Precast Inc. - March/April 2008 - (Page 11)

Surface preparation requires removal of loose and deteriorated concrete, and includes concrete removal behind exposed bars. Figure 1 After completion of material removal, formwork is erected to close the cavity. repaired the product shall be rejected. Major defects shall be evaluated by qualified personnel to determine if repairs are feasible, and if so to establish the repair procedure. Proper repairing procedures and curing shall be inspected.” defect has been noted to minimize the differential shrinkage between the original concrete and the repair concrete. The concrete repair should become an integral part of the product with no delaminations or cracks. Prepare older concrete for repair Determine your repair method With sufficient information determining the cause and the classification (major or minor) of the problem, you are now ready to determine your repair method, depending on the type of repair needed. Major and minor defects require different repair procedures and therefore should be approached differently. Whichever method is used, make the repairs as soon as possible after the Thorough, clean and moist – this is the condition that you want your existing concrete to be in before repair. Preparation of the existing concrete for the application of repair materials is an important issue. If the repair involves some bad concrete, remove the weak materials thoroughly and firmly into sound concrete; stone must fracture and not pull out. Even if you apply the finest repair materials to a concrete product, if the existing concrete is not prepared correctly the results could be substandard. It is essential that the repair material bonds to the existing concrete. The shape of the repair is also important. Don’t feather the edges; instead, saw cut some depth at an angle to develop a wedge action for the new material. Dig out enough material behind exposed rebar to allow the new mix to fully envelop the bar (see Figure 1). Make a plan to get the material where it is needed and to keep it there (repairing an underside or vertical side requires different consistency, forming, placing and possibly curing than a top surface – assuming you can’t rotate the piece). Once the existing concrete has been properly prepared, it must remain clean and protected from damage until the repair materials can be placed and cured. Repeat the preparation process if a repair becomes contaminated or damaged. Also, if it involves any exposed steel, clean any rust or adhered concrete. If you are required to reconstitute galvanized or epoxy rebar surfaces, consult with your bar supplier for specific recommendations; you may need to cut out extra concrete to allow more work space behind the bar. Epoxy-bonded materials generally work better when the mating surface is kept dry. For normal repair mixes, the mating surface needs to be wellsaturated but free of all standing or excess water. Mating surfaces that will be repaired with cementitious materials should be in saturated surface dry (SSD) condition. The repair material should be as dry as possible for workability, but you don’t want the substrate pulling moisture away from it. Remember that the moisture is there to hydrate the cement, so keep the surface moist. Apply repair to the product Repair of Minor Defects. As stated in the NPCA QC Manual, minor defects are essentially cosmetic and are likely to occur more often. However, if minor defects are routinely seen in your concrete products, you haven’t fully addressed the root of the problem. • Cracking – Small cracks that do not penetrate down to the steel MARCH/APRIL 2008 | WWW.PRECAST.ORG 11 http://www.precast.org

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Precast Inc. - March/April 2008 Contents Technically Speaking Concrete Repairs Return-to-Work Programs Marketing in a New World Building Confidence Risk Management Culture Bloquea La Energia – Es Tu Vida! Lock It Out – It’s Your Life! Fiber Optics in Countertops MCPX Wrap-Up Product Profile People & Products Advertisers Index NPCA Calendar

Precast Inc. - March/April 2008

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