The outlook for regulatory support for adoption of efficient concretes

The analysis of data obtained under studying scientific, technical and normative literature in the area of applying efficient concretes, in particular, fiber-reinforced concrete, in the world and national tunnel construction is presented herein. The European regulatory documents, standard metrologically certified methods and procedures for analyzing and testing, laboratory test equipment and measuring instruments are indicated. Steel fiber concrete studies was explored at the Scientific and Educational Center for Innovative Technologies of the Architectural and Construction Institute of the Ufa State Petroleum Technical University. Prism bending tests were made with different fiber content. In addition to laboratory tests, other tests were made in accordance with the Russian standards, where the prisms are tested without a notch. The correlation between the slope of the load-to-displacement curve of the crack mouth (CMOD - Crack Mouth Opening Displacement) and the length of the crack was used in the course of the study. The fiber content enabling to get an average residual bending strength of at least 1.5 MPa at 0.5 CMOD (equivalent to 0.47 mm center deviation) and an average residual bending strength of at least 1 MPa at 3.5 mm CMOD (equivalent to 3.02 mm of center deviation) is found. Statistical distributions of the Grubbs tests are analyzed by methods of statistical modeling. The regulatory framework for fiber concrete is not currently well developed in the Russian Federation, thereby reducing greatly the application of new generation concretes meeting the current knowledge. The application of fiber concrete in tunnel construction will prove to be economically justified and the areas of applying effective materials are to be developed in future with the wide participation of scientific, design, production, construction and other specialized organizations, as well as educational and training centers.

Flexural Tensile Strength of Concrete with Synthetic Fibers

Fiber reinforcement is currently most often used in floors, railway sleepers, prefabricated structural elements such as slabs, beams and tanks, and in small architecture elements. Designing elements or structures made of fiber-reinforced concrete requires knowledge of its basic mechanical parameters. In the case of concretes with metallic fibers, the literature can find many tests and standard guidelines regarding compressive, flexural, tensile strength and fracture energy. The properties of concretes with non-metallic fibers are slightly less recognized, especially concretes with new types of polymer fibers. Additionally, the lack of standardized methods of testing concrete with polymer fibers make their application much more difficult. In the article, the possibility of using the EN 14651 standard to assess the flexural tensile strength of concrete with the addition of 2.0 and 3.0 kg/m3 of synthetic fibers with different geometry and form was presented. There was a 5.5–13.5% increase in the flexural tensile strength depending on the mixture type. Moreover, in the case of fiber-reinforced concretes, the ductility was enhanced and the samples were characterized by significant residual flexural tensile strengths. Additionally, from the workability tests it was concluded that after the incorporation of fibers, the consistency class decreased by one, two or three. Nevertheless, the compressive strengths of concrete with and without fibers were very similar to each other, and varied from 58.05 to 61.31 MPa. Moreover, it was concluded that results obtained from three-point bending tests significantly differed from empirical formulas for the calculation of the flexural tensile strength of fiber-reinforced concretes with dispersed steel fibers present in the literature. As a result, the new formula determined by the authors was proposed for concrete with polymer fibers with a nominal fiber content ≤1.0% and slenderness of up to 200. It must be mentioned that the formula gave a very good agreement with studies presented in different literature positions. In addition, an attempt was made to evaluate the strengths of tested mixes in accordance with the Model Code 2010. However, it occurred that the proposed fiber-reinforced concrete mixtures would not be able to replace traditional reinforcement in a form of steel bars. Furthermore, in uniaxial tensile tests, it was not possible to determine the σ–w graphs, and received results for maximum tensile strength did not show the clear influence of fibers incorporation on concrete. Then, the fracture energy enhancement (from about 16 to 22 times) and dependencies: crack mouth opening displacement–deflection; crack mouth opening displacement–crack tip opening displacement; and crack tip opening displacement–deflection were analyzed. Finally, the results from flexural tensile tests were compared with measurements of the surface displacement field obtained through the Digital Image Correlation technique. It was concluded that this technique can be successfully used to determine the crack mouth and crack tip opening displacements with very high accuracy.

Modelling and Characterizing the Adhesion of Parallel-Grooved Interface between Concrete Lining Structure and Geopolymer by Wedge Splitting Method

A new method for increasing the interface resistance between geopolymer coating and concrete lining structure without applying the organic binder was suggested in this study. Parallel grooves with different depths and orientations were milled on the top surface of concrete block, and well-blended geopolymer mixture (Na-PSS type geopolymer: Sodium poly-sialate-siloxo) was coated upon the grooved interface. The wedge splitting (WS) experiments were conducted to compare the interface adhesion capacity of specimens with different groove width/depth ratios and groove orientations. The average energy release rate (ERR) was calculated by integrating the Pv-CMOD diagram to quantify the interfacial fracture toughness. To understand the interface strength mechanisms and the fracture mode at the front crack mouth, franc 3D simulation was carried out to segregate the mixed fracture mode to determine the initial pure stress intensity factors K I , K II , K III at the crack mouth. Both the experiments and simulation results indicated that the highest interface fracture toughness was reached by the double diagonal parallel grooves with 0.375 width/depth ratio. These findings put forward a promising attaching method for efficient and reliable passive fire protection coating, with the aim of decreasing the risk of layer delamination in highway tunnels.

