Concrete slab crack initiators

Abstract

A crack initiator used with a concrete slab, such that the crack initiator has an alternating pattern of pointed protrusions arranged on a trapezoidal-shaped, round-shaped, or sinusoidal-shaped base. The crack initiator being located under the concrete slab and centered at a location at which a control joint is formed in the concrete slab. The crack initiator allows movement in a plane of the concrete slab that is perpendicular to the control joint, such that cracks that form in the concrete slab are offset from the control joint and have interlocking crack angles.

Description

CONCRETE SLAB CRACK INITIATORSFIELD

[0001] This disclosure relates generally to a control joint formed in a concrete slab that allows cracks to form in the concrete slab that are offset from the control joint and that have interlocking crack angles. More specifically, this disclosure relates to a crack initiator placed under a concrete slab and centered at the location of a control joint.BACKGROUND

[0002] The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.

[0003] A concrete slab generally shrinks as it cures. As a result, tensile stresses build-up within the curing concrete slab. The build-up of such tensile stresses may result in the formation of cracks within the cured concrete slab. Construction joints, typically described as control joints, are commonly created (e.g., cut with a saw, etc.) after the concrete is placed into position. These control joints are implemented in order to control the formation of stress cracks by forcing such cracks to form in the slab at or near the joints.

[0004] The design of a concrete slab on grade (or pavement) is often governed by the amount of load that can be carried by the slab immediately adjacent to the control joints. The design of the concrete slab is done in this manner because the load capacity of the slab is lowest near these joints, unless rebar or other load transfer devices are provided. The use of rebar or other load transfer devices in the construction of a concrete slab is generally not preferred because such solutions increase the overall cost associated with the concrete slab.

[0005] Some amount of weight or load may be transferred without the incorporation of rebar or other load transfer devices into the slab provided that the resulting cracks remain relatively small in size. This result can be accomplished by keeping the joint spacing low so that less shrinkage accumulates between the control joints.

[0006] During construction, control joints are typically placed at a distance from one another that is in the range of 24 to 36 times the thickness of the slab. The distance between control joints typically depends upon on a multitude of factors, such as:- slab design method;- slab thickness;- type, amount, and location of reinforcement;- shrinkage potential of the concrete, including cement type and quantity;- aggregate type, size, gradation, quantity, and quality;- water-cementitious material ratio;- type of admixtures;- con crete te m pe ratu re ;- base friction;- floor slab restraints;- layout of foundations, racks, pits, and equipment pads;- trenches and similar floor discontinuities; and- environmental factors such as temperature, wind, and humidity.

[0007] Proper spacing between the control joints assures that the cracks that form near or at the control joints remain small enough to allow for load transfer. The formation of a control joint effectively divides or separates the concrete slab into multiple adjacent slab panels. If there is not enough load transfer, these slab panels can “rock” at the edge resulting in the surface of the slab panels becoming off-set or uneven. When this happens, wheels from vehicles or carts that travel over the joint between the off-set panels, can degrade the surface of the slab panels.

[0008] When joint spacing increases beyond the range described above or when the loads become very high, it may become necessary to provide load transfer devices in the form of smooth dowels (often oiled to allow for movement) or flat plate dowels (to allow for movement both perpendicular and parallel to the joint). When joint spacing increases even more or when a large amount of traffic across joint areas is expected, the use of armored edges is often advised in order to protect the edges of the control joints.

[0009] However, there remains high interest industry-wide for the continued development of solutions that will allow for an increase in the spacing between control joints in order to reduce the overall cost associated with joint maintenance and to allow for the use of thinner concrete slabs. A reduction in the number of control joints will allow for the use of higher loads for a given thickness of concrete, provided that the increased spacing between joints does not lead to an increase in crack width beyond the point at which the slab panels can efficiently transfer load through aggregate interlock.SUMMARY

[0010] The present disclosure generally provides a crack initiator for use with a concrete slab. This crack initiator comprises an alternating pattern of pointed protrusions arranged on a trapezoidal-shaped, round-shaped, or sinusoidal-shaped base. The crack initiator is positioned under the concrete slab and centered at a location at which a control joint is formed in the concrete slab. The crack initiator allows for movement in a plane of the concrete slab that is perpendicular to the control joint, such that cracks that form in the concrete slab are offset from the control joint and have interlocking crack angles.

