Questions from the Field is a selection of questions the NPCA Technical Services Department receives from calls, emails and comments on blogs or magazine articles on precast.org. If you have a technical question that needs an answer, contact us by calling (800) 366-7731 or visit precast.org/technical-services.
Asiddin writes:
Is it acceptable practice to insert the head of an internal vibrator at an incline? How does inserting the vibrator at an angle affect the concrete?
NPCA Technical Services engineers answered:
Stinger vibrators (internal vibrators) are designed for maximum effectiveness when inserted vertically into concrete. There are circumstances where casting thin elements will not allow this, so the vibrator head may be inserted at an angle or horizontally to keep it completely immersed. It is important the vibrator head does not move laterally while consolidating in this manner. Stinger vibrators should not be used to move concrete during placement.
If the vibrator is inserted at an angle, concrete consolidation can be compromised. Vibrator heads contain an eccentric weight that rotates at high speeds and exerts vibratory action perpendicular to the axis of the head. If the head is inclined, there is a risk the vibrator will not consolidate some portions of the concrete.
NPCA file photo
Seyyed writes:
What kind of solutions are available for improving production management? Do you know any precast plants who use radio-frequency identification?
NPCA Technical Services engineers answered:
Digitizing precast concrete product inventory can be a tough task. But like most automation, once it’s in place and proper training is given, the payback is worth the time and effort. The U.S. is starting to use more advanced processes within the precast industry. And recently, we have seen some government agencies require RFID coding on precast concrete products.
For your continued research, we recommend visiting NPCA’s website and reading the blog post “Change the Way You track Your Product,” or the article “Inventory Control Comes of Age” from the March-April 2011 issue of Precast Inc. In addition, we suggest visiting precast.org/find to search and contact a supplier who has developed inventory and production control software solutions or a producer who uses the system.
Photo provided by Terre Hill Concrete Products
Alex writes:
We want to reinforce a 60-inch diameter reinforced concrete pipe. The wall thickness is 6.75 inches. We want one layer of wire mesh for the outer reinforcement. For the inner reinforcement, we don’t have the exact wire mesh, so we want to use two mats.
One is .24 and the other is .18 square inches per linear foot equaling .42, which is greater than the requirement. In ASTM C76, Section 8.1, “Circumferential Reinforcement,” this is permissible. The wall thickness is less than 7 inches, but for “a line of circumferential reinforcement,” you can use two mats. Is this correct?
NPCA Technical Services engineers answered:
You are correct. It is permissible to double up the welded wire reinforcement to obtain the minimum steel area requirement. As discussed, it is important to keep the two circumferential wires as tight together as possible and certainly within the tolerances provided within the standard. That standard states the (circumferential) layers shall not be separated by more than the thickness of one longitudinal plus ¼ inch.
It may be beneficial to use special steel layouts on large diameter higher strength pipe to save on the cost of heavy steel areas wasted in the compression zones of the pipe. This is accomplished by first rolling an inside “carrying cage” and then rolling quarter section “quad mats,” which are tied to the carrying cage in the crown and invert tension zones. The idea is to optimize the steel usage. However, this “quad” method may be more
labor-intensive than simply rolling two complete cages and binding them to meet the required steel area. It is important to verify any effect this new steel layout may have by conducting a three-edge bearing test and comparing it to other historical data you may have on a single line inside a cage pipe.
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