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Finally, a comparison between optical image scanning and X-ray computed tomography was given. Additionally, the majority of air voids displayed irregular and elongated shapes, which could be attributed to the extrusion and layer-wise manufacturing processes in 3D printing. Large air voids (1000–6000 μm) were enclosed mainly between the printed filaments (interface region), which resulted in the relatively higher local porosity than that of layer region. The results revealed that most of the air voids with the diameter in the range of 10–1000 μm were distributed evenly in the layer region of printed samples. Afterwards, air void metrics in printed cementitious materials were determined, i.e., content, distribution, size, and shape. The micrographs were acquired by using optical image scanning and X-ray computed tomography. Two printable limestone and calcined clay-based mixtures were employed to prepare the printed samples. Therefore, to fill this gap, this study attempts to provide an example of systematically characterizing the typical air void system of 3D printed cementitious materials via different image acquisition and analysis techniques. However, to date, far too little attention has been paid to reveal the air void characteristics in 3D printed cementitious materials. The findings will benefit the development of rejuvenator encapsulation technique and the optimization of the capsule healing system towards a better healing effect in asphalt pavement.įor many 3D printed cementitious materials, air voids may play a dominant role in the interlayer bond strength. Furthermore, these rejuvenators were encapsulated in calcium alginate capsules and the tests on these capsules indicate the diameter, mechanical resistance and thermal stability of the capsules are influenced by the encapsulated rejuvenator. To this aim, three different rejuvenators were studied and ranked based on the performance of their rejuvenated bitumen, including physical properties, rheological properties, chemical properties and the performance after re-ageing. The quality and speed of the damage repair process strongly depend on the quality of rejuvenator, thus it is important to choose a proper rejuvenator with good abilities to restore the lost properties of bitumen from ageing and show a sustainable performance after healing.

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Then the released rejuvenator wets the crack surfaces, diffuses into and softens the aged bitumen, allowing two broken edges to come in the contact, preventing further asphalt pavement deterioration. When the microcrack encounters the capsule, the fracture energy at the tip opens the capsule and releases the rejuvenator. Once the capsules are embedded within asphalt pavement, the healing is activated on-demand via progressing microcrack. Rejuvenator encapsulation technique showed great potential for extrinsic asphalt pavement damage healing. Advanced characterization techniques and concrete technology tools are needed to account for limiting properties of fRCA in concrete mix design.

As such, continuous quality of fRCA is hard to be obtained, even though they may be more continuous in terms of chemistry. By comparing properties of fRCA with properties of fine natural aggregates, the key limiting properties of fRCA are identified as the high water absorption of fRCA, moisture state of fRCA, agglomeration of particles and adhered mortar. This paper sets the foundations for better understanding the quality of fRCA obtained either from parent concrete specifically produced in the laboratory, with controlled crushing and sieving of the recycled aggregates or from field structures. Namely, what is still missing is knowledge on key physico-chemical properties and their relation to the quality of the concrete mix and the concrete performance.

Based on the systematic review of the published literature, it is impossible to deduce without any further research the guidelines and tools to introduce the widespread application of the fRCA in new concrete whilst keeping the cement contents at least the same or preferably lower. This paper discusses the state-of-the-art of the fine recycled concrete aggregates (fRCA), focusing on their physical and chemical properties, engineering properties and durability of concretes with fRCA.

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