1.1. Introduction

Concrete is strong in compression, but weak in tension. In order to reduce or

prevent such cracks from developing, a concentric or eccentric force is imposed in

longitudinal direction of structural element. This force prevents the cracks from

developing by eliminating or considerably reducing the tensile stresses at the critical

midspan and support section at service load, thereby raising the bending, shear and

torsional capacities of the sections. The sections are able to behave elastically and almost

the full capacity of the concrete in compression can be efficiently utilized across he entire

depth of the concrete sections when all loads act on the structure.

The main difference between reinforced and prestressed concrete is the fact that

reinforced concrete combines concrete and steel bars b simply putting them together and

letting them act together as hey may wish. Prestressed concrete, on the other hand,

combines high-strength concrete with high-strength steel in an active manner. This is

achieved by tensioning the steel and holding it against the concrete, thus putting the

concrete into compression. This active combination results in much better behavior of the

two materials. Steel is ductile and now is made to act in high tension by prestressing.

Concrete is brittle material with is tensile capacity now improved by being compressed,

while its compressive capacity is not really harmed. Thus prestressed concrete is an ideal

combination of two modern high strength materials.

The development of presstressed concrete has occurred in application of

posttensioning to buildings, bridges, and pressure containers, including the combination

of pretensioning, posttensioning, and conventional reinforcing to structures and structural

components. Outside the fields of tanks, bridgesn and buildings, prestressed concrete has

been occasionally applied to dam, by anchoring presstressed steel bars to the foundation,

or by jacking the dam against it.

1.2. General Principles of Prestressed Concrete

Three different concepts may be applied to explain and analyze the basic behavior

of this form of prestressed concrete.

First Concept – Prestressing to Transform Concrete into the Elastic Material

Presstressed concrete as essentially concrete which is transformed from a brittle

material into an elastic one by the precompression given to it. Concrete which is weak in

tension and strong in compression is compressed so that the brittle concrete would able to

withstand tensile streses. From this concept the criterion of no tensile stresses was born. It

is generally believe that if there are no tensile stresses in the concrete, there can be no

cracks, and the concrete is no longer a brittle material but becomes an elastic materials.

From this standpoint concrete is visualized as being subject to two systems of

forces: internal prestress and external load, with the tensile stresses due to the external

load counteracted by the compressive stresses due to the prestress. Similarly, the cracking

of concrete due to load is prevented or delayed by the precompression produced by

tendons. So long as there are no cracks, the stresses, strains, and deflections of the

concrete due to two systems of forces can be considered separately and superimposed if

necessary.

In its simplest form, a simple rectangular beam prestressed by a tendon (Fig. ) and

loaded by external loads.

Second Concept – Prestressing for Combination of High-Strength Steel with

Concrete

This concept is to consider prestressed concrete as combination of steel and

concrete, similar to reinforced concrete, with steel taking tension and concrete taking

compression so that the two material form a resisting couple against the external

moment, Fig. This is often the easy concept for engineers familiar with reinfoeced

concrete where the steel supplies a tensile force and the concrete supplies the

compressive force, the two forces forming a couple with a lever arm between them.

In prestressed concrete, high-tensile steel is used which will have to be elongated

a great deal before its strength is fully utilized. If the high-tensile steel is simply buried in

concrete will have to crack very seriously before the full strength of the steel is

developed. Fig. Hence it is necessary to prestretch the steel with respect to the concrete.

By prestressing and anchoring the steel against the concrete, we produse desirable sresses

and strains in both materials: compressive stresses and strains in concrete, and tensile

stresses and strains in steel. This combined action permits the safe and economical

utilization of the two materials which cannot be achieved by simply burying steel in

concrete as is done for ordinary reinforced concrete. In isolated instances, medium-

strength steel has been used as simple reinforcement without presressing, and the steel

was specially corrugated for bond, in order to distribute the cracks. This process avoids

the expenses for prestreching and anchoring high-tensile steel but does not have the

desirable effects of precompressing the concrete and of controlling the deflections.

