Wedge-receiving cavity for an anchor body of a post-tension anchor system

An improved anchor for a post-tension system having an anchor body with an internal wedge-receiving cavity. The cavity has a first portion of constantly diminishing diameter extending inwardly from one end of the anchor body. The first portion has an angle of taper with respect to a center line of the cavity. The cavity has a second portion extending inwardly from an opposite end of the anchor body. The first portion and the second portion are coaxial and communicate with each other. The second portion has an angle of taper which is less than the first portion. The first and second portions are cast with the anchor body. The angle of taper of the second portion is less than seven degrees or a negative angle with respect to the center line.

TECHNICAL FIELD 
The present invention relates to post-tension anchor systems, in general. 
More particularly, the present invention relates to the structure of an 
anchor body for such post-tension systems. Furthermore, the present 
invention more specifically relates to the formation of the cavity within 
the interior of the anchor body. 
BACKGROUND ART 
For many years, the design of concrete structures imitated typical steel 
design of column, girder and beam. With technological advances in 
structural concrete, however, its own form began to evolve. Concrete has 
the advantages of lower cost than steel, of not requiring fireproofing, 
and of its plasticity, a quality that lends itself to free flowing or 
boldly massive architectural concepts. On the other hand, structural 
concrete, though quite capable of carrying almost any compressive 
(vertical) load, is extremely weak in carrying significant tensile loads. 
It becomes necessary, therefore, to add steel bars, called reinforcements, 
to concrete, thus allowing the concrete to carry the compressive forces 
and the steel to carry the tensile (horizontal) forces. 
Structures of reinforced concrete may be constructed with load-bearing 
walls, but this method does not use the full potentialities of the 
concrete. The skeleton frame, in which the floors and roofs rest directly 
on exterior and interior reinforced-concrete columns, has proven to be 
most economic and popular. Reinforced-concrete framing is seemingly a 
quite simple form of construction. First, wood or steel forms are 
constructed in the sizes, positions, and shapes called for by engineering 
and design requirements. The steel reinforcing is then placed and held in 
position by wires at its intersections. Devices known as chairs and 
spacers are used to keep the reinforcing bars apart and raised off the 
form work. The size and number of the steel bars depends completely upon 
the imposed loads and the need to transfer these loads evenly throughout 
the building and down to the foundation. After the reinforcing is set in 
place, the concrete, a mixture of water, cement, sand, and stone or 
aggregate, of proportions calculated to produce the required strength, is 
placed, care being taken to prevent voids or honeycombs. 
One of the simplest designs in concrete frames is the beam-and-slab. This 
system follows ordinary steel design that uses concrete beams that are 
cast integrally with the floor slabs. The beam-and-slab system is often 
used in apartment buildings and other structures where the beams are not 
visually objectionable and can be hidden. The reinforcement is simple and 
the forms for casting can be utilized over and over for the same shape. 
The system, therefore, produces an economically viable structure. With the 
development of flat-slab construction, exposed beams can be eliminated. In 
this system, reinforcing bars are projected at right angles and in two 
directions from every column supporting flat slabs spanning twelve or 
fifteen feet in both directions. 
Reinforced concrete reaches its highest potentialities when it is used in 
pre-stressed or post-tensioned members. Spans as great as 100 feet can be 
attained in members as deep as three feet for roof loads. The basic 
principal is simple. In pre-stressing, reinforcing rods of high tensile 
strength wires are stretched to a certain determined limit and then 
high-strength concrete is placed around them. When the concrete has set, 
it holds the steel in a tight grip, preventing slippage or sagging. 
Post-tensioning follows the same principal, but the reinforcing is held 
loosely in place while the concrete is placed around it. The reinforcing 
is then stretched by hydraulic jacks and securely anchored into place. 
Prestressing is done with individual members in the shop and 
post-tensioning as part of the structure on the site. 
In a typical tendon tensioning anchor assembly in such post-tensioning 
operations, there is provided a pair of anchors for anchoring the ends of 
the tendons suspended therebetween. In the course of installing the tendon 
tensioning anchor assembly in a concrete structure, a hydraulic jack or 
the like is releasably attached to one of the exposed ends of the tendon 
for applying a predetermined amount of tension to the tendon. When the 
desired amount of tension is applied to the tendon, wedges, threaded nuts, 
or the like, are used to capture the tendon and, as the jack is removed 
from the tendon, to prevent its relaxation and hold it in its stressed 
condition. 
