D.—l

For average New Zealand summer temperatures, road-oil cutback to the initial viscosity of M.C. a should be used, for midsummer work in the hotter districts the consistency of the cutback should he that of M.C. 4 , while an M.C. 2 viscosity will be more satisfactory for surfacing in cool weather, or when only a very small chip is obtainable.. The various grades of dust-laying seal-coat binders will be known as " Soft Residue M.C.j," " Soft Residue M.C. a ," and " Soft Residue M.C. 4 ." In other words, the viscosities and distillations will be those of the respective M.C. products, but the residue from distillation will be much softer with a penetration at 77° F. of not less than 300, and a float test at 122° F, of not less than 150 seconds. Except for some attention to drainage, gravel or macadam roads which have been reasonably well maintained may require very little preparation prior to the application of a dust-laying seal-coat. There are instances where little beyond the sweeping of the surface is necessary, or where a short period of intensive grading and planing during suitable weather or in conjunction with sprinkling will be all that is required. In other cases, where the road is out of shape, some scarifying with the addition of a few yards of new metal per chain will often bring the road to a satisfactory condition. " Dust-laying seal-coat" surfacing is not intended to supersede the tar prime-coat followed by the road-oil seal for the more important highways, but, if properly laid, dust-laying seal will provide many years of service without further treatment on highways of lower traffic intensities. Even where conditions warrant the prime-coat and road-oil seal, side-road intersections and turn-outs, private entrances, and haunches of highways through townships can often with advantage be treated simply with this lighter class of surfacing. By this procedure the large 1 in. to f in. chips are all retained for the construction of the bituminous carpet on the important highway. Some form of light or temporary seal is frequently required to serve in the meantime on roads which will be deviated or realigned in four or five years' time. The dust-laying seal will meet these conditions. This position frequently obtains at the approaches to old bridges. Many village roads might be given a dust-laying seal-coat after the minimum of preparation. Even in the case of some of the State highways this lighter treatment will often meet requirements. During the year orders were placed for the supply of New-Zealand-manufactured oil-fired bitumen-sprayers which can be transported by and readily operated from ordinary lorries. It is intended that these items of plant should be used for maintenance, priming, and small construction works which cannot be satisfactorily undertaken by contract. The rapid heating and definite temperature control, together with the general utility of these sprayers, places them much in advance of such machines previously used by the Board. Within the limits of practicability the Board's constant aim is to keep design, construction, and maintenance standards in pace with transport requirements, or, to use the words which have been quoted in other places, every endeavour is made to make roads fit the traffic rather than try to fit the traffic to the roads. Standard Specifications. As indicated in the previous annual report, standard specifications covering the following classes of work have now been printed and bound in the form of a loose-leaf book entitled " Book of Instructions and Standard Specifications " : — M.H.B. 42 : Construction of Formation. M.H.B. 43 : Construction of Base-course. M.H.B. 44 : Top-course Construction. M.H.B. 45 : Tar Priming-coat. M.H.B. 46 : Road-oil Sealing-coat. In addition, a pamphlet, M.H.B. 42a, " Notes on Formation," has been incorporated in the Book of Instructions, and provides a useful guide to present-day practice. With the object of having these specifications adopted for works financed either wholly or in part from the Main Highways Account, a complimentary copy of the bound book was forwarded to each local authority in New Zealand with a request that the relative specification be included in any proposals for the undertaking of work on main highways. Bulletin No. 2, which has until recently been regarded as the Board's text-book on highway construction and maintenance, has been considerably superseded by the foregoing specifications and notes. The distribution of the specifications has brought many appreciative references from roading authorities throughout New Zealand. A further specification and descriptive notes for a light type of sealing, to be known as a " Dustlaying seal-coat," are now being printed. As opportunity offers it is intended to prepare and issue further instructions covering highway work, all of which will be distributed for incorporation in the Book of Instructions and Standard Specifications. Copies of the individual specifications are available to local authorities at a small cost from the Government Printer for inclusion in contract documents. Operations of Magnetic Truck. During-the year the Board's magnetic truck, which is utilized for clearing main highways of iron or steel puncture-producing articles, operated chiefly in the South Island. The length of road actually cleared during the twelve months ended 31st March, 1939, was 2,084 miles, compared with 4,427 miles for the previous financial year ; the weight of material picked

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