INSTALLATION OF POST-TENSIONING ANCHORS AT ANCHORAGE ZONE WITHOUT REINFORCEMENT IN ANCHORAGE ZONE

Abstract

A post-tensioning anchor is configured for use in a structural element formed by surrounding the post-tensioning anchor with concrete, wherein a portion of the structural element surrounding the post-tensioning anchor comprises a concrete block. A post-tensioning anchorage zone, defined by the concrete block and the post-tensioning anchor, is configured to withstand an applied compression and an applied tension from a predetermined post-tensioning force without regard to reinforcement in the concrete block at the anchorage zone, such that the anchorage zone is configured to transfer the predetermined post-tensioning force to a remainder of the structural element without regard to reinforcement in the concrete block at the anchorage zone.

Claims

1. A post-tensioning anchor for use in a structural element formed by surrounding the post-tensioning anchor with concrete, wherein a portion of the structural element surrounding the post-tensioning anchor comprises a concrete block, wherein a post-tensioning anchorage zone, defined by the concrete block and the post-tensioning anchor, is configured to withstand an applied compression and an applied tension from a predetermined post-tensioning force without regard to reinforcement in the concrete block at the anchorage zone such that the anchorage zone is configured to transfer the predetermined post-tensioning force to a remainder of the structural element without regard to reinforcement in the concrete block at the anchorage zone.

2. The anchor of claim 1, wherein applied compression stress due to a prestressing force under the contact area between the anchor and the concrete block exceeds 0.45 times the concrete strength at stressing.

3. The anchor of claim 2, wherein the applied compression stress due to the prestressing force under the contact area between the anchor and the concrete block is between 0.6 and 5 times the concrete strength at stressing.

4. The anchor of claim 1, wherein the applied compression stress due to a prestressing force on the concrete block exceeds 0.45 the concrete compression strength at 28 days.

5. The anchor of claim 1, wherein a size of the concrete block in width (X) and height (Y) dimensions is between 1 and 4 times a size of the anchor in a width or height dimension.

6. The anchor of claim 5, wherein a length of the anchor in a length (Z) direction is between 0.05 and 3 times the dimension of the concrete block in X and/or Y dimensions.

7. The anchor of claim 1, wherein the anchor has a shape and/or size and/or material configured to prevent an applied tension stress due to a prestressing force within the concrete block from exceeding the tensile strength of the concrete.

8. The anchor of claim 1, wherein the anchor has a shape and/or size and/or material configured such that an applied tension stress, due to a prestressing force, within the concrete block does not require additional reinforcement to resist the tension force.

9. The anchor of claim 1, wherein the anchor has a shape and/or size and/or material configured such that an applied compression stress, due to a prestressing force, within the concrete block does not exceed the compressive strength of the concrete.

10. The anchor of claim 1, wherein the anchor has a shape and/or size and/or material configured such that an applied compression stress, due to a prestressing force, within the concrete block does not require additional reinforcement at the anchorage zone.

11. The anchor of claim 1, wherein the anchor has a shape and/or size and/or material configured such that an applied compression stress and/or applied tension stress, due to a prestressing force, within the concrete block does not require additional reinforcement for strengthening of the concrete.

12. The anchor of claim 1, wherein the structural element includes an individually placed anchor.

13. A structural element including a plurality of anchors according to the anchor of claim 1, wherein the plurality of anchors are placed in the structural element such that the anchorage zones of the plurality of anchors are not overlapping one another.

14. A structural element including a plurality of anchors according to the anchor of claim 1, wherein the plurality of anchors are placed in the structural element such that the anchorage zones of at least two of the plurality of anchors overlap one another.

15. The anchor of claim 1, wherein the anchor is made of metal, concrete, a fiber-based compound, and/or a composite material.

16. The anchor of claim 1, where the structural element is made of conventional concrete, fiber reinforced concrete, self-compacting concrete, high strength concrete, or ultra-high-performance concrete, or any combination thereof.

17. The anchor of claim 1, wherein the anchor has any desired shape, size, or material configured to withstand the applied compression and the applied tension from the predetermined post-tensioning force without regard to reinforcement in the concrete block at the anchorage zone such that the anchorage zone is configured to transfer the predetermined post-tensioning force to the remainder of the structural element without regard to reinforcement in the concrete block at the anchorage zone.