Third Concept – Prestressing to Achieve Load Balancing

This concept is to visualize prestressing primarily as an attempt to balance the

loads on a member. In the overall design of prestressed concrete structure, the effect of

prestressing is viewed as the balancing of gravity loads so that members under bending

such as slabs, beams, and girders will not be subjected to flexural stresses under a given

loading condition. This enables the transformation of a flexural member into a member

under direct sress and thus greatly simplifies bith the design and analysis of otherwise

complicated structures..

This application of this concept requires taking the concrete as a freebody, and

replacing the tendons with forces acting on the concrete along the span. Take for

example, a simple beam prestressed with a parabolic tendon. (Fig.)

1.3. Classification and Types of Prestressed Concrete

Prestressed concrete structure can be classified in a number of ways, depending upon

their feature of design and construction. This will be discussed as follows.

Externally or Internally Prestressed. - The method of arch compensation was

mentioned previously, where a concrete arch was prestressed by jacking against its

abutments. Theoretically, a simple concrete beam can also be externally prestressed by

jacking at the proper places to produce compression in the bottom fibers and tension in

top fibers. Fig. , thus even dispensing with steel reinforcement in the beam. Such an ideal

arrangement however cannot be easily accomplished in practice, because, even if

abutments favorable for such a layout are obtainable, shrinkage and creep in concrete

may completely offset the artificial strains unless they can be readjusted. Besides, such a

site would probably be better suited for an arch bridge.

For a statically indeterminate structure, like a continuous beam, it is possible to

adjust the level of the supports, by inserting jacks, for example, so as to produce the most

desirable reactions, Fig . This sometimes practical, though it must be kept in mind that

shrinkage and creep in concrete will modify the effects of such prestress so that they must

be taken into account or else the prestress must be adjusted from time to time.

Linear or Circular Prestressing. - Circular prestressing is aterm applied to

prstressed circular structures, such as round tanks, silos, and pipes, where the prestressing

tendons are wound around in circles. As distinguished from circular prestressing, the term

linear prestressing is often employed to include all other structures such as beams and

slabs. The prestressing tendons in linearly prestressed structure are not necessarily

straight; they can be either bent or curved, but they do not go ground and round in circles

as in circular prestressing.

The example of Circular prestressed concrete is prestressed concrete tanks for

hazardous liquids that circular prestressed concrete tanks are ideal for the storage of

liquids. Although, so far, they have been used mostly for water storage structures and

municipal wastewater treatment structures, they also offer advantages for storage of

liquids detrimental to the environment. The ability of circular prestressed concrete tanks

to resist backfill presure allows them to be buried, the additional security and other

advantages provided make circular prestressed concrete tanks a logical part of double

containment systems for chemicals, hazardous wasters and petroleum storage. The ability

of circular prestressed concrete tanks above grade, and to a greater extent below grade, to

withstand the domino effect in multitank storage facilities is very valuable for decreasing

the economic and environmental consequences of industrial accidents.

Pretensioning and Posttensioning

1. Pretensioning

The term pretensioning is used to describe any method of prestresing in which

the tendons are tensioned before the concrete is placed. It is evident that the

tendons must be temporarily anchored against some abutments or stressing beds

when tensioned and prestress transferred to the concrete after it has set. This

procedure is employed in precasting plants or laboratories where permanent

beds are provided for such constructed.

2. Posttensioning

Posttensioning is a method of prestressing in which the tendons is tensioned

after the concrete has hardened. Thus the prestessing is almost always

performed against the hardened concrete, and the tendons are anchored againts

it immediately after prestressing. This method can be applied to members either

precast or cast in place.

In posttensioning prestressed concrete, we know two types of tendon:

a. Bonded Tendons

Bonded tendons denote those bonded throughout their length to surrounding concrete.

