Autoclaved aerated concrete panels, referred to as AAC panels, are favoured by engineers because of their inherent advantages, which include properties such as light weight, high strength, good thermal insulation performance, non-combustible, sound insulation, good machinability and easy installation. In recent years, China has vigorously developed prefabricated buildings and established a sound green building standard system. Autoclaved aerated concrete, as a green building material, can be used in the prefabricated building envelope system to meet the needs of building industrialization. On the basis of a series of national policies, AAC wall panels have increasingly been applied in various industrial and civil buildings both as internal walls and for the external envelope of the structure.

With the improvement of design Standards related to building energy efficiency, it is difficult to meet the thermal performance design requirements of external walls in northern China by relying on autoclaved aerated concrete alone. Consequently, in regions with cold and severe climatic conditions, AAC panels are usually only used as inner wall panels. In recent years, some enterprises and scientific research institutions have begun to develop high-performance AAC panels with low thermal conductivity, as well as AAC sandwich insulation wall panels, in order to meet the requirements for energy-saving wall design by improving the thermal resistance of wall panels. According to the requirements for external wall panels for buildings made with prefabricated steel structures, a new type of wall panel is proposed in this paper. This proposal relates to AAC composite thermal insulation wall panels, which form an integral part of the thermal insulation and load-beating structure of a building. Through experimental investigations and theoretical analysis, the performance of this new type of composite insulation wall panel was studied.

AAC composite insulation wall panel construction

There are many types of wall panels available for the construction of external wall enclosure systems, each with their own construction process. In order to ensure the engineering quality of external wall enclosure systems, GB/T 51232 "Technical Standard for prefabricated steel structure buildings" puts forward clear and specific performance requirements for the external wall enclosure system, mainly including safety, functionality and durability. In addition, the outer wall panels should also have good economic characteristics and meet the requirements for low environmental impact. Economical considerations require that the raw materials used in the production of wall panels are widely available, locally sourced, and relate to simple design and production in order to reduce production costs. Environmental coordination requires the composition of the wall panels to relate to efficient use of material resources, less energy consumption, less environmental pollution, and high recycling rates. Prefabricated exterior wall panels for the building envelope transfer a significant proportion of on-site work, necessary in the traditional external wall construction method, to the production plant. In the process, the panels are manufactured in the factory and then transported it to the construction site, where they are assembled and installed through a reliable connection method. Therefore, the outer wall panel should also meet the requirements of industrialization, which includes the following aspects:

-       Factory standardization, automation, and assembly line production.

-       Light weight construction, allowing convenient transportation and hoisting.

-       A simple node structure, which facilitates convenient installation and construction.

In summary, the performance requirements for the ideal exterior wall panel are shown in Figure 1.

Based on the performance requirements for exterior panels for prefabricated steel structures used in building construction, the research group proposed a new type of composite insulation AAC wall panel, which is based on autoclaved aerated concrete panels, composite organic insulation boards and 35 mm thick A-grade insulation slurry prefabricated in the factory. The panel combines requirements for thermal insulation and structural capacity. The composite insulation exterior wall panel is shown in Figure 2.

The composite insulation wall panel uses an AAC base panel to bear the weight of the wall panel and the horizontal load acting on it. An organic insulation board is used as an insulation layer to meet the energy-saving design requirements of external walls in cold climates, and insulation paste is applied as a protection layer to meet the fire performance requirements of the wall panel. Figure 3 shows an example of the wall panel in a shear wall structure for a residential project, while Figure 4 shows the application of the wall panel in a steel structure for a residential project. There is no cavity between the structural layers of the composite thermal insulation exterior wall panels. Once the AAC base panel is coated with the interface treatment agent, the adhesive mortar is combined with the thermal insulation board, following which the thermal insulation slurry protective layer is poured on the upper surface of the thermal insulation board. In this process, the thermal insulation slurry completely encases the thermal insulation board. In order to improve the bonding properties and connection strength between the insulation slurry protective layer and insulation board, a key connection directed along the width of the panel is set up on the surface of the insulation board. In addition, the insulation layer and the AAC base panel are mechanically connected using plastic anchors.                                                                                                     

Mechanical testing of AAC composite insulation exterior wall panels

The insulation layer of the composite insulation wall panel is bonded to the AAC base panel with a bonding mortar. As a consequence, the bond strength between the bonding mortar and the AAC panel directly affects the connection safety between the insulation layer and the base panel. In order to study the bond properties between the adhesive mortar and the AAC base, tensile tests were carried out using a total of 20 test specimens. The influences of interface preparation and adhesive mortar thickness on tensile bond strength were analysed, aiming at optimizing the connection between the insulation layer and the AAC base panel of the composite insulation wall system.

