Do we need to add mineral filler in bituminous mixtures?

Mr. Rajib Chattaraj, Executive Engineer P.W.D. West Bengal asked Prof. Prithvi Singh Kandhal some interesting and good questions about the use of mineral fillers in bituminous mixes in India. The following Questions and Answers (Q and A) should be of interest to the highway community in India and therefore are being shared with them.

Prof. Kandhal had the privilege of leading research on mineral aggregates and fillers (including baghouse fines) at the national level in the US [Kandhal and Parker 1998; Kandhal 1981]. The latest Information Manual on Baghouse Fines currently in use in the US has been authored by him [Kandhal 1999]. Therefore, the responses to Mr. Chattaraj’s questions are based on his opinions considering the latest research on this subject.

Before responding to Mr. Chattaraj’s questions, Prof. Kandhal gave some background on mineral fillers.

Background

During the early 1970s, the use of baghouses in hot mix asphalt (HMA) plants was made mandatory in the US due to stringent air pollution regulations. Prior to the 1970s, the HMA plants used to blow the dust from the aggregate dryers into the atmosphere. This generally used to result in deficiency of fines in the bituminous mixes, which required addition of some kind of mineral filler. Once the aggregate fines were started to be collected in the HMA plant baghouses, there generally was no deficiency of fines in the bituminous mixes because these “baghouse fines” were incorporated back into the mix in a controlled manner through a screw conveyer. Therefore, there was no need for any additional mineral filler. Initially, some asphalt technologists had doubts about the quality of baghouse fines. However, many studies in the US have since confirmed that the baghouse fines (which are same as stone dust or mineral dust resulting from the aggregate crushing operations) are good mineral fillers. Therefore, most HMA plants in the US hardly use any additional mineral filler now except 1-2 % of hydrated lime, which is used only when the designed mix does not meet the requirements of moisture susceptibility test such as AASHTO T 283.

Now that the baghouses are also required and used in India, it is not understood as to why some engineers are so keen to add some sort of filler (such as portland cement) when the baghouse fines are abundantly available and are as good as those fillers. It is simply a waste of money. The US has one of the best roads in the world, if a technology works there, why not in India?

Questions & Answers

1. Chattaraj: In the MORTH Orange book, hydrated lime, cement, mineral dust etc. have been recommended as filler. However, limestone powder (LSP) has not been mentioned as so, whereas LSP has been prescribed as filler in Mastic Asphalt. Why, is there any specific reason for this? What is the harm if we use LSP as filler in case of mixes like BC or DBM?

1. Kandhal: Limestone powder can be used as mineral filler in mixes like DBM or BC. However, as mentioned in the background it does not have to be used if baghouse fines (which are also mineral dust) are abundantly available.

Limestone powder must be used in mastic asphalt. This is based on successful experience in Germany where “Gussasphalt” (also mastic asphalt) is used. I had the privilege of supervising and documenting the first-ever usage of Gussasphalt on a major project in the US [Kandhal and Mellott 1977]. We followed the German specification for selecting the mineral filler, which is a major component of the Gussasphalt mastic. The filler must meet the compatibility (miscibility) requirement, which is reported to be essential for obtaining satisfactory mastic asphalt. At least 75 g of filler when mixed with 15 g of bitumen at 163 C should result in a homogeneous mixture to pass this test. The heated filler is added in increments to the hot bitumen while mixing with a spatula. It was possible to mix 93 g of the filler used on that project with 15 g of bitumen. It was a limestone dust, which passed that test for the Gussasphalt. This type of test can also be conducted on the mastic asphalt used in India to determine the suitability of the limestone powder actually being used.

I must mention here that unlike BM and SDBC, which are fundamentally flawed and undesirable mixes and should be deleted from the orange book [Kandhal, Sinha and Veeraragavan 2008], mastic asphalt is a “success” story in India. Mastic asphalt has served us very well in resisting rutting on roundabouts and intersections. However, it cannot be used on highways where speed limits are over 50 km per hour because of its relatively lower skid resistance.

2. Chattaraj: What should be the exact dose of filler? As per orange book it is 2% by weight of the aggregates but as per international standard (Superpave) it should be 0.6 to 1.2% of the effective asphalt content. These two actually differ which I had calculated and shown in the comment on paper no. 530 (published in IRC Journal April-June, 2008).

2. Kandhal: As mentioned in the background, there is no need to add any “filler” (when baghouse fines are available) so the question of adding 2 % filler does not arise at all. This statement should be deleted from the orange book. Only the dosage of hydrated lime is established between 1 to 2 percent based on the results of AASHTO T 283. In the US, generally there is no “filler”. It is called “fines”, which are defined as all material passing the 0.075 mm sieve size (sieve No. 200) quite much of it usually coming from the baghouse again. It is also called P200 material (passing 200 sieve). In Superpave or Marshall for that matter, we should ensure that the ratio by weight of P200 and effective bitumen content (fines/bitumen) in the bituminous mix is between 0.6 to 1.2 so that we do have some minimum fines to stiffen the bitumen and not too much fines which will make the mortar too brittle (dull) and less cohesive and will also reduce the VMA more than we desire. Now you see the difference between the “filler” and “P200 material”. Again, just to emphasize there is no need to add any filler (unless the aggregate is clean and therefore deficient in fines).

