Mitigation of Reflection Cracking in Asphalt Concrete Overlay on Rigid Pavements

. Reflective cracking is one of the primary forms of deterioration in pavements. It is widespread when Asphalt concrete (AC) overlays are built over a rigid pavement with discontinuities on its surface. Thus, this research work aims to reduce reflection cracks in asphalt concrete overlay on the rigid pavement. Asphalt Concrete (AC) slab specimens were prepared in three thicknesses (4, 5, and 6 cm). All these specimens were by testing machine designed and manufactured at the Engineering Consulting Office of the University of Baghdad to examine for the number of cycles and loads needed to propagate the reflection cracking in the asphalt concert mixture at three temperatures (20, 30, and 30°C). It was noticed that the higher thickness AC mixtures increased the reflection cracking performance life of the AC overlay. Also, it was found that the number of crack initiation and failure cycles increased as the temperature increased. In contrast, the increased temperature decreased the required load to crack initiation and failure load in the sample.


INTRODUCTION
The construction of new roads is essential for the economic development of a country.The road has several negative meteorological and natural-climatic influences throughout its useful life, including functional temperature variations, regular wetting by surface and ground waters, seasonal freezing, and unequal thawing of road structural layers.As a result, the water-thermal state of the subgrade changes, and the ultimate strength of the road pavement's structural layers weakens, leading to an instability of the "road pavement -subgrade" [1, 2,3].As maintenance, riding quality, skid resistance, noise, and repair methods, overlays are constructed on the existing pavement.An overlay's cracking occurs in sites with joints in the underlying pavement due to stress concentration when an overlay is put on top of the existing pavement and subjected to heat, shrinkage, or traffic-induced loadings."Reflection cracking" describes this kind of damage.Water may seep into the pavement structure through reflective cracks in the overlay, contributing to several other kinds of pavement degradation, such as raised roughness, spalling, and a shortened fatigue life [4].
Under a passing wheel load, three load pulses at the crack tip can be observed.The first is due to shearing forces when the load approaches the crack, the second is due to bending forces when the load is directly on the crack, and the last is expected to be shearing forces again when the load leaves the crack [5]. Figure 1 presents these pulses of the different locations of the wheel load.
Reflection cracking is the process by which preexisting pavement cracks spread to newly installed overlays or the cracking of a resurfacing or overlay over underlying cracks or joints, with the likely cause being the movement of some kind in the underlying pavement.Also, it is a special kind of pavement cracking in which a fracture that begins in the subgrade or base course eventually reaches the pavement's surface [6].Reflection cracking is seen in Figure 2 here in a simplified schematic.
Figure 2: Propagation of reflective crack from the existing concrete pavement into the new asphalt overlay [7].
To reduce or postpone reflective cracking, several experiments have been done.Many approaches have been tried and assessed, from raising the HMA overlay thickness to using crack-arresting interlayers to utilizing a three-ply composite applied only over the joint/crack location.Although some of these methods have effectively reduced reflective cracking in specific contexts, many still need to be studied, especially in colder climates.
Different methods are concentrated to gain a better solution.One or more of the following procedures are categorized by [8] as below: • Treatment technologies of the surface layer HMA overlays.
• Different rigid pavement rehabilitation techniques.
• Saw and Seal.
• Treatment technologies by the interlayer system.

AVAILABLE LABORATORY TEST SETUP TO SIMULATE REFLECTION CRACKING
Different types of reflective cracking devices are used in the lab, and these categories are based on the effects simulated by the test setups.Some of it includes tools that attempt to simulate the impact of seasonal or daily temperature changes.These apparatuses are uniaxial tension test devices, with the principal components being a fixed portion and a moveable section over which the beam or slab specimens are attached.The fractures open and close because of the horizontal cyclic tensile tension produced by the mobile portion.Overlay testers from the Texas Transportation Institute (TTI) and the Iraqi Overlay Testing Equipment (OTE) [6, 9, and 10].Other devices are used just for traffic load simulation.Devices that apply a wheel load moving back over the specimen, like the Hamburg wheel tracking tester [11], and devices that apply only a vertical point load in pre-determined locations, static and cyclic, on the specimens, like the one used to test geogrid systems at Florida Atlantic University [12].

