The Study On Early Shrinkage Cracking Of Cementitious Materials Based On The Eccentric Ring Test

In response to the needs of sustainable development,China has put forward the specific goals of strengthening the research and application of building materials such as new cementitious materials and low-carbon concrete in the“Carbon Peak Action Plan before 2030" released in 2021.The development direction of the concrete and cement industry turns to "green" and "durability" these two aspects.Reducing the cracking problems as much as possible is essential in the realization of durability of concrete.The appearance of cracks accelerates the intrusion rate of external substances,thus accelerating the deterioration of concrete component.Aiming at the research on shrinkage cracking of concrete,this research summarizes the development of autogenous shrinkage and drying shrinkage of cementitious materials.The eccentric ring test is selected to investigate the shrinkage cracking of cementitious materials.The fracture toughness of cementitious materials is calculated,simultaneously.The results of this research provide important support for the rapid prediction of autogenous shrinkage and drying shrinkage of mortar and concrete,the testing and characterization of the crack resistance of different cementitious materials,and the accurate calculation of fracture toughness of cement paste and mortar.This research can be a significant basis on avoiding early shrinkage cracking of concrete.The main contents of this article are as follows:（1）The autogenous shrinkage and drying shrinkage of cement paste,mortar,and concrete were tested by the prism method.The autogenous shrinkage of mortar and concrete can be predicted by the power function of aggregate volume fraction multiplying by the autogenous shrinkage of pure cement paste with the same water-cement ratio.The exponents of the autogenous shrinkage model of power function are 1.588 and 1.477 for mortar and concrete with the water-cement ratio of 0.4.The drying shrinkage of mortar with water-cement ratios of 0.4 and 0.5 can be predicted by the inverse proportional function of aggregate volume fraction multiplying by the drying shrinkage of pure cement paste when environment temperature is 20℃ and relative humidity is 60%.The autogenous shrinkage of mortar and concrete was simulated by the finite element method.The simulation results show that the exponents in the autogenous shrinkage model of mortar and concrete are 1.521 and 1.357,respectively,which can verify the effectiveness of the autogenous shrinkage model.（2）The eccentric ring test was used to test the shrinkage cracking of cement paste,mortar,and concrete.When water-cement ratios,aggregate volume fraction,and external humidity are high,the cracking times of cementitious materials are longer.Reducing the drying surfaces,adding fibers and polymers can extend the cracking time of cementitious materials.Compared with the traditional ring test,the eccentric ring test can induce cracks to appear on the thin side of the specimens,and experimental results are repeatable.The influence of different factors on the cracking times can be clearly reflected in the eccentric ring test,which contributes much to the crack resistance of different cementitious materials.（3）The mechanical response of the eccentric ring structure when the boundaries of the outer ring and the inner ring are subjected to uniform force can be derived easily in the bipolar coordinate system.When the outer boundary of the eccentric ring is subjected to uniform force,the circumferential tensile stress at the thinnest part of the outer ring can reach more than 4 times the radial stress.The circumferential tensile stress at the thinnest part of the eccentric ring is 1.7～2.7 times greater than that at the thickest position,indicating that the excessive circumferential tensile stress on the thinner side is the main factor causing the cracking of the eccentric ring structure.（4）The nanoindentation method have difficulty in measuring the overall fracture toughness of the composites,while the three-point bending method is more suitable for calculating fracture toughness of cement paste and mortar.A new fracture toughness calculation method based on the transverse force is developed to accurately determine the crack initiation and calculate the initial fracture toughness as well as unstable fracture toughness of cement paste and mortar.The unstable fracture toughness of cement paste with water-cement ratios of 0.3,0.4 and 0.5 are 0.465,0.426 and 0.325 MPa·m^0.5,respectively.Under the condition of 25%and 50%sand volume fraction,the unstable fracture toughness is 17.4%higher than the initial fracture toughness for mortar with a water-cement ratio of 0.4 and 0.5.The calculation results of cement paste and mortar obtained by the new calculation method are lower than those obtained by the traditional calculation method,but the two methods tend to be consistent when the aggregate volume fraction is high enough.（5）The autogenous-drying shrinkage model based on chemical shrinkage and capillary theory is practicable in the simulation of the autogenous shrinkage and drying shrinkage of cementitous materials.The autogenous-drying shrinkage model can be applied to the shrinkage simulation of eccentric ring specimens.The results show that the humidity on the thinner side of the eccentric ring decreases faster and the circumferential stress increases rapidly.The circumferential tensile stress on the outer ring is more than 7 MPa in the early drying stage.The analysis method based on the comparison of the stress intensity factor and the unstable fracture toughness on the thin side of the eccentric ring can accurately predict the cracking time of cement paste and mortar.The simulation results of the cracking times are in line with the test results.（6）Polymer modified mortar has a high water-loss rate and small drying shrinkage,while ordinary mortar has a small water-loss rate and large drying shrinkage.Ordinary mortar is easier to crack than polymer modified mortar.The water-loss shrinkage coefficient,which is defined as the ratio of drying shrinkage and water-loss rate,can characterize the sensitivity of water loss during the drying shrinkage and indicate the crack resistance for different cementitious materials.The larger the water-loss shrinkage coefficient,the worse the crack resistance of cementitious materials when the water-cement ratio is the same.The eccentric ring samples with water-cement ratios of 0.4 and 0.5 tend to crack when the waterloss shrinkage rates reached about 0.3%and 0.15%,respectively.