Quality concrete out of the form is desirable; quality concrete in service is essential.
By Carl S. Buchman, P.E.
No precaster makes every piece perfectly every time, and the need for some correction or repair inevitably arises. What would be best: to repair the problem, or to just scrap the piece and remake it? This decision is an important one, involving intelligent evaluation of the cause and severity of the problem, type of repair needed, your ability to accomplish it correctly, available time to properly complete the repair, and cost comparisons of repair versus replacement. Without your careful consideration of these and other concerns, the repair effort may turn out to be a failure.
The final thought involves your customers: how this may impact your company’s image, your reputation and your guarantee.
Evaluate the problem
You should realize that recognizing a problem is not the same as determining the actual cause of the problem. As an example, cracks may be seen as an obvious problem needing repair; the cause may be drying shrinkage, improper stripping or storage, or misplaced reinforcement.
Thorough investigation and analysis of the cause(s) will benefit you in two ways. First, the repair will be an appropriate one. Second, you can take corrective steps to prevent the problem from recurring.
If you find the same problem recurring, you know your corrective measures were either incorrect or ineffective. This aspect of product repair is up to management; the physical repair is up to qualified people, which usually are yours, possibly even yourself, but sometimes may be a subcontractor.
Even with skilled repair personnel, will the product be acceptable to the customer? Consider these questions:
- Will the expected performance and useful life of the product be compromised?
- Will the aesthetics of the repair be acceptable?
- Is there time to complete the repair thoroughly?
- Will the total cost of repair exceed casting a replacement?
- Will this repaired product upon delivery impact your company’s relationship with the buyer and negatively influence future work? What cost do you put on that?
Keep in mind that perception usually pre-empts reality or fact.
Determine the classification
There are two classifications governing repair: minor defects and major defects. This determination is not made with the toss of a coin; it must be made by a qualified person who also is familiar with the functional use of the product. Quoting from the NPCA Quality Control Manual for Precast Concrete Plants: “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.” Also: “Repairs of minor defects are essentially cosmetic (for example, the product would behave as intended without the repairs).”
As for major defects from the QC Manual: “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.”
You should establish standard repair procedures for all minor or ordinary defects (honeycombing, sand streaking, spalls, surface cracks, omitted or misplace inserts, etc.). This work is then done on an automatic basis, based on the standard procedure. Even these basic repairs should be inspected and recorded to establish that the work was performed properly and the product is ready to ship.
Most major repair situations can also be performed following established procedures. The difference is that major repairs must be approved before the work begins, by your customer in many instances, or perhaps by your own engineering or quality control staff. In addition, the work must be inspected while repairs are made, the work documented and records kept in the file for that piece.
Repair of minor defects
These are the ones you just fix, the routine defects (although if they routinely occur, you haven’t addressed the cause). For example, small cracks – those not penetrating to the steel reinforcement, can be filled by rubbing in dry cement. The moisture in the concrete and in the air will hydrate the cement and fill the cracks.
Small honeycomb areas can be cleared of any unsecured stone by chipping down to solid concrete, blowing or brushing the area clean, and filling in with your standard repair materials (see Fig. 1). This standard repair material is what you list on your standard repair procedure. It can be the same blend of materials used in the product, or it may be your favorite proprietary repair material. Chips and spalls are handled in the same routine manner.
Whether these are minor defects or major, they must be competently installed and finished. This includes proper preparation of the defective area, placing the repair material, adequately curing the repair and acceptably finishing the exposed surface. In the choice of repair materials, always remember that the piece being repaired is older and has cured longer than the new material being added.
The key idea here is that the base material has begun to dry and some small amount of shrinkage has taken place. So unless you are using a proprietary material that does not call for it, the area to be repaired must not only be clean and sound, it must be thoroughly dampened to keep it from pulling moisture from the freshly applied material. And curing the repair material is extremely important to keep the newly applied material from shrinking and obviously separating from the base concrete.
As an example, after placing repair material over a honeycomb area, cover it with wet burlap and a larger sheet of plastic, which is taped on all edges to the base product to hold the moisture in. Depending on ambient temperatures, you may also need to supply heat to the repair to ensure hardening and strength gain.
Spalled corners or edges from rough stripping or handling are all too common. This is normally a minor defect, but should be a major sign to management of a lack of understanding and care requiring additional training and instruction. As with honeycombing, the broken area must be properly prepared before applying repair material. The nature of these defects most often involves angular breaks, which have outer edges tapering to thin edges. A repair material can’t be tapered to a thin edge – it will shrink away or flake off. It is better to saw cut at the edge of this break, then cut out a minimum thickness of material to allow the repair to have some body and a definite line to work against.
After curing the repair, evaluate the finished surface for appearance (remember, perception is often more important than reality). You may have made the most structurally sound and long-lasting repair of all time, but if it is obvious because it is almost black and the rest of the product is very light in color, you have done yourself a disservice. Thus, the repaired surface should be made to blend with the primary piece, and this is hard to do as a finishing step. Normally, a fresh repair will be darker than the surrounding concrete, and it will lighten with time. It is good practice to have various aged samples of different patch materials to compare color with the product to be fixed so that the repair when cured will blend.
