Unconfined compressive strength of MICP and EICP treated sands subjected to cycles of wetting-drying,freezing-thawing and elevated temperature:Experimental and EPR modelling

Microbial-induced carbonate precipitation(MICP)and enzyme-induced carbonate precipitation(EICP)are two bio-cementation techniques,which are relatively new methods of ground improvement.While both techniques share some similarities,they can exhibit different overall behaviours due to the differences in urease enzyme sources and treatment methods.This paper presented 40 unconfined compressive strength(UCS)tests of MICP and EICP treated sand specimens with similar average calcium carbonate(CaCO3)content subjected to cycles of wetting-drying(WD),freezing-thawing(FT)and elevated temperature(fire resistance test e FR and thermogravimetric analysis e TG).The average CaCO3 content after a certain number of WD or FT cycles(ACn)and their corresponding UCS(qn)reduced while the mass loss increased.The EICP treated sand specimens appeared to exhibit a lower resistance to WD and FT cycles than MICP treated specimens possibly due to the presence of unbonded or loosely bonded CaCO3 within the soil matrix,which was subsequently removed during the wetting(during WD)or thawing(during FT)process.FR test and TG analysis showed a significant loss of mass and reduction in CaCO3 content with increased temperatures,possibly due to the thermal decomposition of CaCO3.A complete deterioration of the MICP and EICP treated sand specimens was observed for temperatures above 600C.The observed behaviours are complex and theoretical understanding is far behind to develop a constitutive model to predict qn.Therefore,a multi-objective evolutionary genetic algorithm(GA)that deals with pseudo-polynomial structures,known as evolutionary polynomial regression(EPR),was used to seek three choices from millions of polynomial models.The best EPR model produced an excellent prediction of qn with a minimum sum of squares error(SSE)of 2.392,mean squared error(MSE)of 0.075,root mean square error(RMSE)of 0.273 and a maximum coefficient of determination of 0.939.

Microbial-inspired self-healing of concrete cracks by sodium silicate-coated recycled concrete aggregates served as bacterial carrier

Microbially induced carbonate precipitation(MICP)is a promising technique for the autonomous healing of concrete cracks.In this study,the effect of pH on MICP was investigated.The results indicate that the MICP process was inhibited when the pH was higher than 11.Both vaterite and calcite were produced when the pH was<8,whereas only calcite was produced when the pH was>8.Recycled concrete aggregates(RCA)coated with sodium silicate have been proposed as protective carriers for microbial healing agents.Although the presence of the coated RCA resulted in a loss of the splitting tension strength of the concrete,the loaded healing agents were highly efficient in self-healing cracks.Concrete incorporated with 20%RCA loaded with healing agents exhibited the best self-healing performance.When the initial crack widths were between 0.3 and 0.4 mm,the 7-d mean healing rate was approximately 90%.At 28 d,the crack area filling ratio was 86.4%,while its water tightness recovery ratio was 74.4%and 29.8%,respectively,for rapid and slow absorption.This study suggests that RCA coated with sodium silicate is an effective method for packaging microbial healing agents and has great potential for developing cost-effective self-healing concrete.

Temperature Drop Model Based on Discrete Element Method for Simulating Damage of Bio-Cemented Sand by Cold Wave

The microbially cemented sand(MCS)material is a new building material with a broad research prospect,although the nationwide cold wave affects the mechanical properties of the material in the practical application.The microstructure of MCS is obtained by computed tomography(CT)and scanning electron microscope(SEM);the thermodynamic mathematical model is established by considering the particle shapes and bonding state based on direct element method(DEM).By studying the damage of temperature drop amplitude and cooling duration to MCS material under the effect of cold wave,the following conclusions are drawn.For a given temperature drop range,an increased cooling time can aggravate the material damage.In addition,a rapid drop in temperature can cause serious damage to the material.The cracks generated by the temperature stress propagate in the direction of the weaker component of the material.The DEM model can be better used to analyze the damage of the MCS structure induced by cold wave.