Metallic components within concrete structures may be come exposed to many 
corrosive elements, such as de-icing chemicals, sea water, brackish water, 
or spray from these sources, as well as salt water. If this occurs, and 
the exposed portions of the anchor suffer corrosion, then the anchor may 
become weakened due to this corrosion. The deterioration of the anchor can 
cause the tendons to slip, thereby losing the compressive effects on the 
structure, or the anchor can fracture. In addition, the large volume of 
by-products from the corrosive reaction is often sufficient to fracture 
the surrounding structure. These elements and problems can be sufficient 
so as to cause a premature failure of the post-tensioning system and a 
deterioration of the structure. 
FIGS. 1 and 2 illustrate various components of a typical post-tension 
assembly designated generally at 10. System 10 includes a tendon 12 having 
an exposed end protruding from a sheath 14. The end of the tendon 12 is 
typically fitted through an extension tube 16. Extension tube 16 has a 
diameter slightly larger than sheath 14 such that one end 16a of tube 16 
may overlie sheath 14. The opposite end 16b of tube 16 fits over, and 
communicates with, a rear tubular portion 18 of an anchor 20. Rear tubular 
member 18 includes an aperture (not shown) which communicates with a 
frontal aperture 22. Frontal aperture 22 defines a cavity in which wedges 
24 and 26 are received as shown in FIG. 2, below. 
FIG. 2 illustrates an assembled view (in one-fourth cutaway perspective) of 
system 10 shown in FIG. 1. As known in the art, tendon 12 is disposed 
through extension tube 16 and through anchor 20. In one known embodiment, 
end 16b of extension tube 16 is force-fitted over rear tubular member 18. 
The other end 16a of extension tube 16 is sealed to sheath 14, by use of 
tape or other means. 
After tendon 18 extends through frontal aperture 22 (see FIG. 1), and 
assuming the far end of the tendon (not shown) is fixed in place, tension 
is applied to tendon 16, typically by use of a hydraulic jack. While 
applying this tension, wedges 24 and 26 are forced in place on both sides 
of tendon 12 within the wedge cavity defined by aperture 22. Once in 
place, teeth 24a and 26a of wedges 24 and 26 operate to lock tendon 12 in 
a fixed position with respect to anchor 20. Thereafter, the tension 
supplied by the hydraulic device is released and the excess tendon 
extending outward from anchor 20 is cut by a torch or other known device. 
Wedges 24 and 26 thereafter prevent tendon 12 from releasing its tension 
and retracting inward with respect to anchor 20. Moreover, this tension 
provides additional tensile strength across the concrete structure. 
After years of work with the anchor body of the prior art, it was found 
that the cavity used in the anchor body created many problems. The cavity 
in the anchor body is of a constantly diminishing diameter extending from 
a forward end of the anchor body to a rearward end of the anchor body. 
This internal cavity of constantly diminishing diameter is formed during 
the casting of the anchor body. Unfortunately, the narrow diameter end of 
the cavity creates problems with the installation of tendons in a 
corrosion-resistant environment. 
When the anchor body is used in the formation of intermediate anchorages, 
it is often necessary to move the anchor body over a very long length of 
sheathed tendon. If there is insufficient clearance between the narrow 
diameter end of the cavity and the outer diameter of the sheathed portion 
of the tendon, nicks, abrasions, and cuts can occur in the 
corrosion-resistant sheathing. As such, the integrity of the anchorage 
system is impaired. Furthermore, there are circumstances where the 
sheathing may exceed expected tolerances and will prevent the anchor body 
from easily sliding along the length of the tendon so as to assume its 
position as an intermediate anchorage. Additionally, in recent years, 
there has been a tendency to increase the thickness of the sheathing so as 
to facilitate greater protection of the tendon from corrosive elements. 
An easy solution to this problem would be to expand the diameter of the 
cavity so as to avoid the aforementioned problems. Unfortunately, if the 
diameter of the cavity is expanded, then conventional wedges cannot be 
used. Problems would further occur because of the use of larger wedges or 
of irregular wedges. If the cavity were enlarged, then the wedge 
components would have to be replaced in all such post-tension anchor 
systems. Furthermore, the use of variant sized wedges could create new 
problems associated with the tensioning of the anchor system. 
