Class Of Strength Research Articles

Predicting out-of-plane bending strength of cross laminated timber: Finite element simulation and experimental validation of homogeneous and inhomogeneous CLT

The strength of cross laminated timber (CLT) depends on the stiffness and strength of the lamellas and on the strength of the finger joints. A model for how stiffness and strength vary along and between lamellas is used in combination with a finite element model of CLT and Monte Carlo simulations to calculate out-of-plane bending strength of homogeneous and inhomogeneous CLT. Calculated and experimentally obtained results of characteristic bending strengths, coefficient of variation of bending strength and the proportion of finger joint failures, agree very well for both types of CLT. The characteristic out-of-plane bending strength and the mean bending stiffness were 23% and 16% higher, respectively, for inhomogeneous CLT with outer layer lamellas graded in the strength class C35, compared to homogeneous CLT with all lamellas graded in the class C24. Simulation results give basis for simple equations by which bending strength of CLT can be determined as function of the layup, the strength class of outer layer lamellas and characteristic strength of the finger joints. Furthermore, system effects are investigated. For inhomogeneous CLT, with outer layer lamellas of high strength class, the system effects turn out to be quite different from those of ordinary, homogeneous CLT.

Bond strength characteristics of high strength alkali activated slag concrete mixes

Bond characteristics are influenced by several factors such as concrete compressive strength, type, diameter, and embedment length of the bar and confinement of concrete. Herein, an attempt has been made to evaluate the bond strength characteristics of a new class of high-strength, flowable, alkali activated slag concrete mixes made using the industrial by-products from iron and steel industries. Such evaluations are made using the results of direct pull out tests where again, the bond stress – slip behaviour of the trial mixes at different ages are also studied. GGBFS was used as the principal binder along with fine amorphous quartz powder in the development of the present class of high strength, flowable AASC mixes. While mixtures of steel slag sand and quartz sand were used as the fine aggregates, 12.5 mm down electric-arc furnace slag aggregates constituted the coarse aggregate fractions in all these mixes. The alkaline solutions used consisted of mixtures of specified amounts of sodium hydroxide flakes dissolved in the calculated quantity of water pre-mixed with the liquid sodium silicate solution. The experimental study was planned based on Taguchi’s design of experiments methodology. A set of AASC mixes were developed and evaluated for their workability characteristics along with compressive strengths at the ages of 3, 28, 56 and 90 days. Bond strength characteristics of a set of nine of such mixes were evaluated in an initial calibration phase. Strength prediction equations were then derived based on those initial results, the predictive capability of which were then assessed and ascertained with actual results of experiments on a set of six new mixes, in the prediction phase. Further better bond stress-slip behavior, with higher bond strength values, varying in the range of 12–14 MPa as compared to control OPC based reference concrete mix were obtained for the AASC mixes.

Investigation of Engineering Properties of Lightweight Concrete Made With the Addition of Sodium Salt Based Powder Additive

In the construction sector, which is needed for the rapidly increasing world population, the production of strong and safe buildings is given importance. This is achieved as a result of the functionality, durability and economical material production in construction materials. In order to reduce the dead load in buildings, the use of elements made of lightweight concrete is the most appropriate and alternative method. In the current study, lightweight concrete with 4 different mixtures was produced by using 0%, 0.5%, 1.5% and 2.5% powdered sodium salt-based styrene butadiene polymer admixtures (SBPA). The workability effect of the additive on the fresh mixtures was investigated by the flow table test. The 7 and 28 day compressive and flexural strengths of the obtained samples were tested. Density and porosity ratios of the samples were calculated before the 28-day compressive strength test. According to the results, it was observed that the additive material caused a slight decrease in workability. In addition, decreases were observed in mechanical strengths, but when compared with the literature, considering the decrease in density, it was understood that it is in the class of lightweight concreteswith suitable strength.

