US2180339A - Bituminous nonskid pavement - Google Patents

Description

Patented Nov. 21, 1939 UNITED STATES,"

BITUMINOUS NONSKID PAVEMENT Joseph H. Conzelman, Birmingham, Ala., assignor to Alabama Asphaltic Limestone Company, Birmingham, Ala, a corporation of Alabama No Drawing. Application July 2, 1937,

Serial No. 151,645

5 Claims.

This invention relates to a new and novel cold laid bituminous pavement and the method of laying it and has for its object to produce a highly tractive nonskid surface and to retain therewith the desirable features of good paving mixture design.

Where properly designed cold laid bituminous paving mixtures are used, either for full-thickness pavements or seal coats, it has been found that the resulting surface offers a low tractive resistance, and especially is this true if a slight excess of bituminous material is used in the dense low void mixture of conventional design.

In order to make road and street surfaces as nonskid as possible there has been considerable deviation from good paving mixture design by incorporating in the mixture a high percentage of voids, as it has been found that high void mixtures offer better tractive resistance than dense well designed bituminous mixtures. However, these open mixtures are not satisfactory in that they often lack toughness and stability and are further objectionable for the reason that the excessive voids in the mixture permit more or less rapid disintegration through the deteriorating action of moisture, air and light which penetrate it.

Seal coats on bituminous pavements or base first consisted of applications of bituminous material covered with crushed stone, slag, gravel or sand. Such seal coats contain excess of aggregate in spots that may ravel and excess of bitumen in others that result in slippery areas. These facts have led to the development of premixed seal coats containing a uniform amount of bitumen and offering possibilities for correctly graded aggregate. As in the case of most cold laid bituminous mixtures, premixed seal coats or seal coats mixed in place have been open so that necessary stability through evaporation oflique fier used can be developed and also for the purpose of making the surfaces nonskid- Such seal coat mixtures do not protect the surface or base they are laid on from moisture. They are subject to rapid deterioration from the effects of the moisture, light and air that their high void content allows to penetrate throughout the thickness of material laid.

I have found that the advantages of both the dense and the open mixtures can be combined in a cold laid surface or seal coat by spreading all but a predetermined portion of the dense mixture, then adding to the balance a sufiicient quantity of hard, abrasive aggregate to so change the character of the remaining portion that it will have a coarser texture. The body of the surface constructed in this manner will be tough, stable, and

impervious to moisture and the top portion containing the abrasive agent will be nonskid but still will contain suflicient well designed mixture to prevent the rapid weathering and raveling prevalent in open type mixtures.

In some localities bituminous coated broken stone, usually graded from A to in size, has been distributed over the surface of hot mix pavements during the process of rolling. These chips are then rolled, the intention being to anchor them in the hot, plastic surface to increase the frictional resistance of the pavement. Gen-. eral practice has been to use these stone chips in the quantity of from seven ('7) to twelve (12) pounds per square yard.- This is sufiicient quantity to result in pockets of chips in some areas Where only the bottom particles of aggregate are keyed into the hot mix surface. No effort is made to mix the bituminous coated chips with the hot surface mixture. The resulting surface contains a large percentage of open or high void spots in the pavement where chips segregate that hold moisture, with the. result that the chips not only disintegrate under traific but also through atmospheric action lose their bituminous coating and ravel out under traflic. Under this method of securing a non-skid surface it is difficult to anchor the nonskid aggregate uniformly over the pavement surface. The aggregate is rolled on the surface and is not mixed with the surface mixture as my invention contemplates.

Mixing this abrasive aggregate with the material in the upper portion of the surface mixture as contemplated by my invention can be accomplished in various ways. Mixtures'that can be spread with a motor patrol, or self-propelled road grader, may be bladed into place with the exception of the last five (5), ten (10) or twenty pounds of material to be spread. To this remaining windrow of material may be added the desired quantity of the abrasive aggregate, preferably coated with bituminous material limited in amount to that which will allow the material to separate into individual particles during the subsequent mixing process. The abrasive aggregate can then be thoroughly mixed with the remaining surface mixture and the resulting mix spread uniformly over the pavement surface by means of the motor patrol. The major portion of the surface so laid would be dense, water resistant, well designed bituminous mixture. The surface in contact with Vehicles would contain nonskid aggregate held firmly in place by the properly designed mixture that has been mixed with it.

There are also many other ways that this mixing of abrasive aggregate with the upper portion of cold laid surface mixture could be accomplished. The abrasive material may be mixed with the upper portion of the surfacing mixture with rakes, or it may be distributed over the surface of the cold laid mixture after it is spread and before it is rolled, and can be mixed with the upper portion of surface by use of a drag preferdisplace the upper particles of surface mix, mixing these particles with the abrasive aggregate. The intent of this invention is to mix abrasive aggregate, preferably having a light bituminous coating, with the upper portion of a dense cold laid paving mixture in such manner that the abrasive aggregate is mixed with and not merely imbedded in the surface mix.

