Various Post Tensioning Systems
Achievable Eccentricities
Grouted System
Say at midspan from bottom of the slab; the cable can not be lower than (20mm cover (in slab) + 10mm dia. distribution steel + 17mm (i.e. at top face of the flat duct after the stressing) = 47mm
Un-bonded System
At the same place as above, from bottom of the slab; the cable can not be lower than (20mm slab cover + 10mm dia. Distribution steel + 8mm (i.e. centriod of the cable) = 38mm.
Hence there is at least 9 to 10 mm difference at bottom and if we consider similar situation at high level point than the total difference would be 18 to 20mm in the cable drape. This is a significant value. This requires more amount of cables to achieve same stress and deflection level in case of grouted system when it is compared with the un-bonded system. This is very decisive when the case is of slabs. The slabs are having lesser depth than beams and PT has to work in smaller depths where the deviation of few millimeters can create large difference in the stresses and deflection values.
Friction Losses
Grouted System
The friction coefficients show slightly higher amount of friction which leads to get lesser amount of effective force.
Un-bonded System
The friction coefficients show slightly lesser amount of friction due to greasing which leads to get good amount of effective force.
Achievable Tensile Stresses
Grouted System
The effective force as well as eccentricity values are lesser. This leads to generate lesser amount of weight balancing. This requires more amount of cables to follow the criteria related to stresses given in the codes.
Un-bonded System
The effective force as well as eccentricity values can be taken to its optimized level so that the stresses can be achieved to its optimum level as per the specifications of codes with the lesser amount of cables.
Deflections
Grouted System
Deflections can be more as the balancing effect is not optimum. To achieve the deflection criteria as per codes the requirement of cable is obviously more. IS:1343 specifies to attain the long term deflection values considering creep and shrinkage and without live load shall be either span/350 or 20mm whichever is lesser. This is very hard to achieve with the help of grouted system without increasing slab thickness or cable numbers.
Un-bonded System
Deflection can be managed to its optimized level without increasing the tendons as well as thickness of the slab.
Tendon Spacing and Minimum Precompression Limits
The provision leads to a non-optimal use of the flat duct grouted system in common slab construction because to satisfy above requirement of the code, it would require one or maximum two cables per each duct in the specified spacing. This would be an insufficient use of the duct, grout and anchorage assemblies. Note that each anchor head is made for four to five strands but will be used for once to two strands only. Same thing for the grouting material too. Now the empty space is more in the duct so grouting material consumption would be more than double. This will make the system costlier than un-bonded. In un-bonded system, each strand can be managed individually as far as spacing is concerned and as well as each strand have a separate anchorage.
Grouting related issues
The problems related to grouting has compelled the available grouting system for building construction to make certain changes that can eliminate the grouting and hence all grouting related problems, material as well as cost and time. There has to be vacuum system of grouting along with the pressure grouting. Only pressure grouting does not assure the proper grouting inside the ducts. There is not a proper system available to ensure the effectiveness of grouting. Sometime if the bleeding/segregation occurs due to improper mixing with water, water pocket can get accumulated at lowest point and can trigger corrosion related problems in future. There are certain specifications of grouting material as well as quality of water which is really difficult to match on site.
In case of unbonded system, the grouting is eliminated hence related material, time and cost can be considered as saving. The Encapsulated unbonded cable is well tested and proven system which is effectively secured against corrosion related problems. The entire system (each and every component) is covered with protective sheathing of PE/PP material.
Two Way Slab Systems
The unbonded compares more favorably to a grouted system similarly designed because of several reasons as below :- Due to generally shallow depth of slabs, the loss of drape due to duct size becomes more significant. This places the grouted construction at a disadvantage.
- It is costlier affair for grouted system to achieve all the related criteria of code and standards where as unbonded system is most economic and effective to achieve all the criteria of code.