International Scientific Journals
of Scientific Technical Union of Mechanical Engineering "Industry 4.0"

The report presents the different stages of development and implementation of two similar innovative technical solutions for repair and restoration of defected reinforced-concrete constructions of existing swimming pool and artificial lake using the methods of “wet” shotcreting. In the first case, the object of renovation is a seriously defected outdoor mineral pool of a prestigious hotel in the town of Hissarya. Given the significant leaks of water from the pool in the surrounding area over the years, a specialized technical inspection was initially conducted to determine the causes of defects in the main load-bearing monolithic steel-reinforced concrete construction – walls and bottom slab. An innovative technical solution has been proposed to restore the operational suitability of the pool, based on the possibility of using the existing load-bearing structure as external formwork for a new built-in load-bearing structure of the facility. The proposed solution envisages jointless thickening of the existing walls through additional reinforcement and concrete laying of new high-tech sprayed concrete. The bottom of the pool is being renovated by additional new layer of a hybrid fiber-reinforced jointless concrete slab and machine powerfloated of the finishing surface. A composition of high-tech concrete with the participation of an internal crystallization chemical admixture and shrinkage-compensating one is proposed. A similar solution has been applied for the repair and restoration of the walls and bottom of an artificial lake in West Park, Sofia. Implementation of the project creates a unique art vision of the lake, the rock garden and the surrounding park space. This pilot implementation creates objective preconditions for expanding the range of innovative concrete structural systems, which, in addition to specific functions, are also applicable as opportunities for aestheticizing the urban and park environment.

The development of hydration processes in cement concrete is associated with the release of significant amounts of heat, which leads to a significant exothermic temperature increasing in the concrete body, especially valid for massive structures. This creates a temperature gradient from the inside of the array to the surface. The latter may be re lated to the development of unacceptable internal stresses, often leading to cracking, defects and reducing the durability of the facility. The report presents the individual stages of the search for an optimal technical solution for the construction of a specific hydro-technical facility – a massive hybrid reinforced trapezoidal wall of a water catchment gorge, in the area of Stara Zagora town. In the context of specified geometric dimensions of the facility, the specific features of the exothermic increase of the temperature in the concrete body as a function of the hydration processes are discussed. A reasonable choice of a specific type of cement is proposed in order to limit the amount of heat released – slag cement CEM III-A 42.5 N with a specific heat of hydration up to 280 J/g. A specific concrete mix design has been proposed, providing a balanced increase in temperature within acceptable limits, preventing cracking, in two possible scenarios – winter and summer outdoor temperature conditions. An additional advantage of the mix is the inclusion in the recipe of fiber-reinforcement and high-tech chemical admixtures – deep internal crystallization and shrinkage-compensating one. Specific calculation data for the kinetics of temperature increasing are presented, and the obtained values are critically evaluated in terms of guaranteed cracks eliminating in the structure.

The bearing monolithic reinforced construction of “Open Fuel Storage” of SOLVAY SODY AD, Devnya, has been operated for more than 40 years in conditions of extreme stress and external aggressive factors. All this has logically led to the presence of serious corrosion damages. The basic requirements for developing a technical solution for repair and restoration of defect structures retaining walls (assembly type), supporting beams and columns, is the bringing of the technical characteristics of the reinforced concrete structure in its initial (design) or better condition. The report discusses the features of the proposals as the most effective technological method for repair, based on the possibilities for the formless construction of a new special thin repair concrete layer restoring the original cross-section, applied with the methods of “wet” shotcreting. A composition of high -tech concrete with the participation of an internal crystallization chemical admixture and a shrinkage-compensating one suitable for shotcrete is proposed. Specific details have been developed for the renovation of the individual structural elements, as well as optimal technological regulations for the implementat ion of the repair.

Solvay Sodi JSC, Devnya, Bulgaria, is the biggest calcinated soda production plant in Europe. It is built in 1974 and it works in three-shift production regime up to present days. The exploitation environment is highly influenced by numerous factors, act ing corrosively on main reinforced concrete structures – active atmospheric processes (in continental type moderate climate), sea water born chlorideaerosols, as well chemical attack caused by carbon dioxide and nitric acid, presented in the air in form of evaporation. The most reinforced concrete structures need urgent remedial measures to restore the load-bearing capacity of its components.
The “light soda” silos are reinforced concrete facilities (cylindrical shells) which repair has to be executed dur ing continuous production process. The options for repair of these structures and restoration of their load bearing ability by specially designed innovative high-tech hybrid reinforced sprayed concrete, containing capillary crystalline admixture, are discussed in the report.
It is well known that two basic methods of shotcrete exist – “dry” and “wet” ones.
The report discusses the main advantages of the specially designed innovative hybrid fiber-reinforced “wet” shotcrete for thin repairing overlays, compared to industrially made “dry” one. By purposeful using of standard and direct physical test methods the basic long -term mechanical and structural characteristics of two composites are presented and discussed. The effectiveness of the proposed “wet” method is based to real possibilities to involve in concrete mix design different chemical innovative admixtures – ultra high range water reducing (HRWRA), shrinkage compensating (ShCA), internal crystallization one, etc. The mechanical properties of “wet” one dramatically exceeded respective ones of the “dry” one. The mechanical test results fully correspond to the respective long -term structural characteristics, obtained by using of X-Ray, DTA, SEM and BET-methods.
It could be concluded that the innovative hybrid fiber-reinforced “wet” shotcrete is preferable for high-performance thin repairing overlays
for structural strengthening of damaged concrete structures.

In recent years, the use of internal crystallization chemical admixtures for concrete and mortar to increase their water-tightness and other physical and mechanical characteristics has been of increasing importance in modern construction. These types of chemical modifiers allow for the effective replacement of conventionally performed waterproofing works (membranes, rolls, brushed or sprayed coatings, etc.) by purposefully improving the physical-mechanical characteristics of structural concretes, rendering them, to one degree or another, impermeable to water or/and aggressive agents from different origins. In the specialized world market for such products there are several leading competing companies – producers of internal-crystallization chemical admixtures, which have different activity in Bulgaria. Based on results of purposeful set of structural investigations (DTA, SEM, BET) conducted the purpose of the report is to assist participants  in the construction investment process in understanding the nature, specific characteristics and differences in the performance (effectiveness) of different products in terms of their ability to limit the ingress of water into concrete and reinforced concrete sections, as well as their ability to increase the durability of concrete as the main structural material.

Shotcrete is a technology patented in 1911 by Dr. Carl Akeley – curator at the Field Columbian Museum in, Chicago. The system enables one to place a cement-sand mix or concrete on various surfaces via high pressure and casing by means of special equipment – a shotcrete machine. In fact, this is a method for concrete casting by pressure through a hose and pipe-lines at high velocity into the surface. Two basic methods of shotcrete exist – “dry” and “wet” ones. The report discusses the main technological features of the two methods, analyzing their advantages and disadvantages. Emphasis is placed on the possibilities of rational choice of one of the two varieties of technology depending on multiple factors of complex nature. In a synthesized way are presented their potential for carrying out specialized construction works – repair, reconstruction and strengthening of damaged reinforced concrete constructions, consolidation of rock massifs and slopes, construction of tunnels, swimming pools and other special facilities. The effectiveness of the “wet” method is based to real possibilities to involve in concrete mix design different chemical innovative admixtures – ultra high range water reducing (HRWRA), shrinkage compensating (ShCA), internal crystallization one, etc. Preliminary estimates are presented for their economic expediency under specific conditions.