As an example, in laying a forty pound per square yard seal coat or leveling course of Alabama asphaltic limestone cold mixture the material, spread by a motor patrol, would be bladed into place, building up the surface by wasting the material out light lightly under successive trips of the blade until approximately ten (10) pounds per square yard of the cold mix remained in the windrow. To this windrow would then be added a suilicient amount of the abrasive aggregate to add ten (10) pounds per square yard of aggregate to the material in the windrow. To be most effective the nonskid aggregate should be a hard stone, slag, gravel, or sand. The maximum size particles would preferably be This aggregate to facilitate bonding with the rock asphalt mixture is coated with as heavy a film of bitumen as the aggregate used will held without bonding the particles together. For this example a specially prepared hard, dense slag would be used, graded from to 10 mesh sizes and'coated with from three (3) to four (4) percent of asphalt cutback. When the abrasive, or nonskid aggregate has been added to the windrow it is thoroughly mixed with the cold laid asphaltic limestone by means of the self-propelled motor patrol and the spreading of the surface completed. Rolling of this surface can be carried on during the spreading of the mixture or after spreading has been completed.

It is obvious that any aggregate hard enough to stand abrasion better than the softer grade of paving aggregates would serve as a nonskid aggregate in the example just described if the nonslrid aggregate was graded satisfactorily. Preferably the aggregate used should not contain particles larger than in size and should not be graded so that it contains a low percentage of voids. Pea gravel; concrete sand; finer sand if at least fifty (50) percent is one commercial screen size; crushed hard stone, gravel or slag with the 50 mesh and liner particles removed if more than twenty (20) percent of such particles are present; any of these if mixed as described with the upper portion of a well graded cold mix will increase the frictional resistance of the resulting pavement.

By the expression well balanced dense aggregate employed in the claims, is meant an aggregate which has such a distribution of grades or sizes of particles that there are sufficient progressively finer grades to substantially fill the voids of the coarser ag egate.

By the expression unbalanced relatively open aggregate employed in the claims, is meant an aggregate having a disproportionate amount of some size or sizes of aggregate other than the finest so that substantial voids are present.

I claim as my invention:

1. A bituminous pavement comprising a compacted layer of bituminous mixture having a lower portion containing a well balanced dense aggregate, the particles of which are coated and 'ably containing pegs or teeth that agitate and bonded together with bitumen to form a stable water sealing strata and an upper portion containing an unbalanced relatively open aggregate the particles of which are coated and bonded together with bitumen to provide a nonskid trac tive surface, said upper portion forming a substantial part of said layer and merging into and being integral with said lower portion.

2. A bituminous pavement comprising a compacted layer of bituminous mixture having a lower major portion containing a well balanced dense aggregate with the particles thereof coated and bonded together with bitumen to form a stable water sealing strata and an upper portion containing said aggregate in admixture with an abrasive aggregate and providing an unbalanced relatively open resulting aggregate, the aggregate particles of said upper portion being coated and bonded together with bitumen to provide a nonskid tractive surface, said upper portion being a substantial part of said layer and merging into and being integral with said lower portion.

3. The method of laying a bituminous pavement which comprises spreading a main layer of well balanced dense aggregate having the particles thereof coated with bitumen, forming a second layer of unbalanced relatively open aggregate, having the particles thereof coated with bitumen, over said main layer prior to substantial compacting of said main layer and thereafter compacting said layers to produce a bituminous pavement having a lower portion forming a stable water sealing strata and an upper portion of substantial depth merging into and integral with said lower portion and providing an antiskid tractive surface.

4. The method of laying a bituminous pavement, which comprises spreading a layer of well balanced dense aggregate having the particles thereof coated with bitumen, mixing an abrasive aggregate having particles coated with bitumen with the upper portion of said layer prior to substantially compacting the same to produce a strata of substantial thickness containing an unbalanced relatively open aggregate, and compacting said layer to produce a bituminous pavement having a lower portion forming a stable water sealing strata and an upper portion of substantial thickness merging into and integral with said lower portion and providing an antiskid tractive surface.

5. The method of laying a bituminous pavement, which comprises spreading a main layer of bituminous mixture containing a well balanced dense aggregate having the particles thereof coated with bitumen, mixing another aggregate having particles coated with bitumen with an additional amount of said mixture to produce another bituminous mixture containing an unbalanced relatively open aggregate, spreading a layer of said other mixture over said main layer prior to substantial compacting of said main layer and thereafter compacting said layers to produce a bituminous pavement having a, lower major portion forming a stable water sealing strata and an upper portion of substantial thickness merging into and integral with said lower portion and providing an antiskid tractive surface.

JOSEPHH. CONZELMAN.