IN THE APPLICATION INVENTOR(S) ABUL KALAM AZAD AND IBRAHIM YAHYA AHMED HAKEEM FOR ULTRA-HIGH PERFORMANCE CONCRETE REINFORCEMENT BARS

Transcription

1 Attorney Docket No IN THE APPLICATION OF INVENTOR(S) ABUL KALAM AZAD AND IBRAHIM YAHYA AHMED HAKEEM FOR ULTRA-HIGH PERFORMANCE CONCRETE REINFORCEMENT BARS APPLICANT: King Fahd University of Petroleum and Minerals

2 ULTRA-HIGH PERFORMANCE CONCRETE REINFORCEMENT BARS BACKGROUND OF THE INVENTION. FIELD OF THE INVENTION [000] The present invention relates to reinforcement bars for concrete construction, and particularly to ultra-high performance concrete bars that are reinforced with steel fiber and used for tension reinforcement.. DESCRIPTION OF THE RELATED ART [000] Recently, ultra-high-performance concrete (UHPC) was developed by agencies as a new generation of cementitious material. UHPC is characterized by being a fiber-reinforced cement composite material with compressive strengths in excess of 50 MPa and flexural tensile strength exceeding 0 MPa. UHPC is also characterized by its constituent material make-up, typically including fine-grained sand, silica fume, reinforcing materials and special blends of high-strength Portland cement. It should be noted that UHPC contains no large aggregate. The current types of UHPC in production differ from normal concrete in compression by their strain hardening, followed by sudden brittle failure. Ongoing research into UHPC failure due to tensile and shear failure is being conducted by multiple government agencies and universities around the world. [000] In spite of its advantages, UHPC is not widely used. The only commercial source of a proprietary mix for UHPC widely available in the United States is Ductal, produced by LaFarge Company of France, although other commercial UHPC mixes are commercially available outside the United States. Thus, most concrete construction continues to use

3 convention cement mixes. Nevertheless, it would be desirable to provide a convenient way for concrete construction to take advantage of the superior properties of UHPC concrete material. [0004] Thus, ultra-high performance concrete reinforcement bars solving the aforementioned problems is desired. SUMMARY OF THE INVENTION [0005] The Ultra-High Performance Concrete (UHPC) reinforcement bars are precast concrete bars made from UHPC that can be used as tension reinforcement in concrete construction elements, replacing steel reinforcement bars. The UHPC bars may be made from any known UHPC mix having steel fibers dispersed through the mix. The UHPC mix will typically contain a mixture of cement, micro-silica, fine sand, steel fiber, and a superplasticizer. The UPHC is mixed with water, cast in a mold, and cured into precast bars having suitable cross-sectional dimensions and length for providing reinforcement to beams and other concrete construction elements made from conventional concrete, thereby forming hybrid or composite concrete elements. Although the UHPC reinforcement bars may have smooth faces, preferably the UHPC reinforcement bars are deformed bars, having deformations cast in opposite lateral faces of the bar to provide good bonding between the reinforcement bars and the conventional concrete. [0006] These and other features of the present invention will become readily apparent upon further review of the following specification and drawings.

4 BRIEF DESCRIPTION OF THE DRAWINGS [0007] Fig. is a partial side view in section of an exemplary ultra-high performance concrete reinforcement bar according to the present invention. [0008] Fig. is an end view of a hybrid concrete beam having the tension portion of the beam reinforced with a pair of UHPC reinforcement bars according to the present invention. [0009] Fig. is a load-deflection plot for a first sample of three identical concrete beams (50x50x760 mm) reinforced with a pair (5x50 mm each) of the ultra-high performance concrete reinforcement bars according to the present invention. [000] Fig. 4 is a load-deflection plot for a second sample of three identical concrete beams (50x00x900 mm) reinforced with a pair (5x5 mm each) of the ultra-high performance concrete reinforcement bars according to the present invention. [00] Fig. 5 is a plan view of an embodiment of a UHPC reinforcement bar according to the present invention having deformations formed on opposing lateral faces of the bar. [00] Similar reference characters denote corresponding features consistently throughout the attached drawings. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS [00] The ultra-high performance concrete reinforcement bars are precast concrete member made from ultra-high performance concrete (UHPC) that are dimensioned and configured for use as reinforcing material for construction elements made from conventional concrete, such as concrete beams, replacing conventional steel tension reinforcements and the like to form hybrid or composite construction elements. Although the UHPC reinforcement

