confined concrete
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This paper presents the results of an experimental study on the biaxial compression behavior of concrete prism confined using pre-stressed bars. The pre-stressed bars could provide passive confinement stress, that preventing the lateral strain of the prism from increasing leading to an increase in both the initial modulus of elasticity and prism compressive strength. The confined concrete had a higher compressive strength that was directly proportional to the confinement bar pressing force and lower ductility than the plain prisms. The concrete initial modulus of elasticity is directly proportioned to the confinement lateral pressure of the prestressing bar and inversely proportion with the spacing between prestressing bars. It was simple to find out that the best pre-stressing stress was 10 N/mm2, also the compressive strength of the confined concrete with pre-stressed skew bars was greater than the compressive strength of the unconfined concrete by more 3.3 times.

Axial compression tests were carried out on 72 FRP (fiber reinforced polymer)–stirrup composite−confined concrete columns. Stirrups ensure the residual bearing capacity and ductility after the FRP fractures. To reduce the effect of stress concentration at the corners of the confined square−section concrete columns and improve the restraint effect, an FRP–stirrup composite−confined concrete structure with rounded corners is proposed. Different corner radii of the stirrup and outer FRP were designed, and the corner radius of the stirrup was adjusted accurately to meet the designed corner radius of the outer FRP. The cross−section of the specimens gradually changed from square to circular as the corner radius increased. The influence of the cross−sectional shape and corner radius on the compressive behaviour of FRP–stirrup composite−confined concrete was analysed. An increase in the corner radius can cause the strain distribution of the FRP to be more uniform and strengthen the restraint effect. The larger the corner radius of the specimen, the better the improvement of mechanical properties. The strength of the circular section specimen was greatly improved. In addition, the test parameters also included the FRP layers, FRP types and stirrup spacing. With the same corner radius, increasing the number of FRP layers or densifying the stirrup spacing effectively improved the mechanical properties of the specimens. Finally, a database of FRP–stirrup composite−confined concrete column test results with different corner radii was established. The general calculation models were proposed, respectively, for the peak points, ultimate points and stress–strain models that are applicable to FRP−, stirrup− and FRP–stirrup−confined concrete columns with different cross−sectional shapes under axial compression.

This research investigates the efficiency of using Flax Fibers reinforced bio-sourced polymer by comparison to traditional system based on Carbone Fiber Reinforced Epoxy Polymer in order to confine recycled aggregates concrete. Four concrete formulations have been formulated by incorporating recycled aggregates from demolition waste (0%, 30%, 50% and 100%). An air-entraining agent was added to the formulations to achieve the level of 4% occluded air. The main objective is to discuss and to evaluate the effectiveness of confining them using bio-sourced composite by comparison to traditional ones. To hit this target, the developed approaches are both experimental and analytical. The first part is experimental and aimed to characterize the mechanical behavior of the materials: the composites used in the confining process the unconfined concrete (effect of incorporating recycled aggregates on the overall mechanical characteristics). We establish that bio-sourced composites are efficient in strengthening recycled aggregates concrete especially if they are air-entrained. The second part of this work is dedicated to analytical modeling of mechanical behavior of confined concrete with composite under compression based on Mander’s model. The input parameters of the model were modified to consider the rate of recycled aggregates incorporation. Comparison between experimental results and the modified Mandel’s Model is satisfactory.

This paper presents an experimental study on 11 weak PVC-FRP Confined Concrete (PFCC) Column-strong Reinforced Concrete beam joints reinforced with Core Steel Tube (CST) subjected to axial load. The influences of the joint height, joint stirrup ratio, Carbon Fiber Reinforced Polymer (CFRP) strips spacing, steel ratio and CST length on the failure mode, ultimate strength, and strain behavior of specimens are analyzed and discussed. Test results indicate that the failure mode of specimens is distinguished by the cracking of PVC tube, fracture of CFRP strips, yielding of stirrups, and longitudinal steel bars in the PFCC columns. Both the longitudinal steel bars and CST yield at the joint area, while there is no obvious damage on the joint. The ultimate stress of specimens decreases with the increment of CFRP strips spacing, while the other studied variables have little impact on the ultimate stress. As the CFRP strips spacing increases, the ultimate strain of specimens decreases, and the strain development accelerates. Considering the effect of joint dimension, a modified prediction model for the stress–strain relationship of axially loaded weak PFCC column-strong RC beam joints reinforced with CST is proposed and verified with good accuracy.

Numerous studies have indicated the existence of size effect on axial compression behavior of stirrup-confined concrete columns. However, most of these studies have been stressed in terms of nominal compressive strength. The investigation on the size effect of axial strain (at peak load) and descending branch was limited. In this study, the size effect behavior of square stirrup-confined concrete columns under axial compression was explored, by using 3-D mesoscale simulation method. Based on the numerical and available experimental results, the influence of specimen size on the peak axial stress (i.e., the compressive strength), the corresponding strain and the softening rate were explored. Moreover, the quantitative relationships between specimen size and the peak axial stress, the corresponding strain and the softening rate for circular and square stirrup-confined concrete columns were derived. Finally, considering the size effect of peak axial stress, the corresponding strain and the softening rate, a novel stress-strain model describing the axial compression behavior of stirrup-confined concrete was developed. The proposed model was verified by comparing with the available experimental results and the existing models provided.