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.
Raja writes:
We are using crushed stone and sand, and our moisture content is 11%. Is this usable or not usable?
NPCA Technical Services engineers answered:
Aggregate absorption values and specific gravities can vary widely depending on the geology and geography. This can even vary within close regional pit locations. Even so, moisture content of 11% is very high. American Concrete Institute gives a range between zero and 8% for normal weight aggregate. Aggregate with higher absorption levels are generally seen in lightweight, low-density or recycled aggregate. With such high values, durability and strength loss in hardened concrete may be a concern. A careful mix proportion must be chosen if you use this type of aggregate.
Carlos writes:
How do you avoid the risk of getting a notch in the longitudinal bar due to tack welding stirrups or any transverse reinforcement?
NPCA Technical Services engineers answered:
American Welding Society A3.0, “Standard Welding Terms and Definitions,” defines a tack weld as: “A weld made to hold the parts of a weldment in proper alignment until the final welds are made.”
In the abstract, “Fatigue Tests of Reinforcing Bars-Tack Welding of Stirrups,” American Concrete Institute acknowledged that tack welding reinforcement can cause damage to the longitudinal bars and reduce the quality of the reinforcement. R26.6.3.1 in ACI 318,
“Building Code Requirements for Structural Concrete and Commentary,” states, “Tack welding (welding crossing bars) can seriously weaken a bar at the point welded by creating a metallurgical notch effect. This operation can be performed safely only when the material welded and welding operations are under continuous competent control, as in the manufacture of welded wire reinforcement.”
If tack welding must be done, it is important that the welds follow AWS D1.4, “Structural Welding Code – Reinforcing Steel.” Two other articles for reference are, “Tack Welding Reinforcement Bars,”1 and, “Welding Reinforcement Steel: AWS D1.4/D1.4M: 2011.”2
Resources:
1 www.twi-global.com/news-events/case-studies/tack-weldingreinforced-bars-139/
2 structuremag.org/?p=1342
Ernest writes:
At what area intervals should vibration be done in a 6-inch-thick slab using a poker?
NPCA Technical Services engineers answered:
Table 5.1 of ACI 309R-09, “Guide for Consolidation of Concrete,” suggests using a vibrator with a 1 1/4-to-2 1/2-inch diameter head for thin slabs and a 2-to-3 1/2-inch head for heavy slabs. Section 7.2, “Procedure for Internal Vibration,” recommends systematically inserting the vibrator vertically at a uniform spacing over the entire placement area. The distance between insertions should be approximately 1 1/2 times the radius of influence.
The radius of influence can be obtained from the vibrator manufacturer, or the vibrator can be tested by inserting it into the concrete mix placed in the form. Observe and measure the visible movement of the mix as you move the vibrator directly up and down into the concrete mix. The spacing of the vibrator should not allow dead zones between insertion spots. For more information about proper consolidation best practices, view the NPCA Precast Learning Lab video at precast.org/learning-lab.
Dr. Rao writes:
How effective is limestone powder in slag cement? We use 2/3 ground-granulated blast-furnace slag (GGBFS) and 1/3 ordinary portland cement (OPC) for making masonry concrete blocks. Can we mix limestone powder as a part of GGBFS or OPC?
NPCA Technical Services engineers answered:
As shown in Table 1 in the article, “Portland-Limestone Cement,” cement with 35% slag was able to maintain similar 28-day strength results when using varying amounts of limestone powder. Your strength results may differ depending on the amounts of slag and limestone used so I would encourage carefully monitoring test specimens.1
One bonus when using limestone powder is a reduced environmental impact. The usage of limestone as an ingredient in blended cement at levels of 5 to 15% by mass results in less clinker produced for an equivalent amount of cement. This means less energy is used, and carbon dioxide emissions and greenhouse gases are reduced. As far as mixing limestone powder with GGBFS or OPC, this depends on the application. If you plan to use it for construction, it is important to check with the standards body in your area to see what limits there are on limestone powder usage. If you are mixing limestone powder for testing purposes, there are no limits.
If you are interested in further information, we would recommend reading, “State-of-the-Art Report on Use of Limestone in Cements at Levels of up to 15%,” from the Portland Cement Association2 and “Improvement of the Early- Age Reactivity of Fly Ash and Blast Furnace Slag Cementitious Systems Using Limestone Filler.”3
Resources:
1 precast.org/portland-limestone
2 www.cptechcenter.org/ncc/documents/SN3148_Use%20of%20Limestone%20in%20Cements.pdf
3 link.springer.com/article/10.1617%2Fs11527-010-9637-1
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