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Thixotropy is a distinctively rheological (time-dependent) phenomenon, which is found in many complex materials, especially colloidal systems as fine-grained soils. Clays are materials that can recover their initial strength after remoulding. The estimation of such recovering allows optimising the designs of geotechnical structures and then reducing the construction costs. This paper presents the results of a study that evaluates the thixotropy phenomenon of clayey soils. The work methodology involved an experimental plan composed of five phases: (i) extraction of samples; (ii) physical and mechanical characterisation of the undisturbed samples; (iii) remoulding of samples after testing; (iv) storage of remoulded samples; and (v) evaluation of undrained shear strength recovering for 0, 15, 30, and 60 days after remoulding. Undisturbed samples were collected using Shelby tube from the municipality of Madrid, near Bogotá, in Colombia. Results were analysed in terms of sensitivity degree because this parameter allows estimating the shear strength recovering. Findings show that clayey soils from lacustrine deposits of Bogota high plateau do not loss completely all of their original mechanical properties after remoulding and exhibit a medium sensitive behaviour. Conclusions indicate there is a recovering of undrained shear strength along the time, in which the samples will recover their original state about one year after remoulding.

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Zinc oxide (ZnO) is a wide-gap semiconducting material which is chemically stable at high temperatures and has been shown to be compatible with asphalt binders. Additionally, semiconducting nanoparticles such as ZnO could help to improve urban air quality. This has encouraged the use of this material as a binder and asphalt mix modifier. A review on the use of ZnO as an asphalt binder modifier was conducted in this study. Based on the review which we carried out, the following were mainly synthesized and described: the content of ZnO as a modifier, the mixing processes of the binder and the ZnO, the manufacturing process of the modified mix, the type of asphalt binder and/or modified mix, the tests carried out, general conclusions, and environmental effects. ZnO micro-particles increase the aging resistance of the asphalt binder (mainly to ultraviolet radiation (UV)). ZnO tends to increase rutting resistance and adhesion with aggregates, improving resistance to moisture damage. It also tends to improve binder fatigue resistance. At low service temperatures, the performance of ZnO as a modifier is unclear, and further studies should be performed. Few studies have evaluated the effect of ZnO on the environment when it is used as an asphalt binder modifier. Future studies should evaluate the effect of this technology on the environment, perform economic analyses, evaluate the physical–chemical interaction of the modified binder with the aggregate, and measure the long-term properties of asphalt mixtures. ZnO is viewed as an environmentally friendly material and as a promising modifier of asphalt binders for pavement construction.