Acoustic Emission Activity in Ductile Fiber Reinforced Cementitious Composites

This study was performed in the framework of a research program focused in the development of high tenacity polypropylene (PP) fibres for the reinforcement of cementitious composites. Precedent reports have demonstrated how micromechanical modelling was used to attain significant improvements with respect to the macroscopic properties of cementitious composites. These improvements have been based on the optimized composition of the sheath of bi-component core/sheath fibers that resulted in a better interfacial frictional and chemical bonding of the fibers with the cement matrix. The principal aim of this study has been to evaluate the characteristics of AE data and correlate them with the particular fiber reinforcement. The analysis has led to the observation of clear distinct behaviour for each of the different types of fibers. Those types were namely polypropylene with variant surface coatings and polyvinyl alcohol (PVA) fibres. The results of the ‘source location’ data were compared to visually observed cracks during runtime with a microscope, so as to verify the supposed cracking behavior uniformity of the PP fiber composites in contrast to the localized cracking of the PVA composites. Statistical measures as the number of events and the accumulated energy of the recorded signals were to allow one to infer the underlying reinforcement. Introduction: Cementitious composite materials have met increasing use in civil constructions during the last decade. This mainly comes as a consequence to the quality improvements regarding their macroscopic properties combined with a low cost production. One of the leading efforts in this respect has been focused to polypropylene (PP) fibres that are relatively inexpensive and worldwide available. Moreover, processing with classical melt spinning technologies adds up to make it more attractive as a raw material. Ordinary PP fibres are extensively used for the reinforcement of concrete but their use for cementitious composites was until recently inefficient. The overall mechanical performance had to be modified in order to meet strict design requirements. To this end, understanding the mechanism of cement reinforcement with fibres was needed. On the basis of this understanding, that was gradually developed, micromechanical models [1-4] demonstrated that the macroscopic behaviour could be simulated using various micromechanical properties of the fibre-matrix system. Moreover, it was shown that the crack propagation can be unstable even though the fibre strength is relatively high and that the fractural behaviour is optimised with increasing the interfacial bonding of the fibres to the cement matrix. According to this theoretical analysis, it was straightforward to find the origin of the inadequacy of PP fibre in interfacial bonding with cement into the properties of low surface energy (hydrophobic character) and roughness. Therefore the manufacturing that followed focused on improving these surface properties as well as the strength of the ordinary PP fibres by developing bicomponent sheath/core type of fibres, the technology of which is patented [5, 6]. The objective of this study was to establish the AE characteristics for the different types of reinforcement so as to enable the correlation with the micromechanical properties. One could therefore derive the composition and acquire further insight in the fractural behaviour of the structure. There was a clear indication of the change of the AE activity as the different types of fibres were applied in the composite. Preparation of Samples-Experimental procedure: The composite plates were manufactured by Redco NV in their pilot bicomponent spinning plant. They consisted of various types of PP fibers in terms of sheath composition as well as PVA fibers reinforced cement matrix. The weight content of the fibers for all the plates is 1.7%. The different codenames denote the particular composition and surface processing of the fiber used. PVA corresponds to PVA fibers, and all the others are differently processed PP fibers. The plates were numbered and afterwards cut in 3 specimens named alphabetically. In the relevant nomenclature, the name of a sample is consisted of the number of the plate, then the code and finally the letter denoting the particular part of the plate. After the preliminary tests, it was decided to use end taps which were glued on the edges inserted in the grips so as to avoid stress concentration and for the partial elimination of strong noise sources originated from the same area. The dimensions of the cross-section of each of the specimens were measured so as to calculate the macroscopic stress levels developed. Fig. 1. Experimental Setup. The specimens were subjected in a tensile test with a constant displacement rate equal to 0.3 mm/min. This displacement rate was selected among others after the initial tests in order to optimize the duration of the test. The experimental setup is depicted in figure 1. Two sensors were used to monitor the acoustic emission activity. Unfortunately the size of the specimens did not allow the use of guard sensors, which act as filters rejecting noise originated at the grips. Thus, post processing was needed to perform this filtering. Such signals are few and are characterized by abnormally long duration and high energy. A microscope was also set up so as to enable monitoring the onset of cracking and its evolution. Results-Discussion: The fractural behaviour of the composite material is characteristic for the reinforcing fiber type. The material degradation initiates always with fracture of the brittle matrix. Crack propagation leads to stress concentration around the fibers, which starts pulling them out as the interfacial shear strength is not adequately high. The fibers are also subjected to higher tensile stresses as the matrix is broken. So long as the crack closing stress from fiber bridging does not exceed a certain limit a new crack might initiate elsewhere. Otherwise, further crack propagation entails so high tensile stresses for the fibers that the latter start breaking. PVA composites develop generally one crack, which propagates and becomes fatal (fig 2a). The PVA fibers of PVA are comparatively quite brittle and exhibit high strength. In contrast, for PP-A and PP-B specimens, several cracks are developed around one or more significant cracks due to the redistribution of stresses. These cracks accumulate, evolve and cause eventually failure (fig. 2b). Pictures that were taken with the microscope demonstrate the different behaviour. One of the aims of the study was to find a parameter related to AE activity which would enable to predict at the initial loading stage the final failure behaviour.