Post tensionning
Types of prestressing
1-Post tensioning
• The prestressing steel is either placed in a duct or sleeve before the concrete is cast
• After the concrete is being casted and gained adequate strength, a stressing jack pulls the steel strands while reacting against the body of concrete member
• Over time, a friction of the initial force in the concrete is lost due to creep and shrinkage of the concrete and relaxation of the steel
2-Pretensioning
• The prestressing steel is first stressed and anchored against external bulkheads, concrete is then casted over the stressed steel
• Once the concrete has developed adequate strength, the tendons are released from the bulkheads
• The tendency of the stretched tendons to shorten pre-compresses the concrete
Advantages of post tensioned construction
1-Less steel
• Using higher tensile strength of prestressing steel provides four and half times the capacity of the conventional steel
• The codes require a minimum amount of reinforcement in slabs to control shrinkage and temperature
• The codes require a minimum amount of reinforcement to be distributed through the slab in both directions, so that the slab will have the necessary in plan strength for diaphragm action
• The pre-compression imported by tendons spread rapidly through the slab from the anchorages and is sufficient to prevent cracking from shrinkage and temperature changes
• The pre-compression provided by tendons under service condition is generally more than the minimum reinforcement required for diaphragm action, thus eliminating the requirement of adding reinforcement
2-Thnner slabs “less concrete”
• Once span length exceed 5m, a post tensioned slab will be approximately one third thinner than the reinforced concrete slabs designed for the same loading
3-Large spans
• Post tensioned slabs can span greater than conventional slabs of the same thickness
4-Simple forms “eliminating of beams”
• Application of post tensioning can allow beam to be eliminated, that will reduce the cost of forming as much as one-third of other system’s cost
5-Ability to better span irregular support arrangement
• Post tensioned slabs don’t have to rely on the beam-and-slab framing common in conventional slabs, thus post tensioned slabs are particularly adaptable to irregular geometry
6-Lighter concrete forms “lower seismic demand”
• The post tensioned concrete frame is generally one-third lighter than a conventionally reinforced
• The supports and foundation tank the advantaged of the lighter floor system
7-Shorter concrete frames “reduce floor height”
• Reducing slab thickness and elimination of beams results in shorter floor-to-floor height, and consequently, a reduction in the total height of the building
• The shorter building have lower arm for the overturning moment created by seismic or wind
8-Greater ability to resist concentrated forces
• The post tensioned acts as active load system, the applied force from post tensioning is generally configured to counteract the externally applied force, this reducing their undesirable effects
• The tendons can be profiled to apply on upward force from that when combined with pre-compression from the tendons will counteract the applied force without undue deflection or need for local cracking
9-Reduce deflection
• Post tensioning provides an upward force that balances a high friction of floor self-weight, thus reducing the net downward force that causes deflection
• Post tensioned floors have greater flexure stiffness
10-Reduce cracks
• Because the (ACI 318) imposes a low limit on the allowable tensile stresses, two way post tensioned floor slabs will be essentially crack-free under service conditions
• When using the (EC2) the designer selects the extend of allowable cracking and a design crack width based on the anticipated in-service conditions of floor system
• The designer choice becomes the amount of cracking and design crack width, as opposed to the elimination of cracking that results when using (ACI 318)
11-Improve resistance to water penetration
• Because the post tensioned slabs have fewer cracks, thus provides a greater resistance to water penetration
12-Preception and acceptability of vibration
• Foot fall on large areas supported on thin slabs can trigger unacceptable vibrations; cracking will exacerbate the problem because it lowers the natural frequency of the slab
• Post tensioned slabs are generally thinner than their conventionally reinforced counterparts and have longer spans, thus they are more prone to unacceptable vibrations
• Two benefits of post tensioning help to reduce susceptibility of post tensioned floor to objectionable vibration
• One is the reduction of weight (mass), the other is a larger relative stiffness because of less cracking, both features help to increase the natural frequency of vibration and improve the design
• On the other hand, the longer spans used in post tensioned structure tend to lower the nature frequencies and aggravate the perception of vibration