3. Chattaraj: What is your opinion about recycling of mineral dust to be used as filler? Can it serve the purpose of increasing the viscosity of the binder?

3. Kandhal: Although technically correct, I would not call it “recycling” mineral dust; dust is being added back to the mix as baghouse fines in the same HMA plant. I would not call it recycling but just a routine process in mix production. Now that we do have baghouses in India, we must use these fines. Yes, baghouse fines do stiffen the bitumen, which we do desire to certain extent.

4. Chattaraj: How and in what sequence filler should be added in HMA during production in HMA plant (especially in continuous drum mix plant) keeping in view that “filler is mainly used to increase the viscosity of the binder”?

4. Kandhal: Again, keep in my mind we are not always adding any filler unless it is hydrated lime to resist stripping. In a batch plant, baghouse fines or hydrated lime are best added to the weigh hopper and dry mixed with all other aggregates before adding the bitumen. If the fines are added to the aggregates going up the hot elevator these can form slugs in Hot Bin No. 1 and their content in the mix can vary from one batch to another. Do not be concerned, fines are grabbed by the bitumen first before coating the larger aggregate particles. In a drum plant, these are added near the inlet of bitumen so that these are not blown back to the baghouse and are caught by the bitumen instead. I suggest you visit a hot mix plant and see the operations.

Filler has also other important functions besides increasing the stiffness of the bitumen as mentioned later.

5. Chattaraj: What is your opinion about other materials to be used as filler like marble dust, fly ash, granulated slag, etc?

5. Kandhal: Waste materials like the ones you have mentioned, which are hard to dispose off should always be considered. However, we must ensure that their use is cost effective and not detrimental to the hot mix. In my opinion, marble dust has a potential for causing stripping so an antistripping agent will probably have to be used in the hot mix. Some research is already under way in India to determine the suitability of the marble dust as filler. Some states in the US do not permit the use of fly ash because its particles are generally rounded and, therefore, increase the rutting potential of the asphalt mix when used on heavy-traffic roads. Marble dust or any other waste material should not be used in lieu of baghouse fines because then the disposal of waste baghouse fines will also be problem.

6. Chattaraj: From the National Center for Asphalt Technology (NCAT) textbook on “Hot Mix Asphalt Materials, Mix Design and Construction” by Prof. P.S. Kandhal and others, it is read that the particle size of 10 micron and below, filler materials are used in increasing the binder viscosity and come into play as film around the mineral aggregates, because the asphalt film thickness is generally 6 micron to 8 micron. Then what function does 10 micron to 75 micron sized filler material play – is it only to fulfill the gradation requirement of the mix (below sieve size 75 micron)? If so, then there should be particle size-wise sub-diversion in filler material, are ranging 10 micron to 75 micron and another below 10 micron.

6. Kandhal: There is no such actual thing as 6-8 micron bitumen film thickness around the aggregate particles [Kandhal and Chakraborty, 1996]. It is just a “calculated” average value obtained by dividing the volume of bitumen with the surface area of the

aggregate. We actually do not know where is the dividing line in term of size below which the fines (filler) become part of the bitumen thus affecting its viscosity and above which the filler just fills the voids between the larger particles, which is also needed to reduce the VMA to an acceptable level. We do know it happens that way but we do not know the exact answer.

7. Chattaraj: What are the purposes other than increasing viscosity of binder, served by filler? How much purpose, does the filler serve in keeping the balance of air void, VMA, stability, flow, density of the mix? How far its utility is there in decreasing the permeability of the mix?

7. Kandhal: According to a study [Puzinauskas 1969], the mineral filler has a dual role in HMA: (a) it fills the interstices and provides contact points between larger aggregate particles and (b) when mixed with bitumen binder it forms a high-consistency binder or mastic which cements larger aggregate particles together. Both of these roles affect the Marshall properties (void parameters and stability) of the bituminous mix.

Permeability of the mix depends on the extent of interconnected air voids in the compacted specimen, which are function of so many factors such as gradation (including fines) and bitumen content.

8. Chattaraj: It is also learnt that hydrated lime has the property of antioxidant and antistripping (I believe lime stone powder also have such property), this is an added advantage. If the other materials such as cement, mineral dust, stone dust etc. do not have these properties, why at all those will be used as filler?