Non-end anchored tendons are necessarily bonded ones; end-anchored tendons may

be either bonded or unbonded to the concrete. In general, the bonding of

posttensioned tendons is accomplished by subsequent grouting

b. Unbonded Tendons

In unbonded, protection of the tendons from corrrosion must be provided by

galvanizing, greasing, or some other means. Typically, the unbonded tendons is

greased and wrapped with paper or plastic material to prevent bonding to the

surrounding concrete.

History of unbonded tendons

The first unbonded tendons can probably be attributed to R.E.Dill in 1925. In the

1930s, unbonded rod-type bars were prestressed by the use of turnbluckles for

cylindrical tanks. This exposed bars were either galvanized or painted. The first

unbonded tendons in a building were probably used for lift-slab construction in the

mid 1950s and it consisted of greased and helically-paper wrapped ¼ in. 240 ksi

stress relieved wire anchoraged by individual button heads. In the early 1960s, paper-

wrapped, single-strand tendons started to replace wire tendons. In turn, grease-coated,

plastic-sheated strands, introduced in the mid 1960s, began phasing out the paper

wrapped tendons,. By the mid 1970s they had become the predominant unbonded

tendons.

By 1970, unbonded post-tensioning design concepts were becoming more

commonly known and long spans, relatively shallows depths, and generally superior

crack control provided by post-tensioning were seen as an advantage. This was

particularly true for parking structures, which account for 20 percent of unbonded

post-tensioned buildings.

Tendon types, performance

- Paper wrapped wire tendons

The corrosion performance of paper-wrapped, button headed wire tendons has

had some problem. In some of these cases, it was found that the paper wrapping

was inadequate and that the amount and quality of grease protection around the

prestressing steel and anchorage was less than desirable. This was sometimes

exacerbated by inadequate cover, poor-quality concrete and concrete cracking,

which permitted water and chlorides to find their way to the anchorages, resulting

in corrosion.

These unbonded tendons, which had relatively limited use, have now been in

service for up to about 35 years. Since patterns of problems have occurred,

monitoring the behavior of these tendons is advisable.

- Single strand unbonded tendons

Three different types of greased plastic sheaths are used to enclose prestressing

steel strand (Fig 1).

1. The push-through sheath has a loose fit to thread through

the strand.

2. The lapped, heat sealed sheath can be relatively tight

around the strand, but in past practices it quite often was a loose fit,

sometimes was poorly executed and would open.

3. The extruded sheath is generally in tightly fitted.

- Banded Tendons

Prior to 1969 tendons were not bunched or grouped in slab structural

systems; they were distributed throughout the structure. Then tendons started to

be used in grouped bands of up to four or five tendons in one direction of slab

system and uniformly spaced in the other direction. This method of placing

tendons is now the general practice. Grouping tendons in bands makes the

chairing and placing simpler.

- General structural evaluation

When evaluating an existing structure, the usual structural concerns such

as unusual cracking patterns, through cracks, unanticipated cracks, cracks in areas

of shear transfer, concrete spalling and delamination, excessive deflections, and

signs of reinforcing steel and tendon corrosion should be noted. These than should

be related to what is known about the structure, as found in engineering and shop

drawings and job construction documents and photographs so that cracking

patterns and the incidental crack can be evaluated. Then it can be determined

whether the structure should be analyzed to see if cracking that exists is to be

expected or not.

If the cracking is explainable and there is sufficient ultimate strength

without excessive deflections, then the design can usually be considered adequate.

However, if cracks are found that are not explainable or if excessive restraint

cracks are found that may depreciate the shear strength of the structure or change

the expect behavior of the structure, then the problem obviously has to be looked

into further excessive assuming no overload, indicates some deficiency in the

original design, improperly placed tendons or reinforcing bars, non-stressed or

failed tendons, or an inadequate concrete strength and modulus of elasticity.

