The role of coating in preserving the bonding between steel fibers and concrete is investigated in this paper. Straight
types of fibers with and without chemical coating are used in steel fiber reinforced concrete mixes. The specimens are
tested in bending with concurrent monitoring of their acoustic emission activity throughout the failure process using two
broadband sensors. The different stages of fracture (before, during and after main crack formation) exhibit different
acoustic fingerprints, depending on the mechanisms that are active during failure (concrete matrix micro-cracking,
macro-cracking and fiber pull out). Additionally, it was seen that the acoustic emission behaviour exhibits distinct
characteristics between coated and uncoated fiber specimens. Specifically, the frequency of the emitted waves is much
lower for uncoated fiber specimens, especially after the main fracture incident, during the fiber pull out stage of failure.
Additionally, the duration and the rise time of the acquired waveforms are much higher for uncoated specimens. These
indices are used to distinguish between tensile and shear fracture in concrete and suggest that friction is much stronger
for the uncoated fibers. On the other hand, specimens with coated fibers exhibit more tensile characteristics, more likely
due to the fact that the bond between fibers and concrete matrix is stronger. The fibers therefore, are not simply pulled
out but also detach a small volume of the brittle concrete matrix surrounding them. It seems that the effect of chemical
coating can be assessed by acoustic emission parameters additionally to the macroscopic measurements of ultimate
One of the most frequent problems in concrete structures is corrosion of metal reinforcement. It occurs when the steel
reinforcement is exposed to environmental agents. The corrosion products occupy greater volume than the steel
consumed, leading to internal expansion stresses. When the stresses exceed concrete strength, eventually lead to
corrosion-induced cracking beneath the surface. These cracks do not show any visual sign until they break the surface,
exposing the structure to more accelerated deterioration. In order to develop a methodology for sub-surface damage
characterization, a combination of non destructive testing (NDT) techniques was applied. Thermography is specialized in
subsurface damage identification due to anomalies that inhomogeneities impose on the temperature field. Additionally,
ultrasonic surface waves are constrained near the surface and therefore, are ideal for characterization of near-surface
damage. In this study, an infrared camera scans the specimen in order to indicate the position of potential damage. For
cases of small cracks, the specimens are allowed to cool and the cooling-off curve is monitored for more precise results.
Consequently, ultrasonic sensors are placed on the specified part of the surface in order to make a more detailed
assessment for the depth of the crack. Although there is no visual sign of damage, surface waves are influenced in terms
of velocity and attenuation. The combination of the NDT techniques seems promising for real structures assessment.
The acoustic emission (AE) behaviour of steel fibre reinforced concrete is studied in this paper. The experiments were
conducted in four-point bending with concurrent monitoring of AE signals. The sensors used, were of broadband
response in order to capture a wide range of fracturing phenomena. The results indicate that AE parameters undergo
significant changes much earlier than the final fracture of the specimens, even if the AE hit rate seems approximately
constant. Specifically, the Ib-value which takes into account the amplitude distribution of the recent AE hits decreases
when the load reaches about 60-70 % of its maximum value. Additionally, the average frequency of the signals decreases
abruptly when a fracture incident occurs, indicating that matrix cracking events produce higher frequencies than fibre
pull-out events. It is concluded that proper study of AE parameters enables the characterization of structural health of large structures in cases where remote monitoring is applied.
This work deals with the AE behavior of concrete under four-point bending. Different contents of steel fibers were
included to investigate their influence on the load-bearing capacity and on the fracture mechanisms. The AE waveform
characteristics revealed that, although tension was the dominant mechanism of fracture for the plain material, the
increase in the fiber content resulted in extension of the shear failure due to improvement of the weak tensile properties
of concrete. Appropriate AE indices employed for early warning prior to macroscopic failure can lead to more suitable
design of the reinforcement, in order to withstand the specific stresses.