18.-34. (canceled)

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Embodiments of the invention will now be further described, by way of example only, and with reference to the accompanying drawings, in which:

[0028] FIG. 1 is a perspective view of an exemplary post-tensioning anchor with a schematic representation of reinforcement in a concrete block at an anchorage zone housed in a structural concrete element;

[0029] FIG. 2 is a schematic illustration of two anchorage zones for two separate anchors placed in a structural concrete element;

[0030] FIG. 3 is an illustration of the distribution of compressive bearing stresses and tensile bursting stresses over a length of an anchorage zone;

[0031] FIG. 4 is a graph showing distribution of tensile stresses at an anchorage location over the length of the anchorage zone as a function of the ratio between the size of the concrete block and the size of the post-tensioning anchor;

[0032] FIG. 5 is an illustration of the distribution of stresses across the width of the anchorage zone;

[0033] FIG. 6 is a schematic illustration of anchorage zone size;

[0034] FIG. 7a is a diagrammatic illustration of an exemplary multistrand anchor with and without bursting reinforcement in the anchorage zone; and

[0035] FIG. 7b is a diagrammatic illustration of an exemplary monostrand anchor with and without bursting and back-up reinforcement in the anchorage zone.

DETAILED DESCRIPTION OF EMBODIMENTS

[0036] FIG. 1 illustrates an exemplary anchor at an anchorage zone 110 including conventional reinforcement. The anchorage zone is the region immediately behind the anchor and surrounding the anchor that is affected by the transfer of the post-tensioning force (prestressing). This region includes the anchor components, the surrounding concrete block that defines the dimensional extent of the anchorage zone, and the reinforcement placed for confinement and/or bursting, and/or back-up reinforcement, and/or spalling, and any combination. The concrete block in the immediate vicinity of the anchor can be conventional concrete, fiber reinforced concrete, self-compacting concrete, high strength concrete, or ultra-high-performance concrete, or any combination thereof. The anchor may be metal, concrete, a fiber-based compound, and/or a composite material, and rectangular, circular, or oval with varying cross-sections and depth.

[0037] The post-tensioning force transfers quickly away from the anchorage zone into the structural element and therefore the anchorage zone is a small area very close to the anchor location. Once the post-tensioning force is applied, this zone experiences high tensile and compressive stresses that exceed the strength capacity of concrete. Conventionally, reinforcement is placed at this location to resist the tensile force, typically in the transverse or lateral direction to the post-tensioning force, and/or to increase the compressive strength of concrete through confinement typically in the axial direction parallel to the post-tensioning force, and/or to strengthen the concrete capacity right at the anchors, and/or to strengthen the concrete in a more general aspect at the anchorage zone. The need for reinforcement at conventional anchorage zones comes from the fact that the tensile resistance of concrete may be around 1% and 20% of its compressive resistance and reduces further if the concrete is subject to cracking. If cracking occurs, the reinforcement would ensure the integrity of the structure, minimize concrete cracking and avoid failure of the anchorage zone. In addition, the compressive strength of concrete confined with reinforcement increases the capacity of concrete beyond its compressive strength. An exemplary multistrand anchor 750 and an exemplary monostrand anchor 752 are illustrated with reinforcement in FIGS. 7a and 7b, respectively.

[0038] As shown in FIG. 1, the structural element may include an individually placed anchor. Referring to FIG. 2, the structural element may include a plurality of anchors placed in the structural element such that the anchorage zones of the plurality of anchors are not overlapping one another. It should be appreciated that in some embodiments, the structural element may a plurality of anchors placed in the structural element such that the anchorage zones of at least two of the plurality of anchors overlap with one another (not shown).

[0039] As shown in FIG. 3, within the anchorage zone, the compressive load effects like bearing stresses, and the tension load effects in the lateral direction like bursting stresses can be idealized. Compressive stresses are typically the highest right behind the anchor, and the tensile stresses are distributed over the concrete block length L. In some other aspects, not shown, high tensile stresses can develop right behind anchors. Given the limited resistance of concrete to tension, tensile stresses are conventionally resisted by reinforcement. Similarly given the need to extend the concrete capacity beyond its compressive strength, high compressive stress are conventionally resisted by reinforcement confining the anchorage zone.

