The quantity of construction and demolition waste (C&DW) is being produced annually around the world [1
], which aroused environment pollution and resource depletion. All over Europe, about 320 to 380 million tons of C&DW are generated every year [2
]. By contrast, in China, the amount of C&DW is approximately 640 million every year, with an average increasing rate of 8% per year [3
]. The replacement of natural aggregates (NAs) with recycled aggregates (RAs) can decrease the amount of concrete waste that otherwise would be disposed in landfills. Moreover, it can relieve the requirement of the construction industry on new NAs [5
]. Therefore, the use of RAs is essential for realizing a sustainable construction industry, remarkable in environmental preservation, and meaningful in improving resource efficiency [6
]. There have been a great number of studies on the recycling of aggregates in the structural concrete manufacture.
Ample researches reported that the old mortar particles with relatively high porosity and low strength [9
] lead to the inferior properties of RAs compared with NAs. Among those properties, shrinkage is the most affected factor, which causes a great weakness in the use of this type of material. Pedro et al. [17
] reported that the mechanical performance, durability and shrinkage of the recycled aggregate concrete (RAC) are affected by RAs with different sources. They found that concrete containing the recycled coarse aggregate without mortar has superior mechanical performance compared to the natural aggregate concrete (NAC). However, the shrinkage resistance of recycled coarse aggregate concrete was less than this of natural coarse aggregate concrete. Manzi et al. [18
] researched that short and long-term behaviour of structural concrete with RAs by adjusting and selecting the content and grain size distribution of concrete waste with a high content of RAs (ranging from 27% to 63.5% of total amount of aggregates). They conclude that shrinkage and creep, combined with porosity measurements and mechanical investigations, are fundamental features to assess structural concrete behaviour. Vegas et al. [19
] studied shrinkage of RAs at 1250 h and concluded that the shrinkage value of 0.07 mm/m, more than triple the value obtained in concrete with NAs (0.02 mm/m). Fernández et al. [20
] tested at 203 days and concluded that concrete types with RAC received more than double shrinkage compared to concrete with NAs. Ismail and Ramli [21
] studied the strength and drying shrinkage performances of treated coarse RAC. Although the particle density, water absorption, mechanical strength remarkably and drying shrinkage concrete of RAs are worse than those of NAs, the effect of the combination of these two surface treatment methods improved those performances after treated, almost the same to those of NAs. The large drying shrinkage of RAC inhibits strength development and increases cracks. The shortcomings of RAC can be limited by adding reinforcing fibers, and crack propagation can be controlled [22
There are varieties of fibers can be mixed in the RAC, such as polymeric fibers, steel fibers, natural fibers, etc. [25
]. Akça et al. [26
] studied the usability of polypropylene fiber in RAC to be used primarily in field concrete different combinations generated using the RAs and polypropylene fiber. Different polypropylene fiber contents have been introduced into concrete types that have a different amount of RAC, such as 0, 1% and 1.5% by volume. The result concluded that types of aggregate affected concrete compressive strength and fiber content affected flexural and splitting tensile strength besides aggregate type. Nam et al. [27
] investigated the effects of polyvinyl alcohol fibers and nylon reinforced fibers on the mechanical properties and shrinkage cracking of recycled fine aggregate concrete. The results showed addition of fibers at a small volume fraction in RAC was more effective for drying shrinkage cracks than for improving mechanical performance. Kim et al. [28
] researched the effects of recycled PET fiber reinforcement on shrinkage cracking of cement-based composites through pullout test with a different fraction of volume from 0.1% to 1.00%. The result indicated that the fraction of fiber volume affected plastic shrinkage cracking and the embossed type fiber which had advanced mechanical bond strength also possessed the best resistance to plastic shrinkage cracking. Skarzynski and Suchorzewski [29
] studied the mechanical and fracture performances of concrete reinforced with recycled and industrial steel fibers. Through wedge splitting test and the analyses of the 3D cracking phenomenon with both recycled steel fibers and industrial steel fibers, it concluded that the mechanical performance could be improved with steel fibers and industrial steel fibers and cracking areas could be restrained by concrete with steel fibers and industrial steel fibers. Katkhuda and Shatarat [30
] studied the improvement of the mechanical performances of RAs produced by adding the different volume of chopped basalt fibers (BF), including 0.1%, 0.3%, 0.5%, 1%, and 1.5%. The result showed that using chopped BF minimally enhanced the compressive strength of the concrete, but significantly improved its flexural and splitting tensile strengths. Moreover, the optimum BF content that produced the same splitting tensile and compressive strength as NAs was 0.5% for untreated RAs and 0.3% for treated RAs, while the flexural strength was 0.3% for untreated RAC and 0.1% for treated RAC. Although studies on the effects of using reinforcing fibers in RAs have attracted enough attention, the study about using of waste fibers in RAC is very few.
From the environmental viewpoint, adding the waste fibers into RAC is promising to improve its performance and to reduce potential pollution sources. As a combination type of green concrete, it can be reused for waste fibers and construction waste. Waste fiber recycled concrete (WFRC) not only promotes the application of recycled concrete but also is beneficial for environmental protection. Furthermore, to take economic efficiency into consideration, the utilization of waste fibers in structural concrete manufactures is meaningful to minimize the costs of fibers. Thus, it is necessary to investigate the usability of the waste fibers with RAC, especially its influence on shrinkage performance.
The objective of this study is to investigate the effectiveness of adding waste polypropylene fibers into RAC. Firstly, the plat-ring-type shrinkage test is conducted to study the shrinkage cracking performance under the restrained conditions. Meanwhile, the free shrinkage test is used to measure the rate of shrinkage in the unconstrained state. Then, the X-ray industrial computed tomography (ICT) is carried out to reflect the internal porosity of RAC. Additionally, the compressive strength and flexural strength were tested to evaluate the mechanical performance.