The Damage Of Cementitious Composites Under Freezing Crystallization

Freezing and thawing cycles is one of the main factors influencing durability of cementitious composites and it recently catches much attention.The degree of damage of cement matrix under freeze-thaw cycles is mostly evaluated through the macroscopic performance.In fact,freezing-thawing damage of cement matrix starts from micropores and cumulative damage gradually leads to failure.The purpose of this paper is to establish calculation model to quantitatively analyse the size of cracks and the change of macroscopic elastic modulus of cement matrix through analyzing the saturated micro-nano pores cracking due to freezing.The damage degree is calculated based on increase in volume.The idea,analyzing the change of macroscopic performance through microscopic mechanism of cement matrix under freezing,comes true.At first,the existing results in the literature are given with respect to hydration degree,porosity and pore size distribution of hardened cement pastes,based on which the number of pores are calculated in different pore-size ranges.This work is ready for the calculation of crystallization pressure and crack volume in the following section.According to pore size distribution and crystallization pressure theory,spherical and cylindrical pore model are used to calculate the crystallization pressure.Pores crack due to crystallization pressure.Assumptions include that the type of crack is “penny crack” and hardened cement pastes is elastic-plastic body.Then,based on theories of elasticity and plasticity,elastic limit stress is calculated and one can estimate whether or not pores entering plastic state through comparing elastic limit stress with the corresponding crystallization pressure.Subsequently,plastic radius is calculated and assumption is made that plastic radius is equal to the radius of “penny crack”.According to energy conservation principle,the width of crack is calculated.Now,the volume of a single crack can be achieved through multiplying radius by width and the total crack volume deriving from per unit volume of hardened cement paste can be obtained through multiplying the volume of a single crack by the corresponding the number of pores and summing crack volume of each level of pores.At last damage degree is obtained.Finally,hardened cement paste in the different stages is regard as a composite material made of different phase composition.According to boundary theory of effective elastic modulus of Hashin and Shtrikman,the formula of effective elastic modulus is deduced with respect to two-phase,three-phase and four-phase composites,which are used to calculate the effective elastic modulus of materials under freezing and thawing.The idea,analyzing the change of macroscopic performance through microscopic mechanism of cement matrix under freezing,comes true.