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Quality has become one of the most important consumer decision factors in the selection among competing products and services (Montgomery, 1997). This is true not only for individual consumers but also for large corporations, government organizations and the taxpaying public as a group. Thus, quality is a key factor in pavement construction. But what is “quality”? In its broadest sense, quality is a degree of excellence: the extent to which something is fit for its purpose. In the narrow sense, product or service quality is defined as conformance with requirement, freedom from defects or contamination, or simply a degree of customer satisfaction. In quality management, quality is defined as the totality of characteristics of a product or service that bears on its ability to satisfy stated and implied needs (IQA, 2001). This section discusses a few of the more visible aspects of quality: quality assurance, quality control, acceptance and independent assurance. A deeper discussion of quality as a management philosophy is beyond the scope of this Guide but has been addressed by countless textbooks and seminars throughout the world.
AASHTO and the FHWA subscribe to definitions that designate “quality assurance” as an all-encompassing term, to include “quality control”, “independent assurance” and “acceptance” as its three key components (see Figure 8.1). This Guide uses the same definitions (listed below) for consistency (TRB, 1999):
As defined here, quality control, acceptance and independent assurance are wholly separate components of quality assurance. It is difficult, if not impossible, to influence one component, solely through actions within another component. For instance, acceptance procedures are essentially monitoring methods used to determine whether or not a particular process is meeting quality standards. As such, they can be used to accept or reject material based on its quality but they should never be used as a method to control or improve quality; quality will not necessarily improve based on increased or stricter monitoring alone. In short, no amount of inspection changes the quality of a product or service (IQA, 2001). Quality control (process control) is used to control and systematically improve quality. Furthermore, independent assurance test results should not be used for acceptance or quality control. If they are, the tests are no longer independent and should not be used as if they were.
In a broad sense, every organization has a quality control program; in some manner, they assess production and construction and control their end product quality based on those assessments. Often, this method is not formalized but it exists to some extent in every organization. So then, what is quality control and what is an ideal quality control program?
In the narrowest sense, quality control seeks to control the level of quality being produced in the end product. This level of quality consists of two key components:
Thus, a quality control program consists of (1) the actions and considerations necessary to assess production and construction processes and (2) setting the end product target value and controlling variability. In order for a quality control program to be effective it should (1) base actions and decisions on measurable results and (2) be statistically valid.
Quality control actions and considerations should be based on objective evidence and not subjective opinion. Consider the difference between changing a roller pattern based on (1) an inspector’s opinion that compaction is inadequate or (2) consistently low-density test results. While the first option is subjective, the second option results in unequivocal evidence from which confident actions can be taken. This does not mean that experience and expertise are not valuable but rather that they should be used to determine what measurements to take and how to improve the process. A training manual for Komatsu Ltd. in Japan says it this way (Walton, 1986):
“The first step in quality control is to judge and act on the basis of facts. Facts are data such as length, time, fraction defective and sales amount. Views not backed by data are more likely to include personal opinions, exaggeration and mistaken impressions. Data volume has nothing to do with accuracy of judgment. Data without context or incorrect data are not only invalid but sometimes harmful as well. It is necessary to know the nature of that data and that proper data be picked as well.”
Since variability can only be accurately described and evaluated through statistical methods, quality control involves statistics. Fortunately, these statistics are relatively straightforward. For instance, control charts are frequently used to analyze production results (e.g., production, density, strength).
"Acceptance" is a formal procedure used to decide whether work should be accepted, rejected, or accepted at a reduced payment (Freeman and Grogan, 1998). Specifically, it is the monitoring method used to determine whether or not a particular process is meeting quality standards; it is not, however, a form of quality control. Acceptance procedures simply accept or reject things based on their quality; they do not ensure proper quality standards. Thus, they should never be used as a method to control or improve quality; process controls are used to control and systematically improve quality (Montgomery, 1997).
Acceptance procedures can take one of the following three broad forms: (Montgomery, 1997):
Accept with no inspection is generally used when there is no economic justification to look for defective units or material.
100 percent inspection is generally used where components or material are extremely critical and passing any defective components or material would result in an unacceptably high failure cost.
Acceptance sampling is generally used when there is some economic justification to look for defective material and either (1) some small finite percentage of defective material is acceptable or (2) it is not economical or practical to use 100 percent inspection. Acceptance sampling uses statistics to estimate information about a large amount of material from a small random sample.
Pavement construction typically uses acceptance sampling and thus, many
specifications are statistically based. Section 3, Statistical Acceptance
Specifications takes a more in-depth look at these types of specifications.
Independent assurance is the verification by a third party (not directly responsible for quality control or acceptance) of the product and/or the reliability of test results obtained from quality control and acceptance testing. This independent assurance insures that (1) acceptance test results are accurate and provide a fair and equitable basis for construction acceptance and (2) quality control testing is accurate and thus will properly indicate process quality.
Quality control and acceptance tests are often performed by a multitude of laboratories (agency, contractor and independently owned) within a particular county, region or state. Even if all tests are performed at the same central laboratory, they are at least conducted at different times. Thus, independent assurance insures that sampling variability (the variation in sample characteristics from sample-to-sample that is attributable to variations in sampling technique), testing variability (the lack of repeatability of test results - operators, equipment condition, calibration, and test procedure all contribute to testing variability) and the differences in these variabilities between laboratories and over time do not unduly influence testing measurements. For example, the same sample tested by different laboratories should give roughly the same result.
In simple terms, quality is the extent to which something is fit for its purpose. Quality assurance involves those actions necessary to provide confidence that a product or facility is fit for its purpose. These actions involve quality control, acceptance or independent assurance. Quality control (or process control) involves assessing and adjusting production/construction processes so as to control the level of quality being produced. Acceptance is a formal procedure instituted by the specifying agency used to decide whether work should be accepted, rejected, or accepted at a reduced payment. Finally, independent assurance provides an assessment of the product and/or the reliability of test results obtained from process control and acceptance testing. Together, these three elements form the basis of quality assurance.