This work studies the effect of binder and fibre quantity on the mechanical behaviour of "Baixo Mondego" soft soil, chemically stabilised with binders and reinforced/non-reinforced with short polypropylene fibres. The experimental programme was comprised by four types of tests, one to evaluate compressive strength (unconfined compressive strength test) and three to evaluate tensile strength (direct tensile strength test, split tensile strength test and flexural strength test). The results indicate that increases in binder content cause an increment in the stiffness, the compressive and tensile strength, but have a lower impact on the specimens reinforced with fibres. In general, the addition of a low quantity of fibres to the stabilised soft soil originates a decrease in the stiffness, compressive and direct tensile strength, a reduction of the loss of strength after peak and a change in behaviour, from brittle to more ductile. The results also reveal that the impact of the addition of fibres on the strength depends on the strain mechanism used in each test. Therefore, in flexural strength tests the impact of the fibres is significant, while in the direct tensile strength tests the inclusion of fibres has a negligible effect. Relationships between the compressive and tensile strength are presented at the end of this study, as well as between the tensile strengths evaluated from the direct tensile strength test, split tensile strength test and flexural strength test.

a b s t r a c tThe discovery of the unique properties of carbon nanotubes (CNT) did grow interest to its application in nanocomposites, for a wide variety of purposes. However, the greatest challenge for its application is associated with the natural tendency to aggregate, resulting in the loss of its beneficial properties. To overcome this problem it is common the use of surfactants and/or ultrasonic energy to promote their dispersion in suspension. This work is focused on the influence of surfactants' properties on the dispersion of carbon nanotubes and on the influence of the quality of the dispersions on the mechanical properties of stabilized soil. Two surfactants (Glycerox and Amber 4001 differing in molecular weight and charge) were fully characterized, followed by the study of which surfactant concentrations were more efficient on the dispersion of the CNTs. The characterization methods were based on light scattering techniques: Dynamic Light Scattering (DLS) for the hydrodynamic diameter and Static Light Scattering (SLS) for the molecular weight. Suspensions of CNTs were prepared in solutions of the aforementioned surfactants, with different concentrations, and further dispersion was promoted using ultra-sounds (20 kHz during 5 min). The dispersions of CNTs in these two surfactants were then fully characterized using again DLS. Finally, the dispersions of carbon nanotubes were added to the main agent responsible for soil stabilization, the binder (Portland cement type I 42.5R), and the mechanical behavior of the new composite material was studied by unconfined compression strength (UCS) tests. The results of the UCS tests led to conclude that the introduction of CNTs in the binder can have huge impact on the mechanical properties of the stabilized soil. Furthermore, the quality of the dispersion of CNTs has got a very high impact on the performance achieved. It was verified an improvement up to 77% on the compressive strength of the material and 155% on Young's modulus, referred to the reference test where no carbon nanotubes nor surfactant were added, fundamentally dependent on surfactant type and concentration used.

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