Preparation. - The wearing surface shall be composed of sand filler and asphalt cement of the character elsewhere herein specified and mixed in the proper and definite proportions. When more than one kind of sand is used, each kind of sand shall be kept in a separate pile at the boot of the elevator, and the various sands shall be carefully fed into the elevator in definite and accurate proportions, such as may be required to produce a mineral aggregate of the composition desired.

The sand shall pass through a suitable drier which will heat the material to a temperature of from 300° to 400° F. The drier is to be of such construction that the various sands used shall be thoroughly mixed. Before being run into the storage bin, the heated aggregate shall pass through a suitable screen, which shall reject all material larger than that which will pass a 10-mesh sieve.

The sand, filler and asphalt cement shall all be weighed out in definite proportions. These proportions will depend upon the type of mixture desired, and must be approved by the Engineer. The scales and weighing box must be satisfactory to the Engineer, and their accuracy will be subject to verification at such times as the Engineer may direct.

The mixing shall be done in such a way that a thoroughly homogeneous mixture shall be produced. The type of mixer, known as the "Twin-Pug Mill" shall be used. The sand will first be weighed out in a suitable weighing box, resting upon an approved scale, and from this weighing box shall be run into the mixer. The dust shall then be weighed and added cold, and finally, the asphalt cement, heated to a temperature of 275° to 350° F., shall be added. The materials shall then be mixed until every particle of the material aggregate is thoroughly coated with the asphalt cement. Proper mixing will require at least one minute.

Proportions. - The finished pavement shall show upon analysis a mesh composition and bitumen content within the following limits:

Bitumen ...............................................................................................

10.0

to

13.0%

Passing

200-mesh

sieve

..............................................

10.0

to

15.0%

,,

100- "

"

...........................................

9 0

to

18.0%

,,

80- "

"

.............................................

9.0

to

18.0%

"

50- "

"

..............................................

10.0

to

35.0%

"

40- "

"

................................................

10.0

to

25.0%

"

30- "

"

.............................................

6.0

to

15.0%

"

20- "

"

.............................................

4.0

to

10.0%

"

10- "

"

...........................................

2.0

to

8.0%

The minimum amount of bitumen shall be used only in mixture containing the minimum total passing the 80-mesh sieve. The percentage of bitumen must be increased above the minimum as the total passing the 80-mesh is increased. On streets of very light traffic when the Engineer has approved the use of a coarser sand or mixture than that specified for general use, the surface mixture must contain not less than 6% of mineral matter passing a 200-mesh sieve, and not less than a combined total of 18% passing the 200, 100 and 80-mesh sieves. The maximum amount of 200, 100 and 80-mesh material will be regulated according to the kind of sand and asphalt used and the traffic upon the street on which the pavement is to be laid, subject to the maximum requirements elsewhere herein specified under sand and filler.

The above limits as to mesh composition and per cent. of bitumen are intended to provide for such permissible variations as may be rendered necessary by the raw materials used and by the character of the work to be done. The composition of the wearing surface may be varied within the limits above specified at the discretion of the Engineer.

Laying. - Before any top is laid, the concrete base must have been accepted by the Engineer. This acceptance will be given only when the concrete is thoroughly set and perfectly dry over the entire surface. Under no circumstances will top be laid on damp concrete or binder.

The surface mixture will be hauled to the street in dump wagons, which will be covered with suitable covers in order to protect the contents from loss in temperature. The loads shall have a temperature of from 225° to 350° F. upon reaching the street. The Engineer will determine the permissible range of temperature for each kind of asphalt used. All loads failing to fall within the permissible temperature are subject to rejection at the street. The mixture shall be dumped upon the base at some distance from point at which the material will be used. The mixture will then be shoveled into place and will be spread out by means of rakes. In raking, the mixture must be combed out thoroughly entirely down to the concrete, in order that the wearing surface may be of uniform density at all points The surface shall be spread out to such a thickness that it will have the specified thickness after having received its final compression. The surface will then be rolled with a steam roller weighing not less than eight tons. It is permissible to smooth the surface with a lighter roller, but the final rolling must be done with a roller of the weight specified above. After the first rolling, the surface shall be dusted with cement or other mineral dust. The rolling must be continued at the rate of not more than 200 square yards per hour until no more compression is obtainable and the surface is free from waves, roller marks and honeycombed spots.

Such portions of the completed pavement as are defective in finish, compression or composition, or that do not comply in all respects with the requirements of these specifications, snail be taken up entirely, removed and replaced with suitable material, property made ana Paid in accordance with these specifications, at the expense of the contractor.

Where the width of the roadway will permit, the street is to be cross-rolled. Around manholes and other fixtures, where the roller can not reach, compression may be obtained by using hot iron tampers. All gutters and street fixtures must be painted with hot asphalt cement before the top is laid in order to secure proper adhesion at the joint and to prevent the entrance of water into the joint. Whenever ordered by the Engineer, a space twelve inches on the top of the finished pavement next to the curb shall be coated with asphalt cement which shall be ironed into the pavement with hot smoothing irons.

The asphalt plant shall be shut down each day at such time as will enable the loads to be hauled to the street, laid and completely rolled before dark.

The asphalt plant shall also shut down without delay should rain come up during the work, as no loads shall be laid in the rain or on wet concrete base.

No wearing surface shall be laid when, in the opinion of the Engineer, the weather conditions are unsuitable or unless the binder on which it is to be placed is dry. Excessive use of water on the steam roller when compressing the pavement will not be permitted. The finished pavement must be well protected from all traffic by suitable barricades until it is in proper condition to use.