Experimental determination of the stress evolution in a prebent steel very high-performance concrete beam by means of an active stressmeter

This research programme consisted in validating a design method for the prediction of stress redistribution due to concrete creep within the cross-section of an innovative prebent steel VHPC beam. The knowledge of the compression state in concrete due to the technique of prebending was provided by a direct method of stress measurement in concrete by means of an active stressmeter. The system is composed by an embedded ultra flat jack with an oil pressure control to counterbalance continuously the strain signal from gauges located in the area of jack influence with the signal of the reference gauges located in a cross-section submitted to the same stress state as the one where the jack is. The recorded strain signals are in good agreement with the computed values. The accuracy of the stress variation measurements is about 0.2 MPa. During the 4 months of data recording in the instrumented prebent steel VHPC beam, the difference between the absolute stress value measured by the two active stressmeters and the computed stress has kept lower than 1 MPa