[0011] According to another aspect of the present disclosure, a control joint is provided in a concrete slab that includes a plurality of the crack initiators as described above and as further defined herein.

[0012] According to yet another aspect of the present disclosure a method of forming a concrete slab that contains at least one control joint is provided. This process generally comprises the steps of: providing a plurality of the crack initiators as described above and as further defined herein; placing each of the crack initiators in a predetermined position; the predetermined position for each of the crack initiators being centered with respect to a location at which the at least one control joint is formed; pouring concrete to form the concrete slab, such that the crack initiators reside underneath the concrete slab; and forming the at least one control joint in the concrete slab; The crack initiators allow for movement in a plane of the concrete slab that is perpendicular to the at least one control joint, such that the cracks that form in the concrete slab are offset from the at least one control joint and have interlocking crack angles.

[0013] Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.DRAWINGS

[0014] In order that the disclosure may be well understood, there will now be described various forms thereof, given by way of example, reference being made to the accompanying drawings, in which:

[0015] Figure 1A is a schematic providing a cross-sectional representation of a crack initiator formed according to the teachings of the present disclosure;

[0016] Figure 1 B is a schematic providing a plan view representation of the crack initiator of Figure 1 A and a control joint formed according to the teachings of the present disclosure;

[0017] Figure 2A is a top-down schematic showing the control joint along with cracks that form therein upon placement of crack initiators of the present disclosure below the concrete slab.

[0018] Figure 2B is an angled cross-sectional view of the concrete slab of Figure 2A further showing the pattern of cracks that form in the concrete slab offset from the control joint with interlocking crack angles.

[0019] Figure 3 is a flowchart illustrating a method of forming a concrete slab having a control joint according to the teachings of the present disclosure.

[0020] The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way.DETAILED DESCRIPTION

[0021] The following description is merely exemplary in nature and is in no way intended to limit the present disclosure or its application or uses. It should be understood that throughout the description, corresponding reference numerals indicate like or corresponding parts and features.

[0022] The objective of the present disclosure is to remedy the aforementioned disadvantages and to provide an interlocking joint former or crack initiator, which is placed immediately below a control joint in a concrete slab. The control joint effectively separates the concrete slab into two adjacent slab panels.

[0023] The overall benefit of the crack initiator of the present disclosure is that this crack initiator allows for the formation of crack(s) having offset and interlocking crack angles. This crack initiator not only assures that all crack(s) activate (i.e., open), but also the alternating interlocking pattern of the crack(s) allows for positive load transfer to occur, thereby, minimizing or eliminating the occurrence of any vertical off-set between slab panels. Conventional crack initiators use dowels and do not have an alternating design aimed at creating a crack pattern that promotes positive load transfer from panel to panel.

[0024] Referring to Figures 1A and 1 B, according to one aspect of the present disclosure, a crack initiator 1 with an inclined shape is shown in cross-sectional view (Figure 1A) and in plan view (Figure 1 B) as positioned with respect to a control joint 7 in a concrete slab 3. This crack initiator 1 is made of a stamped metal or polymeric material formed with a triangular, rounded, or sinusoidal base. This crack initiator 1 comprises one or more protrusions or ridges 5 protruding upwards from the bottom of the crack initiator 1.

[0025] The protrusions 5 arising from the bottom of the crack initiator 1 create a crack plane 10 in the concrete slab.3. This crack plane 10 extends from the protrusion or protruding ridges 5 of the crack initiator 1 to the bottom of the control joint 7 formed in the concrete slab 3. This crack plane 10 may make an angle in the range of 60 degrees to 85 degrees with the base 6 of the crack initiator 1 . Alternatively, the angle of this crack plane is at least 70 degrees; alternatively, between 75 degrees and 85 degrees; alternatively, about 80 degrees.

