PhD Abstract

Rajendra Kumar Varma

ABSTRACT

This work deals with material modelling and numerical implementation for nonlinear finite

element analysis of reinforced concrete (RC) structures. Since the behaviour of concrete

and steel being crucial for any RC structure under loading, uniaxial cyclic constitutive

models for both were implemented in FEMIX, finite element software.

Various advanced materials have been developed with specific purposes, like fibre

reinforced concrete (FRC) to improve the resistance to cracking and crack propagation,

carbon fibre reinforced polymer (CFRP) for strengthening and retrofitting. The post

cracking behaviour of FRC can range from strain-softening to strain-hardening. A

parametric study was undertaken to study the effect of post-cracking behaviour of FRC

sections, with the aim of proposing design methodology using DOCROS, software for

design of cross-sections. In the ambit of parametric study, DOCROS and post-processing

software to estimate force-deflection relationship of the beams failing in flexure were

developed further.

To improve the confinement of RC columns, embracing them with wet lay-up CFRP sheets

is one of the modern and superior techniques. The analytical relationship was proposed for

uniaxial constitutive model of CFRP confined concrete under monotonic and cyclic

loading. For this analytical approach, an extensive database derived from experiments

carried out by various researchers was used, and the proposed cyclic constitutive law was

implemented in FEMIX and validated with experimental tests carried by independent

research.

The uniaxial constitutive laws of concrete, steel and CFRP confined concrete were

implemented in FEMIX under fibre model, which is based on Timoshenko beam theory.

The fibre model assumes that the cross-section can be divided into longitudinal fibres of

steel, concrete, CFRP confined concrete etc, and perfect bind exist among fibres. However,

perfect bond between concrete and steel cannot be guaranteed always, hence to idealize

such material interface, bond-slip laws are developed. Special procedure is followed to

simulate interface behaviour in combination with fibre approach, which is critical to

simulate bar-slippage and pinching of columns.

A biaxial concrete model under the framework of the fracture mechanics was developed, to

simulate the RC elements under plane stress field. The concrete response is described by a

nonlinear orthotropic model, whose axes of orthotropy are parallel to the principal strain

directions. Equivalent uniaxial stress-strain relationships of the concrete are used in

orthogonal direction in scope of rotating crack model. To supplement it, a smeared

reinforcement model for steel bars is also developed, in context of plane stress

elements/Mindlin shells. All the implemented models were validated with experimental

results.