The freeze-thaw resistance of concretes was studied. Nine concrete mixes, made with five cements and cement-Class C fly ash combinations, were exposed to freeze-thaw cycling following 110 to 222 days of moist curing. Prior to the freeze-thaw cycling, the specimens were examined by a low-vacuum scanning electron microscope (SEM) for their microstructure. The influence of a wet/dry treatment was also studied. Infilling of ettringite in entrained air voids was observed in the concretes tested. The extent of the infilling depends on the period of moist curing as well as the wet/dry treatment. The concretes with 15% Class C fly ash replacement show more infilling in their air voids. It was found that the influence of the infilling on the freeze-thaw durability relates to the air spacing factor. The greater the spacing factor, the more expansion under the freeze-thaw cycling. The infilling seems to decrease effective air content and to increase effective spacing factor. The infilling also implies that the filled air voids are water-accessible. These might lead to concrete more vulnerable to the freeze-thaw attack. By combining the above results with field observations, one may conclude that the freeze-thaw damage is a factor related to premature deterioration of portland cement concrete pavements in Iowa.
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Recent advances in plasma arc technology have found many civil engineering applications, including in-situ soil vitrification (ISV). The ISV process transforms soils into homogeneous glass-like materials, which possess high compressive and tensile strengths (typically about 1 0 times those of unreinforced concrete), high leaching resistance, and are unaffected by wet-dry or freeze-thaw cycles. These vitrified earthen materials usually weigh about 2300 to 2500 kg per cubic meter. Potential ISV applications include slope stabilization, groundwater removal, subgrade stabilization, and simulated construction materials. A 1 00-kW non-transferred plasma arc torch developed by the Plasma Energy Corporation was used for soil vitrification experiments with operating temperatures at about 4000°C. The soils tested include Piedmont silty sand, kaolin clay, and Tyndall beach sand. 5. 1-cm cubes and 5. 1-cm diameter cylinders were cut from the vitrified soil samples and subjected to compression tests, split-cylinder tension tests, and split-Hopkinson bar impact tests.
This study represents one of the first few attempts to systematically assess the reliability of high-strength concrete (HSC) columns designed according to the American Concrete Institute (ACI) Code provisions. In this study, the reliability of both short and slender HSC columns is assessed using a hybrid probabilistic approach. This approach combines Monte Carlo simulation with a first order reliability method. The reliability of 48 reinforced concrete columns designed according to the ACI Code is evaluated. The effects of the concrete compressive strength, amount of confining steel, amount of longitudinal steel, slenderness, and live to dead load ratio are examined. Numerical examples demonstrate that the reliability of short HSC columns is lower than that of the corresponding normal-strength concrete columns. Larger amounts of longitudinal steel have a beneficial effect on column reliability, especially in the case of slender columns. In addition, at small eccentricities, the reliability of HSC slender columns is generally higher than that of the corresponding short columns. Finally, it is demonstrated that at large eccentricities, very low reliability levels may be found for the combination of high concrete compressive strength, minimum amount of longitudinal steel, and high slenderness ratio. Based on these findings, design recommendations are suggested.
The effect of copper slag on the hydration of cement-based materials is studied. Up to 15% by weight of copper slag was used as a portland cement replacement. Hydration reactions were studied through semiquantitative X-ray diffraction and TGA/DTA. Samples of copper slag and hydrated lime (ASTM type S) were used to test the pozzolanic properties of the slag. The porosity was examined using mercury intrusion porosimetry. A decrease in capillary porosity was observed while the gel porosity was increased. A significant increase in the compressive strength for up to 1 year is observed.
Corrosion rate studies were carried out on carbon steel rebar samples under different pH conditions and in the presence and absence of chloride ions in solution. A known amount of calcium nitrite was added as an inhibitor and the mechanism of inhibition was studied by tracking both the thermodynamic and kinetic properties of the system. The studies indicate that there is a competition between the corrosion and passivation reactions, and the resulting open-circuit potential depends on the relative strength of the corroding and passivating environments. The corrosion rate depends to a great extent on the pH of the solution. Nitrite ions act as anodic inhibitors by increasing the rate of formation of a barrier oxide film. The protective action of the nitrite ions seems to be more pronounced in highly corroding environment. This is due to the mechanism of inhibition, which uses the product of the unwanted corrosion reaction and converts it into a favorable passivating one. For a given amount of chloride, a minimum threshold concentration of nitrite is essential for protecting the steel.
The inhibition of hydrogen permeation by zinc-nickel electrodeposited alloy was investigated using the Devanathan–Stachurski permeation technique. The hydrogen evolution and hydrogen permeation rates for the zinc–nickel alloy electrodeposits on iron are compared with the rates for bare iron, zinc electroplated on iron, and nickel electroplated on iron. Hydrogen evolution rates and hydrogen permeation rates were followed as functions of time at different applied potentials. The hydrogen permeation inhibition for thin zinc–nickel electroplates (20s at 10mAcm−2 and 10s at 20mAcm−2) averaged 80% and intermediate to that of nickel and zinc. This inhibition was considered to be mostly due to kinetic effects. Zinc–nickel electroplated for 20 and 40min. at 10mAcm−2 inhibited the hydrogen permeation greater than 95% as compared to bare iron. This inhibition was due to both kinetics and the barrier effect caused by the diffusion resistance of the membrane.
Steel containing 0.03% carbon, 1.35% copper and 0.84% nickel had yield strength in the 540-625 MPa (78-90 Ksi) range depending on thickness, ultimate tensile strength in the 625-690 MPa (90100 Ksi) range, and 25-30% elongation when air cooled after hot rolling. No brittle heat-affected zone was formed during manual or automatic submerged arc welding without pre-heating or postheating. The fracture toughnesses in the plate and in the heat-affected zone were excellent. Introduction During the past several years an easily weldable, high strength (more than 540 MPa yield), high impact fracture toughness steel (NUCu) has been investigated at Northwestern University with bridge applications in mind. For good weldability without pre-heating and post-heating, the carbon content of the steel was kept low and high strength was achieved by copper precipitation hardening. The steel was designed to be air cooled from hot rolling. Omitting the most expensive alloying elements (Cr and Mo), and eliminating the need for quench and temper heat treatment used in other high strength structural steel alloys reduces the cost of the steel, an important requirement for infrastructure applications. Experimental Methods The initial studies at Northwestern University were conducted on six 220 kg laboratory heats prepared at Inland Steel Company's Research Laboratory by vacuum-induction melting. These steel heats, numbered as NUCul through NUCu6 were hot-rolled to 12.7 mm thick plates and air cooled. A commercial 80 ton steel heat was produced at Oregon Steel Mills, Portland, Oregon. The heat was calcium treated for inclusion shape control. Two slabs were cast using Amsted bottom pressure casting process. The slabs were hot-rolled into plates of several thicknesses 12.7 to 25.4 mm (0.5 to I inch) and air cooled. A hot-rolling temperature of 1150°C or less was specified. The compositions of two laboratory steel heats (NUCu5 and NUCu6) produced by Inland Steel Research Laboratory and the commercial steel produced by Oregon Steel Mills (NUCu-Oregon), are listed in TABLE 1. For good weldability the carbon content in steels was kept low, 0.03 to 0.05%, giving low carbon equivalent (Table 1). Since the steel is not quenched and tempered, Cr and Mo were omitted. Copper concentration was approximately 1.3% to provide targeted better than 485 MPa (70 Ksi) strength through precipitation hardening. Ni was added to prevent hotshortness during hot rolling. Nb and Ti were added to control grain size during hot rolling and welding. TABLE 1. Composition of Experimental NUCu5 and NUCu6, and Commercial NUCu-Oregon Steels (Wt.%) ELEMENT NUCu5-INLAND NuCu6-INLAND NUCu-OREGON
