Preparation And Shrinkage Cracking Behavior Of High Crack Resistant Foamed Concrete

Foamed concrete possesses excellent properties such as high fluidity,low density,low thermal conductivity,and good fire resistance.However,foamed concrete also shows some disadvantages such as unfavorable mechanical properties,high shrinkage,and high risk of cracking owing to its high water/cement ratio,high porosity and low content of aggregate,which limits its application.In particular,the obvious and urgent problem in the application of the foamed concrete is the shrinkage cracking.Therefore,it will undoubtedly be of great significance for future application to study crack resistance of foamed concrete.From the perspective of improving the foam stability by using foam stabilizer and increasing the properties of the matrix using internal curing materials,this study has carried out systematic research on strength,shrinkage and crack resistance of foamed concrete.The main research contents and conclusions are shown as follows:(1)The nano-alumina(NA)has been utilized as foam stabilizer to improve the properties of synthetic surfactants(SS),plant protein-based surfactants(PS)and animal protein-based surfactants(AS).On this basis,the Ca(OH)2 as foam stabilizer is used to increase the stability of nano-alumina modified synthetic surfactants(NA-SS)foam and the stabilization mechanisms were investigated.Results shows that the density and stability of foams fabricated by different surfactants can be enhanced with the addition of NA and the NA presents the best effect on the improvement of SS foam.NA is distributed uniformly in SS foam,but dispersed in AS and PS foam.The carboxyl group on the SS surface after SOCl2 treatment reacts with the hydroxyl group on the nano-alumina surface resulting in the NA adhesion to the air-water interface uniformly.The uniformly distributed NA in the foam can hinder physical drainage and improve the stability of the foam.The appropriate amount of Ca(OH)2 in the NA-SS foam can hinder physical drainage of the foam by the reaction of alumina on the bubble surface with Ca(OH)2,thus improving the stability of the foam.In addition,a finer and more homogeneous pore structure could be achieved with the incorporation of the appropriate amount of Ca(OH)2 in the foam,which accounts for the better performance of the foamed concrete.(2)The superabsorbent polymer(SAP),sisal fiber,and lightweight aggregate as internal curing material were used to increase the properties of foamed concrete.Experiment results reveal that the incorporation of SAP improves the yield stress of cement paste,thus increasing the foam-stability of foamed concrete.In addition,the attraction force of the SAP to the foam(FSAP)can decrease the foam-stability of foamed concrete.The foamed concretes with SAP of shorter water absorption period and higher water absorption capacity tend to have higher foam-stability which accounts for the more homogeneous pore size distribution and better performance of the foamed concrete.Lightweight aggregate can adjust the internal humidity of foam concrete,restrict the deformation of the matrix,thus decreasing the drying shrinkage.The porosity accessed through entrances larger than about 100 nm is sufficiently high to provide efficient internal curing.The lightweight aggregate with more ink-bottle entrances larger than about 100 nm tends to have the stronger water release capacity,better internal curing effect and lower drying shrinkage.The appropriate amount of sisal fiber can also delay the internal drying of foamed concrete,increase the connection effect between the internal structures of foam concrete,thus decreasing the drying shrinkage.Under the combined action of the three kinds of internal curing materials,the drying shrinkage of the foamed concrete is reduced from 1322 με to 454με.The maximum shrinkage reduction rate of drying shrinkage is 66%.(3)The 1000 kg/m3 low shrinkage foam concrete produced by the NA,Ca(OH)2,SAP,sisal fiber,lightweight aggregate and fly ash possesses the 28 d compressive strength of 6.2 MPa,the 90 d drying shrinkage of 469 με,and water absorption rate of 5.0%.No obvious cracks of low shrinkage foamed concrete could be observed at 360 d under accelerated cracking condition.(4)The meso-scale model of foamed concrete has been established by using Python software.The relationship between the macro performance and pore distribution,matrix properties,aggregate properties was analyzed by using ABAQUS.The results shows that the foam concrete with different pore distribution,matrix properties and aggregate properties presents obvious stress concentration.The damage first initiates in the thinnest part of the cell wall in the foam concrete,and gradually expends to the internal foam concrete.The relative deviation of yield stress between the numerical and experimental results of foamed concrete with 600 kg/m3 and 1000 kg/m3 density are 6.7%and 6.8%respectively.The relative deviation between the numerical and experimental results is within 15%,indicating that the failure process and yield stress can be reasonably reflected by using the simulated results.The foamed concrete with higher pore size tends to have the lower strength and crack resistance.With the increase of matrix elastic modulus,the compressive strength of foam concrete increases,while the crack resistance of foam concrete decreases.The determining factor of foamed concrete shrinkage cracking is the tensile strength of the matrix.The shrinkage cracking of the foamed concrete could be effectively prevented by increasing the tensile strength of the matrix larger than 4 MPa.(5)The effect of temperature difference,restraint condition,and size effect on the shrinkage cracking of foamed concrete slab were analyzed by using ABAQUS.Results shows that the foamed concrete with the high temperature difference or strong internal constraint is apt to crack.The concentration of stresses tends to be more obvious with the stronger internal constraint and higher temperature difference,thus shortening the cracking period and decreasing the crack resistance of foamed concrete.The size effect of foamed concrete slab is caused by the change of temperature difference.Adjusting the temperature difference of the foam concrete within 20℃can effectively prevent the shrinkage cracking of the foam concrete slab.