ROAD-MAKING.

Hawera Star, Volume XLV, 23 January 1926, Page 11

ROAD-MAKING.

TTIE USE OF TA IE «• BY NO MEANS A r ALEIT LtE. ” A London correspondent _wriT.es: - At the Public Works, Roads, ana Transport Congress now in progress a paper was read on the use ot tar in road construction. The subject natuiallv raised a good deal of controversy, but those who submitted the Paper on behalf of the Institute of Gas Engineers maintained that tarred granite macadam was by no means a failure where the tar employed was of tue right adhesive quality as applied to tee stone, and was laid under right conditions. ~ . If the question were fairly considered the overwhelming conclusion, it was stated, would bo that tarred road suifaces were a success. Tlic practice ci dressing roads with suitably prepared tar (followed by the application ol stone clappings in sufficient quantity to form a compact wearing layer had resulted in the abolition of dust on tho highways, and it had until lecently been the only means aaviiable to the road engineer for keeping the majority of road surfaces in a usable condition, while tar-dressing of road surfaces had been oractisod in England for over hall a century. In seeking to allay dust by the use of tar, the Paper proceeds, the penetrating quality of the oils in the tar had a distinct attraction for the road engineer, who was then dealing almost entirely with water-bound roads. The adhesive resinous part of the tar, however. was found to have considerable protective qualities in preventing the attrition of'the aggregate forming the road crust, and a demand soon arosa for a quality of tar having a higher percentage of the adhesive resins an ! a lower percentage of the volatile parts of the tar, which was found to be of great value as a binding agent for surface dressing, and in the tarring o" aggregates for tarmacadam w T ork. Local supplies or tar had advantages to engineers of urban communities; and the gasworks which exist in every town of anv size in England, Wales, and Scotland were called upon either to put in distilling plants or get the tar treated otherwise for the removal of highly volatile naphthas and oils. Where the process has been carried out with care to comply with tlic specifica lions which were afterwards forthcoming from the Road Board, the results have been, with few exceptions, highly satisfactory Pollution of Streams. Prejudice has been raised against coal tar on the ground that washings from loads made or surfaced with it iniure Ibh and aquatic vegetation. It; is possible that in some special eases this may be true- but on tlie vast of our roads such is not the case, anc. no sufficient reason exists for excluding tar therefrom. In this connection reference was made to the recent memorandum issued by the River Pollution Committee of the Ministry of Agriculture and Fisheries, which states “that the impression prevails in many quarters that the committee is antagonistic to the use 0.. tar in the preparation of road surfaces, and is anxious to correct this impression. The committee, while desirous that suitable bitumen shall be used on roads in proximity to streams, which obviously are only a small percentage of the whole of the roads ol the country, point out that it is no part of the committee’s policy either to advocate the general use of bitumen, or to deprecate the Use of tar.’’ Broken iron, slag, or stone roads laid as tar macadam are probably l lie most economical surfaces for modem traffic. Successful construction in these materials demands the selection of suitable tar and dry aggregate ami heating the tar to the desired temperaI u re. .Methods of Construction. Tlic aggregate should not be graded, but must consist of all sizes liont 1m largest to the smallest intended to lie used, in correct proportions, to give Ihe densest mixture. Upon a good fouiiua lion a coating of tarmacadam four indies thick rolled down to three, inches is sufficient, and the writers of j the Paper have never found it advantageous to lay thicker coatings. Exxopt where it is feared mud wll work up into the wearing cost during construction the writers have found no advantage i:i two-coat work, but on the other hand the bin. finished tnickness in one coat is more easily laid, and has been known to give better results than two-coal work. Many road engineers prefer two-coat work and obtain equally satisfactory results. To gi v an the -hi.-kness of the. wearing < cut by the condition of the foundation is not good engineering, as it is not economical. -V good foundation must be secured before laying the wearing •.oat.