Questions from the Field is a selection of questions NPCA Technical Services engineers received from calls, emails and comments on blogs or magazine articles on precast.org.
If you have a technical question, contact us by calling (800) 366-7731 or visit precast.org/technical-services.
Nabahe writes:
I have a question in regard to cleaning the forms we use. The forms are made of A36 mild steel. After several uses of the forms, concrete has begun to build up or cake to the sides of the forms. We are unsure how to properly clean the forms without damaging the steel. Do you have any recommendations on this situation? We believe sandblasting or using a chemical dissolver should be used, but we are unsure if sandblasting will damage the forms.
NPCA Technical Services engineers answered:
The best management practice for form cleaning is to clean them immediately after use whenever possible. This means clearing off as much excess concrete as possible. After stripping, the form then should be cleaned with metal scrapers. Since this is freshly hardened concrete, removing the concrete should be easy. And, if a release agent has been used, this will also assist removing the concrete with a scraper. It becomes more difficult to remove concrete buildup on forms when they have been put away dirty and the concrete has been left to sit for an extended period of time.
Sandblasting forms is not recommended. Although this will clean off the concrete, it will etch the form surface and increase the likelihood of concrete sticking on future uses. Some precasters also use a pressurized water spray to clean forms. There are no specific pressure limits, but make sure the water pressure is not high enough to damage the steel. If the concrete buildup is old, then very carefully use a hammer and wide chisel to remove it. But be careful. Denting the form could cause cast concrete to bulge and add to stripping difficulty. There are also concrete softening products available to assist with form cleaning. To find a supplier that offers this product, visit precast.org/find.
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Mark writes:
How long should we keep 15-inch-by-15-inch-by-4.5-inch blocks in the molds? And when we take the blocks out, should we cover them again? If so, for how long? We are located in New Mexico.
NPCA Technical Services engineers answered:
It is typical to keep wet-cast precast concrete products in forms for 24 hours. Then, provided the concrete has reached the designated stripping strength, you can strip them out and use the forms for the next day’s production.
Although initial set and hardening has taken hold, keep in mind the concrete is still curing. Storing this product in the hot New Mexico sun for the next week could lead to some shrinkage cracking due to evaporation of surface water.
As an experiment, place some of your day-old product in normal storage and place others either in a shaded area and/or a spot where they can be kept wet via a sprinkler or mister for a day. If there is no benefit to the added curing process, then you can eliminate that issue. If the shade or moisture eliminates the surface cracking, then you have a solution during these very hot conditions.
For more details about concrete stripping and curing, we suggest obtaining a copy of the latest edition of the Portland Cement Association’s “Design and Control of Concrete Mixtures.” Keep it right next to your NPCA Quality Control Manual for Precast Concrete Plants.
John writes:
Can you weld epoxy-coated rebar? For example, when fabricating a cage, does it always have to be tied with epoxy-coated ties?
NPCA Technical Services engineers answered:
According to the Concrete Reinforcing Steel Institute’s “Manual of Standard Practice,” structurally welding reinforcing steel should be performed according to the American Welding Society D1.4/D1.4M:2011, “Structural Welding Code – Reinforcing Steel.” If the steel used for the coated bars meets ASTM A706/A706M, “Standard Specification for Deformed and Plain Low-Alloy Steel Bars for Concrete Reinforcement,” the bars are intended for welding without preheating and should be specified for applications that require an appreciable amount of welding.
ASTM A615/A615M reinforcing bars can be welded, but may require preheating the bars up to 500 degrees F depending upon the furnished metallurgy of the bar. Whether the weld is a tack weld to hold bars in place or a structural weld to transfer loads, after completion of the welding on epoxy-coated bars, the damaged areas shall be repaired using patch materials meeting ASTM A775/A775M, “Standard Specification for Epoxy-Coated Steel Reinforcing,” or A934/A934M, “Standard Specification for Epoxy-Coated Prefabricated Steel Reinforcing Bars.”
NPCA file photo
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