[0026] A control joint 5 may be formed in the concrete slab 3 through the use of a saw blade or the like. Thus, common terms used in the art to refer to control joints 5 include “sawcuts” or “saw-cut joints”. It is often preferable to create a control joint 5 when the concrete slab 3 is not yet fully cured. Typically, in this case, a wet concrete saw or an early-entry saw is used to create the control joint.

[0027] The depth of the cut or trough formed in creating a control joint as measured from the top surface of the concrete slab 3 to the bottom of the cut or trough created by the saw blade may range from about 1 / 4thto about 1 / 3rdof the thickness of the concrete slab 3.Alternatively, the depth of the control joint 5 (e.g., depth of the trough or cut) is in the range of about 1 inch to 3 inches deep; alternatively, about 1 .5 inches to about 2.5 inches; alternatively, about 1 .5 inches to 2 inches.

[0028] The protrusions 5 arising from the bottom of the crack initiator 1 generally have a cross-sectional area that are triangular in shape. This triangular shape may be described as an isosceles triangle or the like. However, the triangular shape may also be skewed to one side or the other without departing from the scope of the present disclosure. Alternatively, the shape of the protrusions or protruding ridges 5 may include any other shape that includes a pointed or angled surface.

[0029] The protrusions 5 of the crack initiator 1 may have a height that is in the range of about 0.5 inches to about 1 .5 inches. Alternatively, the protrusions 5 are about 1 inch in height. The distance between multiple protrusions 5 located within a single crack initiator 1 is about 1 inch to about 3 inches; alternatively, about 1 .5 inches to about 2.5 inches; alternatively, about 1 .75 inches to about 2.25 inches.

[0030] The crack initiators 1 may be stamped from sheet metal comprising, without limitation, steel, aluminum, or alloys formed therewith. The crack initiators 1 may also be formed from one or more polymeric materials or plastics, including but not limited to, polypropylene, nylon, polyester, or a combination thereof. The crack initiators may be at least partially formed from a plastic material.

[0031] The concrete slab 3 may be formed from plain concrete or concrete reinforced with a steel rebar, fiber reinforced plastic (FRP) rebar, plurality of steel fibers, synthetic fibers, or a combination thereof. The plurality of steel fibers may include twisted steel fibers or twisted bilateral truncated circular-shaped discontinuous reinforcements that are dispersed within the concrete matrix. One specific example of such discontinuous twisted steel fibers that are dispersible within a concrete matrix is the twisted steel micro-rebar (TSMR) commercially available from Pensmore Reinforcement Technologies (Michigan) as Helix® Micro Rebar®.

[0032] Still referring to Figures 1 A and 1 B, each of the crack initiators 1 are centered along the control joint 7 formed in the concrete slab 3 with the protrusions offset alternating to either side of the joint every 4 inches to 8 inches apart from one another. Alternatively, the crack initiators 1 may be placed apart from each other every 5 to 7 inches; alternatively, about 6 inches apart.

[0033] The crack initiators 1 are placed on the ground, grade, or pavement with the protrusions 5 facing the concrete slab 3 formed on top thereof. The offset refers to diagonal placement of the crack initiators 1 across the control or expansion joint 7, such that the protrusions arising from the crack initiator are positioned approximately 0.5 to 1 .6 inches fromthe control joint; alternatively, about 0.8 to 1.4 inches; alternatively, about 1.0 to 1.2 inches; alternatively, about 1.1 inches.

