Concrete coverage of reinforcing steel.
By Greg Stutz
You’ve just pulled your casting from the form and noticed that there is a large area of reinforcing steel (rebar) showing on the surface of your fresh casting. What will you do?
- Ship it and hope it gets buried quickly so no one will notice.
- Break out the concrete in the area where the rebar shows, then reposition the rebar and patch the concrete.
- Scrap the casting and start over.
- Put some wrap or sealant over it to cover it up.
The answer – or the suggested answer, to be more precise – will be revealed at the end of the article. But first let’s cover the basics and some of the finer points of pre-pour inspections and quality assurance.
Two materials – one unit
Reinforced concrete is a logical union of two materials: plain concrete, which possesses high compressive strength but little tensile strength, and reinforcing steel rods embedded in concrete, which can provide the needed strength in tension.
Steel and concrete work readily in combination for several reasons:
- Bond (interaction between bars and surrounding hardened concrete) prevents slippage of the bars relative to the concrete;
- Proper concrete mixes provide adequate permeability of the concrete against bar corrosion; and
- These two materials have sufficiently similar rates of thermal expansion when exposed to atmospheric changes of temperature.
Protective concrete covering around the steel bars, more commonly called “coverage,” is necessary in order to make the bars and the concrete act together in support of flexural stresses incurred during various load conditions. Failure of a concrete structure with no reinforcing steel would be sudden since concrete is a brittle material. Add deformed steel bars to the composite section and position them to offer tension reinforcement, and the composite section will be significantly strengthened. As a result, it is crucial to the structural integrity of the precast unit that the reinforcing steel is placed in accordance with recommended guidelines established within the American Concrete Institute (ACI) requirements or Concrete Reinforcing Steel Institute (CRSI) Manual of Standard Practice.
When two materials such as steel and concrete act together, it is understandable that the analysis of strength has to be continually evaluated. ACI acts as the clearinghouse for all theoretical and experimental research in this area.
Rebar shifting inside of a precast unit can occur due to a number of reasons:
- Poor support of reinforcement steel
- Inadequately tied/welded intersections
- Wire gauge is too light and shifts due to the pressure of the concrete during placement
- Errors and omissions during the pre-pour setup
Fortunately, this situation can be minimized if you follow the practices published in the NPCA Quality Control Manual for Precast Products. QC personnel generally inspect for the following:
- Do the materials conform to the plans and the appropriate standard, or are they approved based on the tests conducted prior to use?
- Are the forms: true in dimension and alignment; tight; braced to prevent bulging or displacement; oiled to prevent sticking; and cleaned of debris before placing of concrete?
- Is the reinforcing steel free from loose scale and form oil; adequately tied, welded and supported to prevent displacement; properly sized, dimensioned, spaced, spliced and bent in accordance to the plans; and checked for coverage tolerances based on plans or applicable standards?
- Are the vibration methods: adequately sized; sufficient in quantity to do the job; and used within recommended duration?
- Was the technique of concrete placement in excess of recommended handling procedures? (This refers to methods of discharge and layers of uniform thickness.)
Lighter gauge wire mesh reinforcing steel cages are known to shift and are difficult to keep within tolerance. The placement of rebar stiffeners on the corners and strategically throughout the body of the cage can minimize this. In addition, the use of rebar chairs (supports) can significantly reduce the risk of this occurring.
Rebar tie wire, plastic supports, welding and more
Precasters choose from a variety of materials that are designed to hold rebar in position. The choice of these materials depends on the application, cost effectiveness and sometimes the specification. Annealed rebar tie wire in 16 and 16.5 gauges available in 3.5-pound coils can be applied manually with wire tie hooks or with an automated wire tie handheld gun. Also included are black annealed 100-pound merchant coils, bulk-packed wire ties in a variety of gauges, and 11-gauge staples.
For a safe, fast and easy way to connect rebar, some precasters prefer stackable chairs made from injected molded plastic. Their simple design allows cross, parallel and vertical applications where multiple layers of rebar are required. These devices do not promote rust and do not scar or scratch the protective coatings on rebar or post-tensioned cable. They also can be applied with or without tools. The noncorrosive properties eliminate tie wires and won’t damage epoxy-coated rebar or scratch the tight scale of the rebar. With heights starting at 1 inch, with 0.5-inch vertical increments, they can fit any size of rebar. Some manufacturers color code them to distinguish the proper size and spacing. When stronger materials are called for (i.e., supporting 2,000 pounds), rebar chairs made of polypropylene copolymers meet CRSI Class I bar supports. Applications where chairs are used include all precast products, slabs, post-tensioned slabs, multilevel buildings, footings and wall panels.
