چكيده به لاتين
The stone column is considered a common and low-cost technique in soil improvement. In this technique, aggregates are compacted within the soil profile using several methods and formed as a column into the soil, leading to the enhanced load-bearing capacity and the reduced settlement. The natural aggregates are usually used to construct these columns. Also, cemented stone columns (rigid columns) are used in sites that the confinement pressure of soil is not sufficient. The cementing agents used in cemented stone columns are usually Portland cement and or lime. This research has considered the possibility of using the microbially induced calcium carbonate precipitation (as cementing agent) for the construction of cemented stone column using recycled aggregates; but because of Interdisciplinary nature, this object has been considered in four researching steps. The first step is the determination of the type of applied microorganism and its performance because the soil improvement projects are performed in various geographical locations and different environmental conditions and the use of a non-native (or non-indigenous) microorganism may change the normal flora of the soil. So, in the first step, the possibility of using an indigenous microorganism in fine aggregate cementation and also its survival time after cementation have been considered. Indeed, the survival and mechanical resistance induced in the soil using indigenous microorganisms in comparison to common microorganisms applied in other researches have been assayed. The results indicate the indigenous microorganism has a good ability in soil improvement (the created resistance was about 2.5 Mega Pascal (MPa) in fine aggregate) and less survival after cementation (the indigenous microorganism survived seven days according to the survival test). The second step was the investigation of mechanical properties and durability of the natural aggregates and recycled concrete through microbially induced precipitation. For this purpose, unconfirmed compressive strength, thermal resistance, water absorbance, and the freeze-thaw cycle of cemented aggregates constructed in the shape of cubes with dimensions 10×10×10 cm, have been assayed. The results of compressive strength of these aggregates (cemented recycled concrete aggregates through microbially induced precipitation) indicate that constructed aggregates are comparable with some aggregates in the building industry and their maximum unconfirmed compressive strength reached 9.33 MPa. Elastic modulus (E) of the stone column is one of the important parameters in design (settlement calculation) and cemented stone columns (rigid columns) are usually constructed with Portland cement and because to preserve the permeability of these columns, the permeable concretes are recently used instead of stone columns, in the third step, the effective and total porosity, unconfirmed compressive strength and Elastic modulus of porous cemented aggregates (by the construction of permeable concrete in three levels of porosity:40, 42 and 45%) have been assayed using Portland cement and microbially induced calcium carbonate precipitation. The results of compressive strength show that in the range of particle sizes between 2.7 and 4.75 mm, 113 Kg microbial precipitation and or 250 Kg Portland cement in m3 is required to achieve the porous concrete with a porosity of 42% and compressive strength of about 2.3 MPa. The results of Elastic modulus (E) investigations show that cemented aggregates through microbial precipitation possess the bigger Elastic modulus than cemented aggregates through Portland cement and microbial precipitation and have brittle behavior. Finally, according to this fact that the considerable part of mechanical properties of cemented aggregates is related to the properties of the microstructure of cementing agent and its connection zone to aggregates, in the fourth step, the microstructure of Portland cement paste, pure microbial precipitation, properties of the transition zone of aggregates and the cementing agent have been assayed by applying nano-scale indentation test used Berkovich tip. The results of these tests show that unlike Portland cement, microbial precipitation doesn't have different constituent phases, and the elastic modulus of microbial precipitation (69 GPa), on average, is twice as big as the elastic modulus of the hydrated calcium silicate phase of Portland cement paste (34 GPa). On the other hand, the obtained results demonstrate that in the cementation through microbial precipitation, there is no transition zone with such a nature defined for Portland cement.
