ASTM Committee C08 on Refractories— An overview of the definitive standard tests for refractories

In this review of current standard tests from “ASTM Committee C08 on Refractories”¹—which includes ruggedness test, interlaboratory study, and precision and bias overview—both published information and opinions of producers and end-users are presented.

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Refractory raw material producers, producers, and users rely on ASTM standards as the definitive standards for comparing refractory materials in the United States and internationally. The standards are well written, easily followed, and lead to repeatable results. However, problems can occur in the use of standards. In many cases, the referenced standard is outdated, and the user needs to access and follow the most current standard. In other cases, the standard is referenced yet not followed. Sometimes, the standard is followed to the letter, but results are not used correctly for a variety of reasons, including

1. A lack of understanding of the ruggedness test that was used to determine what factors are critical to be controlled to obtain valid results.
2. The interlaboratory study has not been completed so results from test to test and especially from laboratory to laboratory are suspect.
3. The standard is correctly followed, all factors are correctly controlled, and reporting is correct. Yet the results are not used correctly due to a lack of understanding of precision within the laboratory and between laboratories as compared to bias.

An overview of ruggedness test, interlaboratory study, and precision and bias should assist in eliminating the before stated difficulties and make the results of standards more useful. A review of most used standard methods of test for evaluating performance and data sheet values for users of refractories should assist test users and results users more fully understand the significance of the results. Both published information and opinions of producers and end-users are presented as hearsay evidence should allow users to determine the tests needed to obtain pertinent results.

Developing a test method

ASTM has identified a procedure to develop new and revised test methods. ASTM requires all test methods to have precision and bias statements that are based on interlaboratory test methods. Although there exists a wide range of statistical procedures, there is a small group of generally accepted techniques that are very beneficial to follow. ASTM E 1488 “Standard guide for statistical procedures to use in developing and applying test methods” is designed to provide a brief overview of the procedures and suggest an appropriate sequence of carrying out the procedures. Figure 1 illustrates the recommended procedure for developing a standard test.

This procedure is generally followed as closely as possible to develop good repeatable test procedures. Other standard organizations do not always follow similar procedures which in part may lead to poor test results and understanding. It is important that test users and test interpreters understand these steps and the implications, which are covered in more detail in the following sections.³

Ruggedness test

The purpose of a ruggedness test is to identify those factors that strongly influence the measurements provided by a specific test method and to estimate how closely those factors need to be controlled.

Because there usually are many environmental factors that might be considered in a ruggedness test, it is customary to use a “screening” type of experiment design that concentrates on examining many first order effects and generally assumes that second order effects such as interactions and curvature are relatively negligible. Often in evaluating the ruggedness of a test method, if there is an indication that the results of a test method are highly dependent on the levels of the environmental factors, there is a sufficient indication that certain levels of environmental factors must be included in the specifications for the test method, or even that the test method itself will need further revision.⁴

Interlaboratory study

Interlaboratory testing of a method is carried out by a chosen group of laboratories that are available and willing to undertake the test work. The coordinator of the program must ensure that every participating laboratory has appropriate facilities and personnel and performs the method exactly as written. If this goal is achieved, the statistics developed during the interlaboratory study will be adequate for determining if the method can produce satisfactory precision in actual use. If the program includes certified reference materials, the test data also provide information concerning the accuracy of the method. The statistics provide a general guide to the expected performance of the method in the laboratories of those who will use it.⁵

Precision and bias

The primary purpose of including results of various studies, including an interlaboratory study, are to provide estimates of precision and bias.³

Precision is a measure of the variability among test results conducted on the same material in the same laboratory by the same person (repeatability) and between laboratories (reproducibility). Usually, the repeatability is smaller than the reproducibility.

Bias refers to the difference between a population mean of the measurements or test results and an accepted reference or true value. Bias is measured for very few refractory related tests as the results of the test are only defined by the test. That is, there is no standard reference value for most refractory materials or properties.³

Repeatability index (r) is the statistic given in the method that estimates the expected range of results reported in the same laboratory on different days, a range that is not exceeded in more than 5% of such comparisons.

