Patent Application
20130247497
The present invention relates to masonry blocks, and more particularly to self-reinforced masonry blocks.
Common masonry walls are made of hollow concrete blocks and mortar; the hollow portions of the blocks are typically referred to as “cells”. The cells reduce the weight of block that the mason must lift into place during construction, and also enable vertical reinforcement to be installed in the wall. For added resistance to lateral loads, grout and vertical reinforcements, such as steel reinforcing bars, are placed in the cells of the block. Filling of the block cells also enhances the compression strength of concrete block walls under vertical loads. Placing vertical steel reinforcing bars in the block cells enhances the flexural strength of the wall to improve ductility through yielding of this reinforcing bar. However, the extent of ductility is limited by compression failure of the concrete block at relatively low compression strain.
In seismic design for earthquake loading, concrete block shear walls that are intended to resist the horizontal forces caused by seismic motion must be reinforced to increase their flexural strength and to develop some ductility and energy dissipation properties. However, it is very challenging to achieve sufficient ductility and energy dissipation prior to compression failure of the concrete block. Reinforced concrete block construction must often be designed for nearly twice as much lateral loading as the more ductile competing construction materials such as reinforced concrete structures and steel structures. Hence, reinforced concrete block construction is often not economically competitive and sometimes not technically feasible. Changes in recent building codes have imposed limitations affecting reinforced masonry construction with the result that use of this most common building material has been significantly limited.
Another aspect of structural design relates to the limit states with which building design must comply, namely the “serviceability limit state” and the “ultimate limit state”. The serviceability limit state deals with the normal course of building performance under expected loads, and requires that in these circumstances the building should not show any sign of distress and should function in the intended manner. The ultimate limit state is directed to providing a margin of safety against failure by designing for a higher load than is actually anticipated and by making allowance for variability in material strength, for example to deal with unexpected overloading or weaknesses that may develop.
More recently, the concept of design to accept damage but prevent collapse has been introduced, particularly in relation to seismic forces and other forces that are more difficult to predict. This design concept is directed to conditions beyond the ultimate limit state, at which point permanent damage is experienced. Where a structure has been appropriately designed to accept damage but prevent collapse, in conditions beyond the ultimate limit state but within the design limit, the structure may be visibly damaged but will retain most (at least 80%) of its original strength and, in the case of earthquakes, the additional accepted damage produces increased ductility and energy dissipation. This additional ductility and energy dissipation allows design for lower lateral forces for cases of low probability of occurrence such as either the 1 in 475 year or 1 in 2500 year earthquake that is currently designed for in most countries. In the event of such an earthquake, damage would occur but the building would not collapse and thus deaths, injuries and collateral damage may be reduced. Depending on the extent of damage, it might be economical to repair the building.
There are two related but separate aspects of behavior of grout-filled hollow masonry block construction subjected to vertical compression, such as is created in concrete block shear walls by gravity loading and by loading resulted from lateral seismic forces: interaction between grout in the cells and the mortared hollow masonry block, and brittle compression failure of grout-filled hollow masonry block.
Reference is first made to the interaction between grout in the cells and the mortared hollow masonry block. In standard hollow block construction, compression failure occurs at stresses well below the compressive strength of individual blocks as a result of incompatibility between the mortar and the block material. Under vertical compression, the larger lateral expansion of the softer mortar creates lateral tension in the blocks which results in development of vertical cracks through the webs and face shells of the block, leading to sudden crushing of the combined material at relatively low levels of vertical strain. Thus, compressive strength of the combination can be predicted based on mortar type and compressive strength of the block. However, when grout is used to fill the cells created in the hollow concrete block construction, addition of this third material creates a more complex condition where the different stress-strain properties of the grout, the discontinuities in the column of grout created by imperfect alignment of the block cells from course to course, wedging action due to the tapered shape of face shells and webs, and shrinkage of the grout, all combine to produce a lower material strength than attained in the ungrouted assemblage. The addition of the grout increases the overall capacity of the structure but, when considering the increased solid area of the grouted cross-section, the stress at failure is typically about 25% lower than for ungrouted hollow masonry, with strength based on failure load (capacity) divided by the effective net area of the assemblage. Increasing the grout strength has only a minor effect on the overall compressive capacity.
