Patent Publication Number: US-8534022-B2

Title: Twisted threaded reinforcing bar

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a national stage entry of PCT/US2011/021463, filed Jan. 17, 2011, which is hereby incorporated by reference. 
     BACKGROUND 
     The present disclosure contemplates that reinforced concrete structures may be prestressed using “post-tension” methods and/or “pre-tension” methods. Post-tension reinforcement of concrete may create compressive forces in a concrete beam and/or plate that may be cast on-site, such as a floor of an office building or a beam of a bridge. For example, in post-tension reinforcement, concrete may be poured over greased cables that may be sheathed in plastic tubing. Although the plastic tubing may become adhered to the concrete, the grease may allow the cables to slide within the tubing. After the concrete cures, special machines and/or fixtures may be used to pull on protruding cable ends, creating tension, and/or to fix the cable ends in position while maintaining the tension on the cables. Thus, the cables may exert compressive forces on the concrete. 
     The present disclosure contemplates that in some pre-tension methods, a solid reinforcing bar (“rebar”) may be held in tension while concrete is poured and/or cures around the rebar. Once the concrete has cured, the externally applied tension on the rebar may be released, thereby allowing the rebar to apply compressive stresses to the concrete. Due to the difficulties of stretching rebar on-site, this method may be performed in a factory. 
     As used herein, “stress” may refer to a measure of the internal forces acting within a deformable body. As used herein, “strain” may refer to the deformation of a physical body under the action of applied forces. 
     The present disclosure contemplates that post-tensioning with cables may not be as effective for increasing the strength of concrete members as cast-in-place pre-tensioned rebar. The difference may be at least partially because compressive stress produced by a post-tension cable may not be applied evenly throughout the concrete (e.g., along the length of the cable), but instead may be applied substantially at the cable ends (e.g., on the outer surface of the concrete where the cables terminate). A post-tension cable, sliding in a greased tube, may not apply substantial forces inside the concrete structure (except, if the cable is curved, it may apply lateral forces generally at right angles to the cable). In contrast, a pre-tension rebar may apply forces to the concrete along the length of the rebar. 
     SUMMARY 
     Techniques for reinforcement of concrete using one or more reinforcement bars are generally disclosed. Some example embodiments may include prestressed concrete structures reinforced by twisted, threaded reinforcing bars. 
     In some example embodiments, a reinforcing bar for a prestressed concrete structure is generally described that may include an elongated, generally cylindrical rod and an external thread disposed on the rod. The external thread may be formed from an elongated, generally nonlinear channel wrapped about a radial surface of the generally cylindrical rod in a generally helical fashion. A base portion of the nonlinear channel may be disposed substantially against the radial surface of the generally cylindrical rod and an upstanding portion of the nonlinear channel may extend generally orthogonally from the radial surface of the generally cylindrical rod. 
     In some additional embodiments, prestressed concrete structures are generally described that may include a hardened concrete body, a reinforcing bar extending at least partially through the hardened concrete body and including a threaded section engaged with the hardened concrete body and a first lock configured to maintain the reinforcing bar in a twisted condition so that the threaded section maintains an axial compressive force on the hardened concrete body. The first lock may be arranged to oppose rotation of a locked section of the reinforcing bar relative to the hardened concrete body. 
     Methods of constructing prestressed concrete structures are generally described. Some example methods may include torsionally stressing a reinforcing bar such that an external reinforcing bar thread engaged with a hardened concrete body applies a compressive force to the hardened concrete body. Some example methods may further include securing the torsionally stressed reinforcing bar to maintain its torsional stress. In some example methods, the compressive force may be applied to the hardened concrete body in a generally axial direction relative to the reinforcing bar. 
     The foregoing summary is illustrative only and is not intended to be in any way limiting. In addition to the illustrative aspects, embodiments, and features described above, further aspects, embodiments, and features will become apparent by reference to the drawings and the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other features of the present disclosure will become more fully apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. Understanding that these drawings depict only several embodiments in accordance with the disclosure and are, therefore, not to be considered limiting of its scope, the disclosure will be described with additional specificity and detail through use of the accompanying drawings. 
       In the drawings: 
         FIG. 1A  is a cross-sectional view of an example prestressed concrete beam; 
         FIG. 1B  is an elevation view of an example external wrench; 
         FIG. 2  is a cross-sectional view of an example prestressed concrete beam; 
         FIG. 3  is a cross-sectional view of an example prestressed concrete beam; 
         FIG. 4A  is an end view of an example channel; 
         FIG. 4B  is a plan view of an example channel; 
         FIG. 4C  is an end view of an alternative example channel; 
         FIG. 4D  is an end view of an alternative example channel; 
         FIG. 5  is a perspective view of an example threaded rebar; 
         FIG. 6  is a perspective view of an example rebar; 
         FIG. 7  is a plan view of an example curved rebar; 
         FIG. 8  is a cross-sectional view of an example beam; and 
         FIG. 9  is a flow chart illustrating an example method for constructing a prestressed concrete structure; all arranged in accordance with at least some embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, reference is made to the accompanying drawings, which form a part hereof. In the drawings, similar symbols typically identify similar components, unless context dictates otherwise. The illustrative embodiments described in the detailed description, drawings, and claims are not meant to be limiting. Other embodiments may be utilized, and other changes may be made, without departing from the spirit or scope of the subject matter presented here. It will be readily understood that the aspects of the present disclosure, as generally described herein, and illustrated in the Figures, may be arranged, substituted, combined, and designed in a wide variety of different configurations, all of which are explicitly contemplated and make part of this disclosure. 
     This disclosure is drawn, inter alia, to methods, systems, devices, and/or apparatus related to reinforcement of concrete using one or more reinforcement bars (“rebar”). Some example embodiments may include prestressed concrete structures reinforced by twisted, threaded rebar. 
     The present disclosure contemplates that when forces are applied against opposing points at the two ends of a beam, the stress may be concentrated at the points of application; however, the stress may not remain concentrated throughout the beam length. Toward the middle of the beam, the stress produced by the end forces may be distributed nearly uniformly over the area of the transverse beam cross-section. At any particular beam transverse cross-section, the integral of the compressive pressure over the cross-sectional area may be substantially equal to the force applied at the ends, but the distribution of the pressure may vary from high pressure over a small area (at the end faces) to low pressure over substantially the entire area (near the middle of the beam). 
     The present disclosure contemplates that this redistribution of force may occur according to Saint-Venant&#39;s principle of rapid dissipation of localized stresses. According to Saint-Venant&#39;s principle, a localized force may dissipate into a substantially uniform pressure at a distance from the application point that is about equal to the width of the member. As mentioned above, a post-tensioned cable may apply forces directly to the beam at two end points. Regardless of where the force is applied on the end faces, near the middle of the beam the compression force may be spread substantially evenly across the beam&#39;s cross-section. 