Reproducibility index (R) is the statistic that estimates the expected range of differences in results reported from two laboratories, a range that is not exceeded in more than 5 % of such comparisons. Use R to predict how well your results should agree with those from another laboratory.

To use R, first obtain a result from the test method, then add R to, and subtract R from, this result to form a concentration confidence interval. Such an interval has a 95% probability of including a result obtainable by the method should another laboratory analyze the same sample. For example, a result of 46.57% was obtained. If R for the method at about 45% is 0.543, the 95% confidence interval for the result (that is, one expected to include the result obtained in another laboratory 19 times out of 20) extends from 46.03 to 47.11%.⁵ Most of the R values for refractories are much larger than this example.

Standard methods of test

Review of most commonly used test standards in the refractories industry. There is no specific order to the review. The author has listed the standards in the order of most used, primarily for technical data sheets and specifications, and most questioned results. The author apologizes if a standard is not covered. Precision statements were simplified by converting values on the standard to percentages and rounding. Do not use this document as definitive precision values. The standard gives much more detail on the precision statement.

C16-03(2012) Standard Test Method for Load Testing Refractory Shapes at High Temperatures

The ability to withstand load at high temperature is a measure of the high-temperature service potential of the material.

Precision
Average percent deformation
Dense 70% alumina brick—% Deformation +/–29% in laboratory; +/–20% between laboratories

This test may have been the first refractory standard developed by ASTM C08. It is still a valid test and could be used for service limit, but it may not be used very often as it is not often requested by end users or shown on data sheets.

C133-97(2015) Standard Test Methods for Cold Crushing Strength and Modulus of Rupture of Refractories

The cold strength of a refractory material is an indication of its suitability for use in refractory construction.

Precision
Cold crushing strength (CCS)
Dense brick— CCS +/–12% in laboratory; +/–45% between laboratories
Dense castable— CCS +/–7% in laboratory; +/–23% between laboratories
Modulus of rupture (MOR)
Dense brick— MOR +/–10% in laboratory; +/–40% between laboratories
Dense castable— MOR +/–9% in laboratory; +/–20% between laboratories

This test is the most controversial. Many users do not follow the standard to the letter, leading to results varying by more calculated precision. Just changing from cardboard to the prescribed cellulose fiber wall board can lead to increase of measured value by 50%. (By the way, the standard does not specify the density of the cellulose fiber board, density of the fiber board changes the values.) The standard allows for specimen sizes that are different than those for which the precision was calculated. It is easy to show by statistical methods that larger specimens lead to lower results with higher reliability as this test result is dominated by the distribution of the largest flaws in the specimen. There is also controversy over whether MOR or CCS is the better method. The author prefers diametral compression (splitting tensile) test, but a standard has not been completed for it by C08, see ASTM C496-11 Standard Test Method for Splitting Tensile Strength of Cylindrical Concrete Specimens by committee C09.61 for concrete.

ASTM C583-15 Standard Test Method for Modulus of Rupture of Refractory Materials at Elevated Temperatures

This test method covers determination of the high-temperature modulus of rupture of refractory brick or monolithic refractories in an oxidizing atmosphere and under action of a force or stress that is increased at a constant rate.

Precision
Modulus of rupture (MOR)
Alumina Brick— MOR +/–22% in laboratory; +/–36% between laboratories

This test is thought to be indicative of service strength. The author is not sure how strong a refractory needs to be at temperature, as it is theorized and seen that stronger materials generally have worse thermal shock and sometimes shorter lifetimes. Most requirements for elevated temperature MOR seem a bit excessive. A correlation study between elevated temperature MOR and service life would be interesting.

C830-00(2016) Standard Test Methods for Apparent Porosity, Liquid Absorption, Apparent Specific Gravity, and Bulk Density of Refractory Shapes by Vacuum Pressure

Apparent porosity, water absorption, apparent specific gravity, and bulk density are primary properties of refractory shapes. These properties are widely used in the evaluation and comparison of product quality and as part of the criteria for selection and use of refractory products in a variety of industrial applications.

