In this work, three different techniques are investigated for measuring and monitoring the crack-mouth opening displacement and the depth of the neutral axis of six beams, such as: electrical resistance strain gauges, digital image processing, and displacement transducers. Steel fibers reinforced concrete beams were subjected to bending and the load, displacement, compressive strain, crack-mouth opening displacement and depth of the neutral axis were monitored. For displacement transducers and digital image processing techniques it is showed equations to be used to correct the crack-mouth opening displacement and the depth of the neutral axis due to the beam displacement and rotation. The results show that electrical resistance strain gauges and displacement transducers techniques are more efficient to monitor the depth of the neutral axis. Using the technique of digital image processing it is observed that a small error in the measurement of grid deformations causes a great error in the measurement of the neutral axis position. The crack-mouth opening displacement is measured only by image processing and displacement transducers techniques, and a good agreement between the results from these two techniques can be seen. It is verified that the best technique is the displacement transducers, because with this it is possible to monitor and measure, both, the crack-mouth opening displacement and the depth of the neutral axis. Additionally, the displacement transducers technique is easier to be applied.
steel fiber reinforced concrete; steel fiber; depth of neutral axis; crack width; flexural test