No abstract

Recently there is a strong trend in engineering researches in the field of engineering materials for the production of high and ultra-high strength concrete. Using different additives which are added to the concrete mix reduced the water cement ratio and caused an increase in the concrete strength. As well as the use of ultra-fine materials such as Nano-silica increase the strength of concrete dramatically. In spite of the success of this research to get to compressive strength up to 150 MPa or more, however, many problems have found in the mechanical behavior of these concretes, which turned into a decrease in the fracture toughness significantly. An experimental program was designed to investigate the effect of cement content, silica fume and Nano silica percent, and curing on the compressive strength of concrete at different curing ages. The results of the present work clearly showed an increase in the value of 28-day concrete compressive strength with existence of silica and Nano silica. Steam curing, also, led to an improvement in compressive strength as compared to water curing.

The creation of cracks, which are the most common cause of structural failure, has a significant impact on the structure's strength and durability. As a result, effective repair and maintenance are vital and unavoidable for treating any of these issues. Self-healing mortar holds promising benefits for reducing the cost of repair as cracks are autonomously repaired without any human intervention. This study investigated the effect of bacteria type, bacteria content, bacteria concentration, and nutrient type on the properties of the self-healing mortar. Three types of bacteria, Bacillus sphaericus, Bacillus Megaterium, and Bacillus subtilis encapsulated in calcium alginate beads, were introduced into the mortar. Two concentrations of bacteria, 2× 108 and   2× 109 Colony Forming Units per milliliter, and different percentages of bacteria of cement weight were selected for the study. In addition, calcium lactate and calcium acetate were used at 0.5% of cement weight as nutrition for bacteria. Tests were performed for compressive strength, bending strength, SEM, EDX, and TGA/DTG. The results show a significant development in the mechanical behaviour of mortar, especially with Bacillus Megaterium using a 2.5% bacterial proportion with a concentration 2× 109 CFU/ml. This can be related to the filling of voids and cracks in microbial mortar by calcite, which was confirmed by SEM and EDX.