Crack healing in concrete using various bio influenced self-healing techniques Wasim Khaliq ⇑ , Muhammad Basit Ehsan National University of Sciences and Technology (NUST), Islamabad, Pakistan highlights  Experimental study was carried out in order to find optimum self-healing technique.  Concrete crack healing was observed for various bacteria incorporation techniques.  Graphite nanoplatelets emerged as good carrier compound for short period healing.  Light weight aggregate depicted as good carrier compound for long period healing.  Light weight aggregate incorporation improved compressive strength of concrete. article info Article history: Received 1 June 2015 Received in revised form 18 October 2015 Accepted 2 November 2015 Available online 13 November 2015 Keywords: Concrete Self-healing Cracks Bacteria Graphite nano platelets Calcium lactate Compressive strength abstract Crack formation and progression under tensile stress is a major weakness of concrete. These cracks also make concrete vulnerable to deleterious environment due to ingress of harmful compounds. Crack heal- ing in concrete can be helpful in mitigation of development and propagation of cracks in concrete. This paper presents the process of crack healing phenomenon in concrete by microbial activity of bacteria, Bacillus subtilis. Bacteria were introduced in concrete by direct incorporation, and thorough various carrier compounds namely light weight aggregate and graphite nano platelets. In all the techniques, calcium lac- tate was used as an organic precursor. Specimens were made for each mix to quantify crack healing and to compare changes in compressive strength of concrete. Results showed that bacteria immobilized in graphite nano platelets gave better results in specimens pre-cracked at 3 and 7 days while bacteria immobilized in light weight aggregates were more effective in samples pre-cracked at 14 and 28 days. In addition, concrete incorporated with bacteria immobilized in light weight aggregate, also exhibited significant enhancement in compressive strength of concrete. Ó 2015 Elsevier Ltd. All rights reserved. 1. Introduction Concrete is most widely used engineering material in construc- tion due to its strength, durability and low cost as compared to other construction materials. The major drawback of concrete is its low tensile strength which makes it susceptible to progression and coalescence in microcracks resulting in low strength and dura- bility. These tensile stresses can be due to tensile loading, plastic shrinkage and expansive chemical reactions [1]. This liability to cracking not only results in strength reduction of concrete, but also makes concrete vulnerable to deleterious environment. Entry of harmful chemicals through these cracks may result in concrete deterioration through chemical attack and can also cause corrosion of steel reinforcement [2]. This corrosion leads to increase in crack damage resulting in loss of strength and stiffness of concrete struc- tures [3]. This deterioration in reinforced concrete for both con- crete and reinforcement results in high maintenance cost. According to report of Federal Highway Administration [4], United States of America spends 4 billion dollars annually in terms of direct cost of maintenance of concrete highway bridges. De Rooij, Van Tittelboom [5] stated that UK spends 45% of its annual con- struction cost on maintenance of existing concrete structures. With the capability of self-healing in concrete, the formation and prop- agation of cracks can be reduced and a concrete with dense microstructure can be obtained. As a result, more durable struc- tural concrete, with reduced maintenance cost can be produced. Different strategies are used to retard crack propagation and bridge cracks leading to increased durability of concrete. However, most of the strategies, such as epoxy systems, acrylic resins and sil- http://dx.doi.org/10.1016/j.conbuildmat.2015.11.006 0950-0618/Ó 2015 Elsevier Ltd. All rights reserved. ⇑ Corresponding author at: Room 103, NICE, SCEE, NUST Campus, Sector H-12, Islamabad 44000, Pakistan. E-mail addresses: wasimkhaliq@nice.nust.edu.pk (W. Khaliq), basit.ehsan@ya- hoo.com (M.B. Ehsan). Construction and Building Materials 102 (2016) 349–357 Contents lists available at ScienceDirect Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat