Application of Acoustic Emission Method to Determine Critical Stress in Fibre Reinforced Mortar Beams

The objective of this investigation was to test the effectiveness of the Acoustic Emission (AE) mea-
surements in determining the critical stresses during four-point bending of mortar beams. Within the
measuring procedure the parameter σcr/σ300 was calculated and analysed. Additionally, the influence of
cement replacement by high calcium fly ash (HCFA) on the process of crack healing was discussed. Mortar
beams with different content of HCFA and reinforced by steel microfibres were prepared for tests. After
curing in standard conditions the beams were subjected to four-point bending test in order to introduce
the pre-cracking. Thereafter the beams were cured in the lime water and loaded after 56 and 112 days
in the same way as for the first time. Additionally the microstructure of mortars was studied in a stereo
optical microscope as well in an electron scanning microscope including the Energy Dispersive X-ray
analysis (EDX). The results of microstructural characterization of mortar containing HCFA from lignite
combustion are presented. The applied load level slightly exceeded the critical stress, producing intense
crack growth processes however did not significant affected the load capacity of the beams. During the
consecutive loading the decreasing tendency of σcr/σ300 ratio was noted. The obtained results confirm
that the latter parameter can be applied as a measure of the composite degradation level for the elements
carrying the repeated loads of amplitude close to the critical stress of the structure and also that the
cement replacement with HCFA influences the process of crack healing.