Influence of the processes taking place in the contact zone “masonry - plaster coating” on the destruction of the wall structure

To create the optimal composition of the plaster solution with the preassigned properties, it is necessary to know the processes taking place in the “masonry-coating” system, including in the contact zone between it and the masonry, the mechanism of its destruction. The purpose of the presented work was to describe the processes of occurrence and growth of cracks in the plaster coating and its contact zone with the masonry, in the process of its application, hardening and operation of the wall structure. In recent years, significant progress has been made in analyzing the reliability of plaster coatings, their behavior during operation, however, there are problems in predicting the evolution of damage in terms of initiation and growth of cracks, which ultimately leads to cracking and delamination of the plaster coating from masonry. The behavior of the plaster coating largely depends on its structure and the structure of the contact zone. The state of the “masonry-coating” system was described and this made it possible to obtain data on the process of its destruction, the appearance and spread of cracks. The system was destroyed according to two main schemes: formation and development of cracks in the coating and in the contact zone. Thus, these researches make it possible to reduce the number of defects in the contact zone with the masonry and increase the service life of the wall structure.


Introduction and literature review
The use of plaster coatings is due to the need to ensure a longer service life of the wall structure. To create a plaster mortar with desired properties, it is necessary to know the processes taking place in the "masonry-coating" system, including in the contact zone between it and the masonry, the mechanism of its destruction.
In recent years, there has been significant progress in the analysis of the reliability of plaster coatings [1,2], their behavior during operation, but nevertheless, there are problems in predicting the evolution of damage in terms of initiation and growth of cracks, and this leads to cracking and delamination of the plaster coating from the masonry [3].
The behavior of the plaster coating largely depends on its microstructure [4,5] and the structure of the contact zone. Plaster mortars are hierarchical composites consisting of fine aggregates and inorganic compounds -brittle minerals, cement hydration products, the structure of which and the contact zone with masonry is formed as a result of processes occurring during application, hardening, operational influence and the environment [6]. Investigation of the state of the "masonry-coating" system allows obtaining data on the process of its destruction, the appearance and propagation of cracks. The article discusses a model of the stress-IOP Publishing doi:10.1088/1757-899X/1164/1/012059 2 strain state of the system. It destruction according to two main schemes: formation, development of cracks in the coating and its contact zone with the masonry.
After applying the mortar mixture to the masonry, cracks form on the surface of the plaster coating, the cause of which is the shrinkage of the cement stone due to its hydration, the suction of moisture by the masonry, its evaporation under the influence of the sun and wind ( figure 1, b, c).
The concept and view of the problem disclosed by the authors in this article is a unique and independent scientific direction in previously published works [7][8][9][10][11], the processes occurring in a plaster coating during its application and hardening, the destruction mechanism of the system "masonryplaster coating" are considered. It was revealed that the system is destroyed according to two main schemes: the formation and development of cracks in the plaster coating and its contact zone with the masonry. The reason for this is the stress caused by: shrinkage of the material due to moisture loss and hardening of cement, structural heterogeneity, deformation of the coating, masonry and their difference, etc.

The aim of the work
The aim of the work is to describe the processes of occurrence and growth of cracks in the plaster coating and its contact zone with the masonry, in the process of its application, hardening and operation of the wall structure.

Materials and research methods
The rate of occurrence of new cracks is determined by the stiffness of the coating. In hard plastering coatings, breaking stress is achieved as a result of less deformation. Therefore, at the initial stage of cracking, the density of cracks grows most rapidly for coatings that are characterized by the maximum modulus of elasticity.
Due to the fact that the masonry restrains the shrinkage deformations of the plaster coating, tensile stresses (σ) appear in it, due to which the cracks formed during shrinkage increase.
Their value is found from the equations [12]: tensile stresses taking into account elastoplastic deformations due to the presence of a gel; and modulus of elasticity and Poisson's ratio of plaster coating; ∆difference in deformations of plaster and masonry; <1coefficient of elastoplastic deformation of the solution under tension.
To calculate stresses ( ) and simplify, we assume that the shrinkage of the aerated concrete masonry has stopped, i.e. ∆ = ( shrinkage deformations of plaster They are significantly higher than the calculated tensile resistance for the calculation of the formation of cracks equal to 0.3 MPa and the destruction stress equal to 0,4 MPа [13].
In the contact zone "masonry-coating", due to the suction of moisture by the masonry from the mortar mixture and its shrinkage, horizontal cracks are also formed (figure 1.a) [15,16].
Cracks formed on the surface of the plaster coating "grow" both along the plane and into its depth, initially vertically, to the intersection with the plane of the "masonry-coating" contact zone, and then horizontally, along the contact zone. The direction of growth of these cracks is determined not only by the geometry of stresses, but also depends on the presence of previously formed shrinkage cracks in this zone.
The reasons for the destruction of the plaster coating and the contact zone are also the deformation of the wall structure due to the effect of temperature In bearing wall structures made of autoclaved aerated concrete, deformations of the masonry are added to these deformations, due to the constant load, which reach 1.2-2 mm / m [17]. As a result, in the contact zone, cracks of normal separation and flat shear are developing ( figure 4). At the initial stages, the crack propagates according to the mechanism of normal separation, then longitudinal shear begins to prevail ( figure 4).

