surfaces formed by the fracture. The danger of structural failure at interior corners is thus greatly minimized. As this would seem to be a most important function of pavement reinforcement it appears logical that the minimum amount of steel used should at least be sufficient to resist the stresses set up by contraction of the pavement, and be proportioned in the manner recommended by the before mentioned committee.

In order to promote economy by reducing the variety of mesh designs called for and in other ways it would seem logical to make transverse joint spacing equal to the width of the pavement so that the area of steel called for in each direction would be equal. By so doing only a few styles and weights of fabric reinforcement, which would be determined by the width of the street and thickness of the pavement, would be called for.

Integral curbs add to the edge strength of a street pavement. Dowling combined curb and gutter construction to the pavement increases the edge strength of the pavement and prevents the settlement of the combined curb and gutter.

TRAFFIC FACILITIES

In street work it would seem wise to provide ten feet of width for each lane to be kept open for the passage of vehicles and at least six feet additional for each strip to be allowed for parallel parking. The minimum curb radius at street corners should not be less than the turning radius of the longest ordinary vehicle (about fifteen (15) feet) and preferably greater on narrow streets to improve sight distance and avoid cramped intersections.

Pedestrian traffic should not be penalized by water-filled gutters at sidewalk crossings. This may be avoided by decreasing the fall from crown to curb as sidewalk crossings are approached and placing a small inlet, midway between the sidewalk extensions at each corner of the intersection. The gutter dip to such inlets from points opposite the center of the walks need be but a few inches and should be made by reverse vertical curves. The gutter grade to the inlets away from the intersection should also be made by easy reverse curves. If judgment is used in laying out the gutter grades the waves in the grade line need be felt by right turn traffic only, and then but slightly at the speed proper in making such turns.

APPEARANCE

While the very nature and purpose of pavement does not permit of much in the way of beautification, certain unsightly features may at least be avoided. Among these may be mentioned, square-cornered curb cross sections, short-radius curb curves at intersections, choppy curb grades, and poor curb alignment, blotchy surface finish due to spot troweling without subsequent general surface belting, irregular alignment of expansion joints or other dividing planes, unsightly irregular contraction cracks caused by dividing planes too widely spaced or the lack of sufficient steel, unsightly inlet designs, poorly graded parkways, etc.

It is obvious that these and many other important features of concrete street pavement design and construction merit further discussion and investigation. It is hoped, however, that this brief discussion may be helpful.

A NEW ABRASION TEST FOR CONCRETE

By C. H. Scholer, Road Materials Testing Laboratory, Kansas State Highway Commission, Manhattan, Kansas.

The satisfactory service which a concrete road or floor will give is dependent upon the resistance of the concrete to wear. A concrete floor which does not have a high resistance to abrasion will dust and thus is very undesirable in any building. A pavement which does not wear well is both unsightly and rough.

The materials which go into work which is carefully done are always tested and an effort is made to produce a concrete which is strong and has good wearing qualities, but the desired results are not always gotten because there are other factors which enter in besides the quality of the materials. The strength of concrete can be predicted before it is made and it is a comparatively simple matter to get data on the actual strength of it as it is in the structure, but the wearing qualities are not so easily obtained.

Tests to determine the actual resistance to abrasion which a concrete made from a given set of materials and under given conditions will have are not ordinarily made because the use of a Talbot-Jones rattler has been necessary. The first cost of this machine is high and the operation expensive. It consists of a cast iron drum around the perimeter of which 10 blocks of concrete eight inches square and five inches thick are fitted in the form of a polygon. The outside diameter of the polygon is 36 inches and the inside 26 inches. The specimen placed in this form presents a nearly continuous surface for wear. The chief disadvantages of this machine are first, that it is necessary to have ten specimens before a test can be run, and, second, that the apparatus has no other use than the testing of the abrasion of concrete.