[0040] As very high concentrated stresses develop within the anchorage zone, it is not uncommon that during the stressing operation the applied post-tensioning force exceeds 1 to 4 times the capacity of the concrete. It is therefore widely recognized that the behavior of force transfer at the anchor location is not readily quantifiable analytically and requires sophisticated finite element analysis which may or may not exactly capture the expected behavior. Therefore, the anchorage zone information may be defined by testing or by conservative simplified calculations.

[0041] The anchorage zone information includes information on the required reinforcement and information on the concrete block. The reinforcement information includes details such as size, strength, spacing and quantity. The concrete block information includes details such as concrete compressive strength and dimensions. The concrete block dimensions can be defined by the concentric block capable to adequately transfer the post-tensioning force to the structural element. It is typically defined in the cross-sectional plane to the post-tensioning force as the minimum distance to the nearest edge of the concrete element and/or the minimum distance between adjacent anchors. The anchorage zone details are generally affected by adjacent anchors within proximity of each other that affect the same concrete substrate, for example, where the anchorage zone of anchors overlap (not shown).

[0042] The required reinforcement and concrete block properties at the anchorage zone are dependent on the post-tensioning anchor device properties. For a given pos-tensioning force, the anchor properties can be the size, shape, and material.

[0043] FIG. 4 shows a well-known relationship model between the lateral force distribution over the length or depth of the anchorage zone for various ratios related to the dimension of the anchor device over the dimension of the concrete block. This model is for simple rectangular members where the post-tensioning force is concentric with the member. The graph highlights the complexity of the force transfer and shows for example that for b/h=0.9, the lateral force is only 0.05p, whereas as for b/h=0 the lateral force increases to 0.5p. In FIG. 4, the parameter h is the dimension of the concrete block, b is the dimension of the anchor device, and P is the post-tensioning force. The strength of the anchorage zone may be determined by testing.

[0044] The extent of the anchorage zone defined by the concrete block and reinforcement are dependent on the transfer of the post-tensioning force across the anchorage zone region and the concrete capacity at the anchorage zone region. the concrete strength without reinforcement may range from 0.3 fc to 1.2 fc, where fc is the concrete compressive strength at time of stressing or application of the post-tensioning force, and/or may range from 0.2 fc to 0.8 fc where fc is the concrete compressive strength at a certain time typically 28 days, or other constraints. The properties of the concrete block are therefore a function of the capacity of concrete to resist the post-tensioning force without relying on reinforcement. An exemplary multistrand anchor 750 and an exemplary monostrand anchor 752 according to the present disclosure are illustrated without reinforcement in FIGS. 7a and 7b, respectively.

[0045] In some aspects, the transfer of the post-tensioning force onto the concrete block is at an angle between 1:1 to 1:4 from the anchor, as shown in FIG. 5. The anchorage zone delimited by the extent of the concrete block where the force transfer happens may be concentric and/or symmetric with the anchor but may be distorted especially with a non-uniform concrete substrate, or changes in the cross-section of the concrete element in the vicinity of the anchorage zone, or several closely placed anchorages where the anchorage zones may overlap, and so on. The length of the concrete block L may be between 1 and 4 times the cross-sectional dimension of the anchorage zone a.sub.x, or a.sub.y.

[0046] As shown in FIG. 6, the cross-sectional dimension of the anchorage zone concrete block a.sub.x, a.sub.y, in each direction may be 1 to 4 times the anchor dimension b.sub.x, b.sub.y. The ratios of ax/bx, and bx/by may be independent of each other. For thin concrete elements like slabs and walls, the dimension of the concrete block in the thickness direction is typically equal to the thickness of the member.

