Unbonded Post-Tension Strand Protector
An unbonded post-tension strand protector (200) and related method of use is provided. The unbonded post-tension strand protector is used to protect one or more unbonded post-tension strands (101) embedded in concrete from damage due to drilling or chipping. Unbonded post-tension strands embedded in poured concrete are tensioned to improve the compressive strength of concrete. The unbonded post-tension strand protector is placed over at least one selected portion of an unbonded post-tension strand (101).
The present invention relates to a device and method for protecting strands used in pre-stressed concrete. More particularly, the invention relates to a protector and method for protecting unbonded post-tension strands.
DESCRIPTION OF THE RELATED ARTConcrete has long been used in the construction industry for its great compressive strength. However, concrete is inherently weak when placed under tension. For general strengthening, concrete has been reinforced by the use of steel reinforcement bars, called rebar, inside poured concrete. The high tensile strength of steel, combined with concrete's great compressive strength provides improved results under both tension and compression when the structure is cast.
To further overcome concrete's natural weakness in tension, methods of pre-stressing concrete with the use of steel have been developed. The principle behind pre-stressed concrete is that compressive stresses induced by high-strength “tendons” in a concrete member before loads are applied will balance the tensile stresses imposed in the member during service. Pre-stressing removes a number of design limitations conventional reinforced concrete places on span and load. Pre-stressed concrete methods are used to produce beams and floors with longer unsupported spans than is practical with ordinary reinforced concrete. This allows architects and engineers to design and build lighter and shallower concrete structures without sacrificing strength.
When referring to a “tendon,” it is a term that encompasses several components. This includes a strand, which is a cable that is conventionally manufactured in ½ inch, or 6/10 inch diameter sizes. This strand is made from very high-tensile-strength steel wire, usually seven wires twisted together. In addition, the term also includes an anchorage having a cast-iron bearing plate and special wedges to secure the strand inside an anchor housing.
There are two main methods for inducing compressive strength in pre-stressed concrete: pre-tensioning and post-tensioning the steel reinforcement. In pre-tensioning, the steel is stretched before the concrete is poured. High-strength tendons are placed between two abutments is poured into molds around the tendons and allowed to cure. Once the concrete reaches the required strength, the stretching forces are released. As the steel of the tendons reacts to return to its original length, the tensile stresses are transferred into a compressive stress in the concrete.
In contrast, post-tensioning stretches the steel after the concrete hardens. Notably, the concrete is cast around but does not contact the unstretched steel. There are two types of post-tension systems: bonded and unbonded. With a bonded system, steel, aluminum, or plastic ducts are positioned in the formed area where tension would otherwise occur in the concrete element. The ducts are then attached to anchorages at either end. Once the poured concrete has hardened to the required strength, the strands are threaded through the ducts and tensioned. The tendons are tensioned by hydraulic jacks that urge against the concrete member itself. When the tendons have been stretched to design specifications, they are locked in position to maintain tension after the jacks have been removed. Thus, the locking action imparts a tensile force to the concrete. The ducts are then filled with a special grout designed to protect the tendons from corrosion.
Unbonded systems do not employ a duct system like the bonded systems described above. Instead, unbonded systems use stands surrounded with special corrosion-inhibiting grease and encased in waterproof plastic sheaths. This assembly is positioned within a mold and then has concrete poured around it, similar to standard reinforced concrete. Unbonded systems are conventionally used for building and floor slab construction, while bonded systems are used mostly for bridge construction.
Once the pre-stressed concrete members have been incorporated into the overall design of a structure, other features of the structure can be installed. Such features can include, but are not limited to: the installation of doors and windows, installing anchor bolts into elevator beams, laying plumbing and electric piping, installation of column and sheer walls, and attaching anchor supports on balconies where personnel hoist braces are attached. These features typically require drilling and/or chipping of the pre-stressed concrete which is tensioned using unbonded post-tension stands. Aside from the plastic sheath, the unbonded strands are relatively unprotected from drilling or chipping when installing the other features of the structure.
Sometimes in the process of drilling and/or chipping, a construction worker will accidentally damage the highly tensioned strands that are hidden within the concrete. Given the extremely high tension force of these strands (typically 30,000 pounds of force), the slightest nick, cut, or impact to these strands can result in the strand popping out of the concrete structure with extreme force. It is akin to a stretched rubber band that snaps. This is a potential safety hazard that has resulted in a great number of construction injuries (i.e. impalements, lacerations, amputations) and deaths.
In addition to the problem of construction worker safety, a cut strand compromises the strength and stability of the concrete structure itself. This could lead to immediate or gradual structural failure as greater stress and strain is applied to the remainder of the structure. When there are apparent indications of strand damage (i.e. the strand popping out of the concrete body), there may be a chance to repair the concrete structure before any additional or irreversible damage is incurred. However, repairing these unbonded post-tension systems is quite expensive and dangerous.
