# Material Characterization for Sustainable Concrete Paving Blocks
## 1 Introduction  
In China, the construction industry has developed rapidly since late 1990s [1]. High-rise buildings are established, and the old buildings are demolished at the same time. Due to the irreplaceability of concrete in the construction industry, large quantities of concrete were used in the past two decades, and this situation leads to the generation of large volumes of construction waste [2]. Using recycled demolished concrete as the aggregate in the new construction products can reduce the utilization of the limited nature resource and reduce the environmental burden at the same time. Relevant studies about using construction wastes in concrete products were investigated by many researchers. Concrete products that incorporates recycled materials can help minimize the CO\({}_{2}\) footprint and potable water usage via the avoidance of using raw materials. Therefore, concrete containing waste material is generally regarded as a type of \"sustainable concrete\". In this case, studying on the maximum possible amount of recycled materials in concrete, and extending the types of waste materials that can be used in concrete are important in the sustainable concrete research area. In early 1985, [PERSON] and [PERSON] [3] studied the effects of using recycled-concrete aggregate instead of natural aggregate. In recent years, many researchers have studied the impact of using different types of recycled aggregates on the properties of concrete products. Not just crushed concrete, some other common construction wastes, such as crushed clay bricks, are used as recycled-aggregate [4]. In general, using recycled aggregates instead of natural aggregates in concrete will reduce the strength of concrete products [3; 4; 5]. As one type of concrete product, blocks are usually composed of 85%-90% fine coarse aggregates and fine aggregates, mainly small limestone particles and sand [6]. Construction wastes not only refer to recycled concrete aggregates, which contain crushed concrete, crushed bricks, and granites, but also contain wasted glass, wood, plastic, and rubber [7]. In order to utilize the crushed glass as aggregate, and produce the concrete incorporating recycled glass, the experiments to incorporate glass waste in concrete have been carried out since the 1960s [8]. Rubber is considered as a type of recycled aggregate that can be used in concrete [9; 10; 11; 12; 13]. In general, the research studies suggested that the amount of rubber shall not exceed 25% of the total aggregate volume [11; 12; 13]. Furthermore, as a commonly found industrial waste material, ground granulated blast furnace slag (GGBS) is also investigated as a material to replace some of the cement during the concrete products' manufacturing [14]. There are no strict requirements on strength and durability for concrete paving blocks, and such products are easy to manufacture and to cure [15]. Besides compressive strength and tensile splitting strength, the water absorption, abrasion resistance and slip resistance of blocks are also important properties for concrete paving blocks [15; 16]. The performance requirements for concrete paving blocks are summarized and shown in Table 1, according to British standard BS EN-1338: 2003 [15] and Chinese standard GB 28635-2012 [16]. The common size of concrete paving blocks is 200 mm \(\times\) 100 mm \(\times\) 60 mm. In that case, recycled aggregates can be used to replace either coarse aggregates or fine aggregates. However, the aggregate crushed value is usually lower than natural aggregates. The utilization of recycled aggregate made from crushed concrete in paving blocks has been studied by many researchers. [PERSON] et al. [6] investigated the use of recycled demolition aggregate in producing concrete paving blocks. It is recommended that, in order to ensure that the strength of recycled concrete block is as same as those blocks that only contain natural aggregates, the replacement level of recycled masonry-derived coarse aggregates shall not exceed 60%.