Asphalt polymer blending requirements

Gordon D A, (1997) studied the factors, which affect the properties of modified bitumen. The properties of the modified bitumen depend on the polymer type, polymer content, and temperature. Each polymer type has its own way of mixing and special treatment. Different methods of mixing polymer with the bitumen were used and this is known from the experience or from old written report. The blending mechanism, which were used to blend bitumen with polymer depend on the level of shear rate, which induces into the bitumen.

Clear definitions of the factors, which affect the properties of the modified bitumen, were presented in this study. There is a recommended shear blending speed is assumed to be 2500 rpm. Final conclusion from the previous study is that the blending time depends on the blender configuration and polymer type.

Al-Abdul-Wahhab, H and Al-Amri, G. (1991) reported that the blending temperatures depend on the molecular weight of the used polymer. It is clear that polymer with higher molecular weight needs higher temperature than the polymer with low molecular weight. The following blending sequence was used to modify bitumen materials with crumb rubber or styrene butadiene styrene:

• Asphalt cement was heated in an oven at a temperature of at least 160 ^oC.

• The stainless steel beaker used for mixing was cleaned and kept in the oven at a temperature of at least 160 ^oC.

• The required amount of asphalt was weighed into the beaker; then the amount of additive required to yield the desired additive-to-asphalt ratio was weighed.

• Eight blends were prepared with 3%, 5%, 7%, and 10% SBS and CR, respectively, by total weight of bitumen.

• The mixer was started, and the prepared amount of additive was added gradually to the beaker while stirring.

• The mixing temperature was controlled during mixing using heater and it is 180 ^oC for crumb rubber and 160 ^oC for styrene butadiene styrene.

• The ready modified bitumen was used to prepare the tests sheet, which were used for making DSR specimen with different diameter using special tools.

There are different methods to determine the optimum blending time. One of the most known methods to determine the optimum blending time is to measure one physical property for the tested material and when this physical property start to be constant with increasing mixing time then this is the optimum blending time. Complex shear modules at uniform interval time were considered during the blending process. The uniform interval time was 5 minutes and when the complex shear module does not show increase with time, the blending process should be stopped. Longer blending time for production of SBS modified binder and rubber-modified binders lead to an increase in the complex shear modulus; this is due to homogenous and stable network formation in the modified binder.

The optimum blending time is illustrated by the relationship between the complex shear modules versus the time as shown in figure (3-7). It is clear from the figure that the longer blending time in the control binder was found to have little effect on the complex shear modulus. The optimum blending time for production of 10% rubber-modified binders is about 60 minutes and the optimum blending time for production 5% SBS modified bitumen is about 45 minutes. The blending time is dependent on the level shear rate of the blender and polymer type, it is recommended to find the recommended time for each modifier.

Figure (3-7) Relationship between the complex shear modules versus the time at 160 ^oC.

Table (3-4) presents the recommended blending time and temperatures for both crumb rubber (CR) and styrene butadiene styrene (SBS). It is clear from the table that crumb rubber needs more blending time and more temperatures than styrene butadiene styrene.

Table (3-4): Recommended blending time and temperatures for crumb rubber and styrene butadiene styrene

Type of Modifier RBTM MBTM

(**)

RBTD MBTD

(*) (***) (****)

styrene butadiene

styrene(SBS) 60 80 160 200

Crumb rubber (CR) 45 65 180 200

(*) - Recommended Blending Time Minutes (**) - Maximum Blending Time Minutes

(***) - Recommended Blending Temperatures Degree (****) – Maximum Blending Temperatures Degree

This is because the blending temperature depends on the molecular weight of the modifier and it is known that the molecular weight of crumb rubber is more than the molecular weight of SBS. The required blending temperature for (CR) is higher than the required blending temperatures for SBS. Different mixers were used to blend bitumen with polymer according the level of shear rate. The level of shear rate is defended as the speed of the blinder head.

The higher shear blending speed is recommended to be 2500 rpm.

3.5.2 Asphalt Binder Specimen Fabrication

The mixing temperature of bitumen with polymer is generally between 150 ^oC and 200 ^oC or above. The binder is heated in the oven for about two hours at pouring temperatures of 135 ^oC for unmodified bitumen and 150 ^oC – 165 ^oC for modified bitumen until the binder is hot enough to pour. Steel plate is covered with non-stick paper then four small steel pieces with 1-mm and 2-mm according to the needed thickness were placed over the non-stick paper. Then the heated asphalt binder is poured in the space between the four small steel pieces.

Another non-stick paper was placed over the binder, which was intercepted between the four small steel pieces. The upper steel plate was covered by the second non-stick paper.

The upper steel plate is pressed by hand to get uniform thickness for the heated binder.

Finally the bitumen sheet was kept in refrigerator at (4 ^oC to 10 ^oC) to be solid enough. DSR specimen was prepared by using special tools of 25-mm and 8-mm diameter to cut the sample from the bitumen sheet.

Figure (3-8) Asphalt blinder specimen fabrication 3.5.3 Specimen Geometry

The specimen geometry was chosen according to the test type, condition and specification. The specimen geometry at high temperature should have big diameter to save the specimen from melting. At low temperature the specimen should have small diameter with high thickness to prevent it from brittle crack. Two type of testing plate geometries are used with the dynamic shear rheometer. The first specimen geometry is 25-mm diameter spindle with 1-mm testing gap for intermediate to high temperature. The second specimen geometry is 8-mm diameter generally used at low temperature from -5 ^oC to 20 ^oC.

25 mm diameter - 1mm thickness 8 mm diameter - 2mm thickness High Temperature Intermediate Temperature

Figure (3-9) (DSR) test samples for high temperature and intermediate temperature