Some producers purchase a weldable grade of rebar to spot weld the rebar intersections. This is done for strength, ease of cage positioning and/or quality image purposes. Some feel that this method is overkill and not cost-effective. Others claim the incidence of rebar cages/mats moving out of specified tolerance is minimized when cages are lifted into position.
Just to be sure
A common question: “The rebar isn’t exposed to the surface of my casting. How do I know everything stayed in place during the pour?” Answer: Devices exist that detect rebar and other ferrous objects in concrete. Portable units provide on-site rebar depth, diameter and location information. Comprised of a scanner and a monitor, these wireless systems have integrated LCD displays, block scan capabilities and data storage memory cards. Units can transfer data to a PC or laptop via USB connection, and headset connection let users record a 15-second voice memo per scan.
Should I fix it?
As all NPCA certified producers know, you need to determine the classification of the repair (minor defects and major defects). A qualified inspector should document a decision based on the intended use of the product. The NPCA Quality Control Manual for Precast Concrete Products states, “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).”
The QC Manual describes major defects as, “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.”
Minor or ordinary defects are found usually in the form of honeycombing, sand streaking, spalling, surface cracks, omitted or misplaced inserts, etc. Follow your standard written procedures to make repairs of these items. Even minor repairs should be inspected and recorded to establish that the work was performed properly and the product is ready to ship.
It is difficult to imagine any poorly positioned rebar situation that wouldn’t qualify as a major repair. If you elect to try and reposition the rebar and pour a patch, this repair must be performed following established written procedures as well. Again, as the QC Manual suggests, major repairs must be approved by your customer or perhaps by your own engineering or quality control staff before the work begins. In addition, the work must be inspected while repairs are made, the repair documented and records kept in the file for that unit.
Exposed rebar defects
One must consider the effect on durability, serviceability or life expectancy before undertaking any repair. Most importantly, rebar exposed to the surface is not properly strengthening the casting for tension forces as required by the design and does not meet specifications. In addition, the same rebar is susceptible to corrosion at an early stage in the product lifecycle, thus reducing the life expectancy. Therefore, groundwater or moisture of any kind present in the installation will penetrate the steel and affect the structural strength. Even if you cover up the defect by applying waterproofing membrane, sealants or finish coatings (not recommended), the casting is not structurally adequate and will most likely fail. These problems render the product unsuitable for use – period.
Despite the classification of repair, all defects require the same preparation, material selection, workmanship and curing that you use in your general production practices. The difference is that major repairs 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.
If by some remote chance you decide that the exposed rebar is repairable (not likely), 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. Be prepared to set aside adequate labor resources or adjust your pouring schedule. These repairs are labor intensive.
While there are ways to repair some major defects, stop and ask yourself if repairing exposed rebar is the best use of resources. For some major repairs, reinforcing steel can be supplemented, core drilled and cemented into proper location; broken parts can be reattached; through cracks can be epoxy injected; surfaces can be coated for appearance or be made waterproof; and fiber-reinforced plastics and polymers can be laminated (an unsightly but effective procedure). The market is also welcoming new and more specialized materials every year. But is it worth it in this case?
Since you have post-pour inspection records for each day’s production in accordance with the NPCA QC Manual, your repair work should already be documented. Next, how do you determine that the repair is in fact acceptable? Depending on the volume of material used, you could make and cure cylinders with the product just like normal. Most plants have the simple 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. However, there is still one tried-and-true method that always proves the repair integrity: Hit the repair with a hammer! If it rings like good concrete does, you have a good repair. If it sounds “dead” – or falls off – you are back to square one. This will be the point where you will be regretting that you didn’t repour the casting.
Why didn’t we take the time to do it right the first time? Yes, it was the fault of “someone” casting; “someone” inspecting the pre-pour setup; or someone pushing the crew to get it done quickly. It is equally if not more the 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 by the Pour Shop crew. Ultimately, it is your company’s image, reputation and guarantee/warranty at stake.
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? Will the product hold up? Don’t give the customer a reason to withhold money – or worse yet go somewhere else next time.
You may think that the product will be buried and no one will ever see it, but a lot of people see it when it arrives and gets unloaded. And if your company name is on the product, you’ve just created a very bad impression. Taking chances with poorly positioned rebar will ultimately cost your company more than lost revenues. You may have to dig up the casting and replace it when it ultimately fails due to performance issues. Or it may end up back in your yard when it gets rejected at the job site upon delivery.
If your answer to the question at the beginning of this article was anything but “C,” please reconsider. Choosing the other answers will put your business at risk. The risk will eventually surface as damaged reputation for passing on less-than-satisfactory product and/or incurring increased costs to remediate the integrity of a repaired casting. In other words, “Pay me now or pay me later.”