ber of endpoint.

Jump lines are used to connect the endpoints of filling scan lines suc- cessfully in accordance with the sequence that is followed. Similarly, the entire nozzle path planning is completed (Fig. 5), including scanning of the profile and the fill realized by the modified scan line algorithm de- scribed above.

5. Process validation

A column is one of the most common and essential components of a building responsible for bearing loads, as well possessing aesthetic fea- tures. A plinth is usually used in a complete column structure for deliv- ering upper load, water resistance, collision avoidance, lateral support and aesthetics. However, stone plinths used in historical Chinese timber buildings are often prone to deterioration; they are often of curved

ene fibers, including 1.25% water reducer. A summary of the cement mortar mix used is presented in Table 1.

For the purpose of the reproduction process, a hand-held structure light 3D scanner, Creaform MetraSCAN 3D [50], is used to obtain the di- mensional information of an intact plinth. Prior to scanning the plinth, a preparatory work is done as instrument calibration for the C-Track dual- camera sensor, which is erected nearby to establish a spatial reference coordinate system for scanning measurement. The scanner is steadily held and revolves around the column, which is repeated several times. The high speed laser measurement and data transmission realizes real-time surface rendering of the measured plinth as well as the col- umn and surrounding environment on the workstation screen when the scanner is working.

Reverse engineering software known as Geomagic Studio processes the measurement signals with a PCD model, which is created and displayed on the screen. An optimized mesh generation treatment for the PCD model is conducted using the software after the redundant PCD of the column and surrounding environment is manually removed. This is an encapsulation and modification process to obtain an accurate

a b c

Fig. 13. Damaged stone plinths of varying degrees, (a) almost intact plinth, (b) partly damaged plinth, (c) completely damaged plinth.

92 J. Xu et al.  /  Automation  in Construction 76 (2017)  85–96

Intact plinth 3D scanning

STL model

Fig. 14. 3D scanning and PCD treatment process of the intact plinth.

and valid plinth solid STL model that can be used for printing. The 3D scanning and PCD treatment process is presented in Fig. 14.

The plinth printing process is dependent on the self-forming and sta- bilization of cement mortar layering. It takes approximately 10 to 15 min to mix the materials, which is able to be cured within one hour. There is no discontinuity in the printing process, as a bucket of the mixture is poured into the feed hopper by manual as soon as the current mixture is almost completely pumped. Before the nozzle de- posits the material along the setting paths, water and neat cement paste are successively pumped and extruded from the nozzle's head, to lubricate the passage, particularly the pipeline.

The intact plinth solid model is pided into two parts before printing. Four similar half plinths have been printed. Two of the half plinths are intended for a compressive strength test. The other two afford the restora- tion task by joining together to form the entire plinth when installed. The footprint of the entire plinth is 0.7 m in diameter and the height is approx. 40– 80 mm. The plinth shape is akin to a curved cup with a circular hollow (diameter of approx. 370 mm) inside it and consists of four layers (Fig. 15). The average printing time of 2 min per layer was    recorded.

The ribbed surface finish of the two printed half plinths need to be polished for an improved smooth surface reproduction effect by using a sander and abrasive paper, especially the surfaces in touch with the timber column and the ground. Then, they can be painted using imita- tion stone paint or undergo an old process treatment. The two printed half plinths enclose the column using a structural adhesive to bond them together. Notably, the diameter of the column is 350 mm so that 10 mm gap between the column and plinth is considered for infilling the structural adhesive. Residual part of the original damaged plinth is removed away before replacement and the completely reproduced plinth can be seen in Fig. 16.