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Ignition Oven Correction Factors in HMA Production
9/15/2008
9/15/2008
THE IMPORTANCE OF USING IGNITION OVEN CORRECTION FACTORS IN HMA PRODUCTION
Robert E. Lee, Materials Engineer, TxDOT
Recently we have had several inquiries concerning ignition oven correction factors their importance and implementation. We asked our partner in quality Robert E. Lee, Materials Engineer, for TxDOT to give us an update on this important subject. We appreciate the following information that he provided.
Introduction
In recent past, it has been observed on a number of hot mix projects that ignition oven correction factors are not being determined or applied correctly. Failure to determine and apply these correction factors can lead to mixes that are too dry as a result of too many fines in the mixture. Dry mixtures are detrimental to the fatigue life of an HMA pavement. The proper application of ignition oven correction factors will help ensure our mixes are as good on the roadway as we design them in the laboratory.
Mix Designs
Designs are based on washed gradations. Test method Tex–204–F and the HMA specifications require them. Because it is important to know exactly what we are putting into a mix, we run a washed gradation on each stockpile in our design. If the aggregate producer’s historical gradation data is used, it is important to verify that the data is from washed gradations.
QC/QA
In QC/QA production, the use of correction factors has become more important since the widespread use of the ignition oven. In the past, when solvent extractions were run to determine AC% and gradation, the nature of the test itself inherently required a washed gradation. But with the eventual transition to the ignition oven for production testing, test method Tex-236-F requires us to run dry gradations. The key is to make sure we correct those dry gradations run in the production phase of a project back to the washed gradations (JMF) ran during the mix design phase. This is where correction factors come into play. It’s the only way to ensure what we put into the hot mix plant is what we get out of it and on to the roadway.
Correction Factors
Correction factors, not to be confused with correlation factors, were once used to correlate belt samples to extracted mix samples. They are now more commonly associated with the use of ignition ovens to determine AC content and gradation. Tex-236-F describes exactly how to determine and use these correction factors for both asphalt content and aggregate gradation. When this procedure was originally developed, the ignition oven correction factors for aggregates were used to correct for degradation effects caused by the ovens with differing materials. Different materials break down differently in the oven. In general, harder materials have less degradation and smaller correction factors, while softer materials have more degradation and larger correction factors. There are also differences between ignition ovens, even between ovens of the same manufacturer. So it’s important to remember, the ovens we use to test production mix, are the same ovens we use to establish our correction factors.
Dry Gradations
When dry gradations were allowed for quality control and assurance, the role of ignition oven correction factors took on additional responsibilities. Not only were they correcting for the degradation effects of the aggregate, but now they are used to correct dry gradations back to washed gradations. We are now correcting for two different conditions. The good part of this is, you can do both at the same time. One set of correction factors for both conditions. But they need to be established for each mix we design. When correction factors are not determined and applied, test results can lead us into thinking we have fewer fines than what is really in the mix. We then adjust the bin percentages going into the plant to compensate for this apparent lack of fines. The results can lead to dryer mixes with less asphalt and lower VMA’s.
Conclusions
In conclusion, it’s important to remember these points:
• All mix designs are based on washed gradations.
• TxDOT specifications require ignition oven correction factors to be applied for the AC content and for aggregate gradations.
• Correction factors for gradation account for aggregate breakdown in the ignition oven as well as for the difference between dry and washed gradations.
• During production, dry gradations are performed and are corrected back to washed gradations by using these ignition oven correction factors.
Robert E. Lee, Materials Engineer, TxDOT
Recently we have had several inquiries concerning ignition oven correction factors their importance and implementation. We asked our partner in quality Robert E. Lee, Materials Engineer, for TxDOT to give us an update on this important subject. We appreciate the following information that he provided.
Introduction
In recent past, it has been observed on a number of hot mix projects that ignition oven correction factors are not being determined or applied correctly. Failure to determine and apply these correction factors can lead to mixes that are too dry as a result of too many fines in the mixture. Dry mixtures are detrimental to the fatigue life of an HMA pavement. The proper application of ignition oven correction factors will help ensure our mixes are as good on the roadway as we design them in the laboratory.
Mix Designs
Designs are based on washed gradations. Test method Tex–204–F and the HMA specifications require them. Because it is important to know exactly what we are putting into a mix, we run a washed gradation on each stockpile in our design. If the aggregate producer’s historical gradation data is used, it is important to verify that the data is from washed gradations.
QC/QA
In QC/QA production, the use of correction factors has become more important since the widespread use of the ignition oven. In the past, when solvent extractions were run to determine AC% and gradation, the nature of the test itself inherently required a washed gradation. But with the eventual transition to the ignition oven for production testing, test method Tex-236-F requires us to run dry gradations. The key is to make sure we correct those dry gradations run in the production phase of a project back to the washed gradations (JMF) ran during the mix design phase. This is where correction factors come into play. It’s the only way to ensure what we put into the hot mix plant is what we get out of it and on to the roadway.
Correction Factors
Correction factors, not to be confused with correlation factors, were once used to correlate belt samples to extracted mix samples. They are now more commonly associated with the use of ignition ovens to determine AC content and gradation. Tex-236-F describes exactly how to determine and use these correction factors for both asphalt content and aggregate gradation. When this procedure was originally developed, the ignition oven correction factors for aggregates were used to correct for degradation effects caused by the ovens with differing materials. Different materials break down differently in the oven. In general, harder materials have less degradation and smaller correction factors, while softer materials have more degradation and larger correction factors. There are also differences between ignition ovens, even between ovens of the same manufacturer. So it’s important to remember, the ovens we use to test production mix, are the same ovens we use to establish our correction factors.
Dry Gradations
When dry gradations were allowed for quality control and assurance, the role of ignition oven correction factors took on additional responsibilities. Not only were they correcting for the degradation effects of the aggregate, but now they are used to correct dry gradations back to washed gradations. We are now correcting for two different conditions. The good part of this is, you can do both at the same time. One set of correction factors for both conditions. But they need to be established for each mix we design. When correction factors are not determined and applied, test results can lead us into thinking we have fewer fines than what is really in the mix. We then adjust the bin percentages going into the plant to compensate for this apparent lack of fines. The results can lead to dryer mixes with less asphalt and lower VMA’s.
Conclusions
In conclusion, it’s important to remember these points:
• All mix designs are based on washed gradations.
• TxDOT specifications require ignition oven correction factors to be applied for the AC content and for aggregate gradations.
• Correction factors for gradation account for aggregate breakdown in the ignition oven as well as for the difference between dry and washed gradations.
• During production, dry gradations are performed and are corrected back to washed gradations by using these ignition oven correction factors.
