Numerical And Experimental Investigation On Cement-based Sensors And Smart Concrete Column

Coupling action of such factors as loading fatigue,erosion of natural disasters and materials aging will lead to damage accumulation and resistance attenuation of concrete structures.As a result,catastrophic accident will be caused,thus resulting in significant property losses and casualties.Health monitoring is necessary in order to ensure service safety of concrete structures.Service states will be sensed in real-time,and emergency situations will be forecasted timely.Because the self-sensing cement-based composite possessing both function of structure and sensing,it is a good candidate for health monitoring of concrete structures.However,there are such disadvantages for the existing self-sensing cement-based composite as complex fabrication technique,high conductive filler content and low sensitivity.In addtion,in order to overcome problems of high cost and poor performance controllability of self-sensing cement-based composite in the large-scale preparation,embedding sensor type of self-sensing cement-based composite without aggregate is adopted when self-sensing cement-based composite is used in structures.However,there is difference of elastic modulus between sensor and concrete.This will affect mechanical properties of concrete structures and monitoring accuracy.In order to address the above issues,self-sensing cement-based composite and embedding sensor was developed by using hybird nano carbon tube and carbon black fillers.Sensor design and mechanical analysis on sensor embedded in concrete were conducted by numerical simulation.Finally,smart concrete column was fabricated by embedding sensor,and its stress and strain self-monitoring was realized.The main contents and conclusions are as follows:(1)Self-sensing cement-based composite with hybird nano carbon tube and carbon black fillers was fabricated with simple fabrication technique.Mechanical properties,electrical properties and sensing properties of the composite were studied.Percolation characteristic relation of the composite was obtained.Conductive mechanisms of the composite were analysed Experimental results showed that compressive strength and elastic modulus decrease with increase of fillers content,while poisson' ratio increases.Electrical conductivity and sensing property of the composite are improved significantly at lower filler content.Percolation threshold of the composite is 3.85vol.%.The composite featrues good repeatability and stability of sensing properties.Fractional change in resistivity(FCR)amplitude and stress/strain sensitivity are 23.4%and 2.32%·MPa-1/225 respectively under repeat compression.The largest FCR amplitude is 72.6%under monotonic compression up to failure.Sensing properties of the composite under different conditions(e.g.dimentions,loading amplitudes and loading rates)show good stability.Sensitivity of the composite decreases with increase of water to binder ratio.(2)Optimum design on dimensions,voltage electrode separation and surface roughness of sensor was carried out by using of finite element method.Stress and strain state of optimum sensor embedded in concrete column was analyzed.Effect of embedding location,small deflection angle of sensor,loading amplitudes,strength grades of concrete and eccentric compression on strain coordination degree between sensor and concrete was studied.Effect of sensor on ultimate strength of concrete column,stress-strain relationships in different parts of concrete and sensor,average stress/strain ralationships between concrete and sensor were also investigated.The research results showed that optimum dimension and voltage electrode separation of sensor are 20mm × 20mm × 40mm and 10mm,respectively.Strain coordination degree between sensor and concrete is decreased due to embedding sensor in surface of concrete column,deflection angle of sensor,increase of loading amplitudes and strength grades of concrete and eccentric compression.Sensor embedded in concrete is nearly subjected to a uniaxial stress state.Embedding sensor does not affect the ultimate strength of C20-C80 concrete column.(3)Self-sensing cement-based sensors with hybrid nano carbon tube and carbon black fillers were embedded in C30 and C50 concrete columns,respectively.Sensitivity of sensors embedded in smart concrete column was investigated.Influence of sensors on ultimate strength of concrete column and relation of strength matching among sensors and concrete were analyzed.Stress and strain monitoring formulas for concrete were set up,and were used to analyze the stress and strain of C30 and C50 concrete columns.Monitoring errors and correction method were obtained.The research results indicated that FCR of sensors features good corresponding relationship with stress and strain of concrete,and strain of sensors themselves.Loading process of concrete column can be monitored by FCR of sensors.When sensors were embedded in C30 and C50 concrete columns subjected to ultimate stress,the largest FCR amplitude of sensors are 51.67%and 52.56%,respectively.The largest strain sensitivity is 143.25 and 135.41,respectively.Ulimate strength of concrete columns does not decrease as sensors were embedded.Stress and strain of smart concrete column can be calculated accurately by monitoring formulas.Monitoring accuracy of stress and strain is increased after correction.Stress and strain self-monitoring of smart concrete can be achieved by embedding self-sensing cement-based sensors.