Patent Application
20140190113
1. Field of the Invention
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.
2. Description of the Related Art
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 150 MPa and flexural tensile strength exceeding 30 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.
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 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.
Thus, ultra-high performance concrete reinforcement bars solving the aforementioned problems is desired.
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.
These and other features of the present invention will become readily apparent upon further review of the following specification and drawings.
Similar reference characters denote corresponding features consistently throughout the attached drawings.
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 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.
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.
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.
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 2011 (FWHA Publication No. FHWA-HRT-11-038) defines UHPC as “a cementitious material composed of granular constituents, a water-to-cementitious material ratio less than 0.25, and a high percentage of discontinuous internal fiber reinforcement. The mechanical properties of UHPC include compressive strength greater than 21.7 ksi (150 MPa) and sustained postcracking tensile strength greater than 0.72 ksi (5 MPa). UHPC has a discontinuous pore structure that reduces liquid ingress, significantly enhancing durability as compared to conventional and high-performance concretes.” The aggregate material in UHPC generally has a particles size less than 1 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 0.25.
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 220 kg, fine sand 1005 kg, steel fiber 157 kg (about 6.2% by weight of the UHPC mix), superplasticizer (Glenium 51) (an admixture that improves workability of the UHPC), and water 162.4 kg (representing water-binder ratio of 0.145). The steel fibers were 0.15 mm in diameter and 12.7 mm in length, and had ultimate tensile strength of 2500 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 de-molded 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.
The experimental program involved testing a series of beam specimens of 150×150×760 mm and 150×200×900 mm in size. Each normal or conventional concrete beam was reinforced with two UHPC bars 10 of rectangular or square cross section having transverse dimensions of 25×25 mm, 25×50 mm and 50×50 mm, and an axial length equal to the length of the beam. The beams were moist cured for 28 days prior to testing in a four-point bend test. The failure load, mid-span deflection and the mode of failure was recorded for each test.
Results show that the failure occurs with the development of cracks in the UHPC bars 10 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 16 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 28-32 MPa, calculated using transformed section properties. No bond-slip problem was noted, and all beams 16 failed in flexure with the development and propagation of a single vertical crack. The deformed shape of the precast bars 10, along with the rough top surface (from leaving the top of the mold open), enables the bars 10 to develop full strength without any bond-slip.
The experimental findings have clearly established the possibility of utilizing the precast UHPC bars 10 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 10 that can be used to replace mild steel bars is about 15-20 times the area of steel bars. As the cross section of a UHPC bar 10 is much larger, the increased area of UHPC bars 10 can be accommodated within the beam section.
For precast one-way slabs, the use of precast UHPC bars 10 appears to be feasible and attractive.
Thus, the ultra-high performance concrete reinforcement bars may provide tension reinforcement for conventional concrete in hybrid or composite concrete beams or construction elements.
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.
