Micro Fracture Characterization Of Cement-Based Materials By Nanoindentation

Cement-based material is one of the most widely used building materials.It is thus essential to understand its mechanical properties,especially fracture properties,in engineering applications.Due to the limitations of test methods and techniques,the micro mechanical properties of cement-based material that shows multi-scale structures,are far from being fully addressed.Nanoindentation technique is a useful test method in nano and micro scales,and so far has been used extensively to characterize the elastic properties of cement-based material.This study attempted to develop a new method to assess the fracture properties of cement-based material in micro scale using nanoindentation.An energy model for estimating the elastic properties of a material and a plastic-energy model for estimating the fracture properties with a holding effect at peak load were established based on the principle of energy conservation and the full load-displacement curves of an indentation test.The hardness,elastic modulus,fracture energy,energy release rate and fracture toughness of the tested material can be calculated directly from the indentation curves via these models.Ordinary Portland cement paste samples at the ages of 3 days and 28 days were prepared for indentation tests.5 main groups(23 sub groups)of the indentation tests,in total 72 test points,were conducted with the considerations of maximum load,the speed of loading and unloading and the duration of holding period.The microstructures of the paste samples were investigated by using scanning electron microscopy and back scattering analysis.The results about the methodology of indentation test can be recapitulated as follows.1 The energy model for estimating elastic properties can accurately provide the parameters of effective depth,unloading stiffness,hardness and elastic modulus that deviate from the referenced values through the Oliver-Pharr method by less than 10%.2 Holding period is so important that cannot be negligible for the plastic-energy model.Through the modified plastic-energy model considering the holding period,the accuracy of fracture toughness results is improved by 25%-80%.Also,the fracture toughness of the cement pastes calculated by the plastic-energy model is rather steady and consistent with that by conventional macro test methods.The analyzing results of indentation tests on the cement pastes can be abstracted as follows.1 As the curing age grows,the fracture toughness of the cement paste increases remarkably.2 in the same cement paste sample,the clinkers have much larger fracture toughness than the hydration products.And 3,the changes in the maximum load,the loading and unloading speed and the time of the holding periods during the indentation tests can hardly influence the test results.The energy-based methods for indentation test established in this study may provide a general way to study the mechanical properties,especially the fracture properties of cement-based material in micro scale.