Although changes in geometry of the cells in the hollow masonry block and use of shrinkage compensating grout can reduce the decrease in observed strength, these approaches are not fully effective and have undesirable economic impact. Reducing the volume of grout to about 25% of the gross volume and improving the vertical alignment of the cells in successive courses of block masonry can help address the undesirable decrease in strength. For example, for a nominal 20 cm (8 inch) block, a 100 mm (4 inch) diameter cylindrical shaped cell occupies approximately 21% of the gross volume and gross cross-sectional area and, combined with positioning of these blocks so that the cells align from course to course, results in higher compressive strength than traditional grouted hollow block construction.
Turning now to brittle compression failure of grout filled hollow masonry block, despite the improved compressive strength created by the block geometry described above, the mode of compression failure remains the same: development of vertical cracks and sudden crushing/crumbling of the grouted assemblage. This brittle property of grouted masonry and of concrete products in general has been understood for some time as a limiting factor in use of concrete block construction, particularly for seismic design where economic design requires ductile behavior.
It has been shown that lateral confinement of brittle materials such as concrete creates a state of tri-axial compression under vertical axial compression loading so that both higher strengths of the material are obtained and much higher vertical strains are reached prior to crushing and crumbling of the block under the vertical compression load. Both the strength increase and the greater deformability can be used to create more ductile reinforced concrete block shear walls to better resist lateral earthquake load.
A number of strategies have been employed in attempts to introduce lateral confinement into grouted concrete block construction. These confining methods are generally passive in that vertical deformation is required to introduce the confining effects. With vertical compression of the material, lateral expansion of the material takes place where the ratio between the amount of lateral expansion and the vertical compression is known as Poisson's Ratio. At low levels of loading, this ratio is about 0.21 but at high levels of stress this can increase significantly and create what is referred to as dilation. Introduction of confining reinforcement to resist the lateral expansion introduces tension in the horizontal (lateral) reinforcement and a balancing amount of lateral compression in the grouted concrete block. The tri-axial state of compressive stress in the confined region is what creates the much higher compressive strength and greatly increased deformability of the confined material.
One method using a confining reinforcement to enhance the compression capacity and deformability of a grouted section involves placing steel wire mesh, perforated plates, and/or fiber reinforced polymer (FRP) fabric/laminates within the mortar bed joints. For example, Priestley (Priestley, M. J. N. Ductility of Unconfined and Confined Concrete Masonry Shear Walls. TMS Journal, July-December 1981, pp. 28-39) studied concrete masonry prisms confined with 3 mm thick stainless steel plates within the mortar beds. The plates were cut to the net shape of the masonry units so that there was no interference with the grouted cells, with a 5 mm edge allowance for pointing the mortar bed joints. The confined prisms showed increased strength, higher strains at peak load, and a much flatter falling branch of the stress-strain curves. PCT Patent Application No. PCT/US2005/25477, published as WO2006/020261, teaches other methods using confining reinforcement.
U.S. Pat. No. 5,809,732 teaches concrete masonry blocks with one or more external plates that are formed with the plates anchored through the block to enable items to be anchored to a wall built with these blocks. A masonry wall can be constructed using masonry blocks with external plates at preselected locations to anchor items to the wall by attaching them to the plates. The external plates are directed to supporting the anchoring function rather than to reinforcing the wall.
Another proposed technique was to provide lateral confinement for just the grout, for instance by using a spiral coil shape of reinforcement placed inside the block cell prior to grouting.
Hart et al. (Hart, G. C. et al. The Use of Confinement Steel to Increase the Ductility in Reinforced Concrete Masonry Shear Walls. TMS Journal, July-December 1988, pp. 19-42) conducted a comprehensive test program to investigate different types of confinement such as wire mesh, a modified “Priestley Plate”, hoops and spirals. In order to maintain consistent vertical reinforcing throughout all prism tests, one No. 6 bar was provided in each cell. The conclusions were: (1) unreinforced and vertically reinforced unconfined prisms behaved identically and failed in a brittle manner; (2) all types of confinement had a positive effect on the descending portion of the stress-strain curve and increased the area under the stress-strain curve; (3) the Priestley Plate provided the greatest confinement; and (4) the open wire mesh confinement type performed well.
