International Concrete Abstracts Portal

This paper shows how wastes can be converted into useful complementary cementing materials instead of being landfilled. Two examples are presented. The first one deals with paper sludge, the residue coming either from deinking of old papers or water treatment. When calcined under 650°C, paper sludge is transformed into a mixture of metakaolin and calcium carbonate which consumes calcium hydroxide very quickly and can be considered as a very reactive pozzolan. When calcined over 7OOC, the calcium carbonate contained in paper sludge starts to decarbonate and yields calcium oxide and calcium hydroxide. A hydraulic product is obtained, which can be used for partial replacement of portland cement, in backfilling mortars or as a viscosity agent in self-leveling concrete. The second example is the development of synthetic slag from different wastes. These wastes are mixed with a source of calcium and molten at 1450°C then cooled and granulated with water. The resulting vitreous product, once milled, presents very effective hydraulic properties.

The practical application of silica fume started in the late seventies. At that time and the following years, opinions on the effect of silica fume on reinforcement corrosion varied quite a lot. Since then a lot of research has been carried out and the results of many years of practical experience are known. The corrosion process of reinforcement steel in concrete may be divided into two stages: the initiation period and the propagation period. Silica fume affects both the two stages. In the initiation period, carbonation is going on or chlorides are transported into the concrete. The carbonation process results in reduced pH values allowing corrosion to start. Silica fume may be expected to reduce the resistance against carbonation due to reduced amount of calcium hydroxide. On the other hand, silica fume will also improve the resistance against CO? ingress. A concentration of chlorides higher that the threshold value at the steel surface may result in reinforcement corrosion. Silica fume reduces the chloride binding capacity of the binder, leaving more chlorides to attack the steel, but again silica fume will also improve the resistance against chloride ingress. In the propagation period, the electrical resistivity of the concrete and oxygen diffusion are the governing factors for corrosion rate. The first is very much increased by silica fume, the second more debatable. Based on literature review, all important factors for the initiation period and the propagation period are discussed. As far as possible, consensus for each factor is given and an overall conclusion for the effect of silica fume on reinforcement corrosion is given. The overall conclusion is that silica fume has both positive and negative effects on the different factors governing the steel reinforcement corrosion. However, the positive factors are dominating by far over the negative factors.

This paper provides results about the effect of using different types of curing on the compressive strength of concrete both with and without large volumes fly ash (FA). In all the concrete mixtures, the portland cement content was 200 kg/m3. The FA amount was varied from zero to 33,43,50 and 56 percent by mass of the total binder, and a superplasticizer was used to obtain 200-220 mm slump. The compressive strength was tested at the age of 3,7, 14,28,56 days and 6 months. The compressive strength of the Portland-cement concrete made at 35°C was reduced by about 11% at 28 days when compared to that of concrete made at 23°C with ASTM standard curing. With continuous moist-curing of fresh concrete, there was no strength loss of concrete made at 35°C. FA concrete specimens that were under intermittent spray-water curing at 35°C in the laboratory (every four hours) for 7 days and then under ambient conditions gave increased compressive strength up to the time of testing i.e. 6 months. Reduced strength was obtained for 3 days intermittent curing. Higher strength was obtained as the amount of FA was increased for a given amount of the portland cement. The FA concrete mixtures cast at 35°C were cured by covering the specimens with membrane curing compound and placed under ambient conditions until age of testing, the strengths were lower than reference concrete by about 20% to 30% at 28 days, and 30% to 50% at 56 days. It is necessary that enough curing water to promote the pozzolanic reaction is used. The membrane curing did not allow the ingress of water to the concrete mass.

Corrosion of reinforcement embedded in concrete is the most common cause of failure in concrete structures. The work is being carried out to investigate the rate and amount of corrosion of steel in concrete where cement is replaced by slag in different proportions. This paper reports a detailed corrosion study, carried out on concrete containing blast furnace slag. The study encompasses slag collected from several premier steel plants in India. Corrosion of steel was examined electrochemically through potentiodynamic study and also by accelerated corrosion study. Micrographic study was also carried out to examine the alteration of pore structure of slag concrete. These studies reveal that increase in slag proportion decreases the rate and amount of corrosion of reinforcement embedded in slag concrete.

Marine durability of 15-year old plain and reinforced concrete cylindrical specimens exposed in marine environments for 15 years is presented here. The specimens were made with ordinary Portland, slag (Type A, B and C) and fly ash (Type B) cements. Water-to-cement ratios were 0.45 and 0.55. Compressive strength of concrete, corrosion of steel bars, and chloride-ion concentrations in concrete were evaluated. After 15-year of exposure, compressive strength of concrete increases compared to its 28-day’s strength for the investigated cements, except fly ash cement. Slag cement of Type C shows the best performance against chloride ingress and corrosion of steel bars in concrete. Accumulation of chloride-ion at the surface of concrete made with slag and fly ash cements is observed. The presence of voids at the steel-concrete interface causes the formation of corrosion pits irrespective of the type of cement. The use of seawater as mixing water causes an earlier strength development at the 28-day and does not cause the strength of concrete to regress after 15-year of exposure. However, it causes more corrosion of steel bars at a lower cover depth. At the deeper cover depth, no significant corrosion of steel bars is found irrespective of the type of mixing water.