As one kind of porous medium, even without external loading, concrete material is still possible to be damaged by the internal pore pressures, such as hydraulic, crystallization and cryosuction pressures during freezing and thawing cycles (FTC). In this paper, a mesoscale model using Rigid Body Spring Method (RBSM) is developed to simulate the defor-mation behaviors of concrete under FTC cycles. On one hand, the macroscopic material is divided into small rigid ele-ments of mesoscale; on the other hand, the microscale internal pore pressures are regarded as average values in mesoscale based on poromechanical theories. The constitutive relation is also developed to reflect deformation compatibility be-tween porous body and ice-water system. Finally, the internal cracking and residual deformation are simulated for mortar and concrete using RBSM, which are found in a good agreement with previous experimental data.
Low-temperature calcination for cement manufacturing by the addition of fluoride compounds has been researched by many investigators. Although it is possible that fluoride ions elute to the suspension after calcination, there are very few studies about the influence of fluoride ion on the fluidity of cement paste with superplasticizer. This paper describes the influence of fluoride ions on the adsorption mechanism of polycarboxylate based superplasticizer in cement paste. When the amount of KF was increased, the viscosity of the cement paste with superplasticizer and the amount of adsorbed superplasticizer increased. The fluidity with the polycarboxylate based superplasticizer containing more functional groups was more susceptive to KF addition and the amount of adsorbed superplasticizer was larger for the polycarboxylate based superplasticizer containing more functional groups. The specific surface area increased with KF addition. In the case of no KF addition, the hydration of alite was retarded by the addition of polycarboxylate based superplasticizer. In contrast, in the case of KF addition, the hydration of alite was not retarded by the addition of superplasticizer. It is supposed that some types of fine particles were generated by KF addition and the fine particles deprived the cement particles of superplasticizer.
The research presented in this paper has been motivated by the need to numerically simulate performance of high strength fiber reinforced concrete (HSFRC) structural elements with given spatially variable fiber volume fraction. The intended applications include prediction of load and deformation capacity of HSFRC members with imperfect fiber distribution or design and verification of functionally graded HSFRC members. In order to achieve a predictive capability, modeling is based on micromechanics of fiber debonding, pullout and crack-bridging. The concept of cohesive crack is employed for implementation in the finite element method (FEM). A strong emphasis is placed on the feasibility of the model identification. To this end, a procedure which uses data from conventional notched-beam fracture tests and inverse analysis to determine the model parameters is proposed. The model and the identification method are verified and validated both on micro and macro scales by comparing predicted results with experimental data.
The effect of montmorillonite on the hydration reactions and fluidity of cement paste was investigated using comb-type superplasticizer or sodium gluconate. When comb-type superplasticizer was added, montmorillonite affected the retar-dation action of the initial hydration reaction in the cement and the dispersion significantly. When sodium gluconate was added, the effect of the montmorillonite on the retardation action was smaller compared with that of the comb-type su-perplasticizer. The amount of comb-type superplasticizer adsorbed on the cement particles is reduced significantly by the addition of montmorillonite. In the case of sodium gluconate, the amount adsorbed on the cement particles is also de-creased by the addition of montmorillonite. However the adsorbed amount of sodium gluconate on montmorillonite is smaller compared with that of superplasticizer on montmorillonite.
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