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.
Vijay writes:
I am making vertical cast concrete pipe with zero-slump concrete with a 0.39 water-cement ratio. All is good, but bug holes are present on the surface. Please inform me how to remove bug holes from the pipe surface.
NPCA Technical Services engineers answered:
You did not mention if your pipe process was using packerhead consolidation or standard dry cast with external vibration (either attached to the formwork, vibrating core or both). Regardless, when casting a zero-slump concrete as you describe, the internal friction within the cement paste and fine aggregate is greater than normal-slump concrete. Consequently, when entrapped air is pressed against the form, many times the paste and fines do not move to fill the void with the given vibration energy. The following are some items that should be checked.
Ensure all the vibrators are functioning correctly. Sometimes, among the noise and commotion of pipe production, a vibrator may stop working. This is difficult to identify without checking.
Ensure the vibrator chosen for this product is adequate for the formwork. You may need to request assistance from your vibrator supplier to ensure the frequency and amplitude of the external vibrator is appropriate for the placement and product being cast.
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If the pipe is being consolidated on a vibrating table that worked well for a smaller diameter pipe, it may not work well for a larger size due to the increased weight of the product and energy increase required to properly reach and consolidate all the areas.
For packerhead production, you may experiment to see if reducing the speed rate of the packerhead or if two passes of the roller head will greatly decrease the bug holes on the exterior surface. Additionally, check if the distribution fins at the top of the packerhead are functioning properly.
For both production methods, the easiest solution is to slow down the rate of concrete entering the form. Give the vibration or packerhead time to consolidate the mix. For dry-cast forms, it is good practice to load the form uniformly and continuously along the perimeter – described as ribbons or layers of concrete – rather than in larger pockets moved intermittently along the form, which can trap substantial amounts of air in the mix.
Lastly, some precasters find the addition of a liquid surfactant admixture provides an enhanced surface texture, reducing internal friction of the concrete components and increasing movement into trapped air voids. It also provides a slick surface, which the form can trowel as it is stripped from the product to remove smaller bug holes. Experimenting to provide the correct amount is critical, as too much will create entrapped bubbles.
Olevis writes:
Can you splice epoxy-coated steel with non-epoxy-coated steel? I’m wondering if this could cause a cathodic reaction.
NPCA Technical Services engineers answered:
You will find the appropriate guidance for your application in Subsection 4.16 of the Concrete Reinforcing Steel Institute Manual of Standard Practice.
We are not aware of any provisions that would exclude splicing of epoxy-coated and black rebar specifically related to your reference to a cathodic reaction. However, the use of uncoated rebar and reinforcing steel or another dissimilar metal to zinc should not be spliced to zinc-coated reinforcing.
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Jeline writes:
What is the frequency of rebar testing in precast?
NPCA Technical Services engineers answered:
Depending on the specifications, reinforcing steel used in precast applications within the United States should conform to ASTM International standards or equivalent American Association of State Highway and Transportation Officials standards. These standards dictate the welded wire reinforcement or steel bars’ composition, size, weight, tensile strength and deformations. The steel must meet these specific requirements at the time of manufacturing.
Testing the steel beyond that is at the discretion of any other governing body or the individual plant, and depends on what the steel is used for.
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The steel reinforcing manufacturer (rebar or wire) is required to conduct material testing in accordance with specific ASTM material standards and at a frequency prescribed within those standards. Typically, that frequency is for each heat of steel produced and those results are recorded on the mill certificates for that lot of steel reinforcement. Most precast concrete plant certification programs, including NPCA’s, require the precaster to maintain records of the specific steel mill certificates received with the steel shipment. Included within those mill certificates are the tested strength and material compositions for the steel within that lot. Any additional testing such as yield or ultimate strength is typically not performed unless it is specified or sampled by the governing body having jurisdiction, or is included within the precaster’s specific quality control plan.
Reinforcing steel manufacturers producing reinforcement in accordance with ASTM A1064, A615 and/or A706 are required to publish the ladle analysis for chemistry and conduct performance tests for the physical attributes of a particular heat of material per the applicable standard.
For specific information regarding how many pieces or what percentage of the lot manufactured is tested by the steel manufacturers, contact a rebar supplier. You can receive a list of rebar supplier contacts by visiting precast.org/find.
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