There are also instances where there could be no immediate indication that the strands and the pre-stressed concrete have been compromised. Unfortunately, in the absence of external indicators there is no non-invasive way, such as X-ray analysis, of detecting whether a pre-stressed concrete form has in fact been compromised. Considering that a typical concrete floor slab can have between 100-300 hidden strands, the task of inspecting each strand can be daunting.
To date, the problem of protecting unbonded post-tension strands from drilling and chipping has gone unsolved. Therefore, what is need in the art is a device and method that can protect unbonded post-tension strands from drilling or chipping. At the same time, the device should be adapted such that it can be strategically and removably placed in a location where selected portions of the unbonded post-tension strands(s) are protected from damage. Such a device and method would minimize both weight and construction cost, while reducing the risk of potentially life-threatening injures.
SUMMARY OF THE INVENTIONThe invention is an unbonded post-tension strand protector. The protector includes a plate that is sized and shaped to be place over at least one selected portion of the unbonded post-tension strand. The one or more selected portions are protected from damage caused by drilling or chipping. Moreover, the one or more selected portions are located in several locations. These locations include under and over doors and windows, adjacent to elevator beams where anchor bolts will be installed, around sleeves for plumbing and electric lines, where column and sheer wall forms are to be installed, on balconies where personnel hoist braces are attached, or at portions of a cast concrete body that require drilling or chipping.
In another aspect of the invention, the invention relates to a protector for protecting one or more unbonded post-tension strands. The protector includes one or more plates having an elongated length and a predetermined width. Moreover, a portion of the plate(s) are located in proximity to one or more unbonded post-tension strands for protecting the unbonded post-tension strand(s) along the elongated length of the plate(s).
In one embodiment of the invention, the plate is formed of steel. In addition, there are several embodiments of the plate's size and shape. According to one embodiment of the invention, the plate is sized and shaped to be removably clipped to at least one unbonded post-tension strand. In another embodiment, the plate includes a planar shape. The planar shape attaches to the unbonded post-tension strand using clamps extending from the plate. Another embodiment, the plate is comprised of at least one recess. The recess partially encases each unbonded post-tension strand. As an alternative, each recess is located adjacent to one another. As another separate embodiment, a first plate is sized and shaped to be removable clipped to at least one unbonded post-tension strand. Moreover, a second plate is shaped and sized to be removably clipped to the first plate.
In another aspect of the invention, the invention relates to a method for protecting one or more unbonded post-tension strands in a pre-cast concrete body. The method includes selecting one or more unbonded post-tension strand having either a half inch, three-eight inch, or six-tenth inch diameter. At least one unbonded post-tension strand is positioned in a pre-cast concrete body before the concrete is cast in the pre-cast concrete body. Selected portions of one or more unbonded post-tension strands to be protected are determined.
The selected portions are placed in a location where said selected portions are protected from damage caused by drilling or chipping. According to one aspect of the method, the selected portions are placed under and over windows, adjacent to elevator beams where anchor bolts are to be installed, around sleeves for plumbing and electric lines, where column and sheer wall forms are to be installed, on balconies where personnel hoist braces are attached, and portions of a cast concrete body that require drilling or chipping.
The selected portions of the one or more unbonded post-tension strands are protected by placing a least a portion of a protective layer, adjacent to the selected portion. According to one embodiment, the protective layer of the protecting step includes a first plate that is sized and shaped to be clipped to one or more unbonded post-tension strands. As an alternate embodiment, a second plate is sized and shaped to be clipped to the first plate. According to another embodiment, the protective layer of the protecting step includes a planar shaped plate that attaches to the unbonded post-tension strand using clamps extending from the planar shaped plate. According to yet, another embodiment, the protective layer of the protecting step is selected to have one or more recesses configured to partially encase one or more unbonded post-tension strands. A plurality of equally-spaced unbonded post-tension strands is uniformly aligned by joining a plurality of protective layers.
At least one region 100 outlines selected portions of the plurality of unbonded post-tension strands 101 that are to be protected from potential damage caused by drilling and/or chipping through the concrete body 105 to the point the it impacts the underlying unbonded post-tension strands 101. Typically locations where region 100 can be found include where drilling and/or chipping occurs. These locations include, but are not limited to, under and over doors and windows, adjacent to elevator beams where anchor bolts are to be installed, around sleeves for plumbing and electric lines, where column and sheer wall forms are to be installed, and on balconies where personnel hoist braces are attached.
