Better Roads Digital Magazine
Issue link: http://read.dmtmag.com/i/165348
RoadScience by Tom Kuennen, Contributing Editor Quality Management How to handle materials, from the source to the site Y ou've designed and are building a pavement to resist fatigue and thermal cracking, stripping and raveling in ways that are suggested in Better Roads' five-part series. But your job isn't done. Now is the time to maintain an iron grip on the quality of the construction materials at the source and of construction practice in the field. Fortunately, a framework exists to make your job easier. Quality control (QC) and quality assurance (QA) practices – adapted from the manufacturing sector – provide a path for staying focused on the way a project is executed in the field. QC Compared to QA Quality control, also called process control, is defined as the material producer's and field contractor's methodology for testing, inspection and oversight of all materials and processes so you can obtain a quality, constructed product that meets specifications. If QC describes the observation techniques and activities used to maintain quality in a project, quality assurance describes activity in the quality-control chain that will ensure specs will be achieved and a quality project attained. In a construction project, QA describes statistically based acceptance specifications, which measure quality characteristics – such as asphalt content, aggregate gradation, concrete compressive strength or soil density. These are related to pavement performance and can be used to pay the contractor for the level of quality provided, according to Moin Uddin, PhD, E.I.T., East Tennessee State University-Johnson City; Paul M. Goodrum, P.E., Kamyar C. Mahboub, P.E., and Arnold Stromberg, University of Kentucky-Lexington, in their 2012 Transportation Research Board paper: A Solution to Non-Normality in Quality Assurance Acceptance Quality Characteristics Data. As such, QA exists within a QC structure, quantifying data obtained in the QC process and using it to calculate payment to the contractor. QA refers to the systematic measurement, comparison to a standard, monitoring of processes and an associated feedback loop that prevents errors. QC, on the other hand, is focused on process outputs. "FHWA's (Federal Highway Administration) recommended [QA] specification compliance measure, percent within limits (PWL), is typically used to quantify quality provided," the professors write in their paper. "PWL is based on the assumption that acceptance quality characteristic (AQC) data obtained from a lot or sub-lots are normally distributed, a lot [being] defined as a quantity of similar material, construction or units of product subjected to either an acceptance or process control decision." The statistical approach is not foolproof and might shortchange contractors, Uddin, Goodrum, Mahboub and Stromberg say. "If the process shows evidence of a significant departure from normality, then the quality measures calculated may be erroneous," they write. "When the population distribution is non-normal, AQC data tend to misdirect contractor payments, which can incorrectly penalize contractors who deliver acceptable construction materials and reward contractors who deliver poor construction materials." Their paper describes a new statistical approach involving a search algorithm that can substantially reduce pay biases due to non-normality, even when lot sample size is small. "The method is robust with high accuracy, and it leads to fair and equitable payment to state agencies and contractors," they say. "[A]ny state highway agency interested in implementing it can easily automate it. Instead of removing or replacing test data that result in non-normality with high spread, state agencies can treat the data as it is and use the proposed method to calculate more accurate pay factors." Getting Compaction QC Right For hot-, warm- and cold-mix asphalt pavements, quality control is required because compaction specifications must be met, and there will be penalties for undercompaction as well as overcompaction. 4 September 2013 Better Roads RoadScience_BR0913.indd 4 8/30/13 8:15 AM