Application of a Novel Crack Mouth Opening Displacement Partitioning Technique to Creep Crack Growth Tests on SEN(T) Geometries of Type 316H Stainless Steel

A new technique has recently been proposed to provide improved estimates of the creep contribution to the crack mouth opening displacements (CMOD) and displacement rates during creep crack growth (CCG) tests. This technique employs finite element analysis that incorporates material specific uniaxial tensile test data to simulate crack growth in an experimental test and can account for strain history and creep stress relaxation effects during CCG tests. In this work, this new methodology is applied to analyse the results of a CCG test performed on a relatively low constraint single edge notched tension, SEN(T), geometry. The proportions of the CMOD due to elasticity and plasticity are quantified, and compared to historic, standardised methods of estimating these values. The new method reduces the over estimation of the contribution of plasticity to the CMOD measurement. The impact of this analysis on CCG test results is discussed.

INFLUÊNCIA DA FIBRA DE AÇO SOBRE A RESISTÊNCIA RESIDUAL À TRAÇÃO POR FLEXÃO OBTIDA SEGUNDO O RILEM TC 162-TDF (2002)

RESUMO: A caracterização de peças reforçadas com fibra de aço à tração pode ser realizada por meio de ensaios de tração direta, ou de compressão diametral ou de flexão. No que diz respeito aos ensaios de flexão, o RILEM TC 162-TDF (2002), além de descrever os procedimentos para a realização do referido ensaio, também avalia o comportamento à tração em termos das curvas carga x deslocamento e carga x CMOD (Crack Mouth Opening Displacement). O presente trabalho tensiona estudar as resistências residuais à tração na flexão ( ). Nesse contexto, coletou-se da literatura um banco de dados formado por 46 ensaios de flexão em três pontos de prismas entalhados, será apresentado e avaliado, através de um estudo estatístico, para que propostas sejam estabelecidas para estimar as resistências residuais à tração na flexão (i = 1, 2, 3 e 4). Além disso, também será analisado a capacidade destas propostas em reproduzir as relações tensão-CMOD e tensão-deslocamento, as quais são regularmente obtidas a partir do ensaio de flexão em três pontos de prismas entalhados. A análise dos resultados quanto à capacidade de estimar as resistências residuais obtidas em ensaios foi favorável. Evidencia-se o desempenho das estimativas de (i = 1, 2 e 4), com MED ? 1,0 e CV < 25 %. A estimativa de fR3 foi mais dispersa, com Média ? 1,0 e coeficiente de variação > 25 %, dado o elevado nível de fissuração do prisma no momento do registro dessa resistência. Por fim, este trabalho tem grande importância no meio cientifico principalmente, pois a fibra de aço é um material que vem sendo estudado a bastante tempo, e por se tratar de um material complexo, necessita-se um estudo prévio do comportamento da resistência á flexão de vigas reforçadas com fibra de aço, tendo como principal objetivo determinar valores (i = 1, 2, 3 e 4) de ensaio de flexão em três pontos de prismas entalhados.

PREVENTION OF METAL BRIDGE SPANS FROM FATIGUE CRACKING

At present, more than four thousand metal bridge spans are in operation all over the Russian railway network. Through the years of operation, about thirty types of fatigue cracking were identified. The dynamics of the types Т-9 and Т-10 cracks formation has increased significantly in recent years. The formation and growth of fatigue cracking is influenced by such factors as residual welding stress, stress-strain state of the bridge structure, defects, damages, and non-observing the operation and maintenance standards, bridge location and bridge span structure. At present, neither measures nor repair are performed to prevent fatigue cracking not exceeding 20 mm in length. Only when a creak reaches a certain length, the regulatory documents require to drill a hole at a crack mouth in order to prevent its further development. The hole diameter should be equal approximately to the doubled wall thickness. In order to prevent further crack growth and cover the hole, a high-tensile bearing type bolt is fixed into it, creating a volume stress. As a rule, such a repair is not enough to stop the cracking process. The article suggests a number of measures based on induction heating that allows to prevent cracking (including forging reducing gaps between connecting plates and horizontal sheets in a truss), to carry out repair (crack mouth soldering) and reinforcement (fixing metal plates onto a wall with a hole).

Mean Stress Correction for Fatigue Life of Carbon Steel: Proposal of Non-Closure Model

Influence of the mean stress on fatigue life was investigated for carbon steel. Uni-axial fatigue tests were conducted by stress and strain-controlled conditions at room temperature. The fatigue life was reduced by applying the mean stress for the same stress amplitude. The fatigue life exhibited better correlation with the strain range rather than the stress amplitude. Increase in strain range caused by applying the mean stress correlated well with the decrease in the fatigue life. It was assumed that the mean stress effect on the fatigue life was brought about by the change in crack growth rate caused by applying the mean stress. The mean stress enhanced crack mouth opening and accelerated the crack growth. The non-closure model, in which the crack mouth is assumed not to be closed even at the minimum peak stress, was proposed.

TEST STUDY ON THE BEST PASTING LAYER OF FRP REINFORCED CONCRETE

To optimize the strengthening method using the fiber reinforced polymer (FRP) for the reinforcement of the concrete structure with cracks, the three-point bending test was conducted on the concrete beams wrapped with different layers of FRP materials. The strain gauges were pasted on the surface of the specimens to measure the initial cracking load. The crack mouth opening displacement (CMOD) was utilized to test the load–crack mouth opening displacement curve. According to the improved calculation formula of the fracture toughness, the critical effect crack length [Formula: see text], initiation fracture toughness [Formula: see text] and instability fracture toughness [Formula: see text] of specimens were calculated. The test results showed that, under the same initial crack depth, the peak load of FRP reinforced concrete decreases with the increase of FRP pasting layer. When there was one layer wrapped over the specimen, the instability toughness of the specimen reached the maximum value and the crack resistance was the best. Based on acoustic emission testing method, the acoustic emission parameters of the above-mentioned concrete during fracture process were identified and collected. The optimal layer of the FRP reinforced concrete with cracks was analyzed from the acoustic emission method.