Method of mixing paving material
US 1788887 A
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Patented Jan. 13, 1931 wrrmmirnnnr mailman, or RALEIGH, norm-n: CAROLINA METHOD 017, MIXING ravine MATERIAL 3110 Drawing ..3 I i The present invention relates to a method of mixing, paving materiahand, in the present disclosure, it will be described in. connection with bituminous paving material.
5 Paving materials, and, particularly, the type in :which .a bituminous binder is used,
are ordinarily composed of an aggregateand a suitable binder, such binder, in the bituminoustype of pavement, being a mineral pitch.
In mixtures of this character, it is essential :to a stable and wear-resisting pavement, that the aggregate and binder be brought into inti gregate be thoroughly and completely] coated 5 andimpregnated with the binder. Failure to do this'will resultin uneven areas orJz'ones i throughoutthe materialwhere; uncoated or insufliciently coated particles of theaggre- .,,gate are found, and this condition of the gamaterial gives an unsatisfactory pavement,
V I for't'he reason that there will be, thro ughout thepavement, areas of material,which, be-
] cause they are not suflieiently bound together, will break up under traflic strain, or dlSlIli tegrate under the action of the elements.
, Paving material of this general character is ordinarily produced by mixing the aggregate, which is composed of different sizes or V grades of material, usually running from the olfiller dust up 'to'larger fragments of a size i suitable to' the work in hand, with a mineral andgrade the aggregate, then mix that aggre-' gate which is, preferably, thoroughly heated 40 to drive ofi the moisture and expandthep'ar -ticles, and to such aggregate mixturethe A binder isthen applied, and the mass is then tumbled or agitated, with a'view jof; coating e the individual particles of the aggregate with a thet binder,
, While paving mixtures, "prepared in ac} cordance with this plan] have I merit, this method ofmixing has been found defective,"
f r, the reason that where the diiferent sizes E0 of aggregate are intermingled before the Application filed December 31,1925." Serial mam.
binder-is applied, there is a tendency of the larger particles of the aggregate to rob the smaller particlesof the binder, and, particularly,t o deprive thefine'r grades, especiallythe dust or filler, of proper impregnation withithebi'nder; Ithas been found that un der the old method, the finer grades of-mater1al,particularly the dust 'orfiller, will ball upin the interstices between the larger particlesiof the aggregate,and the binder will be prevented from gettingto and coat- 1 in and im re hatin the fine individual :mate relation, and that the particles of age 6 p g b particles of the smaller sizes of aggregate,
'Thisinevitably results; in a mixture which, becauseof lnsuflicient and imperfect coating, conta ns. weakened and unbound areas,
which, when laid in a pavement and exposed to the stresses of trafiic andthe attacks of the weather, will quickly disintegrate and breakdown. I f 7 i 4 7 It'is Witha view of'providing a method by following which-the complete coating and impregnating of all of the particles of ag gregate, from the filler'dust to the larger sizes, will be secured, and a mixture, free from faulty and weakened areas, produced, 1
so that,when laid, a pavement of uniform character' and great stability will be produced, that I have devised the present method of mixingaggregate and binder.
Themethod may be practiced with any of the well known types of'apparatus now; 1n use, such as mixers of theipug mill, rotary, or tumblertype, and this being the case, it is unnecessary to illustrate any specific apparatus for carrying out mynew and improved method, 7 c p In practicing the method'which I have invented, the aggregate will first be selected from appropriate material and graded into the desired sizes, it being common tograde material: Which is to form the aggregate from therfiller dust up tovmaterial of approximately one and one-half inch mesh in size.
These various grades of material will be'keptseparated,one from the other, and, as isusual, will be subjected to heat of sufii; cient temperature'to' thoroughly dry them,
and to expand the particles so as to put them in condition for receiving and absorbing'the coating and impregnating binder.
After the aggregate has been prepared, the binder will be put into the mixer of whichever type is selected, and if the binder is of the mineral pitch type, it will, preferably, be heated so as to be in liquid form in the mixer. The aggregate is then introduced in any suitable manner, by delivering to the mixer and the binder thereim thedifferent grades, and this introduction. of the successive grades of aggregate will be effected while the binder is being agitated in the mixer, so that as the successive grades are introduced, each grade will, in succession, be
subjected to a coating action. The mixing step will continue after the introduction of one grade of aggregate, until that grade has been thoroughly coated and impregnated; then another grade of aggregate will be in troduced, and the mixing operation continued until that succeeding grade has been thoroughly coated, impregnated and intermingled with the mass already in the mixing apparatus. By thus successively introducing and thoroughly mixing each grade as it g is introduced, it will be seen that the particles of aggregate under treatment in the mixer at any one time will all be of uniform size, and the tendency of the larger sizes to deprive the smaller sizes of the binder, which is-present where the aggregate is introduced in a mixed mass, is eliminated, the particles of uniform size, where they are successively -introduced, each having an equal opportunity to take up and coat and impregnate themselves with the binder. he particles already coated will not interfere with the uniform coating of the next succeeding grade of aggregate introduced, for, since the particles of aggregate first introduced have been thoroughly intermixed, and have received their share of the binder, the uncoated particles of-the succeeding grade will equally and uniformly take up their share of the binder.
I have found that, preferably, the aggregate should be introduced into the binder in the mixing apparatus by introducing first the line aggregate or filler dust, and then proceeding regularly through the other grades, with the next-grade in order of fineness, although it is conceivable that under some conditions, the order of introduction might be reversed or varied.
The reason for introducing the filler dust or fine aggregate first, is that this has a tendency, under the methods now in use, to a'gglomerate or ball up, so that it will-appear in the mixture in the form of coated pellets or small masses, which will be held by an envelope of binder, but with their interiors uncoated, and, consequently, unstable and friable.
This condition, obviously, interferes with the stability of the mixture, and
ticles of aggregate, and there is norobbing of the small particles of their proper proportion. of binder.
lVhen the filler dust has been thoroughly commingled with the binder, the next coarser grade will then be added, and it will take up its proper proportion of the binder, and the coating and impregnating of this next grade will not be interfered with by the already coated finer grade first introduced. This same condition will obtain for each succeeding grade, for there is not at any time in the mixer, uncoated aggregate of different grades, one of which grades, namely, the larger one, has a tendency to deprive a smaller grade of its proper share of the binder.
The aggregate may be introduced in any suitable or approved manner, but, preferably, it will be conveyed to the mixing apparatus and the binder therein in a diffused and scattered condition, so that it will not be dumped,
as a mass, into the binder, but will be delivered toitwith the particles separated from one another. This introduction of the aggregate in diffused and separated condition may be accomplished by any suitable scattering devices capable of delivering the aggregate to the mixer in a scattered condition. This could be done by delivering it down an inclined plane under the force of gravity, suitable scattering devices being provided to separate the stream of aggregate. It could be delivered by a suitable screw conveyor, having at its delivery end a scattering wheel, which would throw the aggregate over a con siderable area and shower it into the mixing apparatus, It can be projected in a shower, by a sand scattering belt of-familiar design. I have found, however, that'it may readily be conveyed, by air through a suitable pipe, having orifices of sufficient size to handle the largest grade of aggregate,
through which the aggregate is expelled, the
I claim 1. The method of mixing paving material, which consists in depositing in a suitable mixing apparatus a substantially liquid binder,
blowing into said mixing apparatus and to said binder in diffused and scattered condition the diiferent grades of aggregate, and agitating the mixture of binder and aggregate upon the introductionof each grade of aggregate. a
2. The method of mixing paving material, which consists in depositing in a suitable mixing apparatus a substantially liquid binder, blowing into said mixing apparatus and to said binder in successive orderof fineness and in difl'used and scattered condition the different grades of aggregate, and agitating the mixture of binder and aggregate upon the introduction of each grade of aggregate.
In testimony whereof I have hereunto set my hand. 7
WILLIAM TRENT RAGLAND.