(If a penetration apparatus is supplied or is available, the penetration method may be used instead of this flow-method for determining the consistency of the cements. Proper instructions for its use will accompany the apparatus.)
38. Ratio of Sand and Cement.—When the proper ratio of sand and asphaltic cement has been determined for any street you will see that this ratio is carefully adhered to. The correctness of the weighing scales used should be frequently tested.
39. Mixing.—When the sand and asphaltic cement are properly mixed the appearance of the mass will be perfectly uniform throughout, with every grain of sand entirely and evenly coated with cement. The mixing should, however, be continued somewhat beyond the attainment of this condition, so as to insure thoroughness. After the proper result has been arrived at, either the time in the mixer, or the number of revolutions of the mixer shaft should be noted and the standard thus determined substantially adhered to thereafter. With the standard mixers commonly used, the hot sand and the filler (dust) should first be put into the mixer and mixed for about fifteen seconds; then the asphaltic cement added and the mixing continued as long as necessary (in the usual mixer not less than sixty seconds).
40. Records.—You will keep complete records of the results of your observations and tests in a book which will be provided for the purpose, and will make such reports to the Engineer as he may direct.
41. Inspection on the Street.—The leading things to be kept in mind by the street inspector, in the laying of asphalt pavement on the street, are the following:
a. Temperature of mixtures when applied to the street. b. Proper preparation of the street surface before material is laid. c. Laying and rolling of the binder-course. d. Laying of the surface-course. e. Rolling the surface-course.
42. Inspector’s Equipment.—The inspector will be supplied with the following apparatus:
1 armored thermometer reading from 200° F. to 500° F. 1 graduated spatula. 1 twelve-foot straight-edge. 1 fifty-foot tape line.
43. Temperature of Mixture.—If proper care has been taken at the mixing plant the mixture will not arrive on the street at too high a temperature, and your principal care will be to see that it is not laid at too low a temperature. The minimum temperature named in the specifications should be adhered to. While the main or interior mass of a wagon load may be well above that temperature, the top and outer part of the load may be, particularly in cold weather, too cold to be safely used. Some of this colder portion may usually be sufficiently reheated by mixing it with the hotter material, if properly handled in unloading; but any material that is so cold as to be lumpy when unloaded, or, more particularly, when being raked out, should be discarded. This applies to both surface and binder mixtures.
It must not, however, be overlooked that the different kinds of asphalts have different appropriate working temperatures, and for some asphalts the minimum temperature named in the specifications may be too low. The best practical guide is the manner in which the mixture behaves in raking. It must always be so hot that it will, under the rake, break up into a uniform, crumbling or powdery mass. If it does not do this it is (unless the mixture is excessively rich in bitumen or improperly compounded) too cold.