Characterization of stress-dependent ultrasonic nonlinearity variation in concrete under cyclic loading using nonlinear resonant ultrasonic method

Highlights

• Compressive loading effects on decreasing the ultrasonic nonlinearity.

• Cyclic compressive loading effects on increasing the ultrasonic nonlinearity.

• Thermal damage of concrete effects on increasing the ultrasonic nonlinearity.

Abstract

In this work, an experimental study was carried out to investigate the stress-dependent characteristics of concrete under cyclic loading, based on a nonlinear resonant ultrasonic method. Since concrete subjected to cyclic loading accompanies variations in the elastic and plastic characteristics of concrete in relation to microstructural changes, the adoption of a nonlinear ultrasonic approach is required, which has advantages for the evaluation of micro-cracks in concrete. In this experimental study, two types of loading were considered, namely, continuously increased and cyclic repeated, to identify their effects on the ultrasonic nonlinearity of concrete and the load-induced damage. An additional experiment on the exposure of concrete samples to high temperature was conducted to further investigate the effects of cyclic loading accompanied by an increased occurrence of micro-cracking. A comparison analysis was also performed on the experimental results, and the potential to monitor the stress history of concrete by using the nonlinear resonant ultrasonic method was evaluated.

Introduction

Concrete is a popular material in the construction industry due to its advantages of cost-effectiveness, water-resistance, thermal characteristics (similar to the steel), and low thermal conductivity. Generally, the compressive strength of concrete is 10 times higher than its tensile strength. Therefore, for most concrete structures, concrete plays a major role in sustaining compressive forces according to the self-weight of structural members and external loadings. It is also intended to resist compressive stress on concrete structures. However, concrete structures can suffer reduced durability if they are exposed to excessive loading or other damaging conditions, which can cause deterioration in the concrete microstructure and its corresponding mechanical properties [1]. There are several causes of increased micro-cracks in concrete. Most of them are based on chemical mechanisms, such as alkali-silica reaction, sulfate attack, carbonization, and fire-induced damage, while others are based on physical mechanisms, such as freeze-thaw damage and load-induced damage. Generated micro-cracks weaken the binding forces of the cement matrix, and the bonding forces of the interfacial zone between the cement matrix and aggregates [2]. Among them, the stress-dependent characteristics of concrete have been investigated to evaluate the load-induced damage state or to monitor the load level of concrete structures. Several studies have concentrated on monitoring the compressive or tensile load history by determining the acoustoelasticity of concrete, which represents the load-dependent characteristics of concrete. It is mainly evaluated by measuring ultrasonic velocity based on coda wave interferometry, which sensitively reflects changes in ultrasonic velocity compared to the conventional method of measuring ultrasonic velocity [3], [4], [5], [6].

On the other hand, other studies have monitored the stress-dependent characteristics of concrete by using nonlinear ultrasonic methods, such as the higher harmonics method, the time-shift method, and the nonlinear resonant ultrasonic method, for measuring ultrasonic nonlinearity, which is based on the nonlinear acoustic behavior of concrete caused by its stress-strain nonlinearity [7], [8], [9], [10]. These nonlinear ultrasonic methods have also been reported to sensitively evaluate the conditions that may lead to micro-cracks in concrete, such as alkali-silica reaction, fire-damage, carbonation, and load-induced damage [11], [12], [13], [14], [15], [16], [17], [18]. Shah reported that the higher harmonics characteristics of concrete are dependent on the continuous increase of compressive loading, at 20%, 40%, 60%, and 80% compressive strength [7]. Antonaci et al. reported that the compressive stress level of concrete can be effectively monitored by measuring the nonlinear ultrasonic characteristics of concrete by using the time-shift method [8], [9]. Kim et al. reported in their preliminary study the possibility of using the nonlinear resonant ultrasonic method to monitor the compressive stress state of concrete, and the ultrasonic nonlinearity results were compared to experimental results obtained by measuring ultrasonic pulse velocity [10]. Previous studies have identified the stress-dependent characteristics of the nonlinear ultrasonic behavior of concrete, and they have suggested the possibility of its application in monitoring the stress state of concrete. Additionally, it seems that increases in stress-induced damage are related to increased measured ultrasonic nonlinearity.

In this study, an experimental research was performed to characterize the stress-dependent characteristics of concrete based on the measurement of the nonlinearity parameter, the purpose of which was to identify the effects of cyclic loading on ultrasonic nonlinearity and to evaluate the load-induced damage of concrete using the nonlinear resonant ultrasonic method. For this purpose, two types of loading were considered, and the nonlinearity parameter was measured for concrete samples under various levels of compressive loading. Additionally, experimental procedures were performed on the concrete samples after exposure to high temperature to identify the effects of an increasing number of micro-cracks distributed throughout the concrete on the stress-dependent ultrasonic nonlinearity of the concrete. Based on the test results, the effects of continuous and cyclic loading histories as well as an increasing number of micro-cracks in the concrete on ultrasonic nonlinearity and load-induced damage were investigated. Finally, a comparison analysis was performed based on the experimental results.