Fresh concrete monitoring by NDT techniques

Abstract

The most reliable examination for concrete is mechanical testing (usually compression) after hardening. However, ensuring the quality of fresh concrete before hardening, substantially reduces the possibility of performance at standards lowerthan designed. Curing of conventional cementitious material is complicated and becomes even more complicated considering the new generation of additives like super absorbent polymers and nano-reinforcement which may strongly modify cement behaviour. In order to obtain better control on the process from very early age, this study describes a combined approach of several monitoring techniques. Acoustic emission (AE) is applied to record the sources occurring during the first hours when concrete is still in liquid form and later when hardening takes place when shrinkage cracking occurs. In addition, pressure sensors register the development of capillary pressure in the matrix and indicate the moment of air entry into the System, which is the precursor of cracking. Settlement and shrinkage, measured both non-contact by Digital Image Correlation (DIC) and conventionally, as well as temperature shed light into the complex processes occurring into fresh concrete and help to verify the sources of AE. Among other, mechanical sources like aggregate impacts due to gravity and air bubbles release are isolated and tested in a dedicated experiment showing that these produce recordable AE despite the viscous nature of paste. The final aim is to develop a methodology to assess the quality of the fresh concrete from an early age, to possibly project to the final mechanical properties and to ensure a proper service life