Concrete asphalt, which is prepared using crushed materials with continuous granulations and pure bitumen with a degree of penetration between 60 and 100 degrees, can be prepared by several methods, such as the Marshall method, the Haberfield method, and the Wim or Gyratory method. Marshall's method is one of the common and practical methods. In the following article, the Marshall Bitumen test and its purpose will be discussed further.
The best way to produce asphalt concrete
As mentioned, asphalt concrete is one of the most practical and widely used materials, which is produced in many ways. Haberfield method is used to produce asphalt concretes with fine granularity, and all its materials are passed through grade 4 sieve. In the gyratory method or Vim method, the asphalt samples will be pounded together with rubbing, and the largest asphalt grains in this method will be one inch. Marshall's method was also presented by a scientist named Bruce Marshall and today it has changed a lot. The size of the materials in this method is also 1 inch maximum. Basically, the Marshall method is the best and most practical method for asphalt concrete production, and the Marshall Bitumen test evaluates the quality of the produced product.
The purpose and reason of the Marshall bitumen test
As it was said, in the production methods of asphalt concrete, different materials and materials such as Gilsonite etc. are used in different proportions. The Marshall Bitumen test can be performed for various purposes and reasons, the most important of which include the following:
- In this experiment, the relationship between density and bitumen percentage can be obtained.
- You can find the relationship between specific gravity and bitumen percentage.
- Also, in this experiment, the relationship between bitumen percentage and void space, compressive strength and drawing the corresponding curves can be found.
This test is performed on cylindrical samples with a diameter of 10 cm and a height of 6.25 cm. Also, the compaction of the asphalt sample is done with a 4.5 kg hammer and the fall height of the hammer is 45 cm. The number of strokes can also be different according to the amount of traffic of the desired route plan.
The method of doing the Marshall Bitumen test
To perform Marshall's test, various tools are needed, such as Marshall's hammer, Marshall's molds, mixer, molten bitumen, thermometer, scale and various weights, trowel, oil and fireproof gloves. In this method, the samples that have been granulated in advance are placed in the oven for 24 hours at a temperature of 160 to 170 degrees Celsius to completely remove the moisture from the stones. The granulated samples should all weigh 1200 grams in order to eliminate the effect of these two factors in the test.
- In this test, 21 samples will be prepared, 3 of which will be reserved to be replaced if an error occurs during the production of the asphalt sample.
The 18 prepared samples are divided into 6 groups of 3 and each group has the same bitumen percentage. Bitumen percentage values added to aggregates will be 4.0, 4.5, 5.0, 5.5, 6.0, 6.5 respectively. To start Marshall bitumen test, one should first put a mold with all the attachments in the oven so that it reaches the same temperature as the aggregates, and at the same time take out an oven from the oven and pour it into the mixing container and place the container on a scale that has the same weight in the bottom. There is a weight in front of the scale.
How to calculate the required weight percentage of bitumen
The following formula can be used to calculate the weight percentage of bitumen required for testing. Of course, this formula is calculated for 4% bitumen and it should be done in the same way for other percentages.
It should be noted that the above relationship for each group is equal to the bitumen percentage of that group. 50 grams of weight is added to the bottom where the weights are located, and then the molten bitumen will be slowly poured into the mixing container and it will continue until the two bottoms are equal. The temperature of mineral bitumen or used bitumen should not be more than 137 degrees because at this temperature the characteristics of bitumen will change.
Aggregates should be mixed with bitumen using a mixer and a special heater should be used under the mixing bowl to reduce the temperature of the mixture. The mixing process should continue until the aggregates are completely covered with bitumen.
Now, the mold should be removed from the oven and smeared with oil, and first, 2-3 cm of fine-grained materials should be poured into it using spatulas, and then other materials should be added uniformly inside the mold. After pouring all the mixture into the mold, they should be completely mixed in the shortest possible time to prevent the temperature from dropping.
Finally, the sample should be placed under the Marshall compression hammer dipped in oil and the number of blows should be set to 75 blows. After hitting 75 blows, the mold is turned over and 75 blows are applied to the other side, and finally the percentage of bitumen added is noted on the sample and placed in a safe place to reach the ambient temperature.
The final conclusion
Bitumen Marshall test is one of the tests that can be used for different purposes such as finding the relationship between density and bitumen percentage, finding the relationship between specific weight and bitumen percentage, and also finding the relationship between bitumen percentage and void space, compressive strength and drawing curves. It is used and to do it, tools such as Marshall hammer, Marshall molds, mixer, molten bitumen, thermometer, scale, etc. are needed.
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