Construction time and labor intensity of installation

The construction time and labor intensity were compared for the most popular technologies using the example of a two-story house with a usable area of 135 m², using catalogs of labor, materials and equipment expenditures KNR (Table 4). For modular elements, empirically estimated data were used. Based on experiences from existing projects, it can be assumed that in single day, two installers can assembly approximately 150 m² of structural walls from modular AAC panels during one work shift (10 hours). For partition walls, one experienced installer typically achieves an efficiency of about 40 m² wall surface during one work shift (10 hours).

The workload for erecting structural external walls, structural internal walls and partition walls compared to calcium silicate blocks and ceramic hollow bricks is more than five times lower. Walls from AAC are erected faster, but the use of modular elements further reduces labor intensity by almost three times. The difference increases when utilizing the thermal properties of autoclaved aerated concrete and constructing monolithic walls. In extreme cases, erecting walls and finishing the facade with modular AAC panels can shorten the construction time by over 600 man-hours, or nearly 80%. This is a significant difference even for a single-family house project. Considering that the system is currently most commonly used in developer projects where several to a dozen buildings are erected, it can also translate into significant differences in fixed costs associated with maintaining the construction site.

Buildings can be erected faster using typically prefabricated partitions. The advantages of such a solution include the possibility of delivering walls together with installations. The assembly time can be reduced to 0.5 days for a single-story building, which is not a significant difference considering the need to manufacture elements on special order. Flexibility in ordering is crucial for the realization time. The assumption of modular AAC panel systems is the availability of typical solutions from stock. This flexibility is important for design changes and mistakes on construction, which occur more frequently in this segment of applications than in other buildings.

Table 4: Comparison of the labor intensity of constructing partitions of a 135 m² building using different technologies [24-28]

For construction projects commissioned by private investors, estimates are rarely based on bill of quantities and are most often a lump sum for a specified scope of work. This means that the costs of building construction are not proportional to the labor intensity. Despite this, the labor costs for partitions in the modular AAC panels system constitute about 40 to 60% of the costs of constructing partitions using standard technology.

The technology of modular AAC wall panels fits into the mechanization of construction work, which still has a small share in masonry work in Poland. In countries where labor costs are higher, the mechanization of masonry work is already significantly developed. According to estimates, even 95% of partitions in multi-family buildings in the Netherlands are constructed using lifting equipment.

There are many research studies worldwide on the robotization of masonry work. Three directions can be distinguished in the development of construction robotization: printing walls on-site and in production plants, using cable robots to erect walls from small-sized elements, and using a mobile crane (Hadrian X) for autonomous masonry with masonry elements.

In practice, each of these solutions has drawbacks. On-site printing [29] and cable robots [30] require setting up temporary structures, which for small constructions like single-family houses, will likely take longer than the process of installing AAC panels. The mobile robot requires continuous replenishment of material in the loading area [31], so it is not maintenance-free. In practice, only prefabricated structures can have a significant advantage in installation time. The real advantage is not in the installation time but in the ability to deliver the partition together with fitted electrical and sanitary installations, which are prepared not on-site but in the production hall. This allows workers performing tasks at fixed workstations to achieve better efficiency compared to construction conditions.

According to the authors´ opinion the installation time of AAC panels at the level of one working day per single floor will no longer require optimization for projects involving up to several residential buildings. The issue of installing electrical and sanitary installations and potentially finishing the walls remains to be resolved. It is worth noting that setting up panels requires significantly fewer qualifications than masonry walls, and the partitions are significantly more even and easier to construct. In some projects in Poland, due to the lack of available masons, panels were installed by people inexperienced in working with heavy building materials. Setting up panels for partition walls requires slightly more qualifications. In this case, achieving the assumed efficiency usually requires about a month of practical work on the construction site. Full efficiency in installing panels for structural walls using a crane can be achieved by workers after a short training and installing just a few elements.

Benefits of including BIM technology

AAC modular panels can be used even more effectively after including BIM technology in design and assembly phase.

The architect, having developed a preliminary design, sends his design to manufacturers design department, which is then saturated with more detailed information regarding the chosen material. At this stage, manufacturer creates a preliminary model with a layout of specific elements, and also analyzes the construction or fitting of openings to adjust the structure to modular panels. The goal here is to make the most efficient use of materials on site, planned already at the design stage. Such a model goes back for approval by the architect and the investor, where, after final changes, it can be used to place a complete order and plan production and transportation and logistics.

The 3D model itself, however, serves as the beginning of further work with it. Using 4D simulations, manufacturer can create a progress schedule and optimize the way that panels are assembled. It allows to move part of the construction process into the production phase.

