Abstract:
A prescribed prestressing process is employed to construct precast concrete beams and girders. The process is utilized where an early concrete strength is too low for transfer of the full pre-tensioning force on a daily schedule to avert an otherwise serious and costly production delay. The process described provides the producer a reliable way of making beams and girders that are prestressed to take advantage of the higher concrete strength characteristics. The consequent economic advantage of higher structural capacity beams and girders is thereby realized. Additionally, beam or girder camber is controlled by the process, fostering production of a superior quality product.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION 
       [0001]    This application relates to U.S. Provisional Patent Application No. 61/280,109 filed on Oct. 29, 2009, entitled A METHOD FOR PRODUCING HIGH-CAPACITY CONCRETE BEAMS, which is hereby incorporated herein in its entirety by this reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to the prefabrication of structural building materials, and, more particularly, to the prefabrication of concrete beams or girders. Specifically, various embodiments of the present invention provide an apparatus and process to realize economic and quality benefits by producing concrete beams or girders of high structural capacity through practical steps that are easily implemented in most precast beam/girder production plants. 
       BACKGROUND OF THE INVENTION 
       [0003]    Progress in the production of concrete beams (also known as “girders”) for construction of bridges and buildings was greatly stimulated in the 1950&#39;s when the technique of prestressing the concrete was proven to have advantages in the United States. There are two known techniques of prestressing: pre-tensioning and post-tensioning. Both techniques of prestressing employ steel cables or bars to apply and hold a concrete member in compression. Prestressing can be referred to as “active reinforcement” as compared to “passive reinforcement”, such as is obtained with mild reinforcing steel (rebar). 
         [0004]    Pre-tensioning is the predominate form of prestressing employed in the precast concrete industry. This technique involves stretching steel cables with a high tensile force, with the cables held between fixed abutments that are situated at each end of a casting platform, or “bed”, and placing concrete in forms on the bed, which forms encase the cables to form a beam or girder. At a later time, after the concrete has gained sufficient strength and bonded well to the cables, the cables are released from the abutment anchors, the forms removed, and the completed beam or girder is lifted from the bed. 
         [0005]    It is of great economic importance that this procedure be completed on a daily cycle. In order to do this, heating the newly cast concrete at a curing temperature as high as 180 degrees F. to accelerate concrete strength gain has been a common practice. 
         [0006]    The post-tensioning technique is not generally practiced in precast concrete production. Post-tensioning is more expensive per pound than pre-tensioning, so it has been employed sparingly in the production of beams or girders other than in special cases to meet design requirements. 
         [0007]    In recent years, there has been remarkable progress in making concrete that has much higher strength than ever before. Ultimate 56 day compressive strength is now possible in the 10,000 psi to 20,000 psi range, which is up to 10,000 psi higher than strengths attainable a short time ago. 
         [0008]    However, there has not been an advantage taken of higher strength concrete by employing a proportionately higher prestressing force in the design of precast beams or girders. Beam or girder load carrying capacity is increased dramatically when, using the same beam or girder size and shape as those made with “standard” concrete, stronger high performance concrete (HPC) is employed with a substantially greater prestressing force. This fact was demonstrated on an experimental bridge project where HPC beams having a 56 day strength of 13,600 psi were constructed with approximately 60 percent more prestressing force than standard beams made with 6,000 psi concrete. Test results proved that four HPC beams had the same load carrying capacity as seven standard beams for “twin” bridges of an identical span and roadway width. Although the cost per beam was higher for the HPC beams, the cost of the bridge superstructure having four high structural capacity beams was approximately 15% lower than the bridge having seven standard beams. This project confirmed the economic viability of employing higher structural capacity beams made with superior concrete strength and constructed with a high prestressing force. However, industry has not reaped the benefits of these features to achieve an improved and more economic product. There are certain problems that must be solved. 
         [0009]    In addition to the common precautions observed in the design of a concrete mix, there are two important factors that must be dealt with concerning concrete durability. Both of these factors pose potential problems in making durable concrete beams or girders, as well as other concrete members. The first is known as alkaline-silica reaction (ASR); the second is called delayed ettringite formation (DEF). ASR is caused in large part by high alkalinity in the concrete reacting over time with silica in the aggregate. In severe cases, which are not uncommon, this reaction results in cracking and destruction of the concrete. 
         [0010]    On the other hand, it has been learned recently that DEF is promoted principally by curing the concrete at a very high temperature. DEF typically occurs over time in mature concrete. It has been mistakenly identified as ASR in some cases, because its apparent failure mode is similar to the failure mode attributable to ASR. 
         [0011]    The solutions to both problems are now known. Damage due to ASR can be avoided by substituting another cementitious material such as fly ash or slag for a portion of the cement in the mix to reduce net alkalinity. The drawback to this approach is that early concrete strength gain is slowed. Although final strength is typically very high, the concrete strength required for transfer of stress (the “release strength”), is not reached in time for daily recycling on the prestressing bed. Daily recycling of the bed is critical to a beam or girder manufacturer&#39;s economics. 
