crazing

<body topmargin=0 leftmargin=0 marginheight=0 marginwidth=0> <table cellpadding=0 cellspacing=0 border=0 height="100%"><tr> <td valign="top" BGCOLOR="#CCCCCC" TEXT="#080000"> <div> <B>The Crazing of Mastic Asphalt Roofing</B></div> <div> By A.W. Attwooll </div> <div> <I>I.A.T Journal - No 8 September 1969</I></div> <div> 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.</div> <div> <IMG SRC="39085e070.jpg" border=0 width="350" height="263" ALT="example of crazing" ALIGN="BOTTOM" HSPACE="0" VSPACE="0"></div> <div> <B><U>What is </U></B><B><U>crazing</U></B><B><U>?</U></B></div> <div> 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.</div> <div> 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<FONT SIZE="3" COLOR="#00FF00" FACE="Batang" > </FONT>less in the early days of the industry than in the years following the First World War.</div> <div> The reason was that the whole system of specification, manufacture, re melting and application went through a radical change during and as a result of the war years.</div> <div> 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.</div> <div> Manufacture was a protracted process requiring the incorporation of the materials for periods of up to twenty-four hours.</div> <div> 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.</div> <div> 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.</div> <div> 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.</div> <div> <B>1926 Review</B></div> <div> ` 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.</div> <div> ` 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.</div> <div> ` 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'</div> <div> The review covered:</div> <div> The general physical characteristics, together with a summary of the causes and accelerating factors involved.</div> <div> Recommendations for the reduction or entire elimination of “crazing” in mastic asphalt.</div> <bR> <div> <B>1. Physical characteristics, causes and accelerating factors</B></div> <div> ` 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.</div> <div> ` “ 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.</div> <div> <IMG SRC="390a5e070.jpg" border=0 width="350" height="263" ALT="example of crazing 2" ALIGN="BOTTOM" HSPACE="0" VSPACE="0"></div> <div> ` However, irrespective of the physical dimensions and extent of the phenomena, the characteristics and fundamental causes are identical.</div> <div> ` 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 <FONT SIZE="2" COLOR="#0000FF" FACE="Batang" >(see Fig 1)</FONT></div> <div> <IMG SRC="39035e660.jpg" border=0 width="350" height="102" ALIGN="BOTTOM" HSPACE="5" VSPACE="0"></div> <div> Fig. 1 - Section of asphalt roofing as laid</div> <bR> <div> 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 <FONT SIZE="2" COLOR="#0000FF" FACE="Batang" >( see Fig 2)</FONT></div> <div> <IMG SRC="39025e660.jpg" border=0 width="350" height="102" ALIGN="BOTTOM" HSPACE="0" VSPACE="0"></div> <div> Fig.2  - Section showing ridges formed in bitumen skin</div> <bR> <div> ` 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 <FONT SIZE="2" COLOR="#0000FF" FACE="Batang" >(Fig 3)</FONT><FONT SIZE="3" COLOR="#FF0000" FACE="Batang" > </FONT>which mechanically weakens the the skin and accelerates its own development.</div> <div> <IMG SRC="39065e5c0.jpg" border=0 width="350" height="92" ALIGN="BOTTOM" HSPACE="0" VSPACE="0"></div> <div> Fig. 3 - Section showing crazing fissures formed in bitumen skin</div> <bR> <bR> <bR> </td> <td valign="top" width=400 BGCOLOR="#FFFFFF" TEXT="#080000"> <div> ` In every known case of “crazing” the surface crack is situated in the centre of a small ridge or corrugation (which may be easily felt even in the case of minute “hair” cracks by sliding the finger over the surface at right angles to the direction of the crack)</div> <div> ` These ridges or “crater” phenomenon allow for immediate recognition of “crazing as opposed to ordinary cracking - it is, without exception always present in the former but never in the latter.</div> <div> <IMG SRC="390e5e070.jpg" border=0 width="350" height="263" ALT="extreme crazing" ALIGN="BOTTOM" HSPACE="0" VSPACE="0"></div> <div> `There appear to be no doubt, also, from experimental work carried out in the laboratory, that “crazing” is accelerated by the  action of ultra violet light. The latter probably has an oxidising action on the bitumen present in this surface skin, which again weakens it mechanically”</div> <div> <B>2. Reduction in thickness of the bituminous skin.</B></div> <div> ` From the previous section it will be gathered that the phenomiona of “crazing” is essentially and fundamentally dpendant for its origin and existence on the highly bituminous, unstable, and unprotected surface skin on the asphalt. It logically follows, therefore, that if the surface layer can be dispersed and effectively protected from sun irradiation, progress toward the elimination of this defect will naturally result. Both from a practical and theoretical standpoint, its is essential that this bituminous skin should be reduced to a mnimum if “crazing” of the material is to be avoided. The dimensions and bituminous nature of the surface skin will be largely dependant on the bitumen content of the asphalt and, in practise, it is a fact that, other important factors being equal, the extent to which “crazing” takes place, is proportionate to the bitumen content of the material. An improvement undoubtedly results when the bitumen content is cut down, but there is naturally a very definite limit below which it is impossible to go without seriously affecting the consistency of the asphalt. This is especially true where a natural rock aggregate is employed.