Estimating The Cracking Resistance Of Concrete With Different Strength Grades Under Restrained Shrinkage

Due to the uniform restraint,convenient operation and relative low cost,the restrained ring test has been widely used to assess the shrinkage cracking of cement based materials.American Society for Testing Material(ASTM)reduced the thickness of concrete into 37.5 mm based on the standard restrained ring test recommended by American Association of State Highway and Transportation Officials(AASHTO).This improvement can not only improve the degree of restraint but also decrease the non-uniform shrinkage caused by moisture gradient.Many researchers thought that the moisture gradient and non-uniform shrinkage along radial direction can be neglected in the 37.5 mm-thick ring specimen under outer circumference drying.Considering the highly nonlinear moisture distribution within concrete materials and the differentiation of moisture distribution between various concrete materials,this paper is intended to research the fracture mechanism of thin ring specimen(37.5 mm)of three kinds of concretes with compression strength at 28 days around 30 MPa,50MPa and 80 MPa under outer circumference drying.This research can accurately analyze the crack initiation and propagation process of the restrained ring test suggested by ASTM.The main work of this paper is shown as follows:(1)Experimental research on the material properties of C30,C50 and C80 concrete at early ageThe material properties of C30,C50 and C80 concretes were measured at the 3rd,7th 14 th 21st and 28 th days,respectively,including splitting tensile strength,elastic modulus,fracture energy,initial fracture toughness and free shrinkage.The experimental results of the material properties at different ages were fitted into continuous curves,from which the material properties at every day of C30,C50 and C80 concretes can be obtained.(2)The restrained circular and elliptical ring test of C30,C50 and C80 concreteAccording to the standard ring test recommended by ASTM,restrained circular and elliptical ring test of C30,C50 and C80 concretes were conducted.The semi-major axis of elliptical ring was equal to the radius of circular ring,and the minor-semi axis of the elliptical ring was half of the semi-major axis.The cracking ages of the restrained ring test was determined by the hysteresis curve of the strain measured at inner surface of steel ring.The experimental results show that,with the increasing of the concrete strength,the shrinkage cracking occurs at an earlier age,which means that the high strength concrete is more sensitive to shrinkage cracking.What's more,the restrained elliptical ring specimen cracked earlier than corresponding circular ring specimen.The advantage of elliptical ring in improving the degree of restraint and accelerating the shrinkage cracking was effective for all the concretes used in this paper.(3)Numerical research on the crack initiation and propagation process of the restrained circular and elliptical ring testFirstly,the interior moisture fields of C30,C50 and C80 concrete ring specimens under outer circumference drying were calculated based on existing theoretical model.Combining the interior moisture field and the free shrinkage strain,the fictitious temperature field was derived to simulate the non-uniform shrinkage of ring specimen.Then,a numerical model of the restrained ring test was built up using ANSYS codes and the fictitious temperature was applied to the numerical model to analyze the stress distribution.According to the stress distribution,the position where the shrinkage cracking occurs was determined.Finally,the numerical model with an initial crack was established.By introducing the fictitious crack model and the crack propagation criterion based on the initial fracture toughness,the crack initiation and propagation process of restrained ring test was simulated.The numerical result shows that the shrinkage cracking occurs at outer surface of the ring specimen and propagates into inner surface.The cracking ages obtained by experimental and numerical simulation show good agreement,which means that the fracture mechanism proposed in this paper is reasonable.(4)Further analysis on the driving force of the crack propagation and uniform shrinkage assumptionAfter considering the non-uniform shrinkage along the radial direction,the force driving the crack to propagate can be divided into two categories: the self-restraint caused by non-uniform shrinkage and the outer restraint provided by steel ring.As for the thin ring specimen of C30,C50 and C80 concrete,the outer restraint played an important role in crack propagation process.The outer restraint provided by the steel ring increases with the increase of concrete strength due to the relative slight non-uniform shrinkage.In addition,the elliptical ring has a higher outer restraint compared with circular one.The uniform shrinkage assumption was made to study the restrained ring test.The cracking ages under this assumption showed huge difference with that under considering non-uniform shrinkage,which proved the necessity of considering non-uniform shrinkage.