CHAPTER 4. LABORATORY TESTING OF SELF-HEALING MICROCAPSULES IN
4.4 Experimental Program
rejuvenator with 5 g of polyurethane, and mixed the emulsion with 30 mL of ethyl phenylacetate.
The process then added the thoroughly mixed emulsion to the main solution slowly, in order to maintain the pH between 3.0 and 3.5. After a 15-minute stabilization, the process added 6.33 g of 37% formaldehyde solution and heated the solution for four hours at the desired temperature at a rate of 1oC per minute, adding deionized water throughout the heating time to maintain the desired water level. Once the four hours of heating were complete, the solution was cooled under a fume hood and was vacuum-filtered to arrive at the final product.
4.4.1.3 Microcapsules Morphology. The study employed a Scanning Electron Microscope (SEM- FEI Quanta 3D FEG Dual Beam SEM/FIB) to evaluate the morphology of the produced
microcapsules. Having sprinkled microcapsules on top of a double-sided tape attached to a pin stub specimen mount, the researcher sputter-coated the microcapsules with platinum for four minutes before imaging the specimens under a secondary electron mode at an accelerated voltage of 5 kV.
4.4.1.4 Thermogravimetric Analysis. Asphalt binder is typically added to the aggregate blend at around 163oC in a Superpave Hot-Mix Asphalt (HMA). Therefore, the study conducted a Thermogravimetric Analysis (TGA) on the prepared microcapsules as a means to study their degradation at high temperatures.
4.4.2 Self-Healing Mixture Testing
4.4.2.1 Materials. The selected materials in the mixture preparation consisted of a styrene- butadiene-styrene (SBS) polymer-modified binder, which was selected based on the Louisiana specifications for PG 70-22M; 16mm gravel, 6.35mm gravel, coarse sand, and fine sand, thus satisfying the mix design requirements of a 12.5mm Nominal Maximum Aggregate Size
(NMAS). Post-consumer waste shingles (PCWS) were incorporated into the evaluated mixtures
at 5% by total weight of mix. In addition, the experimental program considered the use of an asphalt rejuvenator and the microcapsules in selected mixtures at 5% by total weight of RAS, see Table 4.1.
4.4.2.2 Specimen Preparation. Table 4.1 presents the prepared asphalt mixtures for the study.
The mixture 70CO is a conventional mixture containing no RAS. Mixture 70PG5P used 5%
PCWS by weight of the total mix. The experimental program considered the use of 5% PCWS by weight of the total mix and 5% Rejuvn8 by weight of RAS for the mixture 70PG5Rej8.
Lastly, the MCRej8 mixture contained 5% PCWS by weight of the total mix and 5%
microcapsules by weight of RAS.
Table 0.1. Job-Mix Formula Mixture
Type RAS Rejuvenator (%) Microcapsules Content (%)
70CO - - -
70PG5P 5% PCWS - -
70PG5Rej8 5% PCWS 5% Rejuvn8 -
MCRej8 5% PCWS - 5%
Six specimens were prepared for each asphalt mixture type, with three to be exposed to room- temperature healing conditions and three to be exposed to high-temperature healing conditions after cracking. Cylindrical samples were prepared using a Superpave Gyratory Compactor (SGC) and rectangular specimens with dimensions 40mm x 40mm x 160mm (as shown in Figure 4.1) were obtained by sawing the cylindrical samples. All specimens were prepared to an air voids of 7.0 ± 0.5%.
4.4.2.3 Self-Healing Test Description. A three-point bending setup was used to induce cracks at room-temperature in the prepared rectangular specimens with span length of 100mm without any prior conditioning through a strain-controlled load applied at a rate of 0.25 mm/min, which allowed stopping the test before any sudden failure. The three-point bending setup is shown in Figure 4.1(b).
(a)
(b)
Figure 0.1. (a) Rectangular Specimen Obtained by Sawing Cylindrical Samples and (b) Three- Point Bending Test Setup
The three-point bending test results were used to calculate the stiffness for three different
conditions (undamaged, damaged, and healed). The test was stopped 100 seconds after reaching the peak load in each condition. As shown in Figures 2(a and b), the undamaged stiffness was defined as the slope of the linear equation from the steepest part of the load-deformation plot. A box in Figure 4.2(a) identifies the part of the curve analyzed in Figure 4.2(b). The same
procedure was repeated for a second three-point bending test before healing in order to calculate the damaged stiffness and to increase the severity of the cracks before healing. Following healing, a third three-point bending test was conducted in order to estimate the healed stiffness.
(a) (b)
Figure 0.2. Calculation of Undamaged Stiffness (a) Load-Deformation Plot from First Bending Test (b) and Stiffness Calculation
Utilizing an optical light microscope, the study quantified the healing process of cracked
specimens as a function of time, by adopting a magnification rate of 12x in order to measure the different cracks in the specimens. Immediately after crack characterization, specimens were then subjected to a 6-day healing period under controlled environmental conditions. Specimens were placed horizontally over a flat surface during the healing period. Cracks were observed at healing
0 0.1 0.2 0.3 0.4 0.5 0.6
0 1 2 3
Load (KN)
Deformation (mm) Load-Deformation-Undamaged
Sample
y = 0.3853x + 0.0058 R² = 0.99
0 0.05 0.1 0.15 0.2 0.25
0 0.2 0.4 0.6
Load (KN)
Deformation (mm) Undamaged Stiffness Calculation
periods of 0, 1, 2, 3 and 6 days. The study also performed a digital image analysis to measure the crack width over time.
4.4.2.4 Environmental Conditions for Self-Healing. Three specimens for each of the evaluated asphalt mixtures were exposed to two different dry temperature conditioning. The first
temperature conditioning was applied at room temperature (26 ± 2°C). The second conditioning was conducted at a temperature of 50 ± 2°C, which was maintained for 6 days using a
conventional oven.
4.4.2.5 Quantification of self-healing. Healing of specimens as a function of time was quantified by digital image analysis using light microscopy. The light microscope utilized for data
acquisition was a Zeiss SteREO Lumar.V12. The crack width was measured before healing (day 0), and at day 1, day 2, day 3 and day 6 of the healing period to quantify the healing efficiency of the specimens. The healing efficiency of the specimens at the different healing periods was calculated as follows:
𝐻𝑒 = (1 −𝐶𝑤𝑡
𝐶𝑤0) ∗ 100 (4.1)
where 𝐻𝑒 = Healing efficiency (%); 𝐶𝑤0 = Initial Crack width, mm; and 𝐶𝑤𝑡 = Crack width at the time of analysis, mm. Furthermore, a relationship between damaged and healed stiffness was evaluated to determine the stiffness recovery at the end of the healing period. The undamaged and healed stiffness values were compared by the ratio between the healed stiffness and the undamaged stiffness.