Porous Network Concrete

Abstract

The high energy consumption, its corresponding emission of CO2 and financial losses due to premature failure are the pressing sustainability issues which must be tackled by the concrete infrastructure industry. Enhancement of concrete materials and durability of structures (designing new infrastructures for longer service life) is one solution to overcome the dilemma. Concrete is a quasi-brittle material with properties that are high in compression but weak in tension, therefor concrete is prone to cracking. In the case that a continuous network of cracks is formed, the permeability of concrete will increase and the reinforcement bars may be open to the ambient atmosphere. This opening provides easy means for aggressive substances to enter into concrete and reach rebars which may start to corrode. Further cracks may threaten the tightness of the retaining structures, e.g. liquid containing structures tank wall, aqueducts, underground spaces, tunnels, etc., which undergo tensile forces. In these cases cracks may facilitate the flow of fluid – liquid or gas – into and out of the structures which considerably alters its serviceability, leads to unhealthy environments within a structure, and diminishes its functionality. In case the container or reservoir contains waste, highly toxic materials or radioactive materials, leakage through the concrete is catastrophic and unacceptable. One promising concept to design new concrete structures to achieve higher durability is incorporating self-healing mechanisms that are found in nature into the cement-based materials or the concrete structural element. If unavoidable cracks due to inherent brittleness in concrete could be self-sealed/healed/repaired, concrete will certainly serve longer and be more durable and sustainable. In general, on attempting to solve engineering problems, one can (always) seek inspiration from biology (nature). Though, borrowing nature’s idea to enhance our living environment is as old as humankind, the post-industrial technical advent makes the process more systematic and deliberate, hence makes use of bio-mimicry to solve problems and inspire innovation. Observing the domain of biology, there are several wound healing mechanisms found in nature: cut skin and bone fracture healing in human and animal, and plant response to injury. The present work takes inspiration from studies on bones of present-day mammals and birds and its healing mechanism. Two of appropriate principles that might be constructive are identified; (1) bone morphology comprises of cortical (solid) bone and trabecular (spongious) bone and (2) a feedback loop process is present in the remodelling and healing process. These two principles formed the basis for the development of a healable concrete material and for a method for healing it with healing agents. The idea behind this is that cortical bone may be mimicked with solid concrete and trabecular bone may be imitated by porous concrete. The combination of the two types of concrete resembles Porous Network Concrete, a bone-like concrete able to self-heal by the mechanism of feedback loop. These are the points addressed in the chapter 1 which explores the success story of concrete in serving society and civilization for millennia, the present challenge to make modern concrete more durable, and the bio-inspired solution of self-healing concrete Porous Network Concrete (PNC) is a hybrid system in which high permeability porous concrete is embedded in the interior or exterior of normal dense concrete. The porous network core constitutes alternate means for [1] channelling temporary or permanent materials to form a dense layer in the later stage and [2] distributing healing agent from the point of injection to cracks in the concrete main body. In chapter 2 the concept of the PNC is elaborated by setting up criteria and realized by creating a fabrication procedure. The production process – the making of the PNC – follows the current standard for both of the main and porous part and seemingly there is no complicated fabrication procedure. PNC characterization was carried out to study its pore and mechanical properties. The autonomous healing mechanism in the PNC is designed by incorporating the feedback mechanism; once a certain crack width is sensed, an action to heal takes place. As a proof-of-concept, in chapter 3, a simple and intuitive approach to design a feedback system for PNC self-healing mechanism has been carried out. When a concrete structure receives loads and builds up internal stress, it deflects, cracks and deforms. Once the crack mouth opening reaches a certain prescribed value the healing agent is injected automatically. The proposed working principle is verified by mechanical and leakage (permeability, infiltration) testing. The solidification process of the self-healing agent is important and even critical for the success of the healing strategy and mechanism designed. Instead of developing new healing agent and investigating its behaviour, this present study aims to examine the effectiveness and efficiency of the healing process in the Porous Network Concrete with different classes of agents. Three groups of healing agents are then studied and its healing efficiency is tested by leakage and mechanical testing. The first type of agent is a single- and double-component chemical based which mostly works through poly-condensation or cross-link polymerization upon contact with the atmosphere, with the concrete matrix or within the reactants. In this case, epoxy resin is used. The second healing agent used is grout material made of a cementitious powder mix. Cementitious grout material can be thought of as healing agent for concrete structures since it functions as crack sealant and void filler with the objective to restore structural integrity. The use and healing performance of PNC by both healing agents is discussed in chapter 4. The third agent is bacteria-based repair solution. It contains alkaliphilic bacteria able to facilitate bio-mineralization, nutrients and transport solution. It was originally developed as a bio-based repair material for cracked concrete. This is discussed in the dedicated chapter 5. It has been demonstrated in this thesis that the Porous Network Concrete has a good prospect in making concrete structural elements self-healing. This is the concluding point presented in the final chapter. Some recommendations for improving the work are presented such as; modelling work, larger and realistic experimental campaign and improved damage sensing.