US20080184667A1

US20080184667A1 - Concrete Reinforcement Apparatus and Method

Info

Publication number: US20080184667A1
Application number: US11/596,958
Authority: US
Inventors: Riyadh A. Hindi
Original assignee: Bradley University
Current assignee: Bradley University
Priority date: 2004-05-17
Filing date: 2005-05-12
Publication date: 2008-08-07
Also published as: WO2005116357A3; WO2005116357A2

Abstract

A reinforced concrete column (110) includes a plurality of axially oriented reinforcing steel bars (112) within a surface of the concrete column (110). A first spiral wire (114) within the surface of the concrete column (110), the first spiral wire (114) being around an outer circumference of the plurality of axial reinforcing (112) and a second spiral wire (116), the second spiral wire (116) also being within the surface of the concrete column (110). The second spiral wire (116) is around a circumference of the plurality of axial reinforcing bars and the second spiral wire (116) is opposed to the first spiral wire (114).

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No. 60/571,641, filed May 17, 2004.

BACKGROUND OF THE INVENTION

1. Field of the Invention
This invention is in the field of construction, in particular in the area of construction using reinforced concrete.
2. Related Art
Concrete construction techniques are ancient. In modern times, it is typical to reinforce concrete with steel, sometimes called reinforcing rods or bars.
Concrete pouring techniques must take into account the rapid time in which concrete initially sets, or hardens. When pouring concrete for reinforced construction, care must be taken that the fluid concrete spread to all portions of the form, and also spread completely around the reinforcing bars to the greatest possible extent. This process is called consolidation. If a pour is not fully consolidated, gaps, air pockets or holes will exist in the final concrete structure, making it weaker. The techniques available to promote better consolidation include vibration. However, vibration cannot be used extensively and must be stopped after a short time because continuing vibration of the concrete will case separation of its constituent components (water, cement and aggregate inclusions), again weakening the final structure.
The above construction issues and considerations are further complicated in the construction of long vertical structures, including supports, piles, and the like (which shall be inclusively referred to herein as “columns.”). Consolidation issues are complicated still further when reinforcing bars, or any other metal or reinforcing component used, are too close together, making it difficult for fluid concrete to flow between them and achieve full consolidation.
Of course, there is a continuing need in the industry to increase strength and ductility of concrete structures. It has been shown, since the beginning of the 20^thcentury, by many investigators that axial strength and ductility of reinforced concrete column can be significantly improved by providing sufficient lateral reinforcement. As the concrete in a column is compressed axially it expands laterally. The lateral reinforcement imposes lateral or confining stresses on the concrete core, which reduces the lateral expansion and leads to strength and ductility enhancement.
Considere was the first to show that the axial behavior of a reinforced concrete column can be significantly enhanced when it is confined using a single spiral. See, Considere, A. Experimental researches on reinforced concrete, Leon S. Moisseiff, trans., McGraw Publishing Co., 1903, p. 188. Later, many studies investigated the effect of different confinement techniques such as spiral, hoops, and welded reinforcement grids. See, Mander, J. B, Priestley, M. J. N., and Park R. Observed stress-strain behavior of confined concrete, Journal of Structural Engineering, ASCE, Vol. 114, No. 8, 1827-1849, 1988; Sheikh, S. A. and Toklucu, M. T. Reinforced concrete columns confined by circular spirals and hoops, ACI Structural Journal, Vol. 90, No. 5, 542-553, 1993; Saatcioglu M. and Grira M. Confinement of reinforced concrete columns with welded reinforcement grids, ACI Structural Journal, Vol. 96, No. 1, 29-39, 1999. Using spiral reinforcement to enhance the strength and ductility of reinforced concrete columns was found to be the most adequate method among all other confinement techniques. Spiral reinforcement acts to resist the lateral expansion by applying a uniform pressure on the concrete core surrounded by the spiral. This uniform pressure significantly reduces the lateral expansion, which leads to significant strength and ductility enhancement.
The ACI 318-02 concrete building code (Building code requirements for reinforced concrete and commentary ACI 318-02 and ACI 318R-02, American Concrete Institute, Farmington Hill, Mich., 2002, p. 443) recommends using single spiral reinforcement to confine columns, especially in earthquake resistant structures where ductility is a very important issue. ACI 318-02 specifies a minimum allowable spiral spacing of 25 mm (1 in.) for constructibility reasons. The construction of long and slender reinforced concrete piles, where no visual inspection can be performed, can be very challenging if the spiral is closely spaced.
High-strength concrete (HSC) has the advantage of high compressive strength and high durability compared to normal-strength concrete. However, engineers are still reluctant to use high-strength concrete, especially in earthquake resistant structures, because of its high brittleness. This disadvantage could be overcome if sufficient and proper confinement is provided to improve the ductility of high-strength concrete; however, existing confinement techniques with their limitations are not enough to achieve that.
Earthquake survivability requires that columns, which are the main supporting elements of a structure, are adequately confined to provide sufficient ductility to dissipate energy without collapse, especially when high-strength concrete is used.
There is a continuing need for economy, speed and facility of construction and improved consolidation.