5 bars may have smooth faces, preferably the reinforcement bars are deformed to provide good bonding between the reinforcement bars and the conventional concrete used to form the beam or other concrete construction element. [004] Conventional concrete is generally formed from cement, water, and aggregate material (such as sand, stones, and gravel), which includes both coarse and fine aggregate material. Concrete is generally strong in compression, but its tensile strength is low enough that the concrete may require reinforcement with steel bars, which are often deformed by projections or lugs to improve bonding between the concrete and the steel reinforcement bars to prevent sliding at the interface between the two materials. [005] In recent years, ultra-high performance concrete has been developed that has improved compressive strength, and when reinforced with steel fibers, possesses sufficient tensile or flexural strength that it may be used, often without the need for reinforcement with steel bars. In addition, the material has greater durability than conventional concrete and may be used in environments where conventional reinforced concrete would be subject to weakening by corrosion of the steel reinforcement bars. [006] The term ultra-high performance concrete does not have a standard definition. A technical note published by the U.S. Federal Highway Administration in March 0 (FWHA Publication No. FHWA-HRT--08) defines UHPC as a cementitious material composed of granular constituents, a water-to-cementitious material ratio less than 0.5, and a high percentage of discontinuous internal fiber reinforcement. The mechanical properties of UHPC include compressive strength greater than.7 ksi (50 MPa) and sustained postcracking tensile strength greater than 0.7 ksi (5 MPa). UHPC has a discontinuous pore structure that reduces liquid ingress, significantly enhancing durability as compared to 4

6 conventional and high-performance concretes. The aggregate material in UHPC generally has a particles size less than mm, i.e., there is an absence of coarse aggregate, generally permitting the material to be more tightly packed, requiring less water and producing smaller pores, improving compressive strength compared to conventional concrete. As used herein, the term ultra-high performance concrete refers to concrete thus defined, in which the fiber reinforcement comprises steel fibers, while the term conventional concrete refers to concrete having coarse aggregate and a water-to-cementitious material greater than 0.5. [007] In order to test the viability of using UHPC reinforcement bars to provide tension reinforcement for hybrid concrete construction elements, the inventors tested the concept experimentally as follows. The mix design used for UHPC bars was developed in-house using local materials. It comprised (per cubic meter of concrete): ASTM Type I Portland cement 900 kg, micro-silica 0 kg, fine sand 005 kg, steel fiber 57 kg (about 6.% by weight of the UHPC mix), superplasticizer (Glenium 5) (an admixture that improves workability of the UHPC), and water 6.4 kg (representing water-binder ratio of 0.45). The steel fibers were 0.5 mm in diameter and.7 mm in length, and had ultimate tensile strength of 500 MPa. The UHPC bars, rectangular or square in section, are cast in a wooden mold with built-in vertical grooves to provide the deformation to the UHPC bars. After mixing the materials in a horizontal revolving planetary mixer, the mixture was placed in the mold, leaving the top surface of the bars unfinished. After a few hours, the bars are demolded and then thermal-cured for two days at 90 C, as it was found that heat-curing enables the bars to rapidly gain strength, which allows avoiding longer periods of moist-curing. [008] Fig. shows a side view in section of a deformed UHPC reinforcement bar 0, including the steel fibers 4 dispersed in the matrix of UHPC concrete. Fig. 5 shows a top 5

7 view of the reinforcement bar 0, showing the deformations (or deformities) comprising shallow vertical (orthogonal to the length of the bar 0) projections 0 spaced apart along opposing sides of the bar 0. For a reinforcement bar having a width of 5-50 mm, the projections 0 may extend outward from the bar 0 about -4 mm and may be spaced apart about 00 mm along the length of the bar 0, and extend in length from the top face of the bar 0 to the bottom face of the bar 0, i.e., for the height of the bar 0 as viewed from the side of the bar 0. It will be understood that the deformations may have other cross-sectional shapes or other orientations, so long as the deformations provide sufficient surface area to improve bonding at the interface between the UHPC reinforcement bars 0 and the conventional concrete of the hybrid concrete beam or construction element. For example, the projections 0 may have a rounded end face or be triangular in cross section, and the projections may be oriented diagonally or obliquely on the lateral faces of the bar 0. A typical UHPC bar 0 has flexural tensile strength of about MPa and direct tensile strength of about - MPa. [009] The experimental program involved testing a series of beam specimens of 50 x50x760 mm and 50x00x900 mm in size. Each normal or conventional concrete beam was reinforced with two UHPC bars 0 of rectangular or square cross section having transverse dimensions of 5x5 mm, 5x50 mm and 50x50 mm, and an axial length equal to the length of the beam. The beams were moist cured for 8 days prior to testing in a fourpoint bend test. The failure load, mid-span deflection and the mode of failure was recorded for each test. [000] Fig. shows an end view of a hybrid concrete beam 6 having a pair of the UHPC reinforcement bars 0 embedded in the beam made of conventional concrete 8 in parallel along the bottom face of the beam 6. Generally, the top face of the beam 6 is the 6

8 load bearing face and defines a compression area that extends downward from the top face to a neutral axis where the beam experiences substantially zero bending stress. The region extending from the bottom face to the neutral axis defines a tension area. When a beam is subjected to a bending stress, the beam deflects because its compressive side (adjacent the top face) shortens, and its tension side (adjacent the bottom face) lengthens, which may produce cracks and ultimate failure of the beam. Consequently, in the hybrid beam 6, the UHPC reinforcement bars 0 are embedded in the tension area of the beam 6, along the bottom face of the beam 6, as shown in Fig.. [00] Results show that the failure occurs with the development of cracks in the UHPC bars 0 within the maximum moment region. It should be noted that the behavior and mode of failure of UHPC reinforced beams is different from that of concrete beams reinforced with steel bars. A UHPC reinforced beam 6 attains its peak load at the development of first flexure cracking, following which a declining load path follows with increased deflection, exhibiting a gradual softening mode of failure. The peak load corresponds to the attainment of maximum flexural stress within the range of 8- MPa, calculated using transformed section properties. No bond-slip problem was noted, and all beams 6 failed in flexure with the development and propagation of a single vertical crack. The deformed shape of the precast bars 0, along with the rough top surface (from leaving the top of the mold open), enables the bars 0 to develop full strength without any bond-slip. [00] The experimental findings have clearly established the possibility of utilizing the precast UHPC bars 0 in one-way slab-type construction, for which shear is not a critical design consideration. For flexural design, a normal concrete member can be reinforced with deformed UHPC bars, replacing mild steel bars. The approximate amount of UHPC bars 0 7

9 that can be used to replace mild steel bars is about 5-0 times the area of steel bars. As the cross section of a UHPC bar 0 is much larger, the increased area of UHPC bars 0 can be accommodated within the beam section. [00] For precast one-way slabs, the use of precast UHPC bars 0 appears to be feasible and attractive. [004] Figure shows a typical load-deflection plots of three identical beam specimens (beams of 50x50x760 mm reinforced with UHPC bars of 5x50 mm in cross section). The peak load varied between 5 to 40 kn. The post-cracking behavior shows increased deflection with decline in load. At about 50% of the peak load, the mid-span deflection increased to almost twice the value at the peak load. In Figure 4, the load-deflection plot is shown for three identical UHPC-reinforced beam specimens of 50x00x900 mm, reinforced with UHPC bars of 5x5 mm in cross section. The computed flexural tensile strength corresponding to the peak load varied from 6 to MPa. It should be noted that a UHPC bar 0 does not have much post-cracking elongation in direct tension, unlike steel bars. In flexure, the progressive crack-growth allows UHPC to show a softening mode of failure (Figures and 4). [005] Thus, the ultra-high performance concrete reinforcement bars may provide tension reinforcement for conventional concrete in hybrid or composite concrete beams or construction elements. [006] It is to be understood that the present invention is not limited to the embodiments described above, but encompasses any and all embodiments within the scope of the following claims. 8

10 CLAIMS We claim: 4. An ultra-high performance concrete reinforcement bar, comprising an elongate bar made from ultra-high performance concrete having discontinuous steel reinforcing fibers, the elongate bar being dimensioned and configured for providing tension reinforcement for a conventional concrete construction element.. The ultra-high performance concrete reinforcement bar according to claim, wherein the elongate bar is deformed The ultra-high performance concrete reinforcement bar according to claim, wherein the elongate bar is rectangular in transverse section, having a top face, a bottom face opposite the top face, opposing lateral faces extending between the top face and the bottom face, and opposing end faces, each of the lateral faces having a plurality of shallow projections spaced apart along the length of the bar for improving bonding between the elongate bar and the conventional concrete of the construction element. 4. The ultra-high performance concrete reinforcement bar according to claim, wherein the projections are oriented orthogonal to the length of the elongate bar, the projections having a length equal to the height of the lateral face. 5. The ultra-high performance concrete reinforcement bar according to claim 5, wherein the projections are rectangular in section. 9

11 6. The ultra-high performance concrete reinforcement bar according to claim 4, wherein the projections are oriented oblique to the length of the elongate bar. 7. The ultra-high performance concrete reinforcement bar according to claim, wherein the top face of the elongate bar is rough for further improving bonding between the elongate bar and the conventional concrete of the construction element. 8. The ultra-high performance concrete reinforcement bar according to claim, wherein the elongate bar is square in transverse section A method of providing tension reinforcement to a concrete beam, comprising the step of reinforcing an elongate beam made of conventional concrete with at least one precast ultra-high performance concrete reinforcement bar according to claim incorporated into a tension area of the beam and extending along the length of the tension area. 0. A hybrid concrete beam made from conventional concrete, the beam having a compression area and a tension area, the beam comprising at least one ultra-high performance concrete reinforcement bar according to claim incorporated into the tension area The hybrid concrete beam according to claim 0, wherein the beam has an elongate compression face and an elongate tension face opposite the compression face, the at least one ultra-high performance concrete reinforcement bar comprising a pair of ultra-high performance concrete reinforcement bars extending in parallel along the length of the tension face. 0

12 . A hybrid concrete beam made from conventional concrete, the beam having a compression area and a tension area, the beam comprising at least one ultra-high performance concrete reinforcement bar according to claim incorporated into the tension area The hybrid concrete beam according to claim, wherein the beam has an elongate compression face and an elongate tension face opposite the compression face, the at least one ultra-high performance concrete reinforcement bar comprising a pair of ultra-high performance concrete reinforcement bars extending in parallel along the length of the tension face. 4. The ultra-high performance concrete reinforcement bar according to claim, wherein said steels reinforcing fibers have a diameter of about 0.5 mm, a length of about.7 mm, and an ultimate tensile strength of about 500 MPa. 5. The ultra-high performance concrete reinforcement bar according to claim 4, wherein said steels reinforcing fibers comprise about 6.% by weight of the ultra-high performance concrete. 6. The ultra-high performance concrete reinforcement bar according to claim, wherein said ultra-high performance concrete has a water-to-cementitious material ratio of about The ultra-high performance concrete reinforcement bar according to claim, wherein said elongate bar has flexural tensile strength of about MPa and direct tensile strength of about - MPa.

13 A hybrid concrete beam, comprising: an elongate concrete beam made from conventional concrete, the beam having a compression area and a tension area when subjected to a bending stress; and at least one elongate, deformed reinforcement bar made from ultra-high performance concrete having steel reinforcing fibers, the at least one elongate, deformed reinforcement bar being incorporated into and extending along the tension area of the elongate concrete beam A method of providing tension reinforcement to a concrete beam, comprising the step of reinforcing an elongate beam made of conventional concrete with at least one precast, deformed ultra-high performance concrete reinforcement bar incorporated into a tension area of the beam and extending along the length of the tension area, the ultra-high performance concrete reinforcement bar having steel reinforcing fibers.

14 ABSTRACT OF THE DISCLOSURE The ultra-high performance concrete reinforcement bars replace conventional steel tension bars and the like in the reinforcement of construction elements, such as concrete beams. The ultra-high performance concrete reinforcement bars are formed from ultra-high 5 performance concrete reinforced with steel fiber. The reinforcement bars are preferably deformed to improve bonding at the interface between conventional concrete in the beam and the reinforcement bars. The bars are disposed in the area of the beam known as the tension area when the beam is subjected to a bending stress to provide tensile reinforcement.

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