AU726864B3 - Crack induced concrete slabs - Google Patents

Description

P/00/012 Regulation 3.2

AUSTRALIA

Patents Act 1990

ORIGINAL

COMPLETE

SPECIFICATION

PETTY PATENT Invention Title: "CRACK INDUCED CONCRETE

SLABS"

The following statement is a full description of this invention, including the best method of performing it known to me/us: 2

TITLE

"CRACK INDUCED CONCRETE SLABS" FIELD OF THE INVENTION THIS INVENTION relates to a method of laying concrete slabs using closely spaced crack inducer elements which eliminate a need for conventional formed or sawn control joints.

BACKGROUND OF THE INVENTION Large concrete slabs such as commercial, retail and industrial floors, and continuous pavements such as concrete roadways and paths will crackwhile curing due to drying shrinkage of the concrete and other effects if they are not detailed to accommodate the shrinkage strains. In the absence of shrinkage control joints, cracks will typically occur in concrete slabs and pavements in the first three months after placing, and these cracks will normally meander through the concrete at random locations.

Uncontrolled, visible cracks in concrete slabs and pavements are generally perceived by those observing them at best as ugly, and at worst, as failures.

Furthermore, the uncontrolled cracks are weak regions which may fail under load, and uncontrolled cracks will widen and crumble under heavy traffic.

To remedy this problem in a conventional manner, shrinkage control joints of various types are introduced to provide a structural break in an attempt to accommodate and control the concrete shrinkage in predetermined locations. Although vastly superior to uncontrolled cracking, conventional control joints are expensive to 3 install and they are often the first point of failure in floor slabs and pavements.

The control joints are vulnerable to damage in traffic areas, usually due to impact, and they become unsightly when the slab edges break away and when sealants fail. They can also be a hazard for pedestrians and some random cracks often still occur despite the installation of a pattern of control joints.

There are a number of different control joints that are typically specified by engineers in the construction industry to accommodate shrinkage cracking of concrete slabs and pavements. One of the most popular control joints is a saw cut that is installed once the concrete has cured to the extent that it will support a worker. The depth of a suitable saw cut is typically twenty five percent of the total thickness of the slab and the spacing is typically three to six metres. Such a joint does not prevent cracking, but attempts to limit cracking to the saw cut locations and generally attempts to control cracking to straight lines. To achieve a relatively smooth finish and to seal the joint, saw cuts are usually filled with a suitable elastomeric material.

Unfortunately, this method is time consuming and involves a worker revisiting the slab after it has set to install the saw cut, and yet again to install the sealant. The additional time and material adds to the cost of preparing the concrete slab.

Other traditional and commonplace shrinkage control joints include formed dowel joints, keyed joints and tooled joints.

US Patent No. 6,092,960 relates to concrete joint restraint system which secures dowel bars to a support structure. Use of dowel bars for transferring shear loads at joints in concrete pavement is known, and may provide a means to transfer forces across a joint. Using the invention of this patent, however, requires additional time and materials, and use of joints.

US Patent No.5,857,302 provides a means for controlling concrete slab cracking near walls or columns. The patent describes an outwardly extending vane perpendicular to the wall or column before pouring the concrete. The vane is oriented in line with a saw cut which is made after the concrete has set. Although this invention directs cracking in a straight line near walls or columns, additional time and labour are still required in making the saw cuts.

SUMMARY OF THE INVENTION In accordance with one aspect of the invention a method of producing a large continuous concrete slab is provided including the steps of:arranging an array of crack inducer elements on a surface; pouring concrete onto the surface to cover the crack inducer elements; and allowing the concrete to cure; wherein the crack inducer elements are of a size, shape and spacing to promote fine cracking in the vicinity of the elements and thereby avoid cracking away from the elements.

Preferably, arranging the array of crack inducer elements includes arranging a first group of spaced, substantially parallel elements, and a second group of spaced, substantially parallel elements perpendicular to the first group.

Preferably, the method further includes a step of stabilising the crack inducer elements to prevent excessive movement.

Boxes may be placed at the locations where the crack inducer elements intersect and attach to the intersecting crack inducer elements. The boxes may serve a second purpose of providing support to fabric or bar reinforcement.

Preferably the boxes are junction boxes or fitments.

In accordance with a second aspect of the invention a crack inducer system for concrete slabs is provided comprising: an array of crack inducer elements arranged on a surface, the elements being elongate and of a size and shape to induce fine cracks in curing concrete poured over the array; wherein the array consists of a plurality of regularly spaced crack inducer elements that displace concrete, thereby leaving thinner concrete above the elements.

Preferably, the crack inducer system of the array is a one-dimensional arrangement of spaced parallel elongate crack inducer elements.

Preferably, the crack inducer system of the array is a two dimensional grid of spaced elongate crack inducer elements, the two dimensional grid being formed from a first group of elements and a perpendicular second group of elements.