         [0012]    DEF can be avoided by restricting concrete curing temperature to a maximum of approximately 160 degrees F. Here again, because early concrete strength gain is dependent on curing temperature, the lower temperature requirement makes attaining release strength overnight less likely. 
         [0013]    Thus, there are two factors that have constrained production of superior and more cost-effective beams or girders prefabricated with HPC. Since higher strength concrete beams or girders containing a high prestressing force have been shown to produce a significant lower cost for a completed structure, it is important to have a way of making prestressed HPC beams or girders on a daily production cycle. 
         [0014]    Control of camber in concrete beams or girders can be yet another serious problem. Camber is the arching upward of a beam/girder or slab that is prestressed when the prestressing force is located below the centroid of the concrete. In almost all cases, the pre-tensioning force applied to a beam or girder on a pre-tensioning bed is well below the centroid of the concrete. When a prestressing force (which is a compressive force) is applied to concrete, the concrete immediately shortens elastically as the force is applied. Thereafter, there is an inelastic shortening due to a phenomenon known as “creep” of the concrete. The amount of creep is a function of time, the level of compressive stress, and the modulus of elasticity of the concrete. Camber takes place in a prestressed concrete beam or girder when the concrete fibers in the lower portion of the member are under a higher compressive stress than the fibers in the upper portion. Creep of the concrete continues to shorten the bottom of the member as time passes, causing camber to grow. There have been cases where camber growth has been so great that beams or girders became unfit for use in structures and were rejected. The economic implications of such a problem go well beyond loss of money by the precaster having to manufacture substitute beams or girders. The construction company, depending upon timely delivery of product for constructing the bridge or building, is impacted by delay that ensues while new beams or girders are manufactured to replace the rejected ones. 
         [0015]    One objective of the present invention is to provide a process that can be readily implemented by beam or girder manufacturers to overcome these problems. 
       SUMMARY OF THE INVENTION 
       [0016]    The various embodiments of the present invention provide a process for making precast beams or girders that have a greater load carrying capacity by employing a strategy that also provides additional control of quality. The process described makes it practical to use higher strength concrete that carries a high prestressing force. A substantial advantage is obtained by the following combination of steps to achieve superior load carrying capacity and quality and achieve advantageous economic results. 
         [0017]    First, the full prestressing force required by the design requirements for a beam or girder is not introduced by pre-tensioning, as is now routinely done. Instead, only a portion of the full design force is applied by pre-tensioning while the beam or girder is on a prestressing bed. A pre-tensioning force is applied that is at least a magnitude that will allow the beam or girder to be removed from the bed and withstand stresses experienced in handling and storage. The pre-tensioning force needed is readily calculated as a part of production procedures as is well-known to persons skilled in the art. 
         [0018]    The purpose of applying only a partial prestressing force is to allow earlier release of the pre-tensioning cables or rods, which release is made possible because the prestressing force that is applied to the concrete by pre-tensioning is reduced and thus permits the concrete strength to be lower before release of cables or rods from the abutments. Thus, the concrete beam or girder, although having an initial lower strength, can be removed from the bed earlier. Also, the effects of low early concrete strength that is caused by adjusting the concrete mix to diminish the prospect of ASR, and the lower curing temperature to combat DEF, as well as other factors that result in a concrete strength too low to carry the full prestressing force, are effectively managed, while daily cycling of the bed is achieved. 
         [0019]    Second, after a beam or girder is removed from the bed, the beam or girder is stored on supports near its ends, so that gravity acting on the beam or girder counteracts most of the prestressing force and thus resists camber growth due to flexural stresses. The result is that little, if any, inelastic concrete creep and camber growth occurs over time. 
         [0020]    Third, the remainder of the required prestressing force for the beam or girder is induced by post-tensioning. Post-tensioning can be accomplished at any time of the manufacturer&#39;s choosing, typically just several days before shipping the beam or girder to a customer&#39;s jobsite. By this timing strategy, unwanted camber growth can be eliminated. 
         [0021]    An added operational advantage produced by this process is that post-tensioning is performed away from the casting area at a distance from the prestressing bed, and therefore it is not on a critical production path because it does not affect the high intensity core activity of the beam or girder manufacturer. Also, because a range of post-tensioning forces can be applied, the manufacturer can potentially build an inventory of partially constructed beams or girders and thus supply beams or girders to customers more quickly than if construction of the beams or girders had not yet begun. 