     The present disclosure contemplates that the situation may be different with prestressed construction, where the rebar may be locked to the concrete by the rebar&#39;s surface indentations. Because the rebar may be locked to the concrete along the length of the rebar, the stress may not spread out in the same manner, but, instead, may remain substantially concentrated near the rebar. Inside the beam, the force applied by the rebar may not result in any appreciable strain of the concrete, because contraction may be resisted by the entire beam and there may be much more concrete than steel. When the pre-tension on the rebar is released at the factory, the beam may not contract appreciably. The rebar, being locked to the concrete, may also not contract appreciably. (Some dimpling might take place at the beam ends, but any effect from that may not be felt in the middle, again due to Saint-Venant&#39;s principle.) 
     The present disclosure contemplates that because there may be substantially no strain near the middle portion of the beam, there may also be substantially no Saint-Venant&#39;s principle stress dissipation there. Consider a small cubical element of the beam, near the rebar, that is aligned to the major axes of the beam. The force from the rebar may be generally axial, and that force may be transferred from the element face which is perpendicular to the beam length onto the adjoining face of the next element, and so on along the length of the beam. The force may not be transferred to a laterally-adjoining element unless there is some strain. The adjoining element by itself may not generate any force; instead, if it is deformed, stress may be induced in that adjoining element. 
     The present disclosure contemplates that this effect may be understood by imagining a rectangular pile of children&#39;s cubical blocks (which may be analogous to the elements mentioned above), where the blocks are stacked in a regular Cartesian array on the floor, so that each face of each block is in full contact with a corresponding face of an adjoining block (except on the outside of the pile). Imagine further that the pile is in contact with a vertical wall, and that a force is exerted on one block in the middle of the pile opposite the wall, with the force directed toward the wall. Pressing on the one block may transfer the applied force directly to the wall through a horizontal column of blocks, and the wall may be subjected to force only there (e.g., substantially no dissipation of the force). If the wall is immovable, then the blocks may not substantially change their positions (e.g., there may be substantially zero strain). Next, imagine that the wall is yielding and that the blocks arc glued with a yielding adhesive, such as rubber cement. Now, the horizontal column may move forward and the blocks in the adjoining horizontal columns may also move, spreading out the force according to Saint-Venant&#39;s principle. Finally, imagine that the blocks are glued together but wall is again unyielding. Since none of the blocks move, the force may not spread, even though the rubber cement connects the blocks. Essentially, this example may demonstrate that with minimal strain there may also be minimal stress dissipation. 
     The present disclosure contemplates that another analogy may be useful for understanding stress dissipation. The stress induced by the pre-tensioned rebar may be somewhat like the compressive stress that may be produced by a pipe carrying chilled fluid, which was located in the same position as the rebar. The concrete around the pipe may experience stress due to thermal contraction, but it may not actually contract in the axial direction because the stress in the chilled core may be counteracted by opposing stresses in the other portions. By the principles of statics, the integral of the stress over any transverse cross-section of the beam must be zero (because there is no external force in this example), so the stress in the chilled portion would result in an opposing stress in the rest of the beam, with net force zero. By symmetry, this may happen all along the beam (except the ends) and the contractive stress would remain localized. The beam may contract only very slightly if the chilled core were a small fraction of the entire cross-section. That stress can exist without a corresponding strain may be exemplified by the type of auto glass which is tempered to have permanent tensile strain in the outer layers. When broken, the embedded stress causes the glass to shatter into pebbles, rendering it harmless in an accident. 
     The present disclosure contemplates that another example may be useful for understanding stress dissipation as described above. Take an ordinary rectangular gum-rubber eraser, draw parallel lines across the wider side face with a pen, and stand it end-up on a table. If the upstanding end is compressed with a flat object held parallel to the tabletop, the lines may remain straight. If the upstanding end is compressed with a pencil held parallel to the tabletop and transverse to the lined face, the lines near the pencil may bend (this can be seen by looking at the lines nearly end-on). In the case of the flat object, the compressive force may be transmitted without any strain (except for the vertical direction), and there may be substantially no dissipation. In the second case involving the pencil, dissipation of stress according to Saint-Venant&#39;s principle may be related to strain other than just axial strain. 
     The present disclosure contemplates that the above discussion may be summarized as follows. Compressive forces applied by a pre-tensioned rebar may remain near the rebar, but the forces applied by a post-tensioned cable may not remain near the cable. Also, a post-tensioned cable&#39;s force may be applied substantially evenly over the entire cross-section of the beam, except near the ends. 
     The present disclosure contemplates that if a beam that is suspended at both ends is loaded, for example by a weight near the middle of the beam, the beam may be subjected to bending (flexure). The bending of the beam may be resisted by compression in the upper portion of the beam and by tension in the lower portion of the beam. These compressive and tensile forces which resist bending may be greater near the middle of the length of the beam. Considering the beam&#39;s transverse cross section, at the beam&#39;s “neutral axis” there may be neither compression nor tension, and the compression may increase linearly to a maximum at the upper surface while the tension may increase to a maximum at the lower surface. 
     The present disclosure contemplates that because concrete may be relatively strong in compression but may be relatively weak in tension, rebar may be placed in the lowermost portion of a concrete beam so as to resist the tensile force which may occur there. 
     The present disclosure contemplates that, according to Saint-Venant&#39;s principle, a post-tensioned cable may create substantially the same force at the top of the beam and at the middle of the beam as at the bottom. Therefore, most of the force exerted by the cable may be “wasted,” doing little or nothing to resist bending and thereby stiffen the beam. The pressure in the upper portion, above the neutral axis, may actually make the beam more likely to fail in compression by pre-loading the upper portion the “wrong” way (e.g., instead of opposing the bending induced compressive forces in the upper portion of the beam, the post-tension forces may further increase the compressive forces felt in the upper portion of the beam). In the lower portion of the beam, the effectiveness of the force may decrease from the lower surface toward the neutral axis and may be only partially effective. For example, if the beam has a rectangular cross section, then the force in the lower portion may be about one-half effective, and the compressive force for the whole beam may be about one-quarter effective. Thus, the steel in the cable may be being used at about 25% efficiency. 
     The present disclosure contemplates that, in contrast to post-tension cables, pre-tensioned rebar located in the lowermost portion of the beam may be very efficient because the force it exerts may be substantially confined to, and utilized in, the “right” portion of the beam, namely, where the tension loading in the beam may be counteracted. 
