The incidence and phenomenon of crazing is as old as the mastic asphalt industry itself, and yet comparatively little is known and less is written about its cause and cure than any other practical or technical aspect of asphalt application.

Crazing is solely a surface phenomenon which occurs periodically on mastic asphalt surfaces exposed to direct sunlight. It begins usually as a network of fine vein like lines and these in extreme cases, develop into shallow fissures as much as 1/4 inch in width and 3/16 inch deep.

The appearance is most unsightly and, to the average observer, disquieting, for there is every appearance of potential failure of the asphalt surface. From what is known and from personal experience of over 50 years, the incidence and the extent of crazing was less in the early days of the industry than in the years following the First World War.

Formerly the mastic asphalt industry was in the hands of a few specialist firms of wide experience who based their specifications on raw materials of high and proven quality.

Remelting was also a slower process while the quality of workmanship in laying was an essential feature of the finished asphalt surface, rather than an insistence on the minimum yardage in a limited time.

The First World War changed this. Rock asphalt supplies ceased and other raw materials were difficult to obtain. The result was that many substitute materials had to be used to provide mastic asphalt to meet the demand of the war effort. The experience and the ingenuity of the established asphalt companies overcame the difficulty but this opened the door to a whole range of substitute materials and with the increasing volume of work and the new companies entering the field, there was the combination of circumstances which could and did lead to a greater incidence of crazing. It must be said, however, that it was not the use of new materials in themselves which gave rise to an increase in the defect but rather the changes in the techniques of manufacture and the laying and in some cases to a lack of knowledge and experience.

With the establishment in the 1920's of a large demand, the industry soon gave greater attention to the technical aspects of manufacture of control and of application of mastic asphalt. With new methods of examination allied to new apparatus, investigations were focused on problems of long standing so that in 1926 an investigation was commenced to determine the cause, and if possible, the cure for crazing. Apparatus was developed for this work, but the initial investigation was not very fruitful. However as a result of continued work the results required were achieved and the following notes were taken from the final report.

` Mastic Asphalt, when used as a roofing material and fully exposed to normal weather conditions, in particular, direct irradiation by heat and light from the sun, is prone to suffer from the defect known as “crazing” The effect on the appearance of the roof can be serious. It is possible for a high standard of workmanship to be largely counteracted by these surface cracks, which may also, to the lay mind, give the impression of genuine fracture of the asphalt.

` Although initially incapable of seriously affecting the watertight character of the mastic, crazing can, in extreme cases, reduce the effective “life” of an asphalt roof since it allows water to penetrate the surface “skin” and accelerate the normal weathering of the material.

` A detailed examination of this phenomena has been undertaken and at this point it should be emphasized that the conclusions and statements set forth in this report were arrived at by the co ordination of facts and data gathered from an extensive practical investigation supported by parallel laboratory work'

` Before proceeding to discuss these matters, it would be advisable to emphasize that ”crazing” in mastic asphalt must not be confused with genuine cracking of the material - it is as widely different in physical characteristics as it is in fundamental cause.

` “ Crazing” may take the form either of a network of small, intersecting “hair” cracks or much larger but comparatively widely separated fissures. In every case, however, it is initially purely a skin effect. The cracks are never very deep, even in the most aggravated cases the depth rarely exceeds 1/8 inch to 3/16 inch, although the width of the surface may be greater.

` It is a fact that “crazing” only occurs on material directly exposed to the rays of the sun. Asphalt which is effectively shaded or screened rarely contains any traces of this defect. There can be no doubt that the fundamental cause of “crazing” is the intermittent irradiation of a comparatively unstable and richly bituminous skin on the surface of the asphalt (see Fig 1)

During the process of laying, the wooden float will bring to the surface a skin of richly bituminous material. Owing to the extremely poor heat conductivity of asphalt, this surface material under solar irradiation will at first assume a temperature considerably higher than that of the lower strata. It will tend to expand laterally, but as this is naturally rendered impossible by the fixed dimensions of roof, this surface skin will corrugate slightly ( see Fig 2)

` Laboratory measurement shows that this bituminous skin has a co efficient of expansion three to five times as great as the remainder of the material. When however the source of irradiation is withdrawn, and the surface becomes subject, possibly to the cooling effects of wind or rain, this bituminous skin begins to contract rapidly. The lower strata are still warm and occupy a greater than normal volume. This fact, together with the relatively high rate of contraction of the bituminous skin creates a state of strain in the latter. After numerous repetitions of this cycle of unequal expansions and contractions a minute “tear” crack appears at the crest of the surface corrugation (Fig 3) which mechanically weakens the the skin and accelerates its own development.