International Concrete Abstracts Portal

Paper reports the accelerated carbonation test results to investigate the effect of replacement ratio of fly ash, initial curing period in water, and air content on the carbonation phenomena in concrete. Using these test results, equations for the prediction of carbonation depth of concrete with and without fly ash are proposed, and these effects are also evaluated by these equations. Furthermore, the accelerated carbonation test results are compared with natural exposure test results for 15 years, and a method to predict the carbonation depth of concrete with and without fly ash exposed to natural indoor conditions is proposed. Concrete with fly ash is affected by initial curing period in water rather than concrete without fly ash from the viewpoint of depth of carbonation and compressive strength. The higher the fly ash content of concrete is, the deeper is the depth of carbonation. Depth of carbonation can be evaluated by compressive strength of concrete cured in water for 28 days, irrespective of the fly ash content of concrete. Carbonation depth of concrete with and without fly ash naturally exposed indoors can be predicted by the equation obtained by the accelerated carbonation tests.

Paper reports results of an investigation conducted to evaluate permeability, porosity, and the corrosion-resisting characteristics of fly ash concrete made with finely graded beach/dune sand and crushed limestone (typical aggregates used in the Arabian Gulf countries). The experimental program was designed to include concretes made with water-cement ratios in the range of 0.35 to 0.55. The effect of fly ash addition on water permeability, porosity, and pulse velocity was studied over a period of one year. Accelerated corrosion tests were carried out in the laboratory for a period of about 4 years to study the corrosion-resisting characteristics of these concrete mixes. Specimens were located in the exposure site to evaluate the effect of salt contamination on the corrosion of reinforcing bars in fly ash concrete. Time to cracking, weight loss of reinforcing bars, and pH measurements were also carried out on these specimens. Results show that fly ash incorporation in concrete improves its general quality. Fly ash concretes show significantly better performance than plain cement concrete mixes in terms of resistance against reinforcing bar corrosion. Also, fly ash concrete specimens contaminated with slats do not show a noticeable aggravation in the reinforcing bar corrosion process for 20 percent replacement and for a 9 month exposure period. Reaction between fly ash and calcium hydroxide does not reduce the pH value below the pH of pure saturated Ca(OH)2 solution (12.5), even after partial consumption of the calcium hydroxide by fly ash.

A research program was conducted in which the temperature rise of mortars and the durability of concrete containing fly ash were studied. The study of the effect of fly ash on the temperature rise of mortars included the use of both ASTM C 618 Class C and Class F fly ashes. Control tests were conducted on mortars containing ASTM C 150 Type I, Type I-II, and Type III cements, and comparison tests were conducted on mortars containing 20, 27.5, and 35 percent fly ash by volume of cement. It was found that the use of Class F fly ash resulted in a reduction in the temperature of the mortar, whereas the partial replacement of cement with Class C fly ash did not lower the mortar temperature, regardless of the type of cement used. Resistance to scaling in the presence of deicing salts and abrasion resistance tests were conducted on concrete samples cast from 21 batches of concrete. Variables studied included fly ash type, fly ash content, and curing conditions. Both ASTM Class F and Class C fly ashes were used to replace 25 or 35 percent of the cement by volume, and curing conditions included combinations of 50, 75, and 100 F with 50 and 100 percent relative humidities.

Curing of concrete is impaired by exposure to drying at early ages. Removing water from the surface layers restricts the binder reactions and pore structure development. High porosity in the surface region will allow the ingress of deleterious agents, which can lead to durability problems. Present work reports results obtained by hydrating a flyash blended cement under drying conditions. Comparisons are made with similar results from a portland cement. Small samples of OPC/pfa (70/30) paste with a water binder ratio of 0.59, initially cured under saturated conditions for 7 days, were exposed at 20 C in a CO2-free environment, to various preselected relative humidities. After 28 and 91 days, the extent of reaction and the porosities of the samples were measured by thermogravimetry and methanol adsorption, respectively. Results show the extent of hydration falls when changing from saturated to 70 percent relative humidity (rh) conditions; below this rh, it is virtually constant. From the shape of the TGA curve, it seems that there is little change in the nature of the gel phase. The pozzolanic reaction appeared to cease below 80 percent rh. Total porosity only fell very slightly with increasing relative humidity even after 91 days exposure. Under drying conditions (70 percent rh) the large-diameter porosity was three times greater than large-diameter porosity obtained under saturated conditions. From these tests it is clear that to promote reaction and to effect a decrease in the volume of large pores, the relative humidity must be greater than 95 percent, at least during early-age curing.

Blast furnace slags have been successfully used to reduce the leachability of technetium from cement-based waste forms because the slag produces a less permeable product or reduces the pertechnetate to a less mobile form. Waste contaminated with technetium is of particular concern to the U.S. Nuclear Regulatory Commission, Department of Energy, and Environmental Protection Agency because of its mobility as the pertechnetate ion. Results on the technetium leachability of cement-based waste forms with and without a slag component and for different slags are presented. The mass transfer parameter (e.g., diffusivity) for leaching technetium from these waste forms decreased by five orders of magnitude when slag was used (i.e., using slag can increase the ANS 16.1 leachability index by five). Results of bulk and surface examinations of the slags are presented.