     The present disclosure contemplates that although a beam is discussed above, the same ideas may apply to floors, plates, or other structures. Also, besides the “unbonded” system of greased cables described above, there may also be a “bonded” post-tension system in which ducts may be embedded in the concrete. After the concrete cures, cables may be fished through the ducts, tensioned, and then grouted in place to lock in the applied tension. The bonded cables may have generally the same the stress distribution characteristics as the unbonded cables described above. The tension in the concrete due to tightening the cable ends may be generally the same in both cases. Grouting may preserve, without changing, the stress distribution. 
     The present disclosure contemplates that it may be beneficial to provide the higher structural efficiency of pre-stressed solid rebar but in a post-cure application that may provide at least some of the implementation advantages of post-tensioned cable. Some example embodiments according to the present disclosure may include post-tensioned rebar that produces substantially the same force distribution in hardened concrete as factory-made, pre-tensioned rebar. In some example embodiments, the rebar may be cast in concrete at the construction site and/or may not require curing the concrete while the rebar remains strained (e.g., the rebar may be strained after the concrete cures). 
       FIG. 1A  is a cross-sectional view of an example concrete beam  100 , arranged in accordance with at least some embodiments of the present disclosure. As depicted, beam  100  may be supported by supports  101 A,  101 B. Beam  100  may include a hardened concrete body  102  at least partway through which a threaded rebar  104  may extend. For example, threaded rebar  104  may extend within the lower portion  102 A of the hardened concrete body  102 , generally where the hardened concrete body  102  may experience tensile stress due to bending. External threads (for example, as discussed below) of threaded rebar  104  may threadedly engage corresponding internal threads of hardened concrete body  102 , which may be formed by concrete cured around the external threads of threaded rebar  104 . A lubricant  105  may at least partially interpose threaded rebar  104  and hardened concrete body  102 . 
     Threaded rebar  104  may include a first end  106  and/or a second end  108 . First end  106  may be fixed within hardened concrete body  102 , such as by an internal wrench  110 . In some example embodiments, internal wrench  110  may be disposed within a cavity  110 A formed in the hardened concrete body  102 . Second end  108  may extend externally of hardened concrete body  102 . An external wrench  112  may be coupled to second end  108 . As discussed below, external wrench  112  may be used to torsionally deform threaded rebar  104  to prestress hardened concrete body  102 . In some example embodiments, first end  106  may extend externally of hardened concrete body  102  and/or may include a corresponding external wrench generally similar to external wrench  112 . 
     Some example embodiments according to the present disclosure may utilize “all-thread” rebar, which may be commercially available in various diameters and/or lengths. All-thread rebar may be much like the “threaded rod” found in hardware stores and/or may include a substantially continuous thread along its length. The threads of this type of rebar may mate with external nuts and/or may lock the rebar to the concrete. 
     In some example embodiments, lubricant  105  may remain effective after concrete  102  has set around it. For example, lubricant  105  may include one or more strips of heavily-greased paper disposed around the outside surface of threaded rebar  104 . In some example embodiments, the strips of greased paper may fit rebar  104  closely and/or substantially without gaps, thereby avoiding adhesion of rebar  104  to concrete  102 . In some example embodiments, rebar  104  may be rotated back and forth slightly while concrete  102  is curing to help prevent adhesion. Some example embodiments utilizing rotation to prevent adhesion may not utilize greased paper strips. In some example embodiments, the gap between rebar  104  and concrete  102  may be minimized, which may provide a closer fit and/or improved contact between rebar  104  and concrete  102 . 
     In some example embodiments, cavity  110 A may be formed in the hardened concrete body  102  by embedding a form, which may be generally similar to a plastic bottle, for example, in the liquid concrete with the internal wrench  110  inside the form. Cavity  110 A may allow the internal wrench  110  to dither, to a limited extent, inside the cavity when external wrench  112  is rotated back and forth (e.g., during curing as discussed above). Cavity  110 A may act as a reservoir for lubricant as discussed below. 
     In some example embodiments, internal wrench  110  may be configured to prevent and/or limit rotation of first end  106  of rebar  104 . Internal wrench  110  may include, for example, a metal cross piece welded to first end  106  of rebar  104 . Alternatively, internal wrench  110  may engage rebar  104  generally as an open-end or box-end wrench engages the head of a bolt. For example, a multiple number of flats may be formed on first end  106  of rebar  104  and/or a component generally similar to an open-end and/or box-end wrench may be engaged with the flats. In some example embodiments, the internal wrench  110  may be held in position by concrete  102 ; however, rebar  104 , which may be lubricated, may be able to rotate relative to the concrete, starting from a position near the internal wrench. 
     In some example embodiments, second end  108  of rebar  104  may protrude from concrete  102 . After concrete  102  has cured, second end  108  may be rotated, such as by external wrench  112 . An example external wrench  112  may be an actual wrench (e.g., an off-the-shelf tool) and/or may include any device capable of exerting torque on second end  108  of rebar  104 . 
     Torque applied to second end  108  of rebar  104  may cause rebar  104  to twist (e.g., torsionally deform), which may also have the effect of advancing the threads of rebar  104  relative to corresponding female threads cast into the concrete  102 , which may have molded themselves on the threads of rebar  104  during cure. Neglecting friction, the degree of twist of rebar  104  at any point along the length of rebar  104  may be approximately proportional to the distance from first end  106 . Thus, as discussed below, the advance of the threads of rebar  104  through concrete  102  may also be generally proportional to the distance from first end  106 . The torque may be applied in the direction that will stretch rebar  104  and/or compress concrete  102  (e.g., counter-clockwise for right-handed threads). 
     Stated concisely, in some example embodiments, applying a torque to external wrench  112  may cause rebar  104  to rotate due to torsional deformation; rebar  104  may move relative to the concrete female threads, thereby forcing second end  108  outward; rebar  104  may become longer, may be stressed, and may transfer a generally axial force to the adjacent concrete through the female threads; and the compressive force may be substantially uniform along the length of rebar  104 . The effect may be generally the same as that of factory-made, pre-tensioned rebar if external wrench  112  is kept fixed in position to maintain the torque. The efficiency may be increased several times in comparison with post-tensioned cable because the stress may remain substantially localized in the portion of the beam that resists tension. 
     In some example embodiments, use of solid rebar  104  may provide a stiffer reinforcement than would be provided by tensioned cable. The present disclosure contemplates that due to the low proportion of cross-sectional area of steel (as opposed to air) in a cable and/or the decreased modulus of elasticity because the cable can stretch by winding itself more tightly, the modulus of cable may be substantially less than that of solid rod. For example, a one-inch cable may have an actual cross-sectional area between about 0.380 and about 0.580 square inches, while a solid rod of the same diameter may have an area of about 0.785 square inches. Considering the median value of 0.480, the area of the cable is only about 60% that of the rod. The moduli of steel cable may be roughly half the modulus of solid steel of the same diameter. Thus, the overall modulus for cable may be only about a third of the modulus for rebar of the same diameter. The present disclosure contemplates that a difference in stiffness may become significant if a reinforced concrete structure is loaded enough to neutralize the prestress and/or the steel reinforcement begins to stretch. For example, in some such circumstances, a lack of stiffness may cause beam failure. 
     The following analysis assumes that concrete is incompressible. A rod may have a twist φ (per unit of length) that may be given by
 
