IE43390B1 - Bituminous composition - Google Patents

Abstract

1494279 Mastic asphalt BRITISH PETROLEUM CO Ltd 30 June 1976 [10 July 1975] 29042/75 Headng C3N A mastic asphalt, suitable for coating pipelines, comprises 8-22% wt. bitumen, 8-25% wt. of filler passing a 75 microns sieve and 53-84% wt. of aggregate in the grading range from larger than 75 microns to a maximum particle size of 2À36-37À5 mm., the percentages being by total weight of bitumen, filler, and aggregate, and the aggregate having a grading curve close to a modified Fuller curve for particles of the maximum particle size chosen, said modified Fuller curve being the Fuller curve recalculated to exclude material passing 75 microns. By "close to the Fuller curve" is meant that the end points of the grading curve are not more than 5% above or below the Fuller curve and that any intermediate point on the curve is not more than 10% above or below the Fuller curve. The aggregate larger than 75 microns may be sand with or without gravel. The filler may be limestone, Portland cement or lime. The bitumen may have a penetration of 10-100 at 25‹ C. and a softening point of 40‹-115‹ C. It may be petroleum bitumen or coal tar pitch or a mixture thereof, and may be straight run or blown. It may also contain some rubber. The optimum bitumen : filler is 1 : 1.

Description

This invention relates to a bituminous composition of the ma3tic type, suitable for coating pipelines, particularly underwater pipelines.

Pipelines, particularly underwater pipelines, are usually coated with concrete, for protection and weighting, except at the ends of each length of pipe. The end3 have to be left uncoated to allow the pipes to be welded together on site (e.g. on the lay barge). The gaps in the concrete coating at the welds then haVe to be filled with a relatively quick setting material. A mastic asphalt i3 normally used, the mastic being poured into a cylindrical mould surrounding the gap. The mineral portion of a mastic asphalt is normally crushed limestone,'· but. for pipeline coating, sand is sometimes used. Gravel is also usually incorporated for economic reasons.

The search for crude oil and natural gas in off-shore areas is increasing and hence the need for underwater pipelines is also increasing both for moving oil and gas from well heads to a gathering platform and/or transporting it ashore. The transport of oil products and other liquids or ga3es by underwater pipeline is less well developed but may increase. Depending on the quantity and type of fluid to be transported, the pipelines may have diameters requiring concrete coatings of up to 5 inches (76 mm) thick. The mastic for coating at the joints will have essentially the same thickness as the concrete.

A typical mastic asphalt for underwater pipelines is formed of 2050 irt bitumen / wt limestone or cement filler (passing 200 mesh BSS) 40% wt sand to B3 594 standard / wt pea gravel (^ /4 to ^/8 inch) With such a mix, however, there is a pronounced tendency - 2 43390 for the gravel to segregate both during manufacture and coating.

This problem can be overcome by omitting the gravel but this increases cost.

It has now been found that the BS 594 standard sand grading, used in the manufacture of hot rolled asphalt for paving, is not the best standard for mastic asphalts based on sand.

According to the present invention, therefore, a mastic asphalt comprises - 22% wt of bitumen - 25% wt of filling passing a 75 microns (200 mesh BSS), sieve and - 84% wt of aggregate in the grading range from larger than 75 micron (200 mesh) to a maximum particle size of from 2.36 mm (7 mesh BSS) to 37.5 mm (lj inches BSS), the percentages being by total weight of bitumen, filler and aggregate, and the aggregate having a grading curve close to a modified Fuller curve for particles of the maximum particle size chosen, said modified Fuller curve being the Fuller curve recalculated to exclude material passing 75 microns.

Fuller curves are grading curves which give the minimum void space and closest packing and thus the lowest void space for sands and other mineral aggregates containing particles of varying size.

The shape of a Fuller curve depends on the maximum particle size but will be a single curve for any given maximum particle size. Particle shape also has some effect on packing but the present invention has been found to be effective with aggregates which are a mixture of round and angular particles obtained from both natural and crushed aggregates. The precise particle shape is not, therefore, considered critical.

Fuller curves are based on the observation that, for lowest void space, the particle size distribution follows a certain law, viz 3 3 9 0 τ, , . ___Aperture siae of that sieve Precentage passing any sieve = 100 y-H— Fuller curves can thus be calculated mathematically using the above formula. The law was first enunciated in a paper by Fuller and Thompson entitled The Laws of Proportioning Concrete, published in the Transactions of the American Society of Civil Engineers, 1907, 59. pages 67-172. In the following text, various Fuller curves will be identified by the nominal size of the largest particles present e.g. The 2.36 mm Fuller curve is the Fuller curve for aggregate whose maximum particle size is 2.36 mm.

