The inherent inferior quality of recycled coarse aggregate (RCA) influences the microstructural characteristics and consequently, the macro-mechanical properties of recycled aggregate concrete (RAC). The present paper investigates the influence of aggregate properties, degree of hydration (), and micro and meso level characteristics of concrete on its compressive strength. Moreover, the influence of different mix design methods (conventional and Particle Packing Method) and mixing approaches (normal mixing approach and two stage mixing approach) on , and micro and meso level properties of concrete are analysed. In addition to the crushing value and water absorption of coarse aggregate, thermogravimetric analysis, nanoindentation and image analysis of back-scattered electrons images and X-ray microtomography images are performed to measure , interfacial transition zone (ITZ) thickness, voids content in the ITZ and interface of concrete, respectively. However, none of these parameters can be singled out to demonstrate its major or significant contribution to the compressive strength of concrete. Hence, the influence of each parameter must be appreciated. An expression is proposed by accounting each of these parameters and also the cement content and coarse aggregate fraction to predict the compressive strength of concrete, which exhibits good correlation with the experimental results.
The present investigation is primarily aimed to study the effects of confinement of Glass fibre reinforced polymer (GFRP) and specimen size on the recycled aggregate concrete (RAC). The RAC is prepared by complete replacement of natural coarse aggregates (NCA) with recycled coarse aggregates (RCA). Strain gauges are used to study the variations of axial and lateral strain with axial stress. To examine the size effect the experimental investigation is carried out on specimen size of diameters 50, 100 and 150mm with constant aspect ratio of two. A comparative study has also been drawn between NAC and RAC based on various parameters such as strength, effect of confinement on the stress-strain behaviour, effect of specimen size on strength and strain enhancement. Study reveals that there is effective load transfer from concrete to GFRP jacket resulting in improved strength in confined specimen. The size effect is more pronounced on strength enhancement of RAC compared to NAC for unconfined concrete. However, for confined concrete, pronounced size effect is observed in NAC over RAC on both strength and strain enhancement.
Recycling of concrete wastes obtained from construction and demolition of existing old structures to obtain aggregates have assumed importance in the recent times to pave the way towards a viable method to curb the problems associated with the shortage of resources as well as to provide a sustainable solution for the issue concerning disposal of construction wastes. One of the major problems associated with disposal of these wastes is the unavailability of proper dumping sites. Further, the improper disposal leads to large chunks of debris and dust in the air which promotes respiratory problems. Earlier, several investigations have been carried out to study the potential of recycled aggregates particularly, recycled coarse aggregates (RCA) to utilize for construction purposes [, , , , , , , ]. Researches have shown that RCA can act as an economical and suitable alternative to NCA for use in construction works.
Literature [3,5,9] state that the existing mortar adhered to the aggregate surface greatly determines the quality of RCA and hence, the quality of recycled aggregate concrete. The adhered mortar is generally a porous material and the quantity of adhered mortar depends on the crushing procedure employed as well as on the dimensions of the recycled aggregates. This further affects the water absorption capacity and the durability performance of aggregates for use in structural applications . Due to the higher porosity, the Recycled aggregate concrete (RAC) has a lower density as compared to conventional concrete. The replacement amount of RCA affects the initial slump value and hence the workability. Research studies [11,12] suggest pre-soaking of recycled aggregates and using the aggregates in saturated surface dry (SSD) condition to maintain proper workability during the mixing. In terms of mechanical properties, generally natural aggregate concrete (NAC) shows better performance as compared to RAC. There is no significant change in the compressive strength is observed by Kwan et al.  up to 30% replacement of NCA with RCA. However, at 100% replacement around 10% reduction of compression strength value is observed as stated by Rahal . In the similar manner, no change in split tensile strength values were observed for lower incorporation ratio while there was 10% reduction for 100% replacement . However, no change is observed in the flexural behavior of RAC beams. Further, it is noticed that the modulus of elasticity of RAC is generally lower than that of NAC . The higher porosity of adhered mortar makes the RAC less durable, however research suggest improvement in behavior by the use of superplasticizers [17,18].
In the recent years, the use of confinement of concrete structural member particularly for concrete column gaining popularity due to strengthening and rehabilitation purpose. Confinement of concrete members is provided using ties or spirals which provide lateral pressure thereby increasing the strength, ductility as well as retrofitting the old, cracked and damaged concrete after exposure to fire or being subjected to earthquake [19,20]. Further, it helps improve the energy absorption capacity of concretes specimens under axial loading . In order to achieve improved mechanical properties, wrapping of the concrete specimens by steel plates, fiber reinforced polymers or cement coating may be implemented. Hadi et al.  studied the comparative behavior of steel strands and CFRPs (carbon fiber reinforced polymers) in enhancing the behavior of concrete and reported that maximum load capacity was shown by those specimens with CFRP wrapping. It is further stated that the better efficiency is seen for a circular cross section as compared to square columns. Study by Benzaid et al.  suggest that the effect of confinement can be enhanced by increasing the stiffness by means of applying multiple layers FRP jackets.
