Understanding Typical Values, MARV and Minimum Values
Posted on 11.22.2021 by Chastity Adkins, Technical Manager
Our team is often asked to explain the difference between Minimum Average Roll Value (MARV), Typical Value (Typical), and Minimum Value (MV). Since geosynthetics are engineering materials, it is important that the terminology defining property values and how it applies to design and specification is clearly understood.
This blog post will help engineers, distributors, contractors, and owners in the geosynthetic industry understand these terms and their significance to the design and specification process. Additionally, since all Propex geosynthetics are manufactured according to the following definitions, this post will provide a greater understanding for how these terms apply to Propex products.
RELEVANCE TO MANUFACTURING QUALITY CONTROL (MQC)
All manufacturing processes inherently contain variation, to some extent or another. Additionally, all test methods used to quantify material properties further contribute to the variability. To properly account for this variation and accurately represent the material characteristics of these products, most manufacturers of geosynthetics publish properties as MARV, Typical, or MV values. These terms are defined in American Society for Testing and Materials (ASTM) D-4439, Standard Terminology for Geosynthetics.
When a manufacturer develops a new product, a sufficient quantity of the material must be tested to determine basic material properties and evaluate variation. Using simple statistics, the material’s MARVs, Typical Values, and MVs can be determined.
For example, say we are interested in determining the MARV, MV, and Typical Value for the grab tensile strength of GEOTEX® Nonwoven 601 geotextile. All results from the grab tensile strength test performed during the production period under consideration are compiled on a frequency distribution plot and analyzed. Figure 1 shows grab tensile strength data from GEOTEX 601 geotextile. This data is considered normally distributed.
The Typical Value is defined in ASTM D-4439 as the average or arithmetic mean of all historic test data points. On Figure 1, the Typical Value is 180 pounds.
DEFINING MARV, TYPICAL AND MV
Per ASTM D-4439, the MARV yields a 97.7 percent degree of confidence that any samples taken from quality assurance testing will exceed the value reported. For normally distributed data the MARV can be determined by subtracting two times the standard deviation from the average or Typical Value. The data in Figure 1 shows the MARV is 160 pounds.
ASTM D-4439 defines the MV as yielding a 99.9 percent degree of confidence that any sample taken from quality assurance testing will exceed the value reported. For normally distributed data the MV can be determined by subtracting three standard deviations from the average or Typical Value. Data in Figure 1 shows the MV is 150 pounds.
Once the MARVs, Typicals, and MVs are published, the manufacturer continues to monitor, quantify, and document the properties of the geotextile to assure that the material stays within the published values. This is done by testing a specific frequency determined by the variability of the product and/or process and making process adjustments on-line when necessary.
MARV is the industry standard for published properties of geotextiles in the United States. AASHTO and many other specifications state their requirements in terms of MARV. ASTM D-4439 notes that the definition of MARV applies to strength properties but may not apply to properties such as apparent opening size, permittivity, and Ultraviolet (UV) stability. MARV is relevant to the manufacturer’s quality control program and should not be calculated for a limited volume such as a truck load.
QUALITY ASSURANCE (QA) / CONFORMANCE RELEVANCE
Now that we have learned how these terms are applied by manufacturers, let’s consider how they can be properly verified through conformance or quality assurance testing.
Let’s take a project site where a truckload of 175 rolls of the same style geotextile have been delivered. Although defined differently by each manufacturer, these 175 rolls constitute a single lot of material in this example. It is required in the project specifications that the Construction Quality Assurance (CQA) firm verify the delivered product is in conformance with the project specifications. One of the requirements of the project specifications is as follows:
Grab Tensile Strength ASTM D-4632, Standard Test Method for Grab Breaking Load and Elongation of Geotextiles, of 160 pounds. (MARV)
To properly determine if the materials are in conformance with the specification, the CQA representative refers to ASTM D-4354, Standard Practice for Sampling of Geosynthetics and Rolled Erosion Control Products (RECPs) for Testing. The first table in ASTM D-4354 instructs that a full roll sample with a width of three feet long, needs to be taken from a minimum of six randomly selected rolls. These samples are then sent to a Geosynthetic Accreditation Institute – Laboratory Accreditation Program (GAI-LAP) accredited testing laboratory. At the lab, ten specimens are taken from each sample and tested per ASTM D-4632. In conformance with ASTM D-4759, Standard Practice for Determining the Specification Conformance of Geosynthetics, the results from the ten samples are averaged, thus providing an average sample value (average roll). The lowest of these average roll values is 161 pounds. This is greater than the required project specification value of 160 pounds, therefore the entire truckload is acceptable.
It is important to note that even though nine individual specimen values were below the required 160 pounds, the truckload is acceptable per ASTM D-4759. If the lowest average roll value for the tested material was below 160 pounds, ASTM D-4759 states that the lot would be re-sampled and retested. This holds true except in the case where all of the individual average roll values are below the required value. In this case, ASTM D-4759 states the entire lot should be rejected without further sampling and testing.
Let’s now say that the average roll value for sample #3 and sample #5 were below 160 pounds. Following ASTM D-4759, roll #3 and roll #5 are not included in the re-sampling and are disposed of in a manner agreed upon by the purchaser and the seller. The remaining 173 rolls in the lot are randomly re-sampled and tested. The lot is accepted if the lowest average roll value of the re-sampled rolls is greater than or equal to 160 pounds. The entire lot does not pass if the lowest average roll value is below the specified value.
Propex is committed to providing high quality products with solid documentation. A key element of our Ringgold plant is our onsite testing facility and lab. Each day, nearly 35 tests are performed per material for quality assurance. This also allows us traceability for each product that is shipped from our facility and ensures that we are delivering top quality products to our customers. Our lab is certified by the Geosynthetic Accreditation Institute/Laboratory Accreditation Program and American Association for Laboratory Accreditation.
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