To protect the selected portions of the plurality of unbonded post-tension strands 101, an unbonded post-tension strand protector 200 (
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The U-shaped plate 201 is sized and shaped to be clipped to one or more unbonded post-tension strands 101. In this regard, the word “clipped” means that the unbonded post-tension strand 101 is inserted through the opens side portion of the U-shaped plate 201 and is secured within the U-shaped plate 201. The unbonded post-tension strand(s) 101, when clipped, would be bounded by the first planar portion 202, the second planar portion 204, the arcuate portion 203, the convex portions 304. The U-shaped plate 201 is designed to protect the top side, bottom side and a single side of the unbonded post-tension strand 101 from any drilling or chipping that would otherwise damage the unbonded post-tension strand 101. According to a preferred embodiment, the U-shaped plate 201 is formed of a hard material, such a steel. However, the invention in not limited in this regard and other materials can be used so long as the U-shaped plate 201 can resist penetration by a drill bit 210 or a tip of a chipping hammer (not shown).
The unbonded post-tension strand protector 200 is advantageous in that it permits the construction worker to adjust the position of the unbonded post-tension strand protector 200 relative to the unbonded post-tension strand 101. In addition, it is advantageous to place the U-shaped plate 201 on selected portions of the elongated length of the unbonded post-tension strand 101 as compared to placing the U-shaped plate 201 along the entire length of the unbonded post-tension strand 101. The advantage is that the above arrangement minimizes the additional weight and cost of the overall structure by using the minimum amount of material forming the unbonded post-tension strand protector 200.
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The second U-shaped plate 802 is identically shaped as the first U-shaped plate 801, yet different in size. The second U-shaped plate 802 is comprised of a third planar portion 807, a fourth planar portion 808 that is spaced apart from and parallel to the third planar portion 807, a second arcuate portion 809 that is connected to the third and fourth planar portions 807, 808, and a second open side portion 809 located opposite the second arcuate portion 809. The principal difference between the first and second U-shaped plates is that the second arcuate portion 809 is larger than the first arcuate portion 805. As a result, the distance separating the third and fourth planar portions 807, 808 is larger than the distance separating the first and second planar portions 803, 804. This allows the second U-shaped plate 802 to fit over the first U-shaped plate 801.
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In addition, second distal ends 905 of the second U-shaped plate 802 are each formed with a second undulation 906. Each second undulation 906 is curved away from the other such that each of their respective second convex portions 907 are spaced from and facing each other. Since the second undulations 906 are intended to securely fasten the second U-shaped plate 802 to the first U-shaped plate 801, the closest distance between second convex portions 907 is less than the closest distance between first and second planar portions 803, 804.
The first U-shaped plate 801 is sized and shaped to be clipped to one or more unbonded post-tensions strands 101. A second U-shaped plate 802 is sized and shaped to be clipped over the first U-shaped plate 801. In this regard, the word “clipped” means that the first U-shaped plate 801, which is already secured to a portion of the unbonded post-tension strand 101, is in turn inserted through the second open side portion of the second U-shaped plate 802. The first and second U-shaped plates 801, 802 are clipped to one another such that they are arranged in opposite orientations relative to each other. The first U-shaped plate 801 is secured within the second U-shaped plate 802. The first U-shaped plate 801, when clipped, would be bounded by the third planar portion, the fourth planar portion, the second arcuate portion, and the second convex portions. In effect, the portion of the unbonded post-tension strand 101 secured by the unbonded post-tension strand protector 800 would now be protected on all sides from any drilling or chipping that would otherwise damage the unbonded post-tension strand discussed in embodiment 200 of the unbonded post-tension strand protector, shown in
According to a preferred embodiment, the first and second U-shaped plates 801, 802 are formed of a hard material, such as steel. However, the invention is not limited in this regard and other materials can be used so long as the first U-shaped plate 801 and the second U-shaped plate 802 can resist penetration by a drill bit 210 or a tip of a chipping hammer (not shown).
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All of the apparatus and methods disclosed and claimed herein can be made and executed without undue experimentation in light of the present disclosure. While the invention has been described in terms of preferred embodiments, it will be appreciated by those skilled in the art that variations may be applied to the apparatus, methods and sequence of steps of the method without departing from the concept, spirit and scope of the invention. More specifically, it will be apparent that certain components may be added to, combined with, or substituted for the components described herein while the same or similar results would be achieved. All such similar substitutes and modifications apparent to those skilled in the art are deemed to be within the spirit, scope and concept of the invention as defined. Accordingly, the particular arrangements disclosed are meant to be illustrative only an not limiting as to the scope of the invention which is to be given the full breadth of the appended claims and any and all equivalents thereof.
Claims
1. A method for protecting at least one unbonded post-tension strand in a pre-cast concrete body, comprising the steps of:
- selecting at least one unbonded post-tension strand;
- positioning said at least one unbonded post-tension strand in a pre-cast concrete body before concrete is cast in the pre-cast concrete body;
- determining selected portions of said at least one unbonded post-tension strand to be protected;
- protecting said selected portions of said at least one unbonded post-tension strand by placing at least a portion of a protective layer adjacent to said selected portions.
