Spatio-Temporal Distribution of the Sources of Acoustic Events in Notched Fiber-Reinforced Concrete Beams under Three-Point Bending

Abstract

The acoustic activity, generated in notched, beam-shaped concrete specimens, loaded under three-point bending, is studied in terms of the position of the sources of acoustic events, and the frequency of their generation. Both plain specimens (without any internal reinforcement) and specimens reinforced with various types of short fibers were tested. The target of the study is to investigate the existence of indices that could be considered as pre-failure indicators of the upcoming fracture. In addition, an attempt is undertaken to classify the damage mechanisms activated to tensile or shear nature. Considering comparatively the spatio-temporal evolution of the position of the acoustic sources and the respective temporal evolution of the frequency of generation of acoustic events, it was concluded that for relatively low load levels the acoustic sources are rather randomly distributed all over the volume of the specimens. As the load increases toward its maximum value, the acoustic sources tend to accumulate in the immediate vicinity of the crown of the notch and the average distance between them approaches a minimum value. When this minimum value is attained, the load is maximized and the generation frequency of the acoustic events increases rapidly. The simultaneous fulfillment of these three conditions is observed a few seconds before the onset of propagation of the catastrophic macrocrack for all classes of specimens tested, providing a kind of warning signal about the upcoming fracture. Moreover, the classification of the damage mechanisms to tensile and shear ones revealed a crucial difference between the plain and the reinforced specimens after the maximization of the load applied. Indeed, while for the plain specimens, the prevailing damage mechanism is tensile microcracking, for the reinforced specimens a balance between tensile and shear damage mechanisms is observed after the load applied has attained its peak and starts decreasing.

Keywords: acoustic emissions; acoustic sources; fiber-reinforced concrete; plain concrete; three-point bending.

Figure 1

(a) The spatial arrangement of the eight acoustic sensors; dashed lines and light green color indicate sensors mounted at the rear face of the specimens; (b) a typical specimen before loading.

Figure 2

The mechanical response of typical specimens: The load applied versus the displacement.

Figure 3

(a) The reference system adopted; (b) defining the conventional fracture instant for the reinforced specimens.

Figure 4

The temporal evolution of (a) the frequency of generation of acoustic events, and (b) the average Euclidean distance between the acoustic sources, in juxtaposition to the respective evolution of the load applied. (c) The spatial evolution of the acoustic sources (specimen without reinforcing fibers).

Figure 5

(a) The temporal evolution of (a) the frequency of generation of acoustic events, and (b) the average Euclidean distance between the acoustic sources, in juxtaposition to the respective evolution of the load applied. (c) The spatial evolution of the acoustic sources (specimen with polyolefin reinforcing fibers).

Figure 6

(a) The temporal evolution of (a) the frequency of generation of acoustic events, and (b) the average Euclidean distance between the acoustic sources, in juxtaposition to the respective evolution of the load applied. (c) The spatial evolution of the acoustic sources (specimen with polypropylene reinforcing fibers).

Figure 7

(a) The temporal evolution of (a) the frequency of generation of acoustic events, and (b) the average Euclidean distance between the acoustic sources, in juxtaposition to the respective evolution of the load applied. (c) The spatial evolution of the acoustic sources (specimen with steel reinforcing fibers).

Figure 8

Classification of the acoustic signals detected as tensile (above the diagonal, the slope of which is equal to 10) and as shear (below the diagonal) for the four tests analyzed in Section 4 (grey rectangles correspond to Group I while red ones to Group II).

Figure 9

The temporal evolution of the index $\overline{\mathrm{m}}$ in juxtaposition to the respective evolution of the load applied versus the (t_f − τ) parameter for all four experiments considered.

Figure 10

The temporal evolution of the average distance between the acoustic sources in terms of the time to-failure parameter, for all specimens analyzed, in juxtaposition to the respective evolution of the load applied.