Chapter 3
Materials and Methods
3.1 Materials
One asphalt from vacuum distillation with a 170/220 Pen grade was used in this work. Modification was done using a linear styrene-ethylene-butylene-styrene block copolymer (SEBS) KRATON® G1652 (styrene content = 30 % by weight, MFI = 5 g/10 min (230°C, 5 Kg), elongation at break = 500%).
3.2 Preparation of PMA
The neat asphalt has been modified in asphalt modification Laboratory of University of Pisa. The samples with 2%, 4% and 6% SEBS by weight were blended. The resulting PMAs have been called S2, S4 and S6, respectively. Neat asphalt was heated up to 180°C, removed from
the oven and placed in a heating chamber to keep the temperature constant during modification process. The asphalt was stirred with a SILVERSON L4R high shear mixer at low speed and the polymer was gradually added. The speed of the mixer is then increased to 2500 rpm. The system was kept at 180±1°C for 90 minutes. During the mixing, at predefined times, samples were taken in order to obtain information about morphology (fluorescence microscopy) and to check the softening point. When the mixing time ended, part of the material is poured in a can for the subsequent penetration testing.
3.3 Physical characterization
3.3.1 Softening Point Test
The aim of this test, also called Ring & Ball test because of the particular geometry of the experimental apparatus, is to determine the temperature at which asphalt softens. In our case, samples were taken each 45 minutes during the mixing and poured into two (preheated) brass
rings with standard dimensions, cooled at room temperature for at least 30 minutes, and trimmed with a hot blade. Rings were moved on a standardized ring holder and the whole structure plus two small metal spheres were moved into a beaker filled with distilled water for 15 minutes at 5°C, for expected softening points up to 80°C. Otherwise, the beaker must be filled with glycerine and the 15 minutes conditioning bath must be kept at 30°C. The ring holder was equipped with ball centring guides: after the conditioning bath, asphalt samples were loaded by placing the balls in the centre of the rings, and the temperature was increased at a rate of at least 5°C per minute. Softening temperature was taken as the one at which balls touched the lower plate of the ring holder. The two readings should agree within 1°C. For more information see AASHTO T53-92 or ASTM D36-86.
3.3.2 Penetration Test
The sample was moved into a 25°C water bath and penetrated by a needle with a load of 100g per 5sec. Penetration results are reported in decimillimeters. Small variations in the temperature can lead to very different results. See AASHTO T49 or ASTM D5-97 for more information.
taken each 15 minutes during the mixing, and poured into a cylindrical brass mold preheated in oven, at 180°C, to avoid asphalt quenching. The mold was in the oven for 1 minute, then kept at room temperature for at least 30 minutes and finally stored at -28°C. The sample was then removed from the mold, kept in liquid nitrogen for a couple of seconds to ensure a brittle cracking, cut and then analyzed with a LEICA DM LB optic microscopy. It is important to apply a silicon paste on the inner surface of the mold to allow an easy removal of the sample. For more information about fluorescence microscopy see UNI prEN 13632.
3.4 Rheological Characterization
3.4.1 Samples: conditioning and loading procedure
Asphalt samples were poured into rubberized molds, cooled to room temperature and then stored at -28°C. Before being used for rheological testing, samples were placed onto the lower plate of the rheometer, warmed up to 60°C. The upper plate was then brought to a gap of 1.100 mm, the sample trimmed, and a pre-stress of 25Pa was applied to the sample for 10 seconds. Then, in order to release possible internal tensions, the sample was kept at 60°C for 10 minutes. After that, the test temperature was established and kept for 5 minutes, the gap reduced to 1.050mm, and the sample trimmed again. Finally, the 1 mm gap was reached and the experiment started. Quite often, the same asphalt sample was tested more than once. In this case, the sample was kept at 60°C for 20 minutes between one measurement and another. In dynamic creep experiment, because of very high deformations obtained, each sample was tested only once. No pre-stress was applied before this experiment; each sample was kept at 60°C for 10 minutes and at the test temperature for other 5 minutes before the start up of the experiment.
3.4.2 Simple Creep
Simple creep and recovery experiments were performed in a stress-controlled dynamic shear rheometer (DSR) CVOR from Bohlin Instruments – currently Malvern Instruments. The creep and recovery tests were conducted at three different temperatures: 40, 50 and 60°C for PMAs and 30, 40 and 50°C for Base asphalt, with plate-plate geometry; the diameter was 25 mm and the gap was 1 mm. Creep time was 1 minute, followed by 9 minutes of recovery, and 1 second, followed by 9 seconds of recovery. The levels of the applied shear stress were 25, 50, 100 and 1000 Pa. All experiments were repeated several times and the presented results are averages of the three best runs.
3.4.3 Dynamic Creep
Repeated creep and recovery experiments were performed in a stress-controlled dynamic shear rheometer (DSR) CVO 200 from Bohlin -currently Malvern- Instruments. The creep and recovery tests were conducted at three different temperatures: 40, 50 and 60°C for PMAs and 30, 40 and 50°C for Base asphalt, with plate-plate geometry; the diameter was 25 mm and the gap was 1 mm. A set of one hundred cycles of repeated creep and recovery were taken by the instrument, with a loading time of 1 second and unloading time of 9 seconds. The levels of the applied shear stress applied were 25, 100 and 1000 Pa. All experiments were repeated several times and the presented results are averages of the three best runs.