Recent studies have suggested the use of FRP (Fiber reinforced polymer) composites as confining material around concrete members because of the light weight, high strength and the relatively easier wrapping methodology. FRPs primarily distribute the load along the length of the fibers and provide strength and stiffness. Sezen and Miller  state an improvement of 140% in axial behavior over non-retrofitted or base column with the use of FRP. Further, a brittle failure was observed after attaining the axial capacity. Theriault et al.  studied the influence of slenderness ratio and specimen size on the behavior of axially loaded FRPs and no observable influence of slenderness ratio was seen on medium and large scaled specimens. Yousseff et al.  proposed a unified FRP confined concrete model and report a bilinear stress-strain curve for concrete wrapped with FRP composites. Berthet et al.  studied the various parameters influencing the FRP confined concrete and stated that while the curve is bilinear in the first part, in the plastic zone, the curve depends on the stiffness of the jacket. They further report that ultimate strength of jacket and concrete strength are the most influential parameters in determining the strain and ultimate strength of confined concrete. Similarly, Zhao et al.  studied the effect of FRP confinement on RAC and NAC and found that the replacement ratio has a significant effect in determining the stress strain behavior of the composite under axial compression. Further, it was found that the confinement has similar effect on both NAC and RAC and both exhibited similar behavior. Yang and Han  studied the possibility of using both RAC and NAC filled hollow steel tube columns and report similar behavior in both RAC and NAC. Further, they suggest that the same mechanical model applies for RAC and NAC filled steel tube columns.
Chen et al.  studied the behaviour of CFRP confined recycled aggregate concrete under compression and state that although the compressive strength decreases with the use of recycled aggregates, however, the effectiveness of confinement isn't significantly affected by the replacement ratio. However, a study by Xie and Ozbakkaloglu  report that replacement ratio by RCA has a significant effect on ultimate axial strain and compressive strength. They further report that greater strength improvement is obtained with circular cross section. However, this improvement percentage decreases with increase in strength of unconfined concrete for a given confinement ratio. Gao et al.  made a comparative study of GFRP confined and CFRP confined concrete and observed a higher compressive strength and lower ultimate axial strain in case of concrete with CFRP confinement. Toutanji and Balaguru  also state that CFRP is more effective than GFRP in harsh environments. The influence of fibre distribution on the behaviour of FRP confined concrete under axial compressive loading is studied by Vincent and Ozbakkaloglu . The authors observed that maximum strength is obtained when the fibers are oriented in the hoop direction.
Glass fiber reinforced polymers (GFRP) are gaining popularity over the recent years due to their excellent resistive property against corrosion, their lower cost as compared to CFRP and their high strength-weight ratio. Fonseca et al.  suggest a delay in chloride diffusion rate when concrete specimens wrapped with GFRP were kept in a corrosive environment, where GFRP acts as a physical barrier between the chloride solution and the concrete. Sharma et al.  reported a significant increment of failure load in the range of 47149% with the use of GFRP jackets. Further, they concluded that the axial compressive strength of the columns increases with the increase in number of GFRP layers, which is due to the availability of larger area for hoop action. Further, it has been observed in the literatures that brittle failure occurred in all specimens by bursting of GFRP. Raval and Dave  studied how the properties varied with different shapes of columns. The authors concluded that the ductility of circular columns is more compared to rectangular columns as circular columns show greater axial deformation compared to rectangular columns.
In order to make recycled aggregates particularly, recycled coarse aggregates usable for structural applications, study to improve the behavior of RAC is extremely significant in estimating proper design and performances of structural members. The most of the studies on recycled aggregate concrete are mainly directed towards the enhancement of RAC properties. The significance and complexities associated with the behaviour of confined concrete under compressive loading has attracted the attention of many researchers. It is also a well-known fact that the size effect of concrete members is an important aspect. It can be observed from the review of existing literature that the majority of work on effect of confinement and size effect have been carried out on natural aggregate concrete. In light of the current scientific background there are several critical issues motivated this investigation such as: a) experimental investigation on behaviour of GFRP confined RAC under axial compressive loading is scarcely available in published literature, b) to the best of authors knowledge there is no such experimental studies reported in existing literature demonstrating the size effect in RAC, c) the comparative behaviour of confined NAC and RAC is also scarcely available in existing literature. Thus, the experimental procedure adopted in the present investigation is primarily aimed at exploring the influence of GFRP confinement on the characteristics of RAC subjected to axial compression and also on the size effects of recycled aggregate concrete. The present experimental investigation demonstrated the effects of GFRP confinement on the behaviour of recycled aggregate concrete based on different parameters such as peak stress, peak strain, stress-strain characteristics, strength enhancement and strain enhancement in confirming to the Indian standards. For this purpose, both NAC and RAC were wrapped with different number of GFRP layers. The present investigation also compares the characteristics of NAC and RAC with different confinement ratios.
