Bearing capacity of damaged reinforced concrete beams strengthened with metal casing

Experimental data on the bearing capacity of damaged reinforced concrete beams with the dimensions of 2000×200×100 mm, reinforced with prestressed metal casings, are presented. Damaging in the form of through normal and crossing inclined cracks, as well as excessive vertical moving of the beam were obtained during previous tests for the effect of high-level transverse alternating loads.The authors of the article have developed a method and equipment for restoring and strengthening damaged reinforced concrete beams using a casing. Beams are manufactured and tested in accordance with the three-level design of an experiment.Previously damaged and reduced to the ultimate (pre-emergency) state, the beams were strengthened with the declared method and equipment, and then retested. New data on the bearing capacity of ordinary and damaged beams, as well as reinforced concrete elements strengthened with casings and tested for the action of transverse forces and bending moments were obtained. The research results are presented in the form of experimental-statistical dependences of the bearing capacity of the support areas, deformability and crack resistance of the investigated elements on the ratio of the most significant design factors and external factors. A comparative analysis of the influence of these factors on the main parameters of the bearing capacity of ordinary as well as previously damaged and then strengthened test beams is carried out.The possibility and appropriateness of using the proposed method of strengthening reinforced concrete beams damaged by through normal and cross-inclined force cracks in the conditions of an existing production has been experimentally proved.

Rolling and Rolling-Sliding Contact Fatigue Failure Mechanisms in 32 CrMoV 13 Nitrided Steel—An Experimental Study

The aim of this work is to characterize the rolling and rolling-sliding contact fatigue failure mechanisms on the 32CrMoV13 nitrided steel. During rolling contact fatigue tests (RCF), two general features were observed: specimens presenting short lives and rough and sharpened spalling damage and specimens presenting long lives and only microspalling marks. It was possible to determine a contact fatigue limit of 3 GPa. During rolling-sliding contact fatigue tests (RSCF), a clearly different behaviour between the two specimens in contact has been observed: the driver shows circumferential and inclined cracks and only inclined cracks appear in the follower. This behaviour can be understood if the effect of the residual stress state in near-surface layers is considered. Before RCF tests, the residual stresses are compressive in all near-surface layers. After RCF tests, strong residual stress relaxation and even reversing behaviour was observed in the axial direction, which facilitates the surface crack initiation in the circumferential direction at rolling track borders.

Stress-deformed state of proof areas of reinforced concrete beams

The results of experimental studies of the stress-strain state of the support sections of rein-forced concrete beams from some of the most signifi-cant factors are presented. It was found that during long-term loading such factors are the level of the initial loading; concrete class; the percentage of transverse reinforcement in the shear span; the per-centage of longitudinal reinforcement in the com-pressed zone; the percentage of longitudinal working reinforcement in the extended zone. With an increase in the class of concrete, the moment corresponding to the appearance of normal cracks and the shear force corresponding to the appearance of inclined cracks slightly increase, although they lag behind the growth of the class of concrete. The process of crack-ing in the investigated beams begins, as a rule, with the appearance of normal cracks in the zone of pure bending at a load level (0.15... 0.33) from breaking. Inclined cracks appeared somewhat later, at load levels close to 0.5 of breaking. With a relatively high percentage of longitudinal reinforcement, the first oblique cracks appeared in the middle of the beam height in the shear span. At load levels close to 0.7, the process of formation of new cracks practically stops, and already existing cracks open more inten-sively. During the period of exposure to a constant long-term load, the width of the opening of normal cracks increased on average by 1.2–1.5 times, and the width of the opening of inclined cracks increased by 2.5–3 times. In the process of loading before the destruction of previously long-loaded beams, the width of the opening of normal cracks remained practically unchanged, and the width of the opening of inclined cracks increased by 1.2–1.7 times, and the breaking load for beams loaded with a preliminary long-acting load of 0.85 from the breaking load in-creased by 13–15 % compared to short-term loading. The opening width of inclined cracks along the length was different, its maximum value was ob-served in the middle of the height of the section of the support sections of the beams.

Penetration Form of Inter-Hole Cracks under Double-Hole Blasting Conditions with Inclined Fissures

During blasting construction in tunnel engineering, an inclined fissure near the blast hole produces the “Z” type of over-excavation and subsequently affects the overall blasting effect and stability of the tunnel. In this work, dynamic caustics testing was used to study the burst propagation mode and penetration form of explosive cracks at different positions between holes under double-hole blasting conditions. Results showed that the existence of gently inclined cracks changed the propagation law of explosive stress wave. The dominant fracture surface was formed in the vertical direction between the borehole and the fracture. Finally, the crack penetrates to form “Z”-type over-excavation, which was analyzed by the dynamic caustics test. The expansion velocity of the burst crack reached the maximum under the reflection of the explosion stress wave and then decreased with the attenuation of the stress wave intensity. The peak propagation velocity decreased with the increasing vertical distance between the prefabricated fracture and the borehole, and the stress intensity factor at the crack tip immediately reached its peak value after detonation, and the oscillation then decreased. These research results can serve a basis for reducing tunnel blasting over-excavation under this condition and optimizing blasting parameters.

