Self-Sealing Process Evaluation Method Using Ultrasound Technique in Cement Composites with Mineral Additives

Abstract

The self-sealing process, associated with chemical and microstructural changes inside damaged cement-based composites, leads to the recovery of the original material integrity. Assessing the magnitude of internal changes in samples using non-destructive techniques to capture only the self-sealing effects is difficult. The challenge is evaluating the differences between subsequent observations in time and between samples with different properties. This paper proposes a new approach to the use of an ultrasonic technique for self-sealing investigation. The method allows the quantification of material changes strictly related to self-sealing processes, excluding changes caused by the naturally progressing hydration of binders. The applied ultrasonic pulse velocity (UPV) data processing procedure allows the investigation of material changes inside and near the cracks, the effects of stimulating the self-sealing of cement composites with mineral additives, and the assessment of changes over time. An important aspect of the method is the sample preparation procedure and testing conditions that reduce the impact of moisture content on the UPV measurements. New parameters allowing the quantitative characterization of the self-sealing process are proposed. The method was evaluated using cement mortars modified with siliceous fly ash with induced cracks 0 to 750 µm wide, which were then cured in water for 152 days. The maximum degree of effective crack filling as a result of autogenous self-sealing in the tested mortars was determined to range from 33% to 57%. Observations of the microstructure of the crack surface confirmed that apart from the volume of the newly formed products, the density of these products may have a key impact on the ultrasonic measurements of the self-sealing performance. The studies were supplemented by the examination of the compression strength of mortars, mortar sample scanning and computer image processing, and observations using an optical microscope and scanning electron microscope with energy dispersive spectroscopy.

Key Factors Determining the Self-Healing Ability of Cement-Based Composites with Mineral Additives

“…The lack of self-sealing effects for these cracks near the outer surface of the material is likely to be the result of a significant reduction of water permeability in this area as the consequence of crack sealing in deeper parts of the material. Similar observations were presented in [ 64 ] based on the analysis of material accumulation at the surface of sample fractures. The frequency distributions of full crack closure are very similar among cracks exceeding the width of 200 µm.…”

“…In the case of the quantity defined in Equation (2) based on UPV values, the differential values determined in this way provide a measure of changes caused only by the self-sealing process (called pure self-sealing). A more extensive interpretation and application of the is presented in [ 64 ]. where is the average ultrasonic pulse velocity for a given mortar at time t i ; is the average ultrasonic pulse velocity for a group of samples of a given mortar for the state immediately after cracks were induced; is the average ultrasonic pulse velocity of a given mortar immediately before crack induction at time t 0 ; and are the average ultrasonic pulse velocities for a group of control samples (without cracks) of a given mortar at the time t i and t 0 , respectively.…”

“…At each location, the measurement was repeated twice or more until a stable indication of the time the wave passed through the sample was obtained. The obtained UPV data for samples with defects and control samples were processed according to the procedure described in [ 64 ], which allows quantifying only the changes occurring in the course of crack self-sealing, i.e., changes in the degree of crack filling, neglecting the changes occurring in the course of progressive hydration of binders.…”

“…In the case of the quantity defined in Equation ( 2) based on UPV values, the differential values determined in this way provide a measure of changes caused only by the self-sealing process (called pure self-sealing). A more extensive interpretation and application of the USR t i is presented in [64].…”

Selected Properties of Self-Compacting Concrete with Recycled PET Aggregate

“…When the ultrasound wave passes through different media (materials), it is partially reflected and transmitted; therefore, its velocity decreases. First of all, the decrease in the ultrasound wave velocity is caused by the reduction in concrete density [ 46 ]. Moreover, as noted by Albano et al [ 43 ], due to the fact that the ultrasonic wave velocity is a function of the elastic properties and volume concentrations of the constituents when replacing natural fine aggregate with PET, the ultrasound wave velocity must decrease.…”