For concrete block construction with standard block sizes, placing confining reinforcements within the mortar bed joints, as suggested by Priestly, means using a 200 mm (8 inch) vertical spacing between the confining reinforcements (i.e., the distance between successive bed joints). Such a large spacing limits the effectiveness of the confinement and effectiveness of support against buckling of enclosed vertical compression reinforcement. Reducing the height of the blocks to reduce the spacing distance demands handling more blocks and laying more mortar, which can dramatically increase construction cost. Similarly, increased construction labour is associated with placement of spiral coil reinforcements inside the block cell prior to grouting. In addition, the effectiveness of such reinforcement is limited because, for a typical grouted cell occupying less than 45% of the solid volume, less than 30% of the section can be effectively confined. Following crumbling of the block and grout outside of the spiral, the residual confined area is prone to buckling and cannot develop sufficient extra strength to compensate for the area lost after the material outside of the confined region fails in compression.
Thus, achieving increased ductility in masonry block construction using techniques mentioned above involves practical difficulties and may also involve significantly increased labour costs.
In one aspect, the present invention is directed to a self-reinforced masonry block. The self-reinforced masonry block comprises a main body having opposed substantially parallel stacking surfaces and having at least one tubular cell defined therethrough from one of the stacking surfaces to the other stacking surface. Each cell has a longitudinal axis and a longitudinal length defined by the stacking surfaces. At least one hollow confining reinforcement is embedded in the main body, with each confining reinforcement surrounding a corresponding cell along the longitudinal length thereof Each confining reinforcement extends substantially entirely along the longitudinal length of its corresponding cell and terminates inwardly of the stacking surfaces.
In one embodiment, each confining reinforcement is spaced outwardly from its corresponding cell, and in a particular embodiment each confining reinforcement is porous.
In one embodiment, the main body of the self-reinforced masonry block is formed from concrete.
In one embodiment, each confining reinforcement is tubular. In one particular embodiment, each cell and each confining reinforcement is substantially circular in cross-section, and in another particular embodiment, each cell and each confining reinforcement is substantially square in cross-section.
The confining reinforcements may comprise, for example, cold formed steel, hot-rolled steel, aluminum, glass, carbon fiber composites and fiber reinforced polymer. In one particular embodiment, each confining reinforcement comprises a metal sheet formed into a tube and having perforations therein. In another particular embodiment, each confining reinforcement comprises a mesh material.
In another aspect, the present invention is directed to a method for making a self-reinforced concrete block. The method comprises placing at least one hollow confining reinforcement inside a main cavity of a block mold, inwardly of side walls of the main cavity, introducing concrete mix into the main cavity to fill the main cavity while leaving at least one cell region within the main cavity substantially devoid of concrete, with the confining reinforcement(s) being positioned to surround a corresponding cell region, closing the mold, and vibrating the closed mold and applying compression to the concrete mix to form the concrete block.
In a particular embodiment, the method further comprises placing at least one cell mold element inside the main cavity, inwardly of side walls of the main cavity, so that the cell mold element(s) define the cell region(s). Positioning of the confining reinforcement to surround a corresponding cell region results from the confining reinforcement(s) and the cell mold element(s) being arranged so that for each confining reinforcement, the corresponding cell mold element is disposed inside and inwardly spaced from that confining reinforcement. In one embodiment, the confining reinforcement(s) are placed inside the main cavity after the cell mold element(s) are placed inside the main cavity, and in another embodiment the confining reinforcement(s) are placed inside the main cavity simultaneously.
In another aspect, the present invention is directed to a wall comprising a plurality of self-reinforced masonry blocks as well as a plurality of unreinforced masonry blocks. Each of the self-reinforced masonry blocks and the unreinforced masonry blocks comprises a main body having opposed substantially parallel stacking surfaces and at least one tubular cell defined therethrough from one of the stacking surfaces to the other stacking surface, with each cell having a longitudinal axis and a longitudinal length defined by the stacking surfaces. Each self-reinforced masonry block further comprises at least one hollow confining reinforcement embedded in the main body of the self-reinforced masonry block, with each confining reinforcement surrounding a corresponding cell in the self-reinforced masonry block along the longitudinal length of that cell. Each confining reinforcement extends substantially entirely along the longitudinal length of its corresponding cell in that self-reinforced masonry block and terminates inwardly of the stacking surfaces of that self-reinforced masonry block.
