Post-tensioned slab construction is identified by:
Correct Answer
B) Prestressing steel tensioned after the concrete reaches strength
Post-tensioning tensions the prestressing tendons after the concrete has hardened and attained the specified strength for stressing. The resulting prestress is part of the engineered design. This does not mean waiting for complete curing or that any unverified slab can be safely cut or altered.
Why This Is the Correct Answer
Option B is correct because post-tensioning is a method of prestressing in which the prestressing steel is tensioned after the concrete has reached a specified strength. The tendons are stressed and anchored, putting the slab in compression as part of the engineered design. This is why such slabs should not be cut or altered without qualified review.
Why the Other Options Are Wrong
Option A: Steel columns that support any unreinforced concrete slab poured on grade
Option A describes steel columns supporting an unreinforced slab poured on grade. Post-tensioning involves tensioned steel tendons inside the concrete itself, not columns under an unreinforced slab.
Option C: Wood framing that serves as the slab’s only reinforcement throughout
Option C says wood framing serves as the slab's only reinforcement. Post-tensioned slabs are reinforced with prestressing steel tendons, not wood.
Option D: The complete absence of any reinforcement within the slab
Option D says the slab has a complete absence of reinforcement. A post-tensioned slab is defined by its tensioned prestressing steel, so it is the opposite of unreinforced.
Post means after the pour
Post-tensioning: the concrete is poured and gains strength first, and the steel is pulled tight after.
How to use: Use this to separate post-tensioning from pretensioning on exam questions.
Exam Tip
Read the prefix: post-tensioned means the steel is stressed after the concrete reaches strength, pretensioned means before it is cast.
Common Mistakes to Avoid
- -Confusing post-tensioning with pretensioning
- -Assuming a post-tensioned slab can be cut like an ordinary slab
- -Thinking the steel must wait for complete curing rather than a specified strength
Concept Deep Dive
Analysis
This question tests the definition of post-tensioned concrete. The key is the word post: the prestressing steel is tensioned after the concrete is placed and has reached a specified strength. Options that describe unreinforced slabs, wood reinforcement or steel columns do not match that definition.
Background Knowledge
Prestressed concrete uses tensioned steel to place concrete in compression, improving its ability to resist cracking and loads. In pretensioning, the steel is tensioned before the concrete is cast; in post-tensioning, it is tensioned after the concrete hardens. Post-tensioned slabs are common in areas with expansive soils and in some residential and commercial construction.
Real-World Application
An appraiser sees a stamped notice in a garage warning that the slab is post-tensioned and should not be cut. When the owner mentions plans to cut the slab for new plumbing, the appraiser notes the slab type in the report and does not offer opinions on whether the work is safe, leaving that to qualified engineers.
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