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What to do when things don’t turn out perfectly.
By Evan Gurley
Perfect: “Being complete of its kind and without defect or blemish.”
Even the most experienced precaster who follows all the proper mix design procedures and production steps cannot produce a “perfect” piece every time. This is true in most every industry, not just in precast concrete manufacturing. Repair is therefore an inevitable part of most every industry.
When something doesn’t turn out perfectly, the imperative question is this: When a correction or repair is needed, what appropriate action should be taken? The answer should be in two parts: one to address the cause of the problem, and one to address the problem itself.
Get to the root of the problem
It is almost always obvious to the naked eye when a problem is occurring with your concrete structure. Cracking and honeycombing, for example, are easy to spot. But the real concern is the underlying cause of the problem. Recognizing that a problem exists is not the same as determining its cause. For example, cracking is seen as a problem, but the cause of the cracking (improper design, improper stripping/storage, drying shrinkage) will require a more in-depth analysis of the structure.
Before attempting any repairs, identify the cause and take corrective measures at the appropriate stage of production. This will increase the likelihood that the problem will not occur again.
Once you have found and fixed the cause of the problem, it’s time to turn your attention to the problem itself. The b ottom line is to determine whether to repair the piece or scrap it, but this requires intelligent evaluation of the cause and severity of the problem, type of repair needed, ability to accomplish it correctly, available time to properly complete the repair, and cost comparisons of repair versus replacement.
Consider these questions:
After you have considered these questions and decide that repair is the best alternative, step through your standard repair procedure. If you don’t have a standard repair procedure in place, you are probably not alone – but it’s a good idea to develop one. Here is a synopsis of what a standard repair procedure may contain:
With a standard repair procedure, you can consistently evaluate concrete repairs on a case-by-case basis. It can be used for major as well as minor defects. It is also highly recommended that you have an up-to-date concrete repair manual.
Major or minor?
Determining whether the defect is classified as major or minor is your next step in the repair procedure. A qualified person should judge which defects are minor and which are major. This person must be thoroughly familiar with the product’s function and the environment in which it will exist.
The classifications of major and minor defects are stated in the NPCA 2008 Quality Control Manual for Precast Concrete Plants:
Minor Defects. “Defects not impairing the functional use or expected life of a precast concrete product shall be considered minor defects. Minor defects may be repaired by any method that does not impair the product. Repairs of minor defects are essentially cosmetic; in other words, the product would behave as intended without the repairs.”
Major Defects. “Defects in precast concrete products that impair the functional use or the expected life of products shall be considered major defects. Unless major defects are 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.”
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 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
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 well-saturated 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.
Surface defects, whether honeycombing, sand streaking, form joint offsets or spalling, are among the easiest and most common repairs to make. Even though these are minor defects, they must be competently applied and finished. This includes proper preparation of the defective area, placing the repair material, adequately curing the repair and acceptably finishing the exposed surface.
Unless you are using a proprietary material that does not call for it, the area to be repaired must be thoroughly dampened to keep it from pulling moisture from the freshly applied material. Placing a damp burlap sheet or cover with a larger sheet of plastic on top and taping the edges to keep in the moisture should suffice. The bond strength of new concrete to old concrete develops much more slowly, and the tendency to shrink and loosen is reduced by a long, moist curing period. Applying heat to the repaired areas may also be necessary to ensure hardening and strength gain.
After the repair has cured, evaluate the finished surface for appearance. Appearance holds more significance than it should; even if the repair is structurally sound, it won’t be looked upon as a quality product if the repair doesn’t look flush and similar in appearance as the rest of the product.
Repair of Major Defects. Repairs for major defects require the same preparation, material selection, workmanship and curing as for minor defects. However, according to the NPCA QC Manual, major repairs must be analyzed by a qualified person; they require the repair procedure to be established and written out; they often require approval by an outside agency before work begins; and they must be inspected (witnessed) and documented. This is another reason to have a standard repair procedure in place.
Many different repair products are on the market, and in fact several businesses are making repair materials specifically for the precast industry. Consider several material properties when selecting a particular repair product. These include permeability, strength, adhesion, corrosion resistance and application method. Base your selection of a repair material on its intended purpose. If the customer is a state DOT, always make certain that the supplier uses materials that are state approved for all states in which you supply products.
For most applications it is acceptable to use the same ingredients from your original mix, but proceed with caution. Concrete used in repairs should be essentially the same as the original concrete, except that the repair concrete should contain less water. You should use less water if the repair area is not cut into a nice formed area, and typically it is not. Also, the reduction of water will help reduce shrinkage. The repair batch acts more like a dry-pack material that you force and hammer into place to gain full compaction (see Figure 3). Another factor to consider is the size of the aggregate. The maximum size of aggregate should be as large as possible but not greater than one-half the minimum dimension of the repair.
Consider using a materials supplier that specializes in concrete repair. These suppliers have the newest and best products available and can provide you with valuable information pertaining to your specific application. The repair material’s strength, adhesion, shrinkage, color and appearance need to be compatible with the original concrete, and this is where concrete suppliers can help.
A new product that precast concrete repair suppliers are now offering is called “carvable” (shapeable) cement mortar. This product is used for structural and cosmetic repairs. Formerly these cement mortars either set up too fast or too slow, or they were too light or too dark. Now they are offered in three different set times of five, 15 and 30 minutes to meet the needs of a specific project. These mortars also come in different shades of gray to match the color of the existing concrete. They contain no gypsum cement and therefore will not swell when exposed to rain, snow and sleet.
Many repair products can obtain high strengths, some at a very early age, which is often a selling point. Keep in mind that typically the higher the early strength gain, the lower the service life of the repair. This is because the ingredients used to obtain high early strengths (Type III cement, accelerators, water-reducing agents, etc.) can cause shrinkage cracking and can be more brittle than the concrete it is repairing. As a general rule use a material with the strength appropriate for the repair.
After you have made all the corrections associated with concrete repair, how will you know whether it’s acceptable? Depending on the volume of material prepared, you could make and cure cylinders with the product just as you normally do. Most plants have a cylindrical impact hammer, which evaluates hardness by measuring rebound within the instrument. This can be used to compare the repair area with the base concrete.
The bottom line is that concrete products can’t be perfect every time. But the manner in which these issues are handled will reflect on the integrity of your company and the industry as a whole.