The National Precast Concrete Association reports that it has certified more than 200 precast operations. NPCA Plant Certification is a benchmark increasingly required by state departments of transportation and other specifying authorities. The association established the plant certification program in 1987 to validate high production and quality control standards. It requires participants to pass an initial inspection and then maintain certified status by passing periodic, unannounced on-site inspections performed by nationally accredited engineer and consultant Wiss Janney Elstner. Inspectors audit the plant's compliance with the requirements described in the NPCA Quality Control Manual for Precast Concrete Plants. Additional product-specific requirements apply to plants fabricating manholes, septic tanks, concrete pipe and box culverts.

Last month, NPCA published its revised QC Manual online, reflecting changes from the group's Quality Assurance Committee. The new edition includes:

• Improved requirements for production of self-consolidating concrete.
• Upgraded standards for manhole and concrete pipe production.
• New requirements for box culvert production.
• Gasket, connector and sealant quality assurance requirements.
• The addition of critical requirements for which plants must receive a minimum score of 75 to maintain certification (such as concrete and product testing).
• The creation of a probationary certification status for plants failing to achieve the minimum required score for critical requirement sections or those scoring between 75 and 80 overall, requiring some of them to take immediate corrective action.

A chapter-by-chapter sampling of revisions indicates how the QA committee is addressing new mix agents and technologies, and methods for improving water tightness in drainage products:

CHAPTER 2.1.1 — CEMENT

In an effort to acknowledge the growing popularity of nonportland cementitious materials, the latest edition of the QC manual makes added allowances for ASTM C595-compliant Type IS (portland blast-furnace slag) cement and Type IP (portland-pozzolan) cement, in addition to similarly spec'd ASTM C150. The manual also adds the statement: “When using blended cements, trial batches should be tested to ensure adequate strength is reached prior to stripping the product.”

CHAPTER 2.1.7 — SUPPLEMENTAL CEMENTITIOUS MATERIALS (SCM)

Originally titled “Mineral Admixtures,” this section adds a reference to the standard specification for ground granulated blast-furnace slag, ASTM C989, and a commentary regarding delayed initial strength gain when using SCMs, which can be affected by the cement replacement level.

CHAPTER 3.1.1 — CONCRETE MIX PROPORTIONS

Plants using self-consolidating concrete shall include specific quality control procedures in their plant-specific QC manual. At a minimum, written procedures shall address the steps necessary for an initial mix qualification, including trial batching and in-depth concrete testing; and mix qualification procedures, which shall include developing the range for acceptable test results of daily QC testing used for mixture acceptance (e.g., target slump flow of 22 to 27 in.) and a daily quality control regimen. Subsequent daily test regimen must be the same as that developed during mix qualification.

Because of the current lack of accepted standardized QC procedures, the plant should consult with the admixture manufacturer in developing appropriate QC operations for SCC. The plant also may consider consulting the Precast/Prestressed Concrete Institute Interim Guidelines for SCC.

CHAPTER 5.2.2 — MOISTURE CONTENT

A new subsection has been added to this section covering requirements for various batching and aggregate surface moisture testing when producing conventional, dry-cast, and self-consolidating concrete.

For SCC, when moisture probes or meters are used with automatic mixing-water adjustment systems, the aggregate surface moisture content shall be determined at least once a day prior to making the first SCC batch. Moisture tests shall be performed in accordance with ASTM C70, “Standard Test Method for Surface Moisture in Fine Aggregates,” or ASTM C566, “Standard Test Method for Total Evaporate Moisture Content of Aggregate by Drying.” Drying aggregate using a microwave or hot plate shall be permitted in addition to using an oven. Samples for moisture tests shall be taken as close as possible to the area where the probe is located.

Since SCC is extremely moisture sensitive, precise control of the water content is essential. Verifying aggregate surface moisture will allow for necessary adjustment in mix water as the moisture content of the aggregates changes throughout the day. Slump flow and VSI testing will only confirm whether a SCC mixture is within spec and, if moisture tests are not performed on a regular basis (every three mixes), becomes necessary for this confirmation. In facilities where SCC is produced, the plant should consider performing unit weight of the mix in addition to slump flow and VSI as another verification of proper mix proportioning.

For SCC made without moisture probes or meters and automatic mixing water adjustment systems, the aggregate surface moisture content shall be determined at least once a day prior to making the first SCC batch and then once every four hours of elapsed time after the first batch, while the material is being mixed. Moisture tests shall be performed in accordance with ASTM C70 or C566. Drying aggregate using a microwave or hot plate shall be permitted in addition to using an oven. In addition, slump flow and VSI tests shall be performed for every three batches of SCC produced. (This is to ensure that moisture fluctuations of aggregates are accounted for correctly.) In lieu of this additional testing, moisture tests may be performed every three batches.

CHAPTER 5.3.1 — SLUMP, SLUMP FLOW, AND VISUAL STABILITY INDEX

Having changed its name from simply “Slump,” this category has added subsections covering requirements for slump testing of conventional concrete and slump flow and VSI testing of SCC. Added commentary has also been included explaining that air content of SCC can affect the desired properties of the mixture, and it is recommended that the air content be tested regularly with the slump flow and VSI.

CHAPTER 6.2.6 — GASKET QUALITY CONTROL

This newly added section to the manual requires gasket QC documentation, which only applies in cases where the plant uses gasketed joints. In lieu of documentation received from the gasket maker, the plant must perform in-house testing, according to the following:

• Prelubricated gaskets shall be tested for durometer (ASTM D2240), length (ASTM C497), height, width and splice strength.
• O-ring gaskets shall be tested for durometer, length, volume (ASTM C497), diameter and splice strength.
• Profile gaskets shall be tested for durometer, length, height, width and splice strength.

Added statements dictate that further testing must be done if nonconforming gaskets are discovered. Nonconforming gaskets may not be used.

CHAPTER 6.3.5 — GASKET QUALITY CONTROL DOCUMENTATION

Certificates of conformance shall be received for each shipment of gaskets. In addition to physical property test results, the certificates shall include information listed below for each gasket type (length, durometer, dimensions, etc.). In lieu of supplier certification documentation, in-house gasket testing shall be performed and documentation maintained on file at the plant. One gasket of each type shall be tested for every 300 received, according to the following:

• Prelubricated gaskets shall be tested for durometer, length, height, width and splice strength.
• O-ring gaskets shall be tested for durometer, length, volume, diameter and splice strength.
• Profile gaskets shall be tested for durometer, length, height, width and splice strength.

If any quality control measurements indicated that the gasket is not within acceptable tolerances, additional gasket testing shall be performed to determine if the remainder of the lot should be used. Nonconforming gaskets shall not be used or supplied by the plant for installation on products.
— NPCA, 800/366-7731; www.precast.org