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Federal Highway Administration Research and Technology
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Publication Number: FHWA-RD-02-095 |
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This chapter continues the discussion of Phase II of the specification development process. This chapter is intended to provide "how to use" best practices in the development or modification of the QC issues of QA specifications. The steps that are involved in this part of the process are identified in the flowchart in figure 6. The numbers in boxes before the titles of the following sections refer to the corresponding box in the flowchart.
As defined above and used in this manual, QC activities are those QA actions and considerations necessary to assess production and construction processes so as to control the level of quality of the end product. The QC procedures and requirements are made up of two parts: the QC requirements and the quality characteristics to be measured. These are the main ingredients that constitute the QC plan. It is emphasized that the QC function is the responsibility of the contractor.
9.1. Purpose of the QC Plan
It is important to realize that the purpose of the QC plan is to measure those quality characteristics and to inspect those activities that impact the production at a time when corrective action can be taken to prevent appreciable nonconforming material from being incorporated into the project. The QC efforts should also be able to quickly identify that nonconforming material is being made. These purposes should serve as a guide to the decisions used in establishing the requirements and determining the quality characteristics to measure.
The determination of who establishes the QC plan is an important one. Ideally, the QC plan should be the contractor's plan, and not the agency's. The contractor should know what activities to test and to inspect to produce acceptable material. Generally, two approaches have been used by agencies to specify the QC plan that is required. One is for the agency to stipulate the minimum QC requirements and properties that the QC plan must contain. The other is for the agency to specify all the requirements and properties that must be tested. Both have advantages and disadvantages. By stating the minimum requirements and properties, the agency lets the contractor know the least that is required. The disadvantage is that the contractor may view this as all that is necessary for adequate QC, rather than as the minimum. If a minimum-type QC plan is decided upon, the agency may want to require agency review or approval of the plan prior to construction. On the other hand, by stating all the requirements and properties required, the plan is likely to be all inclusive, but the contractor may view the QC plan as the agency's plan rather than the contractor's plan.
If the QC plan requires agency review and/or approval, there are some legal questions that should be considered when making this decision. For instance, if the agency only reviews the QC plan, what constitutes acceptance or rejection of the plan and what are the consequences to the contractor? If approval is required, then does this mean that once approval is obtained the contractor has no further restrictions? Legal interpretations can vary from State to State, so the agency developing the review requirements must determine what is most appropriate for its State.
If the QC plan is stipulated by the agency, care should be taken to assure that the plan is not "overkill", i.e., that it is not so complicated or involved that it is viewed as being unworkable. This is not likely to be a concern if the contractor determines the QC plan.
Addressing the QC requirements provides the contractor with the necessary information needed for staffing, procuring laboratory equipment, etc. It is important that the QC plan submitted by the contractor shows some thought and planning on the part of the contractor and does not simply repeat what the agency requires. Some of the QC requirements that the contractor may need to incorporate into the QC plan include:
Specific information on how to establish and use control charts is available from many sources.(8, 9, 10, 11)
10.1. Qualified or Certified Technicians
A comment regarding the use of the terms "qualified" and "certified" technicians is warranted here. FHWA 23 CFR 637B (6) uses the term "qualified" personnel, as opposed to "certified." One reason that "qualified" was selected is that some States are prohibited by State law to "certify" technicians unless they are State employees. Also, as noted in chapter 3, technician certification usually implies the use of an ongoing recertification program, while technician qualification could be a one-time event. An AASHTO Draft Standard Recommended Practice for Technician Training and Qualification Programs (12) has been recently developed. This document indicates that the terms "qualification" and "technician" are meant to be generic descriptions. The AASHTO QA Guide Specification (7) uses the term "certified technicians." It is generally understood that technicians must be qualified and that one way to assure this is to require them to have undergone some certification procedure.
