Strength Of Concrete Increases Research Articles

Experimental evaluation of steel fiber effect on mechanical properties of steel fiber-reinforced cement matrix

This paper investigates the possibility of combining steel fibers with different weight percentages along with their functions in increasing compressive strength, indirect tensile strength and bending strength. In this study, two types of steel fibers, hooked and crimped, have been employed in the preparation of samples. These fibers have a length to diameter ratio of 30 mm and 50 mm (L/D=30 and L/D=50). Further, the combination of these fibers caused a reduction in concrete flow and reduced its efficiency. The combination of these fibers caused considerable increase of concrete bending strength compared to fibreless and single-fiber type concretes. In this research, tensile strength and bending strength of samples with 1.5%, 2% and 2.5% fibers were investigated in which the tensile strength and bending strength of all samples increased. However, there were very considerable increases in tensile strength and bending strength of samples made with hybrid crimped fibers. The results indicated that steel fibers did not have much impact on concrete compressive strength.

Fracture toughness and failure mechanism of high performance concrete incorporating carbon nanotubes

Cement and concrete composites are inherently brittle and exhibit very less tensile/flexural strength capacity as compared to their compressive strength. Use of thoroughly dispersed carbon nanotubes in the concrete matrix is one of the possible solution for enhancing mechanical properties in tension/flexure. In the present research work, small fractions of multiwall carbon nanotube (MWCNTs) i.e. 0.05 and 0.10 wt% of cement have been integrated into the cement concrete to study their effect on the mechanical properties of the resultant concrete mixtures. The enhanced performance of the whole mix lies on a single point that MWCNTs must be thoroughly disperse in the mixture. Hence, special arrangement through usage of high energy sonication along with amended acrylic based polymer (performing as a surfactant) was made to have a uniform dispersion of MWCNTs in the concrete mix. The testing of concrete samples includes i.e., flexure, splitting tensile and compressive strengths after 3, 7, 28 and 56 days of curing. After having comparison with the control mix cured for 28 days, it was observed that the addition of 0.05 wt% MWCNTs increased the splitting tensile strength by 20.58%, flexural strength by 26.29% and compressive strength by 15.60%. Through above results, which verify the increase in concrete mix strength after adding MWCNTs, these MWCNTs may be incorporated in the treatment of Nano/micro cracks completed through process of connecting, branching and pinning. Similarly, as proved in threepoint bending tests, MWCNTs also enhances the breaking strains as well as the fracture energy of the concrete mixes, besides, imparting increase to the strength. The investigations have shown that incorporating lesser amounts of MWCNTs i.e., 0.05 and 0.10 wt% of cement to the concrete mixes after insuring there complete dispersion, unusually improve their properties like mechanical strengths and fracture behavior.

Bond performance of deep embedment FRP bars epoxy-bonded into concrete

Abstract With increasing numbers of strength-deficient concrete infrastructure assets, strengthening and repair of concrete structures is becoming an issue of international importance. This study examines the bond behaviour of deep embedment (DE) glass fibre reinforced polymer (GFRP) and carbon FRP (CFRP) bars embedded into concrete prisms using an epoxy adhesive. The experimentally investigated parameters were the embedment length, FRP bar type and diameter, concrete compressive strength and hole diameter. The increase in embedded length enhanced the pull-out capacity for both GFRP and CFRP bar types. However, the bond strength and initial stiffness of the bond stress-slip curves decreased with the increase in embedded length. The specimens with DE CFRP bars had higher pull-out capacities and better bond performance than the corresponding specimens with DE GFRP bars. For the specimens with DE CFRP bars, the pull-out capacity increased with the increase in bar diameter and concrete strength but these two parameters did not affect the behaviour of the specimens with DE GFRP bars. The increase in hole diameter reduced the initial stiffness of the specimens with DE GFRP bars but affected neither the failure mode nor the failure loads. For the first time, this paper presents a mathematical model for predicting the bond strength of DE FRP bars. The mathematical model was validated against experimental results and demonstrated to produce accurate predictions. It is envisaged that both the mathematical model and the experimental results will contribute to the development of future design guidelines for DE concrete shear strengthening.

PECULIAR PROPERTIES OF STRUCTURAL FORMATION OF CEMENT COMPOSITES IN THE PRESENCE OF FUELEROID TYPE CARBON NANOPARTICLES

The paper deals with the application of fulleroid carbon nanoparticles (FCN) for the cement com-posites modification. In particular, the thermokinetic analysis was performed to assess the change in the rate and completeness of the cement hydration when the nanomodifier was introduced into it. The study of exothermic heterogeneous interactions and thermal effects in the binder - water - nanomodifier system allows one to estimate the hydration rate and depth and to assess the degree of nanomodifier’s influence. The conducted studies revealed the increase in the hydration temperature when carbon nanoparticles are introduced into the cement dough, while the induction period is prolonged, that indicates a more complete course of the reactions. It is confirmed by the results of qualitative X-ray structural analysis. The dynamics of decrease in calcium silicate peaks characterizes the completeness of hydration and binding of portlandite, that explains the increase in the modified cement stone strength. Microstructural analysis allows to detect the structure characteristic of control samples with a large number of large chaotically located crystals, the bulk of which is formed on the first day of solidification. Under the same conditions, the modification of the cement stone contributes to the creation of the more dense spatial structure consisting of small crystalline hydrate formations. All that process positively influences the formation of the rigid matrix with a smaller number of pores, which determines the in-crease in strength and durability of cement stone and concrete on its basis.

Bond behaviour of GFRP bars in high-strength concrete: bar diameter effect

The effects of reinforcing bar diameter and concrete compressive strength on the bond behaviour of glass fibre reinforced polymer (GFRP) bars were investigated. The primary experimental parameters were the type of reinforcing bar (steel, sand-coated GFRP and spiral-wrapped GFRP), the concrete compressive strength (20, 40 and 60 MPa) and the bar diameter (19 and 25 mm). The experimental results revealed that as bar diameter increases, the bond strength of the GFRP bars decreases at a greater rate than that of the steel bars. The bond strength was found to decrease linearly with increasing bar diameter for the GFRP bars while, for the steel bars, the bond strength decreased with increasing bar diameter up to 19 mm and then became constant thereafter. For high-strength concrete, a lower reduction rate in bond strength was observed with increasing bar diameter. It is postulated that an increase in concrete compressive strength causes internal interface (fibre–resin) failure of GFRP bars, while an increased bar diameter results in external interface (resin–concrete) failure.

Numerical Study on Section Constitutive Relations of Members Reinforced by Steel-BFRP Composite Bars

Steel-Basalt FRP Composite Bar (S-BFCB) is a new kind of substitute material for longitudinal reinforcement, with high elastic modulus, stable post-yield stiffness and excellent corrosive resistance. Based on mechanical properties of S-BFCB and the plane cross-section assumption, the moment-curvature curves of beam and column members are simulated. Some parameters such as equivalent rebar ratio, postyeild stiffness, concrete strength and axial compression ratio of column were discussed. Results show that the constitutive relation of the cross section is similar with RC member in elastic and cracking stages, while different in post-yield stage. With the increase of postyeild stiffness ratio of composite bar, the ultimate bearing capacity of component improved observably, member may turn out over-reinforced phenomenon, concrete crushing may appear before the fibersarefractured. The effect of concrete strength increase in lower postyeild stiffness ratio is not obvious than in higher. The increase of axial compression ratio has actively influence on bearing capacity of column, but decreases on the ductility.

Evaluation of physical and mechanical characteristics of siderite concrete to be used as heavy-weight concrete

The objective of this study is to investigate the usability of siderite aggregates mentioned above as a heavy-weight aggregate in the heavy-weight concrete production and their effects on physical properties of the heavy-weight concrete mixture. For this purpose, normal-weight aggregate content was decreased at the rate of 20%, 40%, 60%, 80% and 100% by volume of the concrete mixture prepared and mixtures were prepared by its substitution with siderite aggregate at the same rates. Based on the increase in siderite aggregate in the concrete mixture, a considerable increase in unit weight, compressive strength and tensile strength of concrete was obtained. Plate-shaped concrete specimens obtained from combined normal-weight and heavy-weight aggregates were exposed to radiation with different X-ray source (118 keV, 164 keV) and Co-60 (1250 keV) beam source at Çekmece Nuclear Facility. It is clearly seen that the concrete mixture containing siderite aggregate improved 70% radiation absorption characteristics compared to that of the concrete mixture without siderite aggregate.

Low carbon building: Experimental insight on the use of fly ash and glass fibre for making geopolymer concrete

Abstract Due to the environmental impacts resulting from the production of Ordinary Portland cement (OPC), the drive to develop alternative binders that can totally replace OPC is gaining huge consideration in the construction field. In the current study, attempt was made to determine the strength characteristics of glass fibre-reinforced fly ash based geopolymer concrete. Sodium hydroxide (NaOH) and sodium silicate (Na 2 SiO 3 ) were used as alkaline solutions (for activation of geopolymer reaction) at 12, 16, 20 M. Glass fibres were added to the geopolymer concrete in varying proportions of 0.1–0.5% (in steps of 0.1%) by weight of concrete. A constant weight ratio of alkaline solution to fly ash content of 0.43 was adopted for all mixes. British standard concrete test specimens were cast for measuring compressive strength, split-tensile strength, and flexural strength. Concrete specimens were cured by heating in oven at 90 °C for 24 h and natural environment, respectively. From the results, thermally cured concrete samples had better mechanical properties than the ambient (natural) cured samples. Thermally cured concrete specimen, containing 0.3% glass fibre and 16 M NaoH, achieved a maximum compressive strength of 24.8 MPa after 28 d, while naturally cured samples achieved a strength of 22.2 MPa. There was substantial increase in tensile strength of geopolymer concrete due to the addition of glass fibres. Split tensile strength increased by 5–10% in glass fibre-reinforced geopolymer concrete, containing 0.1–0.5% glass fibre and 16 M NaoH when compared to the unreinforced geopolymer concrete (1.15 MPa).

Finite Element Analysis of Horizontal Bearing Capacity of Light Steel Concrete Wall

Based on a large number of reference, by using non-linear FE analytical method to analyze the lightweight steel concrete horizontal bearing capacity. In order to obtain more accurate simulation results, using solid element to simulate the various components of the wall, between the concrete and the wall frame columns are connected by the binding manner. To limit the beam wall vertical wall planar degrees of freedom .The coupling beam at the loading point X direction freedom, which was calculated in the X direction. The analyzing result shows that the light steel structure is the main force members of the composite wall. And the increase of concrete strength can also improves the horizontal bearing capacity of the composite wall, but the concrete is not as the main bearing capacity of the component.

Partial Confinement Utilization for Rectangular Concrete Columns Subjected to Biaxial Bending and Axial Compression

The prediction of the actual ultimate capacity of confined concrete columns requires partial confinement utilization under eccentric loading. This is attributed to the reduction in compression zone compared to columns under pure axial compression. Modern codes and standards are introducing the need to perform extreme event analysis under static loads. There has been a number of studies that focused on the analysis and testing of concentric columns. On the other hand, the augmentation of compressive strength due to partial confinement has not been treated before. The higher eccentricity causes smaller confined concrete region in compression yielding smaller increase in strength of concrete. Accordingly, the ultimate eccentric confined strength is gradually reduced from the fully confined value fcc (at zero eccentricity) to the unconfined value f_{c}^{{prime }} (at infinite eccentricity) as a function of the ratio of compression area to total area of each eccentricity. This approach is used to implement an adaptive Mander model for analyzing eccentrically loaded columns. Generalization of the 3D moment of area approach is implemented based on proportional loading, fiber model and the secant stiffness approach, in an incremental-iterative numerical procedure to achieve the equilibrium path of P–ε and M–φ response up to failure. This numerical analysis is adapted to assess the confining effect in rectangular columns confined with conventional lateral steel. This analysis is validated against experimental data found in the literature showing good correlation to the partial confinement model while rendering the full confinement treatment unsafe.

Using granite siftings for producing vibro-pressed fine-grained concrete

Using stone crushing waste products allows more rational and complex use of mineral resources and increases the ecological safety level of quarry works and production of stone materials. This paper focuses on an experimental investigation of the composition effect of vibro-pressed fine-grained concrete incorporating granite siftings on compression and flexural strength. Analysis of regression equations, obtained using mathematical experiments planning, shows that dust particles included in the siftings can behave as micro-filler. This is especially the case at low cement-water ratios therefore yielding an increase in concrete strength. The positive effect of the dust fraction in siftings decreases as the cement-water ratio and the fineness modulus of sand that is a part of the aggregate become higher. The hardening kinetics of vibro-pressed concrete with granite siftings is analyzed. The results of the present study may be used for preliminary assessment of granite sifting efficiency as aggregates in vibro-pressed concrete and design of their optimal compositions.

Borassus aethiopum mart of Benin used as reinforcement in concrete: Adhesion characterization

Researches carried out in Benin allowed the use of Borassus aethiopum mart as reinforcement in concrete. The aim of this study is to examine the bond of the two materials. The results of pull out tests have shown that the bonding strength is around 1 MPa. This adhesion rate decreases slightly when the bond length increases; on the other hand, the adhesion rate increases slightly when the concrete strength increases. The behaviour of Borassus / concrete interface shows a first phase of perfect adhesion followed by a second phase of progressive loss of adhesion and a final friction phase which continues until the complete output of the reinforcement from the concrete.

Mechanical properties and microstructure of high performance concrete containing stabilized nano-silica

Nanotechnology can significantly improve the mechanical strength and durability of concrete. At the nanoscale, good dispersion of constituent materials is an essential factor for the improvement of concrete properties. In this scenario, the objective of this research was to evaluate the mechanical properties of high performance concrete containing stabilized nano-silica. The incorporation of nano-silica to the polycarboxylate chemical admixture allowed its application in concrete, since the nano-silica was evenly dispersed in the concrete mass. For this study, three different concretes were employed: a reference concrete, one containing stabilized nano-silica and another containing a mixture of silica fume and stabilized nanosilica. The evaluated concrete mechanical properties were the axial compressive strength and secant modulus of elasticity measured in three different ages. The results indicated a substantial increase in concrete compressive strength and modulus of elasticity with the addition of stabilized nano-silica combined with a reduction in cement content. The results also showed that use of stabilized nano-silica proved to be very advantageous, especially when mixed with silica fume. When they were combined, a synergy effect occurred in the concrete that was of chemical nature in addition to the physical effect of better concrete particle packaging. Analysis by scanning electron microscopy showed the improvement in microstructure of concretes with stabilized nano-silica, which contributed to gains in mechanical properties.

Research Into Influence of Ultra- and Nanodisperse Size Additives on the Structure and Properties of Heat Insulating Autoclaved Aerated Concrete

The paper presents the results of the research into the influence of various ultra- and nanodisperse size additives on the structure and mechanical-and-physical properties of heat insulating autoclaved aerated concrete (AAC). Research into interaction mechanisms of such additives as silica gel, high active metakaolin and multi-walled carbon nanotube dispersion (MWCNTs) with heat insulating AAC components consisted in the evaluation of the dynamics of the cellular concrete plastic strength increase of porous mixture, degree of cellular concrete blowup and comparing the strength and density of the check sample and the modified one. The studies found that the most effective method of obtaining the structural strength of AAC is by the use of the MWCNTs. They contribute to obtaining the optimum viscoplastic properties of bulk concrete and stabilizing of the pore formation process with simultaneous formation of the solid homogeneous hexagonal structure.

Resistance of circular concrete-filled tubular sections to combined axial compression and bending

This paper describes strength formulae and simplified strength interaction curves for the direct strength method (DSM) for circular concrete-filled tubular (CFT) sections. A simple axial strength formula and a flexural strength formula for circular CFT sections are proposed to account for the post-local buckling strength of the circular steel skin and the increase in concrete compressive strength caused by confinement of the steel skin. The squash load predicted by the proposed strength formula is compared with test results in the literature, and those predicted by AISC specifications and Eurocode4. A simplified strength interaction curve for circular CFT members under combined axial compression and bending is proposed and compared with test results. The comparison confirms that the proposed axial and flexural strength formulae and simplified strength interaction curves can be used to conservatively predict the resistance of circular CFT columns to an axial load, and combined axial compression and bending.

Structural Behavior of Modified Reactive Powder and Reactive Powder Concrete Wall Panels Subjected to high temperature

This research presents an experimental work to investigate the structural behavior of concrete wall panels subjected to high temperature (3500c) and distributed load. Eight specimens of wall panels were tested in this study and divided equally in to four groups, each group consists of two wall panels first one with slenderness ratio (H/t=20) and the other one with slenderness ratio (H/t=14). The first group with normal strength concrete, the second with high strength concrete, the third with modified reactive powder concrete (MRPC) and the fourth with reactive powder concrete (RPC). The results shows that for wall panels with slenderness ratio H/t=20 the lateral deflection decrease by about 19% as concrete strength (fc') increase from 30.2Mpa to 69.5Mpa and about 58% as concrete strength (fc') increase to 128Mpa at failure load, while for H/t=14 the lateral deflection decrease by about 49% as concrete strength (fc') increase from 30.2Mpa to 69.5Mpa and about 28% as concrete strength (fc') increase to 128Mpa at failure load. For Wall panels with high strength concrete, MRPC, and RPC there is no big difference between lateral deflection at the linear part for panel with H/t=14and panel with H/t=20. This difference increases at the nonlinear part of the curves. The failure load increases by about 62% as the concrete strength (fc') increases from 30.2Mpa to 69.5Mpa and increases to 50% as the concrete strength (fc') increases from 69.5Mpa to 128Mpa for panels with H/t=14. The failure load increases by about 89% as concrete strength (fc') increases from 30.2Mpa to 69.5Mpa and the failure load increases by 56% as the concrete strength (fc') increases from 69.5Mpa to 128Mpa for panels with H/t=20. For all tested panels the failure mode was buckling failure and the cracks for wall panels with RPC and MRPC are close to center of the panel.

