3D mapping of the stiffness evolution of SAP concrete through elastic wave tomography

Early-stage phenomena during concrete curing greatly influence its final mechanical properties, making hydration monitoring crucial. Non-destructive techniques have, therefore, gained great attention for early-age concrete assessment. This study examines the elastic wave velocity and stiffness development of hardening concrete cubes using elastic wave tomography (EWT) at different curing stages. Simultaneously, ultrasonic pulse velocity (UPV) measurements with a pseudorandom noise (PRN) pulse are performed and compared to a standardized UPV device. Monitoring begins 7 hours after mixing and continues until 70 hours. A three-dimensional (3D) velocity map compares conventional concrete with concrete containing superabsorbent polymers (SAPs), a shrinkage-reducing admixture. SAPs reduce the compressive strength of concrete by increasing the porosity, affecting the elastic wave velocity evolution. However, internal curing by SAPs promotes hydration product formation, partially offsetting strength loss. EWT provides global insights into SAP uniformity across specimens, contrasting with traditional single-path UPV measurements. Furthermore, early-age stiffness evolution, linked to UPV, is a good indicator of final mechanical properties. Results show that SAP concrete exhibits slower 3D stiffness development than conventional concrete but supports hydration over a longer period. Continuous wavelet transform (CWT) analysis of PRN signals shows prolonged signal transmission development, indicating a more thorough hydration evolution in SAP concrete.