Some typical defects in unbonded structures are restraint cracks caused by

the presence of stiff elements, which do not permit to floor system to shorten

freely due to prestressing compression elastic shortening and time depend

shrinkage and creep, occur in many ways. Inadvertent misplacement of tendons

during construction has caused major concrete cracking. Distresss can occur

during the post tensioning or after the period of time and manifested by major

spalls or unexplainable cracks in the vicinity of tendons.

- Evaluating unbonded tendons

The most difficult task in evaluating structure is determining the condition

of unbonded tendons. A non-destructive test to determine the presence of

corrosion on the prestressing steel would be a great help. Since the greater number

of corrosion problems have occurred in tendons “loosely” sheathed, the first step

in the evaluation is to determine the type of tendon system used.

To minimize the amount of concrete removal, try to determine areas of

small concrete cover at the tendon low or high point when selecting the tendon to

probe. If the sheath is soft to touch it is probably a push through as possibly heat

sealed sheath. The sealed type can be identified by the seam at the lap weld. To be

more sure of the type, several tendons may have to be exposed for at least 6 in.

This invasive probe can also be used to determine if there is water in the sheath,

particularly at a tendon low point. Also by removing the sheath at the probe, the

quality of grease protection, and the present corrosion, if any, can be observed.

A non-destructive simple field test, using a vacuum head, can supply

information to enhance ones judgment on the condition of tendon anchorages and,

to some extent, the tendon proper. This vacuum test, suitable for quality control

as well as for the evaluation of existing conditions, is simple to use, fast, and

provides a non-destructive evaluation of the concrete plug joint tightness and the

relative porosity of the parent and anchor plug concrete. Fig. 10 illustrated the

transparent testing head in place over an anchor pocket. The seal is obtained by

special gasket. The battery-operated tester provides the vacuum source and can be

used on most accessible anchorage pockets. When the vacuum is applied, the

adequacy of the anchorage plug to protect against corrosion can be determined

both by the level and rate of decay of the maximum vacuum pressure obtained.

Also by applying a special foaming agent first to the anchor plug cold joint and

separately to the mortar plug and applying the vacuum, the vigor and the nature of

the bubble pattern form reveals the conditions. Vigorous bubbling is most likely

an indication that aggressive materials could be enter the anchor and cause

corrosion.

1.4.Materials and Systems for Prestressing

Strength and endurance are two major qualities that are particularly important in

prestressed concrete structures. Long-term detrimental effects can rapidly reduce the

prestresing forces and could result in unexpected failure. Hence, measures have to be

taken to ensure strict quality control and quality assurance at the various stages of

production an construction as well as maintenance. Figure 2.1. shows the various factors

that result in good-quality concrete.

Resistance to wear deterioration

Strength

Good quality of pasteLow w/c ratioOptimal cement contentSound aggregate, grading

and vibrationLow air content

Economy

Large maximum aggregate sizeEfficient gradingMinimum slumpMinimum cement contentOptimal automated plant operationAdmixtures and entrained airQuality assurance control

Appropriate cement type: low C3A, MgO, free lime; Low

Na2O, and K2O

Resistance to weathering and chemicals

Ideal durable concrete

Appropriate cement typeLow w/c ratioProper curingAlkali-resistant aggregateSuitable admixtureUse of superplatisizers or

polymers as admixturesAir entrainment

Low w/c ratioProper curingDense, homogeneous concreteHigh strength Wear-resisting aggregateGood surface texture

Controlled proportions Controlled material

quality control

Controlled placing and curing

Controlled handling

Higher strength is necessary in prestressed concrete for several reasons. First, in

order to minimize their cost, commercial anchorages for prestressing steel are always

design on the basis of high-strength concrete. Hence weaker concrete either will require

special anchorages or may fail under the application of prestressed. Such failures may

take place in bearing or in bond between steel and concrete, or in tension near anchorage.

Next, some of failure that happen in prestressed concrete structure.

