By Evan Gurley
One of the inherent benefits of precast concrete products is that they are manufactured in a controlled environment that eliminates most, if not all, of the quality issues that arise when casting a concrete product on site. Precast concrete has a great reputation for durability, strength, service life, watertightness and aesthetics that can be attained with a high level of consistency – as long as the proper quality assurance measures are in place and followed.
But quality doesn’t mean a thing if the materials that go into the product are not what the design calls out. All materials – including the cement, aggregates, water, chemical admixtures and reinforcing steel – must be as the design calls for, or the integrity of the product may be jeopardized.
Materials certification requirements
Every year, some of the most prevalent recurring deficiencies recorded by inspectors during an audit tie in with Chapter 2 of the NPCA Quality Control Manual for Precast and Prestressed Plants. While these sections currently are not “critical sections” in the NPCA QC Manual, they are critical in another sense.
Sections 2.1 & 2.2 – “Concrete” and “Reinforcement,” respectively – of the NPCA QC Manual address minimum requirements for the materials that are to be used in the production of a precast concrete product (discussed below). Conformance to these minimum requirements, as well as the frequency of documentation and test report retrieval, are outlined in these sections. Without these documents and test reports outlining the conformance to specifications, you cannot be assured that the materials called out by the design will result in a precast concrete product that meets customer requirements.
Minimum requirements for materials
Section 2.1.7 Plant Requirements
1. The following documentation shall be maintained current in the plant records:
- Cement and supplementary cementitious material mill certificates
- Aggregate supplier and test reports
- Mix water potability test reports or other test records indicating the acceptability of the mix water (annually) unless using a municipal water supply
- Chemical admixture and other additive certifications (annually)
2. Documentation of conformance to ASTM C33 (excluding gradation testing) and test reports indicating that the aggregates are nonreactive and stable shall be maintained for each aggregate source used. Such documentation shall be obtained from the supplier, an appropriate state agency or a testing laboratory engaged by the plant, a minimum of once per year for each material used. The maximum aggregate size shall be proper for the products being cast.
3. Records of incoming raw materials and plant materials tests shall be kept current and on file for a minimum of three years.
Section 2.2.5 Plant Requirements
1. Mill certificates and certificates of conformance shall be maintained current for all reinforcement including reinforcing bars, reinforcing wire, bar mats, welded wire reinforcement and coated reinforcment (see the example in Figure 1).
2. The plant QC Inspector shall crosscheck that certificates are on file for all reinforcing heat numbers being used or stored.
3. Certificates shall be maintained in the plant records for a minimum of three years.
Alicia Kamischke, an inspector with Hanson Professional Services Inc. (HPS), NPCA’s engineering inspection agency, addressed the top quality control problems for precast plants. “Many plants are missing material certification documentation, either mill certificates or annual letters,” she said. “All plants I have inspected use materials conforming to the requirements of the NPCA QC Manual; however, sometimes the documentation is not current.” Kamischke went on to state how plants currently meeting the minimum requirements described in the NPCA QC Manual address this issue. “Some plants set up a schedule or Outlook auto-reminders to regularly review or request certification documentation. This is especially true for the annual certificates that can be pulled from the manufacturers’ websites. Plants will set a reminder for the first working day of the year and update all certificates for the year on Day 1.”
“One of the biggest issues we had last year was getting certifications from vendors stating that they conform to specifications,” said one precaster. “You would think that your gravel and sand suppliers would just send you the proper certifications each year for your file, but every year we have to ask them to give us the letter, and it seems to be a big hassle each year.”
One isolated incident started out as a minor problem but ended up being a major headache for the precaster. “We actually had one vendor who gave us mill certifications that did not match the steel he delivered. Unfortunately we did not catch it, but a DOT inspector did,” he said. “All of a sudden, $500,000 worth of product became unacceptable until we could get the proper certifications and prove that the material used in our products was up to spec. The material in question was supposed to be A706 steel, and the real mill certificate had some ambiguous wording on it that the vendor claimed led him to believe that it was equivalent to A706. Unfortunately it was not, and it caused a lot of headaches. If our materials acceptance procedures were tighter, we would not have had the problem.” In this case, the breakdown of the precaster’s materials acceptance procedures led to realized costs.
Various checks and balances can be implemented at a precast plant to ensure that the vendor is in fact supplying the correct materials, and that those materials are meeting a certain level of quality. Some plants opt to place their confidence in their supplier, and there have been no incidents or mishaps during their business relationship. This may work for some precasters and their suppliers, but it may not work for all. Either way, it is always beneficial to have a procedure in place that is documented in the plant-specific QC manual to verify that all incoming materials received meet the specifications and/or minimum requirements.
A plant may choose to simply review the mill certificates and documents it receives and perform a quick visual inspection for each shipment, or a plant may opt to have a more in-depth procedure in place to ensure that all materials match what was ordered. Whatever works best for the precast plant should be outlined in its plant-specific QC manual and enforced to ensure a product not intended for that project/job is not used.
One enterprising NPCA certified plant, Brayman Precast LLC in Saxonburg, Pa., has developed an in-depth receiving inspection and testing procedure at its facility to ensure that all materials received are up to specifications and the materials are what was ordered. In order for materials received by its vendor to be accepted, the plant adheres to the following checks and balances:
- All incoming raw materials shall be accompanied by appropriate material mill certificates, material certifications, and/or vendor inspection and test reports upon receipt and as stated in the purchase agreement.
- Acceptance of these materials is dependent on the compliance of the mill certificates and test reports with the applicable standards.
The plant then takes it a step further and performs additional checks of each type of material received. For example, its reinforcing (deformed bar and mesh) inspections use the following checklist:
- Mill certificates (epoxy coating certificates when applicable) reviewed and logged
- A visual inspection of incoming reinforcing for bar identification, surface condition, finish (plain or epoxy), and/or other visible bar deformities
- Reinforcing incoming inspection record filled out and turned in to the Quality Assurance manager
- The heat numbers by bar type and date received are logged into the incoming reinforcing log
- The reinforcing incoming inspection report and log are filed as part of the Quality Control records
The plant then marks all approved reinforcing bars and welded wire fabric, after having been subject to incoming material inspection and testing, with a green spray paint mark. All nonconforming incoming reinforcing bar and welded wire fabric are segregated from approved material storage and affixed with a red tag. Purchasing is notified of an incoming material nonconformance and then contacts the vendor. The plant stipulates that the material must not be used for production until the material’s disposition is determined through coordination with the vendor and the purchase agreement.
As you can see, this plant does a thorough job on the front end to ensure that all incoming materials are what were intended, and if they are not, immediate corrective action is taken.
Nucor, the largest producer of steel in the United States, responded to an inquiry about what type of documentation it supplies with each shipment of steel, what type of information it includes on the documentation and how a precaster can have a mill certificate re-created if misplaced:
- Mill certifications are created electronically from our main computer system.
- One set of mill certifications is printed at the time of shipment and travel with the steel.
- A second set can be sent electronically to the same order or another location (usually the billing address).
- Mill certifications that were created after Jan. 1, 2009, can be re-created given the correct information.
To ensure the steel/WWR has all the correct properties and is what was purchased, this information must be made readily available.
It is ultimately the plant’s responsibility to have a plan in place to ensure materials are true. If your vendors are not able to supply you with this critical information for every shipment, you could end up with a yard full of unacceptable inventory.
Evan Gurley is a technical services engineer with NPCA.