# Detecting Critical Damage in Concrete by Taking Advantage of Acoustic Events with an Amplitude Exceeding Their Mean Value

## TL;DR

This paper introduces a new method to detect when concrete is about to fracture by analyzing acoustic events with high amplitude.

## Contribution

A novel approach using the slope of cumulative acoustic events in natural time to signal impending fracture is proposed.

## Key findings

- The slope of the cumulative acoustic event series reaches a constant value before peak load.
- This constant slope indicates increased energy in acoustic events and microcrack coalescence.
- Results align with other methods like acoustic event frequency and source distance analysis.

## Abstract

A time series of acoustic events, the amplitude of which exceeds the respective average value, is studied.

The evolution of their cumulative number is considered versus natural time.

The slope of this function attains a constant value slightly before the applied load attains its maximum value.

Attainment of this constant value suggests that acoustic events are now produced with increased energy content.

From this instant on, the networks of microcracks become richer, leading inevitably to their mutual coalescence.

Thus, attainment of the above constant value signals entrance to the critical stage of impending fracture.

A novel approach for detecting preliminary signals designating upcoming entrance of a loaded system to the critical stage of impending fracture is assessed. The approach is based on the analysis of a time series of the cumulative number of acoustic events, the amplitude of which exceeds the respective average value of all the events recorded during loading. Using the “sliding window” technique, the average slope of the evolution of this time series is quantified, either against conventional or natural time (the latter provides a more detailed view of the stage before macroscopic fracture, during which the “information” gathered is very densely packed in a short interval). For the needs of this study, data from a previously published experimental protocol are exploited. The protocol comprised notched, beam-shaped specimens, made of either plain or fiber-reinforced concrete, under three-point bending. It is concluded that the slope of the evolution of the above time series systematically attains a value equal to unity slightly before the applied load attains its peak value. The results of the present analysis are in qualitative agreement with the respective ones based on either the instantaneous frequency of generation of acoustic events or the Euclidean distance between the sources of acoustic signals.