Masters Degrees (Civil Engineering)

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Browsing Masters Degrees (Civil Engineering) by Subject "Aggregates (Building materials) -- Testing"

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    Relationship of pavement layers’ bearing capacity between laboratory DCP tests and field performance
    (Stellenbosch : Stellenbosch University, 2023-03) Oberholzer, Johannes Jacobus; Jenkins, K. .J.; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.
    ENGLISH SUMMARY: This research study forms part of the SANRAL Project 3.5, focused on the improvement of performance-based test methods utilised in pavement engineering in the South African context, specifically aimed at unbound granular material. As most existing test methods in South Africa are based on empirical formulations, a lack of correlation exists between the test results and in-situ material conditions, and therefore the prediction of the material performance has often been inaccurate. The performance-based test methods are aimed at characterising the material strength as well as a prediction of the performance of the material under the influence of repeated traffic loading. This study considers the influence of moisture content, density, specimen geometry, maximum particle size and aggregate packing on the performance on unbound granular materials in South Africa. To investigate the influence of moisture content and density, three different moisture contents were incorporated in the test procedure, combined with two different compaction energies. To investigate the influence of specimen geometry, two specimen sizes are considered in the experimental methodology. The influence of maximum particle size and aggregate packing is considered by implementing two material gradings, with different maximum particle sizes implemented by scalping the material at the required sieve size. The smaller specimen grading, scalped at the 20 mm sieve, was labelled as “S20” material. the larger specimen grading, scalped at the 28 mm sieve, was labelled as “S28” material. The investigation of the material performance was conducted by means of laboratory dynamic cone penetrometer (DCP) testing, as well as laboratory California bearing ratio (CBR) testing on a graded crushed G2 Hornfel material. The DCP testing was conducted on both specimen sizes, with one test conducted per small specimen and three tests conducted per large specimen. The CBR testing was only performed on the smaller specimens due to the availability of test equipment. Preliminary testing was performed on the material to classify the material characteristics. A sieve analysis was conducted on the material to understand the material proportions. A moisture-density analysis was performed on the material to establish optimal moisture contents (OMC) corresponding to the optimal density (MDD) achievable in the material. The test specimens were compacted by means of vibratory energy, using the vibratory hammer to ensure an accurate simulation of in-situ conditions. From the DCP testing and determination of the DCP-DN values, it was evident that specimens prepared at lower moisture contents and higher densities yielded lower DN values, higher DCP-CBR values and higher estimated elastic stiffness values. It was subsequently concluded that the G2 material is sensitive to moisture and density changes, and that these factors will significantly impact the material performance. The effects of soil suction were considered, and it was determined that soil suction has a negligible impact on the G2 material as the degree of saturation in the material was well above 20%. The larger specimens, with larger maximum particle size also displayed lower DN values, indicating that these specimens had improved shear strength when compared to the smaller specimens with smaller maximum particle size. This conclusion was based on the improvement in aggregate packing due to the presence of larger aggregates. This was confirmed by considering the dominant aggregate size range (DASR), which improved the understanding of the dominant aggregate skeleton governing the behaviour of the G2 material. It was therefore concluded that larger laboratory specimens, with larger aggregates, yield improved material performance and are more representative of in-situ conditions. The correlation between DCP-CBR and laboratory CBR was investigated from the tests conducted in this study, as well as DCP-CBR correlations from previous studies. It was evident that various DCP-CBR formulations displayed large variance in the results when compared to each other with a standard deviation of 23% for the 100% MDD specimens and 26% for the 98% MDD specimens, suggesting that a lack of confidence is associated with the DCP-CBR formulations. When comparing the DCP-CBR and laboratory CBR test results, poor correlation was observed. It was therefore concluded that DCP-CBR formulations do not correlate to laboratory CBR values with a minimum required level of accuracy. It was also concluded that CBR values are not suited for performance-based material classification due to the variability observed in these values. In summary, it was concluded that the performance of granular material is influenced by to the moisture content and density at which the material is prepared. It was evident that other factors such as grading and maximum particle size also have a significant impact on the performance of the material. From this study, it was evident that certain current laboratory techniques do not accurately simulate in-situ conditions due to the adoption of a scaled-down approach. It was also concluded that the use of CBR is not suited for the accurate classification of UGM due to the large variability in CBR results and poor correlation to in-situ material performance. The DCP test, using the DCP-DN values, is recommended and is an effective performance-based test procedure to establish estimations for the behaviour and strength of unbound granular material. The DCP has proven to be cost-effective and easy to use and may therefore be a valuable tool in pavement engineering, especially in developing countries.