Evaluation of Antistripping Additives for Arizona Asphalt Mixes
📨 Contact: Hasan Ozer
🤝 Sponsor: Southwest Pavement Technology Consortium (SWPT)
📅 Timeline: 2021 – 2022
Highlights
Introduction
Water is one of asphalt pavement’s most persistent enemies. When moisture infiltrates the bond between asphalt binder and aggregate, the pavement weakens from the inside out — a process known as moisture-induced damage or stripping. In Arizona, this problem is complicated by the extreme thermal environment: pavement temperatures regularly surpass 140°F during summer, and the combination of intense heat with occasional monsoon moisture creates conditions that can accelerate binder-aggregate debonding. Antistripping additives — either liquid agents blended into the binder or mineral admixtures like hydrated lime mixed with the aggregate — are the primary tools pavement engineers use to combat this threat.
Despite the widespread use of liquid antistripping agents (LAS) across the industry, their performance relative to mineral admixtures under Arizona-specific conditions had not been rigorously characterized. With a growing number of commercial LAS products available from multiple suppliers, and with agencies seeking cost-effective alternatives to hydrated lime, there was a clear need to evaluate these products side by side using relevant laboratory performance tests. This study, funded by Solterra Materials LLC through the SWPT consortium and led by Dr. Hasan Ozer at Arizona State University, set out to fill that gap.
Methodology and Framework
Four plant-produced asphalt mixes representative of Phoenix-area construction were selected for the study, using two aggregate sources with distinct mineralogy. The test matrix included five liquid antistripping agents (two from Supplier A and three from Supplier B), along with hydrated lime and Portland cement as mineral admixture benchmarks, and a control mix with no antistrip. The evaluation was structured around three laboratory performance test methods that collectively assess moisture susceptibility from different angles. The Hamburg Wheel Tracking Test (HWTT) applies repeated wheel loads to submerged specimens, measuring both rutting deformation and the onset of stripping-induced failure. The Indirect Tensile Strength ratio (TSR, per AASHTO T283) compares the tensile strength of moisture-conditioned specimens — including freeze-thaw cycling — against dry controls. The Semi-Circular Bending (SCB-CMOD) test measures fracture energy, capturing whether moisture conditioning degrades the mix’s ability to resist crack propagation.
The combined use of three distinct test methods was intentional: no single test captures the full complexity of field moisture damage. HWTT is particularly sensitive to stripping in the later stages of loading, while IDT/TSR reflects early-stage moisture conditioning effects. SCB fracture testing adds a dimension that neither test alone provides — how moisture affects a mix’s fundamental toughness. All mixes were prepared and tested in a consistent laboratory environment, with multiple replicates to ensure statistical reliability. The study was designed to produce a performance ratio framework that allows direct comparison across all additive types and mix designs.
Key Findings
Liquid Antistrip vs. Mineral Admixtures
Across all four mixes and three test methods, hydrated lime and Portland cement outperformed liquid antistripping agents in moisture resistance. This was most apparent in the Hamburg Wheel Tracking Test, where mineral admixture mixes reached significantly higher load cycles before stripping inflection points appeared, and in TSR values after freeze-thaw conditioning. The finding reinforces longstanding industry knowledge that mineral admixtures — particularly lime — provide superior moisture protection through both chemical and physical bonding mechanisms. For agencies in the SWPT region where stripping is a chronic concern, this data supports continuing to specify lime or cement where moisture susceptibility is the primary design driver.
However, the performance gap between lime and compatible liquid agents was not uniformly large. In mixes where the LAS product was chemically compatible with the aggregate mineralogy, the liquid additives achieved TSR values close to the lime threshold and showed acceptable Hamburg performance. This suggests that liquid antistrip products can be a viable option in lower-risk applications or where lime handling and mixing logistics are prohibitive — provided compatibility is verified through laboratory testing before field use.
LAS B vs. LAS A
The most striking finding within the liquid additive category was the consistent performance difference between the two suppliers. LAS B products — all three formulations — outperformed LAS A products in every test method and across all four mixes. The LAS A products showed more variable results, with some mixes exhibiting poor Hamburg performance and TSR values below the commonly used 0.80 threshold even after antistrip treatment. LAS B, by contrast, achieved above-threshold TSR in most mix-additive combinations and showed notably better stripping resistance under Hamburg loading. These differences point to the importance of additive chemistry and aggregate compatibility rather than simply the presence of an antistripping agent.
Fracture Energy Under Moisture Conditioning
The SCB-CMOD fracture tests reinforced the ranking established by HWTT and TSR, while adding nuance about mix toughness. Mixes with hydrated lime retained the highest fracture energy after moisture conditioning, followed by cement and LAS B mixes. The LAS A mixes showed the greatest reduction in fracture energy after conditioning, suggesting that moisture not only reduced tensile strength but also impaired the mixes’ crack-resistance capacity. This has practical implications: in Arizona’s high-thermal-gradient environment, where mixes are already vulnerable to fatigue cracking from daily temperature cycling, starting with degraded fracture resistance due to inadequate antistrip protection could accelerate pavement deterioration significantly.