Portland Cement (OPC) of Grade 43 conforming to the specifications of Bureau of Indian Standard Specifications  was used in the study. Standard tests were conducted to characterise the cement. The obtained results are presented in Table 1. The chemical composition of cement was obtained from X-ray Diffraction (XRD) analysis according to the provisions mentioned in ASTM C 1365-06 . The resulting pattern is presented in Fig. 1. The peaks obtained in the XRD pattern of cement denote that
The present experimental investigation study is carried out to understand the characteristics of GFRP-wrapped recycled aggregate concrete and also to establish a comparison between the effects of confinement on NAC and RAC. The presence of the porous adhered mortar in RAC shows higher water absorption, which adversely affects the workability. This has also been reported by several other researchers [, , ]. Hence in order to maintain a certain workability in all the mixes,
The present paper aimed to explore the behavior of GFRP-wrapped concrete made with recycled aggregates produced from C&D wastes. The influence of specimen size on the properties of unconfined and GFRP-wrapped concrete specimens are also investigated for both concrete prepared using RCA as well as NCA. The main conclusions obtained from the present investigation are presented below.1.The adhered old mortar on to the recycled aggregates has high porosity which results in lower density of RAC as
Tanish Dey: Conceptualization, Formal analysis, Writing - original draft, Supervision. Chandra Sekhar Das: Methodology, Investigation, Writing - review & editing. Naval Mishra: Formal analysis, Investigation.
I am submitting the manuscript titled Behaviour of Confined Recycled Aggregate Concrete under Compressive Loading: An Experimental Investigationfor the possible publication in the Journal of Building Engieneering https://www.worldscientific.com/worldscinet/ijssd. The above manuscript has not been submitted to any other journal either in whole or in part. This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
Carbonation is a typical deterioration phenomenon which accelerates steel corrosion through pH drop in pore water by intrusion of external carbon dioxide (CO2). Many researches have been performed on carbonation process considering carbonic reaction, mixture conditions, and exterior conditions. Recently probabilistic approaches have been adopted for considering engineering uncertainties such as complicated material behavior, simple evaluation system, and limited sample numbers. In the study, the service life variation was evaluated considering several effects of induced tensile stress level, cold joint, and GGBFS (Ground Granulated Blast Furnace Slag) through probabilistic approach. Utilizing the previous test results, the effect of loading conditions on carbonation depth was investigated. Four design parameters like cover depth, exterior CO2 concentration, carbonatable material, and CO2 diffusion coefficient were assumed as random variables for probability analysis. Through MCS (Monte Carlo Simulation) with normal distributions and intended PDF (Probability of Durability Failure) of 10%, variations of service life were simulated for the RC structure under loading conditions. The effects of COVs (Coefficient of Variation) for design parameters were also simulated. The comparisons of the predicted service life from the probabilistic and deterministic methods were quantitatively discussed, and the safety index with service life was also investigated. The results of this study can provide useful information on maintenance priority and service life evaluation for RC structures containing cold joint subjected to tensile stress.
This study presents a semi-analytical solution of nonlinear vibrations of circular cylindrical shells made of carbon nanotube (CNT) fiber-reinforced composite (CNT-FRC). Vibrations are produced by a radial harmonic force and viscous structural damping is considered. The effective properties of a lamina of the CNT-FRC shell are evaluated in two steps. The elastic properties of randomly distributed CNTs in a polymeric matrix (i.e., hybrid matrix) are computed by the Eshelby-Mori-Tanaka/Voigt scheme to consider the CNTs agglomeration effect in the hybrid matrix. Then, the resulting hybrid matrix is reinforced with aligned fibers in order to prepare the lamina of the CNT-FRC shell; its effective properties are estimated by the Halpin Tsai homogenization approach. The CNT-FRC shell is modelled incorporating the von Krmn geometric nonlinearity and first-order shear deformation theory (FSDT). The nonlinear governing partial differential equations (PDEs) of the CNT-FRC shells are derived by the Hamiltons principle. These PDEs are discretized into ordinary differential equations (ODEs) by using the Galerkins method. The ODEs are solved by incremental harmonic balance method (IHB) in conjunction with the arclength continuation method to obtain the frequency-amplitude response of the shell. The effect of different types of CNT agglomeration models, CNT mass fraction, agglomeration parameters and stacking sequence of laminates on the frequency-amplitude curves corresponding to forced and free nonlinear vibrations of the CNT-FRC shell are studied in detail.