φ= T/JG  
 
where T may be torque, J may be the moment of inertia, and/or G may be the modulus of elasticity in shear. For a circular rod, J may be given by ½ π r 4 , where r may be the radius of the rod.
 
     Substituting yields
 
φ=2 T/Gπr   4  
 
where φ may be in radians. The axial advance of the screw of the all-thread rebar may be equal to the thread pitch p times the number of revolutions of the rebar. Since one revolution may be equal to 2π radians, the axial advance z of the rebar threads for an angle φ may be z=pφ2π.
 
     Combining the last two equations yields
 
 z=pT/[G (π r   2 ) 2 ]
 
or,
 
 z=pT/[GA   2 ],
 
where A may be the cross-sectional area of the rebar core (excluding threads), i.e., πr 2 .
 
     The elongation of a rod, per unit of length, due to an axial force may be
 
ε=σ/ E  
 
where σ may be the axial stress and/or E may be the modulus of elasticity in tension. For steel, the modulus of elasticity in tension (E) may be about 30 million psi, while the modulus of elasticity in shear (G) may be about 12 million psi. E may be expressed as 2.5 G and the last equation may be rewritten as
 
ε=σ(2.5) G.  
 
     The force F in the rebar may be equal to σA. Realizing that z=ε, equating, and simplifying yields
 
 F =(2.5) pT/A , or (since  A=πr   2 ),  F= (0.8) pT/r   2 .
 