Surface defects, whether they are honeycombing, sand streaking, form joint offsets, spalls or air pockets, are among the easiest repairs to make. Replacing missing or misplaced inserts may be as simple as installing hardware of a different design and installation method, or it may require an entirely new design solution. Misplaced openings can be filled using thorough repair methods before new openings are cut or drilled in. Depending on size and location, this may also require an engineering analysis to ensure the structural capacity of the member has not been reduced, and it may also need to incorporate supplementary reinforcing. Clearly, such work would be in the major defect category.
Repair of major defects
Why is a small area of honeycombing minor and a large one major? Why is a surface crack minor and a through crack major? The reason again is based on the effect on durability, serviceability or life expectancy. For example, surface cracks are usually small, do not penetrate deep and may be self healing, while through cracks will allow water penetration through the member, rust steel and possibly affect the structural strength.
These are the problems that would render the product unsuitable for use if not correctly repaired. Despite the classification, these repairs are basically the same as for minor defects, requiring the same preparation, material selection, workmanship and curing. The difference is that these must be evaluated by a qualified person; require the repair procedure to be established and written out; often require approval by an outside agency before work is begun; and must be inspected (witnessed) and documented.
In some situations, you must have outside approval as well before shipping the product. If there is any doubt, keep records of the work as performed. This will also help in your planning and scheduling of work and hopefully eliminate hurrying through the repair procedure to meet delivery deadlines.
There is a means to repair any defect or concrete product satisfactorily, but it is between you, your qualified staff and your customer to establish this. Reinforcing can be supplemented; broken parts can be reattached; through cracks can be epoxy injected; surfaces can be coated for appearance or to be made nonabsorptive; and fiber-reinforced plastics and polymers can be laminated (an unsightly but effective procedure). Every year, new and more amazing materials are being developed.
You can use the same ingredients from your original mix, but there are some cautions. You can’t usually use the maximum size stone from the mix, since repairs are sometimes too thin, or you can’t get the material fully behind exposed rebar, etc. And in general, you should use a drier consistency. You probably aren’t pouring the repair material into a nice formed area, and you want to reduce shrinkage of the repair material as much as possible. Think of the repair batch as being more like dry pack material that you force and hammer into place to gain full compaction (see Fig. 2).
If you have standard purchased products that you have been using for this, there is no reason to change. In these cases, definitely follow the manufacturer’s directions. Your goal, in either case, is to incorporate essentially the same concrete into the repair as you removed. The strength, adhesion, shrinkage, color and appearance need to be compatible with the original concrete. The people who are good at this are more artists than technicians; if you have these people, cherish them. It takes time and experience mixed with patience, especially by management.
The key words are thorough, clean and moist. If the repair involves some bad concrete, the removal of the weak materials must be thorough, firmly into sound concrete, and the stone must fracture, not pull out.
The shape of the repair is important. Don’t work to feather edges – saw cut some depth on an angle to develop a wedge action for the new material. Dig out enough room behind exposed rebar to allow the new mix to fully envelop the bar (see Fig. 3). Make a plan to get the material where it is needed, and 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).
A repair mix won’t bond to a dirty, dusty surface. Use high-pressure water or sand or air to clean all mating surfaces just before placing the repair material. If the work site involves any exposed steel, it needs to be cleaned of any rust or adhered concrete. If you are required to reconstitute galvanized or epoxy rebar surfaces, consult with your bar supplier for specific recommendations; expect to have to cut out extra concrete to allow more work space behind the bar.
For normal repair mixes, the mating surface needs to be well-saturated but free of all standing or excess water. As previously mentioned, the repair material wants to be as dry as possible for workability – but you don’t want the substrate pulling away any moisture. And always remember that the moisture you put in there is to hydrate the cement. So keep the area moist with normal curing procedures.
Since you should have post-pour inspection records for each day’s production, repair work needed will already be documented. After routine or minor repairs are made, they should be noted on the appropriate form. The requirement is more detailed for major repairs, but amounts to the same thing in the end: having documentation that the required work, with inspection, was performed properly.
How do you determine that the repair is in fact acceptable? Depending on the volume of material prepared, you could make and cure cylinders with the product, just like normal. Most plants have the simple cylindrical impact hammer, which evaluates hardness by measuring rebound within the instrument. This can compare the repair area to the base concrete. There may be reason to core through the repair, and both evaluate and test the core. And although there are many sophisticated tools and techniques available today
(X-ray, magnetic resonance, ultrasound, etc.), they are rather impractical to everyday repair evaluation. But there is one old method still being used: Hit the repair with a hammer! If it rings like good concrete does, you have a good repair. If it goes “thunk” – or falls off – you had better try again.
Why do these things happen? If we have time to do it right the second time, why didn’t we take the time to do it right the first time? Yes, it was the fault of one or more people casting the product. But it is a greater fault of management if adequate training in proper form cleaning, setup and checking, concrete placement and vibration, finishing, curing and handling are not well-ingrained and shared by those same people making and handling the piece. And be sure to allow enough time to complete every step properly, no matter what else may have happened earlier that day.
Image, reputation, guarantee
Every worker in your operation needs to put himself in the customer’s shoes. Does this piece meet his expectations? Does it make him believe he dealt with a quality supplier? Is there any reason to not expect the precaster will stand behind his product?
The other side of this has to do with collecting payment. Don’t give the buyer reason to withhold money or demand a reduction in the invoice or some other compensation. You may think that the piece will be buried and no one will ever see it, but a lot of people see it when it arrives and gets unloaded. Remember, you only get one chance to make a good first impression.