[0047] In some aspects, the allowable compressive stress at the anchorage zone may be approximated by the simplified equation:

fconc=B1f.sub.c [0048] where factor B1 for concrete resistance ranges between 0.3 and 1.0 without additional reinforcement at the anchorage zone; may be from 0.4 to 4, or 0.6 to 3 with additional reinforcement at the anchorage zone; may be proportional to the ratio of sqrt(Ac/Ab); may be determined by testing; [0049] f.sub.c is the concrete strength at time of stressing [0050] A.sub.c may be taken as the cross-sectional area of the concrete block Ac=a.sub.x.Math.a.sub.y equals a.sup.2 for square configuration; it may also be taken as the net area a.sub.x.Math.a.sub.y minus some void such as the path for the post-tensioning steel. The parameters a.sub.x, a.sub.y being the concrete block dimensions in x and y direction; [0051] A.sub.b may be taken as the cross-sectional area of the anchor b.sub.x.Math.b.sub.y; it may also be taken as the net area b.sub.x.Math.b.sub.y minus some void such as the path for the post-tensioning steel. The parameters b.sub.x, b.sub.y represent the dimensions of the anchor in x and y direction;
The concrete block dimensions a.sub.x and a.sub.y in each direction can be equal to or larger than: [0052] the anchor dimensions b.sub.x, b.sub.y; and when required, b.sub.x, b.sub.y plus the concrete cover requirement like minimum cover required in per ACI 318 or minimum cover required b.sub.y other code regulations; [0053] the reinforcement dimensions, and when required plus the concrete cover requirement like minimum cover required in per ACI 318 or minimum cover required by other code regulations;
and where the applied compressive stress on the concrete fb within the anchorage zone at the anchor interface is limited to:

Stress fb=F/Ab<fconc

[0054] The concrete factor B1 increases with the amount of reinforcement in the anchorage zone, and other factors such as the ratio of Ac/Ab or the sqrt (Ac/Ab) or other parameters. Similarly, the applied stress fb decreases with the increase of the anchor size. The reduction in stresses fb, and/or the increase in the factor B1 can lead to a reduction of the reinforcement or required concrete strength of the anchorage zone, or enhancement in the adjacent anchorage spacings to avoid overlap of the anchorage zones.

[0055] In some aspects, the required reinforcement can be based on the lateral tensile force T that develops at the anchorage zone which may be calculated by the simplified equations within a length of L=a. The maximum tension load occurs between 0.1 L and 1.0 L:

T=K1.Math.F(1b/a) [0056] where [0057] K1 is the tension or lateral force distribution ratio and may range between 0.1 and 0.5, or taken as 0.25, or 0.4, which may be determined by testing;

[0058] The reduction in the compression and tension load effects leads to a reduction in the reinforcement required. Such reduction may be obtained by an increase in the anchor size which in turn leads to an increase in the ratio b/a and/or an increase of Ac/Ab, and then a reduction in the applied load effects. Given the above, as an example for the same concrete block size and concrete strength, an increase in the anchor dimensions will help reduce the compression and tension load effects which can lead directly to a reduction in the required reinforcement. However, an increase of the post-tensioning anchor device will make the anchor less competitive commercially due to cost, or ability to fit within the structural element thinnest dimension (e.g. along a slab thickness), or ability to fit more anchors within a defined dimension of the structural element longest dimension (e.g. along a slab perimeter), and so on.

[0059] Similarly, as an example for the same concrete block size and concrete strength, and same post-tensioning anchor designed or specified for a maximum applied post-tensioning force P, a decrease in the applied post-tensioning force can lead directly to a reduction in the required reinforcement. However, the use of a post-tensioning anchor at less than X % of F, for example, less than 70% of F, is equivalent to using an oversized anchor in comparison to the post-tensioning force being applied which will make the anchor less competitive commercially. This also requires an unjustified and oversized concrete block given the reduced post-tensioning force being applied. The oversized concrete block can limit the ability to fit the block within the structural element thinnest dimension (e.g. along a slab thickness), or ability to fit more anchors within a defined dimension of the structural element dimension (e.g. along a slab perimeter).

[0060] In parallel, while increasing the concrete block strength or dimensions, for example Ac, will reduce the compression and/or tension stress, the concrete block strength and size are typically kept the smallest possible to ensure feasibility and competitiveness. This is the reason why historically and currently post-tensioning anchors have required reinforcement in the anchorage zone. In theory, one can calculate a higher concrete strength or larger concrete block to reduce the amount of reinforcement but this will prevent the post-tensioning anchors from being used on projects since the calculated concrete block will not fit in the structural element or will not meet the concrete strength project specification. Increasing the concrete strength at the anchorage zone implies increasing the strength of the structural element.