Two groups of bonded specimens were designed for the experiment. One group of specimens was coated with a bonding agent on the surface of the AAC slab, and the other group received no interface treatment. The specimen size was 100 mm x 100 mm. Each group of specimens was divided into 5 types according to the thickness of the bonding mortar. Two specimens of each type were produced, resulting in a total of 20 specimens. The specimens coated with an interface bonding agent are shown in Figure 5 and the parameters of each specimen are shown in Table 1. When preparing the test specimens, the adhesive mortar was filled and placed in the forming frame of the AAC base panel. The thickness of the forming frame is divided into 5 types: 2 mm, 4 mm, 6 mm, 8 mm and 10 mm. After the specimens were made, the tensile tests were carried out after 28 days of curing under controlled environmental conditions at a temperature of 23±2℃ and a relative humidity of 50±5%. The test procedure was based on the tensile test method for basic wall panels and adhesives as stipulated in Appendix C.1 of "Technical Standard for external insulation engineering of external walls", JGJ144.  A bond strength detector with digital display was used for the measurements (Fig. 6).

The tensile bond strength of each bonded specimen is shown in Table 1. Article 5.2.7 of the External Insulation Standard stipulates that the tensile bond strength between the base and the adhesive should be no less than 0.3 MPa. As can be seen from Table 1, for specimens without interface treatment agent, the average tensile bond strength between the adhesive mortar and the AAC base was lower than 0.3 MPa, for each thickness tested. For samples coated with an interface treatment agent, the average tensile bond strength between the bonded mortar and the AAC panel was greater than 0.4 MPa, for each thickness tested. Therefore, the tensile bond strength between the bonded mortar and the AAC panel can be significantly improved by applying an interface treatment agent to the surface of the AAC panel, thus improving the connection safety between the thermal insulation layer and the AAC base panel. According to the relationship between the tensile bond strength and the thickness of the bonding mortar (Fig. 7), changing the thickness of the bonding mortar has little influence on the tensile bond strength of specimens coated with the interface treatment agent. Therefore, in order to reduce the production cost of composite insulation wall panels, the thickness of bonding mortar should be reduced as much as possible, thereby reducing the amount of material required. In order to ensure the connection safety and take into account the requirements of production, it is recommended that the thickness of the bonding mortar for the composite insulation wall board should be 4-6 mm.

Table 1: Tensile bond strength of bonded specimens.

Sound insulation and fire resistance of composite insulation wall panels

The ideal external wall panel requires a low self-weight, and its thickness should be reduced as much as possible while still meeting the thermal performance, to reduce transportation and lifting costs. However, a wall with low self-weight and low thickness usually has insufficient sound insulation properties. In order to test the sound insulation performance of the composite insulation exterior wall panel, the sound insulation performance test was conducted according to GB/T 19889.3, "Acoustic insulation measurement of acoustic buildings and building components, Part 3: Laboratory measurement of ambient sound insulation of building components". The size of the specimens was 2480 mm × 600 mm × 230 mm, and the structural layers of the wall panel were successively as follows: 150 mm thick AAC base, 5 mm thick bonding mortar, 40 mm thick insulation layer and 35 mm thick insulation slurry protective layer. The test results show that the ambient sound insulation of the composite insulation wall panel is 45 dB, which conforms to the requirements of the external wall ambient sound insulation of ≥ 45 dB as stipulated in Article 4.2.6 of GB 50118 of the Civil Building Sound Insulation Design Code.

The fire resistance is directly related to the safety of the building, so it is of great significance to test the fire resistance of the composite insulation wall panel. In order to test the fire resistance of the composite insulation exterior wall panels, the fire resistance test was carried out according to GB/T 9978.1, "Fire resistance test methods for building components, Part 1: General requirements". The size of the specimen was 1200 mm × 600 mm × 200 mm, and the structural layers of the wall panels were successively as follows: 120 mm thick AAC base, 5 mm thick bonding mortar, 40 mm thick insulation layer and 35 mm thick insulation slurry protective layer. The test results show that the fire resistance limit of the composite insulation exterior wall panel is greater than 1.0 hour, which meets the requirement of the fire resistance grade of non-load-bearing exterior walls stipulated in Article 5.1.2 of GB 50016 of the Code for Fire Protection of Buildings. 

Applicability analysis of composite insulation exterior wall panels

With the improvement of energy-saving design standards, is difficult to meet the requirements of the heat transfer coefficient in cold climates using a single material wall panel consisting of AAC. An important index to measure in order to evaluate the applicability of the wall panel is therefore its thermal performance. The low weight of the wall panel is not only convenient for transportation and hoisting, but also can reduce the building weight, the structural effects of earthquakes, as well as the costs related to the general engineering requirements. Further, it is also an important indicator for assessing the applicability of the wall panel. In addition, the cost of wall panels is also an important indicator that affects their promotion and application. Therefore, through the analysis of thermal performance, self-weight and economy of the composite insulation wall panels, the applicability of the composite insulation wall panels for cold climates for use in combination with prefabricated steel structures was studied.

