STUDY OF CONCRETE FILLED UPVC TUBES AS COLUMNS IN SEVERE ENVIRONMENT

Abstract

It is difficult to achieve long term durability of reinforced concrete structure in coastal areas. Sea water deteriorates concrete structures due to combined effect of chemical action of sodium chloride and magnesium sulfate present in it. Concrete structures older than 10-15 years show a considerable amount of deterioration in the form of cracking & spalling of concrete and corrosion of reinforcement. This major problem has led to the development of new construction materials which could protect the structure against the cause of deterioration. Concrete filled plastic tubes (CFPT) can be treated as a new construction material which could safeguard both concrete and reinforcement by providing a skin. Plastics have exceptional properties, such as high resistance to severe environmental attack, low cost, and high strength to weight ratio. Their service life goes beyond fifty years. UPVC is a plastic material which is being used for a wide variety of civil engineering applications. UPVC pipes are generally employed for water supply across the world. These pipes are designed for induced internal pressure of flowing water. In this thesis an experimental, numerical and analytical study has been carried out. In which UPVC pipes are filled with concrete and tested as compression members for their suitability for coastal area. Tested 114 specimens were categorized into three categories viz plain concrete filled UPVC tube (PCFUT), reinforced concrete filled UPVC tube (RCFUT) and bare reinforced concrete column (RCC). Bare R.C.C specimens were obtained from RCFUT specimens by removing the outer tube. Casting of specimens was carried out in two stages. In first stage, 90 specimens were cast while in second stage 24 specimens were cast. For the first stage of casting, the UPVC tubes of class 3, 4 and 5 (as per IS:4985-2000) with nominal pressure of 0.6, 0.8 and 1.0 MPa having diameters of 160 mm, 200 mm and 225 mm were taken. For second stage of casting, UPVC tubes of 160 mm diameter of class 1 with nominal pressure of 0.25 MPa were taken. The internal hydrostatic pressure tests were conducted to get the confining pressure of UPVC tube. A numerical simulation study was also carried out using Finite element method by development of three-dimensional Finite element model using ‘ANSYS’ software. The concrete core, UPVC tube, and reinforcement are modeled and then bond are created between them to simulate the modeling of concrete filled UPVC columns. Suitable material constitutive models for UPVC tube, concrete core and reinforcement are adopted. Numerical results were compared with experimental results in the form of load-displacement curves and associated ii modes of failure. Based on the results, it may be concluded that the proposed models can predict the structural behavior and load carrying capacity of such columns with good accuracy. In the study of PCFUT, a stress-strain model for concrete confined with UPVC tube is proposed. The effect of diameter to thickness ratio (D/t) on the failure stress of confined concrete is obtained and discussed with an empirical relationship. To calculate the load carrying capacity of concrete filled UPVC tubular columns, an equation is proposed. An analysis is done to understand the failure mechanism of confined concrete with the help of Mohr-Coulomb failure criterion. Inclination of failure plane for different confining pressure is calculated. The results are verified with experimental results. Since the strength increment due to confinement given by UPVC tube is only 5-10 MPa. Therefore it necessary to opt reinforced concrete to fill the UPVC tube to get sufficient strength of column. Since transverse reinforcement in the form of lateral ties may also provide additional confinement to the concrete core in the form of arch action. This action becomes more effective when the spacing between the lateral ties is smaller, therefore lateral ties are provided at higher spacing to avoid the confining effect. RCFUT column specimens were cast by filling the reinforced concrete in UPVC tube. 50% of RCFUT specimens were converted to bare RCC specimens by removing UPVC tube. A specific study was carried out to understand the confinement through transverse reinforcement in reinforced concrete column under axial compression, a three dimensional Finite element model is developed using ANSYS software and the results are also verified with experimental results. Total eighteen specimens of different diameters and lengths were tested to obtain load-displacement variations. A parameter Atie/S2 (ratio of cross-sectional area of lateral tie to the square of spacing between lateral ties) was taken to check the effective confinement of concrete core in R.C. column. It was concluded that if Atie/S2 is greater than 0.02 then confinement model of concrete proposed by Mander et al. (1988-a) may be used and if it is less than 0.02 then concrete should be modeled as unconfined. In present study, the spacing between lateral ties was more, therefore an unconfined stress-strain model of concrete was found suitable for the modeling of reinforced concrete columns. Even there was no strength gain observed due to confinement though lateral ties but there was an enhancement in the ductility. Some experiments were carried out to study the behaviour of reinforced concrete inside UPVC tube and bare reinforced concrete exposed to artificially made sea water. Compressive strength, ductility, energy absorption capacity and modes of failure of bare RCC and RCFUT column were obtained from experimental data. It was found that concrete showed an increment in iii strength, ductility and energy absorption capacity when UPVC tube was used as outer layer for RCC. In composite column like RCFUT, outer tube may work as a wall between concrete and aggressive environment. UPVC tubes used in composite column (like RCFUT column) may protect the column from corrosion of steel and degradation of concrete against the attack of salts presents in sea water. Moreover, UPVC tubes may work as a weather proof jacket to the concrete during the whole year. When RCFUT specimens with class 1 UPVC pipes were submerged in different concentration of artificial sea water for different durations, it was found that degradation in concrete was almost negligible. Therefore it may be concluded that the UPVC pipes with minimum nominal/working pressure (0.25MPa) may also be used as the protective layer for concrete against sea water. Some token specimens were cast and tested to get the strain maps at different stage of loading with the use of Digital Image Correlation (DIC) system. The digital images, which were recorded during testing of specimen, were processed using Vic-3D commercial code to obtain strain map. These strains are compared with the strains obtained from simulations. The strains obtained from Vic-3D and ANSYS are almost equal at the maximum displacement stage. It was observed that the strains on the surface of specimen along the failure plane were increased while at other locations the strains were nearly equal during compression process between peak load to failure load.