It is also possible to drill out the narrow diameter end of the cavity so 
as to produce a portion of generally constant diameter. However, any past 
attempts at drilling have been unsuccessful for a number of reasons. 
First, the drilling is a very expensive process in comparison with the 
casting of the anchors. Furthermore, the drilling of a constant diameter 
portion in the anchor body can create burrs and deformations which could 
potentially cut the sheathing of the tendon and cause adverse 
corrosion-protection results. Finally, the drilling of the hole can 
intrude into the wedge-receiving area so as to create an uneven and 
irregular contact area between the wedges and the wall of the cavity. 
U.S. patent application Ser. No. 09/007,608, filed on Jan. 15, 1998, by the 
present inventor, describes an improved anchor for a post-tension system 
which has a cavity with a first portion of constantly diminishing diameter 
and a second portion of constant diameter. The first and second portions 
are coaxial and communicate with each other. The first portion extends 
inwardly from one end of the anchor body while the second portion extends 
inwardly from the opposite end of the anchor body. This improved anchor is 
a cast anchor. However, it was found that the formation of the second 
portion of "constant diameter" created problems during the casting 
process. It is known that for making cast objects, it is often difficult 
to cast or form cavities of constant diameter. Constant diameter cavities 
in objects often require complex forms and molds in order to create such 
constant diameter cavities. As such, the constant diameter second portion 
created certain manufacturing problems. 
It is an object of the present invention to provide an improved anchor for 
a post-tension anchor system which allows for the use of existing wedges 
in the wedge cavity while enlarging the narrow end of the cavity. 
It is a further object of the present invention to provide an improved 
anchor body with a cavity with a wide end which requires no machining. 
It is a further object of the present invention to provide an anchor body 
that avoids sharp edges and irregular contact surfaces. 
It is still another object of the present invention to provide an anchor 
body which enhances the corrosion resistance of the post-tension anchor 
system. 
It is still a further object of the present invention to provide an 
improved anchor body which is relatively inexpensive, easy to manufacture, 
and easy to use. 
These and other objects and advantages of the present invention will become 
apparent from a reading of the attached specification and appended claims. 
SUMMARY OF THE INVENTION 
The present invention is an improved anchor for a post-tension system which 
includes an anchor body having an internal wedge-receiving cavity. The 
cavity has a first portion having an angle of taper of constantly 
diminishing diameter extending inwardly from one end of the anchor body. 
The cavity has a second portion extending inwardly from an opposite end of 
the anchor body. The first portion and the second portion are coaxial and 
communicate with each other. The first and second portions are cast with 
the anchor body. The second portion has an angle of taper less than or 
negative to the angle of taper of the first portion. 
The first portion has a wide end opening at the one end of the body. The 
first portion has a narrow end of a diameter no less than the constant 
diameter of the second portion. The first portion is tapered at an 
approximately 7.degree. angle relative to the centerline of the cavity. 
The second portion extends inwardly from the opposite end of the anchor 
body for no less than one-quarter of an inch. In one form of the present 
invention, the second portion has a larger diameter at the opposite end of 
the anchor body than the diameter at the narrow end of the first portion. 
In the present invention, a tendon extends through the cavity. A plurality 
of wedges are arranged in interference fit relationship between a wall of 
the first portion and an exterior surface of the tendon. Each of the 
plurality of wedges has a length not more than a length of the first 
portion. The tendon has a sheathing extending thereover. The second 
portion has a diameter greater than the diameter of the sheathing on the 
tendon.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIG. 3, there is shown at 30 the improved post-tension anchor 
system in accordance with the teachings of the present invention. The 
post-tension anchor system 30 includes an anchor body 32, a tendon 34 
extending through a cavity 36 in the anchor body 32, and a 
corrosion-protection tube 38 extending around an exterior of the tendon 
34. The tendon 34 is retained in proper position within the anchor body 32 
through the use of wedges 40 and 42. Wedges 40 and 42 are arranged in 
interference fit relationship between the wall of the cavity 36 and the 
exterior surface of the tendon 34. The tendon 36 has one end 44 extending 
outwardly of a forward end 46 of the anchor body 32. A tubular portion 48 
is formed in encapsulation material 50 so as to extend outwardly from an 
opposite end of the anchor body 32. The encapsulation material 50 is a 
polymeric encapsulation that surrounds the steel anchor 52. 
