CHARACTERIZATION AND EVALUATION OF MORTAR COMPOSITES USING LINEAR AND NONLINEAR ULTRASONIC TECHNIQUES

Abstract

Engineering structures such as brick walls may start to weaken well before their projected service life due to environmental changes or natural disasters. This can result in untimely collapse of the structure. Mortar is used in construction for various purposes, including binding building materials together, creating a strong and durable foundation for structures, and filling gaps between bricks, stones, or other masonry units. Mortar is specified by proportions of certain volumes of cementitious material and sand combined with water that gives a workable mix. Earlier study shows that if a specific mix design (i.e. proportions of cement, sand and water) is followed, mortar with certain performance characteristics is reliably obtained. Workability is the most important property of mortar. It can be defined as the ability of the mortar to spread, under the small handheld tool (trowel), into all the cracks and gaps of the masonry unit. In mortar composites, the flaws on the micro-scale continue to spread, they eventually come together to create macro-cracks, which then continue to spread at the macro-scale level. In this study we will characterize/evaluate different types of mortar composites using ultrasonic guided waves and compare them to mechanical properties of mortar composites such as flexural and compressive strength. The ultrasonic guided wave propagation-based technique possesses special advantages since it can be effectively employed for large structures, suits for in-situ monitoring and is able to detect small damages/anomalies and change of state (wet mortar to dry mortar). However, when the ultrasonic guided wave propagation technique is employed for heterogeneous material like mortar composites, major challenge comes from the attenuation of the propagating wave. It has been reported that the nonlinear ultrasonic techniques are more sensitive than linear ultrasonic techniques to minute variations in the material properties. Thus, considerable attention is being paid recently to use nonlinear ultrasonic techniques for material characterization. Cement paste is the primary constituent of mortar that keeps all other constituents together and gives mortar composite its strength. During curing, the cement is developed as a binder by going through various chemical reactions. In this experimental study, ultrasonic guided wave testing is carried out on multiple mortar samples in transmission mode (also known as pitch-catch). The signals are generated using tuned ultrasound transducers. After the selection of optimal operational frequency (on the basis of signal to noise ratio). The obtained (or received signals) are processed to analyze the change in signal characteristics due to different types mortar composites. Nonlinear ultrasonic technique (NLU) called the SPC-I which is derived from the frequency spectra is deployed for extracting nonlinear features from the signal. Linear ultrasonic parameters such a shift in change in velocity are calculated and compared to nonlinear ultrasonic technique. Scanning Electron Microscopy images are used to evaluate curing process and are compared to nonlinear ultrasonic parameter SPC-I.