The paper investigates the behavior of reinforced concrete beams reinforced with CFRP sheet under static loading. The experiment involved 22 large-scale samples – reinforced concrete beams with a size of 120х220х1290mm. One part of the specimens was reinforced with CFRP before loading, the other – under load after the appearance of the first cracks in concrete and their injection. The beams were loaded according to the 4-point bending scheme. The deformation state of the beams was assessed using strain gauge and deflection meter. The debonding of the carbon fiber sheet was evaluated by infrared thermography directly in the process of loading.

The effect of CFRP on the bearing capacity and stiffness of beams reinforced before and during loading is evaluated. The ability of CFRP to restrain the opening of a crack was demonstrated.

The effectiveness of the method of reinforcing beams with CFRP in combination with injection of cracks to restore the rigidity of the structure was evaluated. The pattern of crack formation in samples is presented. The influence of the quality of surface preparation on the nature of the debonding of carbon fiber and the features of cracking and destruction is shown. The parameters and features of CFRP debonding were determined for various debonding mechanisms. The parameters and intensity of CFRP debonding for beams reinforced in the unloaded state and under load are compared. The experimental values of debonding strains are compared with theoretical values determined by 8 known methods.

It is shown that the actual deformation of CFRP debonding is 15-75% lower than the values calculated in accordance with Russian Building Codes SP 164.1325800.2014, and the fact of CFRP debonding does not determine the limiting state of the reinforced concrete beam in the presence of reliable anchoring of the longitudinal strip of the composite on supports.

A centrally compressed flexible rectilinear steel rod is considered. Due to the existing initial imperfection in the form of a perish, the rod works as a compressed-curved one. The inhomogeneity of the stress state from bending leads to the constraint of deformations that cause a change in the elastic characteristics of the material. To obtain the equation of the rod in the deflected state, the incremental theory of nonlinear deformation of bodies in inhomogeneous stress fields with induced anisotropy of properties proposed earlier by the authors is used. The inhomogeneity of the stress field causes the variability of the elastic characteristics of the material, which, due to the induced incremental curvilinear anisotropy, lead to a change in the design parameters of the structure. The solution is based on the numerical implementation of the curved axis equation using the method of variable elasticity parameter. The growth of deflections at the stages of sequential loading with increasing force is analyzed. Various variants of initial curvatures, including vanishing small ones, are considered. Regardless of the magnitude of the initial deflection, a noticeable increase in the compressive force was found, corresponding to a significant increase in deflections compared to the bifurcation approach.

A model of plasticity of reinforced concrete structures is considered, based on on the transformations  of  the  intensity  of  the  “stress-strain”  connection  by  projecting  the  tensors  of  this connection, using special transitions for the main angle of deformations, total shear deformations, etc.). At the same time, the modulus of plasticity of concrete, the coefficient of transverse deformations are determined, and complex functions are constructed for linear and angular deformations in sections, taking into account deformation, gradients of deformations during the formation of cracks and stiffness changes. The hypotheses adopted for the calculation model determine the distribution of force flows - blocks for compressed and stretched concrete (first object), "main cracks" from the mechanics of destruction of reinforced concrete, complex functions and a two-cantilever element for modeling the deformation effect of reinforced concrete, developed by the author (second object). Tensile concrete resistance is transferred to the working reinforcement and is modeled using the sum of the average values of the longitudinal and transverse forces, as well as the average reduced coefficient of tension concrete. The "pin (nagel)" effect in the reinforcement crossed by a crack was obtained using the model of the second level of structural mechanics for a reinforcing bar with two pinched ends. The opening of the crack and the shift of the crack edges are simulated. The main force vector in the reinforcement is characterized by the values of longitudinal and transverse displacements (the third object).

The paper illustrates the application of the formulae of general type derived by the author earlier for the definition of surfaces of diagonal translation of superellipses of variable curvature on a rhombic plane. Explicit and parametric equations were derived additionally for the large group of surfaces of diagonal translation of congruent superellipses. For the both cases, the surfaces of velaroidal type are examined on rhombic plane. All of presented surfaces were visualized with the help of methods of computer graphics. Due to availability of arbitrary exponents of powers in explicit equations of generatrix superellipses of the main frame of a translation surface, design of surfaces of diagonal translation was broadened for the case of using plane algebraic curves instead of superellipses in the process of choice of main frame of projected surface of diagonal translation. The presented surfaces can find the application in architecture, civili engineering, and in machine building.

The article describes the statement of a problem of research of the stress-strain state of reinforced concrete monolithic structures with loop joints of rebar using the finite element method. A review of plastic damaged concrete model, a description of its basic parameters and their selection by the example of the problem of verification of mathematical material model by the results of full-scale tests of concrete prism and beam with a loop joint over a static load have been presented.

The detailed description of the model parameters, diagrams of concrete and reinforcement state, including stress-strain diagrams with regard to the models features, as well as graphs comparing the experimental and numerical research of a concrete prism and a bendable reinforced concrete beam with a loop joint, including a model of concrete, and reinforcement model are presented.

The results of numerical calculations obtained stresses in the reinforcement of 514 MPa, which is close to those resulting from the full-scale experiment of 550 MPa (Fig. 8). The value of deflection when the failure load in the experiment was 16.7 mm, in the FE model it was 18.07 mm.

