Influence of confinement on high strength concrete behavior

An experimental program has been made in order to study the confined concrete behavior when its strength changes from traditional values to high strength values and with confinement levels ranging from 0% to values higher than 4%. The specimen shape and size have also been included as variables. With the data experimentally achieved, the parameters that define the stress-strain curve for concrete were adjusted using a statistical methodology that gives us suitable approximation levels. A stress-strain curve model is proposed, which lets us know precisely the confined concrete behavior up to high strain levels and analyze the material ductility. The achieved improvement, thanks to the confinement, was quantified regarding the parameters that define the concrete behavior, particularly the maximum strength, the strain at the peak and the ductility.

---

Résumé Un programme expérimental a été développé pour étudier le comportement du béton confiné avec une teneur en armature transversale de 0% à 4%. L'étude analyse des bétons de résistance entre 25 MPa et 100 MPa. D'autres variables ont été la forme et la dimension des éprouvettes. Avec les résultats expérimentaux obtenus on a réalisé une analyse statistique pour définir les paramètres de la courbe Contrainte-déformation avec une bonne corrélation. On a proposé un modèle de courbe Contrainte-déformation qui montre le comportement du béton à hautes performances confiné jusqu'à des niveaux de déformation très élevés, et qui permet l'analyse de la ductilité du matériau. Les performances obtenues grace au confinement ont été quantifiées d'après les paramètres qui définissent le comportement du béton, en particulier de sa résistance, de la déformation au pic et sa ductilité.

     

Adequately FRP confined reinforced concrete columns under axial compressive monotonic or cyclic loading

The study presents the experimental behavior of reinforced concrete square section specimens, externally confined by carbon or glass Fiber Reinforced Polymer (FRP) sheets. The columns are subjected to axial compressive monotonic or repeated load-unload cycles gradually increasing up to failure. The research focuses on columns with longitudinal bars which are critical to premature buckling while examining their effect on lower limit cases of strengthening through FRP confinement. Experiments include also plain concrete FRP confined columns and columns with bars adequately supported by transverse steel reinforcement for comparison. External FRP strengthening covers a wide range of volumetric mechanical FRP confinement ratios allowing comparative investigations. A significant variation in the behavior of FRP confined concrete comes up when bars are unstable, for a light external strengthening scheme as well as for monotonic or cyclic loading. The lower limits proposed by existing recommendations for adequate FRP confinement strengthening of columns are examined.     

Influence of curing regimes on compressive strength of ultra high performance concrete

The present paper is aimed to identify an efficient curing regime for ultra high performance concrete (UHPC), to achieve a target compressive strength more than 150 MPa, using indigenous materials. The thermal regime plays a vital role due to the limited fineness of ingredients and low water/binder ratio. By activation of the reaction kinetics, the effectiveness of the binder is enhanced which leads to improvements in mechanical as well as durability properties. The curing cycle employed are ambient air curing, water curing and hot air curing. The specimens were exposed to thermal regime at (90°C/150°C/200°C) for duration of 24, 48 or 72 hours at the age of 3rd and 7th day followed with air curing or water curing till 28 days. The results showed a marked difference in compressive strength ranging from 217 to 142 MPa with change in curing regimes. The samples when thermally cured at the age of 3rd and 7th day produced an average ultimate strength of 217–152 MPa and 196–150 MPa, respectively.     

Influence of overlap configuration on compressive behavior of CFRP-confined normal- and high-strength concrete

This paper presents the findings of an experimental investigation on the effect of overlap configuration on carbon fiber-reinforced polymer (CFRP)-confined normal- and high-strength concrete. A total of 33 specimens were prepared and tested under monotonic axial compression. All specimens were cylinders with 152 mm diameter and 305 mm height and confined by CFRP tubes. Two different concrete mixes were examined, with average compressive strengths of 52.0 and 84.7 MPa. The effect of overlap configuration was examined by manufacturing the specimens with different properties at the overlap region including overlap length, continuity and distribution. Axial and lateral behavior was recorded to observe the axial stress–strain relationship and hoop strain behavior for concentric compression. Ultimate axial and lateral conditions are tabulated and stress–strain curves have been provided. Detailed plots of hoop strain development and lateral confinement pressure at ultimate are presented. The results indicate that FRP overlap length has no significant influence on strain enhancement ratio (ε_cu/ε_co), but an increase in overlap length leads to a slight increase in strength enhancement ratio (f′_cc/f′_co), with these observations equally applicable to both continuously and discontinuously wrapped specimens. The results also indicate that continuity of the FRP sheet in the overlap region has some influence on the effectiveness of FRP confinement. Furthermore, it was observed that the distribution of FRP overlap regions for discontinuously wrapped specimens can influence the axial compressive behavior of these specimens in certain overlap configurations. Finally, it is found that the distribution of lateral confining pressure around specimen perimeter becomes less uniform for specimens with higher concrete strengths and those manufactured with overlap regions that are not evenly distributed.     

