assignment prestressed
TRANSCRIPT
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.