Jingyao Cao

COLLOIDAL GRAPHITE AS AN ADMIXTURE IN CEMENT AND AS A COATING ON CEMENT FOR ELECTROMAGNETIC INTERFERENCE SHIELDING

A water-based colloid of submicron graphite particles is an effective admixture for enhancing the EMI shielding effectiveness of cement paste, though it is ineffective for lowering the electrical resistivity. As an admixture, it is more effective for shielding than 15-mm-diameter discontinuous carbon fibers, though it is less effective than 0.1-mm-diameter discontinuous carbon filaments. A shielding effectiveness of 22 dB at 1 GHz is reached by cement paste at a solid graphite content of 0.92 vol.%, compared to a value of 11 dB for a coating made from the graphite colloid and a value of 14 dB for graphite-colloid-coated cement paste (without admixture). D 2003 Elsevier Ltd. All rights reserved.

Carbon fiber reinforced cement mortar improved by using acrylic dispersion as an admixture

Cement mortar reinforced by short carbon fibers was improved by using acrylic dispersion as an admixture in the amount of 15% by mass of cement. The improvement of the tensile properties (particularly strength and ductility) was more than those attained by using methylcellulose, styrene acrylic, or latex as admixtures. Acrylic was effective whether silica fume was present or not. However, for lowering the electrical resistivity, methylcellulose in combination with silica fume was most effective.

Damage evolution during freeze-thaw cycling of cement mortar, studied by electrical resistivity measurement

Damage evolution during freeze-thaw cycling of cement mortar was found by electrical resistivity measurement to involve damage accumulating gradually cycle by cycle until failure. The damage inflicted during cooling was more significant than that inflicted during heating. Damage infliction occurred smoothly throughout cooling from 52 to 20 C. Failure occurred at the coldest point of a temperature cycle. Electrical resistivity measurement allowed simultaneous monitoring of temperature and damage. An increase in temperature caused the resistivity to decrease reversibly, but with hysteresis, which grew with cycling. The effects of freezing and thawing on the resistivity were small compared to the effect of temperature on the resistivity. D 2002 Elsevier Science Ltd. All rights reserved.

Improving the dispersion of steel fibers in cement mortar by the addition of silane

The dispersion of steel fibers in steel-fiber-reinforced mortar was improved by the use of silane as an admixture, as indicated by electrical resistivity measurement conducted at a low fiber volume fraction below the percolation threshold. The use of silane to coat the steel fibers rather than as an admixture was less effective.

DEGRADATION OF THE BOND BETWEEN CONCRETE AND STEEL UNDER CYCLIC SHEAR LOADING, MONITORED BY CONTACT ELECTRICAL RESISTANCE MEASUREMENT

Degradation of the bond between steel reinforcing bar (rebar) and concrete under cyclic shear loading was observed nondestructively by measuring the contact electrical resistance of the joint. Degradation, which caused a decrease in bond strength but no visual damage, was indicated by an abrupt increase in the resistance at a small fraction of the fatigue life.

Defect dynamics and damage of concrete under repeated compression, studied by electrical resistance measurement

The electrical resistance of concrete in the stress direction increased during compressive loading in the first cycle, due to defect generation. It decreased during loading in all subsequent cycles, due to defect diminution. It increased during unloading in the first and all subsequent cycles, due to defect extension. The baseline resistance and the amplitude of resistance variation increased with cycling, due to minor damage. The interface between mortar and coarse aggregate contributed to the defect dynamics, particularly defect diminution.

Minor damage of cement mortar during cyclic compression monitored by electrical resistivity measurement

Minor damage of cement mortar during cyclic compression in the elastic regime was monitored by measurement of the electrical resistivity in the stress direction. Defect generation (irreversible) resulted in an irreversible increase in the baseline resistivity as stress cycling progressed, whereas defect healing (reversible) resulted in a reversible decrease in the resistivity upon compression within a stress cycle. Defect generation was relatively significant in the early cycles and diminished upon cycling. As the cumulative damage increased, the extent of defect healing within a cycle also increased.

Effect of strain rate on cement mortar under compression, studied by electrical resistivity measurement

The electrical resistivity of cement mortar increased monotonically during compression. An increase in strain rate caused the resistivity at any strain level to decrease, in addition to causing the resistivity at failure to decrease. This means that the microstructural change that occurred continuously during loading decreased with increasing strain rate.

Defect dynamics of cement mortar under repeated loading, studied by electrical resistivity measurement

Defect dynamics, as studied by electrical resistivity measurement during repeated compression of cement mortar in the elastic regime, are characterized by defect generation, defect healing and defect aggravation. Defect generation dominates in the first compressive loading and in tensile loading in any loading cycle. Defect healing dominates in all subsequent compressive loading cycles and in tensile unloading in any cycle. Defect aggravation dominates during compressive unloading in any cycle and occurs during tensile loading in any cycle. Both the interface between sand and cement and that between silica fume and cement contribute to the defect dynamics, particularly the defect healing. The defect dynamics give similar effects to the longitudinal and transverse resistivities. Upon uniaxial compression, the resistivity decreases (except for the first cycle); upon uniaxial tension, the resistivity increases.

Degradation of the bond between old and new mortar under cyclic shear loading, monitored by contact electrical resistance measurement

Degradation of the bond between old and new mortar under cyclic shear loading was observed nondestructively by measuring the contact electrical resistance of the joint. Degradation, which was due to fatigue and caused a decrease in bond strength but no visual damage, was indicated by an abrupt increase in the resistance at a small fraction of the fatigue life.