During the construction stage, the model is further used, thanks to augmented reality glasses. Microsoft Hololens 2 edition Trimble XR10 glasses on construction sites are used along with Trimble Connect software for positioning and visualizing models on site.

Augmented and mixed reality technology is developing at a rapid pace, enabling the placement of models on construction sites with a high degree of accuracy. This advancement allows for the layout of walls using large-format elements, replacing traditional 2D paper-based assembly plans. The benefits are undoubtedly free worker's hands, as they can see the model displayed in front of them where the physical object is supposed to be, but also ensuring that the information is always up-to-date. By placing the model in the cloud, any corrections can be applied in real time and are immediately visible in the model available on site.

At the beginning of 2021, Xella also introduced the service of in-house wall installation on site. As a result, the company is able to offer a full range of services, which is called wall-as-a-service. All wall-related activities - from design in BIM, to production, transportation and logistics, to on-site installation - come from a single entity. But the work doesn't end even after the wall is finished. In Xellas’ pilot project, the company also conducted an as-built scan of the entire building. This allowed to compare the as-built model with the actual state, so the future the design can be improved at an earlier stage.

AAC as a material for sustainable construction

Autoclaved aerated concrete is known as a material with a low environmental impact [33-36]. To produce 1 m³ of concrete, between 300 to 600 kg of raw materials are needed, making it a unique construction material in this regard. Its low mass leads to lower emissions from transport and reduced load and reinforcement requirements for structures. The production of AAC needs sand, lime, cement, anhydrite, water, and aluminum paste/powder. Production in Polish plants is zero-waste – no waste is generated during production, as unused parts of the mass are returned to the mold, and after autoclaving, elements can be crushed and included in the recipe. Currently, AAC can be produced with up to 30% recycled material. Emissions from AAC production mainly consist of scope 2 emissions, mainly emissions from the production of lime and cement. AAC industry has developed a roadmap for zero-emission production of autoclaved aerated concrete [33]. The roadmap includes, among other things, reducing lime and cement emissions using Carbon Capture technology. An important property of AAC is its ability to bind atmospheric carbon dioxide in the amount of 70-80 kg CO₂/m³ during the recarbonization process [36]. Over its lifecycle, the emissions of AAC are comparable to those embedded in wooden structures.

Modular panels made of AAC, compared to standard blocks, have slightly higher emissions due to the presence of reinforcement, which is, however, minimal and amounts to about 10 kg/m³. In Scandinavian countries, the technology of building houses from AAC panels with a thickness of only 10 cm is popular. These buildings do not even have ring beams, and the roof structure based on trusses is only connected to the foundation slab with steel pins. In the coming years, the popularity of thinner partitions is expected to increase, which will result in a typical AAC wall having an even lower carbon footprint than wooden structures over its lifecycle.

Conclusions

Modular AAC panels have been used in Europe for many years. Various indicators point to an inevitable increase in labor costs in Poland in the coming years and good prospects for the development of the AAC industry. Western European countries have been facing a shortage of willing construction workers and high labor costs on a larger scale for years, which is why these solutions have a much larger market share there (e.g., in the Netherlands).

In Poland, panels are currently produced in two manufacturing plants. In both cases, the production lines have been redesigned to meet contemporary market and production realities, and the product range includes typical elements. Elements available from stock can be used in virtually any type of building, and modularity does not imply significant changes in the assumed design concepts.

Modular elements for structural partitions are mainly used in single-family and non-residential buildings. In the case of private investors houses, the fixed costs of the investment mainly consist of mortgage servicing, so speeding up the construction of walls compared to small-sized blocks is not as important. Considering that younger investors, in particular, are more inclined to buy houses from developers, in investments involving the construction of several to a dozen single-family houses, whose number has been increasing in recent years, shortening the time has a real financial dimension. Regardless of profitability, the ability to complete the structure (even without insulation) in single day can be a key factor likely to increase interest in this technology. The key in this technology is reducing labor intensity, which will allow meeting housing needs even with a reduced availability of physical workers. Modular wall panels are also intended for use in multi-family buildings as partition walls, where shortening the construction time has a real impact on its costs.

In France, Belgium, Sweden, and Germany, the environmental impact of products increasingly determines the choice of solutions. AAC elements already have a low carbon footprint and require a small amount of primary raw materials for production over their lifecycle. Achieving the goals of the roadmap for zero-emission production [32] will further enhance the attractiveness of the solution and contribute to increasing development prospects in Poland and Europe.

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