MICROMECHANICAL INVESTIGATION OF COLD MIX ASPHALT
Creators: name: Notani, Mohammad Ali
Publisher: name: Purdue University Graduate School
Year: 2025
Subjects: classificationCode: 400505
subject: Construction materials

description: Cold mix asphalt (CMA) is an eco-friendly paving material produced at ambient temperatures, offering energy savings by requiring less energy to decrease asphalt binder viscosity. This technology eliminates the need for heating during the mixing and compaction processes, further magnifying its economic benefits when used as a cold-in-place recycling technique. Unlike hot mix asphalts that gain strength through cooling, CMA achieves its final strength through a curing process involving the evaporation of volatiles and the hardening of the emulsified asphalt binder over time. However, its reliance on a curing process for strength development raises concerns about its short-term performance.A typical CMA mixture consists of four main components: air voids, mineral aggregate, water, and asphalt droplets suspended in water. The presence of water can significantly influence the overall performance of the mixture under both traffic and environmental loads. Most existing studies on CMA have predominantly focused on the behavior of the mixtures after they have fully cured. However, in real-world scenarios, pavements are often subjected to various stresses during the curing process, which takes up to several months. As a result, premature distress can compromise the early performance of the pavement. Asphalt undergoes significant chemical and physical changes throughout this phase that can influence its final characteristics and in-situ performance. Overlooking this crucial stage can lead to a poor understanding of the material's capabilities and limitations. Despite the importance of this phase, the micromechanical and rheological behaviors of CMA during curing remain largely uncharted territories. Therefore, this dissertation aims to investigate the micromechanical performance of CMA during the curing phase.This research study was performed in two study scales: Mastic and Mixture. The first scale focused on the rheological performance of emulsified-cold asphalt mastic (ECAM), considering varying curing levels, different filler-binder ratios, and filler surface treatments. Comprehensive rheological tests, including frequency sweep, temperature sweep, and strain sweep tests, were conducted on fully and partially cured mastic samples, i.e., 20%, 40%, 60%, and 80%, across a wide range of test temperatures. To analyze the physio-chemical adhesion properties between filler and emulsified asphalt, an analytical tool named the “asphalt-filler interaction” theory was formulated to determine the adhesion bond between filler and binder in the presence of moisture. Microscopic images were also captured to analyze the micro-structure and moisture interaction in the CMA’s matrix. Moreover, the presence of moisture in the CMA brings up another complexity during curing time: The water-to-ice phase transition. Normal Force (Nf) was used as a novel measurement parameter to determine water-ice phase transition effects on the rheological study of emulsified mastic. In the mixture scale, mechanical tests were performed on specimens fabricated with two gradations at fully and uncured CMA samples. The mixture experimental tests included the dynamic modulus test, Illinois flexibility index test, Hamburg wheel loaded test, and disc-shaped compact tension test.This dissertation presents a thorough analysis and detailed findings that illuminate the complex relationships and behaviors of CMA, particularly at the mastic scale. A significant observation is the direct influence of the filler-to-binder ratio on the curing time; increasing this ratio prolongs the curing process while using a filler with less surface area accelerates it. Notably, 25% of the filler-to-binder ratio enhances the rheological properties of ECAM, particularly at lower loading frequencies. This study further pinpoints the 60% curing level as a crucial threshold in the CMA curing process. Below this, moisture's effect on rheological performance overshadows that of the primary asphalt material, leading to brittle characteristics in freezing conditions and viscous behavior at intermediate temperatures. In the curing stage, the trapped and blocked waters that emerge during the coalescence phase of the emulsified asphalt breaking contribute to the extended curing time of ECAM.Additionally, freezing temperatures yield a water-to-ice phase change in uncured ECAM, resulting in a brittle behavior. Interestingly, a direct correlation emerges between curing percentage and freezing point; higher curing percentages relate to lower freezing points. Another significant discovery is the appearance of micropores in fully-cured ECAM, likely due to water evaporation and emulsifier presence, which potentially compromises its performance compared to ECAM fabricated with residual asphalt binder. Furthermore, adjusting the pH, especially by treating limestone filler with hydrochloric acid (HCl), showed noticeable improvements in CMA’s rheological behavior. At the mixture scale, the CMA mixture contained a higher filler-binder ratio in the mixture scale, presenting a better viscoelastic performance and higher cracking resistance at intermediate and freezing temperatures. Moreover, a minimum amount of water, 2.5% by total mass, added to the CMA mixture is essential to ensure adequate mixability, workability, and compactibility. Viscoelastic analysis showed that the curing process changes the transition point from elastic to viscous behavior of CMA mixtures. This shift towards lower frequencies results in a CMA mixture with poor resistance to higher temperature performance.