The cost calculation method of construction 3D printing aligned with internet of things

The existing computer network in intelligent construction engineering is the internet of things; it include the external network, the internal network, the bus network, the language network, the intelligent special network, and the metropolitan area network connection. At the construction site, the internet of things can enable construction machinery, equipment, materials, structures, and even templates to “talk” with the central platform to obtain key performance parameters. The technology of construction 3D printing is the application basis of the internet of things in the construction field. It is the internet among construction workers, construction equipment, construction sensors, and so on. This technology can ensure the smooth progress of the buildings on the internet. The buildings built under 3D printing technology are called 3D printing construction. If the cost of the 3D printing construction is low or the output efficiency is far greater than the input, the internet of things under 3D printing technology will be easily accepted by many enterprises in the construction field. This will greatly promote the building about internet of things. On the contrary, it is difficult to develop. Only when we understand the 3D printing construction cost can we accurately plan the development strategy of the internet of things in the construction industry. And the following is to solve the problems of cost accounting method to 3D printing construction.

The construction 3D printing (C3DP) was first introduced by James B Gardiner in 2011 and refers to buildings constructed with the 3D printing technology. There are two types of C3DP. That is onsite and offsite C3DP. In offsite C3DP, the units or components are printed in the factory and then transported to site for assembly. In onsite C3DP, the units are constructed at the site directly. Currently, there are some cases of 3D printed buildings. The Chinese company Yingchuang had successfully printed the office buildings in Dubai, and Huashang Luhai Ltd. had printed the first integrated non-modular house in Beijing. The 3D printing is a new technology which shows its advantage in constructing irregular buildings. To conventional construction, the cost of the flat wall is cheaper than the curved and irregular wall. On the contrary, the cost of C3DP is irrelevant to the shape [1, 2].

As a new construction technology, the cost of C3DP remains controversial on whether the C3DP is higher or lower than conventional construction [3, 4]. There are two reasons why it is still a controversial topic. Firstly, the comparison was made based on the part of the factors influencing the whole cost. Secondly, the cost calculation lacked overall understanding and analysis to construction. Therefore, it is necessary to study the calculating method and cost structure of construction. The cost calculation of conventional construction has been fully developed globally, whereas, for C3DP, there is no structured bill of quantities, budget quota, and pricing basis, which confuse its cost calculation.

Some scholars thought that C3DP shortened the construction duration and reduced the labor cost and material waste [2, 5]. In particular, the cost of formwork is reduced, because C3DP technology does not need to use formwork [6, 7]. The C3DP also improved the buildability, functional integrity, and accuracy of construction [6, 7]. Bos et al. [8] and Hager et al. [9] believed that C3DP could reduce the cost of construction techniques to a great extent. Yingchuang, a high-tech enterprise of C3DP in Shanghai, China, claimed that C3DP could save 30 to 60% of construction materials and shortened 50 to 70% of construction duration while the labor cost could be reduced by about 50 to 80% [6]. Jinghua and Huxin [10] said that materials used in C3DP mainly come from construction waste. Thus, the cost of construction material should be lower than traditional construction. Liping et al. [11] studied the material of the world’s first 3D printed five-storey building in Suzhou, which is constructed by Yingchuang. They proved that the material comes from the construction waste, cement, organic binder, and fibers. However, Gosselin et al. [12] believed that the overall material cost of C3DP was higher than the cost of traditional construction material due to the special properties of concrete and other materials required by C3DP. Nathalie et al. considered that there was not too much difference between the cost of material and labor in C3DP and the cost of traditional in-situ reinforced concrete [13]. They believed that comparing to the traditional construction method, 3D printing technology only revealed its advantage in cost in building the complex shape structures [13].

