Effects of crumb rubber content and curing time on the properties of asphalt concrete and stone mastic asphalt using dry process

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scopus – master journals – JCR

ایمپکت فاکتور

۴٫۲۷۶ در سال ۲۰۲۰

شاخص H_index

۲۶ در سال ۲۰۲۱

شاخص SJR

۰٫۹۰۱ در سال ۲۰۲۰

شاخص Quartile (چارک)

Q1 در سال ۲۰۲۰

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Effects of crumb rubber content and curing time on the properties of asphalt concrete and stone mastic asphalt using dry process

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Effects of crumb rubber content and curing time on the properties of asphalt concrete and stone mastic asphalt using dry process


Along with the rapid increase in the number of road vehicles, a large amount of waste tires have been created, causing negative effects to the environment. Many attempts have been made to effectively reduce this type of solid waste including  the reuse of recycled rubber powder from waste tires as an additive for improving the performance of asphalt mixtures. In this work, the authors aim to study the effects of crumb rubber (CR) on the mechanical properties, especially the rutting resistance, of CR modified asphalt concrete (AC) and stone mastic asphalt (SMA) by varying two factors-namely, the content of additive and the curing time. The dimension of used CR ranges from 0 to 2.36 mm, which is not too coarse for promoting the CR–bitumen interaction and not too fine for facilitating the pro- duction of CR. The content of CR was increased gradually  from 0 to 3% to examine the effects of CR content on the engineering prop- erties and determine the optimal  content in the mixture. It was observed that the optimal  content is 1.5–۲%, while the optimal  curing  time that contributes to the maximal increase in the mechanical characteristics of both mixtures could not be determined. In the range of 0–۵ h of curing time, the longer the asphalt mixture is maintained at a high temperature, the better the performance of mixture will be. Ageing was found to have influence on the performance  of asphalt mixtures; however, its effects are not as important  as those of CR modification.

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  1. Introduction

In recent years, along  with the rapid development of infrastructure,  the  number  of vehicles  is considerably increasing in developing countries. Countries should not only take advantage of these trends to expand their economy but also pay attention to drawbacks that might affect human beings, such as emission, noise pollution  and park- ing lot shortages. According  to Messenger in 2008, there were more than one billion tires discharged into landfills, and approximately  ۴ billion tires had already been stored in stockpiles [1]. The  problem  is that these countries must immediately  reduce the number  of used tires effectively and safely because the environment is likely to be harmed before or during  treatment processes. Therefore, the idea of reusing waste tires in highway construction is believed to be currently feasible.

The first applications of discharged car tires in asphalt concrete occurred  in the  ۱۹۶۰s, and the methods  were named wet process and dry process, corresponding to the method in which CR is added. The wet process was devel- oped by McDonald with the percentage of waste rubber ranging from 5 to 25% by weight in bitumen. CR is added to bitumen to improve the engineering properties of the binder. Specifically, at high temperatures, light non-polar fractions  are absorbed into the polymer networks. As a result,  the  final bitumen  becomes harder,  and swelling CR particles soften. If the temperature is too high or the time is too long, the swelling will stop and be replaced by depolymerization. This results in the dispersion of rubber into bitumen. These phenomena will occur at temperatures in the range of 160–۲۰۰  C and are time-dependent. How- ever, one of the obstacles that restrict CR modified asphalt binder from applications is storage stability.  Due to the reduced  density after swelling,  CR  modified particles migrate to the top of the storage container,  leading  to phase separation [2–۷].

In contrast,  the dry process, in which the additive  is blended with aggregate before mixing with bitumen, is well known for its environmental  benefits. Because the less fuel is consumed the more CR is used  [۸]. This method was invented  and first  applied in  Sweden  and thereafter patented in the USA in 1978 with the name of PlusRide [3].  In addition to PlusRide,  the following  two methods have also applied CR in the dry process: generic dry pro- cess and chunk rubber process. While PlusRide  consumes CR with a dimension  range of 2–۶٫۳ mm  in gap-graded asphalt  concrete, the generic dry technology   uses 3% of 0.18–۲ mm CR by weight of the mixture. The chunk rubber process uses 3–۱۲% CR with sizes ranging from 4.75 to 9 mm in the mixture. The aggregate gradation  used in this process depends on the CR dimensions. This means that the rubber particles will replace the same-sized aggregate particles.

In studies about the dry process, CR is used in hot mix asphalt as a replacement for parts of coarse and fine aggre- gate, resulting  in a preference  for gap gradations, and aggregate does not appear in the dimensions of the added CR. Furthermore, the work of designing a gradation curve of aggregate corresponding  to the added rubber powder is quite complicated because the melting of fine CR particles will occur at high temperatures. For example, the chunk rubber process could consume CR up to 12% by weight of the mixture. The application of CR in hot mix asphalt is a promising solution, but problems of consistency in the  characteristics  of the  mixes  should be  considered. Moreover, Mujibur Rahman   [۴] and Herna´ndez-Olivares et al. [9] noted  that the distance  between the mixing  plant and construction site has a considerable effect on the con- sistency between mixes; at high temperatures, the interac- tion between the rubber  and bitumen continues  during transportation.

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