Xuli Fu

Ozone treatment of carbon fiber for reinforcing cement

Abstract Ozone treatment of isotropic-pitch-based carbon fiber was found to increase the surface oxygen concentration and change surface oxygen from C–O to CO, thereby causing the contact angle between fiber and water to be decreased to zero. Thus, the bond strength between fiber and cement paste was increased and the tensile strength, modulus and ductility of carbon fiber reinforced cement paste were increased. Moreover, the degree of dispersion of fibers in mortar was increased and the effectiveness of the fibers for reducing the drying shrinkage was improved. As a consequence, the strain sensing ability of carbon fiber reinforced mortar was improved in terms of increased gage factor and better repeatability. The ozone treatment did not affect the morphology, tensile strength or volume electrical resistivity of the fiber itself.

Self-monitoring of fatigue damage in carbon fiber reinforced cement

Self-monitoring of slight fatigue damage was demonstrated in cement mortar containing short carbon fibers (0.24 vol.%), as damage (occurring in the first < 10% of the tensile or compressive fatigue life) caused the volume electrical resistivity to decrease irreversibly by up to 2%. The greater the stress amplitude, the greater the damage, the greater the resistivity decrease and the greater the number of stress cycles for which the resistivity decrease monotonically occurred. The resistivity decrease is attributed to the damage of the cement matrix separating adjacent fibers at their junction.

Improving the bond strength between carbon fiber and cement by fiber surface treatment and polymer addition to cement mix

The bond strength between carbon fiber and cement was enhanced by oxidizing chemical treatments, with ozone treatment giving the greatest effect. The effect was accompanied by an increase in the electrical contact resistivity of the interface. These effects are mainly attributed to oxygen-containing functional groups on the fibers due to the treatments. The bond strength and contact resistivity were also increased by polymer (latex, methylcellulose) admixtures in the cement mix, but the effects were less than those of ozone treatment.

Effects of silica fume, latex, methylcellulose, and carbon fibers on the thermal conductivity and specific heat of cement paste

Abstract Due to their poor conductivity, latex (20–30% by weight of cement), methylcellulose (0.4–0.8% by weight of cement), and silica fume (15% by weight of cement) decreased the thermal conductivity of cement paste by up to 46%. In addition, these admixtures increased the specific heat of cement paste by up to 10%. The thermal conductivity decreased and the specific heat increased with increasing latex or methylcellulose content. Short carbon fibers (0.5–1.0% by weight of cement) either did not change or decreased the thermal conductivity of cement paste, such that the thermal conductivity decreased with increasing fiber content due to the increase in air void content. The fibers increased the specific heat due to the contribution of the fiber-matrix interface to vibration.

Self-monitoring in carbon fiber reinforced mortar by reactance measurement

Self-monitoring in carbon fiber reinforced mortar was demonstrated by AC impedance measurement. The reactance Xs was found to be a more sensitive indicator of strain than the resistance Rs. The fractional increase in Xs was 38 at compressive failure, while the fractional increase in Rs was 23, for mortar with methylcellulose, silica fume and fibers (0.35 vol.%).

Effect of curing age on the self-monitoring behavior of carbon fiber reinforced mortar

The self-monitoring behavior of carbon fiber reinforced mortar was found to be affected in two ways by curing age from 7 to 28 days. Firstly, the electrical resistance increased monotonically with increasing compressive strain during first loading at 7 days, but at 14 and 28 days it decreased monotonically. Secondly, the resistance decreased slightly and irreversibly at the end of each cycle as cycling progressed at 14 and 28 days, but not at 7 days. These effects of the curing age are associated respectively with the effects of the curing age on fiber-cement bond strength and on cement compliance.

Effect of corrosion on the bond between concrete and steel rebar

Abstract The effect of corrosion on the bond between concrete and steel rebar was studied by measuring both bond strength and contact electrical resistivity. Corrosion of steel rebar in concrete immersed in saturated Ca(OH)2 solution was found to cause the bond strength to increase, and the contact resistivity increased until 5 weeks of corrosion. Further corrosion caused the bond strength to decrease, while the contact resistivity continued to increase.

Submicron carbon filament cement-matrix composites for electromagnetic interference shielding

Abstract Carbon filaments of diameter 0.1 mm were found to be a much more effective additive than conventional carbon fibers of diameter 10 mm in providing cement pastes capable of electromagnetic interference shielding. With 0.54 vol.% filaments and a shield thickness of 4 mm, a shielding effectiveness of 30 dB was attained at 1–2 GHz. However, the filaments were less effective than the fibers for reinforcing and for providing strain sensing cement-matrix composites.

Strain sensing using carbon fiber

Carbon fiber provides strain sensing through change in electrical resistance upon strain. Due to piezoresistivity of various origins, a single carbon fiber in epoxy, an epoxy-matrix composite with short carbon fibers (5.5 vol%), a cement-matrix composite with short carbon fibers (0.2–0.5 vol%), and an epoxy-matrix composite with continuous carbon fibers (58 vol%) are strain sensors with fractional change in resistance per unit strain up to 625. A single bare carbon fiber is not piezoresistive, but just resistive.

Submicron-diameter-carbon-filament cement-matrix composites

This paper provides a surface treatment method for the submicron diameter carbon filaments. The treatment results in significant improvement of the mechanical properties of the carbon filament cement-matrix composites.