3.2.1. Cracking Analysis of Experimental Panels

The S0 panel was completed on 1 June 2019 and placed in an indoor environment for shrinkage experiments. As a member made of a single material, the S0 was subjected to free shrinkage. Three months later, cracks emerged in the S0 plate. As shown in Figure 9a, the cracks located in the middle of the panel in a cross distribution. These phenomenon indicated that different parts of the panel had different shrinkage and deformation. Thus, the shrinkage stress was generated in the panel. The distribution of cracks indicated that the shrinkage deformation in the middle of the panel was more limited, which led to the shrinkage stress exceeding the tensile limit of the material and finally cracking. Therefore, it was reasonable to place the strain gauge in the center of the panel (Figure 9b). At the same time, S1 to S6 did not crack during the monitoring period, and it can be concluded that the new GRC-PC composite wall panel met the crack resistance requirements.

**Figure 9.** Cracking condition of S0 and S1: (**a**) S0; (**b**) S1.

In order to have a visual display of the shrinkage deformation of all specimens, the strain curves of the S1 to S6 wall panels, shown in Figure 10, were plotted based on the data collected by the strain sensor during the experimental period. Figure 10a shows the strain of GRC layer of wall panels collected by surface strain sensor, and Figure 10b shows the strain of PC layer of wall panels collected by embedded strain sensor. The strain curves of S1 and S2, which represent pure GRC and pure PC panels, respectively, were used as the standard free shrinkage curves for the material. The other wall panels were classified in terms of environment and type of interface, and each set of strain curves was compared with the standard free shrinkage curve of the material as the crack resistance curve. The analysis was judged by the degree of adaptability, i.e., the more closely the crack resistance curve fit the standard free shrinkage curve of the material, the closer was the shrinkage of the corresponding composite wall panel to the standard free shrinkage, the lower was the resulting shrinkage stress, and the lower was the likelihood of panel cracking. Table 6 lists the maximum strain values of each group of wall panels.

**Figure 10.** Strain curve of wall panels: (**a**) Strain curve of GRC layer for each panel; (**b**) strain curve of PC layer for each panel.


**Table 6.** Maximum strain values of the GRC and PC layers for each group of wall panels.

3.2.2. Shrinkage Analysis of Wall Panels with Different Interface Types

Because of the significant influence of environmental factors on the shrinkage of the composite wall panels, the shrinkage of wall panels with different types of interfaces under two environments, indoor and outdoor, were analyzed separately. Figure 11a,b show the strain curves of panels with different interfaces in an indoor environment. Figure 11c,d show the strain curves of panels with different interfaces in an outdoor environment.

**Figure 11.** Strain curves of wall panels with different types of interfaces: (**a**) GRC strain curves of S1, S3, and S4; (**b**) PC strain curves of S2, S3, and S4; (**c**) GRC strain curves of S1, S5, and S6; (**d**) PC strain curves of S2, S5, and S6.

As shown in Figure 11a,b, the strain values of the GRC and the PC layers of S3 and S4 followed approximately the same strain curve trend over the monitoring duration. At the beginning of the experiment, the concrete and the GRC materials expanded in volume

and were pulled under the effect of hydration heat. With the hydration reaction gradually weakening until disappearing, the GRC material began to shrink, the GRC-PC strain value decreased to a negative value, and the panel began to be under pressure. In the middle and the later stages of the test, the strain showed a small wave change, which indicated that the shrinkage of the GRC materials tended to be stable. By comparing the three curves, we found that the GRC strain of the composite wall panel with a smooth interface in the indoor environment was closer to S1 strain, whereas the PC of the composite wall panel with a rough interface changed to S2 strain, indicating that the interface type of the composite wall panel significantly influenced the shrinkage. From the data listed in Table 6, we found that the shrinkage strains of the GRC material and the concrete with a smooth interface decreased by 23% and 19%, respectively, in the indoor environment, and the shrinkage strain of the GRC material with a rough interface decreased by 72% and that of the concrete increased by 22%. Therefore, the use of a smooth interface is more conducive to improving the shrinkage performance of GRC-PC composite wall panels installed in indoor environments.

Figure 11c,d show a fluctuation in the strain curve of the composite wall panel in the outdoor environment. This was attributed to the significant changes in the temperature and the humidity of the outdoor environment, and the shrinkages of both the PC and the GRC layers were significantly affected. The overall trend in the outdoor strain was similar to that in the indoor strain: both types of layers were in a state of tension in the early stages and began to contract under pressure as the hydration reaction diminished. From the data listed in Table 6, we found that the shrinkage strains of the GRC material and the concrete with a smooth interface decreased by 50% and 75%, respectively, in the outdoor environment, and the shrinkage strain of the GRC material with a rough interface decreased by 54% and that of concrete increased by 50%. Therefore, the use of a smooth interface is more conducive to improving the shrinkage performance of GRC-PC composite wall panels installed in outdoor environments.

In summary, composite wall panels with a smooth interface exhibit better shrinkage performance in both indoor and outdoor environments. It can be concluded that the rough PC surface increases the constraint on the GRC layer, which is not conducive to the free shrinkage of the GRC material, and consequently, the possibility of cracking of the GRC layer increases.

