Time-dependent loss of post-tensioned diaphragm and fin masonry walls

This thesis reports an investigation on time-dependent loss of post-tensioned
masonry box and tee sections representing diaphragm and fin walls, respectively. The
prestress loss due to creep and shrinkage of masonry, and relaxation of steel bars
were quantified separately and the main influencing factors considered were geometry
and masonry type. For each type of masonry three diaphragm and three fin walls
were built to determine prestress loss (decreasing load), creep (constant load) and
shrinkage (zero load). The walls were constructed from undocked clay, calcium
silicate and concrete block units with grade (ii) mortar with cementlime:sand in the proportions
of 1:½:4½ and water/cement ratio of 1.27. Creep and shrinkage were also measured on
unbonded masonry units and mortar prisms for predicting the deformations in the
masonry walls by using a previously developed composite model. The mal;onry units
and mortar prisms were partly sealed to simulate the corresponding volume/surface
ratio of the bonded masonry units and mortar joints in the masonry walls.
The calcium silicate walls exhibited the highest prestress loss, creep and
shrinkage compared with the clay and concrete block diaphragm and fin walls. The
current methods of prediction of prestress loss for masonry are only suitable for
specific types of masonry for which they were developed. On the other hand, the
methods developed for prestressed concrete gave reasonable predictions for all the
masonry types investigated, with one particular method being very accurate. For all test results it was confirmed that long-term deformations were influenced by
geometry, fin walls exhibiting greater deformations than diaphragm walls. The
composite model did not predict shrinkage very well in calcium silicate and concretc
block walls because some moisture in the mortar was absorbed by the masonry units.
As a result the partly sealed unbonded mortar prisms had higher water content than
the mortar bed joint in the walls, and thus a higher shrinkage in the partly sealed
mortar prisms occured. Consequently, when the creep and shrinkage of the partly
sealed mortar prisms was applied to the model, the masonry deformation was
overestimated.
A modified water absorption test was carried out which confirmed that for
units laid dry the mortar bed joint had a reduced shrinkage compared to the unbonded
mortar prisms. From the results, creep and shrinkage adjustment factors were
correlated with unit water absorption, and when adjusted creep and shrinkage were
incorporated with the composite model, satisfactory predictions of masonry
deformations were achieved.