The rate of evaporation of cutter (diluent) from a film of cutback bitumen depends on the rate at which the diluent diffuses through the film to its surface and the rate at which it is removed from the surface by air movement. The diffusion coefficient is dependent on the temperature and the local concentration of diluent in the film. At pavement surface temperatures, the rate of evaporation is mainly controlled by the diffusion process and is relatively slow. Rates of evaporation from films of cutback bitumens 1 and 5 mm thick with diluent concentrations of up to 15 per cent by volume of the cutback bitumen were measured for periods of up to ten days. Exposure temperatures were in the range 30 to 36 degrees celsius and wind speeds across the films were up to 4 m/s. The data were fitted to a model for the diffusion controlled evaporation process and estimates of diffusion coefficient level and its dependence on diluent concentration level were obtained. The photo oxidation skin formed on the exposed surface of the bitumen film in a surface seal a few days after laydown virtually stops the evaporation process. Evaporation can only proceed when this skin becomes cracked by water or mechanical action. The relevance of the results of the study to rate of loss of cutter in a sprayed seal is discussed (a).
Abstract Bituminous road binders have been prepared containing dispersed long‐chain polymers, including both natural and synthetic rubbers, and also certain South African coals dispersed in them under high temperature conditions. Examination of the rheological properties of these binders has confirmed that both types of additive produce an improvement of the viscosity/temperature characteristics and a reduction in brittleness at low temperature. If both coal and polymer are incorporated together, however, the improvement is greater than that calculated from the sum of the separate effects of the two additives. Some of the polymers studied were found to stabilise mixtures of pitch and petroleum bitumen which are normally not completely miscible. An apparatus is described for assessing brittleness under stress and loading times of the same order as those occurring on the road under traffic. The methods of test used in the investigation are critically discussed.
A REVIEW IS CONDUCTED OF PRESENT KNOWLEDGE ON THE PERFORMANCE OF BITUMENS IN ROAD AND AIRFIELD PAVEMENTS AND THE ASSEMENT OF THIS PERFORMANCE IS MADE WITH SPECIAL REFERENCE TO AUSTRALIAN CONDITIONS. BASIC ASSESSMENT OF PERFORMANCE AWAITS THE DEVELOPMENT OF A SIMPLE RELIABLE 'MICRO' METHOD FOR MEASURING THE VISCO-ELASTIC PARAMETERS OF THE MATERIAL AT 50 CYCLES/SEC. AND AT THE EXTREMES OF PAVEMENT TEMPERATURE--10 C. AND 60 C. THE PRIMARY CAUSE OF DETERIORATION OF THE MATERIAL IN SERVICE APPEARS TO BE SLOW ATTACK BY ATMOSPHERIC OXYGEN BOTH IN THE PRESENCE AND ABSENCE OF LIGHT. POSSIBLE WAYS OF IMPROVING RESISTANCE TO OXIDATION BY MODIFIED PROCESSING OR BY THE ADDITION OF 'ANTI -OXIDANTS' ARE CONSIDERED. /AUTHOR/
A description is given of a method for the recovery of 'gram' quantities of bitumen from samples of 'spray and chip' surface seals and plant mix pavement layers for measurement of 'apparent' viscosity by the 'shell' sliding plate viscometer. Examples of the application of the method and estimates of its 'within' laboratory precision are given. The method can be used for routine investigation purposes if the arrb durability test equipment is available. /TRRL/
The diffusion controlled reaction of oxygen with films of bituminous binders is the main cause of the (long-term) hardening of bitumens in pavement surfacings. Earlier work on this reaction was confined to studies of the initial reaction rate and the analytical solution for the differential equation which describes the steady state of the system. A finite difference method to evaluate the equation for the unsteady state had been developed so that the effects of extended reaction times could be studied. The reaction was assumed to be retarded by exhaustion of reactive constituents in the binder and by the reaction products increasing binder viscosity and reducing the rate of diffusion of oxygen through it. The effect of binder reactivity, temperature and length of diffusion path were examined, and it was calculated that long-term binder hardening will be very slow at locations in a bituminous construction where the diffusion path through the binder from a surface exposed to air exceeds about 3 mm. More information is needed on the relationship between the amount of oxygen reacting with a binder and the increase of viscosity which this reaction produces before the system can be more precisely defined. (Author/TRRL)
This paper was presented at Session 20 - Pavement design 2. Data on current bituminous hot mix primary compaction practice in Victoria indicated that, to give four passes of a light twintandem steel-wheeled roller to a layer, the time elapsing between laydown and completion of the operation would be at least nine minutes. This time could be reduced to seven minutes if a heavier compactor with a greater rollwidth is used. Calculation using an improved heat transfermodel showed that, for layers less than 50 mm thick laid in cool or cold weather, the layer can cool to below the temperature at which compaction is effective before the primary compaction operation can be completed. Rate of cooling is influenced by layer thickness (most important factor), laydown and substrate temperatures, layer conductivity and wind speed. An equation is given for calculating the (approximate) time required for a layer to cool to a nominated compaction cessation temperature in terms of these five factors. An indication is also given of substrate temperatures below which layers of asphalt of different nominal thickness and laydown temperature should not be laid. This applies only to fine, cool and cold weather conditions and substrate temperatures for these conditions at any locality in Australia can be approximately estimated from monthly mean maximum air temperature (MMAT) data. /TRRL/
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