Investigation on the use of crumb rubber concrete (CRC) for rigid pavements

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In many countries around the world, the adverse environmental impacts of stockpiling waste tyres have led to investigate alternative options for disposal of waste tyres. One option to reduce this environmental concern is for the construction industry to consume a high amount of recycled tyres accumulated in stockpiles. There are different concerns regarding the introduction of rubber into concrete, which were addressed by previous studies. On the one hand, making a homogenous mix containing even distribution of rubber is a challenge. On the other hand, the severe reduction of concrete strength limits the rubber content. Moreover, replacing a portion of fine aggregates with low-stiffness rubber particles raises concerns regarding the generated shrinkage and cracking of rubberised concrete. This thesis investigates these concerns thoroughly and provides a comprehensive know-how of rubberised concrete characteristics, using crumb rubber. In order to improve the strength of rubberised concrete different rubber treatment has been introduced by previous studies. A commonly applied rubber treatment method in the literature termed sodium hydroxide (NaOH) treatment has been assessed in this study. Numerous investigations examined using sodium hydroxide treatment of rubber. However, the level of improvement provided by different studies was not consistent. It was found that the sodium hydroxide treatment method is required to be optimised to achieve the most promising results. Two arrays of concrete specimens were prepared using different water cement ratios and a wide range of rubber contents. Then, the common fresh and hardened mechanical tests were conducted on the prepared samples. The results indicated that the duration of rubber treatment should be optimised based on concentration of the alkali solution and the type of recycled rubber. Consequently, the 24-hour treatment duration for crumb rubber resulted in the most suitable fresh and hardened concrete characteristics. Compared to untreated rubberised concrete, rubberised concrete produced with the optimised sodium hydroxide treated rubber, showed 25% and 5% higher compressive and flexural strength, respectively. Based on a large number of tests, this research introduced a relationship between the strength of rubberised concrete and three key parameters including the water-cement ratio (WC), the concrete age and the rubber content. Using this relationship enables concrete producers to have an accurate estimate of rubberised concrete strength. In addition, this research investigated the effects of applying an innovative method of rubber treatment, named “water-soaking”. Unlike the current methods of adding rubber into a concrete mix, which are conducted in a dry process, this research trialled introducing of rubber particles into the concrete mix in a wet process. Conducting the required sets of fresh and hardened concrete tests, number of mix series with a variety of rubber contents and water-cement ratios were evaluated. In order to measure the effectiveness of the introduced method, the properties of concrete containing water soaked rubber were compared with concrete containing untreated rubber. It was revealed that applying the proposed method resulted in considerable improvement of fresh and hardened properties. Applying the water-soaked method resulted in 22% higher compressive strength, and the formation of stronger bonds between rubber particles and cement paste compared to concrete made with untreated rubber. The effects of using recycled tyre rubber on shrinkage properties of rubberised concrete were evaluated. It was observed that adding rubber into a concrete mix led to minimise shrinkage cracks, if only an optimised content of rubber was applied. Therefore, the optimised rubber content was determined based on the mix design properties, the early-age tensile strength, and the results of plastic and drying shrinkage tests. Accordingly, the early-age mechanical strength tests, toughness test, bleeding test, and the plastic and drying shrinkage tests were conducted. A semi-automated image processing method of crack analysis was introduced in this research. Average cracks width, length, and area were determined accurately by applying the introduced method. In addition, the experimental data resulted from drying shrinkage tests of rubberised concrete were crosschecked with the results of numerical shrinkage formula provided in the Australian Standard AS3600. It was found that the provided relationship in the Australian Standard AS3600 is a valid measure for estimating the drying shrinkage of rubberised concrete. By considering the shrinkage characteristic and the acceptable mechanical performance of rubberised concrete, this dissertation concludes that the most promising results could be achieved for samples prepared with water-cement ratios of 0.45 and 0.40, and rubber contents of 20% and 25%, respectively.
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