MIX DESIGN FOR OPEN MIXTURES. METHOD OF SPECIFIC SURFACE, CKE IMMERSION - COMPRESSION TEST

Practical questions are examined which arise when one attempts to obtain the optimum binder content for a mixture of the open type in the laboratory. For an open asphaltic concrete, the principle factor is the angle of internal friction, while the binder content has not proved critical and may have different optimum values according to the manner in which the pavement is to be constructed. Nevertheless, at the laboratory level, the problem arises of giving a real optimum binder content value for a given particle size distribution. A problem arises when an attempt is made to apply the procedures used in the close mixtures; a conclusive result has not yet been reached. For this reason a critical study is attempted of the different classical methods of calculating the optimum binder content, which depends largely on the aggregate specific surface; these methods are only approximations and contain numerous defects. An analysis is also made of various mechanical tests which have the same aim and which furthermore provide mechanical characteristics. These are the Marshall test (density), tests on test tracks (mechanical behaviour) and immersion-compression test (susceptibility to water), which is particularly stressed because its use can have a decisive influence on the evaluation of bituminous mixtures in general. Also described are the importance of the results on the factors such as the number of samples, the temperature of the mixture and the compacting, the mixing process, the degree of compaction, etc., all of which should figure in the new test standard. This paper was presented at a conference organized by the CEIA on the Mix Design of Bituminous Mixtures, which took place in Madrid from 1st-5th March, 1976. (TRRL)