Impact resistance of porosity-free fiber-reinforced concrete (PFFRC) beams under low-velocity impact loading

Ultrahigh-performance fiber-reinforced concrete (UHPFRC) is an advanced cement-based composite material. Its ultrahigh compressive strength and high ductility can enable the downsizing of structural members, with special application to high-rise buildings. These excellent mechanical properties also allow its application in protective structures to resist high-speed penetration, low-velocity impact, and blast loading. UHPFRC with a compressive strength of approximately 150–200 MPa has traditionally been used to investigate the impact resistance of structural members under low-velocity impact loading. Recently, however, porosity-free concrete of the 400 MPa class of compressive strength has been developed. In this paper, to investigate the effects of the concrete strength and the steel fiber volume fraction on the impact resistance of porosity-free fiber-reinforced concrete (PFFRC) members, static and drop-weight impact loading tests were conducted on PFFRC beams by varying the volume fraction of steel fiber from 1 to 3.5%. As reference beams, 90 MPa high-strength fiber-reinforced concrete (HSFRC) beams with a 2% fiber volume fraction and normal-strength concrete (NSC) beams without stirrups and steel fibers were also tested. The results obtained from this study were as follows: (1) the static load-carrying capacity of a PFFRC beam can be enhanced by more than two and three times that of an NSC beam by adding 1 and 3.5% volume fractions of steel fiber, respectively; (2) a PFFRC beam with 3.5% fiber had the greatest impact resistance of all the beams considered in this study, and the beam with 2% fiber volume had the second-greatest performance, but the difference was small; (3) even though an HSFRC beam with 2% fiber had a smaller static load-carrying capacity than a PFFRC beam with 1% fiber, the former exhibited a slightly greater impact resistance than the latter because the bridging effect of the steel fibers has a greater influence under impact loading than under static loading.

ON THE CLASSIFICATION OF COLD RESISTANT MATERIALS

Low-temperature technologies are increasingly used in various industries. Cold-resistant materials have gained particular popularity in the field of creating low-temperature machinery. The need for a special approach when choosing cold-resistant materials used in the production of machines, equipment and structures, whose operation is carried out under extreme conditions [low temperatures, dynamic/static loads and (or) intense wear], caused the need to systematize them according to a complex of mechanical properties. Despite the fact that a wide variety of products is produced from new cold-resistant materials in Russia and abroad, neither the scientific literature nor the regulatory documentation provides a common unified classification, which covers a wide range of their categories. Research was carried out for shipbuilding cold-resistant steels, which is reflected in the Russian National Standard R 52927–2008 “Steel rolled for shipbuilding from normal, increased and high strength steel”, however, this classification is incomplete and covers strength categories from 265 to 735 MPa. This article presents comprehensive systematization of scientific data of new structural cold-resistant steels of various leading industries of strength category up to 1550 MPa with corresponding characteristics of plasticity and cold resistance. The article considers and summarizes various criteria used to classify cold-resistant materials, as well as preferred classification features, determines numerical ranges of constituent properties. The article develops a classification system of structural steels according to groups of strength, cold resistance and hardness. The use of the proposed classification of structural alloys makes it possible to select material of certain category (class) of strength depending on operational loads of structural elements, machines and equipment of leading industries in conditions of low temperatures, and also to predict their reliability life depending on interval of low-temperature operating threshold

