Influence of vertical steel reinforcement on the behaviour of edge restrained reinforced concrete walls

Freshly cast concrete undergoes volume changes due to thermal and shrinkage mechanisms which can be restrained both internally and externally. Externally applied restraint can be either end, edge or a combination of the two. Tensile stresses are induced as a consequence of restraint of volume changes which often result in cracking particularly during early age. These are ‘through’ cracks and are of particular concern in liquid retaining structures, nuclear containment chambers, tunnels and basements where besides being aesthetically unpleasant, such cracks can also result in water leakage, ingress / discharge of harmful chemicals and corrosion of steel reinforcement. This calls for development of a clear understanding of the mechanism of cracking and the factors involved in restraint formation so that appropriate mitigation measures at the design and construction stage can be taken.
Currently available guidance is based on the end restraint cases and has been evolved from experimental and analytical investigations on axially reinforced prisms. In walls and slabs, reinforcement is present in both longitudinal and transverse directions and the influence of transverse reinforcement on cracking has not been analysed and incorporated in existing guidance. Critical review of previous research revealed that cracking behaviour of members subjected to edge restraint is very different from those under end restraint. Therefore, the need to undertake experimental investigation for determining the influence of major influencing factors like vertical steel dowels and members geometry was realized.
This research investigates the restraint of imposed strains in edge restrained members and in particular, experimentally illustrates the influence of vertical steel reinforcement between the restrained (wall) and the restraining (base slab) members on the mechanism of restraint development. The investigation constructed real scale reinforced concrete walls onto reinforced concrete bases and also illustrated why previous studies, which have mostly utilized steel members to restrain the imposed strain, are inappropriate for gaining an understanding of edge restraint as they fail to reflect the heat transfer between the wall and the base. Thickness of the tested walls was also varied in the tests to ascertain the influence of relative geometries of the two members on degree of restraint. Results revealed that the restraint significantly increased in the presence of vertical steel reinforcement. They also showed that restraint increases with time due to the steel reinforcement and decreases in its absence. Moreover, in order to ascertain the significance of the transverse reinforcement, tests on reinforced concrete panels were performed. The panels were subjected to direct tension and the results indicate that when transverse reinforcement was present, the cracking load for the tested specimens decreased by 25 – 30 %, whereas the crack widths and number of cracks increased.
Due to paucity of time and the resources involved in experimental investigation, finite element analysis has been utilized to study the behaviour of walls subjected to combination of end and edge restraint. Parametric study was carried out to investigate the influence of combined restraint on the number and size of cracks and crack widths. By investigating walls of different aspect ratios, the domination of end restraint in higher parts of the walls having lower aspect ratios was found. Through finite element analysis the significance of correctly incorporating the real time boundary conditions of the restraining base slab was also identified.