A staff engineer reports on pre-assessment audits for the NPCA Plant Certification Program.
By Evan Gurley
Editor’s Note: In the first two parts of this four-part series, we addressed 10 of the most common problems encountered during a typical pre-assessment plant inspection relating to Chapters 1 to 5 in the NPCA QC Manual. In parts three and four of this series, we will focus primarily on product-specific deficiencies relating to Chapter 6, “Special Requirements.”
The requirements in Chapter 6 are in addition and complementary to the requirements in Chapters 1 through 5. Sections 6.1 through 6.5 are intended to demonstrate that the final product is consistent with ASTM International specifications and other industry standards used to verify acceptable product manufacturing and performance.
Proof of conformance
If the plant claims to manufacture certain products that meet ASTM specifications or other industry standards, then the plant should be able to prove such claims. Proof of conformance to specific ASTM or other industry standards should be documented and kept in the plant records for all products that fall under this category. Too many times, a plant will claim that a product is being manufactured to ASTM or other industry standards, but then it will have no proof, documentation or referenced standards/drawings to back up these claims.
When talking about proof of conformance, we’re talking about the plant providing hard documentation that a product is being manufactured to, and adhering to, industry minimum requirements. Proof of conformance consists of one or more of the following:
• Design calculations and drawings
• Documentation of performance testing
• Documentation of the design conditions and specific requirements stated in individual ASTM International standards
If the precast plant is manufacturing a product to a state DOT-approved drawing, has the most current state-approved drawing/specification on hand and is manufacturing the product according to the requirements, this will suffice. If the plant does not have the most recent drawing/specifications on file and is not performing the checks and balances to ensure the product is being manufactured to the specified requirements, then the plant will receive a deficiency in this area.
Proof of conformance with ASTM or other industry standards should be a normal part of the quality control operations, unless more stringent design requirements are specified for products/projects.
Does every product covered in ASTM specifications require that the drawing be stamped by a licensed professional engineer? No, reinforced precast concrete pipe manufactured to ASTM C76 does not require a PE-stamped drawing, so in this case, ASTM requires product testing in order to conform to minimum standards.
For example, ASTM C76 states that pipe manufactured to ASTM C76 must conform to one of two separate alternatives:
• Basis of plant load-bearing tests, material tests, and inspection of manufactured pipe for visual defects and imperfections
• Basis of material tests and inspection of manufactured pipe for defects and imperfections
Independent of the method of acceptance, ASTM C76 states that all pipe shall be designed to meet both the 0.01-in. crack and ultimate strength requirements specified in Tables 1 to 5 of that standard.
Reinforcing steel inspection (pipe, manhole, box culvert)
Critically important but often overlooked, reinforcing steel inspections outlined in Sections 6.2.1, 6.3.1 and 6.4.3 are essential elements within the NPCA grading schedule (requires a 75% or higher score). They are also critical factors in achieving the design structural strength of the final product itself.
Most plants realize that they are required to perform pre-pour inspections on all products (exclusive of dry-cast, three or 3% inspections as noted later) before casting. But not all plants realize that they are required to perform an additional, in-depth reinforcement inspection for products outlined in Chapter 6. Standard pre-pour checks include everything from inspecting form dimensions and release agent application to positioning and securing of reinforcing. While standard pre-pour inspections do touch on the positioning and securement of reinforcing, additional measures and inspections are required for pipe, manholes and box culverts. Additional reinforcing steel inspections include maintaining documentation of cage inspections with information on the required cage design versus the actual cage design used, including WWR style, steel area, wire diameter, cage diameter, cage length and welded/tied wire laps. It is a critical requirement that plants perform a detailed reinforcing steel inspection on a minimum of three reinforcing steel cages or 3% of each fabrication run daily, whichever is greater. Test specimens must be chosen on a random basis by QC personnel, regardless of fabrication method. (For products that do not fall under Chapter 6 requirements but are outlined in Chapters 1 to 5, a detailed inspection is required on one piece or 3% of daily or shift-produced products unless the product is machine-cast or dry-cast.)
So, why is accurate placement of reinforcing steel in the concrete member so important? The answer can be seen easily when calculating the moment for a section of a precast concrete product. To calculate the moment of the specimen, use the simple equation below:
M = F x d
The moment (M) is equal to the force (F) multiplied by the distance (d). We can put in an arbitrary number for the force (F) of the steel reinforcing, so let’s say in this example it is 60,000 lbs and the distance to the properly placed reinforcement (d) is equal to 4 in. Using the simple equation, we find that the moment (M) in this specimen is 240,000 lbs/in. Now let’s say that the reinforcement was improperly placed at a depth of 3 in. instead of the required 4 in. Performing the same calculation, 60,000 lbs multiplied by the distance (d) of 3 in. equals a moment (M) of 180,000 lbs/in. The design calls for a moment (M) of 240,000 lbs/in., but since the reinforcement was improperly placed by 1 in., the design of the structure is totally thrown off. This is why proper placement of reinforcement is so important.
If the location of the steel reinforcement is off, the concrete structure will also not have the proper concrete coverage. This is important, because when the concrete structure is loaded, the concrete will begin to crack. But if the steel reinforcement is located in the proper location, the cracks will extend only to where the steel reinforcement is located, then the stress will be transferred to the steel reinforcement and the cracks should not develop further. If the steel reinforcement is not located in the proper location, the cracks may develop, loads may exceed the design, and structural damage may occur.
Absorption testing (pipe, manhole, box culvert)
Similar to the reinforcing steel inspection testing requirements previously addressed, absorption testing requirements show up in numerous sections under Chapter 6 in the NPCA QC Manual. And similar to the reinforcing steel inspection testing requirements previously addressed, nonconformance related to absorption testing inconsistencies are frequent.
The NPCA QC Manual states: “Verification of conformance to the concrete absorption requirements of applicable standards shall be documented by performance testing in accordance with ASTM C497 (Test Method A or B). Testing shall be performed a minimum of once per year on the mix design with the lowest amount of cementitious material used at each operation or manufacturing station. Both in-plant and laboratory testing shall be permitted.”
This requirement shows up in Sections 6.2, 6.3 and 6.4 of the NPCA QC Manual, and is a critical requirement in Section 6.2. The inconsistency seen with plant adherence to this requirement is related to the amount of testing and what is tested. If a plant has only one standard concrete mix design used for the manufacture of all three products, only one absorption test (total) will be required per year, as the plant has only one concrete mix with one water-cementitious materials ratio. If a plant manufactures precast concrete pipe, manholes and box culverts, and uses three different mix designs for each product, it must perform three separate absorption tests per year. If a plant manufactures only pipe but has 10 mix designs, the plant is required to perform an absorption test on the mix design with the lowest amount of cementitious material once per year.
Often a plant will perform an absorption test on a product using its standard mix design, but not on the mix design with the lowest amount of cementitious material. This will lead to a deficiency. Even if a plant uses the mix design with lower cementitious materials only one time per year, the plant must perform an absorption test using it since it has the least amount of cementitious material.
This requirement is true for a plant that manufactures both wet-cast and dry-cast products. For example, if a precast concrete plant manufactures wet-cast and dry-cast manholes, an absorption test must be performed for the wet-cast mix and the dry-cast product with the lowest amount of cementitious material in the mix.
A careful review of the NPCA QC Manual requirements (Sections 6.2, 6.3 and 6.4) and ASTM C497 (Test Method A or B) for your standard product line and your mix designs will reduce confusion and the number of deficiencies noted during the inspection process.
End of Part 3, to be continued in Precast Inc. July-August 2012.
Evan Gurley is a technical services engineer with NPCA.