#### 3.2.3. Shrinkage Analysis of Wall Panel under Different Environments

The composite wall panels with the same type of interface were used to compare and analyze their shrinkage patterns in both indoor and outdoor environments.

Figure 12a,b show the strain curves of the composite wall panel with a smooth interface in different environments. Figure 12c,d show the strain curves of the composite wall panel with a rough interface in indoor and outdoor environments. Figure 10a shows a similar overall trend in the GRC strains of S1, S3, and S5, with the GRC shrinkage strain of S3 being significantly lower than that of S5. Figure 12b shows that the strain curves of S2, S3, and S5 followed a similar trend in the early stage, whereas the S5 curve exhibited a downtrend in the later stage. The range of variation in the PC shrinkage strains for S2 and S3 was roughly similar, and the maximum shrinkage strains of PC for S2 and S3 were significantly less than those for S5. From the data listed in Table 6, the shrinkage strain of the GRC was reduced by 23% and 50%, whereas the shrinkage strain of the PC was reduced by 19% and increased by 75% for the composite wall panels with a smooth interface in indoor and outdoor environments, respectively. It can be concluded that, compared with the indoor environment, the GRC-PC panel with a smooth interface has a wider variation range of shrinkage strain in the outdoor environment. It was proven that the shrinkage of composite wall panels is significantly affected by the temperature.

**Figure 12.** Strain curves for wall panels in different environments: (**a**) GRC strain curves of S1, S3, and S5; (**b**) PC strain curves of S2, S3, and S5; (**c**) GRC strain curves of S1, S4, and S6; (**d**) PC strain curves of S2, S4, and S6.

Figure 12c shows that the strain of S6 gradually increased, whereas that of S4 did not change significantly, and the shrinkage strains of both S4 and S6 were lower than that of S1. Figure 12d shows that S2, S4, and S6 had similar stress change trends in the early stages, and the stress change in S6 was greater than those in S2 and S4 in the later stages because of the significant change in the outdoor temperature. From the data listed in Table 6, we found that the shrinkage strain of the GRC was reduced by 72% and 54%, whereas the shrinkage strain of the PC was increased by 22% and 101% for the composite wall panels with a rough interface in indoor and outdoor environments, respectively. It can be concluded that, compared with the indoor environment, the GRC-PC with a rough interface has a wider variation range of shrinkage strain in the outdoor environment.

In summary, the shrinkage deformation degree of GRC-PC composite wall panels with two types of interfaces is greater in the outdoor environment than in the indoor environment. Although the GRC-PC has a greater shrinkage strain amplitude in a relatively harsh outdoor environment than in an indoor environment with suitable temperature and humidity, there was no sharp increase or decrease in the strain value due to component cracking, which indicates that the cracking resistance of the GRC-PC composite wall panel made of the new GRC material meets the requirements of the outdoor environment.

#### **4. Conclusions**

This research investigated the crack resistance and the facade effect of the GRC-PC integrated composite wall panels under different environments through an experimental research. The following conclusions can be drawn.

(1) According to the experimental results of S0, the cracks of wall panels are concentrated in the center position, where the shrinkage stress value is also the largest. In addition, fiber is an indispensable material to improve the crack resistance of GRC by comparing the cracks of S0 and S1.

(2) By studying the shrinkage performance of GRC-PC composite wall panels with different types of interfaces, we can conclude that the shrinkage deformation amplitude of the composite wall panel with a smooth interface is lower than that of the composite wall panel with a rough interface in both indoor and outdoor environments. The strain law of pure GRC and PC panels indicates that the processing method with the smooth interface is more beneficial to the crack resistance of composite wall panels in practice.

(3) The shrinkage deformation amplitude of GRC-PC composite wall panels with two types of interfaces was found to be greater outdoors than indoors. The shrinkage strain of the composite wall panels in the outdoor environment was in line with the free shrinkage law of the material, and no cracking occurred in any of the wall panels during the monitoring period, indicating that the crack resistance of the GRC-PC composite wall panels can be ensured in both indoor and outdoor environments.

GRC-PC insulation composite wall panel is a new type of prefabricated wall which can greatly reduce pollution, shorten the construction period, and improve the construction efficiency. The research in this paper provides an experimental basis for the large-scale application of the wall panel.

Due to the complexity of materials and the uncertainty of environmental changes, this paper was not able to find a reasonable and reliable finite element analysis model for the finite element analytical method, which is the research direction of future research.

**Author Contributions:** Conceptualization, D.C., P.L. and B.C.; methodology, D.C., P.L., H.C. and B.C.; formal analysis, D.C., P.L. and B.C.; investigation, D.C., H.C. and B.C.; resources, Q.W.; data curation, P.L. and B.C.; writing—original draft preparation, D.C., P.L., B.Z. and Q.W.; writing review and editing, D.C., P.L. and B.C.; visualization, B.Z. and B.C.; supervision, B.Z.; project administration, D.C.; funding acquisition, D.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by Natural Science Foundation of Anhui Province (19080885ME173), Research & Development project of China State Construction International Holdings Limited (CSCI-2020-Z-06-04), and Science and Technology Project of Anhui Province Housing and Urban-Rural Construction (2020-YF47).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** All data have been included in the manuscript.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

