NATIONAL SLAG NSA ASSOCIATION

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1 NATIONAL SLAG NSA ASSOCIATION SLAG BAGHOUSE FINES IN ASPHALT MIXTURES Air pollution control regulations have forced many asphalt plants to install baghouse dust collection systems. The collected dust has usually been returned to the mix, often without control of uniformity in addition rate. The baghouse fines have been blamed for most of the problems encountered with mixes in which they were used; probably with justification in some cases but not in others. Questions regarding the effects of the fines have led to several research projects in recent years, including use of new tests and efforts to correlate use of fines with field problems in compaction and durability. The most recent of these studies is one carried out by the PennDOT Materials Testing Division and reported by Prithvi S. Kandhal in Evaluation of Baghouse Fines in Bituminous Paving Mixtures, a departmental report on Project No This report was also presented at the February 1981 AssociatIon of Asphalt Paving Technologists meeting, where NSA presented a discussion of it. While no experience is available on the application of some of the testing procedures to slags, it was felt that the baghouse fines were used in an unrealistic manner, and that the implied correlations with inadequate field performance were not justified. The AAPT will publish the discussion with the paper in a Proceedings volume that will be available about February of The PennDOT report has been and will probably continue to be, rather widely circulated prior to the AAPT publication. It may result in questions regarding the use of slag in other areas. This paper has been prepared to inform the NSA members of the contents of the PennDOT report and of the NSA discussion in the event such questions do arise. The first part of the paper is a very brief summary of the extensive PennDOT study (the entire report with tables and figures is about 140 pages). The second part is the NSA discussion as submitted to AAPT for publication. EVALUATION OF BAGHOUSE FINES IN BITUMINOUS PAVING MIXTURES By P. S. Kandhal, P.E. Summary prepared by D. W. Lewis Nine baghouse fines (minus No. 200 fraction only) from operating Pennsylvania plants were tested with Portland cement used as a control mineral filler. The research purposes were to (1) determine properties of the various types of fines, (2) determine their effects on properties of fines/asphalt (F/A) binder systems, (3) determine effects on dense-graded asphaltic concrete, (4) determine detrimental effects (If any) on NATIONAL SLAG ASSOCIATION

2 Slag Baghouse Fines in Asphalt Mixtures P.S. Kandahl and D.W. Lewis Page 2 resistance of mixes to damage by moisture, and (5) develop specifications for control of quality and quantity of fines used in mixes. PHYSICAL PROPERTIES - The types and physical properties of the bag house fines are sown in the accompanying table, reproduced from the PennDOT report. Hygroscopic moisture shown was the weight increase of oven dried samples exposed to 50% relative humidity for 24 days. Apparent specific gravity values were run in kerosene. Bulk specific gravity is determined from the bulk volume of a 95 gm sample of the fines compacted to constant volume In a 100 ml graduate by tapping the graduate with a loop of rubber tubing. Voids reported are those in the fines compacted by the tapping procedure. The activity coefficient is simply the ratio of apparent and bulk specific gravity. EFFECTS OF FINES/ASPHALT BINDERS - The fines were combined with asphalt in various ratios by solid volume, with only ratios 0.3 and 0.5 used for all types. The fines and AC-20 asphalt cement were heated and mechanically mixed at 325 F. Samples were then subjected to a variety of tests: penetration, viscosity, softening point, ductility, etc., and compared to the original asphalt. A volumetric analysis of the fines/asphalt (F/A) systems was made, based on the concept that asphalt filling the voids in the closely packed fines is "fixed" and only the remaining asphalt can contribute to any mix fluidity. A value was calculated for the bulk volume of fines as a percentage of the total solid volume of asphalt plus fines. This is presumably the percentage by volume of the F/A binder system that reacts as if it were a solid. The tables "Mix Proportions of Fines/ Asphalt Binder: and "Change in Asphalt Properties" summarize the major test results in this portion of the study. EFFECTS ON BITUMINOUS CONCRETES - The various baghouse fines were used in bituminous concretes in Marshall tests using two concentration levels (0.3 and 0.5 F/A ratio) and three compaction temperatures (220, 250 and 280 F). The coarse and fine aggregates used for all tests were a 3/8" nominal top size dolomite (100% passing a 1/2" sieve) and a sandstone with 97% passing the No.4. All minus No. 200 material was removed from the aggregates and replaced with the respective fillers, and a constant asphalt content of 6.3 % was used for all tests. EFFECTS ON RESISTANCE TO MOISTURE DAMAGE - Two test procedures were used. The Asphalt Institute test compares the Marshall stabilities of specimens vacuum saturated at 140 F oven for 24 hours. The Idaho test used Marshall specimens compacted with 15 blows for 0.3 F/A ratio and 10 blows for 0.5 F/A ratio. A splittingtensile test is used, comparing the tensile strength of specimens vacuum saturated at room temperature, frozen at O F for 15 hours and thawed for 24 hours in a 140 F water bath followed by three hours in a 60 F bath with the strength of dry specimens conditioned for three hours at 60 F. Neither of those tests is standardized by ASTM or AASHTO. The accompanying figures show the percentage retained stability (at 4% NATIONAL SLAG ASSOCIATION

