Material testing

Corrosion assessment and prevention related to Engineered Cementitious Composites

The durability of reinforced concrete structures is significantly affected by the corrosion of rebars, which can induce cracking and spalling of the concrete cover. Once the rebars are exposed to external environment, the corrosion process is accelerated which leads to rapid cross section loss of the steel. To recover the original safety factor, structural repairs must be performed. Up to date the most common repair method is to splice an additional piece of reinforcement to the corroded rebars to compensate for the area reduction, however this is a time consuming and costly process.

On that sense Engineered Cementitious Composites (ECC) appears to be an attractive option for such repair applications. This kind of materials present high ductility and the formation of closely spaced multiple cracks with opening only about 100 μm until the ultimate tensile strength is reached. This means high deformation capacity, high energy absorption and excellent durability (by controlling the crack width to limit water/chemical penetration). However, as a still new material much is still to be discovered to understand how those materials can limit the corrosion process or fix it for structures already heavily corroded.

Therefore, on my research interests I target to understand by which means the ductility and associated cracking behavior of ECC’s can affect the corrosion process. Moreover, the aim is also to evaluate the effectiveness of using this material as repair methods, being a key challenge to understand the joint between old and new concrete. Not only mechanical tests are part of the investigation, but also understanding the chloride transport and binding mechanisms and microscopical observations of rust formation and crack propagation.

Experimental evaluation of self-healing and crack management of engineered cementitious composites (ECCs)

The longevity of reinforced concrete structures under service conditions depends largely on their ability to withstand environmental influences. In its uncracked state, concrete generally provides good protection for the embedded rebar against the ingress of corrosion-inducing media such as chlorides. Cracks in concrete structures are, however, unavoidable and create a primary path for such media to reach the steel and initiate corrosion. Corrosion initiation as well as corrosion progression depend on several factors such as crack width and concrete cover, for which current construction codes have explicit provisions to provide the desired level of durability. Nonetheless, corrosion remains a leading cause for the degradation of structures resulting in a premature end of their service life. With the continuing innovation in the field of cementitious materials comes the necessity to consider other factors that influence durability such as crack tortuosity, crack spacing, and the ability of cracks to self-heal, which are not considered in current codes.

The focus of this research project lies on engineered cementitious composites (ECCs) with PP and PVA fibers. ECCs have a much larger binder content than regular concrete mixes, have no coarse aggregates, and contain a considerable amount of fibers (around 2 Vol-%). This leads to unique mechanical properties such as a strain hardening response under load and a much higher total strain capacity than regular concrete. The inclusion of fibers in ECCs also promotes a distributed cracking pattern with many small and very tortuous cracks. In addition to that, ECCs have the ability to self-heal these cracks after they form. This is largely due to the low water/binder ratio that leaves a significant amount of unhydrated binder in the matrix. Once a crack appears, the binder reacts with water that is absorbed through the crack to form new reaction products, effectively closing the crack.

The goal of this project is to show the potential engineered cementitious composites (ECCs) have in terms of extending the service life of a structure. ECCs exhibit not only high matrix density in the uncracked state, but also significant self-healing capacity after cracks appear, providing high resistance against the permeation of corrosion-inducing media after cracking. These properties make ECCs highly suitable for structures exposed to aggressive environments as well as retrofitting damaged structures. In newly built structures, it can allow for a significant reduction of the concrete cover while maintaining a high level of durability. When used as a retrofitting material, it can re-establish a strong barrier against corrosive media without adding a lot of self-weight.

The research will focus on evaluating the self-healing behaviour on a microscopic level using X-ray µCT to analyze cracking and self-healing under different conditions. Additionally, the influence of reducing cement content in order to reduce the carbon intensity of the material is to be quantified. The goal is to thoroughly assess the ability of PP and PVA-fiber reinforced ECCs to re-establish the durability properties after cracking.