# Effect of Nylon Fiber Addition on the Performance of Recycled Aggregate Concrete
## 1 Introduction  
Recently, a lack of natural aggregates with a high quality has set up the alarm to find alternative uses for recycled aggregate. Environmental and economic benefits led to a higher production and application of recycled aggregate concrete in many countries. A great amount of demolition and construction waste, almost 67,000,000 tons yearly, was generated in South Korea. For the purpose of the wide utilization of recycled aggregate, it is advised to use recycled aggregate up to 30% in concrete construction sites of 24 MPa grade and below [1]. Due to the increased technology in the aggregate production industry, both the quality and quality of recycled aggregates has increasingly improved [2].  
However, there are still some doubts on the performance of concrete using recycled aggregate. In general, the performance of recycled aggregate concrete greatly depends on the adhered mortars oriented from parent concrete. Until now, there have been many reports on the properties of concrete made with recycled aggregate [3; 4; 5; 6; 7; 8; 9; 10; 11], but most of them resulted in a lower level of concrete strengths. This is mainly due to the residual impurities on the surface of the recycled aggregates, which blocked the strong bond between the cement paste and aggregate. Moreover, it is well known that there are two interfacial zones in recycled aggregate, which negatively affect concrete properties. In fact, this is the reason that the microstructure of recycled aggregate concrete is much more complicated than that of natural aggregate concrete. The old mortar in recycled aggregate includes many micro-cracks, formed during the production of recycled aggregate concrete, and it has a high porosity.  
In order to enhance the performance of recycled aggregate concrete, researchers have come up with several advanced techniques. These techniques include the surface treatment [12], the use of mineral admixtures [13; 14] and the addition of fibers [4; 15; 16], which can improve the performance of recycled aggregate concrete.  
In particular, the reinforcement of recycled aggregate concrete using fibers leads to reduced micro-cracks in the cement matrix as well as to an increase in material density. There have been numerous studies on the applications of steel fiber [17; 18; 19; 20; 21; 22; 23] and polypropylene (PP) fibers [24; 25; 26] in concrete. Moreover, the PP fiber also enjoys popularity in the domain of recycled aggregate concrete [4; 27]. However, although nylon fiber (NF) shows a rising acceptance in the literatures [28; 29; 30], it remains unpopular compared to steel fiber and PP fiber. In comparison with steel and PP fibers, while there is limited literature available on the use of NF, some authors [28; 31] reported that the use of NF stepped up the performance after the presence of cracks in concrete, and sustained high stresses. One needs to examine the applicability of NF for the purpose of the enhanced mechanical properties and the reduced micro-cracks in recycle aggregate concrete.  
This study is therefore aimed at investigating the usability of NF in recycled aggregate concrete in order to be used in structural concrete, since the use of NF in field concrete is gaining popularity nowadays. In order to achieve this goal, measurements of compressive strength, split tensile strength, ultrasonic pulse velocity, and chloride ion permeability of recycled coarse aggregate concretes with or without NF were carried out, and the corresponding test results were compared to those of concretes incorporating crushed stone aggregate. Additionally, the microstructures of 28-day concretes were examined by using the FE-SEM technique.