Original Research Article
Study on mechanical acoustic emission sources in fresh concrete

https://doi.org/10.1016/j.acme.2017.12.004Get rights and content

Abstract

Acoustic emission testing has proven very useful as a non-invasive monitoring method for several material processes. Recently several studies have emerged related to fresh concrete monitoring, as AE has the sensitivity to record waves from many different processes, even though no external load is applied to the specimens. Due to the complexity of the activity including among others formation of hydrates, settlement, water migration, early age cracking, the accumulated AE cannot be easily explained. In the present paper, two mechanisms of mechanical origin (aggregate and bubble movement through the paste) are isolated and the characteristics of their emissions are studied. Experimental results are complemented with numerical simulations to enhance the understanding due to the complexity of the wave propagation problem.

Introduction

The physical and chemical processes taking place during setting and hardening of concrete are extremely complex. Even though concrete technology has made great advancements, the exact interpretation of phenomena and interaction between different physical and chemical parameters are yet to be precisely determined. Monitoring of the early stage of the material is important as this stage defines in a great degree the final properties of the hardened concrete. Within the different monitoring techniques, acoustic emission (AE) has been recently increasingly used as it shows sensitivity capturing numerous elastic wave signals during the setting of the material and as early as from the moment of mixing. As will be discussed below, many processes occur like settlement, formation of bubbles and hydrates, mobility of bubbles and water and shrinkage cracking, among others. Since all of them may overlap in time, it is in general a very difficult task to explain the origin of AE populations. This paper wishes to contribute in the field by isolating and examining two physical mechanisms, namely impact of aggregates due to settlement/segregation and bubble release. These are processes that occur while the cement-based material is still fresh and viscous before the matrix hardens and restrains any mobility. Experiments are conducted by impacting aggregates in paste in a controlled way as well as creating bubbles by inducing air pressure in cement paste through a hose. Results are compared with reference conditions like bubbles in water matrix and aggregate impact on empty mold. In addition, numerical simulations of wave propagation are conducted to enlighten the propagation path and help to understand the conditions under which the different signals are received.

Section snippets

Brief overview of acoustic emission

The AE technique detects stress waves emitted by irreversible processes within a material. These processes may be damage propagation of any form, other specific processes, like leakage from a pipe or the processes occurring in fresh concrete. In a general case, piezoelectric transducers are applied on the surface of the material to transform pressure changes on their surface in electric waveforms. These signals are amplified and driven to the acquisition board where they are stored in a digital

Brief overview of AE monitoring of fresh cement-based materials

This overview will focus on cases where the monitoring starts from the liquid stage, meaning just minutes after mixing. However, there are also studies on hardening cement when the material is not yet fully hardened and is prone to shrinkage cracking, but it can still be considered “fresh”, like [12], [13], [14], [15].

Van Den Abeele et al. [16] used two sensors of 375 kHz resonance placed on top of waveguides to collect signals from the microstructural activity of fresh cement mortar. The main

Objectives

Still many questions are open. A basic one concerns the exact sources of AE in fresh cement and concrete. In addition, the effect of wave propagation between the source and the receiver is also of great importance.

This study aims at making a contribution in the field by studying in isolated fashion two mechanisms that are active during the very early age of cement; this is mechanical impact by aggregates and air bubble formation and movement. Air bubbles are in any case included in the matrix

Experimental setup

The applied AE system used in this study is a Micro-II express of Mistras Group that allows recording of the AE parameters and the full waveform. AE monitoring was applied by means of three piezoelectric sensors (R15, Mistras). Specifically, the AE sensors of 150 kHz were attached on the outer side of the steel mold which has the shape of 40 mm × 40 mm × 200 mm (internal dimensions) and thickness of 10 mm. To improve the coupling between the mold surface and the sensors, a viscous silicon grease was

Aggregate impacts

Each time a sphere was let to drop in an empty mold (reference), it had several bounces, which could be easily measured by the corresponding AE signals. Specifically, each drop was accompanied by three hits (one received by each sensor). The sensor at the bottom (Section 8), which was closer to the impact point, registered each time the highest amount of energy. After the sphere bounced on the bottom surface another sequence of three hits was recorded by all sensors and with the same order in

Simulation of impact

Numerical simulations are conducted to enlighten the conditions of propagation. Using AE sensors at the predefined locations gives information on the energy that arrives on the sensor surface but it provides less information on the followed path. In the specific case, it is important to know if most of the energy propagates through the metal or through the liquid. Herein simulations are conducted with the commercially available software WAVE2000. It solves the 2D elastic wave equations using a

Discussion

Simulation results show that sources in contact to the mold are much more likely to produce a noticeable acoustic wave. Using the same amplitude of excitation directly on the mold (case of Fig. 10), the received signals is hundreds of times stronger than the case of Fig. 12 where the excitation is within the paste. This can be very well understood in terms of the acoustic impedance mismatch between the fresh liquid cement and the stiff metal mold. Taking into account the properties supplied in

Conclusions

Two physical processes occurring in fresh cement-based materials are targeted as to their potential to create AE as well as the possibility to be characterized through it. The basic conclusions can be summarized as follows:

  • Movement of aggregates due to settlement and segregation as well as creation and movement of bubbles through the liquid matrix constitute detectable AE sources.

  • Processes that occur close to the container mold wall are received by the sensors with higher amplitude indicating

Ethical statement

The article was conducted according to ethical standards.

Acknowledgement

The financial support of FWO (Fonds Wetenschappelijk Onderzoek-Vlaanderen) through grant G.0123.15N is gratefully acknowledged.
Funding body

FWO (Fonds Wetenschappelijk Onderzoek-Vlaanderen).

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