The heat transfer coefficient is the main index for measuring the thermal performance of external walls. JGJ26, Design Standard for “Energy saving in residential buildings in cold climates”, stipulates limiting values for the heat transfer coefficient of external walls according to the number of building floors (Table 2). In order to facilitate the current investigation, the applicability of the new composite insulation wall panel is evaluated by selecting composite insulation wall panels and AAC wall panels of the same thickness, taking the limiting value for the heat transfer coefficient of exterior walls in cold climates as the criterion. According to the equation for the heat transfer coefficient of external walls composed of multi-layer homogeneous materials provided in the Code for Thermal Design of Civil Buildings GB50176, wall panels with three thicknesses of 240 mm, 280 mm and 320 mm were selected for calculation. In the calculation example, the thickness of the AAC base panel of the composite insulation exterior wall panel is 15 mm, and the insulation layer consists of extruded board with thicknesses of 50 mm, 90 mm and 120 mm. The thickness of the adhesive mortar is 5 mm, and the thickness of the insulation slurry is 35 mm. The performance parameters for the materials of each structural layer of the wall panel (Table 3), and the heat transfer coefficients of the two types of wall panels calculated according to the equation are shown in Table 4.

Table 2: Limits for heat transfer coefficients for external walls.

Table 3: Material performance parameters for each structural layer.

As can be seen from Table 4, compared with AAC wall panels of the same thickness, the heat transfer coefficient of composite insulation wall panels is reduced by 40~57%. When the thickness of the composite insulation wall panel is 240 mm, it can be applied to buildings in cold climates and buildings with 4 floors and more than 4 floors in cold climates. When the thickness of the wall panel is 280 mm, it can be applied to buildings with various number of floors in cold climates. When the thickness of the AAC wall panel is 320 mm, it can only be used for buildings with 4 floors and more in cold climates. Therefore, the composite insulation wall panel has broad application prospects in cold climates.

Table 4: Comparison of heat transfer coefficient and self-weight of exterior wall panels.

Table 3 shows the bulk density values of the materials of each structural layer, as used in the calculation of the self-weight of the wall panels. For the purpose of comparison, a value of 525 kg/m³ was assumed for the AAC panel, 35 kg/m³ for the extruded panel, 400 kg/m³ for the vitrified microbead insulation slurry, and 1800 kg/m³ for the bonding mortar. The calculation results of the self-weight of the two types of wall panels are shown in Table 4. As can be seen from the table, the self-weight of the composite insulation wall panels is reduced by 18~37%, compared with the AAC wall panels of the same thickness. Therefore, the application of composite insulation wall panels in prefabricated steel structure buildings can give the full range of advantages related to light weight, high bearing capacity and good seismic performance of the steel structure system.

Table 5 shows the price comparison of different external wall enclosure systems per unit area. It can be seen from the table that the lowest cost per unit area is the fly ash block in combination with the rock wool board thin plaster external insulation system. However, this external wall enclosure system is a non-fabricated external wall system, and according to the current Evaluation Standard for Fabricated Buildings, GB/T 51129-2017, the assembly rate is not scored. The assembly rate scores for AAC panels in combination with rock wool board insulation decorative board systems, precast concrete sandwich insulation board exterior wall systems, and AAC composite insulation board systems are all 8 points. However, among these three exterior wall systems, the unit area cost of AAC composite insulation board systems is the lowest, amounting to about 40-60% of the respective costs for precast concrete sandwich insulation board exterior wall systems. Therefore, it has better economic applicability.

Table 5: Price comparison per unit area of different external wall enclosure systems.

Conclusions

Based on the performance requirements for exterior wall panels of steel structure prefabricated buildings, a new type of integrated exterior wall panel is proposed for the thermal insulation of the structure, and the connection mode between each structural layer was determined.

The tensile test results show that the interface bonding agent can significantly improve the tensile bond strength between bonding mortar and AAC panel. The thickness of the bonding mortar for the composite insulation wall panels is recommended to be around 4-6 mm.

The sound insulation performance and fire resistance test results show that the sound insulation capacity of the composite insulation wall board is no less than 45 dB, and the fire resistance limit is greater than 1.0 hour, which meets the sound insulation and fire resistance requirements of the current Code for non-load-bearing wall panels.

Compared with AAC wall panels of the same thickness, the heat transfer coefficient of the composite insulation wall panel is reduced by 40~57%, the self-weight is reduced by 18~37%, and the unit area cost is the lowest when comparing the various panel systems on the basis of equal assembly rate scores.