In the present invention, the cavity 36 has a first portion 54 of 
constantly diminishing diameter extending inwardly from end 56 of the 
steel anchor 52. The internal cavity 36 also includes a second portion 58 
extending extending inwardly from an opposite end 60 of the steel anchor 
52. The first portion 54 communicates with the second portion 58. The 
first portion is coaxial with the second portion 58. The second portion 58 
has an angle of taper which is less than the angle of taper of the first 
portion 54. Alternatively, as will be described hereinafter, the angle of 
taper of the second portion can actually be a negative angle relative to 
the angle of taper of the first portion. As used herein, the term "angle 
of taper" refers to the angle of the wall of the cavity 36 in relation to 
the center line 45 of the anchor body 32. 
As can be seen in FIG. 3, the wedges 40 and 42 are fit within the first 
portion of the cavity 36. Wedges 40 and 42 are standard wedges used in 
conventional prior art anchor systems. The first portion 54 of cavity 56 
will have a diameter similar to the diameter of existing cavities. The 
taper of the first portion 54 is approximately a 7.degree. angle relative 
to the longitudinal axis of cavity 36. 
Unlike prior art systems, the second portion 58 has an angle of taper which 
is less than the angle of taper of the first portion 54. The second 
portion 58 extends from an end of the first portion 54 to the end 60 of 
the steel anchor 52. Prior art anchor systems had a constant taper 
extending from end 56 to end 60 of the steel anchor 52. As can be seen, a 
significant clearance 64 is formed between the wall of the second portion 
58 of cavity 36 and the exterior surface of tendon 34. As such, the steel 
anchor 52 can slide easily along the length of the tendon 34 without 
causing damage to the sheathing on the tendon. Furthermore, since the 
first portion 54 is formed by casting with the second portion 58, no 
machining is required. As such, the expense of the production of the 
anchor body 32 is reduced. There will be no sharp burrs or snarled edges 
which could compromise the integrity of the sheathing of the tendon 34. 
In FIG. 3, it can be seen that the corrosion-protection tube 38 includes a 
forward end 66 having a spearhead-shaped configuration. The wide end of 
the spearhead-shaped configuration is in snap-fit engagement with a 
shoulder 70 formed on the interior of the tubular portion 48. The end 72 
of the tubular portion 48 will abut a shoulder 74 formed on the exterior 
surface of the corrosion-protection tube 38. A sealing member 76 is formed 
on the opposite end of the corrosion-protection tube 38 so as to establish 
a liquid-tight seal with the exterior surface of the tendon 34. In order 
to install the corrosion-protection tube 38, it is only necessary to 
insert the spearhead-shaped end 66 into the opening at end 72 of the 
tubular portion 48. The corrosion-protection tube 38 can then be pushed 
inwardly of the tubular portion 48 until the end 66 snap-fits over the 
shoulder 70. 
FIG. 4 is an isolated view showing the anchor body 32. In FIG. 4, it can be 
seen that the first portion 54 is illustrated as tapering from end 56. The 
narrow end 80 of the first portion 54 connects with the second portion 58. 
The second portion 58 extends from the end 80 of the first portion 54 to 
the end 60 of the steel anchor 52. The second portion 58 has an angle of 
taper less than or negative to the first portion 54. Within the concept of 
the present invention, it is possible that the second portion 80 could 
have a slightly increasing diameter extending from the end 80 of the first 
portion 54 toward the end 60 of the steel anchor 52. The second portion 58 
will have a diameter of between 0.6 and 0.7 inches. In the preferred 
embodiment of the present invention, the second portion 58 will extend 
inwardly from the end 60 for no less than one-quarter of an inch. The 
first portion 54, as can be seen, has a length which is greater than the 
length of the second portion 58. 
Referring to FIG. 1, it can be seen that the steel anchor 52 of anchor body 
32 has a cavity 36 with a first portion 54 that extends inwardly from the 
end 56. It can be seen in FIG. 5 that the first portion 54 has a constant 
diameter extending from the end 56 to the end 80. The first portion 54 
transitions to the second portion 58 at end 80. 