If certification is required, most agencies have a certification program. Information on this subject can be found in several documents, three of which are referenced herein. The National Institute for Certification in Engineering Technologies (NICET) sponsors the first two documents in the fields of Highway Materials (13) and Asphalt, Concrete and Soils. (14) The third (15) is an outgrowth of a workshop sponsored by the National Quality Initiative (NQI). This workshop was the result of the requirement contained in FHWA 23 CFR 637B (6) that "After June 29, 2000, all sampling and testing data to be used in the acceptance decision or the IA [independent assurance] program shall be executed by qualified sampling and testing personnel."
ExamplesExample QC plans for HMAC and both structural PCC and PCC pavement are provided in appendices B, C, and D. (16)
The measurement of some quality characteristics may be more ideally suited for the QC function than for acceptance. For other properties the decision regarding whether to make the quality characteristic part of the QC function, rather than the acceptance function, may be more arbitrary.
In several instances what, in years past, were considered acceptance tests, now are considered better suited for QC tests. One of the reasons for this is that in past specifications the functions of QC and acceptance were not separated. Examples are aggregate gradation from stockpiles for HMAC and slump from fresh mix for PCC, which are now often viewed as QC quality characteristics because they are better early indications as to whether the quality of the product is in control as opposed to being related directly to in-place performance.
Assure that the quality characteristics chosen for testing are suitable for QC purposes. To reiterate, the purpose of QC testing is to measure those characteristics that impact the quality of the product in such a manner that production changes can be made in a timely manner. For example, while it may provide useful information for the agency and contractor, 28-day concrete cylinder strength is not a good QC quality characteristic. By the time this quality characteristic is measured, too much production has occurred to make the strength results useful as a QC tool. If the quality characteristic to be tested is found to be nonresponsive for QC purposes, another characteristic or test method must be found that is appropriate.
11.1. Typical Quality Characteristics for QC Testing
For HMAC, typical quality characteristics that may be tested for QC include:
For PCC, typical quality characteristics that are tested for QC include:
Two determinations must be made for each quality characteristic considered for QC testing: (1) the determination of requirements for sampling and testing procedures and (2) the required testing frequency. As noted above, there are several ways that have been successfully used to implement a QC plan. The plan may be stipulated by the agency, chosen by the contractor, or it may be a combination of the two in which the agency chooses some ingredients and leaves others up to the choice of the contractor. The method chosen will impact the way that the QC plan is implemented and the decisions that must be made throughout the implementation of the plan.
What data are necessary for the agency to consider will be related to what decision has been made regarding which party will develop the QC plan. If the agency decides to stipulate the QC plan requirements, or to stipulate minimum QC requirements, then data will need to be analyzed so that decisions regarding test frequencies and, possibly action or control limits, can be made. If the contractor is to be responsible for developing the QC plan, then it may not be necessary for the agency to evaluate as much data at this point in the process. In an ideal situation in which the contractor is fully responsible for the QC function, the agency would, in theory, not need to analyze any data. However, during the initial implementation of QA specifications, most agencies have not been comfortable giving the contractor total responsibility for developing the QC plan. Therefore, it is likely that the agency will choose to evaluate data for prospective QC quality characteristics testing.
To establish the action limits for control, available data must be analyzed to determine the variability and the ability to hit the "target." Whether these limits are established by the agency or the contractor depends upon the decision regarding whether the agency will stipulate minimum or total QC requirements. If the agency allows the contractor to develop the QC plan, then the plan can be operation-specific, i.e., geared to the contractor's specific plant operation. If the agency decides to stipulate QC requirements, then these requirements will have to be generic in nature so as to apply to many different contractors and different plant operations. The best QC scenario is for the QC plan and the properties tested to be specific to each contractor plant or operation.
If the QC plan is operation-specific, the data should come from prior production of similar product from that operation. In this case, the contractor will be responsible for gathering the data and establishing the operation-specific action or control limits. In general, a measure of the average and the variability must be examined over a period of time and used to establish control/action (and, possibly, warning) limits.
If the QC plan is generic, the agency must evaluate data from a number of typical operations to establish these limits. Published reports of product variability are another source of information that can be used for establishing at least preliminary estimates for action limits. If this approach is taken, the limits will not be as useful since they will not have been developed for each specific plant or operation. For this reason, if the agency does choose to establish initial statewide action or control limits, they should move as quickly as possible toward transferring this responsibility to the contractors so that they can develop operation-specific limits for each plant or operation.