The making of concrete into specimens the same size and shape as paving brick and giving them the standard paving brick test has been tried and is not satisfactory because the surface presented for wear is not similar to those surfaces upon which wear takes place in actual service. Concrete cylinders 8" in diameter and 16" long were tried in this laboratory and

it was found that most of the wear came on the ends of the specimen and none on the center. To avoid this difficulty, cylinders 8" in diameter and 8" high were tried, but the results were no better. Having the above facts in mind the Road Materials Labratory of the Kansas State Agricultural College began experimenting in 1921 on the making of an abrasion test of concrete, using spherical specimens in a standard brick rattler. The first specimens were made in a concrete mold which was made by pouring a concrete block 12 inches square and 6 inches high around a 9-inch hemispherical wooden block nailed to a plane surface. One of these blocks was cast solid and the second with a tapered hole or gate in the top. By placing these two blocks together a spherical form 9 inches in diameter with a 3-inch hole at the top for putting in concrete was formed. The form was well saturated with hot paraffin before the concrete for the specimens was poured. This type of mold gave satisfactory specimens, but was cumbersome and difficult to handle. The College foundry made up a cast iron mold consisting of two hemish peres, with half of the tapered opening for filling on each hemisphere. A dozen of these molds are now in use and are proving satisfactory.

The determination of the number of specimens, the charge of shot to be used in the rattler, and the duration of the test or number of revolutions which the brick rattler should run to give the best results were problems to be worked out before any tests could be made. It was found that the weight of three of the balls would average between 90 and 100 pounds and as this was reasonably close to the weight of a standard brick charge, the standard charge of steel shot, consisting of 10 three and three-fourths-inch shot, weighing 75 pounds and enough one and seven-eighth-inch shot to make a total of 300 lbs. was used. The first test specimens were cured for 28 days and run for 1800 revolutions. The loss on these specimens was so high that it was decided that a longer curing period would be necessary and fewer revolutions of the rattler in order to make the test practical.

A curing period consisting of 28 days in damp sand and 32 days in air was found to give the best results, and the number of revolutions of the rattler was reduced to 900.

After the abrasion test had been made upon the specimens a core approximately 41⁄2 inches in diameter was drilled from

each ball with a calix steel shot core drill. These cylinders were capped with plaster of Paris and cement and tested for compressive strength in a 200,000-pound Olsen compression machine. This procedure eliminated the necessity for making separate compression specimens and gave accurate information as to the relation between compressive strength and the abrasion of concrete.

The first series of tests to be run was on concretes made from stones of varying French coefficients. Stone from different parts of Kansas were obtained through the co-operation of the county and resident engineers. The French coefficients of three stones varied from 2.5 to 19 as shown in table 3.

The object of the series was to study the relation between the wearing quality of the concrete and the wearing quality of the coarse aggregate, and to study the effect of the brand of cement used on the resistance to abrasion of the concrete made from that brand.

The two brands of Portland cement used in these tests were purchased from the Manhattan dealers in sufficient quantities to complete the series of tests. The total quantity of each brand was thoroughly mixed dry in a concrete mixer and stored in air-tight containers. The standard cement tests were made and the results of the tests are shown in table 1.

The fine aggregate used consisted of washed sand from the Blue River and was furnished by the Manhattan Sand Co. The sand was thoroughly mixed before it was stored and a representative sample taken for standard tests. The results of the standard tests of this material are shown in table 2.

The coarse aggregate was shipped in from various points in the State in 400 to 500 pound samples. It was broken by hand to such a size that the mechanical analysis corresponded to that required by the Kansas State Highway specifications for coarse aggregate for concrete for one course concrete pavement or a 1:2:311⁄2 mix by volume.

The mixing water was from the College supply pumped from wells on the campus.

The weights per cubic foot of the aggregates were determined by means of a machined, cast-iron measure having a capacity of 1/10 cubic foot. Since an effort was being made to duplicate field conditions,, the weight per cubic foot loose volume instead of puddled volume, was used. The test was