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

The industrial concrete floorings of a large warehouse for storing various bulk fertilizer substances in the Varna region are made as hybrid fiber-reinforced one by using a high-quality powdered surface hardener (dry shake topping) to increase their surface hardness and abrasive resistance. The floors are sized and constructed to take on the specific operational loads – a bulk uniform distributed load with a large mass and a mobile front-end loader with a metal rake. Given the expected possible corrosion effect of pre-specified heterogeneous fertilizer substances in the working concrete mix design is used sulfate-resistant cement, which should ensure defect-free operation. Regardless of the measures taken after one year of operation, the presence of surface defects on the floor in strictly limited areas of operation is reported. A reasonable question arises as to the reasons for this. The report discusses an innovative experimental-theoretical approach proposed by the author to identify the causes of this defect with the combined use of a complex method based on the combine use of standardized mechanical and structural direct physical methods of investigation. After specialized sampling and examination of the concrete strength in specific areas, as well as after additional application of X-ray structural and differential thermal analysis, as well as scanning electron microscopy, original scientific results were obtained, which revealed the causes and mechanism of the discussed corrosion damage, and these complex activities definitely have a justified arbitration significance.

The basic principles of sprayed concrete mix design are reviewed. The technological options for use of the material for repairs and restoration works, executed by “wet” and “dry” spraying methods, are analysed.
Widespread research program is developed and implemented for assessment of the main physical-mechanical and structural characteristics of sprayed concrete for repairs and restoration works. New (author’s) compositions are prepared for this purpose, which are subjected to series of tests, together with factory made products, in form of dry mixes, by leading producer. Two compositions of sprayed concrete are chosen after preliminary selection – conventional for “dry” spraying and specially designed hybrid fiber-reinforced high-tech sprayed concrete for “wet” application method. The numerous additional components are included in the composition of the latter – high range water-reducing admixture based on polycarboxylate ether, polymeric modifier for enhancing of fresh mix compactness and rebound decreasing and new generation internal deep capillary-crystalline admixture.
Original scientific results are obtained concerning the basic physical-mechanical and stress-strain characteristics of both compositions of sprayed concrete. The critical comparative analysis of the results is presented and the right options for the application of the compositions are suggested.

The use of internal-crystallization chemical admixtures of different types is increasingly practiced as an essential part of new innovative waterproofing systems for buildings and facilities, capable of successfully replacing many conventional waterproofing systems – plaster coatings, sprayed, rolled, membranes, etc. The principle is known of action of different types of internal crystallization additives in the composition of the concrete, as a result of which an additional increase in the impermeability of the concrete section is sought, resp. with a contribution to the general waterproofing of buildings and facilities. In such a connection, especially in the presence of established defects, a case for arbitral assessment of the adequate presence (or not) of such admixtures reasonably arises, provided that they have been previously agreed upon between the parties in the investment process. The report discusses a proposed innovative research method for qualitatively and quantitatively assessment to establish the presence of a crystallization admixture in the composition of hardened concrete used for the construction of the defective building structure with impaired waterproofing ability.
Standard test methods are able to determine basic physical and mechanical characteristics of both types of concrete – without and with additives. The specialized structural methods (low-temperature gas adsorption BET-method, differential-thermal analysis and scanning electron microscopy) in turn have a high information capability regarding the features of the formed internal structure of the two types of concrete in question. On the basis of the obtained integral results, an objective conclusion can be formed as to whether or not such a chemical admixture has been added to the project concretes.

The pressure water supply pipeline from the „Iskar Dam“, which has existed since 1981, is the main one for the water supply of the city of Sofia with drinking water. The water pipeline, for the most part executed as a concrete vault tunnel (mantel), passes through complex mountainous terrain with a large difference in elevation. During the long-term active operation, the steel part of the air passage of the water pipeline over the „Porkolitsa River” suffered a serious accident – internal pressure implosion of the steel pipe. The latter prevented the normal water supply to the city and an urgent repair was required by replacing the air-passing steel pipe of the water pipeline. The report discusses a proposed and successfully implemented innovative technical solution for monolithic installation of a steel pipeline in existing tunnel excavations with concrete lining by using self-compacting concrete with shrinkage compensated admixture and with the participation of special an internal crystallization chemical admixture, tailored to the specific requirements of the site. A prescribed composition of self-compacting concrete has been developed, the necessary control has been carried out during the batching plant production of the fresh concrete with monitoring of technical indicators. A technological regulation has been developed for the transport, laying and care of the concrete. During the site laying, standard sampling and testing have been carried out to prove the compressive strength at different ages of the concrete. Based on the above, in conclusion, the effectiveness of the developed and implemented innovative technical solution is certified.

