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 through 5 in the NPCA Quality Control Manual for Precast and Prestressed Plants. In parts three and four of this series, we are focusing on product-specific deficiencies relating to Chapter 6, “Special Requirements,” of the NPCA QC Manual. The first three parts of this series were published in the January-February 2012, March-April 2012 and May-June 2012 issues of Precast Inc.
Side-stepping the requirement detailed in Chapter 6 of the QC Manual will land you a deficiency for products that are designed with steps cast into the product. Deficiencies are far too common with this requirement, as the specifics are often not fully grasped.
The NPCA QC Manual states:
“Step vertical and horizontal load testing must be performed according to the applicable sections(s) of ASTM C497 once per year, per step design used and whenever a new step supplier is used. The step testing must be performed in the precast plant in the product for its intended use. The testing must be performed or witnessed and results documented by a member of the precasters Quality Control Department. The loads achieved must meet the requirements of ASTM C478.”
This requirement is clearly laid out in the applicable sections in the NPCA QC Manual, but deficiencies in these sections are common.
The mix-up occurs with the actual in-plant testing of the step in its cured state. Just like any other material cast into a concrete product, from aggregates to reinforcing steel to rebar chairs and spacers, the manufacturer/supplier of that product will provide a certificate or data sheet to indicate which industry standards the material is in compliance with. The precaster will see this certificate/data sheet that comes along with the steps and see that the step itself meets the industry standards (ASTM C478 and OSHA). The precaster will then assume that the specific requirement is met. This is a common error.
While the step itself may meet the applicable industry standards, the performance of the installed step must also be checked, in the form of a horizontal and vertical pull-out load test (400 lbs horizontally, 800 lbs vertically), per ASTM standards. This test is required once per year, per step design used and whenever a new step supplier is used.
This typically straightforward test often results in a deficiency. The frequency for performing watertightness testing for septic tanks is outlined in Section 6.5.2 in the NPCA QC Manual:
“Tank watertightness testing shall be demonstrated according to the acceptable section(s) of ASTM C1227 or the requirements set forth by the authority or authorities having jurisdiction, whichever is more stringent. A minimum of one test per year on a septic tank produced in each septic tank form used at the plant shall be performed and documented. If the authorities having jurisdiction require a greater frequency of testing, the plant shall maintain records of all additional tests at the plant.
Forms producing tanks that fail watertightness testing must undergo additional testing commencing with the next production of tanks from the form and continuing until 10 consecutive tanks pass the test.
In cases when multiple tank sizes are manufactured using the same form, watertightness testing should be performed on the largest (tallest) structure, as long as the same reinforcement design and concrete strength are used. Otherwise, testing should be performed on each individual tank design.”
For this requirement, deficiencies result from the lack of testing tank forms. The requirement in Section 6.5.2 in the NPCA QC Manual clearly states that a watertightness test is required one time a year per tank form that is used in production, unless a failure in the tank is discovered. In that case, additional testing is required.
Many deficiencies occur when the precaster has multiple tank forms but manufactures only one size. The precaster will claim that only one tank needs to be tested, because the tank forms are the same size and are from the same manufacturer. This is a common mistake. Even if the precaster manufactures one standard size and has more than one tank form, each tank form will need to be tested for conformance.
Watertightness testing of a tank produced in each form is necessary to ensure that all forming equipment remains within the appropriate tolerances. Keep in mind that this is a critical requirement when manufacturing septic tanks, and the plant must receive a passing score (> 75%) in order to pass the plant certification audit.
Dimensional checks show up in Sections 6.2, 6.3, and 6.4 in the NPCA QC Manual, as well as in the deficiency column on a plant inspection grading schedule. In addition to the standard post-pour dimensional checks required for products that fall under Sections 1 through 5 in the NPCA QC Manual, pipe, manhole and box culvert post-pour dimensional checks require more in-depth documentation.
The NPCA QC Manual outlines the following dimensional checks for precast concrete pipe:
“At a minimum, dimensional checks shall include internal diameter, wall thickness, and length of two opposite sides (measured directly across from each other). Joints must be checked for dimensional conformance with the manufacturer’s specifications, applicable standards, and/or specifying authorities.”
The NPCA QC Manual outlines the following dimensional checks for precast concrete manholes:
“At a minimum, dimensional checks shall include: manhole internal diameter, wall thickness, height of two opposite sides, verification of hole locations and size (when applicable), and verification of the invert dimensions and elevations (when applicable).”
The NPCA QC Manual outlines the following dimensional checks for precast concrete box culverts:
“In addition to standard post-pour inspections required in Section 4.6.4 of this manual, the plant shall also specifically check critical product dimensions including top slab, bottom slab and wall thicknesses, and inside length, width and height. These dimensional checks shall be performed on at least one box culvert produced in each form per day.”
These additional dimensional checks are a requirement for the manufacture of pipe and manholes and a critical requirement for box culverts. Dimensional checks of box culverts are to be performed on at least one box culvert produced in each form per day. Pipe and manhole dimensional checks are to be performed on a minimum of three pieces/sections or 3% of each day’s production, whichever is greater, chosen randomly by plant quality control personnel.
Deficiencies occur when plants do not perform the additional dimensional checks altogether; do not meet the three or 3% requirement; or fail to perform and keep on file the detailed checks. Too often, post-pour dimensional checklists will be full of check marks indicating that the actual dimensions meet the dimensions on the detail drawings. Inspectors are looking for more in-depth records than simple check marks on a grid. Correct documentation consists of marking down the actual recorded dimensions and comparing them with the dimensions on the detail drawings to ensure that the product is within the specified tolerances.
Keep in mind that these additional checks are not only for the inspector’s reference when inspecting a plant; they should also be reviewed by plant personnel to ensure the forms, forming equipment and production processes are able to achieve a certain level of consistency and quality.
For the most part, precasters do a great job preparing their plants for the initial inspection audits and continuing similar production practices and quality control measures going forward. For plants looking to get into the certification program or looking to improve upon their current inspection grade, one of the biggest tools they have at their fingertips is the NPCA QC Manual, Version 9.0. The QC Manual outlines in detail every requirement and the frequency for performing or documenting a procedure or material. It is available as a free download from NPCA’s website at precast.org. If for some reason any requirement is unclear to plant personnel, they have a great resource in the technical staff at NPCA to resolve any concerns.
Evan Gurley is a technical services engineer with NPCA.