The analysis of the non-linear vibration response is carried out for functionally graded (FG) circular cylindrical shells subjected to thermal environment along with mechanical in-plane non-uniformly distributed loading along the edges and harmonic radial force. The temperature dependent material properties of the simply supported shell are assumed to vary in the radial direction according to power-law distribution. Based on the first-order shear deformation theory and von-Krmn type geometric nonlinearity, the strain-displacement relationships are established for circular cylindrical shells. The coupled governing equations of motion for functionally graded cylindrical shells are then derived using Hamiltons principle. Employing Galerkins method, the coupled partial differential equations of motions are reduced to a set of non-linear ordinary differential equations. In order to obtain the free and forced vibration response of the FG shell, the incremental harmonic balance method, in conjunction with the arc-length method, is used. The non-uniform in-plane loading is converted to Fourier series and the pre-buckling analysis is performed to determine the stress distribution within the shell. The non-linear frequency-amplitude response is studied to examine the effects of volume fractions of the constituents, static partial edge loadings, thermal loads, and radial periodic loadings.
This paper primarily focused on the seismic capacity of recycled concrete-encased steel (RCES) frame structure. A two-span three-storied RCES frame which is at 1:2.5 scale was conducted to investigate the acceleration time-history curves, acceleration amplification factors, peak restoring force and displacement, energy dissipation capacity, displacement time-history curves, bearing capacity, stiffness degradation and seismic performance evaluation during experiment. Based on the date analyzed compared with other structures, the hysteretic energy dissipation of RCES structure is better than the common reinforced concrete frame but similar to steel reinforced concrete. The pseudo-dynamic analysis method was adopted in this test. Taft, El Centro and Lanzhou synthesized wave were chosen as the input excitations. It is found that RCES structures show good seismic performance during the test. The beam is yield earlier than column, and the strain of node core area is small. The results indicate that the design of RCES frame meets the requirement of strong column weak beam and strong node and strong anchorage.
This paper describes a numerical parametric investigation for the effects of coarse recycled concrete aggregates (RCA) on the time-dependent vertical deflections of reinforced concrete columns under sustained service loads. The deflections were analyzed for interior (concentric) and exterior (eccentric) column loading conditions, with different volumetric replacement of the coarse aggregate, R, concrete strength, column width, column depth, column length, longitudinal reinforcement area, ultimate shrinkage strain, ultimate creep coefficient, and ambient relative humidity. In addition, the quality of RCA was quantified and varied through the water absorption, Arca (viewed as a measure of aggregate porosity), and deleterious material content, Drca (e.g., wood, asphalt). The results show that increased R resulted in larger deflections, and increased Arca and Drca resulted in larger deflections for the same R. High quality RCA with low Arca and Drca resulted in deflections that were not much greater than those for natural aggregate (NA) concrete columns. The increase in deflection due to RCA was larger for columns with larger concrete strength and cross-section dimensions and smaller longitudinal steel ratios, and larger for interior columns than exterior columns with flexural cracking. RCA was also found to have larger effect on the initial column deflections than the time-dependent deflections.
The present study explores the nonlinear stability characteristics of simply supported carbon nanotubes (CNTs) reinforced composite (CNTRC) cylindrical shell panel subjected to combined axial compressive loading and localized heating using semi-analytical approach. The thermomechanical properties of CNTRC panel are considered to be temperature-dependent and are evaluated using extended rule of mixture method. Higher order shear deformation theory and von Krmn type nonlinearity are employed to model the CNTRC cylindrical panel. Two types of localized heating profiles such as rectangular and circular are considered along with full heating. The pre-buckling stresses generated because of applied localized heating are determined using Airys stress function by satisfying the strain compatibility relations. The governing equations for stability problem of CNTRC panel are derived using variational principle incorporating the obtained pre-buckling stresses. The partial differential equations obtained are converted to a set of nonlinear algebraic equations by employing the Galerkins technique. The buckling temperature and nonlinear equilibrium paths are determined by solving the abovementioned equations using appropriate methods. The obtained results using present semi-analytical approach demonstrated the influence of various dispersion profiles of CNTs, CNT volume fraction and heating profiles on the buckling and post-buckling characteristics of CNTRC cylindrical shell panel subjected to thermomechanical loadings.