Specification limits should not be used for control/action limits. Control limits are based on the variability of the specific operation or process, and are not derived from the specification limits. If the contractor's process variability does not allow control limits that are completely within the specification limits, then this indicates that the contractor cannot consistently meet the specification requirements.
Since QC is recognized as a contractor or material producer function, the agency should be wary of establishing control or action limits to use on a statewide basis. By necessity, to allow for the many different contractors and plant operations in each State, these limits would have to be set very wide, i.e., conservatively. As such, they may be of little actual benefit in establishing useful control limits for a specific plant operation. However, the agency may choose to initially establish such limits until the industry has become sufficiently knowledgeable for each contractor to take over the full QC function and establish operation-specific limits that are likely to be more restrictive than those initially stipulated by the agency.
13.1. Analysis of Data
The collection and analysis of the QC data should be compatible with the intended use. That is, the data must be collected for the specific quality characteristics in the same manner and under the same general conditions as they will be used. For example, historical data on aggregate gradation from one quarry may not be appropriate for use is establishing control limits for aggregate from a different quarry. Or, historical data for dry aggregate gradations would not be appropriate if the new QC plan called for a washed gradation analysis.
The analysis should involve the statistical properties that will be used to establish action limits. This may be the average, moving average, range, standard deviation, etc. Detailed discussions on the use of each of these measures for QC using control charts are available from many references.(8, 9, 10, 11)
13.2. Use of Historical Data
From where can the necessary data be obtained? Care must be taken when using historical data because they may not always be unbiased. In fact, historical data may frequently be biased.
To be valid, the historical data must have been obtained using a random sampling procedure. That is, the sampling locations should have been selected at random, and not systematically or by the judgment of the inspector. When judgment is used for sample selection, bias can be introduced because there may be a tendency to select a sample location where the material looks questionable in an effort to ensure that "bad" material is not incorporated into the project. On the other hand, there could be a tendency to select a sample location where the material looks particularly good in an effort to ensure that the material is accepted. Either of these will provide a biased estimate of the actual properties associated with past construction.
Another potential problem with historical data is the past process of selecting and testing a second sample when the first sample failed to meet the specification requirements. If the second sample met the specifications, and as a result the first sample was disregarded and its value not recorded, then the historical data will tend to underestimate the actual variability associated with the process being measured. The validity of historical data must be scrutinized thoroughly before deciding to use them to establish QC limits.
Are there sufficient data to make the decisions that will form the basis of the QC plan? If the QC plan is operation-specific, the data for the QC properties must be available from the plant operation, and it will be the contractor who should obtain and analyze these data. These data should have been collected in the same manner as that in the QC plan that will be implemented. This is preferably from a random sampling plan, however, under certain conditions, QC data may be obtained in a selected manner. This is particularly true if a "check" sample is obtained to verify "out of control" or nonconforming product. In this case, all data must be recorded and used in the analysis as to whether the product was truly in or out of control.
If the QC plan is to be generic, the data must come from a number of typical operations and should be randomly sampled. In this case, the data will likely be collected by the agency. These data may come from agency historical records, or may be provided by various contractors. In either case, the same stipulations in the previous paragraph regarding random sampling and collection procedures apply.
If there are not sufficient data, the needed data must be obtained. If sufficient data are available, the next step is to perform the analysis. What constitutes sufficient data may vary from agency to agency. Data should be collected from operations of a number of different contractors, and covering all of the districts or geographic regions of the State. The data should not be just from operations considered to be superior, but should cover a range of operations from the best to those that are considered to be just acceptable. The limits should not be established based on the best operations, but should be a compromise such that they can be achieved by those operations that the agency deems to have been providing acceptable material. It is likely that data from at least 10, and preferably up to 20, projects will be needed for analysis.
