**1. Introduction**

A glass-fiber-reinforced cement (GRC) is a type of composite building material made of cement and glass fiber as the main components while also including white sand, metakaolin, and fly ash [1]. Because of its excellent plasticity and durability, a GRC material can be used as an exterior leaf decorative material on the walls of buildings and is preferred over lacquer materials. The application of GRC not only protects the environment but also plays the role of architectural decoration in accordance with the concept of green buildings [2,3]. A GRC-PC exterior wall is a new type of prefabricated wall formed by a composite GRC surface layer on the outer leaf layer of a precast concrete (PC) wall panel, as shown in Figure 1, which not only ensures the structural bearing capacity of the members but also plays a decorative role. The use of this wall panel significantly reduces pollution, shortens the construction period, and improves the construction efficiency, thereby integrating the building, the structural, and the decorative elements through fabricated systems. It also has great advantages in improving the structural optimization of the conventional PC industry [4,5].

**Citation:** Chen, D.; Li, P.; Cheng, B.; Chen, H.; Wang, Q.; Zhao, B. Crack Resistance of Insulated GRC-PC Integrated Composite Wall Panels under Different Environments: An Experimental Study. *Crystals* **2021**, *11*, 775. https://doi.org/10.3390/ cryst11070775

Academic Editors: Cesare Signorini, Antonella Sola, Sumit Chakraborty, Valentina Volpini and Tomasz Sadowski

Received: 27 May 2021 Accepted: 29 June 2021 Published: 2 July 2021

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**Copyright:** © 2021 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/).

**Figure 1.** Schematic of a GRC-PC external wall structure.

In practical engineering, the concrete components without glass fiber are prone to cracking, which shortens the service life of components, affects the aesthetic appearance, and reduces the construction quality [6,7]. Previous studies showed that the basic factors causing cracks in concrete cementitious materials are determined by mechanical properties, shrinkage properties, and ductility. Recently, in the development of new concrete materials, materials that incorporate fibers to improve the crack resistance of concrete, such as glassfiber-cement (GRC) and steel-fiber-cement (SFRC), are gradually being used in engineering applications [8–10].

In a study on the mechanical properties of GRC, Liu and Wu [11] pointed out that adding glass fibers helped reduce the elastic modulus of concrete. Zhao et al. [12] showed that, with the increase in the glass fiber content, compressive strength, splitting tensile strength, and flexural strength of concrete increased first but then decreased, i.e., there is an optimal fiber content. Shen et al. [13] found that an alkali-resistant glass fiber could significantly improve the tension–compression ratio and Poisson's ratio of concrete and enhance its toughness and brittleness. Qian and He [14] studied the influencing factors and the development patterns of glass fibers and fly ash composite cements and concluded that the material strength was most influenced by age and cement content, followed by the glass fiber, and least by the fly ash.

In a study on the shrinkage performance of GRC, Lura et al. [15] found that the shrinkage deformation of materials was the primary factor causing cracks in the process of condensate sclerosis, regardless of whether it was ordinary cement or GRC. The dominant shrinkage mechanism was found to be temperature autogenous shrinkage, which is the shrinkage deformation of a material under the combined action of the hydration heat of cement and the external temperature change [16]. Shrinkage deformation causes shrinkage stress in a material, and cracks are induced when the stress exceeds the maximum tensile stress that the material can withstand [17,18]. In addition, the shrinkage of GRC is affected by curing temperature, humidity, and environment [19,20].

To alleviate the shrinkage deformation of the GRC material and improve its crack resistance, previous studies were mainly carried out from two aspects. The first is the reasonable selection of GRC aggregates. Ye et al. [21] and Nguyen et al. [22] concluded that the incorporation of alkaline aggregates could help increase the amplitude of drying shrinkage of the GRC and the cracking sensitivity of cementation materials by measuring the cracking time and the cracking degree of cement with different alkalinities. Kumarappa et al. [23] reported that the addition of alkaline materials affected the reaction

degree and the surface tension of pore solutions based on shrinkage, heat flow, and surface tension of cements blended with Na2O and SiO2, thus affecting the shrinkage performance of cement. Wu et al. [24] studied the effect of cementation material composition on the shrinkage properties of GRC materials and concluded that GRC materials prepared with sulphate aluminate cement underwent the least amount of shrinkage, whereas GRC materials prepared with silicate cement shrunk to a greater extent. Additionally, the incorporation of fly ash and silica fume could effectively reduce drying shrinkage and self-shrinkage of GRC materials. Chylík et al. [25] studied the effect of modified gum powder on the shrinkage of GRC materials and concluded that the incorporation of gum powder could help reduce the internal gel pores and the macropores in the material, improve the hydrophilicity of cement, and thus improve flow and toughness of the GRC. Guo et al. [26] studied the effect of swelling agents on the shrinkage of GRC materials and concluded that GRC materials with swelling agents had fewer bonding cracks between the hydration products and the aggregates, better interfacial transition zone of concrete, and fewer cracks due to drying shrinkage.

