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4.4.1 Filler Content

Two mastics of limestone and granite, each at three concentrations (35%, 50%, and 65% filler content by mass of mastic) were used for this study, in addition to the base bitumen. These proportions by mass were selected based on a standard asphalt mixture as will be discussed later. Filler concentration by mass in mastic can be calculated based on the following equations:

Fm=

(4.4)

The determination of filler content by volume in mastic is given by:

Fv = (4.5)

Where: Fv= Filler concentration by volume, Fm = Filler concentration by mass, Bm = Bitumen concentration by mass, SG ^and

SG

According to BS EN 4987-1:2005 and for 10 mm dense bitumen macadam (DBM), asphalt concrete closed graded surface course, the bitumen content in the mixture is 5.2% while the filler content is recommended to range from 3% to 8% of the total aggregate by mass. The equivalent filler content by mass of mixture is between 2.85% and 7.6% which corresponds approximately to 35% and 65% content in the bitumen-filler mastic.

Chapter 4.Binder Shear Creep and Recovery Characteristics 72

Consequently, mastics were prepared at these limits plus at 50% as a midpoint representing practical mixtures and to investigate the influence of filler content gradual increase. Table 4.1 presents the ID of each binder type and filler contents based on mass and volumes.

Table 4.1 Samples IDs and filler contents

Code B M35 M50 M65 M35-2 M50-2 M65-2

Filler type _ Limestone Limestone Limestone Granite Granite Granite

% of total

mass _ 35 50 65 35 50 65

% of total

volume _ 17 27 41 16 27 40

4.4.2 Material Preparation

Rheological testing of binders requires careful material preparation as the consistency of testing results is determined by the samples uniformity.

Additionally, the utilised DSR Gemini 200 is regularly calibrated to maintain repeatable and consistent results. Initially, the bulk quantity of bitumen in the container was heated in an oven to 160⁰C (approximately the softening point + 100^oC). The heating was maintained for a sufficient time until all the bitumen was in a liquid form (especially at the bottom of the container) and continuously stirred gently to achieve homogeneity. For pure bitumen samples (0% filler), the heated bitumen was immediately distributed into 10 ml vials and stored at 5⁰C for later testing.

For mastic samples, the required mass of heated bitumen was poured into a new heated dry tin. The amount of filler required was heated to 105⁰C to a

Chapter 4.Binder Shear Creep and Recovery Characteristics 73

constant mass of moisture-free particles. The accurate mass of bitumen inside the tin was placed on a heating plate at 160⁰C. Then carefully and gently, the correct amount of filler was added in small batches with continuous manual stirring to achieve equal distribution and avoid any lump formation possibility. The filler addition was maintained carefully with continuous stirring until the whole mass was mixed properly. Finally, the homogeneous mastic was distributed into several 10 ml vials and stored at 5⁰C temperature for later testing.

4.4.3 Test Sample Preparation

Accurate sample preparation is a crucial requirement for true rheological measurements as results are sensitive to the preparation method and sample geometry. Different methods exist to prepare a DSR sample. To achieve repeatability and consistency it is highly recommended to adopt ‘hot pour’

method (Airey et al., 2003). The list provided below details the followed steps to prepare the sample:

1- Before starting the test, bitumen/mastic inside the vial was heated to 160⁰C for 15 minutes.

2- Because of the filler settlement possibility at the bottom of the vial, the heated mastic was stirred continuously for a sufficient time to equally redistribute the filler in the whole binder and remove any air bubbles.

Meanwhile, a zero-gap between the upper and lower plate was established.

3- A sufficient amount of heated binder was then poured onto the lower plate centre in order to achieve uniform distribution in later squeezing.

Chapter 4.Binder Shear Creep and Recovery Characteristics 74

The lower and upper plates’ temperature, before pouring the binder, were maintained at 60⁰C to obtain firm bond with the sample.

4- The upper plate was lowered automatically to the required gap (1 mm in 25 mm parallel plates and 2 mm for 8 mm parallel plates) plus 50 m.

5- The excess amount of binder that has been squeezed out of the edge is then trimmed carefully with a heated spatula. After careful trimming of the surplus around the sample circumference, the upper plate was again lowered to the target gap.

6- It was found that immediately replacing the chamber cover for temperature control after reaching the required gap results in a lack of adhesion between the upper plate and the sample surface during the test.

The improper adhesion manifests itself as a fast upper plate rotation either at the beginning or continuously through the test. The fast rotation provides misleading strain results (error) at the start of the test, while in the other case the sample fails immediately and should be replaced.

Therefore, after reaching the required gap and before placing the water chamber cover, the sample was left for 5 minutes to give sufficient time for secure adhesion. This last step was essentially important to insure no water can infiltrate between the upper plate and the sample surface, hence, smooth strain transition results.

7- Finally, the water chamber cover was replaced to submerge the upper and lower plates including the sample in water all under one temperature (circulator water bath). Temperature control is essentially required as any temperature gradient may lead to inaccurate results. After reaching the

Chapter 4.Binder Shear Creep and Recovery Characteristics 75

desired temperature (with ^oC limit), 15 minutes period was allowed for thermal equilibrium before starting the test.