If sufficient data are not available, there are several potential sources for these data. If the plan is developed by the contractor to be operation-specific, the most appropriate source is from the operation that is to be controlled. However, when the agency decides to initially develop a generic plan, the data should come from several typical operations. An alternative source for these data may be provided from literature reviews. Caution should be observed in this case because there is often a concern as to how applicable data from other places or sources, such as those provided by literature reviews, are to the operation for which the QC plan is being developed. It is important to ascertain that the data from other sources are applicable since the contractor will be held to the limits established from these data.
The sampling procedures for control, including the point of sampling and the method for selecting the sample, must be established. The quality characteristics, test procedures, and inspection activities that are best related to early indications of process control should be stipulated. These decisions will depend on the type of operation for which the QC plan is being developed. Also, a testing frequency must be established that attempts to create a balance between enough tests to determine if the product is in control but not so many tests as to be impractical. The agency or contractor may decide to use increased testing frequencies initially, and then to reduce these as the operation is shown to be in control.
Normally, the sample should be selected in a random manner. The reasons for this are discussed above and again in chapter 5. The results must be unbiased and all results must be reported. However, there are occasions where nonrandom samples are typically allowed for QC samples. This may happen when an "out of control" result is obtained. The reason for this result could be a sampling or testing problem and therefore not due to a material or production issue. Rather than make an immediate change in the production process, a retest may be taken to verify or dispute the original "out of control" result. When this is done, all test results should be reported and the retest should be noted as such.
16.1. Operation-Specific QC Procedures
Realistically, the QC procedures are different for each operation and, ideally, the decisions should be operation-specific. For example, some operations may require sampling from different locations than others. Likewise, operations with a history of QC problems will require more frequent sampling and testing than operations that typically have had few problems. One of the keys to achieving a balance in testing frequency is to relate the testing frequency to the rate and consistency of production. If the production tends to be continuous and consistent, less frequent testing may be permissible than if there are many interruptions. The contractor should be in the best position to know what tests are the best indicators of control and what frequency is necessary for control.
16.2. Generic QC Procedures
Although the ideal QC procedures are operation-specific, many agencies do not require each contractor to establish these. There are practical reasons that agencies choose to make them generic rather than operation-specific. For instance, an inadequate QC staff to perform the level of testing that should be done may inhibit the contractor. Or the contractor may not want to test more often than the competition because of the impact on staffing and cost. These are some of the reasons that many agencies stipulate at least minimum QC plan procedures. A generic QC plan has potential disadvantages both to the contractor and to the agency. The sampling and testing procedures and the test frequency are stipulated and will not be operation-specific. This is a disadvantage for the contractor that has few occasions of "out of control" product. It also will be a disadvantage to the agency when encountering a contractor that needs more frequent sampling and testing to maintain control.
16.3. Establishing QC Tests and Frequencies
If the agency has opted for contractor-developed, operation-specific procedures, then the choice of testing procedures and frequencies is up to each individual contractor. However, as noted above, many agencies decide to stipulate at least minimum QC testing requirements. The selection of the testing procedures to be required and the testing frequency to stipulate will be done by the joint agency/industry task force that was established during Phase I. While the final decision on the QC requirements to stipulate is made by the agency, input should be sought from the industry representatives on the task force. This input regarding what tests and frequencies are necessary to control an operation should be given careful consideration by the agency when deciding upon QC requirements to stipulate.
Example QC plans, which present suggested QC tests and frequencies for HMAC and both structural PCC and PCC pavement, are provided in appendices B, C, and D. (16)
As a final check, each quality characteristic that was selected for QC purposes should be reconsidered once data have been collected, analyzed, and reviewed by the agency/industry task force. A final decision is then made whether or not to include the quality characteristic in the stipulated QC requirements. If the decision is to not measure the quality characteristic for QC purposes, then the task force should decide whether the quality characteristic should be considered for possible acceptance testing, or should be eliminated from further consideration.
At this point, the QC requirements and quality characteristics for QC sampling and testing have been identified. The QC portion of the specification development process is now ready to proceed to the development of the initial draft QA specification.
