A staff engineer reports on pre-assessment audits for the NPCA Plant Certification Program.
By Evan Gurley
Editor’s Note: We continue with part two of this four-part series about common problems encountered during a typical pre-assessment plant inspection. All deficiencies listed in this series relate to the six chapters of the NPCA Quality Control Manual. Part 1, published in the January-February 2012 issue of Precast Inc. and available at https://precast.org/publications, listed the first five deficiencies. We continue here with five more.
6. Lack of documentation on pre-pour/post-pour inspection sheets
Pre-pour and post-pour inspections are intended to be a useful tool and quality control measure for the plant personnel, not a “pencil-whipping task” that adds no real benefit or continuous quality control improvement. As outlined in Section 4.3 in the NPCA QC Manual, pre-pour inspections shall be performed prior to casting each form.
For each form used, the following must be checked: form dimensions, tightness, cleanliness, release agent application, positioning and securing of reinforcing, embedded items and block-outs.
Section 4.6 of the NPCA QC Manual states that post-pour inspections must be performed on each product. The inspections must document: any damage, excessive bug holes or honeycombing, poor dimensional tolerances, or other problems such as exposed reinforcing. The minimum required checks are heightened for certain products as specified in Chapter 6 of the NPCA QC Manual.
A usual occurrence is that a plant will have a pre- and post-pour inspection sheet in place, but the check sheet developed will not adhere to the minimum requirements outlined in the NPCA QC Manual. Rather, the checklist produced will be vague and will not provide space for comments or additional notes, such as actual dimensions taken of the product. It is not uncommon to see 20 checklist pages of red check marks in boxes indicating that for the last 20-plus production days, not one quality control issue was noted when setting up or stripping precast product.
Plant personnel performing these checks should realize that inspection lists are intended to be a continuous-improvement tool and not a cause for supervisory repercussions if staff noted that something was less than perfect. The important thing is that any quality control issue discovered is properly resolved, and a positive learning experience and greater product quality are the intended results.
7. Improper positioning of reinforcement
Proper positioning of reinforcement is a critical procedure, and starting in 2012 it will also be a critical requirement in the NPCA Plant Certification Program. As stated in Section 4.3.3 in the NPCA QC Manual, reinforcing steel must be positioned as specified by the design, and the concrete cover must conform to product requirements.
Even if the product is not made in accordance with any ASTM or other industry specifications, the NPCA QC Manual states that the concrete cover shall not be < ½ in. at any point, and strongly recommends a cover of ½ in. or greater. This is true for any product. ACI, ASTM and other standards with greater coverage requirements take precedence over what is outlined in the NPCA QC Manual.
The real problem observed in some plants is that staff is simply not aware that serious structural damage can result when proper reinforcement positioning or coverage is not achieved. Proper positioning of reinforcement is a big deal, and an incorrect bar size or placement errors can lead to failure of the precast concrete structure. For instance, some plants try to use rolled welded-wire reinforcement for straight-walled products, and without proper mechanical straightening, this practice causes numerous concrete coverage issues. Liberal use of chairs, spacers and positioning wheels is strongly encouraged, especially for smaller-diameter bars or wire.
8. Improper consolidation of concrete
Concrete must be consolidated to minimize segregation of the concrete. This means that internal vibrators need to be lowered vertically into the concrete, without being forced, downward until the tip of the vibrator reaches the bottom of the form or until it penetrates into a previously consolidated lift. Vibrate the concrete until air bubbles within the vibrator’s field of action stop coming to the surface. Withdraw the vibrator slightly slower than it was lowered. Reinsert the vibrator in an adjacent area, making sure the fields of action overlap. Repeat the vibration process until all the concrete in the product has been consolidated.
This is how concrete should be consolidated with an internal vibrator, but it is not always what one finds during pre-assessment visits. Internal vibrators should not be used to move concrete laterally and should not be dragged horizontally throughout the concrete. Internal vibrators should not be forcefully jammed into the concrete for a brief duration of time and quickly removed before air bubbles stop coming to the surface.