     This equation shows that the amount of concrete compression F may decrease as the square of the radius of the rebar. Rebar that is 3½ inches in diameter may exert about 23 times less compressive force than rebar that is inch in diameter, for the same torque. The force may be proportional to the pitch of the threads, so rebar of high pitch may be useful in larger diameters. 
     The modulus of elasticity of concrete may be about 4.26 million psi, which may be about 7 times less than the modulus of steel. If a one inch radius steel rod is compressing a 2.8 inch radius concrete cylinder (with a cross section seven times as that of the rod), the compression strain of one may substantially equal the elongation of the other. As discussed above, a concrete “core” may not substantially contract in length relative to the rest of a beam because of shear strain, so the compressive force may be resisted by substantially the entire beam, the beam may not contract appreciably, and there may be substantially no internal strain, only internal stress. 
     In some example embodiments, rebar  104  may be vibrated while the torque is being applied (or afterward) to free localized small hang-ups due to thread irregularities or dirt. The vibration may be applied axially (or otherwise) and/or may use various frequencies. An alternative may be to hit second end  108  of rebar  104  with a hammer, which may produce an impulse which, according to Fourier, may contain substantially all frequencies. 
     All-thread rebar may be easily end-coupled to another piece of similar rebar using an internally-threaded connector, such as a “standoff.” If the two rebar ends are joined securely, a long length of rebar may be made up of smaller subsections of manageable length, which may be easy to transport, store, and/or handle. 
       FIG. 1B  is an elevation view of an example external wrench  112 , arranged in accordance with at least some embodiments of the present disclosure. As depicted, external wrench  112  may be mounted to second end  108  of rebar  104 . An example external wrench  112  may include a first end  112 A configured to engage second end  108  of rebar  104 , such as by being welded to rebar  104 . A second end  112 B may extend generally radially from rebar  104  and/or may be configured for application of torque to rebar  104 . A stop  114  may be installed to prevent rotation of external wrench  112  relative to hardened concrete body  102 , such as after threaded rebar  104  has been torsionally deformed and/or when it is desired to maintain such torsional deformation. Stop  114  may include, for example, a removable pin and/or other similar component configured to oppose rotation of external wrench  112 . Any component that maintains rotation of rebar and/or otherwise maintains torque on a rebar (e.g., stop  114 ) may be referred to herein as a lock. 
       FIG. 2  is a cross-sectional view of an example prestressed concrete beam  150 , arranged in accordance with at least some embodiments of the present disclosure. As depicted, beam  150  may be supported by supports  151 A,  151 B. Beam  150  may include a multiple number of internally threaded nuts  152 ,  154 ,  156 ,  158  which may be engaged with threaded rebar  160 . Rebar  160  may extend at least partway through hardened concrete body  162 . External wrenches  164 ,  166  may engage portions of rebar  160  extending beyond hardened concrete body  162 . Internal wrench  168  may engage rebar  160  in a pit  170  provided in hardened concrete body  162 . Pit  170  may be at least partially filled with concrete once access to internal wrench  168  is no longer desired. Some example embodiments may include a sleeve  172  interposing at least a portion of rebar and concrete body  162 . 
     Example embodiments including internally threaded nuts  152 ,  154 ,  156 ,  158  engaged with threaded rebar  160  may reduce friction caused by concrete in contact with rebar  160 , particularly where concrete  162  may adhere to rebar  160 . In addition, internally threaded nuts  152 ,  154 ,  156 ,  158  may be useful where cast concrete internal threads engaged with threaded rebar  160  may be insufficiently strong for the desired application. One or more internally threaded nuts  152 ,  154 ,  156 ,  158  may be spaced along rebar  160  and/or may be arranged so that rebar  160  exerts forces predominantly on the nuts  152 ,  154 ,  156 ,  158 , which may then apply forces to the concrete in which they are embedded. In some example embodiments, individual nuts  152 ,  154 ,  156 ,  158  may include a multiple number of separable components which may be assembled over rebar  160  and then locked together, which may avoid tedious threading of nuts onto long lengths of rebar  160 . For example, an individual nut  152 ,  154 ,  156 ,  158  may include two generally semi-cylindrical sections which may be placed on rebar  160  and/or locked together to provide an internally threaded hole engaged with the external threads of rebar  160 . Some example embodiments may include one or more sleeves  172  which may prevent concrete from adhering to the threads of rebar  160 . For example, sleeve  172  may be generally tubular and/or may be configured to substantially cover at least a portion of rebar  160 . Sleeves  172  may be constructed from any appropriate material, such as plastic, metal, etc. 
     In some example embodiments, individual nuts  156 ,  158  may include a plate  156 A,  158 A embedded in concrete body  162 . An individual plate  156 A,  158 A may be provided with an internally threaded section  156 B,  158 B for threadedly engaging rebar  160 . Internally threaded sections  156 B,  158 B may be bonded to respective plates  156 A,  158 A, such as by welding. 
       FIG. 3  is a cross-sectional view of an example prestressed concrete beam  200 , arranged in accordance with at least some embodiments of the present disclosure. As depicted, concrete beam  200  may include an extension rod  202  and/or a universal joint  204  operatively coupling an external wrench  206  (which may include a ratchet  206 A) to a threaded rebar  208  embedded within concrete beam  200 . Extension rod  202  may extend within a channel  210  provided in hardened concrete body  212 . In some example embodiments, extension rod  202  may be oriented other than linearly arranged (e.g., disposed at a non-zero angle) with respect to threaded rebar  208 . In some such embodiments, universal joint  204  may allow rotational movement of extension rod  202  to cause rotational movement and/or torsional deformation of threaded rebar  208 . Universal joint  204  may be provided within a housing and/or covering, which may prevent concrete from entering universal joint  204 . 
