Patent Publication Number: US-5894003-A

Title: Method of strengthening an existing reinforced concrete member

Description:
RELATED APPLICATION 
     This application claims the benefit of the filing date of Jul. 1, 1996 of provisional application Ser. No. 60/020,921. 
    
    
     BACKGROUND OF THE INVENTION 
     In existing reinforced concrete elements such as concrete slabs, beams, columns and walls, it is sometimes desirable to strengthen the element for one or more reasons. For example, the applied loading requirements may exceed the original design values for the element, or the load carrying capacity of the element may have been reduced due to deterioration, or the element may require increased stiffness for less deflection. The element may also require lower working stresses to reduce fatigue, or may require upgrading to withstand higher seismic and/or blast loading. 
     One form of strengthening existing reinforced concrete elements is by laminating or bonding a mat or strip of composite material with carbon or glass fibers to the surface of the concrete element where bending occurs. However, it is undesirable for the composite mat or strip to be exposed to the weather and/or to traffic such as on the top surface of a concrete bridge slab. For example, if water seeps between the composite mat or strip and the concrete surface, it is possible for the mat or strip to delaminate from the concrete surface if the water freezes. It is also necessary to prepare the concrete surface in order to obtain a good bond of the reinforcing mat or strip to the concrete surface. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an improved method for strengthening existing reinforced concrete members or elements such as concrete slabs, beams, columns and walls after it is determined where the existing tensile reinforcing rods or bars in the concrete are inadequate. In accordance with the invention, one or more parallel spaced grooves are cut within the surface of the existing reinforced concrete element or member in the direction of bending of the member and in the area of inadequate tensile reinforcing. A reinforcing rod, which is preferably a composite rod with continuous fibers, is positioned within each groove after a curable bonding material or epoxy resin is inserted into the groove. The reinforcing rod is twisted or rotated so that the resin completely surrounds the reinforcing element. The bonding material is formed flush with the surface of the concrete member and allowed to cure to bond each rod to the concrete defining the corresponding groove. Each groove and corresponding reinforcing element or rod extend within the top surface of a concrete slab across a support for the slab and extend within the bottom surface of the slab at least fifty percent of the distance between adjacent supports for the slab. Each groove and reinforcing element may also extend within a vertical surface of a masonry or concrete wall in the direction of bending of the wall. 
     The method of the invention eliminates surface preparation of an existing concrete element, a step that is normally required to bond a strip or mat to the element. The method also provides for locating the supplemental reinforcing element or rod below the concrete surface, thereby protecting the reinforcing rod which is completely encased within the epoxy resin or other bonding material. The invention further provides for concentrating the reinforcing rods at the critical stress locations, and the use of a composite rod with continuous fibers for the supplemental reinforcing provides for efficient use of the supplemental reinforcing adjacent the surface of the concrete element. The supplemental reinforcing rods within the grooves may also be pre-stressed before bonding to the concrete, and the concrete element may be deflected in a direction opposite to the direction of deflection caused by loading of the concrete element to provide for an initial pre-stressing of the reinforcing rod. 
     Other features and advantages of the invention will be apparent from the following description, the accompanying drawings and the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a fragmentary section of an existing reinforced concrete slab which has been strengthened in accordance with the method of the invention; 
     FIG. 2 is an enlarged fragmentary section of the concrete slab, as taken generally on the line 2--2 of FIG. 1; 
     FIG. 3 is an enlarged fragmentary section of a supplemental reinforcing element or rod bonded within a groove, as shown in FIG. 2; 
     FIG. 4 is a fragmentary section of an existing reinforced concrete beam which has been strengthened by the method of the invention; 
     FIG. 5 is a fragmentary section of a masonry or concrete block wall which has been strengthened in accordance with the invention; and 
     FIG. 6 is a fragmentary section similar to FIG. 4 and illustrating the strengthening of an existing reinforced beam supported by a column or girder. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates an existing reinforced concrete member or slab 10 which includes a set of integrally cast and parallel spaced concrete beams 12. The slab 10 was originally reinforced by embedded concrete bars or rods 16 and 17 (FIG. 2) which extend within the concrete at 90° to form layers of steel reinforcing grids within the concrete. The bottom portion of the beams 12 may also have embedded reinforcing steel bars or rods 18 which are spaced and positioned along with the rods 16 and 17 within the concrete forms before the slab 10 is poured with concrete. After an extended period of use of the concrete slab 10, it sometimes becomes necessary to strengthen the slab for one or more of the reasons mentioned above and in areas where the existing steel reinforcing rods or bars are inadequate for tensile reinforcing of the slab. 