Behavior of square and rectangular ultra high-strength concrete-filled FRP tubes under axial compression

This paper presents results of an experimental program undertaken to investigate the behavior of square and rectangular ultra high-strength concrete (UHSC)-filled fiber reinforced polymer (FRP) tubes (UHSCFFTs) under axial compression. The effects of the amount of confinement, cross-sectional aspect ratio and corner radius were investigated experimentally through the tests of 24 concrete-filled FRP tubes (CFFTs) that were manufactured using unidirectional carbon fiber sheets and UHSC with 108 MPa average compressive strength. As the first experimental investigation on the axial compressive behavior of square and rectangular UHSCFFTs, the results of the study reported in this paper allows a number of significant conclusions to be drawn. Of primary importance, test results indicate that sufficiently confined square and rectangular UHSCFFTs can exhibit highly ductile behavior. The results also indicate that confinement effectiveness of FRP tubes increases with an increase in corner radius and as sectional aspect ratio approaches unity. It is found that UHSCFFTs having tubes of low confinement effectiveness may experience significant strength loss along the initial portions of the second branches on their stress–strain curves. Furthermore, it is observed that the behavior of UHSCFFTs at this region differs from their normal-strength concrete counterparts and is more sensitive to the effectiveness of confining tube. The second half of the paper presents the performance assessment of the existing FRP-confined concrete models in predicting the ultimate conditions of the HSC and UHSCFFTs. The results of this assessment demonstrate that the existing models provide unconservative estimates for specimens with higher concrete strengths. To address this, a new model that was developed on the basis of a comprehensive experimental test database and is applicable to both NSC and HSC of strengths up to 120 MPa is proposed. The model comparisons demonstrate that the proposed model provides significantly improved predictions of the ultimate conditions of FRP-confined HSC compared to the existing models.     

Critical parameters for the compressive strength of high-strength concrete

This study investigates the influence of several material properties underlying the failure mechanism of high-strength concrete (HSC) under uniaxial compression. An experimental-numerical characterization of a single inclusion block (SIB) – an idealized composite comprising of a granite cylindrical core embedded within a high-strength mortar (HSM) matrix – is first carried out. Parametric studies are next conducted with the calibrated SIB model, to identify the critical parameters governing the failure of the idealized composite. The qualitative understanding obtained from the SIB is then utilized to design a series of experiments, exploring the extent of influence of the identified critical parameters on the compressive strength of HSC. Complementary experimental data in literature are also examined. For the range of specimens considered, it is found that the lateral strain capacity of mortar matrix has the most influence on the compressive strength of HSC.     

新型超高强Al-Zn-Mg-Cu合金热压缩变形行为及微观组织特征

利用Gleeble-1500热模拟试验机对新型超高强Al-Zn-Mg-Cu-Zr-Sc合金进行高温热压缩实验,研究该合金在变形温度370～460℃、应变速率0.001～10s^-1条件下的流变应力以及变形过程中的显微组织。结果表明:流变应力在变形初期随着应变的增加迅速增大,出现峰值应力后逐渐下降并达到稳态,流变应力随着应变速率的增大而增大,随着变形温度的升高而下降;流变应力可以采用双曲正弦形式的关系来描述,通过线性拟合计算出该材料的形变激活能等参数,获得流变应力的本构方程。随着变形温度升高和应变速率降低,原始晶粒变形程度显著增加,再结晶分数明显上升。     

New formulation for compressive strength of CFRP confined concrete cylinders using linear genetic programming

This paper proposes a new approach for the formulation of compressive strength of carbon fiber reinforced plastic (CFRP) confined concrete cylinders using a promising variant of genetic programming (GP) namely, linear genetic programming (LGP). The LGP-based models are constructed using two different sets of input data. The first set of inputs comprises diameter of concrete cylinder, unconfined concrete strength, tensile strength of CFRP laminate and total thickness of utilized CFRP layers. The second set includes unconfined concrete strength and ultimate confinement pressure which are the most widely used parameters in the CFRP confinement existing models. The models are developed based on experimental results collected from the available literature. The results demonstrate that the LGP-based formulas are able to predict the ultimate compressive strength of concrete cylinders with an acceptable level of accuracy. The LGP results are also compared with several CFRP confinement models presented in the literature and found to be more accurate in nearly all of the cases. Moreover, the formulas evolved by LGP are quite short and simple and seem to be practical for use. A subsequent parametric study is also carried out and the trends of the results have been confirmed via some previous laboratory studies.     

Effect of steel and synthetic fibers on flexural behavior of high-strength concrete beams reinforced with FRP bars

Six high-strength concrete beam specimens reinforced with fiber-reinforced polymer (FRP) bars were constructed and tested. Three of the beams were reinforced with carbon FRP (CFRP) bars and the other three beams were reinforced with glass FRP (GFRP) bars as flexural reinforcements. Steel fibers and polyolefin synthetic fibers were used as reinforcing discrete fibers. An investigation was performed on the influence of the addition of fibers on load-carrying capacity, cracking response, and ductility. In addition, the test results were compared with the predictions for the ultimate flexural moment. The addition of fibers increased the first-cracking load, ultimate flexural strength, and ductility, and also mitigated the large crack width of the FRP bar-reinforced concrete beams.     