     The present disclosure contemplates that torque may be transmitted from extension rod  202  to threaded rebar  208  without substantial loss because the torque-transmission factor may be the cosine of the angle between extension rod  202  and threaded rebar  208 . Because the bending stresses on a beam suspended at its two ends may be greatest in the middle of its length, and because the stiffness of a beam (e.g., its moment of inertia) may be proportional to the square of the depth, beams may be made thicker in the middle, as shown in  FIG. 3 . The lower and middle portion of a beam may be where the compressive stress of the rebar may be particularly useful for resisting a load applied on the upper side of the beam. 
     The present disclosure contemplates that rebar may be made of fiber-reinforced resin instead of steel. Such rebar may be used in connection with example embodiments described herein. If the fibers run longitudinally through the rebar, then the rebar may be more resistant to stretching than it is to twisting. Larger diameters of rebar may be useful for uniform-pitch rebar, by the analysis above. In some example embodiments, fiber-reinforced rebar may include fibers that are wound generally helically inside the rebar so as to adjust the ratio of resistance to stretching as compared to resistance to twisting (that is, a ratio of moduli). 
     As shown in  FIG. 3 , in some example embodiments, an external wrench  206  may be covered with concrete after torque is applied, which may prevent corrosion and/or accidental loosening of the rebar. In some example embodiments, an external wrench  206  may include a ratchet  206 A and/or equivalent mechanism to keep the rebar strained in torsion. 
     In some example embodiments, one and/or both ends of a rebar may extend beyond a hardened concrete body. Some example embodiments may include internal wrenches at locations other than ends of the rebar. See, for example,  FIG. 2  which illustrates an internal wrench  168  provided generally near the midpoint of a rebar  160 . 
       FIG. 4A  is an end view of an example channel  300 , arranged in accordance with at least some embodiments of the present disclosure. As shown in  FIG. 5  (discussed below), channel  300  may be wrapped around a generally cylindrical rod in a generally helical fashion to provide an externally threaded rebar. Channel  300  may have a substantially nonlinear cross section, such as a generally L-shaped cross section, which may include a base portion  302  and/or an upstanding portion  304 , where base portion  302  and upstanding portion  304  may be disposed at about 90 degrees with respect to each other. The width of base portion  302  may correspond to the pitch P of the threads formed when channel  300  is wrapped around the generally cylindrical rod (see, e.g.,  FIG. 5 ). The height H of channel  300  may correspond to the height of the threads formed when channel  300  is wrapped around the generally cylindrical rod (see, e.g.,  FIG. 5 ). 
       FIG. 4B  is a plan view of example channel  300 , arranged in accordance with at least some embodiments of the present disclosure. In some example embodiments, the width of base portion  302  may vary over the length L of channel  300 . For example, at a first end  308  of channel  300 , base portion  302  may have a width providing a first thread pitch P 1  when channel  300  is wrapped in a generally helical fashion around the generally cylindrical rod (see, e.g.,  FIG. 5 ). At a second end  310 , base portion  302  may have a width providing a second thread pitch P 2  when channel  300  is wrapped in a generally helical fashion around the generally cylindrical rod (see, e.g.,  FIG. 5 ). In some example embodiments, height H of channel  300  may vary along length L of channel  300 . 
       FIG. 4C  is an end view of an alternative example channel  1300 , arranged in accordance with at least some embodiments of the present disclosure. Channel  1300  may be wrapped around a generally cylindrical rod in a generally helical fashion as mentioned above regarding channel  300 . Channel  1300  may have a substantially nonlinear cross section, such as a generally T-shaped cross section, which may include a base portion  1302  and/or an upstanding portion  1304 , where base portion  1302  and upstanding portion  1304  may be disposed at about 90 degrees with respect to each other. The width of base portion  1302  may correspond to the pitch PT of the threads formed when channel  1300  is wrapped around the generally cylindrical rod. The height HT of channel  1300  may correspond to the height of the threads formed when channel  1300  is wrapped around the generally cylindrical rod. 
       FIG. 4D  is an end view of an alternative example channel  2300 , arranged in accordance with at least some embodiments of the present disclosure. Channel  2300  may be wrapped around a generally cylindrical rod in a generally helical fashion as mentioned above regarding channel  300 . Channel  2300  may have a substantially nonlinear cross section, such as a generally hook-shaped cross section, which may include a base portion  2302  and/or an upstanding portion  2304 , where base portion  2302  and upstanding portion  2304  may be disposed at about 90 degrees with respect to each other. The width of base portion  2302  may correspond to the pitch PH of the threads formed when channel  2300  is wrapped around the generally cylindrical rod. The height HH of channel  2300  may correspond to the height of the threads formed when channel  2300  is wrapped around the generally cylindrical rod. 
       FIG. 5  is a perspective view of an example threaded rebar  400 , arranged in accordance with at least some embodiments of the present disclosure. As depicted, threaded rebar  400  includes a channel  300  wrapped in a generally helical fashion around a generally cylindrical rod  402 . Generally cylindrical rod  402  may be hollow with an axial channel  407  and/or may include a radial surface  404 , which may receive base portion  302  of channel  300 . Channel  300  may form an external thread  406  having a height H and/or a pitch P. 