3 Slag Baghouse Fines in Asphalt Mixtures P.S. Kandahl and D.W. Lewis Page 3 voids) in the A.I. test and the percent retained tensile strength at 0.3 F/A in the Idaho test. FIELD CASE HISTORIES - The PennDOT report contains descriptions of projects having "premature distress" using a traprock, an all-slag mix, and a mix with calcareous sandstone coarse aggregate and slag fine aggregate. The all-slag mix was reported to have poor compaction, poor coating (inferred to be possible moisture-induced stripping and raveling. "Project Investigation Data (Sample 6)" tabulates the reported information on this source. DEVELOPMENT OF SPECIFICATIONS Proposed specifications would include: 1. Sampling materials from the plant hot bins (including filler or fines silo). 2. Use all minus No.200 material from the combined aggregate for test. 3. Bulk volume of fines must not exceed 50% unless the increase in softening point of the F/A binder is not more than Retained tensile strength (Idaho test described above) should be at least 50%. PennDOT CONCLUSIONS - Conclusions stated in the PennDOT report include: 1. Some baghouse fines (such as, trap rock, gneiss, sand and gravel) have a tremendous stiffening effect on F/A systems whicl1 offered resistance to compaction of asphaltic concrete. Bulk volume concentration of fines in F/A system has been determined to be the primary factor causing the stiffening." 2. Determination of the bulk volume of fines is simple and it can be used as a tool for preliminary screening of baghouse fines obtained from different sources, or from the same source at different times during the plant production." 3. Limiting values of viscosity ratio and change in softening point of F/A systems have been developed from the bulk volume of fines values to keep the stiffening effect within desirable limits for a dense-graded bituminous mix. Change in softening point was determined to be more reproducible and practical for specification purposes." 4. Resistance to compaction increased significantly after the bulk concentration of fines in the mix exceeded a limit corresponding to the critical bulk volume of fines in the F/A system." 5. Some baghouse fines (three slags and one carbonate) were determined to be detrimental to the mix making it susceptible to moisture damage. This was indicated by both Asphalt Institute Water-Sensitivity Test and the Idaho Test for predicting moisture-induced damage." 6. Idaho Test was found to be more suitable for evaluation purposes because it gives indication of moisture-induced damage even at low concentrations of detrimental fines, and being less sensitive to the variation in specimen air void content it is more reproducible." NATIONAL SLAG ASSOCIATION

4 Slag Baghouse Fines in Asphalt Mixtures P.S. Kandahl and D.W. Lewis Page 4 7. Field case histories of some bituminous pavements which used three baghouse fines evaluation in this study and exhibited premature distress are given. Usually higher and inconsistent incorporation of fines made the mix stiff, brittle and difficult to compact in the field under normal circumstances. Resulting high air void contents accelerated the asphalt hardening which reduced the pavement durability. Some indication of moisture-induced damage to the mix was also noted. " Donald W. LEWIS' SUMMARY Mr. Kandhal has presented an extensive study of mineral fillers. ASTM D242, Standard Specification for Mineral Filler for Bituminous Paving Mixtures, states that "Mineral filler shall consist of finely divided mineral matter such as rock dust, slag dust, hydrated lime, hydraulic cement, fly ash, loess or other suitable mineral matter." However, the rock and slag dusts listed are not likely to be commercially available in large quantities for wide distribution as is the case with limes, cements and fly ashes. Each will probably be used only at the plant where produced, and only with the parent materials in actual asphalt paving mixtures. The materials, as would be expected, vary in gradation, fineness and chemical composition, and as a result also vary in their affinity for asphalt, the stiffening effects and void filling characteristics. As with any fillers, they should be included in the design of the mixtures, and should be added in a controlled and uniform manner. It is unfortunate that Mr. Kandhal's tests used the fillers in a totally different manner from the actual field applications, thereby severely limiting the value of the data for any correlation with service records. These differences are found in several areas, including: 1. The use of only the minus No.200 portion of the baghouse fines provides an unrealistically fine material. The primary collectors remove material that would contain some of the coarser portion of the minus No. 200 fraction. The total minus No.200 material actually returned to the mix is, therefore, likely to be coarser than that used in the reported tests. This fact was recognized in the investigation reported by Eick and Shook The use of large quantities of one type of fines, in mixes with other types of aggregates, is also unrealistic. If, by some unusual circumstances, slag baghouse fines were used as filler in a mixture with a dolomite coarse aggregate and a sandstone fine aggregate, the fines in the aggregates would make up a large portion of the minus No.200 material in the mix, the filler being used only to augment that quantity. This would greatly decrease any possibility that large quantities of a filler having a great affinity for asphalt could result in thinner films or lack of coating on an aggregate susceptible to stripping. The only conditions under which all slag fines would be used in a mix would be one In which both NATIONAL SLAG ASSOCIATION