[0034] Referring now to Figures 2A and 2B, a control joint 7 is shown in which a plurality of crack initiators 1 formed according to the present disclosure are provided below the concrete slab 3 according to the present disclosure as discussed above and as further defined herein. The expected tensile stresses generated within the concrete slab 3 may be used to model the expected cracking pattern 12A, 12B that occurs in the concrete slab 3. As shown in Figures 2A and 2B, the crack(s) 1 1 that form in the concrete slab 3 traverse from the crack initiators 1 to the bottom 8 of the control joint 7. These cracks 1 1 are offset from the control joint 7 and form interlocking crack angles ©. These interlocking crack angles © occur between the plane defined by the crack joint 7 starting at the bottom 8 of said joint 7 and the crack 1 1 that forms with the crack initiators 1 .

[0035] The crack initiators not only assure that all crack(s) 1 1 activate (i.e. , open), but also provide an alternating interlocking pattern 12A, 12B for the crack(s) 1 1 that form, thereby, allowing for positive load transfer to occur. An alternating crack pattern 12A, 12B is demonstrated in Figures 2A and 2B, wherein the crack pattern 12A occurs on one side of the control joint 7, while the crack pattern 12B occurs on the other side of the control joint 7.

[0036] The alternating angular contact configuration of the crack initiators allows for the transfer of load based on the angle of contact, the thickness of the slab and the strength of concrete. To compute this capacity, an analysis of vertical and normal forces may be performed to determine whether shear and compressive demand(s) exceed the resistance at the location of the control joint. Careful attention to stress concentrations formed where the crack initiators alternate from one side of the joint to the other side is necessary in order to prevent random cracking in the surface of the concrete slab.

[0037] Concrete compressive strength may be compared against the normal force present near or at the control joint. The normal force may be computed using the following two equations (Eq. 1 & Eq. 2). Depending upon the design factors and requirements, the slab thickness, the angle, and the width of the crack initiator segment may be different for any given application. The example shown in Figures 1A and 1 B assumes an angle of about 80 degrees, a slab thickness of 6 inches and a joint initiator segment width of 6 inches (distance between crack initiators).Compression Demand = Load / Sine (Angle) (Eq. 1)Shear Demand = Load / Slab Thickness / Width - Initiator Segment (Eq. 2)

[0038] The values for each variable may be increased to account for stress concentrations from the smallest radii in the geometry in accordance with standard tables for stress concentrations. Such standard tables may be found in known texts, such as Peterson's Stress Concentration Factors (ISBN: 9780470048245; copyright 2008 John Wiley & Sons, Inc.). Resistance may be computed using standard formulas for shear and compressive resistance of plain concrete known in the art (e.g. see Building Code Requirements for Structural Concrete, American Concrete Institute; ACI 318 - Chapter 14).

[0039] According to another aspect of the present disclosure, a control joint located in a concrete slab is provided. Referring once again to Figures 1 A, 1 B, and 2, the control joint 7 comprises alternating angled cracks located near or at a top surface of the concrete slab 3 and a plurality of crack initiators 1 located underneath at a bottom surface of the concrete slab 3. The crack initiators 1 are centered with the control joint 7. Each of the crack initiators 1 includes an alternating pattern of pointed protrusions arranged on a trapezoidal-shaped, round-shaped, or sinusoidal-shaped base. The crack initiators allow movement in a plane of the concrete slab that is perpendicular to the control joint, such that the cracks that form in the concrete slab are offset from the control joint and have interlocking crack angles. The crack initiators may also allow movement of the concrete slab in a direction that is parallel to the control joint.

[0040] According to yet another aspect of the present disclosure, a process of forming a concrete slab that contains at least one control joint is provided. Referring now to Figure 3, this process 100 generally comprises the steps of: providing 105 a plurality of crack initiators; placing 1 10 each of the crack initiators in a predetermined position; pouring 115 the concrete to form the concrete slab, such that the crack initiators reside underneath the slab; and forming 120 the control joint in the concrete slab. The crack initiators comprise an alternating pattern of pointed protrusions arranged on a trapezoidal-shaped, round-shaped, or sinusoidal-shaped base as previously described above and as further defined herein. The placement of each of the crack initiators is predetermined as being centered with respect to the location at which the control joint is formed. The crack initiators allow for movement in a plane of the concrete slab that is perpendicular to the control joint, such that the cracks that form in the concrete slab are offset from the control joint and have interlocking crack angles.

