It is hardly ever possible to secure a natural sand that will conform strictly to the theoretical composition, but substantial compliance should be required unless the Chief Engineer directs otherwise. After any sand has been approved for a given contract or street, all the sand thereafter used for that street should conform closely to the standard so approved; for any material change in the quality of the sand will change the quality of the pavement produced, unless the proportions of the other materials are changed accordingly. Any material change in the sand should therefore be promptly reported to the Chief Engineer.
32. Sand for the sieve tests should be taken from the hot-sand box and allowed to become cold. From the sample thus taken, weigh out 50 grams. Arrange the nest of sieves with the No. 200 at the bottom followed by Nos. 100, No. 80, No. 50, No. 30 and No. 10 at the top.
Place the weighed sample upon the No. 10 sieve and thoroughly shake the nest until all the sand that will, has passed through the various sieves; then weigh the various portions, beginning with that which has passed the No. 200 (deducting in each case the weight of the dish). The portion remaining on the No. 200, is the quantity passing the No. 100, etc. The sum of all these portions should be 50 grams, though it will usually be slightly less because of unavoidable waste. In use, keep the sieves clean and as free as possible from adhering grains by using a bristle brush.
33. Temperature of the Sand.—The temperature of the heated sand should be frequently noted by plunging the thermometer into the mass. It should not fall below 315° F. nor exceed 375° F. It is very important to avoid over-hot sand, because, in the mixer, the very thin films of bitumen covering the hot-sand grains are readily and quickly scorched by the excessive heat in the sand.
34. Refined Asphalt and Fluxing Oil.—Unless otherwise notified by the Chief Engineer you will assume that the stock of refined asphalt on hand at the beginning of the work has been inspected and is satisfactory. You will, however, note the appearance of the material and the character and marking of the barrels or packages, and will not permit the use of any new shipments of materials of different appearance or brand without authority from the Chief Engineer. The above instructions apply also to the oil used for fluxing.
35. Ratio of Asphalt and Flux.—You will frequently note and record the weight of asphalt and of flux used in charging the melting tanks.
36. Temperature in Melting Tanks.—The melting kettles are usually equipped with fixed thermometers, but their reading should be frequently checked with your detached tank thermometer. You will see that the temperatures in the kettles are kept within the limits named in the specifications. See that the melted asphaltic cement is kept constantly agitated as required by the specifications.
37. Uniformity of Asphaltic Cement.—It is very important that the asphaltic cement shall be of uniform composition and consistency, or hardness.
When the composition and consistency of the asphaltic cement to be used for the surface-course of any street has been determined upon and approved by the engineer, you will take out and preserve a sufficiently large quantity to be used thereafter as a standard, and each subsequent tank of cement, before it is used, should be tested by comparing it with this standard in the following manner, using the flow-plate apparatus:
Melt and fill two cartridges with the standard cement, and two with the cement to be tested, marking them for identification. When cold, carefully remove with a wetted dull knife any surplus cement from the ends of the cartridges so as to leave them exactly full and the metal ends clean. Immerse them in cold water until they are all of the same temperature. Then place and fasten these filled cartridges on the grooved plate with their lower ends exactly to the zero line on the plate, the standard samples alternating with the new. Then place the plate, at an inclination of about 45°, in front of a source of uniformly distributed heat, as, for instance, near a steam boiler or large steam pipe, or (when the weather is favorable) in the sunshine, facing the sun. Leave the plate in this position until the bitumen in the cartridges shall have melted and flowed down the grooves an average of from four to five inches. Then measure the lengths of flow of each from the zero line on the plate. If the average flow of the new samples differs more than 5 per cent. from that of the standards, the new cement should be regarded as unsatisfactory and should be corrected by adding more asphalt or flux, as may be required.