• The vibration of post tensioned slabs under foot fall should generally be investigated where spans are relatively large
Application of post tensioning in building construction
1-Floor system “flat slab construction”
a. Application in regions of high seismic risk, high wind forces
• Because of lower weight and height and the improved of diaphragm action that results from pre-compression
b. General building applications
• Optimum design in residential and commercial building flat slabs have spans between (8m to 10m)
2-Floor system “beam and slab construction”
• When the aspect ratio of a slab panel exceeds two, it’s often more economical to use this system
3-Podium slabs in low rise building
• For building up to five levels, post tensioned podium slab resists concentrated loads from posts and walls of the upper levels without requiring a support immediately below each load
• Podium slabs are used in building which the lower level require a support layout that is different from the levels above
4-Transfer slabs
• For high rise building where open space is required at the ground floor, one solution is to terminate the supports of upper levels on a transfer slab that received the loads from supports of super structure and transfer them to a limit number of generally widely spaced supports
5-Mat (raft) foundation
• Post tensioned mat is used when the allowable bearing pressure is not adequate to resist peak stresses below walls and columns, but the total area of the soil below the foot print of structure is large enough to resist the total load
• A well designed post tensioned mat can be as much as 40% thinner than a conventional reinforced concrete
6-Industrial ground support slab
• In industrial storage areas, it’s necessary to provide a smooth ride for forklift and other loading equipment, a conventionally reinforced slab besides being subject to shrinkage cracks cannot accommodate the changes in the underlying soil with the flexibility of post tensioned alternative
• Construction of post tensioned ground support slab
a) Covering the upper layer of underlying soil with two large of plastic sheets
b) The tendons are laid out
c) The concrete is casted
d) The tendons are stressed
7-Slab-on-grade “SOG for residential and light industrial”
• SOG used to limit the effect of seasonal changes on building by limiting the shallow foundation movement
• The objective is generally to limit the deformation in the structure to an amount that doesn’t impair it’s serviceability
8-Retrofit through external post tensioning
• Post tensioning has been used effectively to correct strength and deflection deficiencies in building
• When post tensioning is applied judiciously, its active force can configure the probable failure mode of structure, thus enhancing the structure’s level of safety
9-Post tensioning to restore geometry in seismic frames
• In region of high seismic risk, building are designed to undergo post-elastic deformation, this helps dissipate the seismic energy and reduces the demand on resistance from the building frames
• While building are designed to prevent collapse under anticipated seismic forces, they are expected to sustain damage
• Observation from an earthquake revealed that multi story buildings that have experienced post-elastic deformation may not return to their original plumb position
• Post tensioning can be used to restore a building closer to its original position after post elastic deformation from an earthquake
• This is done by directing and controlling the post elastic deformation to designed location and using the force of prestressing tendons to restore the building to its original plumb position
10-Post tensioning in walls
• When the lateral forces along the length of the wall are large and the vertical axial force is not adequate to prevent the wall from excessive tension, the wall possibly overturning
• Post tensioning along the height of the wall in the vertical direction can be used to reduce tension and keep the wall in position
11-Post tensioning in column
• When the column is subjected to significant bending, post tensioning reduces the axial capacity of the column
Post tensioning material and hardware
• Types of post tensioning tendons systems
a. Unbonded
• The prestressing steel is coated with a corrosion inhibiting grease, then encased in aplastic
Marking and recording of tendons position
• Recording the as-installed location of the tendons is necessary to modify the structure in the future
• This helps to identify where repair or drilling will require special precautions
A. Marking of tendons on finished floors
One way is to spray paint the location on the formwork before the concrete is cast, the paint mark will be transferred to slab soffit
Another option is to paint the tendons location on the slab soffit after removing the forms
B. Photo\video recording of reinforcement
Recording the position of reinforcement through photographs or videos and file the record as part of the as-built documents of the construction
Economics and material quantities