In the practice of calculations of compressed reinforced concrete elements for static effects, a number of questions important for determining the deformations and bearing capacity of these elements still do not have definitive and unambiguous answers. First of all, this is the problem of reliable accounting for the long-term effect of the external load. Unlike bending elements, the bearing capacity of compressed rods depends on the time they are under load.

The article deals with the influence of flexibility, the coefficient of reinforcement under long-term load on the decrease in the bearing capacity of a reinforced concrete compressed rod.

A calculation method is proposed to determine the long-term compressive force that provides a given period of safe operation of reinforced concrete columns.

The dependence of the ratio of the level of long-term action to the short-term breaking load on the deflections of compressed rods is revealed.

An analysis of experimental studies was carried out, indicating that the value of this ratio depends on the eccentricity of the longitudinal force, as well as data on the magnitude of the reduction in the long-term resistance of the columns.

Dependences of the stiffness ratio of the column under long-term and short-term external load on the flexibility of the rods, which is laid down in the current regulatory documents, and which is the determining factor in deciding whether to take into account the long-term nature of loading, are obtained.

This conclusion is confirmed by the data of the above experimental studies.

On an energy basis, the problem of calculating the nonlinear deformation of structures of reinforced concrete multi-storey frames during the destruction of crossbars along an inclined section from special influences caused by structural restructuring of the structural system is solved. Analytical dependences are obtained for determining the parameters of the "moment-curvature" and "transverse force-shear" diagrams in the static-dynamic loading mode of the structures under consideration. The limiting load is determined at which, in the considered structural system of the ring, after applying a special effect in the form of a sudden removal of one of the columns, a special limiting state is achieved with the destruction of crossbars along inclined sections. The obtained design scheme of the destruction of the crossbars of the frame structures under consideration caused by the combined action of bending moments and transverse forces is compared with the destruction scheme obtained experimentally.

The condition of asphalt concrete and its standard service life on public roads are the main indicators of the road surface. The factors of violation of the top layer of asphalt concrete from increased dynamic effects of the wheel load of vehicles are considered. Performance indicators are characterized by strength and deformation characteristics.

The factors providing the necessary deformation and strength parameters during the operation of the roadway are analyzed and justified. The indicators and conditions of contact of the surface of the crushed granulate in the zone of contact with the binding components are determined.

A method of using asphalt granulate of three standard sizes in the formation of the top layer of pavement with a normative, operational service life and rational use of material and technical resources at all stages of the formation and laying of asphalt concrete is proposed. The strength of the dispersed system is achieved during the formation of an asphalt mixture by increasing the density, strength, adhesion of the binder, water resistance and preservation of the structure of the pavement. The experiment established the volumetric and fractional standard size of asphalt granulate in the total volume of the mixture. The volume composition of the granulate content in the total volume of the mixture is from 20% to 25%.

Reinforced concrete floor structures of livestock complexes are subjected to intense impacts during operation. Concretes based on Portland cement are mainly used in floor structures. A comparative assessment of the influence of the organic medium on the strength and deformation properties of concrete based on Portland cement, slag-Portland cement and sulfate-slag binders with various hardening activators was carried out. It has been stated that compositions based on complex-activated sulfate-slag binders have the best resistance in the aggressive organic environment of livestock premises compared with concrete samples based on Portland cement and slag-Portland cement.

The article analyzes recycling of industrial waste. The main direction of construction production development is the use of new materials, reduction of material intensity, providing mechanization and industrialization of construction, increasing the operational characteristics of products and structures, the implementation of wasteless technologies in building materials at the expense of industrial waste and pollution reduction. It is important to produce concretes and mortars with enhanced physical and thermal properties for linings of thermal units operating in difficult operating conditions (high temperature, aggressive environment, contact of lining material with gases, metal melts and fluxes). Heat-resistant concretes are used in ferrous and non-ferrous metallurgy, chemical and oil refining, petrochemical, power, machine building, pulp and paper industry, in building materials The introduction of heat-resistant concrete by using new structural elements for thermal units, the most appropriate from the thermal and technological side, which is not feasible when using piece ceramic refractories. Components of heat-resistant concretes - fine grind additives and aggregates are usually made of expensive materials (chamotte, mullite, chromite, magnesite, zircon, etc.). The production of additives requires energy-intensive milling and sieving operations, which complicate and increase the cost of aggregate production technology. The replacement of scarce and expensive components by local materials and the development of technology for obtaining heat-resistant concrete on chemical binders using non-deficient materials, especially industrial waste is an important task. The prospects of using alumina-containing wastes as additives in heat-resistant concretes, which allows increasing durability and refractoriness of construction materials. The used fine-dispersed catalyst IM-2201, which is used in petrochemistry and is an alumina-chromium waste, was studied. The composition and properties of this waste and the change in the properties of concrete with the introduction of additives have been studied. It was shown that the properties of concrete change after the introduction of alumina-chromium waste in a given amount (5, 10 and 15%). Their average density, thermal strength and other properties are increased. The improvement of physical and thermal characteristics depends on the structure and new formation in the obtained samples. Concrete samples were analyzed using petrographic method and it was shown that the addition of aluminochrome waste contributes to densification of the structure due to filling the pore space with glassy mass and newly formed crystals in the cementitious mass.