PRESTRESSED CONCRETE PART 5

Myth associated with PT

we cannot make an opening in PT slab, this is actually a false statement, a PT slab can be modified with proper calculations and expertise. We have made as big as 4m x5m opening in a PT slab which was required for sky light or escalators
we need to do restressing periodically: this is also one of the things I hear about PT, but this is wrong. The strands are cut flush with the concrete surface and can not be restressed even if we wanted to. There is no requirements for restressing as we have low relaxation strands and a very good wedge anchor system
PT slab can not crack: even though PT slabs are designed to be crack free at service load (at least based on ACI, some codes like EC allow for a certain degree of crack), cracks can be witnessed in PT slabs. This can attribute to one of the two 1. If the actual load exceeds the service load momentary 2. Surface cracks, which are very common
No one can claim to have a crack free concrete by definition, we will still have micro cracks

Design based on ACI Vs Euro Code

The analysis procedure and the load combinations are similar for the two codes.
ACI restricts the tensile stress at any time and section to be way below the cracking moment. While EC allows those limits to be exceeded to some extent, but then gives min rebar requirement to supplement that
Both codes do not have any min rebar requirement for positive moments (bottom) in bonded PT, this means one can design a PT slab with no bottom rebar
Further more; ACI has no top or bottom min rebar requirements for bonded PT slabs up until the 2011 code, so with ACI 2011 or earlier code, one can design a PT slab with no rebar
if a slab is designed with ACI, we will get more PT and less rebar than slabs designed with EC.

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