International Concrete Abstracts Portal

Heavy damage due to alkali-aggregate reaction has been observed in concrete structure in and along the sea. An accelerated test is performed on mortar to evaluate effectiveness of fly ash for controlling alkali-aggregate reaction in the marine environment. Mortar bars using Pyrex as aggregate and cements with 0.6 and 1.1% of equivalent sodium oxide are made. The alkali content in the mixture is adjusted by adding NaOH or NaCl. Specimens are stored in distilled water, NaCl solution, and under more than 95% of relative humidity. The controlling effect of fly ash and the effect of internal and intruded chloride ion in mortar on alkali-aggregate reaction is studied by measuring the expansion of mortar. Expansion of mortar depends on the type of cement and chemical reagents used for alkali adjustment, the amount of fly ash used and the exposure condition. Even with the same equivalent sodium oxide in the mixture, mortar using NaCl for alkali adjustment shows higher expansion than mortar using NaOH. The highest expansion is revealed for mortar cured in NaCl solution. The controlling effect of fly ash also depends on the type of cement and the exposure condition.

Corrosion of reinforcement is one of the major degradation mechanisms of reinforced concrete elements. The majority of studies published on concrete-steel corrosion have been conducted on unstressed specimens. Structural concrete, however, is subjected to substantial strain near the steel reinforcing bars that resist tensile loads, which results in a system of microcracks. Report presents the initial results of an investigation to determine the effect of applied load and microcracking on the rate of ingress of chloride on and corrosion of steel in concrete. Simply supported concrete beam specimens were loaded to give a maximum strain of about 600 æî on the tension face. Chloride ion ingress on cores taken from loaded specimens was monitored using energy-dispersive x-ray analysis techniques. Corrosion current and rate measurements using linear polarization electrochemical techniques were also obtained on the same loaded specimens. Variables investigated included two concrete types, two steel cover depths, three applied load levels, bonded and unbonded reinforcing steel, and the exposure to tension and compression beam faces to chloride solution. One concrete mixture was made with Type 10 portland cement, the other with 75 percent blast furnace slag, 22 percent Type 50 cement, and 3 percent silica fume. The rate of chloride ion ingress into reinforced concrete and hence the time for chloride ion to reach the reinforcing steel is shown to be dependent on applied load and the concrete quality. The dependence of corrosion process descriptors--passive layer formation, initiation period, and propagation period--on level of applied load is discussed.

Compressive fatigue strength of concrete in a submerged condition deteriorates drastically compared with concrete in an air-dried condition. One of the reasons for the lowering of fatigue strength in submerged or wet concrete appears to be the influence of the reduction of the bond at the interface between the aggregate and the cement paste. However, this reduction may be mitigated by reducing the calcium hydroxide content and filling the voids at the interface. In this study, compressive fatigue tests were performed in submerged conditions using concrete composed of blast furnace slag or silica fume. The 2-million-cycle fatigue strength of this submerged concrete improved up to 44 percent of its static strength in water compared to 31 percent for ordinary concrete in water. However, this was found to be smaller than 56 percent for ordinary concrete in air. During these tests, the pH of the water in the test tank and the strain of the specimens were measured, and the amounts of calcium hydroxide that oozed out from the specimen and the strain behavior were investigated. The increase in fatigue strength is due to an improvement in the aggregate interface bond and watertightness. However, the expansion of cracks just before failure, which is a distinct characteristic of fatigue in water, was not checked.

Silica fume is no longer a waste by-product from the silicon metal and ferrosilicon alloy industries, but a well-established pozzolanic material which can contribute unique properties to portland cement products. The use of silica fume in cement and concrete technology has sharply increased in North America in the last 5 years. An overview of recently published literature on the subject is presented. Silica fume modifies physical characteristics of fresh cement paste as well as the microstructure of the paste after hardening. The various mechanisms of action of silica fume that cause physical and chemical changes in concrete are discussed. The role of silica fume in altering engineering properties of concrete is highlighted. In particular, the effects of silica fume on the following properties of concrete are discussed: rheological properties (such as consistency and cohesiveness), mechanical properties (such as compressive, tensile, and flexural strengths; bond strength with reinforcement; creep and drying shrinkage), and durability (such as resistance to deterioration by aggressive chemicals, abrasion-erosion, and freeze-thaw cycles).

Cement pastes of interest for high-strength concrete technology were investigated by high-resolution solid state magic angle spinning (MAS) Si-nuclear magnetic resonance (NMR) in combination with thermal analysis (DTA/TG). NMR reveals the degree of hydration for C3S/C2S in cement, pozzolanic activity of condensed silica fume, and average chain length of the silicate anions in the CSH-gel. A combination of NMR and DTA/TC data gives the empirical formula of the CSH-gel. The binders investigated were made from blended portland cement containing 0, 8, and 16 percent cement replacement with condensed silica fume and water-binder ratios of 0.20, 0.30, and 0.40. The specimens were allowed to cure in sealed conditions for 1, 3, 7, 28, 126, and 442 days. The results confirmed that condensed silica fume is a very reactive pozzolan. The conversion rate of condensed silica fume to hydration products after 3 days of curing was, in fact, higher than for the neat cement at the same age. After 3 days of curing, condensed silica fume reduced the degree of hydration of the cement in the blended cement pastes when compared with pastes without it. The effect was enhanced at later ages when the cement hydration process stopped while the pozzolanic reaction continued to near completion. In regard to the composition of the CSH-gel, it was found that the average chain length for the linear polysilicate anions increased with decreasing w(c + s) and, in particular, with increasing dosages of condensed silica fume. Furthermore, the c/s of the gel decreased considerably with increasing dosages of condensed silica fume. The mechanism of the pozzolanic reaction of condensed silica fume is discussed.