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.
Ron writes:
Can concrete slabs expand due to moisture to a size greater than their original dimensions when poured?
NPCA Technical Services engineers answered:
A slight decrease in concrete volume is possible for a variety of reasons, whereas an increase in volume is uncommon.
If a mix has been batched and calibrated to yield a true cubic yard (27 ft.3), then the addition of more water would add volume to the mix and potentially cause the appearance of expansion (over yield). However, the added water could also contribute to surface cracking, a reduction in concrete strength and increased susceptibility to shrinkage when the added water evaporates.
If a concrete mix has been poured and properly consolidated, and initial set has occurred within accepted temperature ranges, then the addition of water to the concrete surface would not create any measurable increase in volume or dimensions. If this concrete mix was cured with very high temperatures outside of the accepted range, then delayed ettringite formation may occur. During DEF, the hardened concrete generates a gel-like structure in the concrete paste, which creates expansion forces, and consequently tensile stresses, within the concrete. These stresses lead to cracking of the hardened concrete structure. This expansion is on a micro level and may not be measurable by normal devices or formwork.
Sam writes:
Are there any other protective coatings for rebar besides epoxy?
NPCA Technical Services engineers answered:
Another protective coating option is galvanized rebar. Galvanization is the process of applying a zinc coating to the reinforcement, which provides extra protection against corrosion. Epoxy coating is considered a barrier-type coating, while some consider galvanization both a barrier and a sacrificial coating. Proponents of galvanized bar point to the fact that the zinc coating has to be completely eroded in order for the steel to be exposed.
Both epoxy-coated and galvanized reinforcement help protect reinforcing steel from water intrusion, which can oxidize and corrode steel. Coated bars are particularly useful in environments where products are exposed to brackish water and when exposure to salt and chlorides is expected. No matter the type of rebar used, it is imperative to follow best practices and any special measures deemed necessary by the reinforcement manufacturer to protect the coating’s integrity while the reinforcement is in storage, being handled, during fabrication, etc.
NPCA file photo
Steve writes:
I am working on a project using absorptive precast concrete sound walls. In regard to finishing the wall panels, a question came up and I was hoping to get some help: What is a double-raked finish?
NPCA Technical Services engineers answered:
A double-raked finish is sometimes referred to as fuzzy-rake or random-rake finish. It’s often used as an anti-graffiti surface.
Using #57 aggregate, allow the concrete to reach its initial set after screeding. Next, using a garden rake, carefully stir up the coarse aggregate near the unformed surface by using a circular motion with the rake, being careful to only rake as deep as necessary to bring the coarse aggregate to the surface. Then go back and ensure the finish is consistent. Please see the included photo for an example of a double-raked sound wall finish.
Photo courtesy of JBM Solutions Inc.
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