Editor’s Note: This is the final article in a year-long series about how seven common types of waste in manufacturing can create unprofitable activity and how to address them in your plant.
By Eric Carleton, P.E.
A precast concrete defect can be defined as a product not meeting a standard or a customer’s expectation. Like all wastes, defective products create additional problems and add costs throughout the production process. They also generate environmental wastes not identified in this series. For these reasons, many lean quality management experts consider defects to be the worst of the seven wastes.
However, defects are one of the easiest wastes to identify. Does the product meet all aspects of its respective ASTM, Department of Transportation or municipal standards? Does it display the aesthetics and company reputation as intended? If the answer is “no” to one of these questions, you have a defect that needs correction.
Photo courtesy of County Materials.
A “Go/No-Go” gauge allows precasters to verify if cast spigot joint dimensions are within tolerance.
While identifying a defect can be straightforward, understanding the cause and corresponding remedy can prove to be more difficult. When attempting to tackle defects, four guidance activities are often recommended1 by lean manufacturing experts2:
- Determine one defect on which to place your primary emphasis. For example, a precast inlet was stripped from the form and has rough vertical wall edges that appear honeycombed or jagged. This defect not only creates issues with appearance and acceptance, but it can also affect long-term durability and possibly structural integrity. This type of defect often requires repair.
- Determine when in the production process this defect occurs and identify the cause. If the product in question is wet-cast, is the issue related to stripping? Are you experiencing a paste leakage issue caused by forms not being connected or latched correctly, properly maintained or checked for dimensional tolerances? Are forms being damaged during production, crane operation or storage? Use root cause analysis procedures to identify where the problem originates and determine the appropriate solution. Ensure the solution totally corrects the identified problem at the source such that the problem does not proceed in some manner down the production line.
- Revise the process and/or provide training to correct the defect. Investigate the form tolerances, ensure latch mechanisms are working properly and perform the necessary maintenance on the forms. Conduct form dimensional checks and maintenance at a greater frequency or add chamfers at the corner sections, which may prevent the loss of concrete paste at the form joints and improve appearance. Train your employees on proper production techniques and talk about why the issue has been occurring and why it’s imperative to make process changes. You can engage crews in helping to identify what other tools or resources may be needed to prevent the defect in the future.
- Standardize the process to eliminate the defect. Leverage the data gathered during the process revision and training phase and include that information into the new process or method. For instance, incorporate the maximum allowable gap tolerances of form joints into the pre-pour dimensional inspection checklist. Develop simple gauges for your workers to verify tolerances are not exceeded. Ensure form dimensional compliance and latch connections are verified in good working order prior to production activities. When you’ve standardized your new process to eliminate the root cause of the defect, conduct ongoing and refresher training on the new process and correct techniques.
NPCA file photo
A jig fixture with the correct bar spacing specifically cut into steel channels or angle iron fabricated frames helps minimize, or eliminate, the possibility of human error.
Another important technique used by precast plants to reduce defects is “Poka-Yoke.” No, this isn’t a variation of a boxing technique championed by Mohammed Ali, but rather a process to remove the potential for human error and make it “mistake proof.” Poka-Yoke (pronounced PO-ka yo-KAY) is defined by the American Society for Quality as “the use of any automatic device or method that either makes it impossible for an error to occur or makes the error immediately obvious once it has occurred.”3
For a precast operation, Poka-Yoke could be applied to proper placement of reinforcing steel into a vault form as an example. The normal process is to construct the steel cage on the form pallet and have the rebar alignments marked on the inside of the interior form or core. This process is completed by closing off the form with the exterior form or jacket. However, the bar spacing simply placed by eye could be out of tolerance or an incorrect bar diameter could be placed in the cage.
A Poka-Yoke solution to this issue would be to construct a jig fixture with the correct bar spacing specifically cut into steel channels or angle iron fabricated frames. The specific placement areas would then be clearly marked with the correct bar numbers or notched so only the correct bar would fit properly. These steps minimize – or eliminate – the possibility of human error.
Other strategies falling within the Poka-Yoke family are those which make an error immediately obvious, preventing the error from moving down the line. Again, thinking of reinforcing steel fabrication, consider fabricating the perfect bent shapes and angles and keep them for a reference or template on a shadow board in the steel fabrication area. If any of the bends or angles don’t resemble the model, they don’t move down the line to the cage fabrication crew.
Properly calibrated aggregate moisture meters or probes, along with automatic mix water adjustment systems, would also be considered Poka-Yoke strategies as they remove the variability from all processes involved during a manual aggregate sampling, moisture burn test and concrete mix moisture modifications.
Another important tool used in the precast industry to verify if cast spigot joint dimensions are within tolerance for proper gasket deformation is the “Go/No-Go” gauge. Proper use of this device will provide quick confirmation if the joint is out of tolerance for the supplied gasket.
Strive for perfect
As a precaster, your goal should be to identify, analyze and eliminate defects, and implement revised methods and training to prevent them from occurring again. Those who do will realize reductions in other types of waste, resulting in a chain reaction of other improvements as well – including enhanced profits, efficiency and morale.
Eric Carleton, P.E., is NPCA’s director of codes and standards.
References:
1 https://blog.worximity.com/en/industry-4_0/defects-waste-in-lean-manufacturing
2 https://theleanway.net/The-8-Wastes-of-Lean
3 https://asq.org/quality-resources/mistake-proofing
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