Different scholars have different opinions about the buildings built by new construction technologies. Ling et al. [14] proposed that it is correct to separate the building structure part and building filling the material part and list out the prefabricated components for cost calculation. Shuang and Chunyan [15] proposed that for prefabricated and assembled buildings, their cost were the conventional construction cost added the manufacturing, transportation, and assembling cost. Erman [16] proposed that the cost of construction components ought to be counted as the material cost of onsite work and calculated as cost of construction measures. These researches tried to amend method of cost calculation for new construction technologies from the different perspectives, such as the SI system, or the “Specification for Valuation of Bill of Quantities for Construction Projects.” These provided the insights for C3DP.

Industrial and construction products are two most common products in life. Industrial products are manufactured in factories according to the uniform procedures in batches and categories, whereas the construction products are eventually built at construction site directly on the foundation as specified according to the pre-approved designs. Although the components design are standardized and uniform within a certain country or region, for each construction product, the structure, building material, construction management, and construction methods are distinctively different from each other due to the difference of natural resources, techniques, and economic situation. Therefore, each construction product is uniquely built.

Meanwhile, the construction usually exists outdoors because of the huge size of the product which no factory can accommodate. Even in the construction industrialization, the prefabricated components have to be assembled outdoors after being manufactured at factories.

For offsite C3DP, the components printed with automated 3D printers belong to industrial products. But the process that these components are transported to the site and assembled belongs to construction products. For onsite C3DP, the printers are located on the pre-assigned location and print the structured building with automated control, which is typical industrial production process. Therefore, it is obvious that the C3DP is characterized as both industrial and construction production process [13, 16‐18], which shall be reflected in its cost composition and calculation method. The above characteristics of industrial product, construction products, and C3DP are shown in Table 1 in great detail.

The cost calculation formulas of industrial and construction products are the following:

In which, the construction installation cost can be calculated based on either sub-engineering tasks or the composition of cost.

For sub-engineering task-based calculation:

For cost composition-based calculation:

It is obvious that the cost is jointly determined by various factors and the compositions of industrial and construction product are different. A detailed composition structure of cost for these two types of products is presented in Table 2. To simplify the analysis, the discussion in this paper is based on assumption that there is no loan and no unforeseen factors. Thus, the reserve and interest during construction will not be included.

The salaries, insurance, allowance, and the provident fund are included in labor cost. The taxes are determined according to the authority regulations. For the category “charges,” insurance and housing fund are counted into labor cost whereas the pollution discharge cost is listed separately. The cost of transporting printed components can be calculated in two different methods practically due to its dual properties. One option is that the printing company is responsible for the delivery and the delivery cost is counted as manufacturing cost, with value added tax (VAT) is 17%. The other option is that the transportation is subcontracted by a logistic company, which belongs to the transportation and logistics industry, VAT is 11%. The VAT rate shall be calculated according to actual cases.

There are a few number of buildings printed by C3DP method because the C3DP is a newly emerged technology. There is no universal standard or technical system in C3DP. It is difficult for construction companies to initiate mass production with C3DP because of the high cost in the investment and low demand in the market. The research into the cost calculation method is beneficial to promoting the C3DP.

The emerging of C3DP technology has posed challenges to the cost calculation in tradition method, and actually, the cost composition is different from the conventional cost compositions. A summary of the difference in cost compositions among conventional construction, offsite C3DP, and onsite C3DP is presented in Table 5.

From the above analysis, it is obvious that the cost of C3DP is more sensitive to social change. When calculating costs, both offsite and onsite factors need to be taken into consideration. This paper studied the cost compositions of onsite and offsite C3DP with considerations on properties of both industrial and construction products and proposed a new method of cost calculation for C3DP. This method separately analyses the cost calculation of components printing, components assembly, and onsite C3DP to emphasize the fundamental difference between the industrial and construction products in the cost calculation. The summary of influence on cost calculation brought by introducing C3DP to construction cost structure was given at the end.

The time cost of C3DP is an important topic for future study. This is because the construction duration is positively proportional to the company expense. Currently, the time auditing is not included in conventional construction, and the companies are ignorant of such audit, which unconsciously increases the actual cost of construction which is not reflected in the financial statement. The absence of such information affects the scientific decision-making and covers up real competitiveness of C3DP.