Effects of early age differential displacements on concrete–bar bond in the connections of staged constructions

Differential displacements are commonly induced between precast reinforced concrete members during construction such as in connections on staged bridge construction projects. The ultimate bond strength between reinforcing bars (rebar) and an embedment material used in the connections may be affected. A pullout test was devised to compare the early age ultimate bond strength developed in specimens with and without differential displacements. A range of different grout and concrete materials (embedment materials) used within typical connections were tested. Maximum relative displacements at right angles to #13M (13 mm diameter) rebar ranged from 2.5 to 0.13 mm (a bar diameter–displacement ratio ranging from 10 to 200). These displacements were imparted from casting until the embedment materials reached final set. Pullout testing on both static and displaced test specimens was subsequently performed at approximately 24 h after casting. When the rebar displaced 1.3 mm or more (a bar diameter–displacement ratio of 20 or less), reduced bond capacity was observed in all embedment materials tested. When the rebar displaced 0.25 mm or less (a bar diameter–displacement ratio of 100 or more), the results indicate insignificant variations in bond strength regardless of embedment material type. These results show that large displacements applied prior to final set of the embedment material can have a detrimental effect on the pure bond strength.