The shrinkage is an inherent characteristic of Portland cement concretes, leading to their volume changes, which potentially create preconditions for the development of different types of cracking – prerequisites for integral decreasing of their durability. The elimination of the negative processes from the development of shrinkage in all its varieties (autogenous, plastic, thermal and carbonization) is based on optimization of complex mix design and technological factors, including the use of innovative chemical products – mainly minimizing and/or compensating the volume changes during setting and hardening of the concrete.
In recent years, new innovative products have entered the world and Bulgarian practice aimed to compensate the shrinkage by initiating of specific additional hydration interactions. These processes give some attractive possibilities for manageable early age increasing of the concrete volume, leading to partial or complete compensation of the negative effect of later shrinkage.
The report presents full-range complex results for the effective action of a special shrinkage compensating modifier ShCA KEPTONITE on the strength-deformation characteristics of fiber-reinforced concrete for advanced joint-less industrial flooring systems with different purposes, especially for heavy loaded logistic bases.
It is concluded that ShCA KEPTONITE demonstrates very good performance in direction to strongly minimize the final shrinkage of the concrete tested. The different dosage rate of the product is an optimal for Bulgarian cement used for testing.

Current construction practice often implies the need to carry out specific actions related to the specification of current operating characteristics of existing and operated in different periods in different ways fibre-reinforced industrial concrete internal floorings and external pavements. Such problems are most often related to a planned change in the operating load, or to estimate their service life in the future. In this regard, specific parameters of a system approach proposed by the author are specified below, including the necessary preliminary field, laboratory and analytical activities related to establishing the current strength-deformation and other essential characteristics of existing industrial concrete floorings and pavements, as follows: concrete cross-section thickness, concrete strengthdeformation characteristics, type and amount of reinforcement used (including fibres), deformation characteristics of the crushed-stone base – according to the dynamic penetration in number of test points, static calculations for the new load conditions, restoring their integrity after sampling, etc. Given the lack of any normative documents on the subject, the proposed systematic approach to the assessment of the residual operational resource of existing fibre-reinforced industrial concrete floorings/pavements is of important methodological importance and fully corresponds to the needs of real construction practice. Its practical use has been approved by the author through a number of successfully conducted systematic expert procedures at a number of sites in the country and abroad.

The shrinkage is an inherent characteristic of Portland cement concretes, leading to their volume changes, which potentially create preconditions for the development of different types of cracking – prerequisites for integral decreasing of their durability. The elimination of the negative processes from the development of shrinkage in all its varieties (autogenous, plastic, thermal and carbonization) is based on optimization of complex mix design and technological factors, including the use of innovative chemical products – mainly minimizing and/or compensating the volume changes during setting and hardening of the concrete.
In recent years, new innovative products have entered the world and Bulgarian practice aimed to compensate the shrinkage by initiating of specific additional hydration interactions. These processes give some attractive possibilities for manageable early age increasing of the concrete volume, leading to partial or complete compensation of the negative effect of later shrinkage.
The report presents full-range complex results for the effective action of a special shrinkage compensating modifier ShCA KEPTONITE on the strength-deformation characteristics of fiber-reinforced concrete for advanced joint-less industrial flooring systems with different purposes, especially for heavy loaded logistic bases.
It is concluded that ShCA KEPTONITE demonstrates very good performance in direction to strongly minimize the final shrinkage of the concrete tested. The different dosage rate of the product is an optimal for Bulgarian cement used for testing.

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. The new extended research on such several new types of those admixtures were reported and discussed. The complex mechanical and structural tests are conducted and respective results are compared to predict the admixture’s efficiency 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.

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.

The technical design of ETEM factory (Sofia) requires the specialized conveyor stretching machine to be monolithic fitted by using of system of steel anchor rods with a diameter of 30 and 40 mm and different lengths (600-700 mm), which have to be grouted in pre-left holes with a diameter of about 150 mm in steel-reinforced concrete foundation slab.
The technically determined anchoring depth determines the total thickness of the anchor layer within 700-800 mm, reaching the “lower edge of the steel support plate”. The company’s produced grouting compositions generally have significant limitations in terms of the maximum allowable thickness of a single layer – usually up to 30-40 mm, which implies multi-stroke implementation to fill the entire anchoring depth.
Given the above, and mainly, taking into account the conditions of laying and the specifics of the operating mode of the facility, thepossibility to use concrete batching plant produced high-strength self-compacting fiber-reinforced concrete with compensated shrinkage (to some degree of self-stressing) is estimated as an attractive alternative from a technical and financial point of view.
After the innovative mix design of that an innovative composition of concrete and its casting in real production conditions, specialized laboratory tests were conducted to prove its strength-deformation characteristics – compressive strength, tensile strength in bending, modulus of elastic deformation and shrinkage characteristics. The obtained results prove that the required high technical and technological characteristics of the concrete mix and the hardened concrete are completely achieved precise for the specific application ca se.