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|Title:||INFLUENCE OF BITUMINOUSM PAVEMENT THICKNESS IN THE DESIGN OF BITUMINUS MIXES|
|Authors:||Rao, Modekurty Venkata Prakasa|
|Abstract:||A bituminous mix of the same composition may not. remain stable if laid as a surfacing course to any thickness-. For example, a bituminous pavement may not rut or shove if laid to a thickness of, say, 2.5 cm but it may rut or shove if laid to a thickness of, say, 10 cm eventhough the same mixture may be used in either case. The thickness of a bituminous surfacing obviously affe cts the stability of the entire road structure because, insuffi cient thickness may cause subgrade failure or failure of the layers below the surfacing. But, when the thickness of the bituminous surfacing is designed, to prevent the failure of layers below it, care should be taken to see that the surfacing itself does not fail due to lack of stability. Despite the fact that a large number of mix design methods have been devised, pavement thickness was neither incor porated in the criteria for mix design nor was it considered as a parameter while determining the stability of a bituminous mix. While some theoretical information is available on the influence of pavement thickness in the design of bituminous mixes, exper imental data ar.e lacking in this respect. For the above mentioned reasops, it was decided to investigate the effect of thickness of bituminous pavement,, in addition to various other factors, in the design of bituminousmixes. To start with, it was decided to investigate its influence on the stability of a bituminous surfacing laid over a rigid base course for stationary loads only. The entire investigation was divided into four major part s< In the first part various factors affecting the stabi lity and other reguirements of a bituminous mix, and various methods of mix design, their advantages and shortcomings, have been reviewed in brief. The second part of the investigation deals with the experimental work carried out to study the effect of dimensions of specimen, loaded area, testing conditions of temperature and rate of loading, on the strength of the sheet asphalt mix with the help of direct compression and bearing tests. Special eguipment was used to maintain constant temperature throughout the testing period for samples which were tested at temperatures above room temperature. In the third, part of the investigation, bearing tests were conducted on medium and large size slabs of sheet asphalt and bituminous concrete mixes with rigid steel plate and cement concrete, as the underlying layers. Different loaded areas were also used. The tests were carried out at room tem perature and at a rate of loading of .0127 cm/min. For casting the bituminous slabs, a bituminous mixer and a laboratory roller simulating the 8-Ton steel roller were used. Deflection dial gauges were used to determine the deflections of bituminous -inconcrete slabs overlying cement concrete base course at several points. The fourth part of the investigation includes triaxial tests conducted on sheet asphalt and bituminous concrete mixes to determine cohesion and angle of internal friction for differ ent testing conditions* In this part, some of the commonly used bearing capacity methods were reviewed and the experimental bearing capacity values, obtained from the bearing tests, were compared with the bearing capacity values computed from the bearing capacity methods. Based on the above investigation, some of the main results of the present study, despite the limitations of experv. indentation, are as follows: (i) The compressive strength, bearing caoacity and yield values increase proportionately with increasing ratio of diameter of loaded area to the thickness of the specimen with other conditions remaining the same, but for ratios above unity only. An equation has been developed relating the yield value and the above ratio. (ii) Equations have been given establishing arelationship between the compressive strength or bearing capacity and thickness of specimen, diameter of loaded area, unit rate of volume displacement and test temperature* It has been found that there was a good agreement between tne values computed from the equation and the experimen tal values. -IV- (iii) The results of the bearing tests carried out on small size specimens and on the medium and large size slab specimens were in close agreement with each other, (iv) It has been found that conventional bearing capacity methods gave very high bearing capacity values as compared to thosn obtained from bearing tests. The stability values, as obtained from Smith's design chart for the triaxial test properties af the mixes, varied :cram 25% to 3o% of thr corresponding experimental valuos obtained from bearing tests for a ratio of liamater of loaded area to specimen thickness of 3 only. For lower ratios the percentage is more, (v) The resistance due to frictional effect between the specimen and the loaded area and between the specimen and the base constitutes a large percentage of bearing capacity of a pavement, particularly for high ratios of diameter of loaded area to pavement thickness. (vi) A stability equation has been given to determine the stability of a mix in kg/sq.cm. The stability can be determined by conducting direct compression tests in addition to triaxial tests at the highest temperature, which is likely to be attained in the field, and at a rate of loading of 0.127 cms per minute, (vii) The last, but not the least, important result of this investigation is that a mix designed by the above method for a particular thickness of the pavement cannot be use for a thickness greater than that for which it has been designed because the thickness of a bituminous pavement influences the design of bituminous mixes.|
|Appears in Collections:||DOCTORAL THESES (Civil Engg)|
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