Research On Crack Resistance Of Basalt Fiber Foamed Concrete

In recent years,foamed concrete has been more and more widely applied in engineering construction,but it easily cracks in the application process.Adding fiber is an important means of improving the crack resistance of cement-based materials.Therefore,it is of great necessity to study the influence of fiber on the crack resistance of foamed concrete and put forward the corresponding evaluation index of the crack resistance.The thesis aims at studying the effect of basalt fiber on the crack resistance of foamed concretes with different density grades,discussing the regularity of the influence of fibers on the crack resistance of foamed concretes,and meanwhile,seeking the suitable evaluation index for estimating the crack resistance of foamed concretes,so as to better improve the crack resistance of foamed concretes.In this thesis,three kinds of fibers with lengths of 5mm,10mm,15mm and four kinds of volume fractions of 0,0.15%,0.30%and 0.45%were adopted to prepare fiber foamed concretes with two density grades,namely A05 and A07.And then,compressive strength,flexural strength,elastic modulus,drying shrinkage test,elliptical ring restraint test and bending toughness test were conducted to respectively study the effects of fibers with various volume fractions and lengths on the performance of foamed concretes.Based on the above results,firstly,three indexes,K₁,K₂ and K₃,which indirectly reflect the crack resistance of foamed concrete,are obtained to evaluate the impact of adding fiber on the crack resistance of foamed concrete,and then compared with the elliptical ring restraint test to establish the appropriate evaluation index suitable for estimating the crack resistance of foamed concrete.Secondly,the micro structure of the fiber foamed concrete is researched by adopting scanning electron microscope（SEM）,so as to investigate the intrinsic causes of the effect of fibers on the crack resistance of the foamed concrete.The results are:（1）The compressive strength,flexural strength and elastic modulus of foamed concretes with two density grades all show an upward trend with the increasing volume fractions and lengths of the fiber,and the main influencing factor is the fiber volume,followed by the fiber length.While the degree of the drying shrinkage decreases with the increase of volume fractions first and then levels off and rises,meanwhile,with the increase of the fiber length,it presents the tendency of gradual decrease,besides,the effect of fiber volume is the most important factor in the A05 foamed concrete,while in the A07 foamed concrete,the effect of fiber length is the most important factor.（2）The cracking time of basalt fiber foamed concrete with different density grades all increases firstly and then decreases with the increasing volume fractions and lengths of the fiber,and the vital factor is the volume fractions.While the crack width decreases first and then increases with the addition of fiber volume fractions content,and shows a gradual downward trend with the increase of fiber length,and the fiber length is the most important factor affecting the crack width of foamed concrete.（3）The correlation coefficient between the intuitive crack resistance evaluation index K₄ and the indirect crack resistance evaluation index K₁ is the highest,and it is more typical to apply K₁ as the indirect reflection index for evaluating the crack resistance of foamed concrete.（4）The plain foamed concrete shows the situation of brittle fracture,while the fiber foamed concrete presents certain deformation and hardening phenomenon,and the more the fiber content is,the more obviously the fiber foamed concrete is deformed and hardened,and the resulting curve is also more comprehensive.For the foamed concrete with different density grades,the fiber volume fraction is the most important factor to affect the toughness of fiber reinforced concrete,followed by the fiber length.（5）The fiber spacing is the primary factor affecting the anti-crack effect,and the long fibers are more conducive to the crack arrest.Besides,the homogeneous distribution of the basalt fiber in foamed concrete and its good bonding with cement hydrates achieve the better crack resistance.