     In some example embodiments, channel  300  and generally cylindrical rod  402  may be constructed from the same and/or complementary materials. For example, channel  300  and generally cylindrical rod  402  may be constructed of resin and/or steel. In some example embodiments, channel  300  and/or generally cylindrical rod  402  may be welded to fasten channel  300  wrapped around generally cylindrical rod  402 . 
     Channel  300  may be wrapped in a generally helical fashion such that each turn of channel  300  around generally cylindrical rod  402  substantially abuts the previous turn. In other words, base portion  302  of one turn may substantially lie against base portion  302  of the previous turn, and so on. In embodiments including a tapered base portion  302  (see, e.g.,  FIG. 4B ), such an arrangement may produce an external thread  406  having a pitch P that varies substantially uniformly along the length of the rebar. 
     The present disclosure contemplates that threaded rebar  400  including an external thread  406  having a gradually varying pitch may be useful, particularly where it may be difficult to torsionally deform a large-diameter rebar sufficiently to obtain the desired compression. In some example embodiments incorporating thread of a varying pitch, the rebar may not need to deform in torsion to apply the desired compressive forces to the concrete. If the thread pitch changes generally linearly with distance along the rebar, then rotation may exert a generally uniform compressive force on the concrete. As with the constant-pitch threaded rebar, varying pitch threaded rebar may be kept from adhering to the surrounding concrete with small to-and-fro rotations during cure. 
     The present disclosure contemplates that, in a variable pitch threaded rebar, if the variation in pitch is not a uniform function of the length of the rebar, then different amounts of compression may be applied to different regions of the concrete along the length of the rebar, which may be useful in some designs. For example, in a beam that is loaded on its upper side, the requirement for compressive stress may be greatest in the middle, and a rebar&#39;s thread pitch may be chosen so that the stress is greater toward the middle of the beam and is less toward the ends. 
     In some example embodiments, threaded rebar  400  including a variable pitch external thread  406  may be constructed by wrapping channel  300  around generally cylindrical rod  402  such that base portions  302  of consecutive turns do not always abut each other. As will be understood by those of skill in the art, spacing between consecutive turns of channel  300  may determine the pitch P of thread  406 . In some example embodiments, channel  300  may have cross sections other than an L-shape. For example, in some alternative embodiments, channel  300  may have a generally T-shaped cross section. 
     In some example embodiments, channel  300  may be coiled such that abutting turns exert a compressive force on each other. In some such embodiments, abutting turns may have substantially no axial distance between each other, and it may be necessary to exert an axially tensile force to pull abutting turns apart. Such a configuration may be advantageous because it may increase the precision of the thread spacing. 
     Although  FIG. 5  illustrates a hollow generally cylindrical rod  402  including an axial channel  407 , generally, cylindrical rod may be at least partially solid or may be substantially completely solid. The present disclosure contemplates that the hollow generally cylindrical rod  402  may be stiffer than a solid generally cylindrical rod  402  comprising the same amount of steel. 
       FIG. 6  is a perspective view of an example rebar  450 , arranged in accordance with at least some embodiments of the present disclosure. As depicted, rebar  450  may include a generally helically coiled channel  300  forming an external thread  452  and/or a hollow interior  454 . Rebar  450  may not include the generally cylindrical rod  402  ( FIG. 4 ). In some such embodiments, generally cylindrical rod  402  may be used to form channel  300  into its generally helical shape, but may be removed prior to installing the resulting rebar into a beam. 
     In some example embodiments, a curved rod or pipe may be the basis for a curved rebar of small radius, using a channel similar to those described above.  FIG. 7  is a plan view of an example curved rebar  750 , arranged in accordance with at least some embodiments of the present disclosure. As depicted, curved rebar  750  may include a curved core  752  about which a channel  754  may be wrapped in a generally helical fashion. In some example embodiments, channel  754  may be wrapped around but may not be fastened to the curved core  752 . Channel  754  may be allowed to slide on the external surface of the curved core  752  and/or may be lubricated for that purpose. If channel  754  is rotated about the curved core  752 , the channel  754  may tend to spread generally axially, which may exert a compressive force on surrounding concrete, which may follow the line of curved core  752 . Such a method might be used to reinforce an arch of small radius. (An arch of larger radius can be reinforced by straight segments joined by universal joints.) 
     Some example embodiments may be configured for flushing lubricant interposing rebar  504  and hardened concrete body  502 .  FIG. 8  is a cross-sectional view of an example beam  500 , arranged in accordance with at least some embodiments of the present disclosure. As depicted, beam  500  may include a hardened concrete body  502  and/or a threaded rebar  504 . A grease nipple  506  may be fluidicly coupled to the exterior of threaded rebar  504  by a grease conduit  508 . Grease conduit  508  may join the exterior of threaded rebar  504  proximate an internal wrench  510 . A lubricant  512  may at least partially interpose threaded rebar  504  and hardened concrete body  502 . 
