## 6 Conclusions  
The performance of a novel vascular network healing system is evaluated in this study by performing cyclic loading on concrete beams and monitoring the cracks response by advanced non-destructive testing techniques. Among the different healing configurations, the 3D printed polymer network with flexible inlet/outlet tubes achieves effective sealing and the greatest mechanical recovery of the crack after a healing intervention. As an extension to previous work and considering the remarks and herein experimental evidence, this novel, additively manufactured healing network can be implemented in the future when designing large-scale concrete elements configured for repeatable healing. The main findings with respect to the healing design are summarized below:  
* The detrimental impact of embedded bulk agent reservoirs is reported since under bending the cracks deviate from the middle bending zone towards the reservoir sections and failure occurs at the concrete-reservoir interface. The incompatibility of concrete with a stiff polymer reservoir suggests that alternative agent delivery systems should be explored. The use of a thin flexible plastic inlet/outlet tube system proved its efficacy and it is recommended for future designs. The position of the connection tubes into the beam and relative to the vascular networks should be also revised. * The 3D printed polymer network appears equally effective compared to the conventional ceramic tubes since in both cases the tubes break instantly upon cracking and the subsequently healing agent is effectively released and distributed into the cracks. * The healing agent viscosity should be modified (reaching lower viscosity) since the tubes are only partially cleaned by pressurized air after the healing intervention. Future work should explore the tightness of the 3D printed vascular networks as it is observed that cement paste can penetrate through the tubes during concrete casting. Clogging phenomena should be eliminated in the future by printing polymers with a higher thickness. * Repeatable healing could not be reached due to the tubes' internal clogging and to re-opening of the same crack or of a crack in its vicinity at the reloading cycles. The complex fracture process and dense cracks network should be taken into account in future works in order to design a network that performs optimally under concrete service loads. Figure 15: (**a,b**) clogged ceramic (B11–B15) and (**c,d**) 3D printed polymer tubes (B1–B10). Effective crack healing is described by a combination of two fracture phenomena: (1) the healed crack section elastically deforms under load, building great strength and toughness recovery; and (2) as the crack forms, the fracture gradually propagates along the crack contributing to great post-softening. The main observations and updates regarding the well-established inspection methodology are:  
AE and DIC complementarily contribute to the online monitoring of crack formation and reopening after healing. The use of DIC for a precise crack opening analysis appears crucial for the accurate assessment of healing efficacy. The AE hits distribution indicates the transition of damage from brittle to pseudo-ductile for a healed crack. The AE localization analysis can identify the active cracking zones even after healing. In an upcoming study dedicated to the AE as a tool for damage and healing identification, both the damage mode shift and the post-healing crack kinetics will be associated to the AE wave features' trends and variations during testing. UPV inspection before and after cracking and healing intervention provides direct information on the damage state of the concrete and can pinpoint the crack recovery cases. Optical microscopy after each test cycle effectively detects the crack filling by agent and visual inspection at post-testing reports on the tubes clogging, cleansing issues and failures of the network.