Determining the efficiency and parameters of rubble strip reinforcement

The necessity of development and improvement of methods and means for the protection of preparatory roadways, in particular, protective structures, was proved on the basis of ordinary rock with binding surfaces. Analysis of the results of the study on the use of protective structures of ordinary rock and bounding surfaces was performed. It has shown the feasibility of reinforcing rock structures to ensure operational conditions for the protected roadways. Such structures include rubble strips reinforced with partitions made of metal mesh. To determine their efficiency and reinforcement parameters, studies were performed using provisions of structural mechanics, soil mechanics, and bulk media, as well as physical modeling using natural materials. According to the results obtained in the performed studies, the efficiency of reinforcement of rubble strips with a metal grid was proved and a procedure for calculation of reinforcement parameters that need to be considered in designing the above structures was developed. Such parameters include width and height of the strip, class of reinforcement, its diameter and tensile strength, size of the grid cells, angle of internal friction of rocks, and diameter of maximum rock pieces in the strip. It was established that reinforcement of the rubble strip by partitions made of metal meshes can reduce the width of the strip and volume of the rock fill by 1.33…2.66 times without losing the structure rigidity. To do this, the condition of reinforcement strength in grids must be met. It consists of comparing its tensile strength with maximum stresses in the partition. These stresses are determined by the magnitude of the load on the rubble strip from the roof rocks, the diameter of the reinforcement, and the maximum rock pieces, as well as relative extensional strain in reinforcement.

Performance of metakaolin/slag-based geopolymer concrete made with recycled fine and coarse aggregates

In this paper, the potential use of both fine and coarse recycled demolition aggregates (RA) in geopolymer concrete (GC) manufacturing was investigated. The geopolymer binder adopted was a mix of metakaolin (MK) and granulated blast furnace slag (GBFS), activated by an alkaline solution (SiO2/Na2O=1.8) with a water to solid ratio of 0.5. A control GC (0% RA) with C35/45 class of compressive strength and S4 class of workability, and three GC mixtures with 10, 30 and 50% of RA were manufactured. It was found that the introduction of pre-saturated RA in GC mixtures does not affect the class of workability, but the slump loss over time decreased proportionally to the RA percentage. With 10% of RA, a slight decrease was recorded in both water porosity and mechanical performances. Moreover, a drop of 18 and 27% in compressive strengths was recorded with 30 and 50% of RA incorporation, respectively. Based on the obtained results and an extensive number of experimental data from the literature, the validity of empirical relationships between the mechanical performances of GC was assessed, and more adequate predictive equations were proposed.

Shear Behavior of High Strength Self-Compacting Concrete with Lower Transverse Reinforcement Index

ABSTRACT Self-Compacting Concrete (SCC) generally has lesser coarse aggregate contents and also the maximum size of aggregates is limited as compared to Normal Vibrating Concrete (NVC) for the same class of strength. This results in reduced aggregate interlock in Self-Compacting concrete as against NVC, which affects the shear strength of slender beams and thus, SCC might have lower shear strength. In this article, an experimental programme which includes six slender beams of High-Strength Self Compacting Concrete (HSSCC) with compressive strength more of than 90 MPa and with different stirrup spacing is presented. Experimental test results of shear strength of HSSCC beams are compared with high strength NVC beams for different stirrups spacing. The results showed the ultimate shear stress of HSSCC beams is lower than NVC beams and increase in transverse reinforcement index, ρw fy, in HSSCC beams decreases this difference. The results are also compared with different code provisions. Not much work has been done on beams with lower transverse reinforcement index (ρw fy) and in the present work, ρw fy ranges between 0.276 to 0.80.

Modeling and Numerical Validation of Stress-Strain Curves of Maraging Steels, Grades 300 and 350 Under Hydrogen Embrittlement

Maraging steels are ferrous alloys with Ni, Co, Mo, and Ti additions. These materials are a special class of ultra high mechanical strength steels with wide and special applications in strategic areas, which makes their knowledge very valuable. Computational advances allowed to analyze the behavior of these materials numerically, using the finite element method and developing mathematical models that can represent numerically its mechanical behavior. The present work has the objective of surveying the mechanical properties of maraging steels 300 and 350 by slow strain rate tensile (SSRT) tests, after the solution treatment at 1183K for 1h. Additionally, it was evaluated the hydrogen embrittlement in samples tested by SSRT under cathodic protection with a potential -1.2 VSCE in 3.5% NaCl solution. The study was complemented with detailed fractographic analysis. This work also presents the analysis of representative models by use of Hollomon, Swift, Voce and coupled Swift-Voce equations to describe the strain-hardening behavior. Compared to the others, the Voce’s model was the one which best fitted the experimental results, with values of R2 higher than 0.992. Through the variation of the chemical composition found in the different grades of maraging steels, this work contemplates the creation of a generalized Voce model based on the variation of the Ti content. The work concludes presenting the generalized Voce model proposed and a numerical analysis of the SSRT results with a good accuracy of the strain-hardening response.

