Q. We recently experienced a number of minor field problems as we coordinated the testing of fresh properties of ready-mixed concrete shipped from several plants to a single pour. When the project was finished, our QC team analyzed our procedures, trying to determine the source of the problems. On the project we standardized our recipe. We had selected single-source suppliers for cement, fly ash, and admixtures. And we used one ge-logic source of aggregates, supplied by different producers.
The review team focused on the aggregate and discovered the core of the problem was the variability of the aggregate gradations in both fine and coarse sizes. On paper, the aggregate gradation reports submitted by each testing location seemed to be consistent, with results falling within our control band. But when our central lab retested samples from all the aggregate sources, it discovered a difference in what was tested and what was reported.
It seems that many of the field labs had performed gradation tests with worn sieves. While most differences were minor, they introduced a degree of unnecessary variability when combined. How can we can minimize our testing variables on these large projects?
A. In “Quality Management System for Concrete Construction, ACI 121,” the committee provides guidance on how project owners are to develop and implement a quality system for their projects. Once adopted, this quality management system, which complies to national standards, advises all parties of their specific roles in a successful project.
In the ACI quality management system, the owner expects the producer to establish and maintain procedures conforming to the project specification. In addition to other activities, the producer should provide documentation on how it calibrates and inspects measuring and test equipment used to verify their product's quality. The document states, “the accuracy of test equipment and measuring should be established and should be consistent with the measurement capability.”
It's important to note that ACI 121 doesn't indicate what the procedures should be, only that there should be written guidelines and an action plan if something fails. Producers can demonstrate they conform to a testing program by adopting into their quality control program the guidelines outlined in ASTM C 1077, Standard Practice for Laboratories Testing Concrete and Concrete Aggregates for Use in Construction and Criteria for Laboratory Evaluation.
Developed for the concrete industry, C 1077 provides a thorough set of minimal standards producers should include in their QC program. According to the ASTM document's scope statement, the standard practice “identifies and defines the duties, responsibilities, and minimum technical requirements of testing laboratory personnel and the minimum technical requirements for laboratory equipment utilized in testing concrete and concrete aggregates for use in construction.”
Sieve accuracy is one of the technical requirements listed in C 1077. Technicians should perform a sieve accuracy verification at least annually on each sieve used in Test Methods C 117 and C 136. The document doesn't specify which of the common methods of verification is preferred, only that one is used. This annual verification is in addition to the technician's daily inspection of the sieve cloth for punctures or obvious defects.
As you discovered in your project, verifying sieve accuracy is important. The wire strand comprising the sieves can wear with use. Abrasive material can thin a wire's diameter. Or repetitive tests can cause the center sections of the testing surface to have more wear than outer areas.
This annual sieve verification can present a challenge for producers. The sieve recertification method can involve a tedious and error-prone manual measurement method. Or it requires extra testing for already busy labs.
One sieve verification alternative that might aid producers in conforming to the C 1077 practice is a new inspection instrument. The tool combines a special vision measurement system with a computer software program that inspects, records, and analyzes sieve wear.
Introduced in 2008, the calibrator automates the verification process. The system is in full compliance with the ASTM E 11 and ISO 3310 standards.
ASCS is a precision optical instrument that scans and measures openings and wire diameters, and supporting sieves up to 18 inches in diameter, with openings from 20 microns to 12.5 mm. The instrument sends results to a spreadsheet embedded with full analysis, including descriptive statistics, pass/fail information, compliance width standards, and raw measurement data.
The manufacturer reports that some testing labs where test sieve certification is critical for day-to-day operations have found the investment worthwhile. For more information on the ASCS, visit the manufacturer's Web site at www.vision-machines.com.