For thin-wall forms with minimal volume, internal vibrators may be laid horizontally to achieve proper consolidation, but vibrators must not be dragged horizontally through the concrete; they must be lifted and placed vertically throughout the concrete mass. Dragging the vibrator through the mix can cause separation of the aggregate and the cement paste, resulting in unacceptable bug holes or honeycombing in the finished product.
Over-vibration is also frowned upon, as this can cause excessive segregation of the aggregates and the cement paste; reduction in the amount of entrained air in the concrete; and less durability of the concrete exposed to freezing-and-thawing environments.
9. Guessing at moisture content
“I can eyeball the aggregates and determine if I need to add or subtract water from the mix.” This is something we have heard as a way to gauge aggregate surface moisture content. Eyeballing the aggregate surface appearance (wet, damp or dry) is not an acceptable way to accurately measure the aggregate moisture content, even if you have raptor-like eyes.
Nor will eyeballing meet the requirements outlined in Section 5.2.2 in the NPCA QC Manual, which states: For conventional and/or dry-cast concrete, aggregate surface moisture content shall be determined at least once per day in accordance with ASTM C70, “Standard Test Method for Surface Moisture in Fine Aggregate,” by alternative methods such as moisture meters or probes, or by ASTM C566, “Standard Test Method for Total Evaporable Moisture Content of Aggregate by Drying.”
If you’re using self-consolidating concrete (SCC), you’re dealing with a new beast altogether. SCC is very moisture-sensitive, therefore precise control of the water content is essential. If moisture probes are used for aggregates in SCC, the aggregate surface moisture content must be determined at least once per day prior to making the first batch of SCC by tests mentioned previously. If the plant does not use moisture probes or meters, then the moisture content must be determined at least once per day prior to making the first batch of SCC and then once every four hours of elapsed time after the first batch for the duration that SCC is being mixed.
Verifying aggregate surface moisture will allow for necessary adjustments in mix water as the moisture content of the aggregates changes throughout the day. Slump flow and VSI (viscosity stabilizing index) testing will confirm whether an SCC mixture is within specification. Slump flow and VSI will become necessary for confirmation if ASTM moisture tests are not performed on a regular basis (every three mixes).
10. Failure to provide the correct number of testing specimens
Nobody likes being scrutinized for his or her every move, especially when working under limiting time constraints and facing serious consequences for product failure. Therefore, we empathize with the plant QC inspectors who must make cylinders and test for air, slump, temperature and unit weight – all in front of an inspector and probably their boss! It may be a mundane task, but it is essential that the plant QC inspectors perform concrete testing correctly, with the proper frequency, and maintain the required concrete testing records on file.
For the most part, QC inspectors do a great job performing concrete testing and keeping documentation on file. Sometimes, though, worrisome issues arise. For instance, when making concrete compressive strength cylinders, two concrete samples, not one (for each date tested), require a cylinder to be cast. So, if you were to test the concrete specimen at seven and at 28 days, a total of four cylinders would need to be cast (two specimens at seven days and two at 28 days).
That goes for next-day stripping strength breaks as well. Many plants perform next-day tests daily to ensure strengths are met before stripping the product from the forms, while other facilities adhere to the NPCA requirements stating that next-day or stripping-strength breaks are required once per quarter year. More often than not, QC inspectors who test daily will forget about the two-specimen rule and cast only one cylinder. This practice is not in compliance with the NPCA QC Manual and will be pointed out and duly noted during the inspection.
For other testing procedures, it is always strongly encouraged that those who perform the test take a quick minute or two to refresh themselves on the proper way to perform the tests according to the ASTM testing procedures. Just like the ACI Grade I test, the NPCA inspector will want to see all concrete testing correctly performed. If something is done incorrectly, even something as small as the number of roddings in a slump test being off by one stroke, the inspector must issue a testing deficiency just as would occur in the ACI Grade I test.
End of Part 2, to be continued in Precast Inc. May-June 2012.
Evan Gurley is a technical services engineer with NPCA.