Chao-Lung Hwang

Drying shrinkage of portland cement pastes I. Microcracking during drying

Abstract The occurrence of microcracking of portland cement pastes during drying has been studied by comparing the effects of specimen thickness on shrinkage and cracking using light microscopy. Increases in specimen thickness tended to impede drying and wetting, but there were only slight changes (less than experimental errors) in total and reversible shrinkage once equilibrium was attained. Although microcracking occurred at the beginning of drying whenever the thin specimen (thickness

The effects of blast-furnace slag and fly ash on the hydration of portland cement

Abstract In this study, a calorimeter, an ultrasonic tester, an optical microscope, and other pieces of conventional laboratory equipment, such as a Vicat needle, were employed in the investigation of the effect of blast-furnace slag and fly ash on the hydration of fresh cement paste. Test results indicate that a strong relationship exists among the calorimetric curve, the ultrasonic pulse velocity curve, and the penetrative resistance strength curve. A transition zone appearing in the ultrasonic pulse velocity curve corresponds to the period between the end of the dormant period and the deceleration period in the calorimetric curve. In the calorimetric curve during the end of the dormant period, the ultrasonic pulse velocity curve rises rapidly and the penetrative resistance strength curve begins to develop simultaneously. From optical microscopy observations, it is found that CH crystals develop rapidly and contribute to the early strength development of cement paste in the plastic state. The end of the dormant period and the second peak in the calorimetric curve are similar to both the initial and final setting times as measured by the Vicat needle. Although the transition zone of hydration introduced by the blast-furnace slag and fly ash differ slightly, they both appear somewhat later in comparison to that of ordinary portland cement.

A design consideration for durability of high-performance concrete

Abstract Environmental factors, especially the climate, have significant influence on concrete structure. This paper aims to investigate the harmful effects of maritime climate on the durability of concrete structures built in coastal areas. Singly reinforced beam specimens of traditional design and those of densified mixture design algorithm (DMDA) were employed to study the potential problems of concrete structure. Results indicate that cracks on the concrete structure, if go unnoticed, may cause failures. Thus, it is important to know the methodology of achieving high strength and durable concrete in order to avoid formation of cracks in the structural member.

Properties of Cement Paste Containing Rice Husk Ash

This paper describes how rice husk ash (RHA) was obtained under different burning conditions from 400 to 1200°C. The changes in the properties of the RHA were investigated using microanalytical techniques. When RHA obtained at different burning conditions was added to cement paste or mortar, several properties such as hydration, setting time, porosity, and strength revealed changes and these changes are described.

Properties of Concrete Using Copper Slag as a Substitute for Fine Aggregate

This paper will discuss that copper slag, a by-product of copper production, contains large amounts of iron oxide and silicate. It is chemically stable and its physical properties are similar to that of natural sand. The physical and chemical properties of copper slag were investigated. Copper slag, in amounts of 20, 40, 60, 80, and 100 percent, was substituted for fine aggregate in cement mortar and concrete. The fineness modulus of the combination of copper slag and fine aggregate was roughly 2.6, the optimum fineness modulus for concrete mix design. At this value, workability was found to be satisfactory with minimal bleeding. Addition of copper slag also improved the strength of the concrete. When the substitutional amounts exceeded 80 percent, lower strengths were obtained, possibly due to the formation of ettringite. It was also found that the effect of copper slag on long term strength development was also dependent on the amount used and its fineness. It was concluded that copper slag could be used as a fine sand substitute for ordinary reinforced concrete.

The use of rice husk ash in concrete

Publisher Summary This chapter discusses the characteristics, quality, hydration mechanism, and influence of rice husk ashes on the quality of concrete. Rice husks are byproducts of rice paddy milling industries. For rice growing countries, rice husks have attracted more attention due to environmental pollution and an increasing interest in conservation of energy and resources. For developing countries where rice production is abundant, the use of rice husk ash (RHA) to substitute for cement is attractive because of its high reactivity. As the production rate of rice husk ash is about 20% of the dried rice husk, the amount of RHA generated yearly is about 20 million tons worldwide. In addition, properly treated ashes have been shown to be active within cement paste. Hence, the use of rice husk ash in concrete is important.

Recycling waste brick from construction and demolition of buildings as pozzolanic materials

This investigation elucidates the pozzolic characteristics of pastes that contain waste brick from building construction and demolition wastes. The TCLP leaching concentrations of waste brick for the target cations or heavy metals were all lower than the current regulatory thresholds of the Taiwan EPA. Waste brick had a pozzolanic strength activity index of 107% after 28 days. It can be regarded as a strong pozzolanic material. The compressive strengths of waste brick blended cement (WBBC) that contain 10% waste brick increased from 71.2 MPa at 28 days to 75.1 MPa at 60 days, an increase of approximately 5% over that period. At 28 days, the pozzolanic reaction began, reducing the amount of Ca(OH)2 and increasing the densification. The intensity of the peak at 3640 cm— 1 associated with Ca(OH)2 is approximately the same for ordinary Portland cement (OPC) pastes. The hydration products of all the samples yield characteristics peaks at 978 cm—1 associated with C—S—H, and at ~3011 cm—1 and 1640 cm—1 associated with water. The samples yield peaks at 1112 cm—1, revealing the formation of ettringite. In WBBC pastes, the ratio Q2/Q1 increases with curing time. These results demonstrate that increasing the curing time increases the number of linear polysilicate anions in C—S—H. Experimental results reveal that waste brick has potential as a pozzolanic material in the partial replacement of cement.

Strength development of blended blast-furnace slag-cement mortars

Abstract Blast‐furnace slags cooled at different rates were used to study the effect of fineness, mixing method and content of slag on the strength development of blended‐slag mortar. Activator and curing temperature were used to activate the early strength. The microstructure of blended‐slag paste was also investigated in this study to explain the strengthen effect.

Application of High-Performance Concrete to High-Rise Building in Taiwan:

The research and development of high-performance concrete (HPC) that can give both high flow characteristics and high strength have attracted wide interest in Taiwan. The construction of the 101-storey Taipei Financial Center and the 85-storey Tungtxt & Chingtai (T&C) Tower requires the slump to be in the range of 230 – 270 mm, the initial slump flow in the range of 580 – 620 mm and slump more than 230 mm after 45 minutes, as well as a 56-day compressive strength of over 56 MPa. The HPC mix is designed using a densified mixture design algorithm which aims to achieve the lowest cement content. With the addition of pozzolanic materials such as fly ash, the workability is much improved because the shape of fly ash is spherical. After one-year of strict quality control, the HPC achieve consistent workability and excellent strength. This indicates that the appropriate use of local industrial by-product materials can produce HPC of the required design strength.

Effect of silica forms in rice husk ash on the properties of concrete

The strength and durability properties of concrete with or without three types of rice husk ash (RHA), namely, amorphous, partial crystalline, and crystalline RHA, were investigates. The three types of RHA were added into concrete at a 20% replacement level. Consequently, the pozzolanic reactivity of amorphous RHA was higher than that of partial crystalline and crystalline RHA. Concrete added with amorphous RHA showed excellent characteristics in its mechanical and durability properties. The results showed that the higher the amount of crystalline silica in RHA, the lower the concrete resistivity value became. When compared with each other, concretes with 20% of the cement replaced with these types of RHA achieved similar ultrasonic pulse velocity values, but all were lower than that of the control concrete. The incorporation of these kinds of RHA significantly reduced chloride penetration. The results not only encourage the use of amorphous materials, they also support the application of crystalline or partial crystalline RHA as mineral and pozzolanic admixtures for cement.