## 1 Introduction  
Two requirements of agent delivery arise for the optimal design of autonomously healed concrete based on a decade of research [1], namely, (1) the amount of healing agent should be sufficient to fill in and seal macro-cracks wider than 0.1 mm and up to 1 mm; and (2) healing function should be continuous and repeatably provide crack healing during the service life of the concrete. The concept of vascular healing networks attracts the interest of the research community today since their design can fulfil both of these essential requirements. As a result, the research is gradually shifting from encapsulation-based self-healing approaches towards vascular network conceptualization as the need for upscaling and commercialization is imperative. Recent studies demonstrate that encapsulation of the healing agent provides successive repair by reducing the capsules' diameter to the material granulometry [2] and by casting them through materials compatible to concrete (i.e., [3]).  
The main difference to the vascular network is the latter's ability to continuously supply an agent to different cracks and throughout the whole service life of the structure [4].  
The early designs of hand-made ceramic [5], cement [6] and glass [7; 8] tubes connected to an inlet-outlet supply system, to build a one-dimensional vascular network that proved its healing efficacy [9]. The breakthrough idea on healing conceptualization was the 3D printed fabrication of hollow tube networks and supply components [10]. Upon cracking, local stresses act on the tubes that eventually fracture at the crack plane. After fracture, the agent is manually poured into the system [5; 11; 12] or deployed from the embedded reservoir, and is then released through the tubes network, as capillary forces draw the agent into the open crack void [13]. The agent flow is occasionally promoted by pressurizing air throughout the network, an action that additionally facilitates the healing kinetics [13; 14] and the cleansing of the tubes after the healing intervention [11].  
Additive manufacturing is fast, reliable and provides the freedom for any geometry and shape of healing systems [4]. Recently, the research has focused on the fabrication of 2D and 3D vascular 3D printed polymer networks and significant advances are marked on modelling [15] and experimental works [16]. In addition, the biomimetic pattern design inspired by [PERSON]'s law is now established to provide optimal agent distribution and flow [12].  
The recent advances on the autonomous healing of concrete are summarized in [17; 18], and this work aims to complementarily assess the limitations and obstacles that arise in practice on the design of vascular healing networks for concrete. Preliminary tests are performed on concrete beams with embedded hand-made and 3D printed vascular systems and their fracture and recovery are tracked by an integrated inspection methodology that provides insights into the damage-healing interaction. The phenomenon of network thrombosis will be discussed since the agent cleansing after the healing intervention appears the most critical design parameter for the durability of the network. The impact of supply components on the damage is exploited, and the compatibility of reservoir-network materials is critically assessed. Spotlight is set on the cracking zone, where the role of the micro-cracking at the fracture process zone surrounding the prominent crack on the re-opening of the crack after healing is investigated.  
The online and post-testing monitoring of cracks provides, for the first time in the literature, a direct link of the crack size and morphology to the healing efficacy and mechanical properties restoration.