Some tolerance is necessaiy to allow for the fact that in a real aggregate blend whose nominal grading is that of a certain. Fuller curve, there may be a small amount of aggregate present which is actually of a larger size than that designated. Consequently, the aggregate is defined as close to' the Fuller curve and, by that, we mean that the end points . of the grading curve are not more than above or below the Fuller curve and that any intermediate point an the curve is not more than 107a above or below the Fuller curve.

The filler content of a mastic has a dual function, acting partly as the finest portion of the mineral content and partly as a stiffener and modifier of the bitumen. For aggregate close to the 2.36 mm Fuller curve, it so happens that the amount of material smaller than 75 microns on this Fuller curve is approximately that required us filler to give a good mastic. Hence for 2.36 mm maximum particle size either the normal or the modified Fuller curve can be used as a criterion. However, as the maximum particle size increases, so the amount passing 75 microns on the Fuller curve decreases to a point where it may be insufficient in relation to the required bitumen content. For larger maximum particle size aggregates, therefore, the correlation should be between the - 4 43390 aggregate grading (without filler) and the modified Fuller curve.

The amount passing any given mesh on a modified Fuller curve can be calculated from the normal Fuller curve aa follows Amount passing a given size meah on modified Fuller curve _/amount passing that size amount passing 75 ~( mesh on normal Fuller microns mesh on I \ curve normal Fuller curve/ 100 - amount passing 75 microns on normal Fuller curve The aggregate in the range of larger than 75 microns may l0 conveniently be sand with or without gravel and may be natural or crushed material or a mixture of natural and crushed material.

The filler may be any suitable known mastic filler e.g. limestone, Portland cement or lime. The term filler implies that the material has particle sizes Of 75 microns •15 or les3. It ia to be understood also, however, that the filler content is the total filler content, including any contribution from fines present in ths coarser aggregate used.

The bitumen may have a penetration of from 10 to 100 at 25°C, proferably from 20 to 30. It may be petroleum bitumen or coal tar pitch or a mixture, and may be straight run or blown.

The bitumen may contain a minor proportion of a rubber or other polymer to modify its visco-elastic properties. The precise amount of bitumen required will depend on the grading of tha aggregate, particularly its maximum particle size and may be chosen, by experiment if necessary, to give good pouring properties with minimum bitumon contents. In general, for any given maximum particle size aggregate, the present invention allows a reduction in the amount of bitumen used as compared with previously used asphalts. The amount of bitumen is still, however, in excess of that required merely to fill the voids between the mineral particles. - 5 4 3 3 9 0 The softening point (Ring and. Ball) of the bitumen may be from 40 to 115°C and may be varied depending on the temperature the maatio is likely to be subjected to. Crude oil at a wellhead may have a temperature as high as 80°C and mastic for crude oil gathering lines preferably uses blown bitumen with a softening point of 70 - 115°C. In main crude oil transport lines, the crude oil temperatures are likely to be lower and straight run bitumens with softening points of 4θ - 70eC should be adequate.

The relative proportions of bitumen, filler and aggregate within the defined ranges will clearly need to be selected for any given mastic. The optimum bitumen:filler ratio may be 1:1 and the bitumen and filler contents can be decreased as the maximum particle size of the aggregate increases. Experiments have 3hown for example, that mastics of comparable pourability and other qualities can be obtained with the following relationship between bitumen and filler contents and aggregate maximum particle size Aggregate maximum particle size 2.36mm 4.76mm 9.52mm 19.0mm Bitumen & ut 17 15 13 11 Filler 5$ wt 17 15 ' 13 11 Tho maximum particle size of the aggregate should obviously not exceed the thickness of the mastic coating and may preferably be from /2 to /3 of the thickness.

The invention is illustrated by the accompanying drawings Figures 1 to 3 and the following Examples.

Figure 1(a) shows a set of Fuller curves for aggregates having maximum particles sizes ranging from 2.36 mm to 19 mm. These curves were obtained by calculation using the first formula given above. Figure 1(b) shows the Fuller ourves of Figure 1(a) recalculated to exclude the material passing 75 microns. - 6 43390 Example 1 A mastic asphalt was produced by mixing 17.9$ wt straight run bitumen having a penetration of at 25°C and a King and Ball softening point of 65°C 5 17.9% wt limestone filler passing 79 microns 65% wt sand of size greater than 75 microns having a grading curve as shown on the accompanying Figure 2.