Flexural ductility of high-strength concrete columns with minimal confinement

The use of high-strength concrete (HSC) instead of normal-strength concrete (NSC) in columns has the advantage of allowing the column size to be reduced and is thus becoming popular. However, since HSC is more brittle than NSC, its use could result in undesirable brittle failure. To evaluate the ductility of columns, nonlinear moment–curvature analysis taking into account the stress-path dependence of the steel reinforcement is required. Based on such analysis, a parametric study has been conducted to investigate the effects of various factors on the ductility of columns. The results revealed that the effect of concrete strength is dependent on the axial stress level (axial load to area ratio) and axial load level (axial load to capacity ratio). At the same axial stress level, the use of HSC has little or basically no adverse effect on the ductility but if the same axial load level is maintained to reduce the column size, the use of HSC would significantly reduce the ductility. Finally, two formulas for direct evaluation of the ductility of columns are developed.     

Prediction of compressive strength of concrete

The paper, which summarises a survey and an analysis of empirical strength laws, was undertaken with a view to formulating a strength law interrelating the water-cement ratio-which determines the porosity of the hardened cement-paste-and Powers's Gel/Space ratio, giving the relative strength of the cement stone. Accordingly, a graphical procedure is given for predicting the strength of ordinary concretes for a given water-cement ratio and lean air-entrained concretes for a given “equivalent” water-cement ratio. Compressive strength may also be predicted by means of numerical formulae.     

Axial compressive behavior of concrete actively confined by metal strips; part A: experimental study

This paper presents the results of an experimental study on the application of strapping technique for retrofit of concrete compressive specimens. In this technique, standard strapping devices, which are used in the packaging industry, are applied to post-tension high strength metal strips around the concrete columns. Experimental program included axial compressive tests on 72 cylindrical and prismatic compressive specimens, which were actively confined by pre-stressed metal strips. The effects of various parameters on strength and ductility of confined concrete were studied including compressive strength of concrete, mechanical volumetric ratio of confining strips, post-tensioning force in the strip, number of strip layers wrapped around the specimens and details of strip joint. The effects of strength and ductility of confining strips on the behavior of confined specimens were also studied. Longitudinal and lateral strains of concrete and strain of the strips were monitored. Test results showed significant increase in the strength and ductility of specimens due to active confinement by metal strips. It was observed that ductility of confining material plays the most important role in enhancement of concrete ductility. The gain in strength is strongly dependent to the effective mechanical volumetric ratio of confining strips. It was also observed that the active confinement of concrete by post-tensioning the confining element results in stiffer pre-peak response of concrete specimens than the usual passive confinement.     

Influence of free edge stress concentration on effectiveness of FRP confinement

The use of fiber-reinforced polymers (FRP) composite materials have been widely used and have been extensively studied in the last decades in the form of jacketing to enhance axial strength as well as ductility of confined columns. Their effectiveness has been extensively proven in many research programs investigating confined concrete column behavior. Despite a large research effort, a proper analytical tool to predict the behavior of FRP-confined concrete has not yet been established due to the many factors jeopardizing the reliability of such tools. The aim of the present study is to analyze the effect of interlaminar stresses at the free edge of the FRP jacket to determine a potential reason for the premature failure of the strengthening system. Numerical examples and a parametric study are presented to illustrate the governing parameters that control the stress concentrations at the edge of the FRP jacket in the overlapping zone.     

The increase in peak strength and strain in confined concrete for a wide range of strengths and degrees of confinement

By statistically processing a great number of experimental data (about 130) from the literature, we have pointed out that the main relations proposed by various authors to express the increase in strength and strain of concrete at the peak due to confinement agree with the physical reality only if parameters involved over a limited range are included. This has led us to propose a relation which is capable of estimating the increase in strength and strain of concrete due to confinement and which is valid up to very high strengths, for a wide range of confinement and for any specimen shape.

---

Résumé Le traitement statistique d'un grand nombre de données (environ 130) provenant de la littérature expérimentale nous a fait remarquer que les principales relations proposées par divers auteurs pour exprimer l'augmentation de la résistance et de la déformation du béton à la charge de pointe en raison du confinement s'accordent à la réalité physique uniquement dans le cas où les paramètres concernés sont d'une étendue limitée. Cela nous a amené à proposer une relation pour estimer l'augmentation de la résistance et de la déformation due au confinement qui est valable jusqu'à de très hautes résistances, ainsi que pour de larges étendues de confinement, et qui s'applique quelle que soit la forme de l'échantillon.

---

---

Editorial note Prof. E. Pozzo is a RILEM Senior Member.