</div> <div> ` An important point arising here with regard to previous satisfactory examples of natural rock mastic, is that the asphalt was invariably heavily gritted both in the “vertical” and  “flat”.</div> <div> <B>3. Protection of the bituminous surface layer </B></div> <div> ` It has been stated that it is not possible or expedient to eliminate entirely the bituminous surface skin, and it is therefore becomes advisable to protect, as far as possible, the remainder of the bituminous surface layer from the effects of solar radiation. This is effectively done by “sanding” or “gritting”  the surface with a suitable material, “rubbing” material at present in use vary considerably. <IMG SRC="39135e070.jpg" border=0 width="350" height="263" ALT="coarse sand for rubbing mastic" ALIGN="BOTTOM" HSPACE="0" VSPACE="0">Sands (of various grading), granite dust, whin and limestone dusts and even cement have been used at various times in different parts of the country. . It can be stated definitely that limestone dust and cement when used as “rubbing” materials are quite fatal from the point of view of crazing. They do not form an effective protective dressing owing, probably, to their absorbent nature and the fact that they tend to chemical instability when directly exposed to weather conditions. There is no doubt that the best material for this purpose is a hard, absolutely non-absorbent and chemically inert substance such as silica (sands) or granite dust. </div> <div> <IMG SRC="39125e070.jpg" border=0 width="350" height="263" ALT="small area of mastic flat rubbed with coarse sand" ALIGN="BOTTOM" HSPACE="0" VSPACE="0"></div> <div> A light coloured material of possibly a quartz nature would appear to be ideal fro a point of view off efficient diffusion of the incidence of sunlight and the breaking up of the surface skin. The grading of this material, within limits, does not appear to be of vital importance. It should not be fine, otherwise a too intimate physical combination with the bituminous surface layer results, and the latter is “dried out” excessively. A relatively high proportion of coarse material can, in many cases, be considered distinctly beneficial - especially when applied in a rather special manner. In this connection it is interesting to quote the method used some years ago, which gave roofs, which are in extremely good condition; here a comparatively large quantity of coarse, hard grit was beaten into the surface both on the vertical and the flat sections. The result is a stable and an efficient protective dressing.</div> <div> <B>4. Incorporation of grit in the asphalt</B></div> <div> ` The incorporation of grit in the asphalt for both “vertical” and “flat” work is also to be highly recommended as this helps to stabilize the material mechanically and has the important further effect of reducing the bitumen/aggregate ratio, without affecting the consistency. By the incorporation of 15 t0 20% of a coarse grit, such as Bridport grit used in the London area, the soluble bitumen figure of the finished mastic is reduced 1 to 1.5% without any harmful effects as regard the consistency.  </div> <div> It is an interesting fact that the “beaten” asphalt previously referred to, was found to contain the quite remarkably high figure of nearly 40% grit (“10mesh” and “retained on 10 mesh”) - this is irrespective of the material beaten into the surface.</div> <div> <B>5. Use of suitable asphaltic cement.</B></div> <div> In the light of previous work carried out on this subject it may appear extraordinary that more prominence had not been given in this report to a discussion of the suitability of standard asphaltic cements in relation to the crazing problem. Although this matter is, of course, significant, a practical and theoretical examination of the problem shows that it is not of vital importance - provided always that the asphalt cement is up to the standard normally required for mastic asphalt manufacture.</div> <div> “ In the case of roofing mastic asphalt it would certainly appear to be advantageous to reduce the penetration of the asphaltic cement to a minimum consistent with local conditions and always provided that this reduction in penetration is not accompanied by a disproportionate loss of ductility.”</div> <div> <B>Summary</B></div> <div> The investigation for the elimination n of crazing in roofing mastic asphalt can be summarized as follows:</div> <div> 1. Reduction in thickness of the bituminous skin by the use of   the optimum soluble bitumen content.</div> <div> 2. Protection of the bituminous skin.</div> <div> 3. Use of suitable aggregate.</div> <div> 4. Incorporation of grit in the aggregate.</div> <div> 5 . Use of suitable asphaltic cement.</div> <div> Although tabulated in an appropriate order of importance it must be emphasized that these recommendations are largely interdependent and if the risk of crazing is to be entirely eliminated, due consideration must be given to each.</div> <div> Of necessity much has been omitted from this report, but from these extracts the cause of crazing is clearly shown and simple practical recommendations for its elimination are tabulated. </div> <div> Since this report was published in 1926, experience has shown two modifications of the views expressed.</div> <div> The first is that the elimination of the skin that may float to the surface on mastic asphalt is desirable, rather than its reduction, provided that it can be done by simple practical means.</div> <div> The second point is that the use of coarse, sharp rubbing materials is the most effective in reducing or eliminating the bituminous skin. This may not produce such an attractive finish as the finer types of sand but there is little merit in having a pleasant looking surface, which is likely to be marred by crazing soon after it's completion.</div> <bR> <bR> </td> </tr></table></body>