Texas Transportation Institute (TTI) Overlay Tester
Lytton and his colleagues developed The TTI overlay tester in the late 1970s [13] to simulate the opening and closing of joints or cracks, the primary driving factors for reflection crack initiation and propagation.As shown in Figure 3, the composition of the equipment consists of two steel plates, one of which is permanent and the other of which is moveable horizontally to simulate the opening and closing of joints or cracks in the old pavements under an overlay.

The Designed Overlay Testing Machine
The overlay testing machine aims to create a useful tool that engineers can use to measure the resistance of the asphalt mixture to reflective cracks.The overlay testing machine manufactured at the consulting engineering bureau/ University of Baghdad is shown as a schematic diagram in Figures 4 and 5. Sensors, DAQ measuring hardware, and a computer with programmable software are the three main components of a data acquisition system.The lab's device to simulate reflect crack consists of an air compressor that pumps air that passes through an air dryer and air lubricator.After that, the air passes through a flexible tube to the regulator valve and the seined valve with a vent attached to the timing unit.Then, two tubes pass from the valve to the loading jack, providing a loading rate of 7.8 mm/min.The loading jack is placed over the testing table with two plats; one is fixed, and the other is movable.The testing table also includes a load cell and LVDT (Linear Variable Differential Transformer) attached to the two plates, each on a side.LVDT and load cell are then connected with electrode wire to Data acquisition (DAQ) and then to the computer to display the result, including time, displacement, and force, and saved as an Excel file.

MATERIALS PROPERTIES
Aggregates and common local asphalt binders have been selected for making laboratory specimens to reach the most truthful simulation of HMA mixtures in Iraq.The Asphalt Concert (AC) used in this study to represent the overlay layer is made of coarse aggregate, fine aggregate, filler, and asphalt cement.

Aggregate
This study employed one kind of virgin aggregate, one Nominated Maximal Aggregate Size (NMAS), and a dense graded mixture because of limited resources and time.Al-Nibaee quarry provided crushed aggregates and natural sand.According to the criteria of the Iraqi State Corporation for Roads and Bridges (SCRB)R9/2003 for wearing course type IIIA, the aggregate was sieved, separated, and graded in the laboratory to match the specified gradation for surface course type III A (12.5NMAS).In addition, a single filler was chosen from the local market.The physical properties of aggregates are shown in Table 1.

Asphalt Cement
According to previous investigations, the asphalt employed in this research comes from the Dourah refinery and has a penetration grade of 40-50.Table 3 displays the asphalt cement's physical characteristics.All analyses were conducted in the labs of Baghdad University under SCRB guidelines.

OVERLAY TESTING EQUIPMENT RESULTS
Test results are displayed using a laptop, as shown in Figure 6.Analog input channel 1 in Figure 7a shows the relationship between the number of cycles and load in triangular waves that gradually decrease.Firstly, the highest load with the number of cycles was recorded.The point at which the crack begins to spread was recorded, at which the load is low, where 93% of the highest load was previously recorded with the number of cycles at that point.Finally, the load the sample failed was recorded, representing 7% of the highest load with the number of courses.In Figure 7b, analog input channel 2 shows the relationship between the number of cycles and displacements, which are also in the form of triangular waves but are at the same level.

Effect of Thickness of Asphalt Concrete Beams
Based on local observations, an empirical relationship was discovered between the thickness of HMA and the reduction of reflected cracking.Asphalt Concrete (AC) slab specimens were prepared, which have dimensions (37.5*7.61cm) in three thicknesses (4, 5, and 6 cm).These samples were tested under three different temperatures (20, 30, and 40°C).From the typical data obtained from the OT test, the load acquired for each cycle is plotted.The first cycle provides the maximum load where the initial damage occurs.The remaining processes represent the crack propagation phase until the failure limit of 93% of the maximum load is reached.It can be seen in Figure 8 for the sample thickness of 4cm that the number of crack initiation and failure cycles increased as the temperature increased.In contrast, the increased temperature decreased crack initiation and failure load.This sample can be considered the reference.The number of crack initiation and failure cycles increased as the temperature increased.In contrast, the maximum load needed for crack initiation decreased with the increased temperature for the sample has a thickness of 5 cm.This result is similar to the specimen with 4 cm thickness, but the initial load needed for crack initiation and failure and the number of cycles are higher values shown in Figure 9.The effect of temperature in the sample has a thickness of 6 cm, similar to the previous two examples.However, from the results shown in Figure 10, the effectiveness of the thickness is clear; when the overlay thickness increased, the load and number of cycles up to failure increased.

