Show slate dust in in asphalt road surface mixtures BY OLIVER BOWLES it is noteworthy that 80 to 95 per cent of the he gross production of all slate quarries in the he united states is discarded as waste in view of the loss involved in handling so much waste and the consequent added cost of the finished product the bureau of mines has undertaken a study of the slate industry for the purpose of devising the best means 1 of reducing the proportion of waste and of utilizing the unavoidable waste the utilization of waste has received special attention and the operation cooperation co of many prospective purchasers of waste slate has been enlisted in testing its adaptability for various us uses es the manufacture of filler for road asphalt mixtures is a promising Z field for slate waste utilization in the preparation of mixtures for surfacing roads in addition to the sand or sto stone ne and sand aggregates finely Y pulverized pulverize d limestone or portland cement are used as fillers in mixtures in which product byproduct by asphalt is used th the weight of filler and asphalt are substantially the same but when trinidad and e certain other native asphalts asp halts are used a somewhat smaller proportion of filler is required the tonnage of filler used in such asphalt mixture is very great one authority estimating that the city of new york alone requires tons per year i As road building is such an important industry austry and as intimations had been received from several sources that slate flour conati buted ted a superior filler for road asphalt asphalt mix tures wes an all attempt was made to obtain more definite data as to the adaptability of slate flour our for such a use in order er to obtain practical tests the cooperation of manufacturers was requested and nd as a res result alt several companies kindly offered to test the material in their experimental i laboratories samples of finely pulverized VL prized sla slate te were submitted for this pur apse results of the most complete tests are e given in the following paragraphs impact and compression tests the slate flour submitted was of such a fi lieness th that at 3 per der cent would pass a 1 ech sh and about 80 per c ent cent pass a 4 mckno alveral Al technologist bureau of mines fori rr aaion is hereby made of in alil hed on asphalt 1111 mixtures kindly sup publia T IL M milburn of the bureau of roads department of agriculture mesh screen tests were made to determine its suitability as a filler in asphalt pavements both on asphalt bonded bri and on standard sheet asphalt pavement mixtures the bri were prepared by adding 10 per cent of D grade calel asphaltum of 65 penetration to the slate dust and increasing the amount by 2 per cent until the pat test showed only a slight excess the completed bri contained 20 per cent of asphaltum they were made at a temperature of to degrees F the standard sheet asphalt consisted of a mixture mix of D grade 67 penetration asphaltum with slate dust and sand its composition by weight is indicated below asphalt 10 per cent slate dust filler 13 per cent sand retained on mesh screen 13 per cent sand retained on mesh screen 13 per cent sand retained on SO mesh screen 24 per cent sand retained on 50 mesh screen 11 per cent sand retained on 40 mesh screen S per cent sand retained on 30 mesh screen 5 per cent sand retained on 20 mesh screen 3 per cent total sand 77 per cent total per cent the standard sheet surface mixture was tested for impact with a large page impact machine having a two kilogram kilogram hammer dropping from a height which was increased one centimeter with each successive blow for the asphalt bonded bri a small page impact machine was used the cementing value under compression was determined ter mined by subjecting the bri to pressure increasing uniformly at the rate of pounds per square inch per minute similar mixtures with limestone dust and portland cement the materials commonly employed as fillers in asphalt pavement were prepared using the same pr proportions by weight as in the slate flour mixtures given above the results of the tests are shown in the following table results of impact and compression tests ASPHALT BONDED compres im sion lbs ibs material pact per sq in remarks slate flour 59 Bri flattened good cementing value portland cement 59 do limestone dust 58 GOO do STANDARD SHEET SURFACE MIXTURE compres material impact sion slate flour as filler 13 25 portland cement as filler 3 20 limestone dust as filler 13 n 24 from these results it is evident that in asphalt bonded bri slate flour is superior to both portland cement and limestone dust in resistance to compression which is a measure of its cementing value in the standard sheet surface mixture itis it is intermediate between limestone dust and portland cement As regards impact there seems to be little choice between the three substances tested as filler in asphalt bonded bri but in standard sheet surface mixture there is some indication of superiority on the part of the slate flour it is to be noted that the proportion of fillers used was determined on the basis of weight and not of volume and as the materials used vary in specific gravity a different volume was used in each instance if the fillers had been added on the basis of volume somewhat different results might have been obtained particularly for portland cement which is considerably heavier than either limestone or slate the results obtained therefore are indicative only and are not conclusive in any dust used in asphalt mixtures percentage percentage percentage time of of material retaliated Recal Recal material screen test decanted on a on a alesh basis mesh alesh basis 93 thru mesh 15 sec TZ red ed slate mesh pa thru do 1 min do slate thru mesh 15 sec gray light I 1 thru mesh albany pa do I 1 min do 95 thru mesh 15 sec 93 gray slate mesh co pa r a 0 thru northampton do 1 min 82 0 do thru mesh 16 see do thru mesh 1 1 5 8 do 1 min 6 78 do thru mesh sec 97 1 do thru mesh do 1 min do only that part which is finer than mesh is available as filler all material coarser than mesh being regarded as aggregate rather than filler clifford richardson in his book entitled the modern asphalt pavement states that only particles smaller than mm in average diameter are to be regarded as true dust and it is even better to have a considerable proportion of the dust finer than mm in diameter the best way to determine the percentage of very fine material is by settlement and decantation tests such tests of slate dust samples were made in the laboratory of the bureau of mines in accordance with the