     In accordance with the present invention, a series of parallel spaced elongated grooves 22 are cut within the top surface and/or bottom surface of the slab 10, as shown in FIG. 2, with a suitable concrete saw. For example, each groove 22 may have a width and depth of 3/8&#34;, but grooves of other sizes may also be used. Each groove 22 receives a supplemental reinforcing element or rod 25 which is secured within the groove by a curable bonding material 28 such as an epoxy resin so that the rod 25 is secured or bonded around its entire outer surface to the concrete surfaces forming the groove 22. Preferably, each rod 25 is a non-metallic composite rod having longitudinally extending continuous glass or carbon fibers to provide the rod with a very high tensile strength. As an example, a rod 25 having a diameter of 1/4&#34; may be used in the 3/8&#34; groove. 
     As shown in FIG. 1, the grooves 22 and corresponding rods 25 extend continuously within the top surface of the slab 10 across the beams 12 and in areas where the existing reinforcing provided by the steel bars 16 and 17, is inadequate. The grooves 22 and corresponding rods 25 within the bottom surface of the slab 10 extend at least fifty percent of the distance between adjacent support beams 12 and preferably have opposite ends close to the beams 12, as shown in FIG. 1. In the bottom surface of the slab 10, the rods 25 are retained within the corresponding grooves 22 by an epoxy resin 28 which is capable of holding the supplemental reinforcing rods 25 within their corresponding grooves until the resin cures and hardens. The resin is also formed flush with the concrete surface with a suitable putty knife before the resin cures and hardens to form the positive bond of the reinforcing rod 25 to the concrete slab adjacent the surface. 
     Referring to FIG. 4, a modified existing concrete slab 10&#39; has embedded steel reinforcing bars or rods 16&#39; and 17&#39; which extend into an integrally cast beam 12&#39;. To provide the beam 12&#39; with supplemental tensile reinforcing and to strengthen the slab 10&#39; and beam 12&#39;, one or more grooves 22 are cut within the bottom surface of the beam 12&#39; and receive corresponding reinforcing rods 25 each surrounded by a bonding material or epoxy resin 28. The bonded rods 25 substantially increase the bottom tensile strength of the beam 12&#39;, and the grooves 22 may also be easily formed within the bottom surface of the beam. 
     FIG. 5 illustrates using the method of the invention for strengthening an existing solid concrete or masonry wall 50, for example, in the form of modular concrete blocks 52 joined together by joint layers of mortar 54. The blocks 52 have may be solid or have internal cavities 57 which may be open or filled with concrete which may be reinforced with steel rods (not shown) when the wall is constructed. In accordance with the present invention, the vertical concrete wall 50 is strengthened by forming a series of parallel spaced grooves 22 in the outer surface and/or inner surface of the blocks 52. The grooves may be vertical or horizontal or at an angle and extend across the mortar joints 54. Each groove 22 is filled with a reinforcing element or carbon fiber rod 25 and bonded to the concrete blocks by epoxy resin 28 within each groove 22, as shown in FIG. 2. As mentioned above, the grooves 22 and supplemental reinforcing elements or rods 25 are located in the area where the wall tends to bow or bend and where tensile reinforcing is necessary or desirable. 
     FIG. 6 illustrates the method of the invention as applied to a poured concrete slab 60 having an integrally cast beam 62 and reinforced by embedded steel reinforcing rods 16 and 17. When an integral beam 62 is supported by a post or girder or column 65 and the embedded reinforcing steel 17 for the beam is inadequate to provide the necessary or desired tensile strength, a series of parallel spaced grooves 22 are cut within the top surface of the concrete slab 60 in parallel spaced relation. The grooves extend over the support column 65 and preferably at least twenty percent of the distance to the next adjacent support. Each of the grooves 22 is filled with a supplemental reinforcing element or rod 25 and bonding epoxy resin 28, as described above in connection with FIG. 2. 
     As mentioned above, the supplemental reinforcing elements or rods 25 may be pre-stressed before the bonding material or epoxy resin 28 cures. It is also within the scope of the invention to deflect a concrete member in a direction opposite to the direction caused by loading and prior to curing of the bonding material or epoxy resin 28. This locks in an initial pre-stress within each supplemental reinforcing element or rod 25. For example, a hydraulic jack may be used to press upwardly on the concrete slab 10 (FIG. 1) midway between the beams 12 in order to deflect the slab upwardly by a slight amount before the epoxy resin 28 cures and hardens within the grooves 22 which extend within the top surface of the slab 10. 
     While the method steps herein described constitute preferred embodiments of the invention, it is to be understood that the invention is not limited to these precise method steps, and that changes may be made therein without departing from the scope and spirit of the invention as defined in the appended claims.