FIG. 5 illustrates how the present invention differs from the prior art. As 
can be seen by the broken line 82, the constantly decreasing diameter of 
the first portion 54 would normally continue from end 56 to end 60. 
However, since the second portion 58 has a taper which is less than the 
angle of taper of the first portion 54, the second portion 58 will emerge 
on end 68 with a greater diameter than which would occur by the 
continuously tapering first portion 54. As shown in FIG. 5, the angle of 
taper of the first portion 54 is shown by .alpha.. The angle of taper of 
the second portion 58 is shown by .beta.. In accordance with the present 
invention, angle .beta. is less than angle .alpha.. 
FIG. 6 shows an alternative embodiment of the present invention. In FIG. 6, 
the anchor body 90 has a first portion 92 and a second portion 94. The 
first portion 92 opens at end 96 of the anchor body 90. The second portion 
94 opens at the opposite end 98 of the anchor body 90. The first portion 
92 of cavity 100 connects with the second portion 94 at end 102. 
It can be seen in FIG. 6 that the first portion 92 has a constantly 
narrowing diameter extending from end 96 to the end 102. The second 
portion 94 has a constantly expanding diameter as extending from the end 
102 to the end 98. Alternatively stated, the second portion 94 constantly 
tapers so as to narrow in diameter from the end 98 of anchor body 90 to 
the end 102 of the first portion 92 of cavity 100. 
As can be seen in FIG. 6, the first portion 92 has an angle a relative to 
the center line 104. The second portion 94 forms an angle .beta. with 
respect to the center line 104. The angle .beta. is negative to angle 
.alpha.. As such, angle .beta. is also less than angle .alpha. relative to 
the center line 104. 
The embodiment shown in FIG. 6 offers certain advantages. For example, the 
constantly widening diameter of the second portion 94 (as extending from 
end 102 to the end 98) will serve to "funnel" the tendon through the 
cavity 100 of the anchor body 90. This outward taper can, under certain 
circumstances, facilitate the manufacture and casting of the anchor body 
90. Since the second portion 94 has a greater diameter at end 98 than the 
diameter of the first portion 92 at end 102, the second portion 94 will 
suitably accommodate the sheathing on a tendon extending therethrough. 
FIG. 7 shows an alternative embodiment 110 of the present invention. The 
alternative embodiment 110 includes an anchor body 112 having a cavity 114 
extending therethrough. The cavity 114 includes a first portion 116 and a 
second portion 118. A center line 120 is illustrated as extending through 
the interior of the cavity 114. The first portion 116 will extend from end 
122 of the anchor body 112 to the end 124 of the first portion 116. The 
first portion 116 will transition into the second portion 118 at end 124 
The second portion 118 has a generally non-constantly expanding diameter 
extending from the end 124 to the end 126 of the anchor body 112. As can 
be seen, the wall 128 of the second portion 118 is suitably radiused so as 
to expand outwardly at end 126. The second portion 118 has a wide diameter 
end at the end 126 of the anchor body 112 and a narrow diameter end at the 
narrow end 124 of the first portion 116. Generally speaking, the angle 
formed by the second portion 118 relative to the center line 120 will be a 
negative angle relative to the angle formed between the wall of the first 
portion 116 and the center line 120. As such, the angle of taper of the 
second portion 118 will be less than (or negative to) the angle of taper 
of the first portion 116. 
The radiused walls of the second portion 118 are somewhat different to 
configure in a manufacturing process but would offer a different advantage 
than the previous embodiments of the present invention. The smooth curving 
of the wall 128 of the second portion 118 will serve to "funnel" the 
tendon through the cavity 114. Furthermore, while accomplishing this 
"funneling" purpose, the smooth transition offered by the curved wall 128 
will prevent any inadvertent nicking of the sheathing of the tendon at the 
area of the transition end 124. 
The foregoing disclosure and description of the invention is illustrative 
and explanatory thereof. Various changes in the details of the illustrated 
construction may be made within the scope of the appended claims without 
departing from the true spirit of the invention. The present invention 
should only be limited by the following claims and their legal 
equivalents.