A defect-resistant Co\u2013Ni superalloy for 3D printing

Additive manufacturing promises a major transformation of the production of high economic value metallic materials, enabling innovative, geometrically complex designs with minimal material waste. The overarching challenge is to design alloys that are compatible with the unique additive processing conditions while maintaining material properties sufficient for the challenging environments encountered in energy, space, and nuclear applications. Here we describe a class of high strength, defect-resistant 3D printable superalloys containing approximately equal parts of Co and Ni along with Al, Cr, Ta and W that possess strengths in excess of 1.1 GPa in as-printed and post-processed forms and tensile ductilities of greater than 13% at room temperature. These alloys are amenable to crack-free 3D printing via electron beam melting (EBM) with preheat as well as selective laser melting (SLM) with limited preheat. Alloy design principles are described along with the structure and properties of EBM and SLM CoNi-base materials.

THE INFLUENCE OF CLT PANEL PRODUCTION TECHNOLOGY ON THEIR STRENGTH AND RIGIDITY

In this paper considered the technological features of the production of CLT panels with focus on overview of the thicknesses of the boards and their width in cross-section in the panel on their character of pressing and selection of adhesive systems. The quality of wooden building materials based on boards is highly dependent on many technological operations, and above all on the method of pressing and the type of adhesive system. Generally accepted technological features of the production of glued laminated timber are significantly different from the technology of production of CLT panels, since the latter work as plates, and the glued laminated timber elements are the core elements of frames of different types of buildings. For completeness of analysis of CLT panels as a structural material methods of research of panels at different types of stress states are given according to the results of which characteristics of strength and rigidity of panels are assigned and correspond to a certain class of strength of CLT of panels. As a rule, at the present stage of the development of panels, all boards have the same strength class, which greatly simplifies the calculation and increases the homogeneity of the panels, as the structure with mutually transverse layers of boards. Large dimensions and specificity of production of panels of panels has a number of tolerances and boundary parameters that affect the quality of panels and their strength characteristics, which is largely due to the tight control of all technological operations in the manufacture of panels of different thicknesses. The cross laminated timber or CLT in the construction market has sparked a new expansion. The manufacturing practice of PKD panels has been around for more than 20 years in European countries, most notably in Austria, Germany and Switzerland. Keywords: cross laminated timber, CLT, producing technology, stress reliefs, boards edge gluing, rolling shear, board thickness, glue systems.

Perfection of the technology of corrosion-resistant sheet rolled stock production at JSC “Ural Steel”

Manufacturing of field oil pipelines require mainly pipes of 09ГСФ and 13ХФА steels, which correspond to the K52 class of strength. The pipe products and connection details of field oil- and oil products pipelines must meet increased requirements to their resistance against aggressive media, containing water and hydrogen sulfide. At the JSC “Ural Steel”, a unified comprehensive technology of production sheet rolled stock of 09ГСФ and 13ХФА steels in a wide range of assortment. The steels ensure the requirements to hydrogen fracturing as per NACE TM0284 and sulfide corrosion fracturing under stress method A of NACE TM0177 standard. The regimes of controlled sheet rolling with accelerated after-deformation cooling following by a high tempering elaborated. The technology regimes ensure necessary level of mechanical characteristics, corresponding to K52 class of strength. To ensure a satisfactory flatness of sheets, variants of the steels chemical composition depending on finished sheet thickness recommended. The elaborated production regimes ensure metal high resistance at low temperatures and actual value of the impact toughness KCV –40 exceeding considerably the standard requirements. It was shown, that decrease of relation Ca/S in the metal down to ~0,6–1,0 at Sulphur content less than 0.002% does not result in corrosion resistance decreasing and also minimizes the steel liquation inhomogeneity and contamination by non-metallic inclusions. The technology elaborated at JSC “Ural Steel” ensures the possibility of industrial production of corrosion-resistant rolled stock of 09ГСФ and 13ХФА steels in a wide range of assortment.