method outlined by richardson and termed by him the elutriation method five grams of slate dust were placed in a cc beaker about mm high the beaker was then nearly filled with distilled water at a temperature of exactly twenty degrees C and the contents were agitated with an air blast until the dust and water were thoroughly mixed care being taken to produce no cyclonic currents 1 the liquid was then allowed to stand for a fixed period of time after which the water and suspended dust were decanted leaving the sediment in the bottom of the glass this was repeated three times it is regarded that particles thus held in suspension for a fifteen second period have a diameter of approximately mm and those that remain in suspension for a period of lone one minute have a diameter of mm although these figures may vary somewhat depending on the specific gravity of the materials employed tests were made at both fifteen second and one minute periods the results of which are shown in the preceding table results of settlement and decantation tests it is to be noted that the tests were made with materials various percentages of which failed to pass a mesh screen As mesh material only is to be considered as effective filler it seems important to eliminate from the determination the percentage of oi material coarser than mesh and to recalculate the fine dust on the basis of that which passed mesh thus in the fifth column of the table is given the percentage of mesh slate flour which remains in suspension at the end of a fifteen second period for purposes of comparison a recalculation was made indicating the percentage of mesh slate flour which remains in suspension at the end of one minute and this is shown in the sixth column it may be observed that the material held in suspension at the end of a fifteen second period is a little finer fined though almost equivalent to that which passes a mesh screen material held in suspension at the end of one minute is considerably finer than that which passes a mesh screen in order to arrive at an intelligent corn com parison of slate flour with other filler materials te rials the following table has been prepared from richardsons Richard sons results as given on page 94 of the book to which reference has already been made the percentages have been recalculated on a mesh basis material at slate quarries tests were 14 to determine ine its value as a filler in asak asphar 1 l hi road surface mixtures actual acqua I 1 tests in ill i bora tories of companies preparing boadas road as s ahalt mixtures indicate that for resistance to impact slate flour is about equal to othu other E elutriation TESTS AS GIVEN BY richardson time of 15 seconds Percenta percentage gere re or percentage calculated calculate don oni material screen test decanted mesh baa x limestone through mesh trap rock through mesh portland cement through mesh W from this table it may be seen that for limestone trap rock or portland cement only 75 to 85 per cent of the mesh material is to be regarded as fine dust suitable as a filler while as shown in the earlier table the fine dust in slate flour approximates per cent of all that passes the mesh screen slate dust is according to these tests a much more economical filler than either lim limestone estone or portland cement in that it provides a larger amount of effective filler for each ton of pulverized material pur chased it is to be observed that in modern practice portland cement is somewhat finer than that used in the above tests and consequently may give better results volume weight filler materials are purchased by weight and other considerations being equal that filler is the most economical which occupies the grea greatest test space per weight purchased A very heavy filler fills a relatively small space as compared with a light one As specific gravity is the weight of a unit volume of a material compared with that of an equal volume of water specific gravity may be regarded as a measure of relative volume weight A comparison of the fillers under consideration indicates that portland cement has a specific gravity of about 31 average limestone about and the average slate of northampton county pennsylvania about slate is tie therefore about equal to limestone in this respect and has an advantage over portland cortlan d cement by about 10 per cent in volume for a given weight cost it is difficult to make any statement as to relative costs of fillers for the production of pulverized slate is an industry not yet established on a sufficiently large scale to permit uniform price quotations in bulk shipments for the limited shipments now made in barrels and bags k in i carload lots prices vary greatly in air localities it is safe to estimate however that slate flour can be placed placed on the market at little more than hali half the present price of portland cement current prices of pulverized limestone indicate a decline almost to prewar pre war levels and it is yet an open question whether producers of slate flour can compete successfully in price with producers of pulverized limestone summary and conclusions in order to find a useful outlet for waste fillers in bonded bri and somewhat si 21 14 in sheet surface mixtures in cement ing value it is superior to both limestone aaa and portland cement in asphalt bonded bri and intermediate between them in standage stan dari sheet surface mixture elutriation tests indicate that slate floar contains approximately 15 to 25 per ceil L more of bf the fine dust that constitutes effet tive filler than either limestone trap arroo root gr or portland cement i in volume weight slate is about aquin lent to limestone and approximately 10 ps cent superior to portland cement the cor N of slate flour is little more than half that of E portland cement but its ability to compete in price with limestone is not yet estal dished n W the tests already made a are re not sufficient idelt 2 ly comprehensive to give conclusive results but they are definite enough to suggest PC pos ties of such importance as to indue further and more extended research both I 1 qt slate producers and by manufacturers of 2 road mixtures the preliminary tests teso recorded above indicate that the use of slat ix flour as a filler in asphalt road surface M as tures would result in improved higby and that a wide use of such filler would al ford a profitable outlet for waste slate witta consequent advantage to the slate produs industries it Ls is highly desirable that th tal conclusions reached in laboratory testo tests kj I 1 verified by practical tests on eap exp so roads |