5 Slag Baghouse Fines in Asphalt Mixtures P.S. Kandahl and D.W. Lewis Page 5 slag coarse and fine aggregates were used, the gradation would not be the same as that tested in this study, the mixture (if properly designed) would not include the high F/A ratios, and neither of the aggregates would be of a type susceptible to stripping. The discussion of Field Case Histories in the paper implies that the test results explain some of the field performance problems cited. Since we are the most familiar with the use of Baghouse Fines No.6 (Slag T) its performance will be examined in some detail. Table 15 in the report lists 3 projects, the first two constructed in October, The mix contained slag coarse and fine aggregates and slag baghouse fines. It should be noted that these materials had been used very successfully for some time, with an asphalt content of 9%. The October, 1974 projects were the first under a new procedure for designing the mixture - a procedure that determined an "optimum" asphalt content of 7.8 percent. The mixture, deficient by more than one percent in asphalt compared to previous experience, was placed in cold, wet weather, largely at night. Temperatures were often around 40 F. Memoranda 2,3 of inspections and investigations of Project A only a few months later established the following points. 1. Performance varied from good to bad, evidenced by loss of material from the wheel paths, ranging from loss of fines to progressive raveling of the entire surface course. Distressed areas showed varying evidence of moisture in the pavement, ranging up to standing or flowing water in some cases. 2. Cores taken and tested by PennDOT showed pavement thicknesses as low as 1/2" in the poor-performing areas (design thickness was 1"). Sections in fair and good areas were usually over 1" thick, but consisted of two layers, in some cases separated by a dirt layer. 3. Voids determined for the cores were 16.3 percent in the "poor" areas, 14.2 percent in the "fair" areas, and 12.0 percent in those areas classified as "good". 4. Gradations of the extracted cores indicated that the mix failed, on the average for all performance areas, to meet the job mix specifications for percentage of material passing the No.8-57 percent rather than the design for 50%. Average amounts of minus No.200 material (7.3 %) were within specification limits for all sections. 5. Inspection of the cores indicated that the coarse aggregate was not well coated and "Most of the asphalt seems to have been taken by the fine aggregate in the mix." Lab efforts to use the design asphalt content and 57% through the No.8 sieve resulted in a mix that "appeared on the dry side." It was concluded that the asphalt content needed to be increased by 0.3 percent "to get good coating." The use of such mixtures, seriously deficient in asphalt content, high in fine aggregate (passing No.8, not that passing the No.200), and placed in thin layers under extremely adverse weather conditions is likely to result in inadequate compaction (job mix formula laboratory tests for this project showed Marshall stabilities of 4,080 pounds, indicating NATIONAL SLAG ASSOCIATION

6 Slag Baghouse Fines in Asphalt Mixtures P.S. Kandahl and D.W. Lewis Page 6 densities difficult to obtain in the field under ideal conditions). After four months of exposure to heavy traffic and poor drainage during the winter, it is perhaps surprising that any of the surface could be rated as "good". There is no need to hypothesize any effects of mineral filler tested under the totally different conditions in order to explain the performance. Our information on the projects listed in Table 16, where slag fine aggregate was used with a calcareous sandstone coarse aggregate, is less complete. However, past experience would indicate that the asphalt contents are lower than they should be for adequate performance. We would also point out that the baghouse fines sample No.8 was slag, whereas that added in construction of the projects would have been partially slag and partially from the calcareous coarse aggregate - far different from the material tested in this study. Mr. Kandhal does indicate that, after these performance problems were encountered, "the minus 200 content in these mixtures has been reduced and the asphalt content has been increased." In addition, each District now has a pavement management engineer, responsible for overlay designs, correction of existing drainage problems, etc. Minimum thicknesses have been increased, and construction is more closely monitored. It is our understanding that these steps have alleviated the problem, at least insofar as slag mixes are concerned. In fact, we are not aware of slag pavements subject to detrimental erosion and raveling unless the mix was (a) deficient in asphalt, or (b) placed under conditions such that adequate compaction was not attained, or (c) both. Corrective measures have not been related to the type of baghouse fines. In conclusion, we believe that proper design of mixtures includes consideration of the proportions of aggregate, filler and asphalt to provide appropriate void content, stability and workability (or placability) of the mix. This must be followed by controlled and reasonably uniform blending of the ingredients and construction under conditions such that proper field compaction can be obtained. We do not agree with general classifications of aggregates, asphalts or fillers on the basis of laboratory tests under conditions far different from those of actual use. The performance in practical, economical and properly proportioned mixtures is all important. Judgment of quality should not be based on any other combinations or circumstances. REFERENCES 1. Eick, J. H. & Shook, J. F., The Effects of Baghouse Fines on Asphalt Mixtures, The Asphalt Institute Research Report 78-3, November R. C. Zellers, Inspection Report - LRO2324, LR330-26M and lr330-1r, unpublished memorandum, January 29, Bureau of Materials, Testing & Research, PennDOT, Investigational Report - Resurfacing with ID-2 A Wearing Course, LR330, Section 26M, Allegheny County, unpublished report, March 11, NATIONAL SLAG ASSOCIATION

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