Sustainable Design of 'Green' Concrete Overlays - Shear Failure at Cracks and Inadequate Resistance to Reflection Cracking

摘要

Concrete overlays have been used as a rehabilitation technique since 1913 - the first practice of using Portland cement concrete to resurface existing pavements, took place in United States. Today, the need for durable and environmentally-friendly construction technologies with low life-cycle costs has brought the world vision to bonded concrete overlays (BCO), which is recognized as a sustainable and effective solution by making maximum use of the existing, structurally damaged concrete pavement. Most research and investigations on BCOs has been carried out in several American states but generally with normal ordinary Portland cement concrete. There is no method readily available for a fast and sound bonded repair of damaged concrete pavements which both fully utilises the potential of the worn concrete pavement and enhances the mechanical properties to give a performance which should equal or even surpass the original.A Pavement Repair Management System (PRMS) has been developed at Coventry University offering a pioneering approach to concrete road rehabilitation. It aims to enhance the structural and functional deficiencies and extend the life of the pavement and at the same time introduce substantial benefits, such as savings in construction material, time, labour and costs, by bonding a layer of high strength concrete on top of the existing, damaged pavement. The utilization of the residual structural potential of the existing worn pavement makes it more sustainable in both environmental and economic terms, as an alternative to the wholesale demolition and reconstruction of the existing pavement.Shear failure at cracks/joints is a major cause of degradation of concrete pavements. Not only it leads to serviceability problems but also introduces reflective cracks and becomes an issue of structural integrity, durability, riding quality and safety for the users. The optimized overlaid material benefits from its high strength and stiffness. However, it raises some concerns regarding its shear performance due to the potential brittleness and relatively smooth fracture surface. This research aims to make a contribution in understanding the behaviour of a concrete pavement overlay under shear loading, and to control and evaluate reflective cracking due to shear by means of utilisation of steel fibres to provide adequate resistance to reflective cracking.IIIA new mix design approach was developed and described in Chapter 3 to produce mixes with high bond strength to the underlying pavement and to facilitate the rapid construction process. The shear capacity of the developed mix was scrutinized experimentally in Chapter 4 employing the purposely developed single notch shear beam test. Not only it enabled the evaluation of the material performance under predominant shear mode but also allowed for analysis on a progressive failure process (crack development). It was shown that the superior quality of the new overlay material achieved high early strength and provided an efficient resistance to reflective cracking. The developed mix design method, laboratory testing data and recorded performance are instructive for the industry, and for future development of pavement overlay design guidance.Last but not least, the progressive failure process was successfully simulated using finite element modelling techniques, as presented in Chapter 6. The cohesive zone model (CZM) was adopted in fracture simulations of the test to reflect the fibre bridging effect and aggregate interlock at the crack interface. Nomographs deduced from finite element analysis showed increasing overlay thickness can effectively reduce the susceptibility to shear failure and reflective cracking and minimise the differential displacement at underlying joints/ cracks. The multi-cracking feature of steel fibre reinforced concrete (SFRC) overlay can provide a safe buffering zone and an effective crack control