than diphasic conductive admixture.The multiple cracking behaviour of the beam were self-monitored by Load-Time-FCR graph.It is concluded after the experimental test results that the multiple cracks can be detect with the relationship of Load-Time and FCR-Time graph.The results indicate that when the first crack occurs,the FCR of conductive concrete beams shows a linear increment in the FCR-time curve,however when the second crack occurs a clear sign of change in FCR increasing rate is observed.This change in the FCR-time curve is an evidence of the second crack which is counter checked by the visual inspection during the experimental process.