8. Kandhal: Yes, hydrated lime retards the oxidation (aging) of bitumen to some extent and also is a very good antistripping agent. However, limestone powder is no match for hydrated lime. The latter has its active lime content (CaOH2 + CaO) above 90 percent as specified in AASHTO M 303, “Lime for Asphalt Mixtures”. Unfortunately, we do not have an Indian Standard for hydrated lime to be used in asphalt mixtures.

As I mentioned earlier, there is no need to add other mineral fillers unless the aggregate is deficient of fines, which happens rarely unless the fines are blown into the atmosphere and are not collected in a baghouse. Hydrated lime should be added if the designed mix does not meet the requirements of AASHTO T 283. The State of Georgia in the US has predominantly granite aggregate, which is prone to stripping, so every ton of hot mix there is treated with hydrated lime. That is what we should do in areas, which have a problem aggregate like granite or quartzite. For example, quartzite aggregate is used in and around Jaipur and therefore every ton of hot mix produced there must be required to have hydrated lime in it to prevent stripping.

9. Chattaraj: If we consider that dose of filler as “0.6 to 1.2% of effective asphalt content” what are the criteria by which the exact amount of filler in JMF is fixed?

9. Kandhal: Again, refer to my previous responses. The range of 0.6 to 1.2 pertains to the ratio: P200/effective bitumen content by weight. Let’s not confuse it with filler. When the total P200 in the job-mix formula is used within this range of ratio based on the gradation of the available aggregates, it is ensured that there are sufficient fines to stiffen the bitumen and not excessive enough to make the fines/bitumen mortar brittle.

Normally, the percentage of the material passing 0.075 mm sieve (P 200) in the job-mix formula should be kept within 4 to 6 percent. Personally, I would never use less than 4 percent so that the mortar contributes adequately to mix rut resistance. The percentage of this material given in the orange book for DBM and BC should be revised to 3 to 7 percent.

It should be mentioned here that this range of 0.6 to 1.2 for fines/bitumen ratio is based on weight. Since it is simple, practical, and easy to implement it is generally specified as such. However, we know different fines cause different degree of stiffening of bitumen although used on an equal weight basis. The stiffening effect of fines is influenced by voids in the dry compacted fines (called Rigden voids). Higher the Rigden voids, more is the stiffening effect. The Information Manual on Baghouse Fines [Kandhal 1999] currently being used in the US gives a nomograph by which one can determine the maximum amounts of fines based on their Rigden voids so that the fines/bitumen mortar is not too stiff or brittle. This manual also gives two more alternate criteria to choose from so that the mortar is not too stiff: (a) the viscosity of fines/mortar should not be more than 11.5 times that of the neat bitumen used, and (b) the softening point of the fines/bitumen mortar should not be more than 11 C compared to that of the neat bitumen.

10. Chattaraj: As per the prevailing norms of orange book (4 th Revision), if mineral aggregates fail to satisfy the stripping criteria (if failed, then water sensitivity test has to be done, even then if the aggregates fail to satisfy this test), filler is recommended to be used. Many stone aggregates easily satisfy these criteria, which means, filler is not required to be used at all, how far this is justified?

10. Kandhal: I drafted IRC:111 Specifications for Dense Graded Bituminous Mixes to include a mandatory test for stripping which is AASHTO T283, “Resistance to Moisture Induced Damage of Bituminous Mixtures”, which will be a part of Marshall mix design. This test is conducted on the final complete mix (including fines or filler). Filler or fines are not added (except for hydrated lime) specifically to reduce water sensitivity.

REFERENCES

Kandhal, P. S. (1981). Evaluation of Baghouse Fines in Bituminous Mixtures. Proc. Association of Asphalt Paving Technologists, Vol. 50, 1981.

Kandhal, P. S. Evaluation of Baghouse Fines in Hot Mix Asphalt. National Asphalt Pavment Association, Information Manual 127, 1999.

Kandhal, P. S. and D. B. Mellott. (1977). Pennsylvania’s Experience with Design, Construction and Performance of Gussasphalt. Proc. Association of Asphalt Paving Technologists, Vol. 46, 1977.

Kandhal, P. S. and F. Parker (1998). Aggregate Tests Related to Asphalt Concrete Performance in Pavements, Transportation Research Board, National Cooperative Highway Research Program Report 405, 1998.

Kandhal, P. S. and S. Chakraborty (1996). Effect of Asphalt Film Thickness on Short- and Long-Term Aging of Asphalt Paving Mixtures. Transportation Research Board, Transportation Research Record 1535, 1996.

Kandhal, P. S., V. K. Sinha and A. Veeraragavan (2008). A Critical Review of Bituminous Mixes Used in India. Journal of the Indian Roads Congress. Volume 69-2, July-September 2008.

Puzinauskas, V. P. (1969). Filler in Asphalt Mixtures. The Asphalt Institute Research Report 69-2, February 1969.

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