The present invention relates to a method for manufacturing granular mastic asphalt, in which filler and heated aggregate, in a first stage, are mixed with a partial amount of bitumen preheated into a liquid state, and the mixture allowed to cool to ambient temperature to form a granular semifinished product, whereupon the said semifinished product, in a second stage, is mixed with a remaining amount of bitumen heated into a liquid state, to form a granular mastic asphalt end product. Such mastic granular is primarily intended for use as a protective coating over welded seams between pipe sections in subsea pipelines, particularly for conveying hydrocarbons. However, it is suitable also in other applications where there is a need for transporting mastic asphalt in a cold state over longer distances.
The word \”filler\” is the technical term used to denote rock flour or stone dust, while \”aggregate\” is the technical term denoting mineral materials in the form of crushed rock, sand, etc.
GB patent 334 588 discloses a method for preparing mastic asphalt in blocs, intended for use as an ordinary paving asphalt. Here, firstly a coarse aggregate fraction is mixed with bitumen, then a fine aggregate fraction is added, optionally together with a remaining amount of bitumen, and finally filler material. The entire operation is performed in one stage without any intermediate cooling, and the resulting product is in the form of blocks – not granular. GB patent 1 494 279, like the above GB 334 588, discloses a method of fabricating mastic asphalt in the form of solid blocks, where the essential aspect is the choice of the proportional amounts of the various components, bitumen, filler and aggregate, and the particle size of the latter.
A method of the introductohly stated kind, for the manufacture of granular mastic asphalt, is known from PCT patent application, publication no. WO 97/24410. The mastic asphalt product resulting from this prior method is an improvement of the 2
mastic asphalt product described in GB 1 494 279, the improvement being that the product according to the PCT application appears in the form of a granular which, compared to the bloc-shaped mastic asphalt, requires less consumption of energy in manufacture and is more easily handled in use, such as described in the latter document. A drawback of this prior method, however, is a non-uniform composition of the mastic asphalt granular, tending to result in the formation of lumps.
It has now surprisingly been found that the above drawback of the technique according to the PCT application is practically eliminated by retaining, in the first mixing stage, not only a partial amount of the total amount of bitumen, but also a partial amount of the total amount of filler to be contained in the end product, as stated more fully in the appending claim 1. Thereby the undesirable lumping tendency is obviated and the ease of handling is correspondingly improved. More particularly, the method according to the present invention possesses the following advantages over the prior art according to the above PCT application: