Strength Enhancement Of Concrete Research Articles

Characterization of the mechanical properties of eco-friendly concrete made with untreated sea sand and seawater based on statistical analysis

Natural sea sand and seawater, as alternative sustainable materials for mitigating environmental pollution and resource shortage of traditional concrete production using river sand and freshwater, are gaining increasing attention from researchers worldwide. In this paper, an innovative and sustainable eco-friendly concrete using different kinds of sea sand and seawater has been investigated. The influence of incorporated ions by sea sand and/or seawater on the mechanical properties of concrete has been explored. Due to the accelerated effect of additional ions on cement hydration process at early age, the volume of intrusion capillary pores, which are responsible for the macroscopic strength of concrete, is found to be relatively higher within seawater sea-sand concrete (SWSSC). The statistical analysis of compressive strength shows that the three kinds of studied SWSSC exhibit no intrinsic difference compared to normal concrete. Although the incorporation of sea sand and especially seawater impedes somewhat the concrete strength enhancement with aging, the global mechanical behavior of SWSSC is still comparable to that of ordinary concrete. With proper selection and preliminary test, the incorporation of sea sand and/or seawater in plain concrete as well as in concrete with corrosion-free reinforcement is promising for the environmentally friendly production of concrete.

Novel approach to improve crumb rubber concrete strength using thermal treatment

The concept of using crumb rubber as a partial replacement of natural aggregate in concrete to produce rubberised concrete and reduce environmental impacts has been a subject of research for many years. A plethora of studies have investigated various methods to improve the rubberised concrete strength using different pre-treatment methods for the rubber particles and/or using other additives for general concrete strength enhancement. However, the efficiency and applicability of these methods have been quite inconsistent and in some cases in conflict with each other. This study presents a novel approach to pre-treating crumb rubber particles using thermal treatment at 200 °C before incorporation into concrete. Heating time, rubber size, and rubber content were the variables in this experimental investigation. Scanning electron microscope (SEM) investigation was carried out on both as-received and thermally-treated rubber particles, as well as crumb rubber concrete (CRC) specimens. The results showed promising enhancements in concrete performance compared with the previous work findings. At 20% rubber content using size #40 mesh thermally-treated rubber, the compressive strength recovered by 60.3%.

An experimental study on effect of Bacillus sphaericus bacteria in crack filling and strength enhancement of concrete

Abstract Objective of the present investigation is to develop a durable cement concrete by incorporating varying proportions of Bacillus Sphaericus bacteria in crack filling and strength enhancement through biomineralisation. Test samples were tested at different ages for cube compressive strength, split tension, flexural strength, modulus of elasticity and also for durability tests with Bacillus Sphaericus bacteria concentrations of 1000, 100,000, 107 cells per 1 ml, then verified by the reference test samples made without bacteria. The crack filling ability of bacteria are studied and found that at all the concentrations, the percentage loss of strength for cracked concrete specimen with bacteria was low compared to cracked concrete specimen without bacteria. For bacterial dosage of 100,000 cells/ml better results were noticed for strength and crack filling of specimens. But with further increasing the dosage of bacteria, the mechanical properties were decreased. Findings revealed that concrete with Bacillus sphaericus is much efficient for increasing the durability of concrete as percentage loss of compressive strength is low when exposed to acid attack. The development in the parameters tested for concrete with bacteria can be attributed to the precipitation of calcium carbonate which was found in SEM analysis. Incorporated bacterium into the concrete mixture do not harm in any way. Hence, it can be stated that the amalgamation of bacterium in concrete reduces the cracks there by adds durability to the concrete.

A Parametric Study of Confinement Effects to the Interaction Diagram of P-M for High-Strength Concrete Columns

In reinforced concrete columns design, P-M interaction diagram is used as axial load control and column bending without taking into account the effect of lateral reinforcement bars. Design principles that ignore the effect of reinforcement bars will result in low value of actual axial capacity of column. This paper presents the effect of lateral reinforcement with a case study of square-sectional columns on high-strength concrete. The equation of unconfined concrete strength enhancement is based on Mander, Legeron, Imran, Antonius and Muguruma models. The reviewed parameters include compressive strength of concrete, reinforcement configuration, and spacings of confinement. The result of the analysis is a comparison of P-M diagram based on confinement models. It shows that confinement significantly influences axial capacity of column, yet it has a certain collapse point at point P = 0 in all models. This is caused by longitudinal reinforcement system. Configuration of reinforcement, spacing and confinement models greatly affects the collapse behavior of column, whether the collapse is classified as compressive collapse or tensile collapse based on the P-M diagram.

Повышение прочности бетона пластифицирующей добавкой на основе наноструктурированного углерода

Повышение прочности бетона пластифицирующей добавкой на основе наноструктурированного углерода

Influence of silpozz and rice husk ash on enhancement of concrete strength

This paper presents the results of a study undertaken to investigate the enhancement of concrete strength using Silpozz and Rice Husk Ash (RHA). The total percentage of supplementary cementitious material (SCM) substituted in this study was 20%. Six different concrete mixes were prepared such as without replacement of cement with silpozz and RHA (0% silpozz and 0% RHA) is treated as conventional concrete, whereas in other five concrete mixes cement was replaced by 20% of silpozz and RHA as (0% silpozz and 20% RHA), (5% silpozz and 15% RHA), (10% silpozz and 10% RHA), (15% silpozz and 5% RHA) and (20% silpozz and 0% RHA) with decreasing water-binder (w/b) ratio i.e. 0.375, 0.325 and 0.275 and increasing super plasticiser dose. New generation polycarboxylate base water reducing admixture i.e.,Cera Hyperplast XR-W40 was used in this study. The results of this research indicate that as w/b decreases,super plasticiser dose need to be increased so as to increase the workability of concrete. The effects of replacing cement by silpozz and RHA on the compressive strength, split tensile strength and flexural strength were evaluated. The concrete mixture with different combination of silpozz and RHA gives higher strength as compared to control specimen for all w/b ratios and also observed that the early age strength of concrete is more as compared to the later age strength. It is also observed that the strength enhancement of concrete mixture prepared with the combination of cement, silpozz and RHA is higher as compared to the concrete mixture prepared with cement and silpozz or cement and RHA.

Flexural behavior of reinforced lightweight concrete beams under reversed cyclic loading

This paper presents the results of an experimental investigation on the flexural behavior of doubly reinforced lightweight concrete (R.L.C.) beams tested under cyclic loading. A total of 20 beam specimens were tested. Test results are presented in terms of ductility index, the degradation of strength and stiffness, and energy dissipation. The flexural properties of R.L.C. beam were compared to those of normal concrete (R.C.) beams. Test results show that R.L.C. beam with low and medium concrete strength (20, 40MPa) performed displacement ductility similar to the R.C. beam. The ductility can be improved by enhancing the concrete strength or decreasing the tension reinforcement ratio. Using lightweight aggregate in concrete is advantageous to the dynamic stiffness of R.L.C. beam. Enhancement of concrete strength and increase of reinforcement ratio will lead to increase of the stiffness degradation of beam. The energy dissipation of R.L.C beam, similar to R.C. beam, increase with the increase of tension reinforcement ratio. The energy dissipation of unit load cycle for smaller tension reinforcement ratio is relatively less than that of beam with higher reinforcement ratio.

An experimental study on flexural strength enhancement of concrete by means of small steel fibers

Cost effective improvement of the mechanical performances of structural materials is an important goal in construction industry. To improve the flexural strength of plain concrete so as to reduce construction costs, the addition of fibers to the concrete mixture can be adopted. The addition of small steel fibers with different lengths and proportion have experimentally been analyzed in terms of concrete flexural strength enhancement. The main objectives of the present study are related to the evaluation of the influence of steel fibers design on the increase of concrete flexural characteristics and on the mode of failure. Two types of beams have been investigated. The force level, deflection and time to failure of beams have been measured. The shear crack, flexural crack and intermediate shear-flexural crack have been studied. The steel fiber content controlled crack morphology. Flexural strength and time to failure of fiber reinforce concrete could be further enhanced if, instead of smooth steel fibers, corrugated fibers were used.

RETRACTED: Effects of fly ash and TiO2 nanoparticles on rheological, mechanical, microstructural and thermal properties of high strength self compacting concrete

Abstract In the present study, strength enhancement and durability-related characteristics along with rheological, thermal and microstructural properties of high strength self compacting concrete (HSSCC) containing nano TiO 2 and industrial waste ash namely as fly ash (FA) have been investigated. With this respect, Portland cement was replaced by up to 15 wt% waste ash and up to 5 wt% TiO 2 nanoparticles and the properties of HSSCC specimens were measured. It was found that with the aim of energy saving and recycling of waste materials, addition of FA as a natural pozzolan can improve the rheological, mechanical and durability properties of concrete at higher ages. TiO 2 nanoparticles as a partial replacement of cement up to 4 wt% could accelerate C–S–H gel formation as a result of increased crystalline Ca(OH) 2 amount at the early age of hydration and hence improve the microstructure of concrete leading to improved durability-related properties and strength enhancement of the concrete. Several empirical relationships for predicting flexural and split tensile strength of concrete based on compressive strength for HSSCC containing FA and nano TiO 2 at different ages have been obtained. Finally, an energy-based assessment of strength enhancement of nano-containing concrete has been presented.

Nonlinear failure prediction of concrete composite columns by a mixed finite element formulation

This study focuses on developing a mixed frame finite element formulation of reinforced concrete and FRP composite columns in order to give more accuracy not only to predict the global behavior of the structural system but also to predict the local damage in the cross-section. A hypo-elastic constitutive law of concrete is presented under the basis of a three-dimensional stress state in order to model the compressive behavior of confined concrete wrapped with FRP jackets. To predict the nonlinear load path-dependent confinement model of FRP-confined concrete, the strength enhancement of concrete was determined by the failure surface of concrete in a tri-axial stress state, and its corresponding peak strain was computed by the strain-enhancement factor proposed in this study. The behavior of FRP jacket was modeled using the two-dimensional classical lamination theory. The flexural behavior of concrete and composite members was defined using a nonlinear fiber cross-sectional approach. The results obtained by developed mixed finite element formulation were verified with the experiments of concrete composite columns and also were compared with a displacement-based finite element formulation. It is shown that the proposed formulation gives e more accurate results in the global behavior of the column system as well as in the local damage in the column sections.

Prediction of nonlinear bending behavior for FRP concrete beams based on multi-axial constitutive laws

This study focuses on a predictive model of the flexural behavior of FRP-confined concrete structural members. A hypoelasticity-based constitutive law of concrete is presented on the basis of a three-dimensional stress state in order to model the compressive behavior of confined concrete wrapped with FRP jackets. The strength enhancement of concrete was determined by the failure surface of concrete in a triaxial stress state, and its corresponding peak strain was computed by the strain-enhancement factor also proposed in this study. The behavior of FRP jackets is modeled using two-dimensional orthotropic lamination theory. Therefore, the newly proposed model is a nonlinear load path-dependent confinement model of FRP-confined concrete. To model FRP-confined concrete bending members, a procedure is presented for the prediction of the flexural behavior of FRP-confined concrete structural members using a nonlinear fiber cross-sectional approach. The developed model is validated with test results obtained from several bending experiments. The results demonstrate that the proposed model is able to capture the flexural behavior of FRP-confined concrete structural members as well as the axial and lateral strains of the sections.

A microscopic approach to rate effect on compressive strength of concrete

The enhancement of concrete strength under uniaxial dynamic compression is investigated in this paper. The influence of the microcrack density to the concrete compressive strength is also discussed. Based on the sliding crack model, the compressive strength is obtained by considering the interaction between microcracks using the Kachanov method. Both free water viscosity and inertia effect are included by considering their influences on the dynamic stress intensity factor of crack tip under linearly increasing load. The relationship between the dynamic strength increase factor and the strain rate is obtained. The comparison between the results by the model proposed in this paper and those by available experiments indicates a favorable agreement.

Concrete with highly active rice husk ash

The overall aim was to investigate the effect of highly active rice husk ash (RHA) produced by an industrial furnace on some properties of concrete. The strength, pore volume and pore distribution of concrete and the Ca(OH)2 content in concrete were investigated by JIS A 1108 (Method of test for compressive strength of concrete), a mercury instrument porosimeter, and the thermogravimetric analysis, respectively. The results show that, with RHA replacement of cement, the compressive strength of concrete is increased evidently; the average pore radius of concrete is greatly decreased, especially the portion of the pores greater than 20 nm in radius is decreased while the amount of smaller pores is increased, and the more the RHA replacement, the less the amount of Ca (OH)2 in concrete. The latter two results are the main reasons for the strength enhancement of concrete.

External Prestressed Carbon Fiber-Reinforced Polymer Straps for Shear Enhancement of Concrete

The use of fiber-reinforced polymers (FRPs) for the strengthening and repair of existing concrete structures is a field with tremendous potential. The materials are very durable and, hence, ideally suited for use as external reinforcement. Although extensive work has been carried out investigating the use of FRPs for flexural strengthening, a fairly recent development is the use of these materials for the shear strength enhancement of concrete. The current system investigates the use of posttensioned, nonlaminated, carbon fiber-reinforced polymer (CFRP) straps as external shear reinforcement for concrete. Experiments were carried out on an unstrengthened control beam and beams strengthened with external CFRP straps. It was found that the ultimate load capacity of the strengthened beams was significantly higher than that of the control specimen. Existing design codes and analysis methods were found to underestimate the ultimate resistance of the control specimen and the strengthened beams. Nevertheless, the modified compression field theory provided insight into possible failure mechanisms and the influence of the strap prestress level on the structural behavior. It is concluded that the use of these novel stressed elements could represent a viable and durable means of strengthening existing concrete infrastructure.

Small-Column Compression Tests on Concrete Confined by WWF

Tests on 127 × 127 × 381 mm concrete columns were carried out to study the efficacy of welded-wire fabric (WWF) as confining transverse reinforcement in columns. Three different wire spacings and a total of five wire sizes were used. The WWF was placed transversely in columns at close longitudinal spacing. The longitudinal spacing to column width ratio (S/D) varied from 0.1 to 0.3. For each of the five wire sizes, four different S/D ratios were used. This resulted in 20 reinforced column specimen designs. Each design was tested under displacement-controlled uniaxial compression with three identical specimens. The axial stress-strain curves were recorded. For the range of parameters tested, the WWF reinforcement provided varied degree of confinement to the concrete, as reflected in the varied level of enhancement of concrete strength and ductility. The S/D ratio seems to have a more dominant effect on confinement than the volumetric ratio of WWF. From the test results, a relationship between the S/D ratio and the volumetric ratio of WWF was deduced to serve as a boundary separating ductile behavior of the confined concrete from brittle behavior. It is concluded that, with proper design, WWF can be effective in providing lateral confinement to concrete in column construction. Further study using large columns is suggested.

A constitutive model for analysis of reinforced concrete solids

This paper describes formulations intended to expand the applicability of the modified compression field theory for general three-dimensional analysis of reinforced concrete solids. The modified compression field theory, which was specifically developed from tests on cracked reinforced concrete panels, has primarily concentrated on describing the stress-strain response of reinforced concrete under compression and tension stress states. Recent work has been directed at refining finite element programs based on the modified compression field theory for problems in which confinement and lateral expansion effects are important. Proposed stress-strain relationships are based on an orthotropic nonlinear elastic model that acknowledges the asymmetric response of concrete under multiaxial stress conditions. Lateral expansion of concrete subjected to compression is modelled under all stress states. The formulation accounts for complex behaviour such as concrete strength enhancement due to confinement, concrete strength degradation due to transverse cracking, tension stiffening, and crack slip. The models assume short-term, monotonic loading conditions. A sample analysis of reinforced concrete shear wall is presented to illustrate the improved modelling with the consideration of lateral expansion and confinement effects.

Properties of cementitious systems containing silica fume or nonreactive microfillers

The object of this work was to explore the effects of silica fume on the microstructure of hardened paste and of the transition zone between paste and aggregates in concrete. The significance of aggregates as reinforcing fillers and their impact on some properties of the transition zone and bulk paste were resolved. The experimental procedure was based on simultaneous studies of model concretes and paste matrices extracted from fresh model concrete mixes. In addition, continuously graded aggregate concretes were prepared. Three sizes of a nonreactive microfiller (carbon black) and one reactive microfiller (silica fume) were applied. On the basis of microstructural studies and compressive strength tests, it was concluded that the primary effect of silica fume was generated by its physical (microfiller) properties, since the strengthening provided by reactive silica fume was similar to that obtained with nonreactive carbon black of similar size and shape. This effect was more significant from the point of view of the concrete strength enhancement than the chemical (pozzolanic) activity of the silica fume. In concretes containing either silica fume or carbon black, aggregates of high quality could serve as reinforcing filler. This could take place when sufficient densification of the transition zone occurred in the presence of silica fume or carbon black. Significant refinement of pore structure was observed in both types of paste matrices (containing silica fume or carbon black). However, this led to a relatively small influence on the paste strength. Concretes containing reactive silica fume or an inert carbon black microfiller behaved as a composite material, unlike conventional concrete.

The influence of microfillers on enhancement of concrete strength

The research presented in this paper was intended to clarify the nature of concrete strengthening in the presence of silica fume. For this purpose, compressive strength development of high strength concrete and its paste-matrix was investigated. Carbon black was introduced as an alternative microfiller to silica fume. The results indicated that carbon black was effective in modifying the basic concrete-matrix strength relationship to an extent similar to that obtained by silica fume. It is suggested that the mechanism by which silica fume affects the concrete behaviour is of a physical origin and is based on microfiller effect, prior to its action as a pozzolanic material. The microfiller effect is of greater significance to strength enhancement.

Reinforced Concrete Columns Confined by Circular Spirals and Hoops

Twenty seven short concrete columns reinforced with longitudinal steel and circular spirals or hoops were tested to failure under monotonic axial compression. Effects of different variables, such as amount and type of lateral steel, lateral steel spacing, etc., were investigated. The relation between lateral pressure on concrete and concrete strength enhancement and the variation of spiral steel stress and confinement effectiveness coefficient k with respect to the amount of spiral steel were also investigated. Requirements of the ACI 318-89 Building code related to the minimum volumetric ratio ofspiral reinforcement and the maximum spiral pitch of 80 mm were critically examined. An increase in the volumetric ratio of spiral steel was found to significantly improve strength and ductility of confined concrete, the effect on ductility being more pronounced. These and other study results are presented and discussed.