Theoretical Investigation of the Load Transfer across Slab Joints in Rigid Aircraft Pavement Structures

Sean Jamieson

doi:10.25907/00987

Rigid aircraft pavements generally comprise unreinforced concrete slabs constructed on a bound or granular sub-base layer, over the natural or constructed subgrade or fill. Important to any rigid aircraft pavement design and construction are the joints between the concrete slabs. The joints control shrinkage cracks during curing, allow for thermal expansion and contraction during daily and seasonal temperature cycles, they isolate concrete slabs from structural penetrations, and important to this research, is that they provide load transfer between adjacent slabs. Load transfer is the ability for a joint or crack in a concrete pavement to transfer load from one slab to the next when trafficked by an aircraft landing gear. Load transfer relies on joint stiffness, which contains three physical contributors; stiffness from point devices, such as dowels, stiffness from aggregate interlock, and apparent stiffness caused by elastic solid base effects. Because edge stresses can be the critical stress for pavement thickness determination, effective load transfer reduces the required slab thickness, where it is generally assumed that joints provide 25% load transfer to adjacent slabs. However, load transfer is variable, and affected by many factors, including loading regime, sublayer support, joint opening, aggregate type, maximum aggregate size, and joint details. Consequently, any study investigating load transfer should consider these variables. Construction joints are the joints that connect paving lanes, and generally rely on round steel dowels for load transfer. The use of round steel dowels has been successful for many years, but the placement of the dowels can be cumbersome due to strict tolerances, and any misalignment can lock the pavement which in turn will lead to concrete cracking. Alternate jointing solutions exist in other pavement applications, including plate dowels and sinusoidal keyed joints. These alternate solutions simplify constructability and should be investigated against aircraft loads to determine if they satisfy minimum load transfer requirements. Contraction joints are situated within paving lanes, and generally rely on aggregate interlock for load transfer, making them susceptible to significant load transfer losses when large joint openings are present. This has resulted in some designers specifying all contraction joints to be dowelled, complicating the constructability of the pavement. Furthermore, aggregate interlock joint stiffness is typically modelled as a spring constant, which may oversimplify the complex interaction between the two slabs connecting the joint. Consequently, there is opportunity for jointing practices and joint modelling to be improved. This research aimed to investigate load transfer across slab joints in rigid aircraft pavement structures, to determine if current jointing practice can be improved. This was achieved through finite element methods, by investigating round dowelled construction joints, diamond-shaped plate (DSP) dowelled construction joints, sinusoidal construction joints and aggregate interlock contraction joints. Each joint was assessed against a range of load-related, sublayer-related and joint-related factors through several parametric studies. Using statistical methods, the statistically significant factors for each joint were determined and regression equations produced to predict load transfer outputs per joint type. This allowed for comparison of innovative joint types to typical joints used in current aircraft pavement practice. It was concluded that for all joint types, weaker subgrades and complex landing gears generally provide better load transfer than stiff subgrades and simple landing gears. That is because weaker subgrades and complex landing gears increase slab deflection, allowing for more joint stiffness to be engaged either through dowels, sinusoidal formed faces or aggregate interlock. Sub-base type was found not to be statistically significant for dowelled construction joint load transfer, but was significant for sinusoidal construction joints and aggregate interlock contraction joints, with unbound sub-bases providing better load transfer. Dowel looseness was found to be the most significant factor for round dowelled and DSP dowelled construction joints, and joint opening the most significant factor for sinusoidal construction joints and aggregate interlock contraction joints. Consequently, design and specification should focus on minimising the effects of these variables on load transfer. It was also concluded that DSP dowelled construction joints perform similarly to round dowelled construction joints, but only if thick plate dowels are used to minimise the risk of steel yielding. Additionally, sinusoidal construction joints can achieve aircraft pavement load transfer requirements, even with a large joint opening. However, this is dependent on loading condition, sublayer support condition and sinusoidal wave shape. It was determined that the optimal sinusoidal wave shape for load transfer is one that minimises wave quantity and wavelength, and maximises wave amplitude. Finally, relationships were developed that better relate field load transfer characterisation values to design load transfer characterisation values, which enables better field determination of joint serviceability. The findings from this research can inform designers and specifiers in optimising joints systems for rigid aircraft pavements. Specifically, industry is recommended to include the innovative jointing solutions in any future design guidance. Furthermore, it is recommended that industry employ the load transfer characterisation relationships to better determine joint serviceability from in-field measurements. Finally, future research is recommended that includes long-term field assessment of the innovative joint types investigated.

Theoretical Investigation of the Load Transfer across Slab Joints in Rigid Aircraft Pavement Structures

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