At airfields, the potential for reflective cracking presents a major challenge for rigid pavement rehabilitation involving asphalt overlays. Reflection cracks most commonly result from temperature cycling-induced contraction (opening) and expansion (closing) in the concrete slab. Traffic loading exacerbates the thermally-induced reflection cracks, but the likelihood of spalling at the cracks and the potential for foreign object debris (FOD) to aircraft is a greater concern to the airport operations. The current airport pavement design and evaluation advisory circular, AC 150/5320-6F, which incorporates the design procedure FAARFIELD, is considered to be suitable for thickness design for the current fleet of heavy aircraft with a high degree of confidence in the thickness computations as affected by gear configurations and wheel loads. Unfortunately, FAARFIELD does not explicitly consider this important distress mode. To properly design HMA overlays on rigid pavements, a reliable performance prediction model based on reflection crack growth is essential. The performance model is expected to relate the required thickness of HMA overlay to several input variables, including aircraft loads, climatic effects, and the condition of the existing pavement.
The research objective is to develop a set of fully validated equations (the failure model) that can be directly implemented in the overlay design procedure in all future versions of FAARFIELD. The failure model will relate the required thickness of asphalt overlay to several input variables, including projected traffic, climatic data (temperature cycles), and the condition of the existing pavement. The failure model aims to:
· Address both aircraft and temperature loads
· Include all fracture modes
· Consider three-phase fatigue process
· Take full advantage of full-scale test data
· Utilize advanced computational mechanics
· Integrate performance characterization of HMA materials
The research approach includes three parallel tracks.
Full-scale Tests (Track 1)
To properly design asphalt concrete (AC) overlays on rigid pavements, a reliable performance prediction model based on reflection crack growth is essential. The performance model is expected to relate the required thickness of AC overlay to several input variables, including aircraft loads, climatic effects, and the condition of the existing pavement. One key element of developing such a sophisticated model is to acquire full-scale test data, particularly, crack propagation rates, under controlled loading conditions to mimic the temperature cycles occurring in nature. Failure in this case is considered to be the appearance of bottom-up cracks on the overlay surface that initiate from underlying concrete joints.
NAPTF Reflective Cracking Rig Experiment
Prior to full-scale tests, a theoretical study was first carried out to predict the pavement temperature using Enhanced Integrated Climate Model (EICM) and calculate the joint opening using Finite Element Analysis (FEA). An experimental study was then performed to characterize the viscoelastic properties of HMA, asphalt concrete (AC) – portland cement concrete (PCC) interface bond strength, fracture resistance, and fatigue performance of AC at low temperatures. Findings from both theoretical and experimental studies lead to the development of Temperature Effect Simulation System (TESS).
Temperature Effect Simulation System (TESS)
The Temperature Effect Simulation System (TESS), was designed, built, and installed at the FAA National Airport Pavement Test Facility (NAPTF). The TESS consists of hydraulic and temperature units, as shown in Figure 1a. The function of the hydraulic unit (HU) is to generate forces that create precise and repeatable horizontal displacement to simulate joint opening and closing induced by temperature changes. The main components of the HU are two cylinders that can generate a maximum total joint opening/closing force of 700,000 lb (Figure 1b). The force is fully adjustable via the motion controller for monitoring the overlay stress relaxation. The cylinders, along with MTS Position Sensors provide a closed loop position control (resolution of 0.001 in) with dynamic synchronization from side to side. The temperature unit (TU) is designed to maintain the test temperature with a variation of no more than ±1.0oF. The TU consists of a 40-ton chiller and refrigeration grids. As shown in Figure 1c, flexible crosslinked polyethylene (PEX) tubes are placed at three depths within the PCC slabs to ensure a uniform temperature.
(b) hydraulic cylinder
(c) PEX tubing
Figure 1. Temperature Effect Simulation System (TESS)
NAPMRC HVS-A Experiment
Comprehensive full-scale tests under different aircraft and temperature loads are needed so that the crack propagation model derived from Reflective Cracking Rig tests can be validated and calibrated to the field conditions. The Heavy Vehicle Simulator-Airfields (HVS-A) provides a unique opportunity to isolate and compare load and temperature effects on reflective cracking:
· Case I: Temperature load only
· Case II: Aircraft and temperature loads
At National Airport Pavement & Materials Research Center (NAPMRC), test data collected under outdoor conditions will be also used to assess the realism of the overlay temperature gradient induced by the cooling scheme integrated in TESS.
Finite Element Modeling (Track 2)
Discrete vertical discontinuities are essential three-dimensional geometric features for reflective cracking. The purpose of Track 2 is to obtain both a linear elastic solution for the stress intensity factor (SIF), K and a linear viscoelastic solution for the energy release rate, ΔG. First, the 3D finite element (FE) modeling considers Mode I temperature load for a given set of pavement structure, temperature gradient, material properties, and crack geometry. The developed FE model will then be used in conjunction with full-scale rig experimental data to computer crack propagation parameters. The last step of FE modeling will incorporate Mode II aircraft loads using the outdoor HVS-A experiment. In addition, the interaction between different fracture modes will be investigated. The final reflective cracking model will take both crack initiation and propagation into consideration.
Laboratory HMA Characterization (Track 3)
Although the causes and mechanisms of temperature-induced and load-induced reflective cracking are not the same, it is well accepted that certain bulk viscoelastic and fracture properties, along with the ability to withstand repeated loading forms of AC materials, are strongly correlated to field observations. Track 3 is a comprehensive testing suite that aims at characterizing the fracture, fatigue, and viscoelastic bulk properties of AC materials. The uniaxial compressive testing is the standard testing mode for evaluation of the AC stiffness. However, obtaining a field core meeting the specimen size requirement (6-in. height) for this testing mode could pose some challenges for AC overlaid PCC pavements. Therefore it is proposed to obtain the viscoelastic bulk properties in the IDT mode rather than in the uniaxial mode.