Micro-damage test of concrete working stress using single-groove method
The concrete working stress is the stress increment relative to the stress-free state. To accurately measure the concrete working stress,a micro-damage method for measuring concrete working stress,i.e.,the single-groove method,was proposed based on the concept of 'strain partial release'. The single-groove method released stress by cutting a shallow and narrow curved groove and calculates stress by measured strains. The method's effectiveness was verified using numerical simulations,and the influences of the parameters,such as the elastic modulus of concrete,on the strain release rate were analyzed. The strain release rate was the ratio of the strain released by cutting to the initial strain in concrete. The effects of the parameters,including the stress magnitude and groove depth,on the testing accuracy were investigated by experiments. The results show that the strain release rate is uniquely determined by the shape and size of the groove and is independent of the elastic modulus,stress magnitude,and groove width. Therefore,given the shape and size of the groove,the corresponding strain release rate can be determined by numerical simulation or experiment. The release strain is distributed in an 'M' shape along both sides of the groove,and increases first and then declines with the distance away from the groove. The strain gauge shall be arranged where the strain is significant and monotonic. Using the single-groove method to measure the concrete working stress,it is recommended to utilize a 50 mm strain gauge and arrange it 25 mm away from the edge of the groove. The deeper the groove,the greater the disturbance strain generated by cutting. The disturbance strain is compressive and ranges from 12 to 18με when the groove depth is less than 35 mm. The disturbance strain can be deducted from the test strain to eliminate its impact on testing accuracy. The deeper the groove,the higher the testing accuracy,but the greater the damage to the concrete. Using a cutting disc of a diameter of 125 mm,the groove depth ranging from 30 to 35 mm is recommended. The single-groove method can measure the concrete working stress and has high accuracy. For the test samples,the measurement error is lower than 8.5％.
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