Papers by Author: You Jun Xie

Abstract: In precast concrete elements manufacturing, steam-cured concrete incorporating 30% fly ash encountered the problem of a too low demoulding compressive strength. To resolve it, this paper developed a new steam-cured concrete (AFSC) incorporating fly ash and a chemical activator. Experiments were conducted to investigate the mechanical properties of AFSC. The corresponding mechanism was also discussed by testing the microstructure of concrete. Results indicate that the demoulding compressive strength of AFSC can meet production requirements, and compressive and flexure strength of AFSC at later ages increase well. Compared with that of ordinary steam-cured concrete, AFSC has a higher tensile strength, and the capability of AFSC to resist cracks is enhanced remarkably. At an early age, addition of the chemical activator can distinctly accelerate the extent of hydration of the fly ash cement systems, and thus the microstructure of concrete becomes denser.

Abstract: The chloride ions and sulfate ions precipitating rates in surface layer of different mortars was experimentally measured. And the concept of precipitating rate of ions and corresponding equation in cementitious materials was proposed. Further, some factors affecting precipitating rate of ions were investigated. Results indicate that the precipitating rate of ions in surface layer of mortar can be expressed well by an exponential function of time variable. And the exponent in corresponding equation is about 0.42 in investigation scope, which cannot be influenced greatly by addition of mineral admixture or type of ions such as chloride ions or sulfate ions. However, the precipitating rate coefficient of ions changes remarkably with compositions of mortar or types of ions. Addition of mineral admixture can reduce the precipitating rate coefficient of ions.

Abstract: The physical crystallization action of sulfate in cement based material, which often occurs by dry-wet cycle, capillary rising and evaporating action, can damage seriously cement-based materials as compared to chemical corrosion by sulfate. The deterioration mechanism of cement-based materials by sulfate crystallization attack, experimental investigations of the process of sodium sulfate crystallization and the factors affecting the process, and the invading track of salt solution by capillary effect are presented in this paper. Results show that the crystal type and crystallization velocity of sodium sulfate is influenced greatly by relative humidity and ambient temperature. There is a close relationship between invading depth and the porosity of concrete. Addition of mineral admixture to concrete can significantly reduce the invading depth of sodium sulfate solution and thereby enhance the resistance of concrete to sulfate crystallization attack.The profile of invading track of salt solution by capillary effect is similar to the shape of concave parabola.

Abstract: Hardened cement paste and concrete are porous materials. Most engineering properties of hardened cement pastes and concrete, such as strength, shrinkage, permeability, etc., are influenced or controlled by their pore structure characteristics. The ingress of chloride ion into concrete can cause steel corrosion and damage of steel reinforced concrete structure. This paper reviews the pore structure characteristics and transport mechanisms of chloride ion, such as hydrostatic advection, capillary suction, diffusion, thermal migration and electrical migration, in hardened cement pastes and concrete.