[0061] For example, when designed in accordance with the following assumptions, a two-strand square anchor (similar to the rectangular anchor shown in FIG. 1) can be installed at an anchorage location with reinforcement in the anchorage location: [0062] Post-tensioning tension steel data: Low-relaxation 0.6 inch strand with ultimate strength fpu=270 ksi, area/strand=0.215 in.sup.2 [0063] Post-tensioning stressing percentage of ultimate strength: 80% of fpu [0064] Post-tensioning force: F=80%27020.215=93 kip [0065] Anchor dimension and area: b=3.5 in; Area=12.25 in2 [0066] Concrete strength at stressing fc=4.5 ksi [0067] Anchorage zone concrete block dimensions: a=7 in [0068] Anchorage zone concrete block cross sectional area: Ac=49 in2

[0069] Applied compressive stress within the anchorage zone [0070] Applied stress at the interface: fb=93/(12.25)=7.6 ksi [0071] Design factor B1=7.6/4.5=1.7

[0072] Applied stress fb=1.7 fc>>1.2 fc therefore reinforcement is required

[0073] A redesigned two-strand anchor can be installed at an anchorage zone without reinforcement being placed in the anchorage zone for the purpose of resisting and distributing the prestressing force at the anchorage zone. The redesigned post-tensioning anchor system eliminates the need for reinforcement in the anchorage zone for the purpose of resisting and distributing the prestressing force at the anchorage zone by relying on the strength, shape and size of the anchor device. Such post-tensioning anchor ensures a smooth distribution of the tensile and compressive stresses away and within the anchorage zone such that the tensile and compressive stresses do not require additional reinforcement in the concrete block. Post-tensioning anchors according to this disclosure can be used for monostrand and multistrand systems, and used for bonded or unbonded applications.

[0074] The redesigned post-tensioning anchor acts as a post-tensioned force transfer device as well as a substitute to the reinforcement being placed in the anchorage zone for the purpose of resisting and distributing the prestressing force at the anchorage zone while staying within the commonly acceptable range of the concrete block strength and size.

[0075] In some aspects of the invention, the post-tensioning anchor is performing like a 2 in 1 device that delivers optimal distribution of the stresses at the anchorage zone such that the stresses do not require additional reinforcement in the concrete block.

[0076] The redesigned anchor shape and size for example follows the path of the tensile stress distribution across the length of the concrete block and absorbs some of these stresses within the body of the anchor.

[0077] The redesigned anchor shape and size for example direction absorbs some of the compressive stresses and transfers the compressive strength such that the anchor is acting as a confinement and/or a strengthening mechanism.

[0078] In some aspects of the invention, the anchor shape is designed to absorb the peak stresses at the anchorage zone by having a localized increase of the anchor's cross section.

[0079] In some aspects of the invention, several ribs along the length of the anchor device help key-in the concrete substrate to smooth out the stresses that develop within the anchorage zone.

[0080] In some aspects of the invention, the anchor is made of a material or a composite material or an assembly of several material items such as reinforcement, barrels, steel fibers or any combination embedded within the body of the anchor to strengthen the anchor and help it resist and transfer the stresses throughout the anchorage zone region.

[0081] Additional embodiments include any one of the embodiments described above, where one or more of its components, functionalities, or structures is interchanged with, replaced by or augmented by one or more of the components, functionalities, or structures of a different embodiment described above.

[0082] Although several embodiments of the disclosure have been disclosed in the foregoing specification, it is understood by those skilled in the art that many modifications and other embodiments of the disclosure will come to mind to which the disclosure pertains, having the benefit of the teaching presented in the foregoing description and associated drawings. It is thus understood that the disclosure is not limited to the specific embodiments disclosed herein above, and that many modifications and other embodiments are intended to be included within the scope of the appended claims. Moreover, although specific terms are employed herein, as well as in the claims which follow, they are used only in a generic and descriptive sense, and not for the purposes of limiting the present disclosure, nor the claims which follow.