• The economy of post tensioned slab versus a conventionally reinforced slab is a function of span length, the span of (7m) is the cross-over point between the two options
• Compliance with different building codes and local practice can override the general case
• When (ACI 318) is used for the design of column-supported floor system, a minimum amount of prestressing is required and spacing between tendons is limited
• These requirements do not exists in (EC2)
• These and other requirements such as allowable stresses, mean that the design quantities are often a function of the building code
• Engineering principles, local perception of good practice, can play a significant role in the quantities commonly used
1-Material quantities
A. Practice and project examples
The quantities consists of concrete, prestressing strands and non-prestressing reinforcement
The conversion to post tensioning reduce the total weight of reinforcement (combined PT and rebar ) by over 2.5 times
B. Base quantities for code compliance (ACI 318 AND EC2)
The quantities used in each construction derive from two requirements
i. First, the reinforcement to comply with the in-service and safety requirements for the applicable code
ii. Second, the reinforcement for structural detailing used for crack control at discontinuities
C. Reinforcement for detailing
2-Construction cost
• The bulk of the construction cost of a post tensioned floor consists of material, field labor, and equipment
A. Material cost
Cost of rebar, purchased, bent, and placed
Cost of prestressing material including hardware, placing, and stressing
B. Labor cost
The benefits of post tensioned structures includes
i. Less reinforcement, which results in lower labor cost for handling and placing
ii. Simplification in construction, which reduces the cost of labor for forming
iii. Shorter construction cycle
The construction cycle is typically one week
This includes stripping and re-shoring the previously cast concrete, moving the forms to next pour and installing them, placing rebar and prestressing tendons, inspection, placing concrete, and stressing tendons3
Repair, Retrofit, Maintenance, and life cycle
1-Floors reinforced with grouted tendons
• For slab with grouted tendons, the procedure for repair or remodeling are similar to conventionally reinforced slab
• The prestressing steel is bonded to the concrete the same way that non-prestressing reinforcement is, but there are some important differences
I. At the strength limit state (USL), prestressing strand is capable of developing three to more than four times the force of non-prestressing reinforcement of the same cross-sectional area, thus cutting a prestressing strand can be more detrimental than cutting a rebar of comparable size if the design relies on the full strength of the cut strand.
As a result, the loss of effectiveness of the strand extends over a longer distance from the location of the cut
II. Bonded strand provides local precompression that is beneficial to crack mitigation, this precompression is lost if a section of the strand is removed
The precompression from each strand is dispersed into the slab when the strand is stressed, once the tendon is grouted, the precompression is locked into the floor system
When the tendon is cut, there will be a local reduction in precompression, but the precompression imported from the remainder of the strand will remain
• It is not necessary to re-stress and re-anchor a strand that has been cut, the grout that is injected into the duct, once hardened, locks the force into the tendon
2-Floors reinforced with unbonded tendons
• When an unbonded strand is cut, it losses it force a long its entire length, thus its contribution to both serviceability and safety of the structure is completely lost
The cut tendon’s contribution to the precompression in the floor system will be lost over the entire length of the tendon
• The loss of force and effectiveness of a tendon over its entire length require that the contribution of each tendon to be fully evaluated and, where necessary, compensated for the repair or retrofit
• The repair/retrofit procedure generally consists of
1. Exposing the affected tendons
2. Carefully cutting them
3. Re-stressing tem and re-anchoring them at the face of the cut
The sequencing of the work may vary depending on the number of tendons need to be cut and the location of the new opening or the repair
• From the standpoint of a structure’s serviceability and safety, the loss of a single strand either inadvertently or by design, is usually not critical
Structural analysis for the effects of a lost tendon is often demonstrates that the structure has enough redundancy that it can lone one or more tendons without compromising its intended performance
• If it is necessary to replace a lost strand, one option is to extract the damaged portion of the strand and rethread the sheathing with a smaller diameter, but higher strength
The new strand is coupled to the existing strand and the tendon is restressed