Both the self-reinforced masonry blocks and the unreinforced masonry blocks are arranged in a stacked configuration in which the cells of vertically adjacent masonry blocks are in registration with one another to define vertically extending tubular cavities. The wall comprises edge portions and intermediate portions between the edge portions, with the intermediate portions comprising the unreinforced masonry blocks and at least base regions of the edge portions are composed of the self-reinforced masonry blocks. All of the outermost vertically extending tubular cavities in the edge portions are filled with grout and have a resilient reinforcement member extending vertically therethrough and embedded in the grout.
In one embodiment, at least some of the vertically extending tubular cavities in the intermediate portions are filled with grout and have a resilient reinforcement member extending vertically therethrough and embedded in the grout.
The edge portions may comprise opposed vertically extending ends of the wall, and may further comprise vertically extending portions of the wall adjacent an opening therein.
In one embodiment, the self-reinforced masonry blocks and the unreinforced masonry blocks are concrete blocks.
In one embodiment, the confining reinforcements in the self-reinforced masonry blocks are spaced outwardly from their corresponding cells, and in a particular embodiment the confining reinforcements are porous.
In one embodiment, the wall further comprises mortar disposed between the stacking surfaces of vertically adjacent masonry blocks.
These and other features of the invention will become more apparent from the following description in which reference is made to the appended drawings wherein:
Reference is now made to
inches) and has the same external size and shape as, and is therefore compatible with, standard conventional concrete blocks of the same dimensions. Other suitable shapes and sizes may also be used for self-reinforced masonry blocks according to aspects of the present invention. A pair of hollow circular tubular cells 108A are defined through the main body 102A from one of the stacking surfaces 104A to the other stacking surface 104A, with each cell having a longitudinal axis AA that is substantially perpendicular to the stacking surfaces 104A and a longitudinal length LA defined by the stacking surfaces 104A. The exemplary masonry block 100A shown in
Two hollow confining reinforcements 112A are embedded in the main body 102A. Each of the confining reinforcements 112A surrounds a corresponding cell 108A along the longitudinal length LA thereof, and extends substantially entirely along the longitudinal length LA of the corresponding cell 108A to terminate immediately inwardly of the stacking surfaces 104A. The confining reinforcements 112A are circular in cross-section so that their cross-sectional shape corresponds to the circular cross-sectional shape of the cells 108A.
In the exemplary embodiment shown in
Some manufacturers may prefer to manufacture “half length” masonry blocks having only a single cell, and in such cases the “full length” blocks with two cells would not need to be “splitter” blocks.
Referring now to
Depending on the geometry of the masonry block and the cells thereof, confining reinforcements for use with masonry blocks according to aspects of the present invention may have other cross-sectional shapes besides circular. For example, confining reinforcements may have an oval cross-section or a square or other polygonal cross-section, or may comprise a spiral. Additionally, the cross-sectional shape of a confining reinforcement need not be identical to the cross-sectional shape of the corresponding cell. The size and shape of the cells will impact aspects such as compactability of the concrete mix used in manufacture, the size and shape of the confining reinforcement that will fit within the self-reinforced masonry block, and the concrete cover over the confining reinforcement, which may affect corrosion protection (if applicable) and fire resistance (if required). Selection of appropriate cell size and shape is within the capability of one skilled in the art, now informed by the herein disclosure.
Preferably, both the cross-sectional shape of the cell and the cross-sectional shape of the confining reinforcement are substantially circular. Also preferably, the confining reinforcement is sized and positioned so that approximately 75% of the gross area, including the concrete of the main body and the cell that will be filled with grout, will be confined by the confining reinforcement.