[0041] For the purpose of this disclosure the terms "about" and "substantially" are used herein with respect to measurable values and ranges due to expected variations known to those skilled in the art (e.g., limitations and variability in measurements).

[0042] For the purpose of this disclosure, the terms "at least one" and "one or more of’ an element are used interchangeably and may have the same meaning. These terms, which refer to the inclusion of a single element or a plurality of the elements, may also be represented bythe suffix "(s)" at the end of the element. For example, "at least one control joint", "one or more control joints", and "control joint(s)" may be used interchangeably and are intended to have the same meaning.

[0043] Furthermore, any range in parameters that is stated herein as being “between [a 1stnumber] and [a 2ndnumber]” or “between [a 1stnumber] to [a 2ndnumber]” is intended to be inclusive of the recited numbers. In other words, the ranges are meant to be interpreted similarly as to a range that is specified as being “from [a 1stnumber] to [a 2ndnumber]”.

[0044] The specific examples provided in this disclosure are given to illustrate various embodiments of the invention and should not be construed to limit the scope of the disclosure. The embodiments have been described in a way which enables a clear and concise specification to be written, but it is intended and will be appreciated that embodiments may be variously combined or separated without parting from the invention. For example, it will be appreciated that all preferred features described herein are applicable to all aspects of the invention described herein.

[0045] Those skilled-in-the-art, in light of the present disclosure, will appreciate that many changes can be made in the specific embodiments which are disclosed herein and still obtain alike or similar result without departing from or exceeding the spirit or scope of the disclosure. The skilled person will understand that the concepts presented herein may be modified without exceeding the scope of the present disclosure in order to address various issues commonly encountered during the construction of a concrete slab.

[0046] One skilled in the art will further understand that any properties reported herein represent properties that are routinely measured and can be obtained by multiple different methods. The methods described herein represent one such method and other methods may be utilized without exceeding the scope of the present disclosure.

[0047] The foregoing description of various forms of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Numerous modifications or variations are possible in light of the above teachings. The forms discussed were chosen and described to provide the best illustration of the principles of the invention and its practical application to thereby enable one of ordinary skill in the art to utilize the invention in various forms and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.

Claims

CLAIMSWhat is claimed is:

1. A crack initiator for use with a concrete slab, the crack initiator comprising an alternating pattern of pointed protrusions arranged on a trapezoidal-shaped, round-shaped, or sinusoidal-shaped base located under the concrete slab and centered at a location at which a control joint is formed in the concrete slab; wherein the crack initiator allows movement in a plane of the concrete slab that is perpendicular to the control joint, such that cracks that form in the concrete slab are offset from the control joint and have interlocking crack angles.

2. The crack initiator according to claim 1 ; wherein the control joint separates the concrete slab into two adjacent slab panels; the control joint being formed using an early-entry saw.

3. The crack initiator according to claim 1 ; wherein the control joint separates the concrete slab into two adjacent slab panels; the control joint being formed using a wet concrete saw.

4. The crack initiator according to any of claims 1 to 3; wherein the crack initiator is at least partially formed of a plastic material.

5. The crack initiator according to any of claims 1 to 4; wherein the concrete slab comprises fiber reinforced concrete that includes a plurality of steel fibers, synthetic fibers, or a combination thereof.

6. The crack initiator according to any of claims 1 to 5; wherein the concrete slab includes steel rebar reinforcement.

7. The crack initiator according to any of claims 1 to 5; wherein the concrete slab includes fiber reinforced plastic (FRP) rebar reinforcement.

8. The crack initiator according to claim 5; wherein the plurality of steel fibers comprises twisted steel fibers.

9. The crack initiator according to any of claims 1 to 8; wherein the pointed protrusions have a cross-sectional area that is triangular in shape.

