When designing airport pavements, our engineers must study and analyze pavement distress. A very common issue in pavement distress is reflective cracking of asphalt overlays. Reflective cracking is caused by temperature cycling-induced contraction (opening) and expansion (closing) of the concrete slab joints and exacerbated by large traffic loads. These cracks in the pavement can lead to spalling, the breaking of pavement into smaller parts, which increases the potential for foreign object debris (FOD). Beyond reducing the lifespan of the pavement, FOD on an airport runway can pose significant risks and hazards to aircraft and airport operations.
ATR’s extensive reflective cracking research is conducted using a one-of-a-kind Reflective Cracking Rig. The Rig is being used to develop design parameters to minimize pavement issues caused by reflective cracking. Our research objective is to develop a set of fully validated equations that can be directly implemented in the creation of future pavement overlay design procedures within FAARFIELD. This failure model will relate the required thickness of asphalt overlay to several input variables, including projected traffic, climatic data, and the condition of the existing pavement. The failure model aims to:
- Include two fracture modes
- Mode I (opening mode): Temperature Load
- Mode II (sliding mode): Aircraft Load
- Consider the fatigue process
- Phase 1: Initiation
- Phase 2: Propagation
- Phase 3: Failure
- Take full advantage of full-scale test data
- Utilize advanced computational mechanics
- Integrate performance characterization of hot-mix asphalt (HMA) materials
Types of Testing
ATR’s multifaceted reflective cracking research includes both indoor and outdoor scenarios. The indoor research is focused on temperature induced reflective cracks due to existing cracks or joints in the underlying pavement.
Outdoor testing is conducted at the National Airport Pavement & Materials Research Center (NAPMRC), where test pavements are built to isolate and compare the effects of aircraft loads during cold weather temperatures versus unloaded pavements. The testing takes place during winter months using the FAA’s Heavy Vehicle Simulator for Airports (HVS-A) to simulate aircraft loads. This portion of studying reflective cracking combines the vertical movement of aircraft loads and the horizontal movement caused by temperature fluctuations.
Finite Element Modeling
Another aspect of reflective cracking we focus on is discrete vertical discontinuities. These are important three-dimensional geometric features related to the cracking phenomenon. The goal of Finite Element Modeling is to achieve two types of solutions: a linear elastic solution for the stress intensity factor (SIF) and a linear viscoelastic solution for the energy release rate (ΔG).
To begin, we use a three-dimensional finite element (FE) model that considers a specific type of loading called Mode I temperature load. This model considers factors such as the pavement structure, temperature gradient, material properties, and the shape of the crack. By using this FE model, we can analyze how cracks propagate under these conditions. To validate our model, we compare its results with data obtained from experiments conducted on the full-scale rig.
Next, we expand the FE model to include another type of loading called Mode II aircraft loads. We do this by incorporating data from an outdoor HVS-A experiment. By studying the interaction between different fracture modes, we can better understand how cracks initiate and propagate in the pavement.
Our goal is to develop a reflective cracking model, validated through full-scale testing, that considers both the initiation and propagation of cracks. This model will be based on the insights gained from the FE modeling, experimental data, and the analysis of different loading conditions.