Investigation Of Ultra-early Stage Elastic Modulus Of Cement-based Materials By Indentation Method

The cracking of cement-based materials will seriously affect the durability of structures,and more and more attention has been paid by academic and engineering circles.The ultra-early stage,namely the initial setting time to the hardening stage,is an important period for the deformation of cement-based materials,and its elastic modulus can not be measured by the experimental method in the national standard.The ultra-early stage elastic modulus has an important influence on the deformation and cracking of cement-based materials,and it is an important parameter for studying the cement-based materials' crack resistance.Meanwhile,the ultra-early stage elastic modulus also provides the necessary mechanical parameters for numerical simulation.Based on the analysis of the indentation principle,this paper applied the indentation method to the cement-based materials,and put forward a test method for the ultra-early stage elastic modulus of cement-based materials.The cylindrical low carbon steel indenter and conical low carbon steel indenters with semiconical angle of 70.3°,45°,30° and 15° were designed and manufactured.The ultra-early elastic modulus of the cement paste with water binder ratio of 0.25 was tested under different indenters by the micro tester,then it was calculated and compared with the hardening stage elastic modulus measured by the dial indicator method.Finally the indenter with the most suitable angle of 45° for the ultra-early elastic modulus test of cement paste was selected.The verification experiment of the cement fly ash pastes with water cement ratio of 0.35 and fly ash content of 30% and 60% was carried out by the most suitable indenter.The ultra-early elastic modulus was calculated,and the feasibility of this method was analyzed.The relationship between ultra-early elastic modulus of cement paste and the time was explored,and a method for predicting the ultra-early elastic modulus of cement paste was proposed on this basis.In addition,the relationship between ultra-early elastic modulus and indentation hardness was analyzed from the two aspects of theory and experiment.The indentation method research on the ultra-early elastic modulus of cement-based materials and the existing research were analyzed comparatively.The results show that the indentation method could be used to test the ultra-early stage elastic modulus of cement-based materials,and the conical low carbon steel indenters with semiconical angle of 45° was the most suitable indenter for cement paste testing.Its range of application was that the indentation hardness of the cement paste was not more than 25 MPa.The ultra-early stage elastic modulus of cement paste had a linear function relationship with time.Based on this conclusion,a method for predicting the ultra-early stage elastic modulus of cement paste was proposed.It was to test the initial setting time and the hardening stage elastic modulus by the dial indicator method,and to predict the elastic modulus at any time of the ultra-early stage through the straight line where the two points were located.The ultra-early stage elastic modulus of cement paste had a power function relationship with the indentation hardness,and this conclusion proved the accuracy and feasibility of the indentation method for testing the ultra-early stage elastic modulus.Meanwhile,the ultra-early stage elastic modulus of cement paste could be predicted by the indentation hardness.At present,the ultrasonic method and theoretical derivation method were generally used to study the ultra-early stage elastic modulus of cement-based materials.Compared with the existing methods,the indentation method was simple and time-consuming,and compared with the hardening stage elastic modulus of cement paste by the dial indicator method,it was more accurate.