**1. Introduction**

Formwork is a temporary structure on a construction site into which concrete or a similar substance is poured, while the falsework supports the shuttering moulds [1]. In building construction, formwork plays a major role in determining the duration and schedule of construction activities [2]. In fact, the cost of formwork construction (forming cost) accounts for 10% of the overall cost and erection and assembly of formwork systems (forming time) spend 50% of the overall construction time of the entire project [3]. Recently, with developments in the building industry, high formwork is used more widely. High formwork projects are more significant than general formwork projects due to their complex structures. Their large size and complex structure make them more vulnerable to potential safety risks.

**Citation:** Zhao, L.; Mbachu, J.; Wang, B.; Liu, Z.; Zhang, H. Installation Quality Inspection for High Formwork Using Terrestrial Laser Scanning Technology. *Symmetry* **2022**, *14*, 377. https://doi.org/10.3390/ sym14020377

Academic Editors: Yang Yang, Ying Lei, Xiaolin Meng and Jun Li

Received: 10 December 2021 Accepted: 1 February 2022 Published: 14 February 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

In the installation of high formwork structures, errors are unavoidable, resulting in defects that differ from code requirements or design specifications. In general, small errors are permitted, but they must be within an acceptable error range. Otherwise, they can create potential safety risks. Any such safety risks can adversely affect subsequent processes, lower the efficiency of the construction work and increase construction costs, or worst of all, cause accidents [4]. Such accidents can lead to significant loss of life as well as financial losses. Hence, it is necessary to conduct proper, high-quality inspections for high formwork after installation. However, the inspection process for high formwork has not been dealt with systematically due to the lack of an appropriate inspection technology for such projects and a shortage of primary data necessary to establish a plan. In fact, inspecting these factors on construction site using common methods like tape measurements or plumb monitoring is exhausting and laborious.

The main concern of the inspection of installation quality for high formwork includes: (1) to check the position of the main components of the formwork such as vertical poles, horizontal tubes, bottom reinforcing tubes and the diagonal bracing; (2) to check whether the spacing of vertical pole and lift height (the distance between the two neighbour tubes) are in accordance with the requirements of the building code. Based on the investigations of [5–7], these factors are the main determinants of the proper function of a high formwork. According to China's building codes related to high formwork [8–10], they are the main inspection terms during the inspection process. Hence, the study proposed a method for effectively checking them.

Current installation quality assessment of high formwork is mainly based on manual inspection using traditional measurement instruments like measuring tapes, which require close-up access, and are not ideal for detecting defects or for accurately measuring the dimensions and recording the data. The inspection process is mainly based on human inspection and judgement, which is labour-intensive and inefficient, especially for largescale structures.

Different approaches have been adopted, including theodolites, joint meters, and inclinometers, as well as tachymetry, global navigation satellite system (GNSS), and robotic total station [11,12]. They can offer accurate results, but they only provide discrete monitoring points and fail to attain full area coverage. Some measurement techniques are also too expensive. Moreover, not all available techniques are suitable for inspection of high formwork due to the size and complex shape of the structure, and level of required accuracy. When choosing a suitable measurement system, factors such as cost, time required for operation, and the required skills of the user should all be considered [13–15].

Inspection accuracy and efficiency can be improved by using terrestrial laser scanning (TLS) technology. Another benefit of TLS is its long range, which allows data collection in inaccessible fields. The TLS system can be regarded as the best monitoring technique for high formwork owing to its ability to provide high spatial resolution and accurate measurements. A major benefit of TLS is that it provides rapid and dense measurements in inaccessible areas [16]. TLS can produce point clouds with high level of details and accuracy, which is particularly good for detailed investigations on a large scale [17,18]. The large amount of accurately measured data has grea<sup>t</sup> potential to deliver measurements that are difficult to achieve by other methods in such a short period of time, with simple installation requirements. TLS measurements require only a laser scanner, a laptop computer with data processing software, and several reference targets.

Previous studies have provided sound evidence for the efficiency and effectiveness of inspection operation using TLS. Gordon et al. [19] and Akinci et al. [20], for example, demonstrated that manual inspection can miss important information such as site changes and defects. However, the adoption of TLS can improve efficiency in construction inspection via timely provision of comprehensive, as-built data. Although there are advantages to adopting TLS, some studies [21–23] also point out the challenges that adversely influence the effective use of TLS, including difficulties in effectively extracting the required information from scan data and in generating required accuracy results.

This study provides a new method for conducting quality inspection using TLS technology. The proposed method can effectively extract important elements such as poles and tubes in high formwork from the point cloud data. A GA-ANN model was developed to increase the accuracy of the TLS measurements. To achieve these, some techniques are first proposed to rapidly extract the important points from the dense data. Secondly, a small-scale high formwork model set with set dimensions and simulated defects was built to help in collecting sufficient data to train the ANN model. The obtained measurements based on the results from the ANN model were compared to the set dimensions to evaluate the performance of the ANN model. Moreover, the genetic algorithm was adopted to optimised ANN model. The performance of the GA-ANN model was compared with other ML algorithms such as support vector machine and random forest. Next, a laboratory test was conducted to validate the proposed method. Finally, the proposed method was used on a real high formwork to validate its effectiveness.

The uniqueness of the study lies in: (1) the consideration of TLS as an effective tool in quality inspection for high formwork after installation; (2) the rapid extraction of the standard poles and tubes from the point clouds; and (3) the development of a GA-ANN model that can generate the coordinates of important points to accurately calculate the distance between the important points and thus detect defects that fall out of the acceptable error range.

The organization of the study is as follows: the literature review and the highlights of the contribution of the current study are described in Section 2. The proposed method for installation quality inspection for high formwork are illustrated in Section 3. The validation of the proposed method on a real high formwork are shown in Section 4. Section 5 provides the conclusion and future research plans.
