Questions from the Field is a selection of questions NPCA Technical Services engineers received from calls, emails and comments on blog posts or magazine articles posted on precast.org. If you have a technical question, contact us by calling (800) 366-7731 or visit precast.org/technical-services.
Andrew writes:
A large part of my business is painting precast wall panels. Could you provide some guidance on how to measure moisture content prior to painting as well as any best practices to consider?
NPCA Technical Services engineers answered:
Painting concrete requires some preparation to avoid issues with peeling or blistering. Concrete surfaces may have remnants of form release agents on the surfaces, which can cause issues with adhesion when it comes to coatings and paint. Wash and prepare concrete surfaces in accordance with the painting or coating manufacturer’s instructions.
Depending on how old the wall is, concrete can contain varying amounts of moisture in the form of liquid and vapor. Go beyond just the concrete surface when assessing the wall’s moisture level prior to painting. While the surface may be dry, moisture that may exist within the concrete could eventually make its way to the surface. Paint manufacturer Dunn Edwards recommends that, “Moisture content should be under 12% for coatings work to proceed.” Various test methods are available to determine concrete humidity. The method outlined in ASTM D4263, “Standard Test Method for Indicating Moisture in Concrete by the Plastic Sheet Method,” utilizes an 18-inch-by-18-inch plastic sheet applied to the concrete for 16 hours to indicate capillary moisture. There also are moisture meters.
As for priming, using an acrylic-based sealer or primer on concrete may be helpful. From the PCI Architectural Manual:
“Coatings applied to exterior surfaces should be of the breathing type, permeable to water vapor but impermeable to liquid water. Typically, latex paints are suitable for most exterior applications.
Typically, latex paints or epoxy, polyester or polyurethane coatings may be applied to the interior surface of exterior walls if a vapor barrier (paint or other material) is necessary. See Section 3.5.16 for finishing procedures for interior surfaces to be painted. The coating manufacturer’s instructions regarding mixing, thinning, tinting, surface preparation, and application should be strictly followed.”
Barry writes:
What is the recommended vibrator head size to be used in a 14-inch wall with a double mat of rebar and 4-inch slump concrete?
NPCA Technical Services engineers answered:
There are many things to consider when selecting the appropriate internal vibrator. The first two considerations are the width or thickness of the concrete being consolidated (in your case 14 inches) and the slump of the concrete (4 inches). The radius of action of an internal vibrator, determined by its amplitude setting, should not be greater than the width of the section being consolidated. The stiffest mixes, those with less than a 3-inch slump, likely need the largest vibrator head diameters – greater than 2 to 3 inches. As a general rule, the head diameter should be 25% of the wall thickness – or 3.5 inches in your case. That’s among the larger sizes, so we understand why you’re reading up on next steps. Another consideration is the spacing of the rebar mats.
The vibrator head should be allowed to lower into the concrete without contacting reinforcing steel. Therefore, the head size should be smaller than the spacing between mats.
If you employ a vibrator that allows for adjustment in amplitude and frequency settings, you can find the right balance between effectively consolidating the concrete and preserving the formwork. But that does take some calculation to determine the settings to start your trial-and-error testing. Your vibrator supplier can assist with testing the efficacy of their equipment in your particular mix.
Charles writes:
Does cube strength accurately represent the strength of in-situ concrete?
NPCA Technical Services engineers answered:
The short answer is: It depends. It sounds like you are talking about concrete cubes (not mortar cubes) and cast-in-place concrete. If so, concrete cube strength can be similar to the concrete strength in-situ if the concrete in the cubes and the concrete in-situ come from the same batch; the concrete for the cubes is sampled correctly; the cubes are cured in the same conditions as the in-situ concrete; and everything is tested at the same age. Realistically, the consolidation and curing processes for the cubes and the in-situ concrete likely vary. This, in turn, could cause the cubes and the in-situ concrete to have different compressive strength results and could cause significant variations between the cubes and the in-situ concrete in terms of durability, service life and other characteristics.
There are many factors that impact an in-situ concrete product’s strength that are likely different for the cubes, because the cubes are cast and cured in controlled, laboratory environments while cast-in-place concrete is cured in a much less carefully controlled environment. Namely, the curing temperature and curing moisture conditions at which the cubes and the in-situ concrete are cured will vary, and this could cause significant differences in the hardened concrete properties of the cubes and cast-in-place material.
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