The results of research
The materials forming the system i.e. masonry and coating differ from each other in their properties. The difference in their coefficients of thermal expansion, moisture deformation, elastic characteristics, causes the occurrence of longitudinal shear stresses along the interface, which also leads to the development of cracks in the contact zone.
∆ ∶ 0 = ∶ (14) where: ∆relative extension or reduction of the wall structure; 0length of the wall structure at the time of erection; -stress in Н/mm 2 ; -modulus of elasticity in Н/mm 2 .
The difference in the values of deformations, elasticity moduli of masonry and plaster coating are the cause of shear deformations and stresses (τ), in the contact zone between them (figure 5), which predetermine the development of a crack in the contact zone. The shear stress in the contact zone is equal to: where:shear stress due to thermal deformations, kgf/cm 2 ; ∆ 1 , ∆ 2temperature difference at the time of installation and operation of the plaster coating and masonry, ºС; 1 , 2coefficient of thermal expansion of masonry and plaster coating; 1 , 2moduli of elasticity of masonry and plaster coating, kgf/cm 2 ; The contact zone works for shear along the axis of the interface and for tension-compression, perpendicular to this axis. a b Figure 5. Difference between deformations of masonry and coating (a) and shear deformation in the contact zone "masonry-plaster coating" (b) c,d Figure 8. Types of loads on the contact zone of the system "masonry-plaster coating" Deformations of the plaster coating and the "coating-masonry" contact zone are the sum of three components: elastic deformation, reversible in phase with stress, completely irreversible -residual, highly elastic, reversible, but not in phase with stress. The competition of the above described fracture mechanisms in the coating -masonry system with an increase in the degree of deformation leads either to the formation of secondary cracks in the coating of normal separation or to the formation of shear cracks at the interface.
Due to the fact that the strength of the connection between the coating and the masonry at shear k and uniform separation σk depends on the destructive load and the area of their contact, then with a decrease in the contact area, the crack growth and delamination of the coating from the masonry becomes easier = ⁄ ; = ⁄ (18) where:destructive load,contact area.
The growth of the main crack occurs due to merging with pore-like cracks, which originate in the zone in front of the apex. This leads to the propagation of a crack along the coating-masonry interface, a decrease in the area of their contacts, as a result of which there is a widespread delamination of the coating (figure 9). The development of the main crack occurs until the plaster coating is detached from the base and the denudation of the masonry (figure 9).

Conclusion
The choice of the components of the mixture, the selection of its composition of the mixture must be made, taking into account the processes of destruction occurring in the system "masonry-plaster coating" during the operation of the wall structure. The article discusses a model of the stress-strain state of the system. Its destruction occurs as a result of the growth of many cracks formed in the zone where the local stress exceeds the local strength of the material. It collapses according to two main schemes: formation, development of cracks in the coating and its contact zone with the masonry. The reasons for the appearance and development of cracks are shrinkage of the plaster coating during hardening, deformation of the wall structure due to the effect of temperature, static and dynamic loads and stresses caused by them. Therefore, for the selection of the components of the mortar mixture, their ratio, it is necessary to take into account the factors. It is necessary to increase the water-retaining capacity of the mortar mixture and reduce the stresses in the coating and the contact zone during operation. The proposed actions will make it possible to reduce the number of shrinkage cracks at the stage of coating hardening, prevent the formation and development of cracks in it and the contact zone during the operation of the wall structure, slow down the rate of its destruction, and increase the time of its operation.