BONDED vs UNBONDED POSTTENSIONING SYSTEM Comparison by Hemal

BONDED vs UNBONDED POSTTENSIONING SYSTEM Comparison by Hemal Mistry Surat 1

(1) RATE OF INCREASE OF STREES IN TENDONS: n Rate of increase of stress in steel (tendons) is larger in Bonded beams than in Unbonded beams, both in Pre-cracking and post-cracking stages. 2

(2) ULTIMATE LOAD CAPACITY: n Steel (Tendons) does not reach its ultimate strength at failure in Unbonded beams, the Ultimate load supported by Unbonded beam is lesser than that of Bonded beam in which steel attains its ultimate strength at the failure stage of the member. 3

(3) FLEXURAL STRENGTH (MOMENT CAPACITY) : n n n There is no bond between tendon and concrete in Unbonded members, so tendons are free to sleep. Strain in tendon is more or less equalized along its length and strain in tendon at critical section (e. g. at location of maximum B. M. ) is lessened. So, stresses in member at critical section will be less, resulting in lesser flexural (moment) capacity. Final strain in Tendon is very difficult to determine accurately in Unbonded member. So, Stress in tendon at ultimate stage (failure) will be significantly less than estimated/calculated stress value in Unbonded Members. IS: 1343 -1960, suggests rigorous analysis or field tests to estimate stress in Unbonded tendons. Ultimate strength (Flexural Capacity) of Unbonded members may be 10 to 30% lesser than strength of equivalent Bonded beam. 4

(4) CRACK PATTERN AND CRACK WIDTH : n In the post-cracking stage, Bonded beam develop small cracks which are well distributed in zone of larger moments. Unbonded beam develops only a few cracks which are localized at weaker sections and crack widths are correspondingly larger and less serviceable in comparison with the Bonded beams may be cracked or uncracked depending upon the permissible tensile stress while Unbonded beams are always cracked like Reinforced concrete. 5

(5) DUCTILITY: n n In Unbonded members, Tendons are free to sleep. Strain in tendon is more or less equalized along its length rather than gradual along the member in line with B. M. diagram. So, strain in tendon at critical section (at location of maximum B. M. ) is lessened. At critical sections, concrete reaches its crushing strength (permissible compressive stress) before Tendons reaches its Ultimate strength (permissible tensile stress), resulting in Brittle failures are Sudden and gives no warning before failure. In Bonded members Tendons reaches its Ultimate strength before concrete reaches its crushing strength, resulting in Ductile failure gives sufficient warning before failure. 6

(6) ANCHORAGE DEPENDENCY : n n Unbonded tendons remains totally dependent on integrity of their anchorages, if anchorages suffers damage, accident or corrosion, all prestress in tendon will be lost and flexural (moment) capacity due to tendon will be zero. Bonded tendons are bonded to concrete section and if anchorages suffers damage, bond should be able to retain prestressing force beyond its bond length. 7

(7) EFFECT OF DAMAGE OF TENDON IN CONTINUOUS SPAN / FLOOR: n n If an Unbonded tendon is damaged in a continuous floor then whole series of associated span will suffer from the loss. If unbonded tendons are deliberately cut, for instance, to make a hole, the adjacent continuous span will require propping. If an Bonded tendon is damaged in a continuous floor then loss is confined to a particular span where the damage is occurred. 8

(8) FIRE RESISTANCE: n n In case of fire, Bonded tendons has the protection of additional concrete cover, because the duct is much larger than the tendon area and stressing tend to pull the tendon into concrete mass, away from the concrete surface. The duct itself act as hit-sink to small extent, resulting in higher fire resistance. In case of fire, an Unbonded tendon has less concrete cover and sheathing has limited insulating value, resulting in lesser fire resistance. 9

(9) RESISTANCE TO OVERLOADING : n n In case of overloading, the distribution of cracks in bonded floors are similar to Reinforced concrete floor. In unbonded floors, cracks are wider and further apart when overloaded. Requires more bonded rod reinforcement to distribute the cracks. 10

(10) TIME OF TENDON STRESSING: n n In Bonded system, all tendons in a duct are stressed in one operation. In Unbonded system, tendons are stressed one at a time. 11

(11) EFFECT OF TENDON FAILURE : n n In case of tendon failure, anchorages are subjected to sudden release of strain energy stored in Tendon, If tendons are unbonded. In case of tendon failure, strain energy released from tendon will be absorbed by surrounding concrete along with anchorages, if tendons are bonded. 12

(12) DEMOLITION OF STRUCTURE : n n In case of demolition of structure with Unbonded tendons, force in the tendon may be transferred to jack, depending on the type of anchorage, and then gradually released to zero by releasing the jack hydraulic pressure. In case of Bonded tendons, Even after transferring tendon force to jack, considerable force is locked in tendon due to bond between tendon and concrete. This is advantageous during demolition, since the tendons may be cut into smaller length, with each section now behaving as a pretensioned tendon. 13

(13) TENDON REPLACEMENT: n n In case of damage to unbonded strand in a completed floor, for short lengths it is possible to withdraw the unbonded strand from its extruded sheathing and compact strand with smaller diameter may be inserted. Replacement of strand is not possible in case of Bonded tendons. 14

(14) TENDON ECCENTRICITY: n n Bonded tendons usually consist of several strands placed in a common duct. The duct is larger in diameter and when placed in position it gives lesser eccentricity. Unbonded tendons consists single sleeved strands, which gives larger eccentricity. 15

(15) GROUTING : n n Bonded system requires proper grouting to avoid tendon corrosion, bursting of duct and spalling of surrounding concrete due to trapped water and air. Unbonded system do not require grouting. 16

REFERENCE : n n n Post tensioned concrete floors by Sami Khan & Martin Williams. Design of prestressed concrete structures by T. Y. Lin & Ned H. Burns. Prestressed concrete design by M. K. Hurst. Prestressed concrete design by N. Krishna Raju. Design of prestressed concrete by Arthur H. Nilson. Design of prestressed concrete by R. I. Gilbert. 17