2. The method for protecting at least one unbonded post-tension strand in a pre-cast concrete body according to claim 1, wherein said selecting step includes selecting at least one unbonded post-tension strand having a half inch diameter.
3. The method for protecting at least one unbonded post-tension strand in a pre-cast concrete body according to claim 1, wherein said selecting step includes selecting at least one unbonded post-tension strand having a three-eighth inch diameter.
4. The method for protecting at least one unbonded post-tension strand in a pre-cast concrete body according to claim 1, wherein said selecting step includes selecting at least one unbonded post-tension strand having a six-tenth inch diameter.
5. The method for protecting an unbonded post-tension strand in a pre-cast concrete body according to claim 1, wherein further including the step of placing said selected portions in a location wherein said selected portions are protected from damage caused by drilling or chipping.
6. The method for protecting an unbonded post-tension strand in a pre-cast concrete body according to claim 5, further including the step of placing said selected portions under and over doors and windows, adjacent to elevator beams where anchor bolts will be installed, around sleeves for plumbing and electric lines, where column and sheer wall forms are to be installed, on balconies where personnel hoist braces are attached, and portions of a cast concrete body that require drilling or chipping.
7. The method for protecting at least one unbonded post-tension strand in a pre-cast concrete body according to claim 1, further comprising the step of selecting a first plate that is sized and shaped to be clipped to said at least one unbonded post-tension strand.
8. The method for protecting at least one unbonded post-tension strand in a pre-cast concrete body according to claim 7, further comprising the step of selecting a second plate that is sized and shaped to be clipped to said first plate.
9. The method for protecting at least one unbonded post-tension strand in a pre-cast concrete body according to claim 1, further comprising the step of selecting a planar shaped plate that attaches to the unbonded post-tension strand using clamps extending from said planar shaped plate.
10. The method for protecting at least one unbonded post-tension strand in a pre-cast concrete body according to claim 1, further comprising the step of selecting at least one recess partially encasing said at least one unbonded post-tension strand.
11. The method for protecting at least one unbonded post-tension strand in a pre-cast concrete body according to claim 10, further comprising the step of joining a plurality of protective layers, uniformly aligning a plurality of equally-spaced unbonded post-tension strands.
12. An unbonded post-tension strand protector, comprising:
- a first plate having an elongated longitudinal axis; and
- at least one recess configured for receiving at least one selected portion of an unbonded post-tension strand.
13. The unbonded post-tension strand protector according to claim 12, wherein said at least one selected portion is located under and over doors and windows, adjacent to elevator beams where anchor bolts will be installed, around sleeves for plumbing and electric lines, where column and sheer wall forms are to be installed, on balconies where personnel hoist braces are attached, or at portions of a cast concrete body that require drilling or chipping.
14. The unbonded post-tension strand protector according to claim 12, wherein the first plate is sized and shaped to be removably clipped to at least one unboned post-tension strand.
15. The unbonded post-tension strand protector according to claim 12, wherein the first plate is sized and shaped to be removably clipped to at least one unbonded post-tension strand.
16. The unbonded post-tension strand protector according to claim 15, wherein a second plate is sized and shaped to be removably clipped to said first plate.
17. The unbonded post-tension strand protector according to claim 12, wherein the first plate comprises a planar shape that attaches to said unbonded post-tension strand using clamps extending from said first plate.
18. The unbonded post-tension strand protector according to claim 12, wherein each unbonded post-tension strand is partially encased by said at least one recess formed in said first plate.
19. The unbonded post-tension strand protector according to claim 18, wherein each of said at least one recess is located adjacent one another.
20. A protector for protecting one or more unbonded post-tension strands, comprising:
- at least one plate having an elongated length and a predetermined width, and
- a portion of said at least one plate located in proximity to one or more unbonded post-tension stands for protecting said one or more unbonded post-tension strands along said elongated length of said at least one plate.
21. The protector according to claim 20, wherein said plate is adapted to be removably clipped to said one or more unbonded post-tension strands.
22. The protector according to claim 20, wherein the plate consists of a planar shape that attaches to said one or more unbonded post-tension strands using clamps extending from said at least one plate.
23. The protector according to claim 20, wherein each unbonded post-tension strand is partially encased by a recess formed in said at least one plate.
Type: Application
Filed: Dec 7, 2006
Publication Date: Jun 12, 2008
Inventors: Anthony Rizzuto (Davie, FL), Charles Rizzuto (Davie, FL)
Application Number: 11/567,775
International Classification: E04C 5/08 (20060101);