Department of Civil Engineering

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Browsing Department of Civil Engineering by Author "Agnello, Nina Maria"

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    Modelling the shrinkage behavior of recycled concrete aggregate and cement stabilised materials
    (Stellenbosch : Stellenbosch University, 2018-03) Agnello, Nina Maria; Rudman, Chantal; Jenkins, Kim; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.
    ENGLISH ABSTRACT: Cement stabilisation of subbases is an effective method to increase the strength, permeability, stability and performance of a pavement structure. Stabilisation is used to improve marginal quality materials, by recycling the in-situ material which conserves non-renewable gravel sources and lowers transportation costs. An alternative to stabilisation of virgin aggregates is to use Recycled Concrete Aggregate (RCA) which can have, some inherent self-cementation properties. RCA has the potential to be used as a lightly cemented material without any additional cement. RCA can be harvested from demolished buildings or old concrete pavements through crushing of the concrete and forming concrete aggregates. This saves quarrying for natural aggregate and reduces associated costs. Other environmental benefits include fewer landfills and the reduction in cement usage. Cement production is estimated to contribute 5% of global CO2 emissions, (Worrell et al., 2001). The primary failure of a stabilised layer is load induced fatigue cracking and shrinkage cracking. As RCA has some self-cementing properties due to the active latent cement particles present, there is potential for RCA to undergo similar failure mechanisms. The challenge with RCA is that it can be considered an inconsistent material in comparison with natural aggregates. Factors such as parent material mineralogy, time and environment of construction, and level of hydration of the material vary from different RCA sources. This can lead to difficultly in properly characterising the material. This study focuses on the characterisation tests and cylindrical shrinkage tests of a stabilised Malmesbury Hornfels and Metamorphic Andesite, as well as RCA. RCA contains a significant percentage of active latent cement which influences the properties of the material. It is an objective of this study to evaluate the active latent cement content of RCA and equate that to an equivalent stabilised granular material. It is difficult to accurately quantify the amount of active latent cement present. Therefore, the extent of active latent cement content was analysed by comparing a RCA that has undergone some exposure through a repeated wetting and drying procedure. This exposure activates some of the active latent cement and decreases the self-cementing properties. The preliminary tests provide the characteristics of the material needed for causal analysis associated with the cylindrical shrinkage tests. The maximum axial shrinkage is addressed and compared. The influence of cement, host material, aggregate size and humidity on shrinkage of RCA and Cement Treated Material (CTM) is evaluated and compared to literature. As drying shrinkage cracking is the common failure mechanism of CTM, the study sets out to predict the shrinkage crack pattern i.e. the width and spacing of cracks within a RCA and CTM pavement layer. A shrinkage cracking model for concrete pavements was established by Houben (2008). The model estimates shrinkage crack widths and spacing. It includes variables such as time and temperature variations, tensile and compressive strengths of materials, stress relaxation and friction between pavement layers. The Houben model works on the basis that cracks form when there is a build-up of tensile stresses from the fluctuations in temperature and humidity. Cracks will form when the tensile strength of the material is less than that of the tensile stresses developed (Houben, 2008). Three materials are modelled with an adapted Houben Shrinkage Crack Model, namely Andesite (2%), Unexposed RCA (0%) and Exposed RCA (0%). It is found that the Unexposed RCA showed higher levels of self-cementation than the Exposed RCA. However, the potential for shrinkage cracking of Unexposed RCA is a reality and must be taken into consideration when designing.