Jiang-Jhy Chang

Effect of aggregate properties on the strength and stiffness of lightweight concrete

Abstract An experimental program was carried out to obtain the compressive strengths and elastic moduli of cold-bonded pelletized lightweight aggregate concretes. Three types of aggregates were made with different fly ash contents. Experimental data were analyzed statistically. Test results of multivariate analysis of variance (MANOVA) with 95% confidence level ( α =0.05) show that the properties of lightweight aggregates and the water/binder ratio are two significant factors affecting the compressive strength and elastic modulus of concrete.

BOND DEGRADATION DUE TO THE DESALINATION PROCESS

Abstract In this paper, bond degradation due to the desalination process of chloride-contaminated reinforced concrete is investigated. Four different constant voltages were applied to study the effect of the current on the desalination. Two constant current densities were applied for comparison. Two different electrolytes, NaOH and Ca(OH)2, were used to examine the effect of electrolyte on the bond characteristics. Pullout specimens were cast and tested, using a single rebar pullout test. Three bond parameters of the bond strength, the critical debonding shear load per unit embedded rebar length and the shear stiffness per unit embedded rebar length were obtained to evaluate the bond quality after desalination. It is found that the bond characteristics decrease dramatically as the duration and/or the voltage of desalination increase. Using Ca(OH)2 solution as the electrolyte, less bond loss is observed than using NaOH solution. A micro-hardness experiment shows that softening of concrete near the rebar becomes more apparent as the desalination duration and/or desalination voltage increase. A chemical titration experiment indicates that the accumulation of potassium and sodium ions increases as the desalination duration and/or desalination voltage increase. A pH value measurement shows that the desalination current increases the pH value of concrete near the rebar. More than two-thirds of chloride ions migrate out of the rebar–concrete interface in two weeks of desalination with desalination voltage greater than 10 V. Bond strength decreases approximately 40–60%.

Correlation between corrosion potential and polarization resistance of rebar in concrete

The relationship between corrosion potential thermodynamics and polarization resistance (electrode kinetics) was analyzed. The influence of cement replacement materials (fly ash and slag) on corrosion resistance of steel in concrete in 3.5% NaCl solution was also investigated by corrosion potential and polarization resistance measurements. Replacement of cement by 10% fly ash shows improved corrosion resistance by reinforcing steel in concrete while concrete with 15 and 30% replacement of cement with slag results in a significant reduction in corrosion resistance of reinforcing steel.

A pullout test for determining interface properties between rebar and concrete

In this study, the rebar-concrete interface properties were investigated by conducting the single rebar pullout test. Using a combination of the stress approach and fracture mechanical theory, the corresponding material parameters were obtained and the model was verified as being suitable for reinforced concrete composite material. Advanced Cement Based Materials 1997, 5, 57–65.

Enhancement of bond strength for epoxy-coated rebar using river sand

Abstract In this paper, improvement of bond strength for epoxy-coated rebars is studied. Although the epoxy-coated rebar is well accepted as an alternative corrosion prevention method in engineering practice, it still suffers from its poor bond performance, which requires modification of design parameters such as development length and splice length. Following our previous study [Proceedings of the Third Structure Engineering Conference (1996) 651–659], it is proposed to mix the river sand in the epoxy to improve the bond strength of epoxy-coated rebar. Specimens of several sizes of river sand and sand/epoxy weight ratios were studied. The single-rebar pullout test was performed to examine the bond strength. The open circuit potential method and the linear DC polarization method were conducted for corrosion rate measurements to ensure for keeping the merit of epoxy-coated rebar, i.e. the ability of corrosion. Experimental results show that the proposed method is very promising.

Effect of silica fumes on the permeability of fiber cement composites

Abstract In this study, the pore systems of cement‐based composites were investigated using absorption, rapid chloride penetration, rapid migration, ponding, mercury intrusion porosimetry (MIP), and scanning electronic microscopy (SEM). Direct current polarization resistance and other permeability parameters were used as indices to justify the effects of fiber and silica fume addition on the permeability of cover concrete. Cement‐based composites containing 10% silica fumes were found to have higher compressive strength, lower absorption, lower coefficient of chloride ion diffusion, lower penetration of chloride ions and higher polarization resistance than those of control specimens. Composites with a combined addition of fiber and silica fume further increased polarization resistance and compressive strength. Addition of silica fumes further improves the permeability of fiber cement composites due to the positive effect from pozzolanic reaction, which produces denser and more homogeneous structures in the matrix. Use of silica fumes or lower w/cm ratio instead of adding fiber in the composites would more effectively increase durability indices such as diffusion coefficient, penetration depth and polarization resistance.

The Influence of Mixture Variables for the Alkali-Activated Slag Concrete on the Properties of Concrete

In this study, the influence of three mixture variables named Sand/Aggregate ratio, Liquid/Binder ratio and Paste/Aggregate ratio on the concrete properties were studied. The properties of fresh concrete including the slump, air content and unit weight were examined. In addition, the mechanical properties such as the compressive strength, elastic modulus and splitting tensile strength were studied. Results showed that the alkali-activated slag concrete has superior strength. The 28-day compressive strength can reach 80% of the 90-day compressive strength. In addition, the influence of the Liquid/Binder ratio on the 28-day compressive strength is not as apparent as the water/cement ratio is for the ordinary Portland cement concrete. The trends of influences on the concrete properties for these three mixture variables are similar to those for the ordinary Portland cement concrete. It means that the experiences for making the ordinary Portland cement concrete should be able to be used for the alkali-activated slag concrete. This paper also provides substantial fresh-concrete and mechanical properties results for future development of the alkali-activated slag concrete mix design.

Enhancement of the bond strength of epoxy coated steel by the addition of fly ash

Abstract In this paper, the bond strength of epoxy coated steel enhanced by the addition of fly ash added is examined. The experimental data shows that when the weight ratio of fly-ash/epoxy is 0.5 the largest improvement in bond strength is achieved. Other combinations using different weight ratios will result in bond improvement compared to a plain epoxy coating, but only specimens with a 0.5 fly-ash/epoxy weight ratio develop a bond strength at the level of that of uncoated rebar. Furthermore, it was found that the shear stiffness per length and the critical debonding shear force per length follow the same trend as bond strength.

Effect of concrete cover thickness on hydrogen permeation through cathodically protected steel membranes

Hydrogen permeation through cathodically protected steel membranes with various concrete cover thicknesses was measured using the electrochemical permeation technique. Experimental results showed that the permeation current through cathodically protected steel membranes increased with the concrete cover thickness. In addition, rich calcium deposits were found on the cathodic side of the membrane.

Tracking the Moisture Expansion of Carbon/Epoxy Composite Exposed to Varying Humidity

The moisture expansion of composites exposed to humid air was investigated. Tests for moisture expansion were performed on carbon/epoxy laminates exposed to humid air of different relative humidity levels and temperatures. The expansion strain data were employed to derive the materials diffusivity and the saturated expansion strain. The result indicated that the saturated expansion strain is about proportional to the level of humidity and inversely proportional to absolute temperature. An algorithm was then employed to track moisture expansion of a laminate in air with varying humidity level based on the diffusivity and the saturated expansion strain. The theoretical predictions were confirmed experimentally.