Composition of Mixture. The relative amounts of asphalt cement filler, and sand required for a surface mixture must of course depend upon the properties of 'these materials. The fineness of the sand and of the filler, and the amount of mineral matter in the cement, are all important in determining the proper propor tions. The proportions of materials are determined by weight, the purpose being to secure a proper amount of bitumen and of dust, as compared with the sand in the resulting mixture. The amount of bitumen required varies from about 9 per cent to 13 per cent, most commonly between 10 per cent and 11 per cent. Mr. Richardson gives* the following mixture as a standard to be used for surfaces of Trinidad asphalt pavements: This he regards as an exceptionally good mixture. As the sands used in practice vary in the proportion of fine grain which they contain, the amount of bitumen must be correspondingly varied. When a larger portion of the sand passes the 8o and 100-mesh sieves, a larger amount of bitumen and of filler may be introduced. When the sand is coarser, a smaller amount of bitumen is necessary in order that the pavement may not be soft enough to mark under the horses' feet. In sand which lacks the finer grains the percentage of bitumen which can be used without marking is often so low as to leave the material too porous and liable to the action of water. It is desir able that the mixture contain all the bitumen that it will carry by the addition of filler without becoming too soft. A lack of bitumen may cause cracking of the surface. The quantity of bitumen required is also somewhat affected by the character of the sand grains and the extent to which the bitumen may adhere to and coat the grains. Some sands will. more bitumen than others of the same grading of sizes without leaving a surplus of bitumen to render the material too soft.
The amount of bitumen to be used in a surface mix ture is commonly tested by the pat test. This consists in pressing a pat of the surface material in a piece of brown manila paper and observing the stain left upon the paper; the depth of the stain indicates to the experienced eye whether the right amount of bitumen has been used and whether the mixture has been prop erly prepared. An impact test is also sometimes made to determine the resistance of the surface material to marking, and frequent analyses are made to test the correctness of the mixture.
The traffic to which a street is subjected has much to do with the consistency required in the surface mixture. For streets of light traffic a softer mixture
should be employed than for one with heavy traffic. The rolling out and working of the surface by heavy traffic will admit of a hard surface material which might crack under light traffic. The surface mixtures must in every case be suited to the local conditions of traffic and weather, that it may neither mark under the impact of traffic nor crack from shrinkage in cold weather.
Method of Mixing. In the preparation of the surface mixture, the sand and asphalt cement are heated separately and then mixed while hot. When two or more sands are used to obtain the proper grading of sizes, this mixing must first be accomplished, and great care is necessary in handling the sand in mixer and heater to prevent the segregation of sizes and bring the sand in uniform mixture at proper temperature (about to 35o° F.) to the final mixture. The asphalt cement is also heated in a large heater where it is agitated by steam jets to maintain the uniformity of mixture.
The surface mixture is prepared in a mixer of small size which mixes to to 15 cubic feet at one operation, and is so arranged as to load directly into the wagon which takes it to the street. The mixing is accom plished by blades revolving on shafts in the mixing tanks, requiring about one to two minutes to make a complete mixture. The proportioning of the ingredi ents is accomplished by weighing the proper quantity of each of the materials for a batch; the sand and filler are first introduced and mixed dry, and the asphalt cement then added and the whole mixed together. The mixture is then carried to the street at a tempera ture above 300° F.
Rock Asphalt. The preparation of surface material with rock asphalt consists only in crushing and grinding the rock to powder, and heating the powder to drive off the water and soften the bitumen, so that it may be compacted in the pavement. The powder is, heated to a temperature of 200° to 30o° F. and is applied hot in laying the surface.
In determining a mixture of asphalt rock, as in the case of other asphalts, the local conditions of climate and traffic must be considered and the quantity of bitumen be so proportioned as to remain solid in sum mer and not to become brittle and lose cohesion in winter. Experience with the material and exercise of great care in the determination of proper proportions are therefore essential to success in the construction of any asphalt pavement.