3.4.4 Stress Relaxation
Stress relaxation experiments were performed in a strain-controlled Rheometric Scientific -currently Ta Instruments- ARES A-33A rheometer. The test geometries were plate-plate with diameters of 25 and 50 mm and a gap of 1 mm. Step-strain measurements were conducted at
very low values of the strain (i.e. in the linear interval), but low enough so that no overload of the transducer occurs and that there is no risk of macro-fractures or slipping of the sample for high values of the strain. Slipping is one of the main problems in rheological measurements. In the case of asphaltic binders, there is usually a strong adhesion between the sample and the plate, thus in the presented results, slip should not to be considered as a significant cause of errors.
3.4.5 Viscometry
Direct viscosity measurement was performed in the Rheometric Scientific – currently TA Instruments ARES A-33A rheometer. The test geometries were plate-plate with diameters of 25 and 50 mm and a gap of 1 mm. Measurements were conducted at temperatures of 30, 40, 50 and 60°C. All experiments were repeated several times and the presented results are averages of the three best runs. Those averages were interpolated with the three parameters Carreau model, described in paragraph (1.4); the commercially available software TableCurve 2D [40], was used for the viscosity fitting.
3.4.6 Fitting of Experimental Data
3.4.6.1 Creep
The following linear viscoelastic models have been used to fit simple and dynamic creep data, by means of the TableCurve 2D software [40]. The first is the model generated by the superposition of Voigt elements:
0 1 ( ) g n i 1 exp i i t t J t J J λ η = − = + − + (3.1)
where, Jg is the glassy compliance, Ji are weights of individual modes, λi are retardation times and η0 is the zero-shear viscosity. This model has been used with n = 2 or n = 3 (two or
three elements), for a total of 6 and 8 parameters, respectively. The second is Burger’s model, popular in civil engineering, where the creep compliance is given by a Maxwell element connected in series with a Voigt element, i.e. one mode has the following form:
1 0 0 0 1 1 1 ( ) 1 exp t t J t G G G − λ λ = + − + (3.2)
where G0 and G1 are the elastic moduli, λ0 is a characteristic time (η0=G0 0λ ) and λ1 is the
retardation time, (in total 4 parameters). The third model is generated by a continuous spectrum ( L ) of retardation times. For the gamma-type distribution [41], [L( )Λ =α λ2 exp(−αλ)] the compliance is given as: [10]
(
)
2
0
( ) g D 1 2 2 t
J t =J +J − αtK αt + η (3.3)
whereK2 is the modified Bessel function and JD is the delayed compliance. This model is
fully described by 4 parameters. Simplified forms of Burger and Voigt models have also been used. Both models are linear in time and described by two parameters only:
0 0 0 0 1 ( ) g t t J t J G G η λ = + = + (3.4)
In the dynamic creep experiment the stress has the the form of pulses of height σ0 and
duration of t . Each pulse starts at time, nt1(n = 0,1,2,…,N) where t1 is the total time of one complete cycle. The stress ( )σ t′ is then given as:
where H is the unit step function. Combining equation (3.5) with the classical linear viscoelastic equation (1.3), one obtains the strain as:
[
]
0 1 1 1 1 0 ( ) N ( ) ( ) ( ( )) ( ( )) n t H t nt J t nt H t nt t J t nt t γ σ = = − − − − + − + (3.6)Equation (3.6) was used to fit only part of the cycles with the above described linear viscoelastic models and to propagate the fitting to the whole experiment (100 cycles).
3.4.6.2 Stress Relaxation
By repeating the step-strain test with various strains, one can construct the nonlinear relaxation modulus, G(t,γ), as a function of two variables. Data can be presented in two different ways.
The majority of authors treat G(t,γ) as a function of time and a parameter γ, i.e. they describe
G by a one parametric family of curves G(t;γ). The other possible approach is to construct a
discrete grid, {ti, γi}, from experimental data and to study the three dimensional surface of
G(t,γ) both in a nonparametric or parametric way. As far as the parametric description is
concerned, one should use a relatively simple model for G(t,γ) and/or the memory function
M(t,γ). A two stretch exponential model [42] of linear relaxation modulus was used to fit
experimental data:
( )
exp exp b f t t G t c d a e = − + − (3.7)This is a modification of the generalized Maxwell model; the strain doesn’t appear in equation (3.7), so that any nonlinear contribution is taken in account. Of course, in order to fit data on a discrete grid, the presence of the strain in the model is required. If each relaxation mode is
“attenuated” by another simple stretch exponential factor, one can consider the following parametric model of the nonlinear relaxation modulus:
( )
, exp 2 exp 2 h j b f t t t t G t c d a g e i γ γ γ = − + + − + (3.8)Note that the strain appears as 2 in order to keep the dependence of the memory function on
strain [43]. In case of separable nonlinear viscoelasticity, i.e. where the use of the damping function was allowed, the following model were used:
( )
, exp t exp t exp tG t c d b a g e i f j γ γ γ γ = − + + − + + − + (3.9) and:
( )
, exp exp h b f j t t G t c d a g e i γ γ γ = − + + − + (3.10)where an exponential structure of the damping function has been assumed. Note that each exponential term, corresponding to a “nonlinear Maxwell element”, has its own dependence on the strain, i.e. there is a damping function per each mode. For base asphalt, a single stretch model was also used:
( )
, exph b
t