Precision
Apparent porosity (P)— +/–3% in laboratory; +/–4% between laboratories
Bulk bensity (B)— +/–0.5% in laboratory; +/–0.6% between laboratories

This test is noncontroversial. Gives great results and is easy to complete with a minimum of expensive equipment. Vacuum or boiling water; vacuum is preferred for castables.

C704/C704M-15 Standard Test Method for Abrasion Resistance of Refractory Materials at Room Temperature

This test method measures the relative abrasion resistance of various refractory samples under standard conditions at room temperature. The abrasion resistance of a refractory material provides an indication of its suitability for service in abrasion or
erosive environments.

Precision
High alumina brick (A)— +/–26% in laboratory; +/–47% between laboratories
Conventional castable (A)— +/–55% in laboratory; +/–78% between laboratories

Do not talk to American Petroleum Institute (API) about this method! Modifications for API have made this method much better, but more work needs to be done before API can reliably use it as a specification. This standard may be the most edited and worked on standard in C08.

C201-93(2013) Standard Test Method for Thermal Conductivity of Refractories

The thermal conductivity (k-values) determined are “mean temperature” measurements rather than “at temperature” measurements.

or

C1113/C1113M-09(2013) Standard Test Method for Thermal Conductivity of Refractories by Hot Wire (Platinum Resistance Thermometer Technique)

The k-values determined are “at temperature” measurements rather than “mean temperature” measurements. Typically values by hot wire are 10–20% higher than values obtained by C201.

The thermal conductivity k-values determined at one or more temperatures can be used for ranking products in relative order of their thermal conductivities. Estimates of heat flow, interface temperatures, and cold face temperatures of single, and multicomponent linings can be calculated using k-values obtained over a wide temperature range.

Precision
Hot plate
Mean thermal conductivity (k)— +/–3.4% in laboratory; +/–9% between laboratories
Hot wire
Thermal conductivity (k)— +/–?% in laboratory; +/–?% between laboratories

Both methods work well. Equipment is expensive in either case. It is important to keep in mind that hot plate measures mean thermal conductivity while hot wire measures thermal conductivity at temperature. The author has had better luck estimating cold face temperature of refractory structures using hot wire results. But, what about when a specification states that thermal conductivity is not exceed a maximum, but does not define the test method? Hot plate give the lower value.

C1171-16 Standard Test Method for Quantitatively Measuring the Effect of Thermal Shock and Thermal Cycling on Refractories

This test method indicates the ability of a refractory product to withstand the stress generated by sudden changes in temperature.

Precision
MOR lost— +/–59% in laboratory; +/–65% between laboratories
Sonic velocity lost (V)— +/–17% in laboratory; +/–23% between laboratories

Do not get me started. I can vary the results on this test in so many ways and follow the standard to the letter. No wonder the precision is so high. There should be a better way!

C1445-13 Standard Test Method for Measuring Consistency of Castable Refractory Using a Flow Table

This test method covers the procedure for determining the consistency of castable refractory mixes by the flow table method.

Precision
Percent flow (F)— +/–25% in laboratory; +/–40% between laboratories

This standard is a great internal quality control test. It quickly shows if major batching problems occurred. Also, you need C230/C230M-14 Standard Specification for Flow Table for Use in Tests of Hydraulic Cement, which describes the flow table and how to keep it in calibration. The easiest way to keep the flow table in calibration is to occasionally replace the cam and lifting shaft.

Conclusion

Refractory raw material producers, producers, and users rely on ASTM standards as the definitive standards for comparing refractory materials in the United States and internationally. Hopefully, this overview has increased understanding of why precision and bias statements after ruggedness testing makes ASTM tests the optimum standard tests. The author hopes that this document will help refractory producers educate end users on data sheet values and specifications that rely on values that cannot be statistically measured according ASTM Standard Test Methods. Of most importance, one should always remember that laboratory testing is only indicative of service. If the primary mode of failure in service is thermal shock look for materials with a high thermal shock resistance, not a high cold crushing strength or bulk density. It seems like end users always want stronger refractories, but how many failures are due to exceeding the strength of a refractory (okay, impact damage, maybe).

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This article was first published in the ACerS St. Louis Section/Refractory Ceramics Division Refractories Symposium Proceedings 2017. Republished with permission of the ACerS St. Louis Section.