ULTRASONIC PULSE VELOCITY FOR MONITORING THE SUSCEPTIBILITY OF CONCRETE TO SODIUM SULPHATE AND WETTING-DRYING CYCLES

Abstract

The  external  sulphate  attack  of  concrete  is  related  to  the  formation  of  expansive  sulphate-containing  compounds  in  the hardened state of the concrete samples. The aim of this research is to study the use  of the technique of ultrasonic pulse velocity (UPV) to estimate the mechanical and physical changes during an accelerated attack of sodium sulphate combined with  wetting-drying  cycles.  Each  cycle  consisted  in  wetting,  drying  in  oven  and  cooling.  This combination tries to replicate field conditions for concrete samples in sulphate-rich environments under climatic changes (wetting/drying). The experimental  program  consisted  of  visual  inspection,  development  of  cooling  curves,  X-ray  diffraction  analysis,  UPV measurements,  mass  and  volume  changes  as  well  as  compressive  strength  of  concrete  specimens.  Standard concrete cylinders of 100 x 200 mm were used and the accelerated attack was monitored each 15 cycles with a total of 120 cycles. The  results  showed  that  both  density  (physical)  and  compressive  strength  (mechanical)  measurements  agreed  with  the UPV behavior registered through the external sulphate attack. In addition, mathematical models relating the compression analysis  and  the  density  values  with  the  UPV  measures  were  constructed.  Finally, the  X-ray  diffraction  results  were successfully correlated with the developed mathematical models.