Current challenges and opportunities in microstructure-related properties of Advanced High Strength Steels

This is a viewpoint paper on recent progress in the understanding of the microstructure-property relations of Advanced High Strength Steels (AHSS). These alloys constitute a class of high strength, formable steels that are designed mainly as sheet products for the transportation sector. AHSS have often very complex and hierarchical microstructures consisting of ferrite, austenite, bainite or martensite matrix or of duplex or even multiphase mixtures of these constituents, often enriched with precipitates. This complexity makes it challenging to establish reliable and mechanism-based microstructure-property relationships. A number of excellent studies already exist about the different types of AHSS (such as dual phase steels, complex phase steels, transformation induced plasticity steels, twinning induced plasticity steels, bainitic steels, quenching and partitioning steels, press hardening steels etc.) and several overviews appeared in which their engineering features related to mechanical properties and forming were discussed. This article reviews recent progress in the understanding of microstructures and alloy design in this field, placing particular attention on the deformation and strain hardening mechanisms of Mn-containing steels that utilize complex dislocation substructures, nanoscale precipitation patterns, deformation-driven transformation and twinning effects. Recent developments on micro-alloyed nanoprecipitation hardened and press hardening steels are also reviewed. Besides providing a critical discussion of their microstructures and properties, vital features such as their resistance to hydrogen embrittlement and damage formation are also evaluated. We also present latest progress in advanced characterization and modelling techniques applied to AHSS. Finally, emerging topics such as machine learning, through-process simulation and additive manufacturing of AHSS are discussed. The aim of this viewpoint is to identify similarities in the deformation and damage mechanisms among these various types of advanced steels and to use these observations for their further development and maturation.

Enhancing adhesion properties between binder-free copper nanoink and flexible substrate using chemically generated interlocking structure

Organic residues formed when using organic binders to increase the adhesive strength of Cu ink to PI substrates degrade the electrical conductivity of Cu electrodes. Binder-free Cu ink can be used to fabricate highly conductive Cu electrodes but lead to poor adhesion owing to inherently weak adhesion between the polymer substrates and metals. We therefore propose a facile method to enhance adhesion between a printed copper electrode formed by flash light sintering and a polyimide (PI) substrate without sacrificing the electrical conductivity by forming an interlocking structure at the interface between the Cu electrode made of binder-free ink and PI substrate through simple chemical treatment. The surface of the PI substrate was converted to polyamic acid (PAA) via potassium polyamate formation after sequential chemical treatment using potassium hydroxide and hydrochloric acid. During the flash light sintering process, the PAA layer, which is much softer than PI, could easily migrate into the cavities between the sintered Cu nanoparticles due to the pressure difference originating from evaporation of H2O released through reduction of the oxide shell of Cu nanoparticles. Thus, an interlocking structure was formed and the substrate surface quickly reverted to PI by absorbing thermal energy during sintering. The increased contact area between Cu nanoparticles and the substrate surface of the interlocking structure could explain the dramatic improvement of the adhesion strength of the Cu electrode from 0 B to 5 B, which corresponds to the highest class of adhesion strength. In addition, the resistivity of the Cu electrode fabricated in this process was measured to be 2.5 times larger (4.17 μΩ·cm) than that of bulk Cu, which has not been achieved for printed Cu electrodes before. Our results could lead to new perspectives in the field of printed electronics.