Creep

Creep, or lateral material flow, is the increase in strain with time due to as

sustained load. The initial deformation due to load is the elastic strain, while the

Figure 2.1. Principal properties of good concrete

additional strain due to same sustained load is the creep strain. This practical assumption

is quite acceptable, since the initial recorded deformation includes few time-dependent

effects. The increase in creep strain as in the case of shrinkage, it can be seen that creep

rate decreases with time. Creep cannot be observed directly and cannot be determined

only by deducting elastic strain and shrinkage strain from total deformation. Although

shrinkage and creep are not independent phenomena, it can be assumed that superposition

of strain is valid.

Shrinkage

Basically, there are two types of shrinkage; plastic shrinkage and drying

shrinkage. Plastic shrinkage occurs during the first few hours after placing fresh concrete

in the forms. Exposed surface such as floor slabs are more easily affected by exposure to

dry air because of their large contact surface. Drying shrinkage, in the other hand, occurs

after concrete has already attained its final set and good portion as the chemical hydration

process in the cement gel has been accomplished.

Several factors effect the magnitude of drying shrinkage:

1. Aggregate.

The aggregate act to restrain the shrinkage of the cement paste; hence concrete with

high aggregate content are less vulnerable to shrinkage.

2. Water / cement ratio

The higher the water/cement ratio, the higher the shrinkage effects.

3. Size of concrete element

Both the rate and total magnitude of shrinkage decrease with an increase in the

volume of the concrete element. However, the duration of shrinkage is longer for

larger members since more time needed for drying to reach internal regions.

4. Medium ambient conditions

The relative humidity of he medium greatly affects the magnitude of shrinkage; the

rate of shrinkage is lower t high states of relative humidity. The environment

temperature is another factor, in that shrinkage become stabilized a low temperatures.

5. Amount of reinforcement

Reinforced concrete shrinks less then plain concrete, the relative difference is a

function of the reinforcement percentage.

6. Admixtures

The effect varies depending on the type of admixture. An accelerator such as calcium

chloride, use to accelerate hardening and setting of the concrete, increase the

shrinkage.

7. Type of cement

Rapid-hardening cement shrinks somewhat more than other types, while shrinkage-

compensating cement minimizes or eliminates shrinkage cracking if used with

restraining reinforcement.

8. Carbonation

Carbonation shrinkage are caused by reaction between the carbon dioxide (CO2)

present in the atmosphere and that present in the cement paste.

Corrosion

Protection against corrosion of prestressing steel is more critical in case of

nonprestressed steel. Such as precaution is necessary since the strength of the prestressed

concrete element is the function of the prestressing force, which in turn is the function of

the prestressing tendon area. Reduction of the prestressing steel area due to corrosion can

drastically reduce the nominal moment strength of the prestressed secton, which can lead

to premature failure of the structural system. In postensioned members, protection against

corrosion is provided by the concrete surrounding the tendon, provided that adequate

concrete cover is available. In post-tensioned members, protection can be obtained by full

grouting of the ducts after prestressing is completed or by greasing.

The strength of prestressed-concrete flexural member is dependent upon the

conditions of its tendons throughout their service life; so they must not experience serious

deteriorations due to corrosion. Prestressing steel is subject to normal oxidation in

approximately in same degree as structural-grade steels. Because wire and strain tendons

are normally of small diameter, it is essential that they be protected against significant

oxidation. Bar tendon should be protected too, but because of their relatively large

diameter, normal oxidation is of somewhat less concern with them than with wire and

strand tendons.

Protection against corrosion is effected in pretensioned construction by the

concrete that surrounds by tendons. In bonded post-tensioned construction, the tendons

are protected by grout injected into the duct containing the tendons after the tendons have

been stressed. Unbonded tendons normally are coated with grease, wax, or bituminous

materials and cover with pastic tubing or waterproof paper in factory before being

shipped to the construction site; chemicals that inhibit oxidation of ferrous metals

sometimes are included in the coating applied to tendons used in unbonded construction.

Cathodic Protection

Cathodic protection is being considered for control of corrosion in prestressing

steel. The differences between conventionally reinforced structures and prestessed

structure, in terms of the properties and arrangement of the materials, complicated the

design of cathodic protection system for prestressed structures. Cathodic protection was

used on prestressed concrete pipes, bridge decks, parking garages, under ground tanks,

and marine structures.

Cathodic protection controls the corrosion of metal structures by altering the

electrical potential of the metal so as to restrict the flow of positively charged metal ions

away from metal. This is typically done by means of a cathodic protection circuit as

shown in Fig.1.

+DC -

Fig.1 – A cathodic protection circuit alters the electrical potential and restricts the flow of positively charged metal ions away from the metal.

The negative terminal of a direct current rectifier is connected to the steel to be

protected, and the positive terminal is connected to anode that are placed n or on the

concrete itself. The anode may be series of discrete, closely space elements, linear strips,

a conductive polymer mesh, or some combiation of these. The protective current flows

from the rectifier. In corrosion control terminology the steel is “structure,” and the

concrete is the “electrolyte.”

Preventing hydrogen embrittlement

Hydrogen embrittlement is of particular concern in corrosion of high-strength

steels and the related issue of stress corrosion, cracking, are very complex, poorly

understood phenomena.

Hydrogen is generated at the cathode of an electrochemical cell, when the

potential of the cathode is depressed to the hydrogen evolution potential. The value of

this potential depends on the chemical environment. The risk of hydrogen embrittlement

is therefore increased, as a metal is cathodicall protected. While its has not been a

problem with mild, ductile, reinforcing steel, there is valid concern for this loss of the

limited ductility of high strength prestressing steels.

Hydrogen embrittlement is of particular concern in corrosion of high-strength steels and

the related issues of stress corrosion, cracking, are very complex, poorly understood

phenomena.

Monitoring corrosion activity

Electrical potential measurements conducted in accordance with ASM C-876,

using a copper/copper sulfate half-cell, can provide a valuable indication of the corrosion

activity of steel embedded in concrete. This test is also used to provide ongoing

evaluation of the operation of a cathodic system.

In order to carry out the test, an electrical connection is made from the voltmeter

to the steel, and the circuit is completed through the half-cell placed on the concrete.

(Fig.2). With unbonded steel that may be electrically insulated from the concrete

environment by a plastic or paper sheath, the potential test is of questionable value. Not

only does this inhibit the ability of the engineer to determine where corrosion is

occurring, but it also complicates evaluation of the efficiency of the cathodic protection.

Distribution of the current

Unsheathed strands fully bonded to the surrounding concrete present the greatest

possibilities for cathodic protection that can flow unimpeded from the embedded anode

material to the steel. Prestressing tendons may be electrically insulated from the

surrounding concrete by paper, plastic, grease, or a grouted tube. This not only increases

the difficulty in taking and interpreting the electrical potential measurements, but also

makes it more difficult to insure that the cathodic protection current is distributed to the

corroding areas of the reinforcement.

As shown in Fig.3 there could be air, water, or grease filled gaps between the

sheats and tendon Successful application of cathodic protection requires that current be

passed through the sheath (or more likely through tears and voids in the sheath) to the

according areas of the steel. Cathodic protection of sheated tendons may involve some

modification to the existing structural system (such as the grouting of unbonded tendons)

to provide a direct electrolytic path from the embedded anodes to the corroding steel.

7 wire tendons

Grout

Grout tube

Void space

Plastic sheat

Void space

Fig.3 – Voids between sheath and tendon in bonded or unbonded prestressing steel could restrict current flow and hinder the effectiveness of the cathodic protection

Durability of Post-Tensioned Prestressed Concrete Structure

Durability research

The surveys of U.S. experience in these reports support the conclusion that

corrosion of post-tensioning tendons is not significant problems in highway bridges.

Accelerated laboratory tests at the University of Texas at Austin have demonstrated that

post-tensioning can provide substantial improvement of durability by eliminating cracks

and limiting crack width.

The research further shows that high-quality concrete and generous cover over

reinforcement cannot protect reinforcement in the presence of cracks. The exposure tests

of unbonded tendons specimens and other tests show that properly fabricated and

installed unbonded tendons provide excellent protection against corrosion in aggressive

environment. NCHRP Report 313 has documented the excellent corrosion resistance of

conventional post-tensioning details and the increase in corrosion resistance possible

through use of newer materials including epoxy coated strand and polyethylene duct.

Durability experience with post-tensioned structures

Reports on the durability of actual post-tensioned buildings and bridges are

presented separately since bridge construction uses mainly grouted or bonded tendons

and buildings are constructed mainly with unbonded and greased and shealthed tendons.

Notwithstanding the preceding favorable discussion, it must be acknowledge that

significant corrosion problems have occurred in isolated bridges and buildings with

unbonded tendons, primarily parking structures in aggressive and corrosive

environments. Loosely sheathed tendons with damaged sheath, poor grease coverage, or

poor end anchorage protection have been the greatest source of corrosion problems with

the extruded-type sheath have experienced comparatively few corrosion problem.

Post-tensioned bridges

Field experience concerning the durability of post-tensioned bridges in the U.S. validates

the high corrosion resistance and durability potential indicated by the research results

noted previously. Serious durability problems with post-tensioned bridges have been rare

and generally have been associated with obvious detail or construction deficiencies. The

use of post-tensioning essentially eliminates cracking in highway bridges, allowing the

corrosion protection of the high-quality concrete used in bridge construction to work.

There is no evidence of corrosion in the stay cables of any U.S. cable-stayed bridges, and

current technology provides cable-stays with a high degree of resistance both during

construction and in the completed structure.

Post-tensioned buildings

Enclosed buildings -

Rehabilitating Parking Structures with Corrosion Damaged Button-Headed Post-

Tensioning Tendons

The boom of prestressing parking structures in the United States began in 1960s

and early 1970s. Many of the early long-span concrete parking structure were built using

the paper-wrapped button headed wire post-tensioning system (or BBRV system).After

10 to 15 years of service, many of these parking structures heve been found to be in

deteriorates conditions with varying degrees of damage to post-tensioning system.

Typical problem experienced with unbonded paper-wrapped button headed post-

tensioning system in parking structures

Typical structural system characteristics

Many of these structures were designed with a higher degree of post tensioning. Design

of that period often produced either a “zero tension” slab and beam design, or limited

tensile stresses under working loads less than 3 fc’. This meant that service load stresses

rather than ultimate strength considerations governed quantity of reinforcement

provided………..

Corrosion protection of unbonded tendons

Generally, parking structures with unbonded button-headed post-tensioning were

designed utilizing a one-way slab and beam system. The slab tendons were usually shop

fabricated using the varying number of ¼ in. (6mm) diameter, 240 ksi (1655 Mpa) stress

relieved wires. Most slab tendons contain six or seven wires, however, the number can

vary from four to ten. The wires were greased, sometimes by hand and spirally wrapped

with a reinforced kraft paper. The purpose of the sheathing, that is the reinforced kraft

paper, was primarily to act as a bond breaker to prevent concrete from coming in contact

with the wires of the tendons, thereby allowing them to move freely in the slab when the

tendon was tensioned. The craft paper also held the grease in place on the wires during

shipping, handling and concrete placement. The function of the grease was seen to be

primarily to lubricate the tendon to allow for lower friction losses during tensioning.

Corrosion protection of the wires was secondary function of the grease and was thought

to be required for a limited duration until the tendons were “safely” embedded in the

concrete.