Quality Control Flashcards
What is quality control?
Quality control: Those quality assurance actions and considerations necessary to assess production and
construction processes so as to control the level of quality being produced in the end product. This
concept of quality control typically includes sampling and testing by the contractor to monitor the process
but usually does not include acceptance sampling and testing by the agency/owner. Also called process
control.
This level of quality consists of two key
components:
Target value. This is the goal set for a certain material characteristic. As a minimum it should
conform to standards and be achievable. For example, on a specific contract the specified inplace
HMA density might be 92-percent of theoretical maximum density. Therefore, a
contractor may set an in-place density target value at 93-percent of TMD. This meets (in fact,
it exceeds) the standard.
2. Variability. This describes how much a process varies from item-to-item (or location-tolocation).
A process that meets the target value on average but is highly variable will result in
pavement locations where the material characteristic is unacceptable.
Quality control program consists of (2) :
the actions and considerations necessary to assess production and construction processes
(2) setting the end product target value and controlling variability.
r 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.
QUALITY ASSURANCE – QA
All those planned and systematic actions necessary to provide confidence that a
product or facility will perform satisfactorily in service. Quality assurance addresses the overall problem
of obtaining the quality of a service, product, or facility in the most efficient, economical, and satisfactory
manner possible. Within this broad context, quality assurance involves continued evaluation of the
activities of planning, design, development of plans and specifications, advertising and awarding of
contracts, construction, and maintenance, and the interactions of these activities.
PURPOSE OF QC PLAN
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 HMA 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.
Typical Quality Characteristics for QC Testing
For HMA, typical quality characteristics that may be tested for QC include:
• Aggregate quality, including fractured faces, sand equivalency, cleanliness, etc.
• Nuclear density.
• Gradation of critical sieve sizes.
• Plant and discharge temperatures.
• Degree of aggregate coating.
• Moisture content of fine aggregate and/or of finished mix.
Sampling by Attributes Using Binomial Distribution
• Attribute data is the simplest kind.
• You sample some number of items and you classify each item as either
having some attribute, like being defective, or not.
• The attribute plan decision rule will reject if too many points are “out”.
Typically, the maximum number of defectives allowed in the sample is
calculated with the binomial distribution.
• Uses Binomial Distribution
Sampling by Variables Using Normal Distribution
• Variables data contains more information than attribute data per data-point.
This is because it allows you to assess “how much” or “how bad” or “how
good” rather than just “yes its defective” or “no its not defective”.
• Uses the original measurements.
• The variables plan decision rule will reject if the sample average of the
measurements goes outside of some calculated acceptable range. Typically,
the limits of that acceptable range are calculated with the normal distribution.
• Uses Normal Distribution.
Types of Quality Control
- Control of Methods
2. Control of End‐Results
Control of Methods
to be on the site and supervise the Contractor during the execution of the works.
• At the same time the field staff will perform simple measurements, such as the
recording of the thickness of fill layers, the temperature of asphalt material, and the
slump of cement concrete.
• Method control is carried out according to the type of work. Where the work
method is of considerable importance and requires constant supervision to achieve
the quality, or where In some case, the quality Is difficult to Improve on, there
should always be a field engineer on the site. Examples are the ramming of piles or
the laying of asphalt, and concreting etc.
• Where work methods are of less Importance or quality Is constantly being achieved
by the contractor, there may be no need for continuous surveillance. Examples are
excavation and compaction of soil.
Control of End‐Results
End‐result control includes field tests e.g. Control of the evenness of completed
pavement layers and laboratory tests, e.g. Marshall tests on asphalt materials.
• Other tests are a combination of field and laboratory tests. An example of this is
the compaction control of earthworks where the achieved density is determined by
means of a field test. And where the AASHTO density with which the result should
be compared is found by means of a laboratory test. End results control is carried
out by laboratory technicians, and most of the work consists of laboratory tests.
• The frequency of end‐result control depends on the quality parameters that are to
be checked.
• Parameters which can vary considerably are continuously controlled. Examples are
the composition of asphalt materials and the compaction of asphalt courses. As
regards regulating laboratory tests the specification usually determines the
number of tests.