The above described invention is relatively uncomplicated and easily monitored in construction. It can provide extensive areas of controlled fine cracked concrete, with crack width varying upward from practically invisible to an unaided eye depending on the spacing of the crack inducer grid. Cold joint pour breaks, otherwise known as construction joints, can be used to break up the construction into manageable daily portions.

Completed lengths or areas of joint-free concrete slabs and pavements could be limitless in a controlled environment or suitable internal applications. External concrete slabs and pavements may require expansion joints to cater for temperature variations.

Additionally, the crack inducer grid could be used to reticulate services where suitable conduits are used as the crack inducer elements.

Surprisingly, the inventors have found that a concrete slab poured using the crack inducer system and/or poured by the method of the invention has an amazing resistance to significant cracking during curing and other effects. It is found that a multitude of fine cracks propagate around the crack inducer elements. The fine cracks are remarkably narrow, and in some cases, virtually invisible to an unaided eye.

The method is suitably used with conventional machinery, thus the method does not require additional specialised equipment beyond that of the specific material of the system.

The controlled fine crack inducer system and method has the following advantages: adaptability and is suitable for most slab on ground applications.

The crack inducer system and method can be used with conventional fabric reinforcement and fibre reinforcement, and it can also be used without reinforcement.

easy installation and does not require skilled labour.

economical due to reduced or eliminated reinforcement requirements.

formed or sawn control joints are not require and hence a cause of most serviceability problems with slabs on grade is eliminated.

flexible by maximising an ability of the slab on grade to accommodate minor ground movements.

7 versatile as construction joints at pour breaks can be installed at short notice. Length and breadth of a typical pour is generally limited only by the ability or capacity of the contractor to place and finish the concrete.

documentation is simple and can be adequately performed with typical details only.

cost savings in reticulation of electrical services through the crack inducer elements.

Savings compared to conventional slabs on grade can be substantial due to less use of reinforcement, bar chairs are generally not required, larger concrete pours are possible, reduced concrete pump use, compressible material at re-entrant corners is not required, formed or sawn control joints are not required, backing rods and sealants are not required, there is no need to reflect control joints in finishes and construction time is reduced.

Throughout this specification unless the context requires otherwise, the word "comprise", and variations such as "comprises" or "comprising", will be understood to imply the inclusion of the stated integers or group of integers or steps but not the exclusion of any other integer or group of integers.

DESCRIPTION OF THE FIGURES In order that the invention may be readily understood and put into practical effect, preferred embodiments will now be described by way of example with reference to the accompanying drawings wherein like reference numerals refer to like parts and wherein:- 8 FIG. 1 is a plan view of a first embodiment of the crack inducer system.

FIG. 2 is a cross section view of the crack inducer system shown in FIG.1.

FIG. 3 is an isometric view of a second embodiment of the crack inducer system.

FIG. 4 is a plan view of a third embodiment of the crack inducer system.

FIG. 5 is a cross section view of the crack inducer system shown in FIG.

4.

DETAILED DESCRIPTION OF THE INVENTION Shown in FIGS. 1 and 2 is a plan view and cross section view respectively of a first embodiment of crack inducer system 1 used with a continuous narrow pavement or path. Crack inducer system 1 minimises the potential for, or prevents the problem of, random large cracks forming in concrete during curing or other effects.

Crack inducer system 1 comprises crack inducer elements 11 which can be held in place relative to subgrade 12 using a suitable attachment means 2, shown a pegs or staples. Crack inducer system 1 provides a means of controlled fine cracking of concrete in predetermined locations and permits natural shrinkage stress relief of concrete without unsightly or dangerous crack formation and failure in load transfer due to loss of aggregate interlock in wide cracks.

Because crack inducer system 1 does not use joints, concrete may be layed as a single continuous pour without a need to position joints prior to placing concrete. Further, this system does not require revisiting a pour site to make saw cuts or bolster or tool indent joints.

In FIG. 1 crack inducer elements 11 are generally shown as cylindrical 9 and are oriented across a concrete pavement 3 to provide sites at which cracks may form. Crack inducer elements 11 are positioned on top of a subgrade 12 and below a surface of the concrete 16, shown in FIG. 2.

The inventors speculate that the crack inducer elements 11 create a more narrow section of concrete 17 which is more likely to form cracks than adjacent thicker regions between the crack inducer elements 11. A rounded upper surface of the crack inducer 11 may provide a broad surface from which cracks may originate in a discontinuous or segmented pattern. Crack inducer elements 11 may have cross sectional shapes other than round or oval, for example square or rectangular 11a, triangular 11 b, or a triangular shaped structure 11 c comprising three stacked cylindrical shapes which combine both triangular and rounded shapes. Accordingly, it will be appreciated by those skilled in the art that crack inducer elements 11 may have a suitable shape or configuration other than those shown by way of example and alternative shapes have been contemplated.