Nonlinear analysis of circular high strength concrete-filled stainless steel tubular slender beam-columns

Concrete-filled stainless steel tubular (CFSST) slender columns are increasingly used in composite structures owing to their distinguished features, such as aesthetic appearance, high corrosion resistance, high durability and ease of maintenance. Currently, however, there is a lack of an accurate and efficient numerical model that can be utilized to determine the performance of circular CFSST slender columns. This paper describes a nonlinear fiber-based model proposed for computing the deflection and axial load-moment strength interaction responses of eccentrically loaded circular high-strength CFSST slender columns. The fiber-based model incorporates the accurate three-stage stress-strain relations of stainless steels, accounting for different strain hardening characteristics in tension and compression. The material and geometric nonlinearities as well as concrete confinement are included in the computational procedures. Existing experimental results on axially loaded CFSST slender columns are utilized to verify the proposed fiber-based model. A parametric study is conducted to examine the performance of high-strength slender CFSST beam-columns with various geometric and material parameters. It is shown that the fiber-based analysis technique developed can accurately capture the experimentally observed performance of circular high-strength CFSST slender columns. The results obtained indicate that increasing the eccentricity ratio, column slenderness ratio and diameter-to-thickness ratio remarkably decreases the initial flexural stiffness and ultimate axial strength of CFSST columns, but considerably increases their displacement ductility. Moreover, an increase in concrete compressive strength increases the flexural stiffness and ultimate axial strength of CFSST columns; however, it decreases their ductility. Furthermore, the ultimate axial strength of CFST slender columns is found to increase by using stainless steel tubes with higher proof stresses.

Analysis of interfacial bonding characteristics of CFRP-concrete under fatigue loading

The paper presents the results of an experimental and analytical study involving the bond properties of nine specimens externally strengthened with carbon-fiber-reinforced polymer (CFRP) sheets under fatigue loading. The purpose of this study is to investigate the influence of concrete strength and fatigue amplitude on the bond performance of CFRP-concrete interface under fatigue loading. Firstly, modified beam specimens with different concrete strengths were designed and constructed. Secondly, the specimens were tested under fatigue loads with different fatigue amplitudes. The test results show that the fatigue life increased with an increase in concrete strength, but decreased as the fatigue load amplitude increased, and a formula was proposed to predict the fatigue life. Moreover, the concrete strength and fatigue loading amplitude have little effect on the effective bond length under fatigue loading. The test results of bond-slip relationship show that the bond-slip curves continuously degraded with an increase in the number of load cycles, that is, the maximum shear stress decreased with an increase in the number of load cycles. Finally, a bond-slip model was developed considering the effects of concrete strength and fatigue loading amplitude. The comparison between experimental results and this model demonstrated that this developed bond-slip model can be used to describe the bond-slip relationship of the CFRP-concrete interface under fatigue loading.

Impact of variability of mechanical and thermal properties of concrete on predicted performance of jointed plain concrete pavements

The performance of jointed plain concrete pavement is affected by various design parameters including mechanical and thermal properties of concrete. Out of these, coefficient of thermal expansion, elastic modulus and modulus of rupture are a few important ones and to evaluate the effects of these material properties on pavement performance, simulations were carried out in MEPDG. All other design parameters such as traffic, design life, climate and road bed soil conditions were considered as constant and pavement performance was evaluated. The simulation results appreciated that with an increase in coefficient of thermal expansion of concrete, the pavement performance was adversely affected. In addition, with an increase in elastic modulus and modulus of rupture values of concrete, the strength of concrete increases and resultantly an improved pavement performance was obtained. It became evident that these material parameters should be carefully considered while designing a rigid pavement.

Use of ultrasonic pulse velocity to estimate strength of concrete at various ages

This study used the ultrasonic pulse velocity (UPV) technique to evaluate the in-place strength of concrete at various ages. Both UPV and strength of concrete increase with age, and the relationship between UPV and compressive strength has been investigated for decades. Previous research results have indicated that the density of concrete is linked positively to UPV and compressive strength. It has also been found that the amount of coarse aggregate and the moisture content of concrete have a significant influence on pulse velocity, but little effect on strength. In this work, a study was carried out to develop a general relationship between UPV and compressive strength of concrete in consideration of multiple variables such as age, coarse aggregate content and moisture content. The suitability of the established relationship for estimating the strength of concrete at various ages was also evaluated. The results obtained from the verification programme carried out on a concrete plate specimen show that the effect of moisture content on UPV must be taken into account for a better estimate of concrete strength with the UPV technique.