A Self-Learning Manual - Institution of Engineers Mauritius

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A Self-Learning Manual Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU Level Two (Advanced FAQs) Part II: Pavement Design 1. What is the principle of asphalt mix design? (AMD1) The main objective of asphalt mix design is to achieve a mix with economical blending of aggregates with asphalt to achieve the following [61]: (i) workability to facilitate easy placement of bituminous materials without experiencing segregation; (ii) sufficient stability so that under traffic loads the pavement will not undergo distortion and displacement; (iii) durability by having sufficient asphalt; (iv) sufficient air voids In asphalt mix design, high durability is usually obtained at the expense of low stability. Hence, a balance has to be stricken between the durability and stability requirements. 2. What is Marshall Mix Design for bituminous materials? (AMD2) The Marshall Mix Design method was originally developed by Bruce Marshall of the Mississippi Highway Department in 1939. The main idea of the Marshall Mix Design method involves the selection of the asphalt binder content with a suitable density which satisfies minimum stability and range of flow values. The Marshall Mix Design method consists mainly of the following steps: (i) Determination of physical properties, size and gradation of aggregates. (ii) Selection of types of asphalt binder. (iii) Prepare initial samples, each with different asphalt binder content. For example, three samples are made each at 4.5, 5.0, 5.5, 6.0 and 6.5 percent asphalt by dry weight for a total of 15 samples. There should be at least two samples above and two below the estimated optimum asphalt content. (iv) Plot the following graphs: (a) Asphalt binder content vs. density (b) Asphalt binder content vs. Marshall stability 274
A Self-Learning Manual Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU (c) Asphalt binder content vs. flow (d) Asphalt binder content vs. air voids (e) Asphalt binder content vs. voids in mineral aggregates (f) Asphalt binder content vs voids filled with asphalt (v) Determine the asphalt binder content which corresponds to the air void content of 4 percent (vi) Determine properties at this optimum asphalt binder content by reference with the graphs. Compare each of these values against design requirements and if all comply with design requirements, then the selected optimum asphalt binder content is acceptable. Otherwise, the mixture should be redesigned. Fig. Design graphs for Marshall Mix Design 3. What is the importance of Marshall stability and flow test? (AMD3) Marshall stability measures the maximum load sustained by the bituminous material at a loading rate of 50.8 mm/minute. The test load is increased until it reaches a maximum. Beyond that, when the load just starts to decrease, the loading is ended and the maximum load (i.e. Marshall stability) is recorded. During the loading test, dial gauge is attached which measures the specimen's plastic flow owing to the applied load. The flow value refers to the vertical deformation when the maximum load is reached. Marshall stability is related to the resistance of bituminous materials to 275
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A Self-Learning Manual - Institution of Engineers Mauritius

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