The specific gravity of the fraction between 315 degrees C. and 355 degrees C. shall not be less than 1.09 at 38°/15.5° C.

6. The oil shall yield not more than 2% of coke residue.

Oil "B" Coal-Tar Paving Oil. - The oil shall be a coal-tar product of which at least 65 per cent. shall be a distillate of coal-gas tar or coke-oven tar, and the remainder shall be refined or filtered coal-gas tar or coke-oven tar. It shall comply with the following requirements:

1. It shall not contain more than 3% of water.

2. It shall not contain more than 3% matter insoluble in benzol.

3. The specific gravity of the oil at 38 degrees C. shall not be less than 1.07 nor more than 1.12.

4. The distillates based on water-free oil shall be within the following limits: Up to 210 degrees C, nor more than 5%.

Up to 235 degrees C, not more than 25%.

The residue above 355 degrees C, if it exceeds 35%, shall have a float test of not more than 80 seconds at 70 degrees C.

5. The specific gravity of the fraction between 235 degrees and 315 degrees C shall not be less than 1.02 at 38°/15.5° C.

The specific gravity of the fraction between 315 degrees and 355 degrees G. shall not be less than 1.09 at 38°/15.5° C.

6. The oil shall not yield more than 2% coke residue.

Treatment. - The timber may be either air seasoned or green, but should preferably be treated within three months from the time it is sawed. Green timber and seasoned timber shall not, however, be treated together in the same charge. The block shall be treated in an air-tight cylinder with the preservative heretofore specified. In all cases, whether thoroughly air-seasoned or green, they shall first be subject to live steam at a temperature between 220 and 240 degrees F., for not less than two hours nor more than four hours, after which they shall be subjected to a vacuum of not less than 22 inches, held for at least one hour. While the vacuum is still on, the preservative oil, heated to a temperature of between 180 degrees and 220 degrees F., shall be run in until the cylinder is completely filled, care being taken that no air is admitted. Pressure shall then be gradually applied not to exceed 50 pounds at the end of the first hour, nor a hundred pounds at the end of the second hour and then maintained at not less than 100 pounds nor more than 150 pounds until the wood has absorbed the required amount of oil.

After this a supplemental vacuum shall be applied, in which the maximum intensity reached shall be at least 20 inches and shall continue for a period of not less than 30 minutes. This vacuum shall be either preceded or followed by a short steaming period.

In any charge the blocks shall contain at least 16 pounds of water-free oil per cubic foot of wood at the completion of the treatment. The blocks after treatment shall show satisfactory penetration of the preservative, and in all cases the preservative must be diffused throughout the sapwood. To determine this, at least 25 blocks shall be selected from various parts of each charge, and sawed in half at right angle to the fibres through the center, and if more than one of these blocks show untreated sapwood the charge shall be retreated. After retreating, the charge shall again be subjected to a similar inspection.

The surface of the blocks after treatment, shall be free from deposit of objectionable substances, and all blocks that have been materially warped, checked or otherwise injured in the process of treatment shall be rejected.

Handling Blocks After Treatment. - Blocks shall preferably be laid in the street as soon as possible after being treated. If they can not be laid within two days, provision shall be made to prevent them from drying out by stacking in close piles and covering them, and, if possible, sprinkling them thoroughly at intervals. The blocks shall be well sprinkled, under the direction of the Engineer, about two days before being laid.

Inspection. - All material herein specified and processes used in the manufacture of the blocks therefor, shall be subject to inspection, acceptance or rejection at the plant of the manufacturer, which shall be equipped with all the necessary gauges, appliances and facilities to enable the inspector to satisfy himself that the requirements of the specifications are fulfilled.

Each time the manufacturer of the blocks is ready to make blocks, he shall notify the Engineer sufficient time in advance to allow inspector to reach the plant. No blocks shall be manufactured except when the inspector is at the plant.

The City shall have the further right to inspect the blocks after delivery upon the street, for the purpose of rejecting any blocks that do not meet these specifications, except that the plant inspection shall be final with respect to oil and treatment.

Concrete Base. - Where the street to be improved has no old concrete base, or where, if same exists, it is deemed so defective as to require replacement, the subgrade for the concrete base shall be prepared and rolled as specified under the head of Grading, Item 1, so as to be --------- inches below and parallel to the grade of the finished paving. On this subgrade will be deposited a layer of concrete of the required class and to a depth of---------inches.

The top of the concrete foundation shall be brought by screeding or otherwise to a grade which shall not vary more than one-quarter (1/4) inch at any point from a surface parallel to and four and one-quarter (4 1/4) inches below the finished surface of the pavement.

Where the street to be improved has an existing concrete base which is to be retained, an additional layer of concrete of the required class shall be spread over the old base to such depth as is necessary to form a surface of the same grade as is above specified for a new concrete base. This work shall be done as specified under the head of Concrete, Items 30-32.