International Concrete Abstracts Portal

In the design of concrete structures, carbonation is one of the most important factors determining service life. Environmental conditions, mix proportions, and cement type are also significant. It has been generally indicated that carbonation rate of concrete made with blast furnace slag or fly ash cements is greater than that provided by portland cement alone. In this study, an accelerated carbonation test was conducted on concrete made with binary and ternary blended cements containing large quantities of admixture. The binary and ternary cements used in this study consisted of three types of portland cement (normal, moderate heat, and belite low-heat), blast furnace slag, and fly ash. The influence of portland cement type and blending ratio of cementitious materials on carbonation rate is discussed. The study focused particularly on calcium hydroxide content in concrete, microstructural density, and the relation between these factors and carbonation rate. The following conclusions were drawn. In binary and ternary blended cement, when the blending ratio of blast furnace slag and fly ash increased, the carbonation constant increased. When the blending ratio of blast furnace slag was more than 60 to 70 percent, the carbonation rate increased rapidly. When fly ash was blended within the range of up to 30 percent, the carbonation constant increased in proportion to the blending ratio, and this tendency did not change, regardless of the portland cement type. Also, to evaluate the carbonation rate of blended cement, both the effective water-cement ratio and calcium hydroxide content, determined by the balance between the amount of calcium hydroxide produced by the hydration of portland cement and the amount required for the complete reaction of blast furnace slag, must be taken into account.

The mechanisms underlying physical disintegration of concrete by crystallization of mirabilite (Na 2SO 4 10H 2O) and thenardite (Na 2SO 4) were studied by a series of laboratory experiments. In contrast to chemical sulfate attack, which manifests itself in the formation of gypsum or ettringite, the deterioration investigated in this study did not involve chemical attack on the cement paste in concrete. Rather, the damage was strictly of a physical nature, caused by phase changes within the sodium sulfate-water system. Several possible mechanisms of distress were investigated, including pressure caused by hydration, evaporation, and temperature effects. Rapid temperature changes were found to be the dominant mechanism of deterioration. In particular, rapid decreases in temperature resulted in supersaturation, rapid crystallization, and a net increase in the sodium sulfate-water system. Consequently, significant hydraulic pressure, similar to that observed in the classical freeze-thaw phenomenon, would develop if drainage conditions within the concrete were not adequate to allow for the volume increase of the sodium sulfate-water system.

In this study, structural-grade concretes with characteristic strength of 20 to 45 MPa were made with general purpose portland cement (ASTM Type I) and fly ash blends. High volumes of fly ash (ASTM Class F) in the range of 40 to 50 percent by weight of total binder were used. It was found that for an equivalent 28-day strength and slump, structural concretes with high-volume fly ash can provide a number of advantages over plain cement concretes, including lower drying shrinkage and better creep characteristics. Similar flexural strengths and elastic modulus were observed between equivalent plain cement and high-volume fly ash concretes. Experience obtained in field trials of high-volume fly ash concretes showed that they can be mixed, transported, placed, and finished using conventional concreting equipment and techniques. Laboratory studies of blended cements with high percentages of fly ash as cement replacement material indicated that steel passivation characteristics improved with age of hydration and that there was no negative effect caused by pozzolanic reaction. Electrochemical data using polarization resistance techniques on paste samples immersed in NaCl solution are given. The results indicated that, even with limited initial curing of 7 days, the corrosion rates of steel in 40 percent fly ash blend by weight were very similar to that of plain cement at high water-to-binder ratio (>0.6) and were lower than that of plain cement at low water-to-binder ratio ( 0.6). Data obtained from mortar samples subjected to sulfate environments suggested that the use of blended cements with high fly ash replacement could be beneficial in the case where the pH of the environment is low, such as that experienced by concrete structures in sewerage works.

In this study, results are reported on classified fly ash with a maximum particle diameter of 10 m. Durability of concrete with this fly ash has been verified experimentally. Silica fume, which is well known for its improvement of concrete durability, was used as the basis of comparison. Durability tests included water permeability, freeze-thaw resistance, air-void spacing factor, pore size distribution, and pozzolanic activity. Based on the experimental results, remarkable reduction of the water diffusion coefficient, enhancement of freeze-thaw resistance, densification of air-void structure, increase of noncrystalline products, and other features were noted, and it was determined that classified fly ash is an effective admixture that contributes to the improvement of concrete durability.

The effects of steam-curing at atmospheric pressure were investigated on three kinds of concrete: pozzolanic portland cement concrete, high-strength portland cement concrete, and high-performance concrete manufactured with high-strength portland cement containing a superplasticizer and retarder. The early and final compressive and flexural strengths, coefficients of capillary absorption, and apparent porosities were determined on prismatic specimens stored for 140 days in water and ammonium nitrate solution. The dynamic moduli of elasticity were also measured every week during the storage period. Two steam-curing cycles with maximum temperature of 60 and 80 C, respectively, were used. The steam-curing processes were found to be suitable and less detrimental for high-performance concrete compared to pozzolanic portland cement and high-strength portland cement concretes.