Cracking in Ground-Supported Post-Tensioned Slabs on
Expansive Soils1
Post-tensioned
slabs are widely used as foundations for light residential construction in
areas of expansive soils. Common design
methods for these slabs assume an uncracked section.
The successful performance of many
thousands of post-tensioned ground-supported slabs built over the past several
decades, combined with the fact that some cracks can be found in virtually all
of them, suggests that cracking is not detrimental to their structural
behavior.
Categories, Causes, and Characteristics
of Cracking
Two broad
categories of cracking can occur in ground-supported post-tensioned slabs. These categories
of cracks are
caused by entirely different phenomena and have very different
characteristics. They are:
- Cracks caused by the restraint of axial slab shortening (RTS cracks).
- Cracks caused by applied loads (flexural or shear cracks).
RTS cracks have the following general characteristics, which can be
helpful in their diagnosis:
- They occur shortly after the concrete is placed, often before the tendons are stressed.
- They are independent of applied load, and occur in locations which have no direct relationship to points of maximum slab moment and shear.
- They are substantially vertical and penetrate the entire depth of the concrete slab.
Ramifications of RTS Cracking
It is concluded, then, that because of their normal orientation, RTS
cracks are of no structural consequence in post-tensioned slabs on ground. This conclusion is consistent with the actual
performance of existing post-tensioned slabs on ground, many of which have
significant RTS cracking and are performing in a completely functional manner.
Applied Load Cracks
Flexural cracks have the following characteristics:
- They are located at points of maximum slab moments.
- They do not penetrate the full depth of the slab. They can be seen only on the slab surface (top or bottom) where tension is produced by applied loads.
- They are independent of the age of the concrete slab. They are only a function of applied loads.
Ramifications of Applied Load
Cracking
Flexural cracking caused by applied loads has no significant effect on
the strength of ground-supported
post-tensioned slabs. This is true
because of the increase in soil support which is mobilized after cracking
occurs in the slab. The soil, in effect,
provides the factor of safety which is required by code strength provisions.
However, applied load B-zone cracking can cause excessive slab
deformations which may affect the serviceability of the slab.
Summary Conclusions
This Technical Note has addressed the following:
- The general behavior of post-tensioned slabs-on-ground.
- The categories and characteristic of cracking (restraint-to-shortening cracks and applied loading flexural cracks) which occur in post-tensioned slabs-on-ground and their ramifications.
It is concluded that most cracks
in post-tensioned slabs on ground are not detrimental to their structural
function. Restraint-to-shortening
(shrinkage) cracks, because of their typical orientation within the slab
dormant zone, have no effect on slab strength or differential slab
deflections. Flexural cracking has no
effect on the strength of ground-supported slabs because of the soil support
available after cracking occurs. Flexural cracking, however, can have a detrimental effect on slab
serviceability by increasing post-cracking deformations. Slabs which exhibit applied load flexural
cracking must be evaluated on an individual basis by a structural engineer
experienced in post-tensioned ground-supported slabs.
These conclusions are consistent
with the observed behavior of many thousands of post-tensioned slabs on ground
built since the late 1960’s, most of which have some level of visible cracking
and are functioning in a satisfactory manner.
- PTI Technical Notes –Issue 6-August 1995 – For
Professionals engaged in Post-Tensioning Design