A combination of the three-dimensional (3D) scanning and cement mortar-based 3D printing technology is used to develop a novel process for reproducing a historical building ornamental component, which is traditionally labor intensive and expensive to construct. A hierarchical algorithm for model slicing and a modified scan line algorithm for nozzle path are developed and presented. To demonstrate the feasibility of the proposed digital re- production process, a damaged cup-shaped inpidual plinth from the campus at the Huazhong University of Sci- ence and Technology (HUST) in China, is 3D scanned, re-modelled, and re-constructed using specific 3D printing technology. An estimation is implemented to the façade of the printed plinth as well as the scanning accuracy. The compressive strength of the printed plinth is tested and calculated, which resulted in 19.8 Mpa and

15.6 Mpa for its vertical and lateral directions, respectively. The reproduction evaluation indicates that the devel- oped process provides the foundation and impetus for future work in the area of the digital reproduction of his- torical building ornamental components using 3D scanning and cement mortar-based 3D printing.70107

1. Introduction

Construction is an ancient human activity; the structures created have engendered society's ability to function and prosper. Some of them are deemed to be of outstanding historical, aesthetic, or cultural importance and often provided with a special status (i.e. landmark des- ignation) ordaining their conservation. Since the emergence of civiliza- tion, generally associated with the final stages of the Neolithic Revolution (9130 BCE), types of materials, methods and technologies used for construction have changed significantly. The materials used to construct buildings of historic significance are invariably subjected to an array of factors contributed to their decay and failure such as air pollution [1,2], salts [3,4], biodeterioration [5,6] and mechanical loads (usage and traffic) [7]. Consequently, historical structures need to be conserved for future generations to connect with their ancestors and re- spect their achievements. Such conservation also allows people from other cultures to understand the values and beliefs that have shaped a civilization.

In the conservation work of a historical building, there are a standard of ethics that highlights the requirements of proposed interventions and encourages a minimum effective intervention. Seven essential degrees of interventions are made at various scales and levels of intensity ac- cording  to  the  physical  condition,  causes  of  deterioration  and

prospective future environment of the cultural property under treat- ment. These interventions, which are likely to occur simultaneously in a major conservation project, are successively [8]: (1) Prevention of dete- rioration; (2) Preservation; (3) Consolidation; (4) Restoration; (5) Reha- bilitation; (6) Reproduction; and (7) Reconstruction. The seven degrees of conservation interventions are adopted in different application sce- narios. The three interventions of restoration, reproduction and recon- struction involve manufacturing or replacing missing or damaged elements or components of historical buildings. A virtual reproduction process of a building element or component often serves as a restoration task. There are however two challenges that curb the reproduction process:

a) Drawings and other documentation are often missing or are scant, which renders it almost impossible to restore or reconstruct them to their original condition [9] and incompatible conservation inter- ventions without aforehand measurement can enhance the decay in historical buildings [10]. Due to the idiosyncrasy of their construc- tion and location, conventional measurement methods such as man- ual physical mapping or “expert-naked-eye” analysis are laborious and inefficient [11], while some cultural properties are forbidden to directly touch or intervene for fear of erosion by a pollution of de- tection devices or human skins [12]. This leads to a demand of an ef- ficient digital non-contact or indirect measurement method; and