STIFFNESS OF REINFORCED CONCRETE STRUCTURES UNDER BENDING WITH TRANSVERSE AND LONGITUDINAL FORCES

The authors developed a model for single reinforced concrete strips in block wedge and arches between inclined cracks and approximated rectangular cross-sections using small squares in matrix elements. From the analysis of the works of N.I. Karpenko and S.N. Karpenko the "nagel" forces in the longitudinal tensile reinforcement and crack slip , as a function of the opening width and concrete deformations in relation to the cosine of the angle . The experimental " nagel " forces and crack slip dependences for the connection between and in the form of an exponent for the reinforcement deformations and spacing are determined. The forces have been calculated for two to three cross-sections (single composite strips) of reinforced concrete structures. On the bases of accepted hypothesis, a new effect of reinforced concrete and a joint modulus in a strip of composite single local shear zone for the difference of mean relative linear and angular deformations of mutual displacements of concrete (or reinforcement) are developed. The hypothesis allows one to reduce the order of the system of differential equations of Rzhanitsyn and to obtain in each joint the total angular deformations of concrete and the "nagel" effect of reinforcement. The curvature of the composite bars has a relationship from the total bending moment of the bars to the sum of the rigidities. The stiffness physical characteristics of the matrix from the compressed concrete area and the working reinforcement are obtained in a system of equations of equilibrium and deformation, as well as physical equations.

IMPROVEMENT OF THE METHOD OF CALCULATING THE STRESS-STATE AND STRENGTH OF REINFORCED CONCRETE STRUCTURES OF HYDRAULIC CORNER RETAINING WALLS WITH INTER-BLOCK JOINTS TAKING

Corner retaining walls are one of the most common structures of waterworks. Most of them were designed and built several decades ago and have been in operation for a long time. In some cases, there is a deviation from the design prerequisites and the strengthening of reinforced concrete structures of retaining walls is required. The main reason for these deviations is incomplete consideration of the characteristic features of retaining wall structures (including horizontal inter-block joints and secondary inclined cracks), as well as the nature of the loads acting on them. As a result, design horizontal transverse reinforcement is practically not installed in retaining walls that is not required by calculation based on traditional calculation methods.Traditional reinforcement schemes for retaining walls do not provide for the presence of horizontal inter-block joints and horizontal transverse reinforcement. As a result of the research carried out,the method for calculating the stress-strain state and strength of reinforced concrete structures of corner retaining walls with inter-block joints has been improved taking into account secondary stresses. Reinforcement schemes for retaining walls have also been improved.

Effects of randomly orienting penny-shaped cracks on the elastic properties of transversely isotropic rocks

Fractured reservoirs, as one kind of unconventional reservoirs, have great potential for oil and gas development, and their accurate characterization requires the development of rock-physics models that better simulate real fractured rocks. However, current models focus mainly on the elastic properties of rocks with aligned cracks, while the effects of randomly orienting cracks in transversely isotropic (TI) rocks are poorly studied even though such conditions are frequently encountered in the earth. To address this problem, we have derived models for the elastic properties of rocks with a TI background permeated by 3D inclined cracks and randomly orienting cracks. Then, based on the developed models, we comprehensively study the effects of the two inclination angles (i.e., the dip angle between the cracks and the isotropic plane and the rotation angle between the cracks and the plane normal to the isotropic plane, respectively) of 3D inclined cracks on the elastic properties of TI rocks. We determine that the two angles have significant influences on the elastic coefficients and hence the elastic velocities, and that their influences on the elastic properties are varying in different directions. We further investigate the effects of crack density and aspect ratio of randomly orienting cracks on the elastic properties of the fractured rocks with a TI background. The results show that the increasing crack density and crack aspect ratio reduce the elastic coefficients and velocities for rocks with randomly orienting cracks, in which the relations between compressional-wave velocities and the crack properties (i.e., crack density and crack aspect ratio) are obtained to aid the interpretation of the acquired acoustic exploration data. The proposed new models can greatly improve the modeling capability for the elastic properties of rocks with a TI background permeated by inclined and randomly orienting cracks.