         [0022]    The foregoing and other objects, features, and advantages of the present invention will become more readily apparent from the following detailed description of various embodiments of the present invention, which proceeds with reference to the accompanying drawing. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    The various embodiments of the present invention will be described in conjunction with the accompanying figures of the drawing to facilitate an understanding of the present invention. In the figures, like reference numerals refer to like elements. In the drawing: 
           [0024]      FIG. 1 , comprising  FIGS. 1A ,  1 B, and  1 C, illustrates the basic process flow for beam or girder production in accordance with one embodiment of the present invention. 
           [0025]      FIG. 2 , comprising  FIGS. 2A and 2B , shows an embodiment of a beam or girder produced in accordance with the process of the invention illustrated in  FIG. 1 . 
           [0026]      FIG. 3  illustrates a cast-in-place concrete deck comprising the beam or girder shown in  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0027]      FIGS. 1A ,  1 B, and  1 C illustrate a flowchart depicting a non-limiting example of the manufacturing process for a high strength concrete beam or girder, which synergistically allows for the use of high strength concrete combined with a rapid and economical cycling of the manufacturing bed, while providing a beam or girder strength that takes full advantage of the high strength concrete. 
         [0028]    The example process of  FIG. 1  begins in a step or operation  100 , as shown in  FIG. 1A , and continues in an operation  102  in which the manufacturing bed is prepared for use. Then, in an operation  104 , reinforcement and pre-tensioning strands, for example, cables or rods, are installed in place on the bed, along with post-tensioning ducts, and anchorages. Next, in an operation  106 , tensile force is applied to the pre-tensioning strands that were deployed in operation  104 . In an operation  108 , the elongation of the pre-tensioning strands is measured and recorded. An operation  110  is then performed which assembles the forms in place on the bed to provide a structure that determines the beam or girder shape. 
         [0029]    In an operation  112 , the high strength concrete is mixed. In certain example embodiments, a high cementitious content is used, along with water-reducing and plasticizing admixtures. The cementitious material in certain example embodiments comprises, for example, Portland Cement. In certain example embodiments a portion of the cementitious material comprises an alkalinity reducing material such as, for example, fly ash or slag to prevent the alkalinity of the mixture from being too high. Otherwise, alkalinity can cause a very serious reaction with silica in the aggregate, resulting in severe cracking of the concrete. While the introduction of the alkalinity reducing material does not materially affect the ultimate strength of the resultant concrete, the strength of the concrete is reduced in the short term, and other measures described herein are preferably taken to compensate for the short term lack of strength and its impact on the manufacturing process. The result is a synergistic combination that provides a beam or girder that takes full advantage of the strength of the highly cementitious mixture, avoids a silica reaction, and yet allows a rapid cycling of the manufacturing bed. 
         [0030]    In an operation  114  the concrete mixture is poured into the form. A portion of the mixture is also poured into a number of cylindrical or cubical forms which allow the strength of the concrete to be sampled at various times. The curing apparatus is put in place in an operation  116 . Then, in an operation  118  the concrete is cured. 
         [0031]    As shown in  FIG. 1B , the strength of the concrete is measured in an operation  120 , typically using one or more of the concrete cylinders or blocks mentioned in the description of operation  114 . A decision operation  122  determines whether the concrete has achieved a strength that is at least adequate to endure pre-tensioning (the “release strength”), removal from the bed, and storage. If it is determined that the strength of the concrete is not adequate, then a wait operation  124  is performed to allow the concrete to gain more strength, and the process returns to operation  120 . If it is determined in operation  122  that the strength of the concrete has achieved a strength that is at least adequate to endure pre-tensioning, removal from the bed, and storage, then the process continues with operation  126  which releases the pre-tensioning strands from their abutment anchorages, thus placing the beam or girder under compression. 
         [0032]    Next, in an operation  128 , the curing apparatus is removed to allow access to the beam or girder. Thereafter, in an operation  130  the forms are removed and cleaned for reuse. Then, in an operation  132 , the beam or girder is moved to storage. In certain example embodiments, the beam or girder is placed on supports proximate to the beam&#39;s or girder&#39;s respective ends, which allows the beam or girder to avoid camber growth, since the force applied in operation  106  is of less than full magnitude. The beam or girder, having gained sufficient strength to support its own weight and avoid deflection can be stored indefinitely. This removal of the beam or girder from the bed permits the bed to be re-used, and allows the beam or girder to gain strength over a period of time in storage. The timing of the removal from the bed is earlier than would otherwise be possible, and this early removal allows the bed to be used again for making another beam or girder. The removal of the beam or girder from the bed can be performed when the high strength concrete is relatively weak, because the pre-tensioning strands that were released in operation  126  have imparted only a portion of the total eventual prestressing force, yet a sufficient force for removing the beam or girder from the bed. The pre-tensioning in operation  126 , which imparts only a portion of the full prestressing force, thus synergistically allows high strength materials to be used even though those materials are relatively weak on the day after casting. 