     The present disclosure contemplates that it may be beneficial to replace lubricant  512  between rebar  504  and concrete body  502 . Over time, lubricant  512  may degrade and/or change its pH, which may leave steel rebar  504  vulnerable to corrosion, attack by microorganisms, or other degradation. In some example embodiments, replacement lubricant may be slowly pumped through the gap between rebar  504  and concrete  502 . This operation may be accomplished by something as simple as periodically applying a grease gun to grease nipple  506 , which may be fluidicly coupled to the exterior of rebar  504 . An air space, perhaps built into the handle of internal wrench  510  (and/or provided by cavity  110 A of  FIG. 1A ), may be used to maintain pressure while injected lubricant slowly moves along the thread and extrudes at the other end. Periodically replacing the lubricant may maintain surface coverage, decrease pH changes, and/or flush out microorganisms. 
     Some example embodiments according to the present disclosure may provide advantages over cable reinforcement systems. For example, because solid rebar may have a modulus of elasticity of about three times that of a cable of the same diameter, solid rebar may more effectively prevent collapse. 
       FIG. 9  is a flow chart illustrating an example method  700  for constructing a prestressed concrete structure, arranged in accordance with at least some embodiments of the present disclosure. Method  700  may include operation  702 , which may include applying a compressive force to at least a portion of a hardened concrete body in a generally axial direction relative to a reinforcing bar extending at least partially through the hardened concrete body by torsionally stressing the reinforcing bar such that at least one external reinforcing bar thread operatively engaged with the hardened concrete body applies the compressive force to at least the portion of the hardened concrete body. Operation  704  may follow operation  702  and may include securing the torsionally stressed reinforcing bar to maintain torsional stress of the reinforcing bar. 
     The herein described subject matter sometimes illustrates different components contained within, or connected with, different other components. It is to be understood that such depicted architectures are merely examples, and that in fact many other architectures may be implemented which achieve the same functionality. In a conceptual sense, any arrangement of components to achieve the same functionality is effectively “associated” such that the desired functionality is achieved. Hence, any two components herein combined to achieve a particular functionality may be seen as “associated with” each other such that the desired functionality is achieved, irrespective of architectures or intermedial components. Likewise, any two components so associated may also be viewed as being “operably connected”, or “operably coupled”, to each other to achieve the desired functionality, and any two components capable of being so associated may also be viewed as being “operably couplable”, to each other to achieve the desired functionality. Specific examples of operably couplable include but are not limited to physically mateable and/or physically interacting components and/or wirelessly interactable and/or wirelessly interacting components and/or logically interacting and/or logically interactable components. 
     With respect to the use of substantially any plural and/or singular terms herein, those having skill in the art may translate from the plural to the singular and/or from the singular to the plural as is appropriate to the context and/or application. The various singular/plural permutations may be expressly set forth herein for sake of clarity. 
     It will be understood by those within the art that, in general, terms used herein, and especially in the appended claims (e.g., bodies of the appended claims) are generally intended as “open” terms (e.g., the term “including” should be interpreted as “including but not limited to,” the term “having” should be interpreted as “having at least,” the term “includes” should be interpreted as “includes but is not limited to,” etc.). It will be further understood by those within the art that if a specific number of an introduced claim recitation is intended, such an intent will be explicitly recited in the claim, and in the absence of such recitation no such intent is present. For example, as an aid to understanding, the following appended claims may contain usage of the introductory phrases “at least one” and “one or more” to introduce claim recitations. However, the use of such phrases should not be construed to imply that the introduction of a claim recitation by the indefinite articles “a” or “an” limits any particular claim containing such introduced claim recitation to inventions containing only one such recitation, even when the same claim includes the introductory phrases “one or more” or “at least one” and indefinite articles such as “a” or “an” (e.g., “a” and/or “an” should typically be interpreted to mean “at least one” or “one or more”); the same holds true for the use of definite articles used to introduce claim recitations. In addition, even if a specific number of an introduced claim recitation is explicitly recited, those skilled in the art will recognize that such recitation should typically be interpreted to mean at least the recited number (e.g., the bare recitation of “two recitations,” without other modifiers, typically means at least two recitations, or two or more recitations). Furthermore, in those instances where a convention analogous to “at least one of A, B, and C, etc.” is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., “a system having at least one of A, B, and C” would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.). In those instances where a convention analogous to “at least one of A, B, or C, etc.” is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., “a system having at least one of A, B, or C” would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.). It will be further understood by those within the art that virtually any disjunctive word and/or phrase presenting two or more alternative terms, whether in the description, claims, or drawings, should be understood to contemplate the possibilities of including one of the terms, either of the terms, or both terms. For example, the phrase “A or B” will be understood to include the possibilities of “A” or “B” or “A and B.” 
     While various aspects and embodiments have been disclosed herein, other aspects and embodiments will be apparent to those skilled in the art. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims.