In spite of scientific advancement, the ability to control time still remains a far-fetched dream, but it would be surprising to know that scientists and engineers are able to predict the future with the help of accelerated tests carried out in the laboratory. These tests comprise material susceptibility to environmental degradation, product testing to verify expected life, or remaining life of broken components in a specific application or environment.
Accelerated Laboratory Evaluations
Accelerated laboratory evaluations are performed based on the understanding of the service environment being generated and recognizing the degradation mechanisms that are generally associated with that service environment.
One way to realize the acceleration possible is by implementing more stringent conditions in a controlled manner without actually modifying the nature of the degradation mechanism being evaluated. This is usually accomplished by either increasing the concentration of chemical components or increasing the temperature in the test environment. But in case of extreme test conditions, failure modes could affect the results and can make the test pointless.
However, test results that start impractical failure modes could lead to wastage of resources used to correct nonexistent product performance problems. In case the test results are used incorrectly for quality control, the entire sets of perfectly acceptable products could be rejected. Recognizing the field failure modes of products and knowing the physical mechanisms that cause those failures can help in developing suitable accelerated test design and methodology.
Conversely, a number of accelerated testing methods have gained wide acceptance. Technical companies including ASTM International have standardized these techniques. Such standards can prove useful when comparing information from different materials and studies. However, these techniques fail to capture the nuances of each application.
Often, it is necessary to examine analytical results so as to ensure that they offer a precise description of real-world behavior. Applying a standard test for any material or test conditions does not reveal the future behavior in all cases, especially when field failures cannot be simulated. Predicting life cycle without proper interpretation of accelerated testing results could result in failure.
Concrete Railroad Ties
Concrete railroad ties are one instance of applying accelerated tests for quality control and as a prediction of performance in the field. They were first utilized in the late 1800s and enhanced thereupon through focused research. They constitute approximately 20% of the ties in service on important railroads. While concrete railroad ties are not economically feasible, they have extended life span and need fewer ties per mile. Also, they require minimum maintenance and are capable of carrying heavier loads when compared to conventional wooden ties.
To predict performance in the field, concrete ties are made to undergo accelerated freeze-thaw testing through the ASTM standard C-666 Rapid Freezing and Thawing in Water, which happens to be a popular and controversial technique used in the concrete industry. This technique is generally employed to test a variety of materials that range from high performance concrete to concrete building slabs. The dispute over the test comes up when the test results do not match to field performance and when laboratory results are not consistent from lab to lab.
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A sample bar cut according to freeze-thaw testing specifications. The arrows point to a crack formed during cutting of the bar from a high performance concrete railroad tie. Depending on size and location of the sample, cracks may occur from mechanical stresses, which can give incorrect results that do not indicate the performance of the end-product.
Freeze-Thaw Testing
In the concrete industry, freeze-thaw is a popular damage mechanism. However, it is not the failure mode of importance in concrete railroad ties. Actually, the Federal Railroad Administration (FRA) of the U.S. Department of Transportation does not recognize freeze-thaw testing. Rather, it concentrates on safety issues related to risk of derailment and loss of gauge.
Deterioration limits for ties in track are published by FRA, and depending on failure modes in the field such as rail seat deterioration (RSD), FRA stipulates that railroads must perform automated inspections of all tracks made of concrete ties. RSD refers to the slow and steady wearing away of the concrete under the tie pad, which accommodates the track to the tie and can be mended in the field. Other observed failure modes comprise rail fastener failure and flexural cracking from the center binding.
The American Railway Engineering and Maintenance-of-Way Association (AREMA) gives the performance specifications for concrete railroad ties and their constituent components and can be further improved at the discretion of the buyer or manufacturer. Certain specifications concentrate on freeze-thaw testing by utilizing ASTM C-666.
Investigators across the globe have identified a difference in laboratory freeze-thaw behavior for quality concrete, which is not simulated in the material’s field performance. This indicates that the ASTM test does not produce real field performance. It is a well-known fact that damage to concrete increases with faster cooling rates.
ASTM does not offer information on lab-to-lab inconsistency in view of the likelihood that two or more labs will be conducting freezing-and-thawing tests on the same concretes. Recognizing the problems on an industry-wide scale is made complicated because of the viabilities on how the test can be applied in the industry. For instance, unlike ASTM standard, AREMA suggests that more rigorous passing criteria and smaller sample dimensions should be employed. Additional inconsistencies in the industry arise from individual manufacturer specifications with different needs.
Conclusion
Recent advancements in cement materials and technology have resulted in the development of quality concrete that is ideal for placing in track as a railroad tie. The railroad industry benefits from the high compressive strengths that are now possible today. However, such aspects present a number of difficulties in assessing the life-span of these materials through ASTM C-666. The present standards for high performance concrete railroad ties are built on an accelerated test with a low degree of accuracy.
Such limitations can lead to the removal of acceptable ties from service. The development of a more practical accelerated test, which precisely and reliably predicts the field performance of concrete ties, can significantly benefit the railroad industry. This can be accomplished by reviewing the range of suitable test parameters in ASTM C-666 to exclusively address these new materials.
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