Although the use of unbonded tendons as the major prestressing technique in design and rehabilitation has been favored for reasons of ease of construction, cost-effectiveness, and maintenance, the overall structural behavior of prestressed with externally unbonded tendon under flexural loading is yet to be well understood, as little work has been done in this field .
in which [f.sub.pe] and [f.sub.py] are, respectively, the effective and yield stresses in the steel tendons, [[OMEGA].sub.u] is the bond reduction factor which is equal to kl(L/[d.sub.p]), k = 1.5 for one-point loading and 3.0 for third-point and uniform loading, L is the span length, [d.sub.p] is the depth from the extreme compression fiber to the centroid of the prestressing steel, [E.sub.p] is Young's modulus for the steel tendons, [[epsilon].sub.cu] is the ultimate compressive strain in concrete and is taken equal to 0.003, c is the neutral axis depth, [L.sub.1] is the length of loaded span or sum of loaded spans affected by the same unbonded tendon, and [L.sub.2] is the total length of the unbonded tendons between end anchorages.
"In comparison, if damage was to occur to an unbonded tendon
it would lose force to the entire length of the tendon as it is reliant upon the gripping wedges (at either end).
The guidelines by which the plants are certified are continually evaluated by CRT-20: Unbonded Tendon Plant Certification Subcommittee to ensure that unbonded post-tensioning tendons are being produced to the highest industry standard.
"The 55 plants that are currently certified, the CRT-20: Unbonded Tendon Plant Certification Committee, and PTI staff played a pivotal role in reaching this milestone and ensuring the program's growth throughout the next quarter century."
This indicated the importance of the anchoring performance at the extremity of the unbonded tendon
because of the even distribution of the stress along the tendon.
Active reinforcement can be made with unbonded tendons
or with bonded tendons that are glued to the timber with adhesives.
Feng, "Experimental study on seismic performances of precast prestressed concrete beam-to-column connections with unbonded tendons
," Journal of Beijing University of Technology, vol.
At present, it has been usually used in stress monitoring of external tendons (i.e., unbonded tendons
), such as the stayed cables, suspenders, and external tendons of PSC bridges, as shown in Figure 1.
Naaman, "A new methodology for the analysis of beams prestressed with external or unbonded tendons
," External Prestressing in Bridges ACISP120-16, American Concrete Institute, Detroit, Mich, USA, 1990.
The model provides a unified approach for RC and PC elements with bonded or unbonded tendons
. The model has already been validated for RC structures .
Besides the use of bonded tendons, other solutions have also been studied with unbonded tendons
that are tensioned when the structural element is bearing loads.