CONCLUSIONS
When applying an overlay on the damaged pavement, reflection cracking is a serious issue.Reflection cracking is caused by daily temperature cycles of opening and reopening joints and fractures.This process is being recreated in the lab at the University of Baghdad using a modified overlay tester.Following extensive overlay testing and application investigations, the following conclusions and recommendations are made: The number of crack initiation and failure cycles increased as the temperature increased, while the maximum load of crack initiation decreased with the increased temperature.This means that the effect of stiffness on ACO performance is critical in determining how soon the reflection cracks will develop.

Figure 4 :
Figure 4: Schematic diagram of the design test machine.The components of the overlay testing machine:• Air compressor: A pneumatic device, an air compressor transforms mechanical energy into the potential energy of compressed air.When the air in the tank is let out, the pressure drops and the kinetic energy it contains may be used in several ways, including using a pneumatic tool.• Air dryer: Compressed air systems often have issues with moisture condensation.It shows itself throughout the distribution network, blocking filters and machinery and causing failures.Damage to air-based machinery from water vapor condensation is possible.A pneumatic system may benefit from an air drier, eliminating the need for moisture to collect in the air.The air dryer's primary objective is to remove the surrounding atmosphere.• Air lubricator: In order to keep pneumatic tools and other equipment running smoothly, a pneumatic lubricator injects an aerosolized stream of oil into an airline.• Regulator valve: A pipeline medium of any kind requires precise pressure control.Sustaining, lowering, and relieving pressure valves are all part of a system of pressure-regulating valves that ensures a stable working environment and automatically adjusts to any changes in pressure.Pressure-sensing valves monitor pressure and adjust the flow of air to either indicate or control a process pneumatically.• An LVDT (Linear Variable Differential Transformer) may be considered an electromechanical passive inductive transducer.An LVDT is a composite electronic device that stores electrical radiation and converts it into a readable signal to represent linear displacement or the movement of an object along a single axis.An external source powers it and has both electrical and mechanical operations.One of the best and most accurate ways to measure linear distances is using an LVDT Sensor.• A load cell is a device that measures and transmits pressure.It measures and standardizes mechanical forces by transforming them into electrical signals.The load cell's electrical signal varies directly with the applied force.A load cell has a metal body to which strain gauges have been secured.The voltage read by the strain gauges represents the tension or compression force.When the load delivered to the cell causes a voltage change, the load cell's output may be used to determine the pressure applied.• Data acquisition (DAQ): It is the technique of determining the value of a physical or electrical quantity.Sensors, DAQ measuring hardware, and a computer with programmable software are the three main components of a data acquisition system.The lab's device to simulate reflect crack consists of an air compressor that pumps air that passes through an air dryer and air lubricator.After that, the air passes through a flexible tube to the regulator valve and the seined valve with a vent attached to the timing unit.Then, two tubes pass from the valve to the loading jack, providing a loading rate of 7.8 mm/min.The loading jack is placed over the testing table with two plats; one is fixed, and the other is movable.The testing table also includes a load cell and LVDT (Linear Variable Differential Transformer) attached to the two plates, each on a side.LVDT and load cell are then connected

Figure 6 :
Figure 6: Test results shown on the computer.

Figure 7 :
Figure 7: (a) The relationship between load and number of cycles, (b) the relationship between displacement and number of cycles.

Figure 9 :
Figure 9: Results of specimen height 5cm at different temperatures produced by (40-50) asphalt grade, tested by overlay testing machine.

Figure 10 :
Figure 10: Results of specimen height 6cm at different temperatures Produced by (40-50) asphalt grade, tested by overlay testing machine.

Table 1 :
Physical properties of used aggregates.

Table 2 :
Basic properties of Portland cement.

Table 3 :
Physical properties of Asphalt cement.

Table 4 :
The characteristics (stability, flow, % Va, % VMA, and %V.F.A) of the conventional mixture at the optimum asphalt binder content.