Shear strength of reinforced concrete beams with recycled aggregates

This article presents an experimental program on the shear behavior of beams without transversal reinforcement manufactured with natural aggregate concrete and 100% recycled aggregate concrete. The beams were tested under four-point bending for a shear span-to-depth ratio ( a/ d) equal to 1.5 and 3.0. The mechanical properties of two mixes were characterized in terms of compressive strength, splitting tensile strength, and elastic modulus. Three-point bending tests were performed on plain pre-notched samples in order to determine the fracture properties by an inverse analysis of experimental force–crack mouth opening displacement curves using the analytical nonlinear hinge model and a power law strain-softening relationship. The strain-softening law is described by two parameters being, respectively, the power n and the critical crack opening displacement wc. The experimental results show that, for the same class of compressive strength, tensile strength, fracture energy, and the shear strength of recycled aggregate concrete are lower than natural aggregate concrete. The decrease in the fracture energy and the shear strength is consistent with the decrease in the splitting tensile strength of the recycled aggregate concrete mixes compared to the natural aggregate concrete. Critical shear crack theory was adopted to model the shear behavior of beams tested with a/ d = 3.0. For an accurate evaluation of the deformation capacity of tested beams, the nonlinear hinge model for recycled concrete members was extended to recycled concrete sections. For deep beams ( a/ d = 1.5), the strut-and-tie model was used. Finally, comparisons of prediction models to a wide range of experimental data are presented.

Research into structure formation and properties of the fiber­reinforced aerated concrete obtained by the non­autoclaved hardening

We have investigated the influence of the ratio cement:fly-ash and the temperature of mixing water on the properties of aerated concrete mixes and aerated concrete. It was established that the rational cement-fly ash ratio is 1:1; the mixing water temperature is 40 °C. Experimental research confirmed that the introduction of waste from salt processing and metakaolin to the formulation of binding compositions leads to the formation, rather than the metastable hexagonal calcium hydro-aluminates, of the stable compounds in the structure of partitions between pores of the hydrocalumite and hydrocarboaluminate type. That allowed the targeted structure formation of partitions between pores of the non-autoclaved aerated concrete, which improves the density of partitions and the strength of aerated concrete. It is shown that the introduction of polypropylene fibers to composition of aerated concrete does not affect the kinetics of swelling of the aerated concrete array. However, the introduction of polypropylene fibers improves the strength of aerated concrete based on the modified binding composition containing metakaolin by 47 %, the modified binding composition containing carbonate-containing waste ‒ by 32 %. For the aerated concrete of the В1.5–В2 class of strength, at a density within 615‒625 kg/m 3 , the estimated coefficient of thermal conductivity is 0.16 W/(m∙K), which makes it possible to reduce heat losses through external enclosures. Thus, there is reason to assert the possibility of the targeted control over the processes of forming a strong structure of partitions between pores using the modified binding compositions containing supplementary cementitious materials. The application of polypropylene fibers enables the reinforcement of aerated -concrete array, forming a strong structural frame of partitions between pores, and ensuring greater strength of the non-autoclaved aerated concrete.

Influence of recycled coarse aggregates incorporation on the fracture properties of concrete

This paper presents the mechanical and fracture properties of a structural concrete incorporating coarse recycled aggregate. Four mixes were designed according to the standard EN 206-1 to achieve S4 consistence class and C35/45 class of strength. Concrete specimens were prepared with different recycled gravel content of 0%, 30%, 65% and 100% by volume of the total coarse gravel.Three points bending tests were performed on 10×10×40cm pre-notched samples in order to determine the flexural strength and fracture properties according to the RILEM recommendations. From experimental Force-CMOD curves, the intrinsic fracture properties were obtained by an inverse analysis using an analytical model and a power law strain-softening relationship. The strain-softening law is described by two parameters being respectively the power n and the critical crack opening displacement wc. It was found that the difference between the calculated fracture energy, GF, and the RILEM fracture energy, GFRILEM, is more pronounced for recycled aggregate concrete.The evolution of n and wc, the parameters of the power strain-softening law, was also studied. It appeared that the power, n, depends on the compressive strength, the aggregate size and the water to binder ratio. For the same class of compressive strength, it remains substantially unchanged when the recycled aggregates content increases. For the critical crack opening wc, it decreases when the substitution ratio increases. Finally, the relationship between fracture properties and the tensile strength has been established.