SUMMARY OF THE INVENTION

A reinforced concrete column includes a plurality of axially oriented reinforcing bars inside the surface of the concrete column. A first spiral wire is wound around an outer circumference of the plurality of axial reinforcing wire and a second spiral wire is also wound around the circumference of the plurality of axial reinforcing bars. The said second spiral wire is opposed to said first spiral wire in the direction it turns.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:

FIG. 1 is a schematic side view of prior art spiral wire construction;

FIG. 2 is a schematic side view of the dual opposing spiral reinforcing wire construction in the present invention;

FIG. 3 is a top view of a column using the present invention;

FIGS. 4A, 4B, 4C and 4d are axial strain result charts; and

FIG. 5 is an experimental materials table.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.
FIG. 1 depicts the prior art. It is a side view of a concrete column 10 having vertical steel reinforcing bars 12 placed roughly in a circle and running axially through the column. The circle of axial (typically vertical) reinforcing bars is bound together with a spiral strand of a metal wire 14. Wire spiral is typically constructed of steel wire of a heavy gauge, for example, wire that is 4 mm in diameter.
FIG. 2 depicts the pair of opposing spiral wires of the present invention. Concrete column 110 includes vertical reinforcing bars 112 arranged in a circle. The circular axial reinforcing bars 112 are bound together first by a first spiral of metal wiring 114. In the present invention, the axial reinforcing bars are also bound together by a separate opposing spiral of metal wire 116. “Opposing” means that the first wire advances in one axial direction with a change in a first rotational direction and the second wire advances in the same axial direction with a change in an opposite rotational directions. “Spiral” means a curvilinear wire of at least about 180°.
In FIG. 2 the spacing of each opposing spiral 114 and 116 is twice the spacing shown in FIG. 1 of the single prior art spiral. The double spacing of the two opposing spiral wires of the present invention provides the same strength and ductibility as a single spiral in the prior art. However, due to the increased spacing between the spirals, speed of construction is promoted, and the extent of consolidation during pouring of the concrete is improved.
When two wire spirals are used, each with the same spacing as the single spiral version for example, a 1″ pitch, strength and ductibility of the double spiral invention is substantially increased compared to the single wire version.

Experimental Testing

Ten reinforced concrete circular columns were constructed and experimentally tested subjected to concentric axial compressive loading.

Details of Test Specimens

Ten reinforced concrete circular columns were built. Six of them were confined with two opposing spirals (cross spirals, XS) and four columns were confined with single spiral (regular spiral, RS). Different spiral spacing (pitch) were used to confine the columns; 25, 35, 50 and 70 mm. The columns were 152 mm diameter and 406 mm long. All specimens had the same longitudinal reinforcement consisted of 6-Ø9.5 mm deformed steel bars spaced evenly around the column. This resulted in a longitudinal (vertical) reinforcement ratio, p_l, of 0.0236. The lateral spiral reinforcement used in all specimens was made from smooth steel wire with a diameter of 4 mm. The spacing of the spiral was reduced by 50% at the top and bottom end 76 mm of each specimen to force the columns to fail in the middle. Also, the longitudinal bars were welded to a steel ring at the top and a steel plate at the bottom to provide more lateral support to prevent bar buckling at the ends. The specimens were poured in two groups using two concrete batches (1 and 2). FIG. 5 show the details of the column specimens. Each specimen was named using first the spiral spacing in millimeters, second the RS for regular spiral or XS for cross spirals, and finally the concrete batch (1 or 2). For example 25RS2 refers to a column confined with regular spiral spaced at 25 mm and cast using concrete batch 2. The 50XS columns are confined with two cross spirals spaced each at 50 mm, which is twice the spacing of the single spiral used to confine the 25RS columns. This resulted in almost the same amount of spiral reinforcement (volumetric spiral ratio, ρ_s) as shown in FIG. 5. The same can be said for the 70XS (cross spirals with 70 mm pitch) columns and the 35RS (regular spiral with 35 mm pitch) columns. For the comparison to be complete, two columns (35XS1 and 35XS2) were constructed using cross spirals with 35 mm pitch. Each of the two 35XS columns contained twice the amount of spiral reinforcement provided in the 35RS or 70XS columns as shown in FIG. 5. The volumetric spiral ratio (ρ_s) and the longitudinal reinforcement ratio (ρ_l) for each specimen are shown in FIG. 5.

Material Properties

A concrete mix was designed for a compressive strength of 35 MPa. Standard cylinders (152×305 mm) were prepared to measure the axial load-deformation behavior of plain concrete. Normal weight concrete with a 9.5 mm maximum aggregate size (pea gravel) was used in all specimens. The column specimens and standard cylinders were cured for 28 days in a humidity room. As discussed earlier, the specimens were cast using two batches (1 and 2). The average compressive strength (f′_c) of the concrete cylinders at the testing date (53 days old) was 39.2 MPa and 36.1 MPa for batch 1 and 2, respectively.
The deformed steel bar (longitudinal bar) and the smooth steel wire (spiral) were tested in accordance with the ASTM A-370 to obtain their force-deformation relationships. The yield strengths of the longitudinal bar and the spiral wire were 33.2 kN (f_y=467 MPa) and 8.5 kN (f_yh=680 MPa), respectively as shown in FIG. 5. The yield strength of the spiral wire was estimated based on the 0.2% strain offset method.

Instrumentation and Testing

The testing was performed using a one million pound capacity, four posts, and servo hydraulic load frame. A displacement-controlled loading was followed in order to capture the post yield behavior. The relative displacement between the top and bottom ends of the specimens was measured using a linear variable displacement transducer (LVDT). Four strain gages were installed on the reinforcement. Three of them were installed on one spiral over the central 200 mm region of the specimens to measure the tensile strain in the spiral. The fourth strain gage was installed on a longitudinal steel bar at the middle of the specimen to measure the compressive strain in the longitudinal bars. Data were recorded using a computer-based data acquisition system at time interval of 1 second. The columns were loaded monotonically to failure at a displacement rate of 0.025 mm/second.

Experimental Results and Discussion

General Observations

Similar damage progression and failure mode under axial compression occurred with both columns confined with regular (single) spiral and two cross (opposing) spirals spaced twice as far apart. The concrete cover severely cracked and started to show signs of spalling at about 0.002 axial strain. At about an axial strain of 0.003, the cover was severely spalled. Before yield, the measured strain in the longitudinal reinforcement was almost similar to the concrete strain; however, significant increase in the strain of the longitudinal bars was observed beyond yield. Small tensile strain was observed in the spiral up to 0.002 axial strain when the concrete cover started to spall. After spalling, the strain in the spiral increased rapidly as the axial strain increased to reach the peak axial load. Beyond the peak axial load, the strain in the spiral significantly and rapidly increased until the spiral fractured and the longitudinal bars showed significant buckling. In columns confined with two opposing (cross) spirals, the fracture of the two spirals occurred at different times, which gave an indication of the complete failure while maintaining significant axial strength. After this stage, significant buckling of the longitudinal bars occurred which led to excessive deformation and failure.

Axial Behavior

Columns With Equivalent Confining Reinforcement

FIG. 4 summarizes the axial behavior and the experimental results of the tested columns. The test results showed that the axial behavior of the columns confined with cross (opposing) spirals was almost similar to the behavior of the columns confined with regular (single) spiral when they had the same volumetric spiral ratio, ρ_s. As shown in FIG. 4, the behavior was similar in terms of stiffness, yield, strength, and ductility. As shown in FIG. 4A, the axial force capacity of column 50XS1 (cross spirals) was 5.3% less than the axial force capacity of its equivalent column 25RS1 (single spiral). The axial force capacity of column 50XS2 (cross spirals) was 1.2% higher than the axial force capacity of its equivalent column 25RS2 (single spiral) as shown in FIG. 4B. FIG. 4C shows that the axial force capacity of column 70XS1 (cross spiral) is 5.5% less than the axial force capacity of its equivalent column 35RS1 (regular spiral). Finally, the axial force capacity of column 70XS2 (cross spirals) was 1.5% less than its equivalent column 35RS2 (regular spiral) as shown in FIG. 4D. The overall average axial capacity ratio of the columns confined with cross spirals and their equivalent columns confined with regular spiral was 0.97. This shows that the columns confined with two cross (opposing) spirals have almost the same axial force capacity as the columns confined with equivalent volumetric ratio of single spiral. The new confinement technique (cross spiral) has the advantage of providing double the spiral spacing (pitch) that the single spiral provides, which facilitates the construction of the columns.
In terms of ductility (displacement capacity), the tested specimens showed that the displacement capacity of the columns confined with cross spirals was very similar to their equivalent columns confined with regular spiral as shown in FIG. 4. The columns confined using the new technique of cross (opposing) spirals have the advantage of fracturing the two spirals at different times, which gave better indication of the complete failure compared to the columns confined with regular spiral. For example, the spiral in column 25RS2 fractures at about 0.026. The first spiral of its equivalent column 50XS2 fractured at about 0.028 axial strain as shown in FIG. 6 b while the second spiral fractured at about 0.033 axial strain, which kept the concrete core holding about 410 kN for about 18% more displacement after the first spiral fractured. The same behavior can be seen in the other columns as shown in the rest of FIG. 6; however, the amount of axial force and displacement after and before the fracture of the first spiral were different from one case to another.

Columns With Different Confining Reinforcement

Columns 35XS1 and 35XS2 were confined using two opposing spirals spaced at 35 mm each. This resulted in twice the amount of spiral reinforcement provided in columns 35RS1 and 35RS2 as shown in FIG. 5. If the same amount of confining reinforcement was to be provided as a single spiral then the single spiral must be spaced at 17.5 mm, which may not be recommended in accordance with the ACI building code since it is less than the minimum allowable spacing (25 mm).
As shown in FIG. 4C, the axial force capacity of column 35XS1 (cross spirals) was 28% more than the axial force capacity of column 35RS1. In terms of displacement, the displacement of column 35XS1 at the spiral fracture was about 45% more than the displacement of column 35RS1. The axial force capacity of column 35XS2 (cross spirals) was about 21% more than the axial force capacity of column 35RS2 as shown in FIG. 4D. In terms of displacement, the displacement of column 35XS2 at the spiral fracture was about 65% more than the displacement of column 35RS2. These extensively confined columns using cross spirals had in average about 25% more strength than columns 35RS1 and 35RS2. Therefore, columns 35XS1 and 35XS2 showed significant enhancement in strength and ductility, as shown in FIGS. 4C and 4D, compared to columns 35RS and 70XS that have half their confining reinforcement. This shows the advantage of using the new confinement technique to provide double the volumetric spiral ratio, which significantly improves the ductility and strength without reducing the spiral spacing and violating the ACI building code requirements.
The following was concluded based on the experimental testing of ten reinforced concrete circular columns under monotonic axial compressive loading. The columns confined using the proposed double spaced cross spirals or the conventional regular (single) spiral showed similar damage progression and failure mode under axial compression. Columns confined with regular spiral and cross spirals that have similar spiral volumetric ratio showed similar axial load capacity. The overall average axial capacity ratio of the columns confined with a double width cross spiral and their equivalent columns confined with regular spiral was 0.97. The displacement capacity of the columns confined with double spaced cross spirals was very similar to their equivalent columns confined with regular spiral. The columns confined using the new technique of cross (opposing) spirals had an advantage of rupturing the two spirals at different times, which delayed complete failure better than the columns confined with regular spiral. The new confinement technique can be used to improve the strength and ductility and/or to facilitate the construction of reinforced concrete columns and piles. The proposed confinement technique may be used to increase the spacing of the confining spiral without jeopardizing the strength and ductility of the column or may improve the ductility and strength without reducing the spiral spacing and e flow of concrete during construction.
As various modifications could be made to the exemplary embodiments, as described above with reference to the corresponding illustrations, without departing from the scope of the invention, it is intended that all matter contained in the foregoing description and shown in the accompanying drawings shall be interpreted as illustrative rather than limiting. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims appended hereto and their equivalents.

Claims

1. A reinforced concrete column comprising:

a concrete column having an outer surface;

a plurality of axially oriented reinforcing bars within said surface of said concrete column;

a first spiral wire, said first spiral wire being within said surface of said concrete column, said first spiral wire being around an outer circumference of said plurality of axial reinforcing bars;

a second spiral wire, said second spiral wire being within said surface of said concrete column and said second spiral wire being around a circumference of said plurality of axial reinforcing bars and said second spiral wire being opposed to said first spiral wire.

2. The apparatus of claim I wherein said first spiral wire and said second spiral wire are steel.

3. The apparatus of claim 1 wherein said first spiral wire and said second spiral wire are 4 mm in diameter.

4. A method of concrete construction comprising:

constructing a form;

orienting a plurality of reinforcing bars within said form;

circumscribing said plurality of reinforcing bars with a first spiral wire;

circumscribing said plurality of reinforcing bars with a second spiral wire, said second spiral wire opposing said first spiral wire; and

pouring concrete into the form.

5. A reinforcement for a concrete column having an outer surface and an interior, said reinforcement comprising:

a plurality of axially oriented reinforcing bars disposed to be within the concrete column;

a first spiral wire, said first spiral wire being disposed to be within the concrete column, said first spiral wire being around an outer circumference of said plurality of axial reinforcing bars;

a second spiral wire, said second spiral wire being disposed to be within the concrete column and said second spiral wire being around a circumference of said plurality of axial reinforcing bars and said second spiral wire being opposed to said first spiral wire.

6. The reinforcement of claim 5 wherein at least one of said first spiral wire and said second spiral wire has a one inch pitch.

7. The reinforcement of claim 5 wherein at least one of said first spiral wire and said second spiral wire is continuous for greater than 180°.

8. The column of claim 1 wherein each loop of one of said first or said second spiral wire is spaced substantially 25 millimeters from a next loop of the same one of said first or second spiral wire.

9. The column of claim 1 wherein said concrete is high strength concrete.

10. The column of claim 1 wherein an axial force capacity of said column is at least about twenty-one percent greater than an otherwise equivalent second column having only a single spiral wire at the same spacing as one of said first or said second spiral wires.

11. The column of claim 1 wherein said column has a displacement of at least about forty-five percent more than an otherwise equivalent second column having only a single spiral wire at the same spacing as one of said first or said second spiral wires.

12. The column of claim 1 wherein said column has an axial capacity ratio that is substantially 0.97 of an otherwise equivalent second column having a single spiral only, said single spiral having a spacing substantially half a spacing of one of said first or said second spiral wire.

13. The method of claim 4 further comprising the step of spacing each loop of one of said first or said second spiral wire substantially 25 millimeters from a next loop of the same one of said first or second spiral wire.

14. The method of claim 4 wherein said concrete is high strength concrete.

15. The method of claim 4 wherein an axial force capacity of said column is at least about twenty-one percent greater than an otherwise equivalent second column having only a single spiral wire at the same spacing as one of said first or said second spiral wires.

16. The method of claim 4 wherein said column has a displacement of at least about forty-five percent more than an otherwise equivalent second column having only a single spiral wire at the same spacing as one of said first or said second spiral wires.

17. The method of claim 4 wherein said column has an axial capacity ratio that is substantially 0.97 of an otherwise equivalent second column having a single spiral only, said single spiral having a spacing substantially half a spacing of one of said first or said second spiral wire.