The crack inducer elements 11 are spaced at desired distances from each other along the pavement 3 to form appropriate sized cracks. Closely spaced crack inducer elements 11 are likely to create smaller cracks than crack inducer elements 11 which are spaced further apart. Crack inducer elements 11 spaced sufficiently close limit cracks to fine cracking which will be practically invisible to an unaided eye.

Preferably, the fine cracks are of 0.17 to 0.40 mm in size or smaller. Closely spaced crack inducer elements 11 may permit omission of additional structural support for concrete, for example fabric or bar reinforcement. Also the thickness of the narrow section 17 may vary depending on the situation. It will be appreciated that there will be a practical limitation to a minimum spacing between crack inducer elements 11 below which structural integrity of the concrete slab may be compromised.

The crack inducer elements 11 may be formed by any suitable means, for example extrusion, milling or may be a natural product for example bamboo. The crack inducer elements 11 may be composed of any suitable material, but preferably are composed of plastic. A suitable material for the crack inducer elements 11 is PVC conduits.

Shown in FIG. 3 is an isometric view of a second embodiment of the crack inducer system 10 used in association with a concrete slab or pavement. Controlled fine crack inducer system 10 is similar to the system shown in the first embodiment for a continuous pavement/pathway; however, in the second embodiment the crack inducer elements 11 are suitably arranged to accommodate the dimensions of the concrete slab in both directions. The crack inducer elements 11 are arranged such that a first group 18 and second group 19 of crack inducer elements 11 are respectively located at suitable distances 20 and 21 from each other. Crack inducer elements 11 in each group 18 and 19 respectively are substantially parallel to each other. The crack inducer elements 11 of groups 18 and 19 respectively are oriented perpendicular to each other and intersect forming a grid which is square or rectangular. Actual spacings of the grid will vary depending on acceptable crack width. It would be appreciated by one skilled in the art that other configurations of the crack inducer elements 11 are possible, for example a triangular configuration. Triangular and other configurations are contemplated as other embodiments of the invention.

The crack inducer elements 11 are placed directly on a subgrade 12 or on a plastic membrane 13 laid on the subgrade as appropriate for a given circumstance. The crack inducer elements 11 are located below a surface level of 11 concrete 16 which is to be poured over the crack inducer elements 11, as shown in FIG.

Shown in FIGS. 4 and 5 is a plan view and cross section view respectively of a third embodiment of the crack inducer system 10 wherein a box 14 is placed at locations along the grid where the crack inducer elements 11 intersect. Box 14 may be a junction box, chair or similar device, and may be of any suitable shape and/or dimensions. Box 14 is shown as a cylindrical junction box in FIGS. 4 and 5. The box 14 may function as bar chairs for fabric or bar reinforcement 15. Additional boxes 14 or bar chairs can be used depending on the bar size and grid dimensions of the fabric or bar reinforcement The crack inducer grid may be set out on a spacing which coincides with a dimension multiple of the reinforcement fabric to suit support requirements of standard fabric reinforcement sheets. The spacing of the grid in each direction may also be varied as required for an acceptable crack width for each individual application.

Preferably, the distance between crack inducer elements 19 and 20 is 800 mm to 1 m from each other in each direction of the grid.

A similar crack inducer system with the crack inducer elements placed between top and bottom reinforcement of slabs on grade and suspended slabs is also contemplated in another embodiment.

The crack inducer system may provide at least the following advantages: minimises or eliminates a need for traditional tooled, sawn or formed shrinkage control joints and dowelled and/or keyed load transfer joints; minimises or eliminates the need for steel reinforcement, depending on the spacing of the crack inducer grid pattern and other design requirements; facilitates the placing and finishing of large areas of slabs 12 in a single pour; and minimises or eliminates a need for joint sealants normally installed in control joints in large areas of slabs and the accommodation of such joints in floor finishes. The advantages reduce both time and materials which results in an overall cost saving.

It is understood that the invention described in detail herein is susceptible to modification and variation, such that embodiments other than those described herein are contemplated which nevertheless falls within the broad spirit and scope of the invention.

Claims (3)

1. A method of producing a large continuous concrete slab including the steps of:- arranging an array of crack inducer elements on a surface; pouring concrete onto said surface to cover said crack inducer elements; and allowing said concrete to cure; wherein said crack inducer elements are of a size, shape and spacing to promote fine cracking in the vicinity of said elements and thereby avoid cracking away from said elements.

2. The method of claim 1 wherein arranging said array of crack inducer elements includes arranging a first group of spaced, substantially parallel elements, and a second group of spaced, substantially parallel elements perpendicular to said first group.

3. The method of claim 1 further including a step of stabilising said crack inducer elements to prevent excessive movement.