Confining reinforcements for masonry blocks according to aspects of the present invention may be made of any suitable material, including cold formed or hot-rolled steel, galvanized steel, aluminum or special alloys, each of which may be corrugated, glass, carbon fiber composites, or different types of fiber reinforced polymer (FRP) products such as laminates. The perforation or aperture pattern and the cross sectional area of the confining reinforcements will be selected according to the design requirements or the class of the masonry block of which it will form a part, i.e., the required level of lateral confinement. The choice of shape and thickness of material used to fabricate the confining reinforcements will also be affected by the ability to form the material into a circumferentially continuous hollow tube capable of resisting lateral tension created by confining the enclosed material such as concrete and/or grout. Additional factors affecting the choice of shape and thickness of the material used to fabricate the confining reinforcements relates to the process of manufacturing the masonry blocks, and are discussed in more detail below.
Reference will now be made to
Like the self-reinforced concrete masonry blocks 100B, 100C, the unreinforced concrete masonry blocks 300 each comprise a main body 302 having opposed substantially parallel stacking surfaces 304, flat ends 306, and at least one tubular cell 308 defined through the main body 302 from one of the stacking surfaces 304 to the other. However, the unreinforced concrete masonry blocks 300 do not include a confining reinforcement of the type shown in
The walls 340, 440, 540 are formed by arranging the self-reinforced masonry blocks 100B, 100C and the unreinforced masonry blocks 300 in a stacked configuration wherein the respective cells 108B, 108C, 308 of vertically adjacent masonry blocks 100B, 100C, 300 are in registration with one another to define vertically extending tubular cavities 342A, 342B (
Typically, as shown in
The walls 340, 440, 540 each comprise respective edge portions 344, 444, 544 and intermediate portions 346, 446, 546 between the edge portions. In general, the edge portions 344, 444, 544 correspond to the critical regions of the respective wall 340, 440, 540 at which the grouted cells are prone to crushing due to high levels of compressive stress. In each of the walls 340, 440, 540, the intermediate portions 346, 446, 546 are composed of the unreinforced masonry blocks 300 and the edge portions 344, 444, 544 are composed of the self-reinforced masonry blocks 100B, 100C. The walls 340, 440, 540 also comprise respective transition regions 349, 449, 549 in which the unreinforced masonry blocks 300 and the self-reinforced masonry blocks 100B overlap. Optionally, instead of using the self-reinforced masonry blocks 100B in which both cells 106B are reinforced, the self-reinforced masonry blocks that straddle the edge portions 344, 444, 544 and the transition regions 349, 449, 549 may have only one confining reinforcement reinforcing only one cell, with the cell that overlaps the unreinforced masonry blocks 300 being unreinforced.
Reference is now made specifically to
Referring now to
In the exemplary walls 340, 440, 540 shown in
As noted above, the respective cells 108A, 108C, 308 of vertically adjacent masonry blocks 100B, 100C, 300 are in registration with one another to define vertically extending tubular cavities 342A, 342B (
Reference is now made to
As shown in
The confining reinforcements 612 are positioned to surround corresponding cell regions 608 (
Referring now to
Referring now to
With the use of no-slump concrete which essentially does not flow, where the confining reinforcement is porous, that is, has a plurality of apertures therethrough, the thickness of the confining reinforcement must be limited so that the vibration and compacting pressure can force the concrete mix to fill the apertures and any space between the confining reinforcement and the stacking surfaces. For example, with a mesh confining reinforcement, use of circumferentially extending elements that are too thick may result in voids under those circumferentially extending elements, which would weaken the concrete and reduce the confining effects.
The confining reinforcement should provide sufficient vertical stiffness to prevent any substantial rebound effect as the compaction pressure is released at the end of the manufacturing cycle (
As noted above, the size and external geometry of self-reinforced masonry blocks according to aspects of the present invention are preferably the same as those of commonly used conventional unreinforced concrete masonry blocks. In a preferred embodiment, the size and shape of the cells, such as cells 108A, 108B, 108C differ from the size and shape of the cells of common unreinforced concrete masonry blocks. As shown in
One or more currently preferred embodiments have been described by way of example. It will be apparent to persons skilled in the art that a number of variations and modifications can be made without departing from the scope of the invention as defined in the claims.
This application claims priority to U.S. Provisional Application No. 61/382,964 filed on Sep. 15, 2010.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/CA11/01043 | 9/14/2011 | WO | 00 | 6/6/2013 |
| Number | Date | Country | |
|---|---|---|---|
| 61382964 | Sep 2010 | US |
