PSI - Issue 5

Annamaria Cividini et al. / Procedia Structural Integrity 5 (2017) 1072–1077 Cividini / Structural Integrity Procedia 00 (2017) 000 – 000

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Fig. 3. Variation of the Young modulus E with the maximum deviatoric stress q max for adobe specimens taken from existing constructions in Aviero district (Portugal): the results of the unconfined compression tests are presented in Silveira et al. (2012).

3. Influence of fiber content

The investigation on re-constituted composite samples intends to verify the influence of the fiber content on the compressive strength. The specimens were prepared remolding kaolin at water content equal to the liquid limit and adding small quantity of natural fibers (with percentage of 1%, 2% and 5% by mass of dry kaolin) and the corresponding amount of water. To represent the limiting case of very 'short' inclusion, some specimens were prepared using, instead of fibers, wood sawdust (in percentage from 5% to 25%). The Atterberg limits of the Speswhite kaolin used in this investigation are PL=30% and LL=59%. It is worthwhile to remark the difference with Aliano finer part, leading to specimens characterized by lower resistance, however the manufactured kaolin powder is immediately available for the specimen preparation. The cylindrical molds were stored at a temperature of about 40. C for one month, leading to specimens having unit weight variable from 15.9 to 13.8 kN/m 3 when containing plant fibers and from 16 to 12 kN/m 3 with sawdust. Then the uniaxial compression tests were carried out at a constant displacement rate of 0.03 mm/min and included two or more unloading-reloading cycles. Assuming linear elastic behaviour, two different values of the Young modulus can be evaluated, that is E o for the initial loading branch of the stress-strain curve, and E ur considering the unloading-reloading cycle. Their variation with the compressive strength q max is presented in Figure 4 on the left and it can be observed that E ur values are, in the average, three times larger than E o ones. With reference to the initial Young modulus E o , the diagram on the right hand side in Figure 4 shows that the data for the composite specimens lie on a straight line, since both elastic modulus and shear resistance are influenced by the fiber content. Note also that the presence of sawdust up to 20% slightly modifies the kaolin mechanical properties evaluated in uniaxial compression, while even a small amount of fibers leads to a non negligible reduction of the mechanical properties (see for instance Readle et al., 2016). Consequently, for a better understanding of the influence of the fiber content, it seems necessary to continue the investigation, carrying out triaxial tests on cylindrical specimens or, at least, direct shear tests on the composite material. The paper discusses some aspects of the geotechnical testing suitable for the mechanical characterization of composite materials like adobe used in historical buildings and monuments. The laboratory investigation allows (a) to assess the evolution of the shear resistance, providing the parameters for modelling the strain softening behaviour in the finite element analysis of structural and geotechnical remedial provisions and (b) to verify the not negligible influence of the fiber content. The last aspect will require further experimental investigation in the laboratory and in the field, in order to properly account for the inhomogeneous behaviour in the numerical modelling of adobe structures. 4. Concluding remarks