Integrity of Bored Piles Under Tension

Instrumented pile loading tests can give a better understanding about the pile load-transfer mechanism. It gives also
opportunity to assess the integrity and durability issues of the bored pile under tension. When a reinforced concrete
bored pile subjected to high tension forces, it might reaches to tensile stress capacity of concrete and cracking occurs
at a weakest section. The formation of cracks influences the load-displacement behaviour of piles under tension load
and it causes difficulties during interpretation of pull-out tension test results. The full shaft resistance capacity can
only be taken into account when it reaches the required relative displacement along the pile for fully mobilized shaft
resistance and crack deformation. Especially for long piles this cannot be achieved in the acceptable pile
displacement range. Thus there are different tendencies in various design manuals to limit the pile capacity under
tension by applying higher safety factors or by lowering the resistance factors. However, these approaches might not
always guarantee crack free design under service load.
In this study an instrumented pile pull-out tension test results are briefly presented from crack formation and
propagation aspect. Crack opens when the tensile deformation from applied loads reaches the tensile deformation
capacity of concrete (Borosnyoi and Balazs, 2005, Bicocchi, 2011). In addition to this study, authors have also
experienced between 50-150 microStrain values for crack formation at piles during pull-out tension test with
different concrete characteristics and different steel content (Inanir, 2018). Bicocchi, (2011) also reported similar
findings with strain gauge measurements while flexural bending crack formation at piles. The position of the SG and
the influence zone of the cracked section might affect the measured value during first crack formation.
It might also be a good practice in design to limit the elongation of reinforced concrete body of the pile which is
subjected to tension. In other words limiting the mobilized strain (between 50-100strain) in the pile for crack free
design might ensure an improved pile performance and material durability, but on the other hand this is preventing
from taking advantage of the reinforcement tensile capacity.