Post Tensioning In Rehabilitation Of Structures
The use of post tensioning has become increasingly popular in recent times due to its numerous advantages. Even small structures with only two floors are now being constructed using post tensioning techniques. Post tensioning is not only used for new constructions but is also being applied to restore, rehabilitate, and strengthen existing structures. A case study was conducted where post tensioning was used to remove alternate interior columns from a building originally constructed for residential purposes but later intended to be remodeled as a commercial building.
The problem faced was that the span between eight columns was only four meters, which was unsuitable for the intended commercial purpose. The engineers considered various options such as providing an eight-meter long structural steel beam below the existing beam, encasing the beams with concrete, among others. However, all of these options had drawbacks such as affecting the headroom of the floor, making the beam heavier with high volumes of reinforcing steel, and being too costly.
As a solution, the engineers decided to use the external post-tensioning technique to remove three columns in a row of eight. The beams connecting these columns were encased with 75mm thick micro-concrete, and the system shown in the figure below was adopted. Although the figure only shows the removal of one column, continuity was maintained for all three alternate columns. The external post-tensioning technique was successfully applied to this case, and the intended commercial use of the building was made possible.
Figure 1: Removal of column using external post tensioning technique
Figure 2: Section 1 – 1
Figure3: Section 2 – 2
Figure 4: Column removed using post tensioning technique
The following procedure was adopted to remove the column:
The process began by removing the existing plaster on the beams and preparing the surface by hacking. Holes were drilled at specified intervals for shear connectors and 8mm diameter shear connectors were fixed in them using polyester resin grout. Core holes were drilled in the slab along the beam at specified intervals for the pouring of free flow micro concrete.
Reinforcement was then fabricated and fixed, along with post-tensioned mono strands encased in HDPE pipes, to the profile. The end anchors of the post-tensioned strands were fixed in place. Shuttering was then fabricated and fixed in position, and micro concrete encasement was carried out throughout the core holes in the slab. The complete beam of seven spans was carried out in a single operation.
After the micro concrete attained a strength of 45 MPa in seven days, the post-tensioned strands were stressed to 60% of UTS. The selected alternate columns were then chipped off using an electrically operated chipping machine with care. The beam was supported on screw jacks during this operation.
All the screw jacks were released carefully, and the deflection in the beam was monitored. Final stressing to 90% UTS was then carried out, and strands were grouted.