On the other hand, it is necessary to control the content and the composition of glass fibers in a GRC material. Fiber is added to increase toughness and ductility of the cement base, improve the tensile strength of cement, reduce cracks, and prevent cracks from developing [27]. He et al. [28] studied the influence of fiber geometry on the cracking resistance of GRC and found that, with the increase in the fiber length and the decrease in the fiber diameter, the total plastic shrinkage cracking area of a cement mortar showed a downward trend, and the cracking resistance improved correspondingly. Shen et al. [29] found that the shrinkage strain of fiber-reinforced concrete decreased with the increase in the fiber volume percentage and put forward a prediction model for the early selfshrinkage strain of fiber-reinforced concrete. Kasagani and Rao [30] studied the effect of fiber grading on the crack resistance of GRC, suggesting that short fibers mainly controlled the expansion of microcracks and improved the ultimate strength, while longer fibers inhibited macrocracks and alleviated the deformation of concrete. Consequently, the combination of long and short fibers could help prevent microscopic and macroscopic cracks from developing, thus improving the crack resistance of concrete.

In summary, previous studies on the mechanics and shrinkage performance of GRC mainly focused on pure GRC prefabricated components, and most of the experiments were carried out in a relatively constant temperature and humidity environment. The GRC-PC composite wall panel studied in this paper was to be used both inside and outside the building, with great changes in temperature and humidity. In addition, the shrinkage rate of concrete was less than that of GRC; in this scenario, the concrete layer would hinder the shrinkage of GRC layer when they were combined, which would increase the tensile stress in the GRC layer, causing GRC layer cracking. To improve the crack resistance of the GRC-PC composite wall panel, the research idea of this paper was as follows. First of all, the GRC material formula was adjusted, and the compressive strength and the elastic modulus of the material were measured. Secondly, according to the different interface types of GRC layer and PC layer, seven wall panels of 1 m × 1 m were prepared, and the shrinkage experiment was carried out for 365 days in environments with different temperature and humidity. Among them, the crack resistance of the GRC layer was the core of the test. Finally, according to experimental results, the reasonable interface type between the GRC layer and the PC layer was determined. Findings from this research contribute to application of GRC-PC composite wall panels and promotion of prefabrication.

#### **2. Materials and Methods**

#### *2.1. Experimental Raw Materials and Equipment*

The raw materials required for the experiment were C30 concrete and GRC mortar. Among them, C30 concrete was produced by a concrete factory, and the GRC material was prepared by mixing in the experimental site; GRC materials are prepared by mixing the raw materials listed in Figure 2 in the experimental site.

**Figure 2.** GRC raw materials.

Portland cement, sand, fly ash, water reducing agent, and glass fiber are the traditional GRC formulations. In order to improve the crack resistance of the GRC material, the formula was adjusted, that is, we added rubber powder, expanding agent, metakaolin, and titanium dioxide.

The role of rubber powder was to reduce GRC internal porosity and improve the hydrophilicity of cement so as to increase its mobility and toughness [25]. The function of the expanding agent was to reduce the bond crack between hydration products and aggregate [26]. The role of metakaolin was to improve the pore structure of the cement mortar and improve uniformity and compactness of the mortar structure [31]. Titanium dioxide was used to improve the brightness to achieve the effect of decoration.

Table 1 lists the mix proportion of mortar of GRC, in which the cementing material and the sand were 8:9, and the water–binder ratio was 0.28.


**Table 1.** Mix proportion of mortar of GRC.

Figure 3 shows the instruments and the equipment used in the experiment, including a compression testing machine, which was used to measure the compressive strength and the elastic modulus of the raw materials (C30 concrete and GRC materials). An embedded strain sensor DH1204 and a surface strain sensor DH1205 were used to measure the strains of the GRC and the PC layers, respectively. A DH3818Y static strain tester was used for strain data collection and recording.

**Figure 3.** Equipment used in the experiment: (**a**) DH1204 embedded strain sensor; (**b**) DH1205 surface strain sensor; (**c**) DH3818Y static strain tester; (**d**) compression testing machine.
