Even though the materials may be the best, there is an abundant opportunity for failure through improper manipulation in heating and mixing the materials.
The asphalt may have been damaged by overheating or " burning." The burning of the asphalt causes the pavement to disintegrate on the surface in spots during cold weather, and may be revealed by a brittleness and a tendency to crack while being rolled. Excessive heat converts the petroline, or cementi tious constituent of asphalt, into asphaltine which is devoid of cementing properties, and by so much reduces the cementing quality—the vital element—of the asphalt. This overheating may take place during the refining (see § 590), or during the fluxing (see § 609), or in mixing the asphaltic cement and the sand (see § 627).
In practice there is much carelessness in melting the asphalt. Not infrequently the kettle is mounted within brick walls directly over a fire which comes in contact with only a comparatively small part of the heating surface, in which case it is highly improbable that the firing will be done so evenly and slowly as not to burn at least part of the material. The fire should not be allowed to come in direct contact with the melting kettle or tank, thereby guaranteeing that no portion of the asphalt can be burned. When the asphalt has been badly burned, it will be revealed by a brittleness during roll ing; but there is no way of determining a lesser degree of burning, although it still may be sufficient to cause a serious defect which will finally develop into cracks and rotten spots. Therefore the inspector should insist upon a method of melting that will insure an unburned product. It is sometimes specified that the asphalt shall be heated by steam.
The overheating of the asphalt may be produced also by over heating the sand (see § 627). Every precaution should be used to have each batch of sand heated uniformly throughout, and its tem perature should be taken before mixing it with the asphalt. As a further check, the temperature of each load of paving compound sent to the street should be taken and recorded at the mixing plant.
Improper Consistency. The paving cement may have been mixed too hard or too soft (see § 613). If the cement is too hard,
the pavement will have a tendency to crack during cold weather; and if it is too soft, it will push out of place and form rolls or waves under the traffic.
The wearing coat may not have contained sufficient cementing material. It should contain at least 9 per cent of bitumen soluble in carbon bisulphide (see § 611). Within the limits imposed by the proper softness and hardness of the pavement, the greater the per cent of asphalt the greater the life of the pavement; and consequently contractors in laying a pavement under a long-time guarantee use the maximum amount of asphaltic cement, but when the maintenance period is short they generally use the minimum. Owing to improper manipulation the amount of bitumen is likely to be too small, since in fluxing the tendency is for the bitumen to rise and the mineral impurities to settle; and consequently if the tank is worked too low, there is a likelihood that the last material taken from the tank will contain too small a pro portion of bitumen and too large a proportion of sediment. In sufficient bitumen has substantially the same effect upon the pave ment as improper asphalt.
The ingredients of the wearing coat may not have been sufficiently mixed. It is important that each grain of sand shall be entirely surrounded by the cementing mate rial, so that no two pieces shall come into actual contact. If the mixing is not well done, the pavement will disintegrate in spots.
If an excess of asphalt or coal is used in the binder course, it is likely to work to the surface of that course and then being absorbed by the wearing coat cause it to disintegrate. This cause of failure manifests itself by irregular blotches on the surface of the pavement.
The mixture for the wearing coat may become chilled while being transported from the mixing plant to the street. To prevent this possibility, the temperature of each load should be taken just before it is laid. The material may also be come chilled by a delay in tamping and rolling, or by attempting to work during too cold weather or during the prevalence of a high wind. A batch of chilled mixture will cause a weak spot in the pavement.