Experimental and analytical study of bond behavior between recycled aggregate concrete and steel bars using a pullout test

The paper presents the results of an experimental program carried out on 96 concrete pullout specimens prepared with natural and recycled aggregates using 10 and 12 mm diameter deformed steel bars. Steel rebars were concentrically embedded in pullout specimens with two embedded length of 5 and 10 times the rebar diameter. The present work includes six recycled concrete aggregate mixtures and two conventional concrete mixtures with C25/30 and C35/45 target class of compressive strength and S4 class of workability. For both series, specimens were fabricated with different incorporation ratios of fine and coarse recycled aggregates and only coarse recycled aggregates.Tensile load was applied gradually until the pullout failure occurred and the slip between the rebar and concrete was measured at the free and loaded ends for each loading level. Furthermore, systematic observations of failure surface have been made at both macroscopic and microscopic scales. Test results showed that for the same class of compressive strength the bond strength and related failure mechanisms remain very close and the obtained values are, at least, four times higher than the predicted values by Eurocode 2.The experimental results of this study, together with an extensive number of results reported in the literature, were used to describe the variation of bond strength with concrete compressive strength, embedded length, and concrete cover. In addition, an analytical bond‐slip relationship is proposed and the parameters of bond‐slip law were calibrated by an optimization algorithm.

Mix Design and Properties of Recycled Aggregate Concretes: Applicability of Eurocode 2

This work is devoted to the study of fresh and hardened properties of concrete containing recycled gravel. Four formulations were studied, the concrete of reference and three concretes containing recycled gravel with 30, 65 and 100 % replacement ratios. All materials were formulated on the basis of S4 class of flowability and a target C35 class of compressive strength according to the standard EN 206-1. The paper first presents the mix design method which was based on the optimization of cementitious paste and granular skeleton, then discusses experimental results. The results show that the elastic modulus and the tensile strength decrease while the peak strain in compression increases. Correlation with the water porosity is also established. The validity of analytical expressions proposed by Eurocode 2 is also discussed. The obtained results, together with results from the literature, show that these relationships do not predict adequately the mechanical properties as well as the stress–strain curve of tested materials. New expressions were established to predict the elastic modulus and the peak strain from the compressive strength of natural concrete. It was found that the proposed relationship E–fc is applicable for any type of concrete while the effect of substitution has to be introduced into the stress–strain (ec1–fc) relationship for recycled aggregate concrete. For the full stress–strain curve, the model of Carreira and Chu seems more adequate.

The study of the conformity criterion for compressive strength of concrete based on order statistics

The test of concrete compressive strength conformity with current regulations PN EN 206-1:2003 is the sampling inspection based on a numerical evaluation. Satisfying the compound criteria, including the adoption of statistical quality control plan, confirms the conformity of the examined batch of concrete defined by the declared class of compressive strength defined by characteristic value of compressive strength. The conformity criteria recommended by EN 206-1 for the initial production are not without flaws and they are critically evaluated by many authors. This paper presents a new criterion of conformity based on order statistics. A preliminary evaluation of the criterion was made for the series with a small number of the test results with the use of probability of acceptance determined by means of the Monte Carlo method with the assumed 5% fraction defective. The analysis of the results showed that the presented criterion does not depend on the dispersion of results whereas the probability of acceptance is maintained at a constant level approached to the appropriate one at the stage of continuous production.