Figure 2 also shows a curve for sand having a maximum particle size of 2.56 mm. This curve is the appropriate Fuller 10 curve recalculated to allow for the absence of material passing the 75 microns sieve. The sand used had a small proportion of material up to 4.76 mm, but this was not sufficient to have a significant affect on the asphalt produced. Also shown on Figure 2 is the grading curve envelope for sand according to B3 594 and its median line. It will be seen that the grading curve for the sand used was relatively close to the 2.36 mm modified Fuller curve and considerably different from the BS 594 sand, having a more continuous grading throughout the particle size range.

Figure 3 shows tha grading curve for the sand when combined with filler (the proportions by weight of total aggregate and excluding the bitumen being 78.8$ yt sand and 22,2% wt. filler) and the 2.36 mm Fuller curve. Again the close similarity of the curves will be apparent.

The asphalt produced was tested for pourability and segregation both in the laboratory and by full scale tests.

In the laboratory, pourability was assessed visually and subjectively at 150° - 190°C, there being no standard test for pourability. Segregation of aggregate was assessed by pouring 500-1OOOg of the asphalt into a cylindrical steel mould. This - 7 4 3 3 9« was maintained in an oven at 160°C for 2 hourB, without stirring, it was then allowed to cool after which the steel cylinder was removed. The sample was out in half hy a diamond saw and the extent of segregation assessed visually.

The full scale tests were carried out using a 36 inch diameter pipe and a 40 inch diameter cylindrical mould around the pipe and filling the space between with asphalt by pouring at 170°C. After cooling, the mould was removed and segregation assessed hy analysing portions of the asphalt removed from various positions around the pipe.

In both the laboratory and the full scale tests, the pourability of the asphalt was as good as the known asphalt based on BS 594 sand and pea gravel. There was also negligible segregation of the larger particles. With the previously knovm asphalt, however, there was extensive segregation of the gravel in both tests.

Example 2 A mastic asphalt was produced by mixing H’A wt bitumen as in Example 1 11'i£ wt limestone filler passing 75 microns 78i? wt sand and gravel (75 microns up to 19.0 mm) The aggregate (sand and gravel) had the grading of a modified Fuller curve for 19=0 mm maximum particle 3ize material recalculated to exclude sand passing 75 microns, ns shown in Figure 1(b) When tested for pourability and segregation in the laboratory as described in Example 1, pourability was good and there was negligible segregation of aggregate despite the fact that particles as large as 19.0 mm were present. - 8 43390 Example 3 A mastic asphalt was produced by mixing 1T% wt blown bitumen having a penetration of 26 at 25°C and a Ring and Ball softening point of 85°C. 19·5’4 wt limestone filler passing 75 microns 63wt sand as in Example 1 The mastic had the same qualities of pourability and non-segregation as the mastic of Example 1 and in addition, could bs heated to 75°C without significant deformation or slumping, as compared with a temperature of 60°C for the mastic of Example 1.

Claims (8)

1. A mastic asphalt, suitable for coating pipelines, comprising 8 - 22% wt of bitumen 0 - 25% wt of filler passing a 75 microns sieve and 5 53-84% wt of aggregate in the grading range from larger than 75 microns to a maximum particle size of from 2.35 mm to 37.5 mm, the percentages being by total weight of bitumen, filler and aggregate the aggregate having a grading curve close to a modified Fuller curve for particles of the 10 maximum particle size chosen, said modified Fuller curve being’the Fuller curve recalculated to exclude material passing 75 microns.

2. A mastic asphalt as claimed in claim 1 wherein the bitumen has a penetration of from 10 to 100 at 25°C and 15 a Ring and Ball softening point of 40 to 115°C.

3. A mastic asphalt as claimed in claim 1 or 2 wherein the bitumen ha3 a penetration of from 20 to 30 at 25°0.

4. A mastic asphalt as claimed in claim 1, 2 or 3 wherein the bitumen is a straight run bitumen having a Ring and 20 Ball softening point of 4θ to 7θ°0.

5. A mastic asphalt as claimed in claim 1, 2 or 3 wherein the bitumon is a blown bitumen having a Ring and Ball softening point of 70 to 115°C.

6. A mastic asphalt as claimed in claim 1 substantially 25 as described in the Examples.

7. A pipeline coated, at least in part, with a mastic asphalt as claimed in any of claims 1 to 6.

8. A pipeline as claimed in claim 7 which is an underwater pipeline coated at its joints with a mastic as claimed in any of claims 1 to 6. Dated this 7th day of duly 1976, /fwf/INS & CO ,