Comparison of the different fiber/polymer-soil matrix behavior
In a recently published study (Rivera-G(Smez et al., 2014) a new approach based on analyzing microscopic structural changes in stabilized specimens was introduced to try to understand the behavior of the swelling and shrinkage phenomena that occur in natural polymer stabilized soils when dried at room temperature, analyzing natural and nonnatural fibers. Four different types of soils from the Andalusia region of Spain, with varying clay mineral contents, and different plasticity index values, were combined with two different types of fibers: one was a synthetic fiber PP and the other was a natural fiber, wool (W). These samples were also combined with a form of alginate and then compressed into brick molds under laboratory conditions. After drying and curing, these samples were then subjected to a series of characterization and mechanical tests and then the results were compared and analyzed.
All the specimens were carefully dried to their initial water content, in an oven for 24 h and then at room temperature, to evaluate partial shrinkage and the relationship of this change was compared for different amounts of natural fiber within different soil samples. Microscopic structural changes in stabilized specimens were then studied with a scanning electron microscope (SEM) and the results from this analysis were compared with the results of mechanical tests. In addition, physical changes were compared with information relating to the plasticity index, Atterberg limits, and the chemical composition of each soil.
SEM has been shown to be a useful tool for the direct study of polymer—soil matrix interfaces. In particular, SEM studies have helped to illustrate the spatial relationships between the various components of matrices and reinforcement fibers. As can be seen in Figs. 4.6 and 4.7 different shrinkage degrees around the PP and wool fibers were measured depending on the type of fiber used. The soil retraction ranges were of a smaller margin in PP fibers (Fig. 4.6A—D) than in wool ones (Fig. 4.7A—D), giving a variation in these samples between 15 and 40 q,m.
When the samples were examined by SEM, different shrinkage degrees around the wool and PP fibers were detected depending on the type of fiber used. For PP fiber samples, it was observed that the clay polymer matrix produced fewer voids around the fiber independent of the type of soil. For the wool fiber mixes, similar shrinkage of the matrix around the fiber surface was observed in all soil types, although it was slightly increased within the white soil specimens.
Subsequently, the main factors, which affect the adhesion between the fibers and soils are: (1) the cohesive properties of the polymer—soil matrix; (2) the compression friction forces appearing on the surface of the reinforcing fiber due to shrinkage of the soil; and (3) the shear resistance of the polymer—soil matrix, due to the surface form and roughness of the fiber. The dimensional changes of natural fibers due to moisture and temperature variation also have an influence on all three of these adhesion characteristics, because during the mixing and drying of the soil, the natural fibers absorb water and expand. This swelling of the fibers initially pushes away the soil (at the microscopic level) and then at the end of the drying process the fibers lose the moisture and shrink back almost to their original
Figure 4.6 (A) Different samples of SEM of Polypropylene fiber in the white soil mix (X300); (B) SEM of Polypropylene fiber in the yellow soil mix (X400); (C) SEM of Polypropylene fiber in the black soil mix (X300); and (D) SEM of polypropylene fiber in the red soil mix (X300).
dimensions leaving very fine voids around themselves. This leads to an increased level of porosity within the polymer matrix and a degree of friction loss between fiber and soil. These results show that the observed loss of strength is not only due to the variation in fiber type but also more importantly the effect of the differing properties of different soil types.