10. The crack initiator according to any of claims 1 to 9; wherein the crack initiator further allows movement in the plane of the concrete slab parallel to the control joint.1 1. The crack initiator according to any of claims 1 to 10, wherein the crack initiator is configured such that a crack plane is formed in the concrete slab; the crack plane extending from the pointed protrusions to the bottom of the control joint: wherein the crack plane makes an angle with the base of the crack initiator that is in the range of 75 degrees to 85 degrees. .

12. A control joint located in a concrete slab, the control joint comprising alternating angled cracks located near or at a top surface of the concrete slab and a plurality of crack initiators located underneath at a bottom surface of the concrete slab; wherein the crack initiators are centered with the control joint; each of the crack initiators comprising an alternating pattern of pointed protrusions arranged on a trapezoidalshaped, round-shaped, or sinusoidal-shaped base; wherein the crack initiators allow for movement in a plane of the concrete slab that is perpendicular to the control joint, such that cracks that form in the concrete slab are offset from the control joint and have interlocking crack angles.

13. The control joint according to claim 12; wherein the control joint separates the concrete slab into two adjacent slab panels; the control joint being formed as an early-entry control joint.

14. The control joint according to claims 12 or 13; wherein the crack initiator is at least partially formed of a plastic material.

15. The control joint according to any of claims 12 to 14; wherein the concrete slab comprises fiber reinforced concrete that includes a plurality of steel fibers, synthetic fibers, or a combination thereof.

16. The control joint according to claim 15; wherein the plurality of steel fibers comprises twisted steel fibers.

17. The control joint according to any of claims 12 to 16; wherein the pointed protrusions have a cross-sectional area that is triangular in shape.

18. The control joint according to any of claims 12 to 17; wherein the crack initiator further allows movement in the plane of the concrete slab parallel to the control joint.

19. The control joint according to any of claims 12 to 18, wherein the crack initiator is configured such that a crack plane is formed in the concrete slab; the crack plane extending from the pointed protrusions to the bottom of the control joint: wherein the crack plane makes an angle with the base of the crack initiator that is in the range of 75 degrees to 85 degrees.

20. A method of forming a concrete slab that contains at least one control joint; the process comprising the steps of: providing a plurality of crack initiators; each crack initiator comprising an alternating pattern of pointed protrusions arranged on a trapezoidal-shaped, round-shaped, or sinusoidalshaped base; placing each of the crack initiators in a predetermined position; the predetermined position for each of the crack initiators being centered with respect to a location at which the at least one control joint is formed; pouring concrete to form the concrete slab, such that the crack initiators reside underneath the concrete slab; and forming the at least one control joint in the concrete slab; wherein the crack initiators allow for movement in a plane of the concrete slab that is perpendicular to the at least one control joint, such that the cracks that form in the concrete slab are offset from the at least one control joint and have interlocking crack angles.

21. The method according to claim 20, wherein control joint separates the concrete slab into two adjacent slab panels; the control joint being formed as an early-entry control joint.

22. The method according to any of claims 20 or 21 ; wherein the crack initiator is at least partially formed of a plastic material.

23. The method according to any of claims 20 to 22; wherein the concrete slab comprises fiber reinforced concrete that includes a plurality of steel fibers, synthetic fibers, or a combination thereof.

24. The method according to any of claims 20 to 23; wherein the pointed protrusions have a cross-sectional area that is triangular in shape.

25. The method according to any of claims 20 to 24; wherein the crack initiator further allows movement in the plane of the concrete slab parallel to the control joint.

26. The method according to any of claims 20 to 25, wherein the crack initiator is configured such that a crack plane is formed in the concrete slab; the crack plane extending from the pointed protrusions to the bottom of the control joint: wherein the crack plane makes an angle with the base of the crack initiator that is in the range of 75 degrees to 85 degrees.

Images