DIFFERENCE BETWEEN SRT AND LRT IN RELAXATION LOSS (PRESTRESSED CONCRETE DESIGN) 1

[adoorable]

Goodmorning Engrs. I would like to ask on what is the difference between srt and lrt in terms of relaxation loss? In addition to my question, why anchorage slip and frictional loss is not present in pre-tensioning?

 

[Ingenuity]

Seldom are such acronyms used in my experience, but SRT would be STRESS-RELIEVED and LRT would be LOW-RELAXATION. Not sure what 'T' is - maybe tendon.

LR strand is stress-relieved, but SR strand is not low-relaxation. Just about all strand manufactured today is low-relaxation, which is a process, after stranding, where the strand passes through a induction furnace under tension - so heat + tension - resulting in low-relaxation properties. Low-relation strand has lower long-term relaxation properties compared to stress-relieved.

In PRE-tensioning there is no "anchorage" within the element, as such (c.f POST-tensioning) - it relies upon bond of strand to concrete to transfer the prestressing forces. There is, however, strand slippage that occurs when the strands are cut from the precast bed and transferred to the strand, and hence the concrete element. When the strand is cut, there is also the Hoyer effect (reverse dialation of strand due to Poisson's ratio and helical wire 'compaction') that provides some 'wedge-like' behavior to the strand as it tries to slip.

Most PRE-tensioning strands are straight from end-to-end, so no friction, as compared to POST-tensioning, where there is intentional tendon curvature (drape) and unintentional tendon curvature (wobble). For those precast elements that have draped/harped tendons - like a bridge girder or maybe a deep DT - there are friction losses at the mechanical deviators in the precast bed, and such losses are accounted for in the prestress calculations by the precaster.