Abstract
Bacteria-based self-healing concrete is an innovative concrete that contains a self-healing agent that provides the material with enhanced autonomous crack-sealing performance. A specific type of this concrete, based on a healing agent composed of bacterial spores and lactate as carbon source, has been developed and applied by the Delft University of Technology for over ten years. Under laboratory conditions it was proven that, depending on the dosage of healing agent, self-healing of cracks up to 0.8 mm widths occurs. As such the material potentially allows reduction of steel reinforcement used for crack width limitation in watertight constructions. Application of self-healing concrete would therefore not only result in a reduction of costs but also in improvement of environmental performance (lower CO2 footprint) and ease of in situ casting due to reduction of use of steel in waterproof applications. However, according to the EN 1990 Eurocode (Basis of structural design), customary application of a novel type of concrete must be preceded by full scale demonstrators proving evidence for safe and functional performance. In this contribution we portray full scale application of bacteria-based self-healing agent as developed by the Delft research group in two repair mortar- and in two concrete construction demonstrator projects. These demonstrator projects show that addition of the bacteria-based self-healing agent to the concrete mix is safe as no negative side effects on construction performance was observed. However, it also proved difficult to find evidence for increased crack-healing performance as cracking in the demonstrator constructions hardly occurred. In further full scale demonstrators we therefore plan to drastically reduce amount of crack width-restraining reinforcement to show crack-healing capacity and potential to save on use of reinforcement steel in watertight concrete constructions.
Highlights
Cracking in concrete is an accepted phenomenon and does not have to lead to problems if it remains 'within limits'
Eurocode EN 1992‐1‐1 Design of concrete structures' recommends maximum crack widths of 0.4 mm for concrete structures exposed to environments classified as X0 and XC1 and 0.2 mm for more aggressive environments (XC2‐4: corrosion induced by carbonation; XD1‐3: corrosion induced by chlorides and XS1‐3: corrosion induced by chlorides from seawater) [1]
In the last two decades many studies targeting the development of concrete with enhanced self‐healing properties appeared in literature which are mostly covered by recent review papers [7,8,9,10,11,12, 22,23, 28,29]
Summary
Cracking in concrete is an accepted phenomenon and does not have to lead to problems if it remains 'within limits'. First full scale application of the bacteria‐based self‐healing agent as developed by the Delft research group was in form of a self‐healing repair mortar (see paragraph 6.1 below) This healing agent‐ and PVA fibre‐based mortar, developed by Sierra‐Beltran et al, is characterized by a low modulus of elasticity and additional strain‐hardening behaviour [25, 26]. Applying PVA fibres in combination with the bacteria‐based self‐healing agent ensures quick autonomous healing of cracks formed under tensile‐ or flexural loading and high bond strength between repair mortar and underlying concrete. Watertight constructions are commonly designed in such a way that only cracks of smaller than 0.1 mm width are allowed to occur as these are expected to self‐heal due to autogenous healing capacity of the concrete [3] Such designs, require a lot of crack width restraining reinforcement in addition to structural reinforcement steel. Water flow was temporarily stopped through application of a 5‐mm thick layer of fast setting cement, immediately followed by manual application of the repair mortar by filling up the crack cavities to level concrete surface (Fig. 6)
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