Carbonation of Functionally Graded Concrete
In this thesis, the carbonation of functionally graded concrete is investigated through a combination of experimental and analytical approaches. The properties of functionally graded concrete vary throughout the cross-section to satisfy performance requirements in a resource-efficient manner. In this research, functional grading of concrete elements is achieved by casting discrete layers of concretes possessing different properties. Layers are cast in the fresh state to prevent formation of a cold joint and associated interface weakness.
Carbonation is the process of environmental CO2 diffusing into and reacting with concrete, thereby changing its chemical composition. It is a durability concern for concrete structures, since carbonated concrete does not provide adequate corrosion protection to embedded reinforcement. Increased resistance to carbonation has traditionally been achieved by increasing the cement content in concrete elements, which increases embodied carbon. Carbonation presents as a surface phenomenon. As such, the concrete mix adopted at the outside face is of most concern when designing carbonation-durable elements. Given the ability to relate properties to performance, functional grading presents a promising approach to providing carbonation durability with reduced cement content. In such an element, a high-cement carbonation durability layer could be placed in exposed areas, but retaining low-cement concrete as the material in the bulk remainder. This would have theoretically equivalent durability to a single-mix element cast from the high-cement mix, but significantly lower cement content.
The presented research seeks to answer the question “Is layered functional grading a viable method of reducing cement contents in concrete elements without sacrificing carbonation durability?''. This is achieved by first understanding the carbonation behaviour of single-mix concrete specimens, thus establishing baseline processes for accelerated carbonation testing and the influence of material parameters on carbonation performance of different concrete mixes. Next, machine learning is applied to predict properties of concretes from mix composition, such as carbonation performance. Following this, the carbonation of layered functionally graded specimens is experimentally investigated, and the results are used to validate a derived model for the carbonation depth in layers. Finally, it is acknowledged that structural cracking could provide a route for CO2 penetration beyond the depth of a carbonation durability layer. Therefore, a further experimental series investigates the cracking behaviour of layered prisms under loading and their carbonation performance when subjected to simultaneous cracking and carbonation. Digital image processing methods are implemented in multiple areas to improve the detection and analysis of carbonation fronts from phenolphthalein indication tests.
Overall, the results demonstrate that layered functionally graded concrete elements offer a viable route to cement reduction, without loss of carbonation durability. Therefore, it is recommended that practitioners consider this approach to design durable structures with reduced cement consumption.