         [0033]    As described above, in the operation  132  the beam or girder is moved to storage and placed, for example, on supports proximate to the ends of the beam or girder in order to limit camber growth. An operation  134  shown in  FIG. 1C  is performed wherein the beam or girder is kept in storage while it gains strength sufficient for the full prestressing force. The amount of storage time can vary dependent on the formulation of the materials of the concrete, and also can vary with strength requirements for the beam or girder. The beam or girder can be allowed to gain strength over any desired amount of time in order to take advantage of the strength potential of the materials used, or meet time constraints that call for beams or girders of lesser strength in a relatively short amount of time. Next, in an operation  136  a post-tensioning force is applied. Then, in an operation  138  cement grout is injected into the tendon ducts employed in post-tensioning. Next, in an operation  140  the grout is allowed to cure over a period of time. Finally, the process is concluded in an operation  142 . 
         [0034]    One example embodiment of the beam or girder that is the product of the process described in conjunction with  FIG. 1  is shown in  FIG. 2 . As shown in  FIG. 2A , a beam or girder  200  comprises prestressed high strength concrete. The beam or girder is cast on a manufacturing bed (not shown) using a set of forms which determine the shape of the beam or girder. The example beam or girder shown in  FIG. 2A  has a resulting shape generally referred to as an “I-beam.” 
         [0035]    As shown in  FIG. 2A , the beam or girder  200  is prestressed during initial manufacture of the beam or girder on the bed using pre-tensioning strands  202  described earlier in conjunction with operations  104  and  106  illustrated in  FIG. 1A . The strands  202  are preferably installed on the manufacturing bed prior to the erection of the forms used to contain the high strength concrete. Semi-flexible post-tensioning ducts  204  are also installed as described earlier in conjunction with operation  104  illustrated in  FIG. 1  A. The post-tensioning ducts  204  terminate at post-tensioning anchorages that may be installed employing reusable blockout forms  206 , as shown in  FIG. 2B . As shown in  FIG. 2B , there may be one or more post-tensioning ducts  204  which are placed into an approximate parabolic curve. Tensile force is then applied to post-tensioning strands inserted through the post-tensioning ducts  204  in operation  136  described earlier to provide the remainder of the required prestressing force for the beam or girder  200 . 
         [0036]    In accordance with another aspect of the present invention, a beam or girder having sufficient area at the beam or girder ends for accommodating post-tensioning tendons that pass through more than one beam or girder is provided to connect with another beam or girder which is aligned with the first and is located in an adjacent span to form a continuous structural frame. A continuous frame, in which two or more spans are connected, reduces structure cost and makes longer spans possible. Precast beams and girders that are connected by post-tensioning tendons at support points such as piers or columns to make a continuous frame require an area at the beams&#39; or girders&#39; ends to permit “through” tendons to connect adjacent spans. If the area at beam or girder ends is not available due to the presence of post-tensioning anchorages previously placed at the ends of girders in “end blocks”, as is the present practice, there is insufficient room for the through tendons to pass through to make the connection. The described beam or girder shape permits locating previously placed tendon anchorages at a distance away from beam or girder ends, thus creating room for tendons to pass through to make a continuous frame. 
         [0037]    In accordance with another aspect of the present invention, a plurality of beams or girders  200  can be deployed to construct a cast-in-place concrete deck  300 , as shown in  FIG. 3 . The deck  300  comprises at least two beams or girders  200 . The spacing between adjacent beams or girders  200  varies according to loading and length of a span to a maximum spacing, for example, 15 feet. Additionally, the deck  300  comprises one or more deck panels  302 . For example, each deck panel  302  may be a four-inch thick prestressed concrete slab. Also, each deck panel  302  may further comprise a continuous neoprene strip  304  at each end of the deck panel in contact with the beams or girders  200  that support the deck panel. Additionally, the outside beam or girder  200  at each edge of the deck  300  is provided with a flange  210  that is preferably precast with the beam or girder. The flange  210  completes the concrete form for the deck  300  and thus retains concrete poured to construct the deck  300 , as well as supports a finishing machine (not shown) employed to smooth the surface of concrete poured to complete the deck. As shown in  FIG. 3 , the flange  210  may also be subsequently employed to support an attached barrier rail or curb  306  of the deck  300  installed at the edge(s) of the deck. The modular elements shown in  FIG. 3  enable a bridge superstructure to be built quickly with high quality at low cost. By fabricating beams or girders  200  of higher concrete strength than in the past and using a commensurately higher prestressing force to produce greater structural capacities, significant economy is achieved by requiring fewer beams or girders for a given span and by the elimination of overhang forms and most on-site superstructure formwork by employing the modular elements shown in  FIG. 3 . 
         [0038]    While the foregoing description has been with reference to particular embodiments and contemplated alternative embodiments of the present invention, it will be readily appreciated by those skilled in the art that changes in these embodiments may be made without departing from the principles and spirit of the invention.